Accepted Manuscript DFT simulation, quantum chemical electronic structure, spectroscopic and structure-activity investigations of 2–benzothiazole acetonitrile V. Arjunan, S. Thillai Govindaraja, Sujin P. Jose, S. Mohan PII: DOI: Reference:

S1386-1425(14)00386-2 http://dx.doi.org/10.1016/j.saa.2014.02.187 SAA 11818

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received Date: Revised Date: Accepted Date:

7 January 2014 24 February 2014 25 February 2014

Please cite this article as: V. Arjunan, S. Thillai Govindaraja, S.P. Jose, S. Mohan, DFT simulation, quantum chemical electronic structure, spectroscopic and structure-activity investigations of 2–benzothiazole acetonitrile, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa. 2014.02.187

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

DFT simulation, quantum chemical electronic structure, spectroscopic and structureactivity investigations of 2–benzothiazole acetonitrile V. Arjunan1, S. Thillai Govindaraja2, Sujin P. Jose3 and S. Mohan4 1

Department of Chemistry, Kanchi Mamunivar Centre for Post–Graduate Studies, Puducherry 605 008. India.

2

Research and Development Centre, Bharathiar University, Coimbatore 641 046. India. 3

4

School of Physics, Madurai Kamaraj University, Madurai 625 021. India.

School of Sciences and Humanities, Vel Tech University, Avadi, Chennai 600 032. India.

Abstract The Fourier transform infrared and FT–Raman spectra of 2–benzothiazole acetonitrile (BTAN) have been recorded in the range 4000–450 and 4000–100 cm−1 respectively. The conformational analysis of the compound has been carried out to obtain the stable geometry of the compound. The complete vibrational assignment and analysis of the fundamental modes of the compound are carried out using the experimental FTIR and FT–Raman data and quantum chemical studies. The experimental vibrational frequencies are compared with the wavenumbers derived theoretically by B3LYP gradient calculations employing the standard 6–31G**, high level 6–311++G** and cc–pVTZ basis sets. The structural parameters, thermodynamic properties and vibrational frequencies of the normal modes obtained from the B3LYP methods are in good agreement with the experimental data. The 1H (400 MHz; CDCl3) and

13

C (100 MHz; CDCl3) nuclear magnetic resonance (NMR) spectra are also

recorded. The electronic properties, the energies of the highest occupied and lowest unoccupied molecular orbitals are measured by DFT approach. The kinetic stability of the molecule has been determined from the frontier molecular orbital energy gap. The charges of the atoms and the structure–chemical reactivity relations of the compound are determined by its chemical potential, global hardness, global softness, electronegativity, electrophilicity and

1

local reactivity descriptors by conceptual DFT methods. The non–linear optical properties of the compound have been discussed by measuring the polarisability and hyperpolarisability tensors. Key words: 2–benzothiazole acetonitrile; FTIR; FT–Raman; DFT; NBO; NLO. *Author for correspondence E.mail: [email protected] (Arjunan) Fax: +91 413 2251613; Tel.: +91 413 2211111; Mobile: 9442992223.

2

1. Introduction Heterocyclic compounds are widely distributed in nature and are essential to life in various ways [1]. The chemistry and biological studies of heterocyclic compounds has been an interesting field for a long time in medicinal chemistry. A number of heterocyclic derivatives containing nitrogen and sulphur atom serve as a unique and versatile scaffolds for experimental drug design [2]. Benzothiazole is one of the most important heterocyclic compound having varied biological activities with great scientific interest. The compounds containing benzothiazole ring showing numerous biological activities such as antimicrobial [3–7] anticancer [8–11], anthelmintic [12], antidiabetic [13] activities. They have also found application in industry as antioxidants, vulcanisation accelerators. Various benzothiazoles such as 2–substituted benzothiazole received much attention due to its unique structure and its uses as radioactive amyloid imaging agents [14], and anticancer agents [15]. The 2– aminobenzothiazole scaffold is one of the privileged structure in medicinal chemistry [14,16] and reported cytotoxic on cancer cells [16]. It must be emphasised that combination of 2– aminobenzothiazoles with other heterocyclic compounds is a well known approach to design new drug like molecules, which allows achieving new pharmacological profile, action and toxicity lowering. Extraction and analysis of various benzothiazoles from industrial waste water have been studied [17].

Ultrafast branching of reaction pathways in 2–(2–

Hydroxyphenyl)benzothiazole in polar acetonitrile solution was investigated [18]. Absorption and fluorescence properties were studied for bis(4–vinylphenyl)acrylonitrile and two dodecyloxy bis(4–vinylphenyl)–acrylonitrile [19]. Photochemical Synthesis of s– triazolo[3,4–b]benzothiazole and mechanistic studies on benzothiazole formation has been carried out [20].

Accelerators for the vulcanisation of rubber are based on 2–

mercaptobenzothiazole [21]. This ring is a potential component in nonlinear optics (NLO) [22]. In spite of the importance of biological and industrial significance of the compound a

3

detailed infrared, Raman, NMR spectral studies, structure–chemical reactivity relations and nonlinear optical properties of 2–benzothiazole acetonitrile (BTAN) have been undertaken for the first time. 2. Experimental The polycrystalline sample of 2–benzothiazole acetonitrile (BTAN) was purchased from Aldrich chemicals, U.S.A and used as such to record the FTIR, FT–Raman and NMR spectra. The FTIR spectrum is recorded by KBr pellet method on a Bruker IFS 66V spectrometer equipped with a Globar source, Ge/KBr beam splitter and a TGS detector in the range of 4000 to 450 cm–1. The spectral resolution is 2 cm–1. The FT–Raman spectrum is also recorded in the range 4000 to 100 cm–1 using the same instrument with FRA106 Raman module equipped with Nd:YAG laser source operating at 1.064 μm with 200 mW powers. A liquid nitrogen cooled–Ge detector is used. The frequencies of all sharp bands are accurate to 2 cm–1. The 1H (400 MHz; CDCl3) and

13

C (100 MHz; CDCl3) nuclear magnetic resonance

(NMR) spectra are recorded on a Bruker HC400 instrument using CDCl3 solvent. Chemical shifts for protons are reported in parts per million scales (δ scale) downfield from tetramethylsilane. 3. Computational details The LCAO–MO–SCF restricted B3LYP correlation functional calculations of BTAN have been performed with Gaussian–09 [23] program, invoking gradient geometry optimisation [24]. The gradient corrected density functional theory (DFT) [24] with the three–parameter hybrid functional (B3) [25,26] for the exchange part and the Lee–Yang–Parr (LYP) correlation function [27], with the standard 6–31G**, cc–pVTZ and high level 6–311++G** basis sets have been used for the computation of molecular structure optimisation, its energy, vibrational frequencies, thermodynamic properties and reactivity descriptors of the compound. The optimised structural parameters of the

4

compound BTAN are used for harmonic vibrational frequencies calculations resulting in IR and Raman frequencies together with intensities and Raman depolarisation ratios. The Raman scattering activities (Si) of the fundamental modes are suitably converted to relative Raman intensities (Ii) using the following relationship [28]

Ii 

f ( 0  i ) 4 Si  i [1  exp( hc i / kT )]

where, v0 is the exciting frequency (cm−1), vi is the vibrational wavenumber of the ith normal mode, h, c and k are universal constants, and f is the suitably chosen common scaling factor for all the peak intensities. The molecular electrostatic potential surface (MEP) and electron density surface [29] are simulated by using 6–311++G** basis set. The molecular electrostatic potential (MEP) at a point „r‟ in the space around a molecule (in atomic units) can be expressed as: 

V (r )   A

ZA 



RA  r



 (r ')dr ' 



r ' r

where, ZA is the charge on nucleus A, located at RA and ρ(r′) is the electronic density function for the molecule. The first and second terms represent the contributions to the potential due to nuclei and electrons, respectively. V(r) is the resultant electric potential at each point r, which is the net electrostatic effect produced at the point r by both the electrons and nuclei of the molecule. The molecular electrostatic potential (MEP) serves as a useful quantity to explain hydrogen bonding, reactivity and structure–activity relationship of the molecules [30]. The total electron density surface mapped with electrostatic potential depicts the shape, size, charge density distribution and the site of chemical reactivity of a molecule. GaussView 5.0.8 visualisation program [31] has been utilised to construct the MEP surface and the shape of the frontier molecular orbitals.

