Accepted Manuscript Spectroscopic properties, NLO, HOMO-LUMO and NBO of maltol V. KrishnaKumar, D. Barathi, R. Mathammal, J. Balamani, N. Jayamani PII: DOI: Reference:

S1386-1425(13)01225-0 http://dx.doi.org/10.1016/j.saa.2013.10.068 SAA 11197

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received Date: Revised Date: Accepted Date:

6 February 2013 9 October 2013 17 October 2013

Please cite this article as: V. KrishnaKumar, D. Barathi, R. Mathammal, J. Balamani, N. Jayamani, Spectroscopic properties, NLO, HOMO-LUMO and NBO of maltol, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2013), doi: http://dx.doi.org/10.1016/j.saa.2013.10.068

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Spectroscopic properties, NLO, HOMO-LUMO and NBO of maltol b

c

d

V. KrishnaKumarª*, D.Barathi , R.Mathammal , J.Balamani , N.Jayamani

e

a

Department of Physics, Periyar University, Salem-636011, India

b

Department of Physics, N.K.R. Govt. Arts College (W),Namakkal-637001, India

c

Department of Physics, Sri Sarada College for Women (Autonomous) Salem-636016, India

d

Department of Chemistry, Chikkanna Govt. Arts College, Tiruppur-641602, India

e

Department of Physics, Vivekananda College of Arts and Sciences, Tiruchengode for Women

(Autonomous) Namakkal-637205, India

Abstract Maltol (3-hydroxy-2-methyl-4pyrone) is widely known as metal ions chelator with many practical applications in catalysis, medicine and food chemistry. The FTIR and FT-Raman spectra of maltol have been recorded in the region 4000– 400 and 4000–50 cm−1, respectively. The conformational analysis, optimized geometry, frequency and intensity of the vibrational bands of maltol were obtained by the density functional theory (DFT) with complete relaxation in the potential energy surface using 6-31G* basis set. The observed and the calculated frequencies are found to be in good agreement. The 1H and and 1H and

13

C NMR spectra have been recorded

13

C nuclear magnetic resonance chemical shifts of the molecule were

also calculated using the gauge independent atomic orbital (GIAO) method and their respective linear correlations were obtained. The electronic properties HOMO and LUMO energies were measured. Thermodynamic properties (heat capacity, entropy and enthalpy) of the title compound were calculated. The Mulliken charges, the values of electric dipole moment (µ) of the molecule were computed using DFT calculations. The first order hyperpolarizability (βº) and related properties (β, αo and ∆α) of both are calculated using B3LYP/6-31G* method on the finite-field approach. The calculated first hyperpolarizability shows that the molecules are an attractive 1

molecule for future applications in non-linear optics. The intramolecular contacts have been interpreted using Natural Bond Orbital (NBO). Keywords: Maltol; DFT; FTIR; FT-Raman; Vibrational spectroscopy; HOMO and LUMO; Hyperpolarizability. 1. Introduction 3-Hydroxy-2-methyl-4pyrone (maltol, Hma) is a natural compound. It was found in many food products. Maltol is also used as a food additive due to its flavour [1] and antioxidant properties [2]. In coordination chemistry, maltol is known as a potent monoanionic, bidendate metal chelator. Various complexes of maltol have been extensively studied because of their catalytic and biochemical properties. For example, the complex of maltol with oxovanadium(IV) ion is known as an orally active insulin mimetic [3], ferric tri-maltol enables effective iron delivery with low side effects in the iron deficiency anaemia [4], aluminium complex exhibits a strong neurological activity [5], bismuth(III)–maltol complex acts as an antibacterial agent [6], complexes with indium(III) and gallium(III) [7] are suggested as new radiopharmaceuticals. Computational

studies

seem

to

constitute

a

valuable

tool

in

pharmaceutical solid-state research. The theory has to be supported by different spectroscopic and diffraction methods. Among them, vibrational spectroscopy is one of the most commonly used for the molecular structure determination. A standard protocol that involves experimental-theoretical technique is based on a comparison of the experimental vibrational spectra with the harmonic frequencies calculated ab initio and scaled by an appropriate factor [8]. The aim of this work is to study vibrational (FT-Raman,FT-IR and NMR) spectra and investigated the optimized geometry, atomic charges and vibrational spectra for the title molecules.

2

2. Experimental The crystalline sample of maltol was obtained from Lancaster Chemical Company, UK with a stated purity of 99% and was used as such without further purification. Fourier transform infrared spectra of the title compound were measured at the room temperature in the region 4000–400 cm−1 using a BRUKER IFS-66 V FTIR spectrometer at a resolution of ±1 cm−1 equipped with a MCT detector, a KBr beam splitter and globar source. The FT-Raman spectra of title molecule were recorded on a BRUKER IFS – 66 V model interferometer equipped with FRA-106 FT-Raman accessory in the 4000–50 cm−1 Stokes region using the 1064 nm line of a Nd:YAG laser for excitation operating at 200 mW power. The reported wave numbers were believed to be accurate within ±1 cm−1. 1

H and

13

C nuclear magnetic resonance (NMR) (400 MHz; CDCl3)

spectra were recorded on a Bruker HC400 instrument. Chemical shifts for protons are reported in parts per million scales (d scale) downfield from tetramethylsilane. 3. Quantum chemical calculations The quantum chemical calculations have been performed by using B3 [9] exchange functional combined with the LYP [10] correlation function resulting in the B3LYP density functional method at 6-31G* basis set. All the computations were performed using Gaussian 03W program [11] and Gauss-View program package [12]. The most optimized structural parameters, energy, and vibrational frequencies of the molecule have been evaluated for the calculations of vibrational frequencies by assuming Cs point group symmetry. At the optimized geometry for the title molecule no imaginary frequency modes were obtained, therefore there is a true minimum on the potential energy surface was found. As a result, the unscaled calculated frequencies, infrared intensities and Raman activities are obtained. In

3

order to fit the theoretical wavenumbers to the experimental, the scaling factors have been introduced by using a least square optimization of the computed to the experimental data. The scaling of the force field was performed according to the SQMFF procedure [13], the Cartesian representation of the force constants were transferred to a nonredundant set of local symmetry coordinates, chosen in accordance to the recommendations of Pulay et al. [14]. The descriptions of the predicted frequencies during the scaling process were followed by the potential energy distribution (PED) matrix. The characterization of the normal modes using potential energy distribution (PED) was done with the MOLVIB – 7.0 programs written by Sundius [15, 16]. Furthermore, in order to show nonlinear optic (NLO) activity of title molecule, the dipole moment, linear polarizability and first hyperpolarizability were obtained. The prediction of Raman intensities The Raman activity (Si) calculated by Gaussian 03 and adjusted during scaling procedure with MOLVIB were converted to relative Raman intensity (Ii) using the following relation from the basic theory of Raman scattering [17, 18]

where  o is the exciting wavenumber (in cm-1 units), i the vibrational wavenumber of the ith normal mode, h, c, and k are the universal constants and f is a suitably chosen common normalization factor for all peak intensities. For the plots of simulated IR and Raman spectra, pure Lorentzian band shapes were used with a bandwidth (FWHM) of 10 cm−1. The theoretically simulated spectra are more regular