5

The stabilisation energy E(2) associated with i (donor) → j (acceptor) delocalisation is estimated from the second–order perturbation approach [32] as given below

E

( 2)

F 2 (i, j )  qi  j  i

where qi is the donor orbital occupancy, εi and εj are diagonal elements (orbital energies) and F(i,j) is the off–diagonal Fock matrix element. The 1H and

13

C NMR isotropic shielding are calculated using the GIAO method

[33,34] using the optimised parameters obtained from B3LYP/6–311++G** method. The effect of CDCl3 solvent on the theoretical NMR parameters is included using the PCM model. The isotropic shielding constant values are used to calculate the isotropic chemical shifts (δ) with respect to tetramethylsilane (TMS) using the relation δiso(X) = σiso TMS(X) – σiso(X), where δiso – isotropic chemical shift and σiso – isotropic shielding constant. Various reactivity and selectivity descriptors such as chemical hardness, chemical potential, softness, electrophilicity, nucleophilicity and the appropriate local quantities employing natural population analysis (NPA) scheme are calculated. Both the global and local reactivity descriptors are determined using finite difference approximation to reveal the reactivity sites of the molecule. The vertical ionization potential (I), electron affinity (A) and the electron populations are determined on the basis of B3LYP/6–311++G** method. The energy of the N electron species of the BTAN has been determined by restricted B3LYP method while N–1 and N+1 electronic species were done using restricted open B3LYP method using the geometry optimised with B3LYP/6–311++G** method. The site– selectivity of a chemical system can be determined by using Fukui functions [35,36] which can be interpreted either as the change of electron density ρ(r) at each point r when the total number of electrons is changed or as the sensitivity of chemical potential (μ) of a system to an external perturbation at a particular point r. 6

     (r )   f (r )       N v ( r )  v(r )  N

Yang and Parr introduced local softness s(r) to predict the reactivity [36]. The s(r) describes the sensitivity of the chemical potential of the system to the local external perturbation and is obtained by simply multiplying Fukui function f(r) with global softness S. The local softness values are generally used in predicting the reactivities such as electrophilic, nucleophilic and free radical reactions and regioselectivity etc.

  (r )   and s(r )  f (r )S s (r )      v ( r ) where, S is the global softness which is inversely related to global hardness (η). The generalised philicity descriptor, ω(r) contains almost all informations about hitherto known different global and local reactivity and selectivity descriptors, in addition to the information regarding electrophilic/nucleophilic power of a given atomic site in a molecule [37]. The local quantity called philictiy associated with a site k in a molecule can be calculated as ka  f ka , where    2 / 2 and a = +, – and 0 represents local philic quantities describing nucleophilic, electrophilic and radical attacks respectively. The condensed philicity summed over a group of relevant atoms is defined as the group philicity n

[38] can also be determined by using the relation  ga   ka where, n is the number of k 1

atoms coordinated to the reactive atom,  ka is the local electrophilicity of the atom k. 4. Results and discussion 4.1. Conformational analysis To predict the most stable geometry and other possible conformations of the compound, the potential energy surface (PES) has been determined by B3LYP/6– 311++G(d,p) method using the dihedral angle N3–C2–C14–C17. During the analysis all the

7

geometrical parameters are simultaneously relaxed while the N3–C2–C14–C17 torsional angle is varied in steps of 15o upto 360o. The potential energy profile which reflects the stability of the possible conformers of the molecule is shown in Fig. 1. The PES reveals that there are four possible conformers for the compound I, II, III and IV shown in Fig. 2. In the most stable geometry (I) of 2–benzothiazole acetonitrile all the atoms present in the benzothiazole moiety lie in the molecular plane but the acetonitrile group does not lie in the molecular plane. It deviates by a mean angle of 35.37o from the plane of the ring. The atoms C14, C17 and N18 present in the acetonitrile group lie in a linear manner. This is confirmed by the angle C14–C17–N18 and is equal to 179.1o. The bond angle C2–C14–C17 is 114.3o. This clearly indicates that the conformer I corresponds to the global minimum. There are two transition structures II and IV. The transition structure II has the acetonitrile group almost in the molecular plane of benzothiazole moiety and it deviates only by 3.9o. The conformer II is 0.24 kcal mol–1 less stable than the more stable conformer I. In the transition structure IV the acetonitrile group deviates more (47.7o) from the benzothiazole moiety and is least stable than that of the more stable conformer (I) by 1.19 kcal mol–1. The conformer III is almost planar and the acetonitrile group deviates from the benzothiazole ring by an angle of 3.4o. Here the nitrile (C≡N) takes up the orientation just opposite to that in conformer II. This conformer is less stable by 1.09 kcal mol–1 than that of the conformer I. The conformer III is not the minima and considered the saddle point structure. From Fig. 1, it is possible to determine the barrier energies of the internal rotation of the acetonitrile group in BTAN. It needs 0.24 kcal mol –1 to enable the planar acetonitrile group and 1.19 kcal mol–1 to have internal rotation perpendicular to the benzothiazole ring. 4.2. Molecular geometry

8

In order to provide the accurate structural parameters of the compound, the most stable conformer is optimised with B3LYP method using 6–31G**, 6–311++G** and cc– pVTZ basis sets. The molecular geometry of BTAN possesses C1 point group symmetry. All the modes are found to be IR and Raman active suggesting that the molecule possesses a noncentro symmetric structure, which recommends the title compound for non–linear optical applications. The optimised molecular geometry of BTAN is shown in Fig. 2(I). 4.3. Structural properties The optimised geometrical parameters of BTAN obtained by B3LYP method with 6– 311++G**, 6–31G** and cc–pVTZ basis sets are listed in Table 1. The computed bond length, bond angle and dihedral angle of the title molecule were compared with the X–ray diffraction data of similar compounds [39,40]. From Table 1, it can be observed that the computed geometrical parameters are agreed well with the single crystal XRD data. The average bond length of aromatic benzene ring is 1.40 Å. The bond length of S1– C2 and S1–C9 are 1.77 Å and 1.75 Å, respectively this slight difference in bond length is due to the presence of acetonitrile group at C2 carbon atom. The bond length of C8–N3 is 1.39 Å shows an excellent agreement with that experimental data. The bond length of C2–C14 and C14–C17 are 1.51 Å and 1.46 Å, respectively shows the delocalisation is not towards the acetonitrile group and the shorter bond length of C14–C17 is due the inductive effect (–I) of the nitrile group. The smaller bond angle of S1–C2–N3 (116.1o) than that of N3–C2–C14 (122.3o) and S1–C2–C14 (121.6o) signifies the ring strain. The bond angle C14–C17–N18 (179.1o) reflects the linear shape of these atoms while the bond angle C2–C14–C17 (114.3 o) shows the bent structure of acetonitrile group. The computed thermodynamic parameters namely total energy, heat capacity, entropy, rotational constants, dipole moments, vibrational and zero–point vibrational energies

9

of the compound are presented in Table 2. Dipole moment reflects the molecular charge distribution and is given as a vector in three dimensions. Therefore, it can be used as descriptor to depict the charge movement across the molecule. Direction of the dipole moment vector in a molecule depends on the centers of positive and negative charges. The total dipole moment of BTAN determined by B3LYP level using 6–311++G**, 6–31G** and cc–pVTZ as basis set are 3.70, 3.63 and 3.75 Debye, respectively. 4.4. Analysis of molecular electrostatic potential Molecular electrostatic potential (MEP) is very useful in the investigation of the molecular structure with its physiochemical property relationships [41–43]. The molecular electrostatic potential surface (MEP) displays electrostatic potential (electron + nuclei) distribution, molecular shape, size and dipole moments of the molecule and it provides a visual method to understand the relative polarity. The reactive sites of the molecules can be determined with the help of the molecular electrostatic potential. Total self consistent field (SCF) electron density surface mapped with MEP of the compound is shown in Fig. 3. The MEP displays molecular shape, size and electrostatic potential values. The extreme limits of the total electron density are –4.943e × 10–2 to +4.943e × 10–2. The colour scheme for the MEP surface is red–electron rich or partially negative charge; blue–electron deficient or partially positive charge; light blue–slightly electron deficient region; yellow–slightly electron rich region, respectively. The electron density varies significantly around the title molecule such as the most electron rich region is N18 where the electron density is –4.943e × 10–2 and slight electron rich region for N3 and S1 are –2.525e × 10–2 and –1.400e × 10–2, respectively. Likewise the most electron deficient region around methylene hydrogen where the electron density is 4.943e × 10–2 and the slightly electron deficient region is around phenyl hydrogen atoms where the electron density is 2.2989e × 10–2. The electron density reveals the polarity of the molecule. The MEP clearly indicates the electron rich region

10

(nitrogen atoms) of the compound. The electrostatic potential surface of BTAN is shown in the supplementary Fig. S1. The contour map of electrostatic potential has been constructed by the DFT method is shown in Fig. 4 also confirms the different negative and positive potential sites of the molecule in accordance with the total electron density surface. The iso–electron density and MEP surfaces clearly indicates the probable sites readily available for the electrophilic and nucleophilic reactions. 4.5. Frontier molecular orbital analysis The HOMO → LUMO transition implies an electron transfer from benzothiazole ring to acetonitrile group. The frontier molecular orbitals are shown in Fig. 5. The frontier molecular orbital energy gap (ELUMO−EHOMO) of BTAN determined by using 6– 311++G** and cc–pVTZ basis sets are 5.3101 eV and 5.3449 eV, respectively. The energy gap reflects the chemical activity of the molecule. Therefore, an electron density transfer occurs from the more aromatic part of the π conjugated system in the electron–donor side to its electron–withdrawing part. Moreover, a lower HOMO–LUMO energy gap explains the fact that eventual charge transfer interaction is taking place within the molecule. 5. Vibrational analysis The molecule BTAN possesses C1 point group symmetry with 48 fundamental normal modes. The vibrational assignments are made on the basis of the spectral regions of the respective modes and with the help of GaussView 5.0.8 visualisation program [31]. In C1 symmetry all the modes are infrared and Raman active. The simulated infrared and Raman spectra by B3LYP method using 6–311++G**, 6–31G** and cc–pVTZ basis sets are compared with the experimental spectra and presented in Figs. 6 and 7, respectively. The observed and theoretical wavenumbers of the fundamental modes of BTAN are summarised in Table 3. For comparison purposes the theoretical data of B3LYP/cc-pVTZ method is used unless otherwise stated.

11

5.1. Scale factors A better agreement between the computed and experimental frequencies can be obtained by using different scale factors for different kinds of fundamental vibrations. To determine the scale factors, the procedure used previously [50–58] have been followed that minimises the residual separating experimental and theoretically predicted vibrational frequencies. The scaling factors 0.955 and 0.96 are used for C–H stretching frequencies obtained by using 6–31G** and 6–311++G**/cc–pVTZ basis sets. The frequencies in the range 1600–1000 cm–1 are scaled by 0.97 and 0.96 in B3LYPmethod by using 6– 311++G**/cc–pVTZ and 6–31G** basis sets. The theoretical frequencies less than 1000 cm– 1

are scaled by 0.97 in B3LYP/6–31G** and B3LYP/cc–pVTZ methods while 0.975 is used

for B3LYP/6–311++G** method. The resultant scaled frequencies are listed in Table 3. The correlation diagram for the calculated and the experimental frequencies of BTAN are shown in the supplementary Fig. S2. The root mean square deviation (RMSD) determined for B3LYP method with 6–311++G**, 6–31G** and cc–pVTZ basis sets are 11, 12 and 10, respectively reveals the better agreement with the experimental data. The RMS deviation reveals that the wavenumbers determined by B3LYP/cc-pVTZ are more reliable than the other methods. 5.2 C–H Vibrations The C–H stretching vibrations are assigned in the expected range to medium intensity IR bands at 3061 and 3051 cm–1 and very strong intensity Raman bands at 3063 cm–1 [44– 47]. The predicted scaled wavenumbers are very well agreed with the observed bands. The C–H in–plane bending vibrations are substitution sensitive, normally showing the bands in the region 1300–1000 cm–1 [44,45]. Medium-to-very strong intensity bands observed at 1240, 1164, 1059 and 1012 cm–1 in IR and strong-to-weak intensity bands at 1241, 1166 and 1015 cm–1 in Raman spectra are assigned to the C–H in–plane bending vibrations.

12

Bands

involving the out of plane C–H vibrations appear in the range 1000–675 cm–1 [44,45]. These vibrations are assigned to strong intensity IR bands at 932 and 761 cm−1 and very weak intensity Raman bands at 979 and 769 cm–1. 5.3 C–C vibrations The C–C stretching vibrations occur in a wider spectral range covering 1650–1200 cm–1 [46]. The very strong-to-medium intensity IR bands at 1590, 1556, 1453, 1431 and 1307 cm–1 and Raman bands formed at 1555, 1456 and 1435 cm–1 are assigned to the C–C stretching vibrations. The strong intensity IR bands observed at 878 and 732, 706 cm–1 and very weak-to-medium intensity Raman bands at 881, 740 and 708 cm–1 are assigned to the CCC in–plane bending vibrations. Two weak intensity IR bands observed at 587 and 507 cm–1 and three medium-to-weak intensity Raman bands observed at 505, 386 and 300 cm–1 are attributed to the CCC out of plane bending vibrations [48,49]. 5.4 Carbon–nitrogen vibrations The characteristic C≡N stretching vibrations are assigned to a strong intensity IR band at 2251 and medium intensity band in Raman spectra at 2252 cm–1. The C≡N in–plane bending vibration is assigned to a very strong intensity Raman band at 130 cm–1, which is in the expected range [44,46]. The C–C≡N stretching vibration is assigned to a weak intensity IR band observed at 955 cm–1 and a very weak intensity Raman band at 949 cm–1. The C– C≡N in–plane bending vibration is assigned to a weak intensity Raman band at 403 cm–1. The C–C≡N out of plane bending vibration is attributed to the weak intensity Raman band at 356 cm–1. The C=N stretching vibration is assigned to a very strong intensity IR band at 1513 cm– 1

and a strong intensity Raman band at 1516 cm–1. The C–N stretching vibration is expected

to be very weak. 5.5 Methylene group vibrations

13

The asymmetric methylene stretching vibration a(CH2) is assigned to a strong intensity IR band at 2953 cm–1 and a strong intensity Raman band at 2954 cm–1. The symmetric stretching methylene vibration s(CH2) is assigned to a strong intensity IR band at 2920 cm–1 and a very strong Raman intensity band at 2922 cm–1. The methylene deformation (CH2) is attributed to a very strong intensity IR band at 1382 cm–1 and a medium intensity Raman band at 1383 cm–1. The methylene wagging mode is attributed to a medium intensity IR band at 1275 cm–1 and a medium intensity Raman band at 1277 cm–1. The methylene twisting modes are assigned to a medium intensity IR band at 1206 cm–1 and a very weak intensity Raman band at 1213 cm–1. The methylene rocking mode assigned to a strong intensity IR band at 903 cm–1, respectively. 5.6 C–S Vibrations The C–S stretching vibrations are assigned to a very strong intensity IR bands observed at 1108 and 1059 cm–1 and a medium intensity Raman band at 1124 cm–1, respectively. 6. NMR spectral studies NMR spectroscopy has proved to be an exceptional tool to elucidate the structure and molecular conformation. The “gauge independent atomic orbital” (GIAO) method [59–62] has proven to be quite accepted and accurate. Density functional theory (DFT) shielding calculations are rapid and applicable to large systems. To provide an explicit assignment and analysis of

13

C and 1H NMR spectra, theoretical calculations on chemical shift of the title

compound were done through GIAO method at B3LYP/6–311++G** level [63] using CDCl3 solvent. The 1H and

13

C theoretical and experimental chemical shifts, isotropic shielding

constants and the NMR spectral assignments are presented in Table 4. The experimental 1H and 13C NMR spectra of the compound are represented in Figs. 8 and 9.

14

Unsaturated carbons give signals with chemical shift values from 100 to 200 ppm [64]. The external magnetic field experienced by the carbon nuclei is affected by the electronegativity of the atoms attached to them. The chemical shift of the carbon increases when it is attached to an atom like nitrogen and sulphur. Thus, the carbon atoms C2, C8, C9 and C17 in BTAN show downfield effect and the corresponding observed chemical shift of both C2 is observed at 168.35 ppm and the carbon atoms C8 and C9 in the benzothiazole ring are assigned to the chemical shift 162.82 ppm and the carbon atom C17 in the acetonitrile group is assigned to the chemical shift 123.37 ppm. The chemical shift values of other carbon atoms of BTAN observed at 135.48 ppm is attributed to C5 and C6 while C4 and C7 carbon atoms have the chemical shift value of 126.72 ppm. The methylene carbon atom C14 of BTAN shows NMR signal at 23.32 ppm. 1

H chemical shifts of BTAN are obtained by complete analysis of their NMR spectra

and interpreted critically in an attempt to quantify the possible different effects acting on the shielding constant and in turn to the chemical shift of protons. The hydrogen atom H10, H11, H12 and H13 attached with the aromatic carbons of BTAN are in the different chemical environment and shows peak at 8.04, 7.52, 7.44 and 7.87 ppm, respectively. In the acetonitrile group, the methylene (–CH2–) hydrogen atoms are in the same chemical environment and shows peak in the downfield 4.23 ppm. The calculated and experimental chemical shift values given in Table 4 shows very good agreement with each other. The linear regression between the experimental and theoretical 1H and

13

C NMR chemical shifts

are presented in the supplementary Fig. S3. The correlation between the experimental and theoretical 1H and 13C NMR chemical shifts are presented in Fig. 10. 7. Atomic charge distribution analysis The atomic charges of the neutral, cationic and anionic species of BTAN calculated by natural population analysis (NPA) using B3LYP/6–311++G** method are presented in

15

Table 5. The high electronegativity of sulphur and nitrogen atoms in BTAN is evident by decrease of electron density on C2, C7, C9 and C17 atoms and increase of electron density on C4, C5, C6, C8 and C14 atoms. The negative charges on sulphur and nitrogen atoms show electron rich centres. In the anion of the compound the C2 carbon has negative charge indicates the delocalization of lone pair of electrons towards it. Similarly, the C14 attains a positive charge while C17 has a negative charge in the anion due to the same reason. The correlation of the atomic charges of BTAN is depicted in Fig. 11. 8. Analysis of structure – activity descriptors The understanding of chemical reactivity and site selectivity of the molecular systems has been effectively handled by the conceptual density functional theory (DFT) [66]. Chemical potential, global hardness, global softness, electronegativity and electrophilicity are global reactivity descriptors, highly successful in predicting global chemical reactivity trends. The global parameters ionization potential (I), electron affinity (A), electrophilicity (ω), electronegativity (χ), hardness (η), and softness (S) of the molecule are determined and displayed in Table 2. The site selectivity of a chemical system, cannot, however, be studied using the global descriptors of reactivity. Fukui functions and local softness are extensively applied to probe the local reactivity and site selectivity. The formal definitions of all these descriptors and working equations for their computation have been described [65–68]. The Fukui functions of the individual atoms of the neutral, cationic and anionic species of BTAN calculated by B3LYP/6–311++G** method are presented in Table 5. The molecule under investigation mainly gives substitution reactions. It is clearly understood that the atoms S1, C4, C5, C14 and hydrogen atoms H10, H11, H12, H13 and H16 are favorable for nucleophilic attack. The other atoms of BTAN are favorable for electrophilic attack.