4

than the experimental ones, because many vibrations presenting in condensed phase leads to strong perturbation of infrared intensities of many other modes. 4. Results and discussion 4.1. Molecular geometry In order to find the most optimized geometry, the energy calculations were carried out for maltol (3-hydroxy-2-methyl-4pyrone), using B3LYP/6-31G* basis set for two different conformers and are shown in Fig. 1(a)&1(b) along with calculated total energies. The conformer shown in Fig. 1(b) is found to have a minimum energy and its structural parameters (bond length, bond angle) by B3LYP with 6-31G* basis set were shown in Table 1. The molecular structure of best conformer is shown in Fig 2. In maltol, the hydrogen bonded O–H bond length is 0.9866A and coincides with the reported hydrogen bonded O–H distance (0.98) of 2quinoxaline carboxylic acid [19]. The C2–C7 bond length is longer than other C-C bondlengths, due to the pulling of sidechain in the position of C2 in maltol. Thereby decreases in force constant and increase in bond length in that place. According to quantum topological analysis performed the intramolecular hydrogen bond OH…O=C in 3-hydroxy-2-methyl-4pyrone (maltol) is weak and has ionic nature. X-Ray analysis (XRA) of a large number of crystal compounds with intramolecular hydrogen bonds has shown that in many of them in the proximity of the hydrogen atom at the distance less than the sum of the van der Waals radii (vdW) there is one more electronodonor heteroatom (O, N, S) [20]. The authors classified such mutual arrangement of atoms as bifurcated (three-centered) hydrogen bonds (BHB). From the data of quantum chemical calculations and XRA, nonvalent distances between the hydrogen atom of the hydroxyl group and carbonyl

5

oxygen in these dimers is less than the sum of the vdW radii (~2.7 Aº [20]). From the title compound, the intramolecular hydrogen bond O11–H12…O13=C4 makes to produce the distance between O13 and H12 is 1.99344 Aº. 4.2. Structural properties The title molecules possessing CS point group symmetry. The 39 fundamental modes of vibrations of 3-hydroxy-2-methyl-4pyrone (maltol) are distributed into the irreducible representations as 27 in-plane vibrations of A' species and 12 out of plane vibrations of A" species, i.e., Г vib = 27A' + 12A", All vibrations are active in both the Raman scattering and infrared absorption. In Raman spectrum the A' vibrations give rise to polarized bands while the A" ones to depolarized bands. The internal coordinates and local symmetry coordinates are tabulated in Supplementary tables 1s&2s. For visual comparison, the observed and calculated (simulated) FTIR and FT-Raman spectra are presented in Figs. 3&4. Comparison between the calculated and observed vibrational spectra helps us to understand the observed spectral features. The results of our vibrational analysis, viz., calculated unscaled vibrational frequencies and IR intensities, SQM frequencies, potential energy distributions (PED) and assignment of the fundamentals, for the title compound are collected in Table 2. 4.2.1. Carbon–hydrogen vibration The heteroaromatic structure showed the presence of C-H stretching vibrations in the region 3100–3000 cm−1, which was the characteristic region for ready identification of C-H stretching vibrations [21]. Hence in the present investigation the calculated frequencies which lie in the region 3098–3082 cm−1 were assigned to the C-H stretching vibrations of the title compound as shown in Table 2 and their experimental counterpart appear in the region 3100–3080 cm−1 of the

6

Raman spectra. Thus there was a good agreement between experimental and calculated frequencies. The in-plane aromatic C–H deformation vibrations occur in the region 1300–1000 cm−1 [22]. The C–H in-plane bending vibration computed at 1211 and 1379 cm−1, while the experimental observation at 1225 and 1371 cm−1 in FT-IR shows excellent agreement with theoretical values. C–H out-of-plane bending vibrations are strongly coupled vibrations and occur in the region 900–667 cm−1 [23]. The calculated frequency at 825 and 936 cm−1 is assigned for the C–H out-of-plane bending. In general the aromatic C–H vibrations (viz, stretching, in-plane and out of plane vibrations) calculated theoretically are in good agreement with experimentally accepted values. 4.2.2. Methyl group vibrations For the assignments of CH3 group frequencies, basically nine fundamentals could be associated to each CH3 group [19] namely CH3ss – symmetric stretch; CH3ips – in-plane stretch (i.e. in-plane hydrogen stretching modes); CH3ipb – in-plane bending (i.e. in-plane hydrogen deformation modes); CH3sb – symmetric bending; CH3ipr – in-plane rocking; CH3opr – out-of-plane rocking; tCH3 –twisting modes. In addition to that, CH3ops – out-of-plane-stretch and CH3opb – out-of-plane bending modes of CH3 group would be expected to be depolarized for A" symmetry species. The CH3ss frequencies were established at 2920 cm−1 in IR spectrum; the CH3ips were assigned at 3063 cm−1 in IR spectrum of the title compound maltol. We observed the symmetrical methyl deformation modes CH3sb at 1464 cm−1 in IR spectra, in-plane methyl deformation modes CH3ipb at 1374 cm−1 in IR. The bands at 2930 cm−1 in Raman and 1465 cm−1 in IR were attributed to CH3ops and CH3opb respectively. The methyl deformation modes were mainly coupled with in-plane