16

The local softness, relative electrophilicity ( sk / sk ) and relative nucleophilicity ( sk / sk ) indices, the dual local softness Δsk and the multiphilicity descriptors (Δωk) have also

been determined to predict the reactive sites of the molecule and are summarised in Table 6. From the dual local softness Δsk and the multiphilicity descriptors (Δωk) one can understand that the atoms S1, C4, C5, C14 and hydrogen atoms H10, H11, H12, H13 and H16 are favorable for nucleophilic attack. The other atoms of BTAN are favorable for electrophilic attack. The local reactivity descriptors of the individual atoms of the molecule ska  f ka S , ka  f ka and f ka where, a = +, – and 0 represents local philicity quantities describing for

nucleophilic, electrophilic and free radical attack, respectively presented in Tables 5 and 6 are clearly express the electron rich/deficient nature of the individual atoms. The plot of Fukui function (fk+ – fk–) versus atoms of BTAN is shown in Fig. 12. Table 7 presents the group philicity values of the BTAN compound. Thus, the atoms S1, N3, C7, C8, H12, H13,C14 and N18 favours for nucleophilic attack and rest of the atoms favours for electrophilic attack. Thus, group philicity provides additional insights into the electron transfer process taking place between the coordinating atoms. The correlation of Fukui functions fk+, fk– and fk0 of BTAN is given in Fig. 13. 9. NBO analysis NBO analysis provides the most accurate and possible Lewis structure and gives useful information about interactions in both filled and virtual orbital spaces that could enhance the analysis of intra and intermolecular interactions. The second order Fock matrix was carried out to verify the donor–acceptor interactions in the NBO analysis [69]. Delocalization of electron between occupied Lewis type (bonding or lone pair) NBO orbitals and formally unoccupied (antibonding or Rydberg) non–Lewis NBO orbitals corresponds to a stabilising donor–acceptor interactions.

17

In NBO analysis large E(2) value shows the effective interaction between electron– donors and electron–acceptors and the extent of conjugation of the whole system. The possible effective interactions are given in Table 8. The intramolecular charge transfer (ICT) causing stabilisation of the system. These interactions are observed as increase in electron density (ED) in anti–bonding orbital that weakens the respective bonds. The electron density of conjugated bond of aromatic ring clearly demonstrates strong delocalization. The strong intramolecular interactions of the π electrons of C–C and C–N bonds to the π* antibonding orbitals of C–C and C–N bonds leads to more stabilisation (more than 10 kcal mol–1) of some part of the ring and is evident from the E(2) energies. The interaction of BTAN such as bonding π(C8–C9) to the antibonding π*(C2–N3) is stabilised by 12.12 kcal mol–1. Similarly the stabilisation energies of the intramolecular interaction between π(C4–C5)→π*(C8–C9) is 20.95, π(C4–C5)→π*(C6–C7) is 19.93, π(C6–C7)→π*(C4–C5) is 18.40 and π(C6– C7)→π*(C8–C9) is 18.85 kcal mol–1, respectively. The highly stabilized (27.64 kcal mol–1) intramolecular hyperconjugation interaction of BTAN is between non bonding of S1 to the antibonding π*(C2–N3) orbital. 10. First hyperpolarizability The expected potential application of the molecule in the field of nonlinear optics demands the investigation of its structural and bonding features contributing to the hyperpolarizability enhancement. The first hyperpolarizability (β) of this novel molecular system of BTAN is calculated using DFT method, based on the finite–field approach. In the presence of an applied electric field, the energy of a system is a function of the electric field. The first hyperpolarizability is a third–rank tensor that can be described by a 3×3×3 matrix. The 27 components of the 3D matrix can be reduced to 10 components due to the Kleinman symmetry [70]. The components of β are defined as the coefficients in the Taylor series

18

expansion of the energy in the external electric field. When the electric field is weak and homogeneous, this expansion Becomes E= E0–Eiμi – ½ αij FiFj – 1/6 βijk Fi Fj Fk – 1/24 γijkl FiFjFkFl + …… where, E0 is the energy of unperturbed molecules, Fi is the field at the origin, μi, αij, βijk and γijkl are the components of the dipole moment, polarizability, first hyperpolarizability and the second hyperpolarizabilities, respectively.

The static dipole moment (μ), the mean

polarizability (α0), the anisotropy of the polarizability (∆α) and the mean hyperpolarizability (β0) using x, y and z components are determined by using the relations μ= (μx2 + μy2 + μz2 )1/2 α0 = (αxx + αyy + αzz)/3 α = [(αxx – αyy)2 + (αyy – αzz)2 + (αzz – αxx)2 + 6 α2xx]1/2/(2)1/2 β0 = (βx2 + βy2 + βz2) βx = (βxxx + βxyy + βxzz) βy = (βyyy + βyxx + βyzz) βz = (βzzz + βzxx + βzyy) The first hyperpolarizability of BTAN computed by DFT methods are presented in the supplementary Table T1. The large value of hyperpolarizability, β which is a measure of the non–linear optical activity of the molecular system, is associated with the intramolecular charge transfer, resulting from the electron cloud movement through π conjugated frame work from

electron

donor to

electron acceptor

groups.

Molecules with

high

hyperpolarizability have chromophores group, since the compound BTAN is a conjugative system with nitrile chromophore and hence its hyperpolarizability value determined by all the methods are in the order of more than 10 × 10–30 e.s.u. [71]. The asymmetric molecule BTAN has large values of molecular hyperpolarisability, β due to the electron delocalisation along a

19

conjugated backbone. Thus, the BTAN molecule is an attractive agent for future studies of nonlinear optical properties. 11. Conclusion The complete molecular structural parameters, thermodynamic properties and fundamental vibrational frequencies of the optimised geometry of 2–benzothiazole acetonitrile have been reported for the first time using DFT calculations. The computed geometrical parameters are in good agreement with the observed X–ray diffraction data of similar compound. The geometrical structure shows a little distortion due to the substitution of highly electronegative nitrile group. The conformational analysis carried out by determining PES using the dihedral angle N3–C2–C14–C17 and the stable conformer with the minimum energy has been determined. The difference between the observed and scaled wavenumber values of most of the fundamentals is very small and shows the reliability of quantum chemical method used in this investigation. The NMR data clearly indicates the downfield effect in chemical shift of C2, C8, C9 and C16 atom of BTAN due to partial ionic character of nitrogen and sulphur atoms. The MEP provides the active sites of BTAN molecule and clearly indicates the electron rich and deficient centres within the molecule. In the MEP surface, the nitrile group of BTAN shows red colour which is electron rich of the molecule. The HOMO and LUMO energy gap explains the eventual electronic transition taking place within the molecule, particularly from benzothiazole group to acetonitrile group for BTAN. Furthermore, the nonlinear optical, first order hyperpolarizabilities and total dipole moment properties of the molecule show that the title molecule is an attractive agent for future studies of nonlinear optical properties. NBO result reflects the intramolecular hyperconjugative interaction of electrons of the molecule. The structure–chemical reactivity relations of the compound were determined through chemical potential, global hardness,

20

global softness, electronegativity, electrophilicity and local reactivity descriptors by conceptual DFT methods.

References [1]

A. Achson, An introduction to the chemistry of heterocyclic compounds, 3rd ed., Wiley –Intersciences, India, 2009.

[2]

N.B. Patel, F.M. Shaikh, Sci. Pharm. 78 (2010) 753–765.

[3]

S. Gupta, N. Ajmera, N. Gautam, R. Sharma, D. Gauatam, Int. J. Chem. 48B (2009) 853–858.

[4]

R.M. Kumbhare, V.N. Ingle, Int. J. Chem. 48B (2009) 996–1000.

[5]

Y. Murthi, D. Pathak, J. Pharm. Res. 7 (2008) 153–155.

[6]

B. Rajeeva, N. Srinivasulu, S. Shantakumar, Eur. J. Chem. 6 (2009) 775–779.

[7]

M. Maharan, S. William, F. Ramzy, A. Sembel, Molecules 12 (2007) 622–633.

[8]

S. Kini, S. Swain, A. Gandhi, I. J. Pharm. Sci. (2007) 46–50.

[9]

H.L.K. Stanton, R. Gambari, H.C. Chung, C.O.T. Johny, C. Filly, S.C.C. Albert, Bioorg. Med. Chem. 16 (2008) 3626–3631.

[10] M. Wang, M. Gao, B. Mock, K. Miller, G. Sledge, G. Hutchins, Q. Zheng, Bioorg. Med. Chem. 14 (2006) 8599–8607. [11] I. Hutchinson, M.S. Chua, H.L. Browne, V. Trapani, T.D. Bradshaw, A.D. Westwell, J. Med. Chem. 44 (2001) 1446–1449. [12] M. Sreenivasa, E. Jaychand, B. Shivakumar, K. Jayrajkumar, J. Vijaykumar, J. Arch. Pharm. Sci. 1(2) (2009) 150–157. [13] S. Pattan, C. Suresh, V. Pujar, V. Reddy, V. Rasal, B. Koti, Int. J. Chem. 44B (2005) 2404–2408. [14] P. Reddy, Y. Lin, H. Chang, Arcivoc. 16 (2007) 113–122.

21

[15] Y. Heo, Y. Song, B. Kim, J. Heo, Tetrahedron Lett. 47 (2006) 3091–3094. [16] F. Piscitelli, C. Ballatore, A. Smith, Bioorg. Med. Chem. Lett. 20 (2010) 644–648. [17] O. Fiehn, T. Reemtsma , M. Jekel, Analytica Chim. Acta 295 (1994) 297–305. [18] O.F. Mohammed, S. Luber, V.S. Batista, E.T.J. Nibbering, J. Phy. Chem. 9A (2011) 8–17. [19] P.V. Govardhana Reddy, Y.–W. Lin, H.–T. Chang, ARKIVOC 16 (2007) 113–122. [20] G. Jayanthi, S. Muthusamy, R. Paramasivam, V.T. Ramakrishnan, N.K. Ramasamy, P. Ramamurthy, J. Org. Chem. 62 (1997) 5766–5770. [21] H.–W. Engels, H.–J. Weidenhaupt, M. Pieroth, W. Hofmann, K.–H. Menting, T. Mergenhagen, R. Schmoll, S. Uhrlandt, Rubber, 4. Chemicals and Additives in Ullmann's Encyclopedia of Industrial Chemistry, Wiley–VCH, Weinheim, 2004. [22] P. Hrobarik, I. Sigmundova, P. Zahradnik, P. Kasak, V. Arion, E. Franz, K. Clays, J. Phys. Chem. C114 (2010) 22289–22302. [23] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery, Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin,

R. Cammi, C.

Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009.

22

[24] P. Hohenberg, W. Kohn, Phys. Rev. B 136 (1964) 864–868. [25] A.D. Becke, J. Chem.Phys. 98 (1993) 5648–5652. [26] A.D. Becke, Phys. Rev. A 38 (1988) 3098–3100. [27] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785–789. [28] V. Krishnakumar, G. Keresztury, T. Sundius, R. Ramasamy, J. Mol. Struct. 702 (2004) 9–17. [29] J.S. Murray, K. Sen, Molecular Electrostatic Potentials, Concepts and Applications, Elsevier, Amsterdam, 1996. [30] S. Chidangil, M.K. Shukla, P.C. Mishra, J. Mol. Model. 4 (1998) 250–257. [31] R.I. Dennington, T. Keith, J. Millam, GaussView, Version 5.0.8, Semichem. Inc. Shawnee Mission, KS, 2009. [32] J.P. Foster, F. Weinhold, J. Am. Chem. Soc. 102 (1980) 7211–7218. [33] R. Duchfield, J. Chem. Phys. 56 (1972) 5688–5691. [34] K. Wolinski, J.F. Hinton, P. Pulay, J. Am. Chem. Soc. 112 (1990) 8251–8260. [35] R.G. Parr, W. Yang, J. Am. Chem. Soc. 106 (1984) 4049–4050. [36] W. Yang, R.G. Parr, Proc. Natl. Acad. Sci. USA 82 (1985) 6723–6726. [37] P.K. Chattaraj, B. Maiti, U. Sarkar, J. Phys. Chem. A 107 (2003) 4973–5004. [38] R. Parthasarathi, J. Padmanabhan, M. Elango, V. Subramanian, P.K. Chattaraj, Chem. Phys. Lett. 394 (2004) 225–232. [39] Z. F. Chen, Y. Z. Tang, S. M. Shi, X. W. Wang, H. Liang, K. B. Yu, Acta Cryst. E59 (2003) 1461–1463. [40] S. Vijayakumar, S. Murugavel, R. Selvakumar, M. Bakthadoss, Acta Cryst. E68 (2012) 2362–2363. [41] I. Fleming, Frontier Orbitals and Organic Chemical Reactions, John Wiley and Sons, New York, 1976, pp. 5–27.

23

[42] J.M. Seminario, Recent Developments and Applications of Modern Density Functional Theory, Vol. 4, Elsevier, 1996, pp. 800–806. [43] T. Yesilkaynak, G. Binzet, F. Mehmet Emen, U. Florke, N. Kulcu, H. Arslan, Eur. J. Chem. 1 (2010) 1–5. [44] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, Inc., London, 1964. [45] G. Socrates, Infrared Characteristic Group Frequencies, John Wiley & Sons, New York, 1980. [46] L.J. Bellamy, The Infrared Spectra of Complex Molecules, Chapman and Hall Ltd., London, 1975. [47] G. Varsányi, Vibrational Spectra of Benzene Derivatives, Academic Press, New York and London, 1969. [48] V. Arjunan, M. Kalaivani, S. Senthilkumari, S. Mohan, Spectrochim. Acta 115A (2013) 154–174. [49] Neeraj Misra, O. Prasad, Leena Sinha. A. Pandey, J. Mol. Struct. (Theochem.) 822 (2007) 45–47. [50] V. Arjunan, S. Mohan, J. Mol. Struct. 892 (2008) 289–299. [51] V. Arjunan, S. Mohan, Spectrochim. Acta 72A (2009) 436–444. [52] H.F. Hameka, J.O. Jensen, J. Mol. Struct. (Theochem.) 362 (1996) 325–330. [53] J.O. Jensen, A. Banerjee, C.N. Merrow, D. Zeroka, J.M. Lochner, J. Mol. Struct. (Theochem.) 531 (2000) 323–330. [54] A.P. Scott, L. Radom, J. Phys. Chem. 100 (1996) 16502–16513. [55] M.P. Andersson, P. Uvdal, J. Chem. Phys. A 109 (2005) 2937–2941. [56] M. Alcolea Palafox, M. Gill, N.J. Nunez, V.K. Rastogi, Lalit Mittal, Rekha Sharma, Int. J. Quant. Chem. 103 (2005) 394–421.

24

[57] M. Alcolea Palafox, Int. J. Quant. Chem. 77 (2000) 661–684. [58] V. Arjunan, P. Ravindran, T. Rani, S. Mohan, J. Mol. Struct. 988 (2011) 91–101. [59] P.V.R. Schleyer, N.L. Allinger, T. Clark, J. Gasteiger, P.A. Kolmann, H.F. Schaefer, P.R. Schreiner, The Encyclopedia of Computational Chemistry, John Wiley and Sons, Chichester, 1998. [60] R. Ditchfield, Mol. Phys. 27 (1974) 789–807. [61] M. Barfiled, P. Fagerness, J. Am. Chem. Soc. 119 (1977) 8699–8711. [62] J.M. Manaj, D. Maciewska, I. Waver, Magn. Reson. Chem. 38 (2000) 482–485. [63] A.J.D. Melinda, Solid State NMR Spectroscopy; Principles and Applications, Cambridge Press, 2003. [64] R.M. Silverstein, G.C. Bassler, T.C. Morrill, Spectrometric Identification of Organic Compounds, 5th ed., 1991, p. 245. [65] P. Geerlings, F. De Proft, W. Langenaeker, Chem. Rev. 103 (2003) 1793–1873. [66] R.G. Parr, W. Yang, Density Functional Theory of Atoms and Molecules, Oxford University Press, Oxford, 1989. [67] R.G. Pearson, Chemical Hardness – Applications from Molecules to Solids, VCH Wiley, Weinheim, 1997. [68] P.K. Chattaraj (Ed.) Special Issue of J. Chem. Sci. on Chemical Reactivity, Vol. 117, 2005. [69] A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. 88 (1988) 899–926. [70] D.A. Kleinman, Phy Rev. 126 (1962) 1977–1979. [71] W. Wu, C. Ye, D. Wang, ARKIVOC (2003) 59–69.

25

Fig. 1. The potential energy surface profile of 2–benzothiazole acetonitrile.

Fig. 2. The optimised geometry (I) and other possible conformers of 2–benzothiazole acetonitrile.

Fig. 3. The total electron density surface mapped with molecular electrostatic potential of 2–benzothiazole acetonitrile.

Fig. 4. The contour map of molecular electrostatic potential surface of 2–benzothiazole acetonitrile.

Fig. 5. The frontier molecular orbitals of 2–benzothiazole acetonitrile.

Fig. 6. (a) Observed FTIR and (b) Theoretical infrared 6–311++G** and (c) 6–31G** and (d) cc–pVTZ spectra of 2–benzothiazole acetonitrile.

Fig. 7. (a) Observed FT–Raman and (b) Theoretical Raman 6–311++G** and (c) 6–31G** and (d) cc–pVTZ spectra of 2–benzothiazole acetonitrile.

Fig. 8. 1H NMR spectrum of 2–benzothiazole acetonitrile.

Fig. 9. 13C NMR spectrum of 2–benzothiazole acetonitrile.

Fig.10. The correlation between the experimental and theoretical 1H & 13C NMR chemical shifts of 2–benzothiazole acetonitrile.

Fig. 11. Correlation of the atomic charges of 2–benzothiazole acetonitrile determined by NPA analysis.