7

bending vibrations. The in-plane and out-of-plane rocking vibrations of the title compound maltol had been identified at 1080 cm−1 in IR and at 1032 cm−1 in the IR spectrum. The tCH3 (methyl twisting mode) vibrations were assigned within the characteristic region 100 cm−1 and is reported in Table 2. 4.2.3. Oxygen–hydrogen vibrations The O-H stretching vibration is normally observed at about 3300 cm−1. The O-H in-plane bending vibration is observed in the region 1440–1260 cm−1 [24]. In maltol, the bands appeared at 3261 cm−1 are assigned to O-H stretching modes of vibrations. The in-plane bending vibrations of hydroxy group have been identified at 1280 cm−1. Due to intramolecular bonding, the out-of-plane bending vibration of hydroxy groups is shifted at 565 cm−1. 4.2.4. Carbonyl vibrations Susi and Ard [25] identified the C= O stretching mode at 1645 and 1614 cm−1, respectively, in 5-iodo uracil and 5-methyl uracil. On referring to the above findings and on the basis of the results of the normal coordinate analysis, the IR bands appeared at 1650 cm−1 have been designated to C= O stretching modes for maltol in this study. 4.2.5. Ring vibrations Usually an in-plane deformation vibration is at higher frequencies than the out-of-plane vibration [26]. In the present study, the bands observed at 853, 520 and 480 cm−1 in IR and Raman respectively are attributed to ring in-plane bending modes. The ring out-of-plane bending mode frequencies are established at 497, 180 and 115 cm−1 in Raman spectra as shown in Table 2. 4.2.6. Carbon–carbon vibrations

8

The ring carbon–carbon stretching vibrations investigation occurs in the region in 1625–1430cm−1 [27]. In the present, the bands of C–C stretching vibrations in benzene ring appear at 1656, 1565, 1400, 1029 and 690 cm−1 in FTIR are also consistent with the calculated one. 5. NLO properties The second-order polarizability or first order hyperpolarizability β, dipole moment µ and polarizability α was calculated using B3LYP/6-31G* basis set on the basis of the finite-field approach. The complete equations for calculating the magnitude of total static dipole moment µ, the mean polarizability αo, the anisotropy of the polarizability ∆α and the mean first hyperpolarizability β, using the x, y, z components from Gaussian 03W output is as follow µ= (µx2+µy2+µz2) ½ αo =1/3 (αxx+ αyy+ αzz) ∆α=½{[ ( αxx- αyy)2+( αxx- αzz)2+( αzz- αyy)2]}½ βtot = (βx2+βy2+βz2)1/2 βtot = {(βxxx+βxyy+βxzz)2+(βyyy+βyzz+βyxx)2+(βzzz+βzxx+βzyy)2}1/2 Since the values of the polarizabilities (α) and hyperpolarizability (β) of Gaussian 03 output are reported in atomic units (a.u.), the calculated values have been converted into electrostatic units (esu) (α: 1 a.u. = 0.1482×10−12 esu, β: 1 a.u. = 8.6393×10−33 esu). The total molecular dipole moment and mean first hyperpolarizability of maltol is 3.3637 Debye and 0.851×10−30 esu respectively shown in Table 3. The mean polarizability of maltol is 3.78651×10−24 esu. 6. Mulliken population analysis The total atomic charges of maltol and 4-pyrone obtained by Mulliken population analysis [28, 29] by B3LYP method with 6-31G* basis set were 9

listed in Table 4. The Mulliken atomic charges are plotted in Fig. 5 for maltol and 4pyrone. The charge distribution on the molecule has an important influence on the vibrational spectra. In 4-pyrone, there is a decrease of C2 charge due to attachment of one side electronegative O1atom [40]. This C2 charge again decreases the C3 and it becomes negative. But in maltol, C3 is positive due to the attachment of hydroxyl group. For maltol and 4-pyrone, all the hydrogen atoms have a net positive charge and all the electronegative oxygen atoms have negative charges. For maltol, H12 atoms near position to O11 atom accommodate higher positive charge than the other hydrogen atoms. This is due to the presence of electronegative oxygen atom, the hydrogen atom attract the positive charge from the oxygen atom. The maximum atomic charge is obtained for C4 when compared to other atoms. This is due to the negatively charged oxygen (O13) atom in C=O [30]. The charge on the acceptor oxygen atom (O11) in the hydrogen bond becomes more negative and the charge of the donated hydrogen (H12) becomes more positive. 7. HOMO and LUMO analysis Many organic molecules that containing conjugated π electrons are characterized hyperpolarizabilities and were analyzed by means of vibrational spectroscopy [31, 32]. In most cases, even in the absence of inversion symmetry, the strongest bands in the Raman spectrum are weak in the IR spectrum and vice versa. But the intramolecular charge transfer from the donor to accepter group through a single-double bond conjugated path can induce large variations of both the molecular dipole moment and the molecular polarizability, making IR and Raman activity strong at the same time. The experimental spectroscopic behavior described above is well accounted for by ab initio calculations in π conjugated systems that predict exceptionally large Raman and infrared intensities for the same normal

10

modes [31]. It is also observed in our title molecule the bands in FTIR spectrum have their counterparts in Raman shows that the relative intensities in IR and Raman spectra are comparable resulting from the electron cloud movement through π conjugated frame work from electron donor to electron acceptor groups. The electron absorption corresponds to the transition from the ground to the first excited state and is mainly described by one-electron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The LUMO: of π nature is delocalized over the whole keto group. By contrast, the filled orbital (HOMO) is mostly located over oxygen (12 and 13) atoms which is a lone pair orbital and is also slightly located over methyl and hydroxyl groups; consequently the HOMO→LUMO transition implies an electron density transfer to aromatic part of π conjugated system from methyl and hydroxyl group and is seen in fig.6. Moreover, these orbital significantly overlap in their position for O13. The HOMO-LUMO energy gap of maltol was calculated at the B3LYP/6-31* level and are shown in Table 5 and the energy gap reflect the chemical activity of the molecule. LUMO as an electron acceptor represents the ability to obtain an electron, HOMO represents the ability to donate an electron. HOMO energy = −6.14977eV LUMO energy = −1.19730eV HOMO–LUMO energy gap = -4.95247eV The HOMO and LUMO energy gap explains the eventual charge transfer interactions taking place within the molecule. The electronic properties of the molecules are calculated from the total energies and the Koopmans’ theorem. The ionization potential is determined from IP = –EHOMO while the electron affinity is computed from EA=–ELUMO, respectively. The

11

other important quantities such as electronegativity (χ), hardness (η), softness (ζ), and electrophilicity index (ψ) were deduced from ionization potential and electron affinity values. Electro negativity (χ) µ= –χ= – Chemical hardness (η) = Softness (ζ) = Electrophilicity index (ψ) = The values of electro negativity, chemical hardness, softness, and electrophilicity index are 3.6735, 2.4762, 0.20192 and 2.7248 eV in DMSO, respectively, for the title molecule. 8. Other molecular properties The calculated several thermodynamic parameters have been presented as shown in Table 6. The zero-point vibrational energies (ZPVE), rotational constants, and entrophy were calculated by B3LYP/6-31G* basis set. The relations among energetic, structural and reactivity characteristics of the molecules may be obtained from the thermodynamic parameters of the compounds. The dipole moment in a molecule is another important electronic property. For example, the bigger the dipole moment, the stronger will be the intermolecular interactions. The calculated dipole moment value for the molecule is 3.3637D also given in Table 6.