Fig. 12. Plot of Fukui functions fk+ – fk– versus atom of 2–benzothiazole acetonitrile

Fig. 13. Combined plot of Fukui functions fk+, fk– and fk0 of 2–benzothiazole acetonitrile

Table 1. Structural parameters of 2–benzothiazole acetonitrile determined by B3LYP method using 6–311++G(d,p), 6–31G(d,p), and cc–pVTZ basic sets. 2–benzothiazole acetonitrile

Structural

B3LYP/

B3LYP/

B3LYP/

6–311++G(d,p)

6–31 G(d,p)

cc–pVTZ

S1–C2

1.77

1.77

S1–C9

1.75

C2–N3

Expta

Exptb

1.76

1.77

1.73

1.75

1.75

1.75

1.72

1.29

1.29

1.29

1.30

1.29

C2–C14

1.51

1.51

1.51

N3–C8

1.39

1.39

1.38

1.39

1.39

C4–C5

1.39

1.39

1.38

1.39

1.37

C4–C8

1.40

1.40

1.40

1.39

1.39

C5–C6

1.40

1.40

1.40

1.38

1.38

C6–C7

1.39

1.39

1.39

1.38

1.37

C7–C9

1.40

1.40

1.39

1.38

1.40

C–H (ring)c

1.08

1.08

1.08

0.93

0.93

1.41

1.41

1.41

1.40

1.40

1.10

1.10

1.09

C14–C17

1.46

1.46

1.46

C17–N18

1.15

1.15

1.15

parameters Bond distance (Å)

C8–C9 C14–H (methylene)

c

Bond angle (o) C2–S1–C9

88.3

88.3

88.4

88.1

88.9

S1–C2–N3

116.1

116.1

116.0

116.8

116.6

S1–C2–C14

121.6

121.9

121.9

N3–C2–C14

122.3

121.9

122.0

C2–N3–C8

111.3

111.3

111.3

109.5

110.1

C5–C4–C8

118.9

118.9

118.9

119.7

118.8

C5–C4–H10

121.7

121.7

121.7

120.1

120.6

C8–C4–H10

119.4

119.4

119.4

120.1

120.6

C4–C5–C6

120.9

120.9

120.9

120.7

121.2

C4–C5–H11

119.7

119.7

119.7

119.6

119.4

C6–C5–H11

119.4

119.4

119.4

119.6

119.4

C5–C6–C7

121.1

121.1

121.1

121.0

121.6

C5–C6–H12

119.6

119.6

119.6

119.5

119.2

C7–C6–H12

119.3

119.3

119.3

119.5

119.2

C6–C7–C9

117.9

117.9

118.0

118.0

117.6

C6–C7–H13

120.8

120.8

120.8

121.0

121.2

C9–C7–H13

121.3

121.3

121.2

121.0

121.2

N3–C8–C4

125.2

125.3

125.3

125.4

125.6

N3–C8–C9

115.1

115.1

115.1

116.2

115.0

C4–C8–C9

119.7

119.7

119.7

118.4

119.4

S1–C9–C7

129.3

129.2

129.3

128.5

129.2

S1–C9–C8

109.2

109.3

109.2

109.4

109.4

C7–C9–C8

121.5

121.5

121.5

122.1

121.4

C2–C14–H15

110.0

109.4

109.8

C2–C14–H16

107.3

107.6

107.4

C2–C14–C17

114.3

114.8

114.5

H15–C14–H16

107.4

107.1

107.2

H15–C14–C17

108.3

108.2

108.1

H16–C14–C17

109.3

109.5

109.5

C14–C17–N18

179.1

179.3

179.3

C9–S1–C2–N3

–0.15

–0.13

–0.22

–0.80

0.67

C9–S1–C2–C14

177.39

176.53

177.11

C2–S1–C9–C7

179.68

179.53

179.67

–178.80

179.76

C2–S1–C9–C8

0.09

0.09

0.14

–0.20

–0.69

S1–C2–N3–C8

0.16

0.13

0.22

C14–C2–N3–C8

–176.36

–176.54

–177.10

S1–C2–C14–H15

–70.87

–84.66

–77.16

S1–C2–C14–H16

172.59

159.31

166.57

S1–C2–C14–C17

51.22

37.14

44.71

N3–C2–C14–H15

106.53

91.81

100.01

Dihedral angle (o)

–0.41

N3–C2–C14–H16

–10.02

–24.22

–16.26

N3–C2–C14–C17

–131.39

–146.39

–138.12

C2–N3–C8–C4

–179.82

–179.73

–179.84

178.90

179.35

C2–N3–C8–C9

–0.09

–0.05

–0.11

–1.70

–0.16

C8–C4–C5–C6

–0.07

–0.09

–0.08

–0.30

0.30

C8–C4–C5–H11

–179.99

179.97

179.99

H10–C4–C5–C6

179.90

179.90

179.90

H10–C4–C5–H11

–0.03

–0.04

–0.03

C5–C4–C8–N3

179.74

179.68

179.74

–179.80

179.67

C5–C4–C8–C9

0.02

0.01

0.02

0.80

–0.80

H10–C4–C8–N3

–0.23

–0.32

–0.25

H10–C4–C8–C9

–179.95

–179.98

–179.97

0.01

0.00

0.02

–0.30

0.40

C4–C5–C6–H12

–179.93

–179.93

–179.93

H11–C5–C6–C7

179.93

179.94

179.94

H11–C5–C6–H12

–0.01

0.01

–0.01

0.11

0.17

0.12

0.40

–0.50

C5–C6–C7–H13

–179.86

–179.86

–179.87

H12–C6–C7–C9

–179.96

–179.90

–179.93

0.07

0.07

0.08

–179.70

–179.63

–179.67

178.60

179.49

C4–C5–C6–C7

C5–C6–C7–C9

H12–C6–C7–H13 C6–C7–C9–S1

C6–C7–C9–C8

–0.16

–0.25

–0.19

H13–C7–C9–S1

0.27

0.40

0.32

H13–C7–C9–C8

179.81

179.78

179.80

N3–C8–C9–S1

–0.03

–0.05

N3–C8–C9–C7

–179.65

C4–C8–C9–S1 C4–C8–C9–C7 a,b c

0.10

0.00

–0.05

1.10

0.64

–179.54

–179.62

179.80

–179.77

179.72

179.66

179.70

–179.40

–178.90

0.10

0.17

0.12

–0.70

0.70

Experimental values are taken from Ref. [39,40].

Mean value

Table 2. The calculated thermodynamic parameters of 2–benzothiazole acetonitrile employing B3LYP method with 6–311++G(d,p), 6–31G(d,p) and cc–pVTZ basic sets. 2–benzothiazole acetonitrile Thermodynamic parameters (298K)

B3LYP/

B3LYP/

B3LYP/

6–311++G(d,p)

6–31G(d,p)

cc–pVTZ

86.77

87.20

86.99

36.87

36.73

36.64

99.66

99.34

99.60

85.00

85.42

85.21

80.69

81.14

80.95

–854.39714

–854.26282

–854.44593

X

2.55

2.55

2.51

Y

0.49

0.49

0.49

Z

0.42

0.42

0.43

μx

–3.23

–3.14

–3.17

μy

0.94

1.07

0.80

μz

1.56

1.48

1.85

Total Energy (thermal), Etotal (kcal.mol–1) Heat Capacity at const. volume, Cv (kcal.mol–1.K–1) Entropy, S (kcal.mol–1.K–1) –1

Vibrational Energy, Evib (kcal.mol ) Zero –point vibrational Energy, E0 (kcal.mol

–1

)

SCF Rotational Constants (GHz)

Dipolemoment (Debye)

μtotal EHOMO (eV)

3.70 –6.9939

ELUMO (eV)

–1.6839

–1.5856

EHOMO – 1 (eV)

–7.1371

–7.0764

ELUMO + 1 (eV)

–0.9331

–0.8066

ELUMO – EHOMO (eV)

5.3101

5.3449

Ionisation Potential(I)

8.8787

Electron Affinity (A)

0.0531

Chemical Potential (μ)

–4.4659

Hardness(η)

4.4128

Global Softness(S)

0.1133

Electrophilicity(ω)

2.2599

Electronegativity(χ)

4.4659

Electrofugality(∆Ee)

11.1386

Nucleofugality(∆En)

2.2067

3.63

3.75 –6.9305

Table 3. The observed FTIR, FT–Raman and calculated frequencies using B3LYP/6–311++G(d,p) , B3LYP/6–31G(d,p) and B3LYP/cc–pVTZ force field along

Assignment

intensity

Raman

intensity

(cm–1) IR

(cm–1) Scaled

wavenumber

Depolarization ratio

B3LYP/cc–pVTZ Calculated

Unscaled

Depolarization ratio

intensity

Raman

intensity

Calculated wavenumber

(cm–1) IR

intensity

Raman

intensity

(cm–1) IR

Unscaled

FTR

FTIR

(cm–1) Scaled

Calculated wavenumber

(cm–1)

B3LYP/6–31G(d,p)

(cm–1) Scaled

B3LYP/6–311++G(d,p)

wavenumber

Unscaled

Observed

Depolarization ratio

with their relative intensities, probable assignments and potential energy distribution (PED) of 2–benzothiazole acetonitrile.a

3200

3072

7.97

100.00

0.12

3220

3075

10.00

100.00

0.13

3202

3074

9.25

100.00

0.13 νCH

3061 m

3194

3066

9.68

30.00

0.33

3213

3068

13.52

40.91

0.26

3196

3068

10.76

29.86

0.35 νCH

3051 m

3184

3056

5.06

43.10

0.68

3202

3058

7.16

51.53

0.66

3186

3058

5.19

42.97

0.68 νCH

3172

3045

1.22

18.47

0.75

3190

3047

1.46

22.84

0.75

3174

3047

1.20

18.17

0.75 νCH

3063 vs

2953 s

2954 s

3092

2968

0.57

25.29

0.47

3113

2973

0.26

29.19

0.56

3093

2970

0.58

23.80

0.47 νaCH2

2920 s

2922 vs

3032

2911

0.31

83.72

0.09

3046

2909

0.95

80.66

0.11

3032

2911

0.52

76.16

0.10 νsCH2

2251 s

2252 m

2361

2267

8.30

46.10

0.21

2376

2269

4.71

36.95

0.27

2364

2270

6.69

38.93

0.23 νC≡N

1637

1588

8.24

3.55

0.72

1654

1588

6.77

4.04

0.68

1639

1590

5.14

3.02

0.69 νCC

1596

1548

4.88

8.93

0.72

1613

1548

4.75

8.28

0.71

1599

1551

4.62

8.12

0.69 νCC

1513 vs 1516 s

1570

1523

47.40

58.95

0.31

1589

1525

42.48

65.38

0.32

1567

1520

42.61

49.12

1453 m

1456 w

1488

1443

5.14

5.35

0.38

1500

1440

4.54

6.15

0.37

1494

1449

4.64

4.33

0.36 νCC

1431 s

1435 m

1464

1420

18.65

5.81

0.61

1477

1418

17.56

5.33

0.52

1470

1425

18.40

6.05

0.51 νCC

1382 vs 1383 m

1451

1408

13.28

3.97

0.50

1460

1402

10.28

6.68

0.56

1454

1410

12.07

3.66

0.49 δCH2

1590 m 1556 m

1555 m

0.31 νC=N

1307 s 1275 m

1277 m

1240 vs 1241 s

1346

1306

14.19

3.45

0.20

1364

1309

13.13

4.88

0.22

1347

1307

13.39

2.55

0.22 νCC

1329

1289

17.95

11.51

0.24

1334

1280

15.79

9.68

0.29

1330

1290

14.63

10.45

0.23 ωCH2

1300

1261

0.86

10.18

0.31

1305

1253

1.53

13.41

0.33

1305

1266

1.14

8.49

0.33 βCH

1261

1223

4.82

31.02

0.26

1270

1219

3.25

30.89

0.30

1265

1227

5.11

28.48

0.27 νC–N

1206 m

1213 vw

1232

1195

2.66

0.84

0.73

1233

1184

4.28

1.86

0.70

1235

1198

2.75

0.39

0.73 τCH2

1164 m

1166 w

1186

1150

0.09

1.04

0.73

1190

1143

0.07

1.90

0.74

1189

1153

0.09

1.11

0.69 βCH

1108 vs 1124 m

1162

1127

45.18

1.21

0.66

1163

1116

45.22

2.01

0.36

1158

1123

43.65

1.21

0.45 νCS

1090 s

1145

1110

5.38

6.11

0.22

1150

1104

7.04

5.50

0.27

1148

1114

5.65

5.33

0.24 βCH

1059 vs

1074

1041

7.15

5.00

0.16

1075

1038

6.54

4.22

0.19

1079

1047

7.93

4.86

0.20 νCS

1015 m

1036

1005

7.72

11.85

0.05

1044

1007

4.73

8.61

0.11

1040

1009

5.82

9.12

0.09 βCH

979 vw

991

966

0.00

0.02

0.21

992

963

0.00

0.03

0.75

1005

975

0.02

0.01

0.75 γCH

949 vw

969

945

4.04

0.50

0.12

970

941

4.14

0.64

0.28

968

939

1.36

0.02

0.16 νCC≡N

932 s

957

934

1.70

0.05

0.51

953

924

1.55

0.23

0.73

968

938

3.58

0.61

0.08 γCH

903 s

926

903

2.64

1.28

0.18

930

902

2.47

1.48

0.20

928

900

2.99

0.97

0.17 ρCH2

872

850

4.48

1.48

0.08

874

847

1.16

1.67

0.69

883

857

0.82

0.19

0.75 βCCC

866

844

0.96

0.01

0.75

873

847

5.33

1.51

0.20

874

848

5.10

1.39

0.14 γCH

1012 s

955 w

878 s

881 w

761 vs

769 m

773

753

53.21

0.26

0.66

777

754

40.59

0.52

0.73

785

761

34.86

0.10

0.67 γCH

732 s

740 vw

736

718

21.85

0.03

0.26

743

720

11.65

1.25

0.75

755

732

28.86

0.30

0.75 βCCC

706 s

708 m

718

700

1.82

2.28

0.19

718

696

2.97

2.62

0.25

720

699

3.06

2.56

0.18 βCCC

709

691

6.14

3.66

0.08

708

687

5.95

3.14

0.15

711

690

5.50

2.58

0.12 βCCC

628

613

19.72

3.23

0.08

632

613

19.88

3.24

0.18

634

615

18.83

2.62

0.13 βCCC

609

593

0.31

0.80

0.73

611

593

0.17

1.07

0.54

617

598

0.26

0.79

0.42 γCCC

584

569

0.53

0.98

0.68

583

566

0.15

1.31

0.69

587

569

0.19

0.81

0.72 γCCC

511

498

0.64

2.17

0.43

511

496

0.48

2.42

0.53

517

501

0.89

0.66

0.62 γCCC

508

495

0.58

2.41

0.25

508

493

0.44

3.09

0.31

510

495

0.20

3.48

0.31 γCCC

436

425

4.60

0.10

0.73

437

424

2.84

0.24

0.45

442

429

4.46

0.19

0.46 γCCC

403 w

407

397

3.25

2.09

0.08

408

396

3.24

1.76

0.15

408

395

3.42

1.92

0.12 βCCN

386 w

390

380

0.03

1.57

0.67

391

379

0.04

1.55

0.69

391

379

0.03

1.18

0.69 γCCC

356 w

363

354

0.64

1.35

0.74

367

356

0.72

2.05

0.75

363

352

0.69

1.03

0.75 γCCN

300 vw

300

293

0.62

0.13

0.70

307

298

0.98

0.57

0.68

309

300

0.73

0.24

0.63 γCCC

241

234

4.51

0.55

0.67

241

234

4.62

0.46

0.75

241

233

4.56

0.38

0.74 ωCC≡N

194

189

2.26

0.19

0.74

196

190

1.58

0.62

0.75

196

190

2.09

0.25

0.75 γCCC

130 vs

141

137

6.62

0.47

0.62

147

143

5.48

0.77

0.67

147

143

5.93

0.46

0.63 τCC≡N

108 vs

84

81

5.11

1.34

0.69

81

79

4.47

2.10

0.72

81

79

4.62

1.49

0.71 βC≡N

19

19

5.39

0.58

0.75

21

21

4.26

1.14

0.75

18

17

4.70

0.87

0.75 γC≡N

587 vw

507 w

505 m

RMSD a

11

12

10

ν–stretching; β–in–plane bending; δ–deformation; ρ–rocking; γ–out of plane bending; ω–wagging and τ–twisting. RMSD–Root mean square

deviation

Table 4. The experimental and theoretical 1H and 13C isotropic chemical shifts (ppm) with respect to TMS of 2–benzothiazole acetonitrile. 1

H Assignment

σiso

Theoretical

Experimental

δ (ppm)

δ (ppm)

13

C

σiso

Assignment

Theoretical Experimental δ (ppm)

δ (ppm)

H10

23.65

8.32

8.04 C2

12.42

172.11

168.35

H11

24.17

7.80

7.52 C4

53.84

130.69

126.72

H12

24.29

7.68

7.44 C5

51.33

133.20

135.48

H13

23.77

8.20

7.87 C6

51.65

132.88

135.48

H (methylene)

27.76

4.21

4.23 C7

56.37

128.16

126.72

C8

25.21

159.33

162.82

C9

34.78

149.75

162.82

C14

157.54

26.99

23.32

C17

60.77

123.76

123.37

δiso – isotropic chemical shift and σiso – isotropic shielding constant.