9. NMR spectra The molecular structure of the title compounds was optimized. Then, gauge – inpendent atomic orbital (GIAO) [33, 34] 12

13

CNMR chemical shifts

calculations of the title compounds had been carried out using B3LYP/functional with 6-31G* basis set. Experimental and theoretical chemical shifts of maltol in

13

C NMR

and 1H NMR spectra were recorded and the obtained data are presented in Table 7. The coefficient of correlations between calculated and experimental data of

13

C NMR

and 1H NMR spectra were determined as 0.87886 and 0.99638for maltol. The correlations of

13

C NMR spectra and 1H NMR spectra are presented in Fig. 7 for

maltol. The agreement between the experimental and calculated data was satisfactory [35, 36]. In the present study, the

13

C NMR chemical shifts in the ring for

maltol are >100 ppm, as they would be expected. The Oxygen atom more ectronegative property polarizes the electron distribution in its bond to adjacent carbon atom and decreases the chemical shifts value. So, the C7 atom in maltol which bonded to methyl showed the determined

13

C NMR shifts very low. The H12

atom in hydroxyl group has very high value due to electron withdrawing oxygen atom. On the basis of

13

C NMR spectra, in which the ring carbon (C4) attached to

the O atom have bigger chemical shift than the other carbon atoms. The computed chemical shift values of 172.04 ppm (B3LYP) for carbon atom (C1) are in good agreement with the measured values (173.22 ppm). 10. Natural bond orbital (NBO) analysis NBO analysis provides the most accurate possible ‘natural Lewis structure picture of ψ, because all orbital details are mathematically chosen to include the highest possible percentage of the electron density. A useful aspect of the NBO method is that it gives information about interactions in both filled and virtual orbital spaces that could enhance the analysis of intra- and inter molecular interactions. The second-order Fock matrix was carried out to evaluate the donor– acceptor interactions in the NBO analysis [37]. The interactions result is a loss of

13

occupancy from the localized NBO of the idealized Lewis structure into an empty non-Lewis orbital. For each donor (i) and acceptor (j), the stabilization energy E (2) associated with the delocalization i→j is estimated as

where qi is the donor orbital occupancy, εi and εj are diagonal elements and F(i, j) is the off diagonal NBO Fock matrix element. Natural bond orbital analysis provides an efficient method for studying intra- and intermolecular binding and interaction among bonds, and also provides a convenient basis for investigating charge transfer or conjugative interaction in molecular systems. Some electron donor orbital, acceptor orbital and the interacting stabilization energy resulted from the second-order microdisturbance theory are reported [37, 38]. The larger the E (2) value, the more intensive is the interaction between electron donors and electron acceptors, i.e. the more donating tendency from electron donors to electron acceptors and the greater the extent of conjugation of the whole system [39]. Delocalization of electron density between occupied Lewis-type (bond or lone pair) NBO orbitals and formally unoccupied (anti-bond or Rydberg) non-Lewis NBO orbitals correspond to a stabilizing donor–acceptor interaction. NBO analysis has been performed on the molecule at the DFT/B3LYP/6-31G* level in order to elucidate the intramolecular, rehybridization and delocalization of electron density within the molecule. The strong intramolecular hyper conjugation interaction of the σ and π electrons of C–C and C–O to the anti C–C, C–H, C–O and O–H bond leads to stabilization of some part of the ring as evident from Table 8. The intramolecular interaction are formed by the orbital overlap between π*C4 - O13→ π* C2 - C3 have 14