Table 5. The local reactivity descriptors (f) of 2–Benzothiazole acetonitrile by B3LYP/6– 311++G** method. Atom

f k

f k

f k0

∆f(k)

Neutral

Cation

Anion

S1

–0.5226

–0.3059

–0.6218

–0.0992

–0.2167

–0.1580

0.1176

C2

0.2027

0.1754

–0.0163

–0.2191

0.0273

–0.0959

–0.2464

N3

0.1525

0.2607

0.0107

–0.1418

–0.1082

–0.1250

–0.0336

C4

–0.7133

–0.6125

–0.7572

–0.0439

–0.1009

–0.0724

0.0570

C5

–0.3233

–0.3120

–0.2771

0.0462

–0.0114

0.0174

0.0576

C6

–0.0639

–0.0398

–0.1134

–0.0495

–0.0241

–0.0368

–0.0254

C7

0.2323

0.3905

0.0648

–0.1675

–0.1582

–0.1628

–0.0093

C8

–1.5964

–1.6431

–1.5959

0.0005

0.0468

0.0236

–0.0463

C9

1.7886

1.7798

1.7223

–0.0663

0.0088

–0.0287

–0.0751

H10

0.1897

0.2737

0.1162

–0.0735

–0.0840

–0.0788

0.0105

H11

0.1729

0.2508

0.1141

–0.0588

–0.0779

–0.0684

0.0191

H12

0.1718

0.2489

0.1108

–0.0610

–0.0771

–0.0691

0.0161

H13

0.1790

0.2601

0.1203

–0.0588

–0.0811

–0.0699

0.0223

C14

–0.1498

–0.1341

1.0043

1.1542

–0.0157

0.5692

1.1699

H15

0.2178

0.2559

–0.6102

–0.8280

–0.0381

–0.4331

–0.7899

H16

0.2045

0.2358

0.1882

–0.0164

–0.0313

–0.0238

0.0150

C17

0.1560

0.1489

–0.0842

–0.2403

0.0072

–0.1165

–0.2474

N18

–0.2986

–0.2332

–0.3755

–0.0769

–0.0654

–0.0711

–0.0115

Table 6. The local reactivity descriptors (s and ω) of 2–Benzothiazole acetonitrile by B3LYP/6–311++G** method.

Atom

sk

sk

sk0

Δsk

 k

 k

 k0

Δωk

Relative

Relative

Eletrophili

Nucleop

city

hilicity

S1

–0.0112

–0.0246

–0.0179

0.0133

–0.2241

–0.4898

–0.3570

0.2657

0.4576

2.1855

C2

–0.0248

0.0031

–0.0109

–0.0279

–0.4950

0.0617

–0.2167

–0.5568

–8.0203

–0.1247

N3

–0.0161

–0.0123

–0.0142

–0.0038

–0.3205

–0.2446

–0.2825

–0.0760

1.3106

0.7630

C4

–0.0050

–0.0114

–0.0082

0.0065

–0.0992

–0.2279

–0.1636

0.1288

0.4351

2.2985

C5

0.0052

–0.0013

0.0020

0.0065

0.1044

–0.0257

0.0394

0.1301

–4.0664

–0.2459

C6

–0.0056

–0.0027

–0.0042

–0.0029

–0.1118

–0.0544

–0.0831

–0.0574

2.0559

0.4864

C7

–0.0190

–0.0179

–0.0184

–0.0011

–0.3785

–0.3575

–0.3680

–0.0210

1.0588

0.9445

C8

0.0001

0.0053

0.0027

–0.0052

0.0012

0.1057

0.0534

–0.1046

0.0109

91.9037

C9

–0.0075

0.0010

–0.0033

–0.0085

–0.1498

0.0199

–0.0649

–0.1697

–7.5275

–0.1328

H10

–0.0083

–0.0095

–0.0089

0.0012

–0.1662

–0.1898

–0.1780

0.0236

0.8756

1.1421

H11

–0.0067

–0.0088

–0.0077

0.0022

–0.1328

–0.1761

–0.1545

0.0433

0.7544

1.3256

H12

–0.0069

–0.0087

–0.0078

0.0018

–0.1379

–0.1743

–0.1561

0.0363

0.7916

1.2633

H13

–0.0067

–0.0092

–0.0079

0.0025

–0.1328

–0.1833

–0.1581

0.0505

0.7247

1.3798

C14

0.1308

–0.0018

0.0645

0.1325

2.6083

–0.0356

1.2863

2.6438

–73.3121

–0.0136

H15

–0.0938

–0.0043

–0.0491

–0.0895

–1.8713

–0.0861

–0.9787

–1.7852

21.7222

0.0460

H16

–0.0019

–0.0035

–0.0027

0.0017

–0.0370

–0.0708

–0.0539

0.0338

0.5222

1.9151

C17

–0.0272

0.0008

–0.0132

–0.0280

–0.5430

0.0162

–0.2634

–0.5592

–33.4490

–0.0299

N18

–0.0087

–0.0074

–0.0081

–0.0013

–0.1738

–0.1477

–0.1607

–0.0261

1.1766

0.8499

Table 7. Group philicity values for nucleophilic and electrophilic attacks with different atoms of 2–benzothiazole acetonitrile. Atom S1 C2

N3 C4

C5

C6

C7

C8

C9

H10 H11 H12 H13 C14

H15 H16 C17 N18

Coordinating atoms C2 C9 S1 N3 C14 C2 C8 C5 C8 H10 C4 C6 H11 C5 C7 H12 C6 C9 H13 N3 C4 C9 S1 C7 C8 C4 C5 C6 C7 C2 H15 H16 C17 C14 C14 C14 N18 C17

ωg+ –0.495 –0.1498 –0.2241 –0.3205 2.6083 –0.495 0.0012 0.1044 0.0012 –0.1662 –0.0992 –0.1118 –0.1328 0.1044 –0.3785 –0.1379 –0.1118 –0.1498 –0.1328 –0.3205 –0.0992 –0.1498 –0.2241 –0.3785 0.0012 –0.0992 0.1044 –0.1118 –0.3785 –0.495 –1.8713 –0.037 –0.543 2.6083 2.6083 2.6083 –0.1738 –0.543

ωg– 0.0617 0.0199 –0.4898 –0.2446 –0.0356 0.0617 0.1057 –0.0257 0.1057 –0.1898 –0.2279 –0.0544 –0.1761 –0.0257 –0.3575 –0.1743 –0.0544 0.0199 –0.1833 –0.2446 –0.2279 0.0199 –0.4898 –0.3575 0.1057 –0.2279 –0.0257 –0.0544 –0.3575 0.0617 –0.0861 –0.0708 0.0162 –0.0356 –0.0356 –0.0356 –0.1477 0.0162

∆ωg 0.7264 –2.8337 0.6612 –0.0492

–0.1146

–0.1455

0.1766

0.1169

–0.1402 –0.1287 –0.1301 0.0574 0.021 2.8673 –2.6439 –2.6439 –2.6178 0.5592

Table 8. Second order perturbation theory analysis of Fock matrix of 2–benzothiazole acetonitrile by NBO method. Donor (i) → Acceptor (j)

E(2)a (kJ mol–1)

E(j) – E(i)b (a.u.)

F(i, j)c (a.u.)

σ(S1–C2)→σ*(C7–C9)

4.53

1.23

0.067

π(C2–N3)→π*(C8–C9)

14.83

0.35

0.071

σ(N3–C8)→σ*(C2–C14)

5.79

1.15

0.073

σ(C4–C5)→σ*(N3–C8)

4.69

1.16

0.066

π(C4–C5)→π*(C6–C7)

19.93

0.28

0.067

π(C4–C5)→π*(C8–C9)

20.95

0.26

0.069

σ(C4–C8)→σ*(C8–C9)

4.63

1.24

0.068

σ(C4–H10)→σ*(C8–C9)

4.34

1.05

0.061

σ(C6–C7)→σ*(S1–C9)

5.94

0.91

0.066

π(C6–C7)→π*(C4–C5)

18.40

0.29

0.065

π(C6–C7)→π*(C8–C9)

18.85

0.27

0.066

σ(C7–C9)→σ*(C8–C9)

4.02

1.26

0.064

σ(C7–H13)→σ*(C8–C9)

4.13

1.06

0.059

σ(C8–C9)→σ*(C4–C8)

4.26

1.27

0.066

σ(C8–C9)→σ*(C7–C9)

4.09

1.27

0.065

π(C8–C9)→π*(C2–N3)

12.12

0.26

0.051

π(C8–C9)→π*(C4–C5)

16.12

0.30

0.063

π(C8–C9)→π*(C6–C7)

18.71

0.30

0.067

σ(C14–H15)→π*(C2–N3)

5.21

0.53

0.050

σ(C14–H15)→π*(C17–N18)

7.16

0.63

0.060

σ(C14–H16)→σ*(S1–C2)

5.74

0.71

0.057

σ(C14–H16)→π*(C17–N18)

5.50

0.63

0.053

σ(C14–C17)→σ*(C17–N18)

5.77

1.66

0.087

σ(C17–N18)→σ*(C14–C17)

4.95

1.49

0.077

n(S1)→π*(C2–N3)

27.64

0.25

0.075

n(S1)→π*(C8–C9)

18.75

0.27

0.065

n(N3)→σ*(S1–C2)

17.48

0.54

0.087

n(N3)→σ*(C8–C9)

6.23

0.90

0.068

10.92

0.94

0.091

n(N18)→σ*(C14–C17)

a

E(2) means energy of hyperconjugative interactions.

b

Energy difference between donor and acceptor i and j NBO orbitals.

c

F(i, j) is the Fock matrix element between i and j NBO orbitals.

1. Stable geometry of 2–benzothiazole acetonitrile has been found. 2. The extreme limits of the electrostatic potential is ± 4.943e × 10–2. 3. The energy gap (ELUMO−EHOMO) is high as 5.3101 eV. 4. Acetonitrile group deviates by a mean angle of 35.37o from molecular plane. 5. The nS → π*C2-N3 interaction has strong stabilisation of 27.64 kcal mol−1.