the highest E (2) value around 131.53 kcal/mol which results intramolecular charge transfer (ICT) causing stabilization of the system [40]. The other significant interactions giving stronger stabilization to the structure are π C2 - C3→ π* C4 – O13, π C5 – C6→ π* C4 – O13, lone pair LP (2) O1→ π* C2 – C3, LP (2) O1→ π* C5 – C6, LP (2) O11→ π* C2 – C3. 11. Conclusion A complete vibrational analysis of 3-hydroxy-2-methyl-4pyrone (maltol) were performed by DFT-B3LYP method with 6-31G* basis set. The influences of carbon–oxygen bond, hydroxy group and carbonyl ring to the vibrational frequencies of the title compound were discussed. Mulliken atomic charges show that the substitution of methyl and hydroxyl group leads to redistribution of electron density. The HOMO-LUMO energy gap calculated at the B3LYP/6-31* level reveals the chemical activity of the molecule. The observed and scaled wavenumber values of O–H fundamental are equal, due to the fact that the presences of the intramolecular hydrogen bonds in the solid state. The calculated first order hyperpolarizability of title molecule was about 4.3 times greater than that of urea. The above results show that maltol was the best material for NLO applications. An NBO result reflects the charge transfer mainly due to OH and CO group of maltol molecule. References [1] Y. Kato, Food Sci. Technol. Res. 9 (2003) 264. [2] K. Yanagimoto, H. Ochi, K.G. Lee, T. Shibamoto, J. Agric. Food Chem. 52 (2004) 592. [3] P. Caravan, L. Gelmini, N. Glover, F.G. Herring, H. Li, J.H. McNeill, S.J. Rettig, I.A. Setyawati, E. Shuter, Y. Sun, A.S. Tracey, V.G. Yuen, C. Orvig,

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J. Am. Chem. Soc. 117 (1995) 12759. [4] D.M. Reffitt, T.J. Burden, P.T. Seed, J. Wood, R.P.H. Thompson, J.J. Powell, Ann. Clin. Biochem. 37 (2000) 457. [5] V.J. Johnson, R.P. Sharma, Neurotoxicology 24 (2003) 261. [6] F.E. Hinds, J.R. Aldrich-Wright, P. Leverett, S.L. Hazell, J. Inorg. Biochem. 96 (2003) 144. [7] B.L. Ellis, A.K. Duhme, R.C. Hider, M.B. Hossian, S. Rizvi, D. van der Helm, J. Med. Chem. 39 (1996) 3659. [8] J.A. Pople, R. Krishnan, H.B. Schlegel, D. DeFrees, J.S. Binkley, M.J. Frisch, R.F. Whiteside, R.F. Hout, W.J. Hehre, Int. J. Quantum Chem. 15 (1981) 269. [9] A.D. Becke, Phys. Rev. A 38 (1988) 3098–3100. [10] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785–789. [11] Gaussian Inc., Gaussian 03 Program, Gaussian Inc., Wallingford, 2004. [12] A. Frisch, A.B. Nielsen, A.J. Holder, Gaussview Users Manual, Gaussian Inc., Pittsburgh, 2007. [13] G. Fogarasi, P. Pulay, in: J.R. Durig (Ed.), Vibrational Spectra and Structure, vol. 14, Elsevier, Amsterdam, 1985, p. 125. [14] (a) P. Pulay, G. Fogarasi, F. Pang, J.E. Boggs, J. Am. Chem. Soc. 101 (1979) 2550–2560; (b) G. Fogarasi, X. Zhou, P.W. Taylor, P. Pulay, J. Am. Chem. Soc. 114 (1992) 8191–8201. [15] T. Sundius, J. Mol. Struct. 218 (1990) 321–326. [16] T. Sundius, Vib. Spectrosc. 29 (2002) 89–95.

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[29] R.S. Mulliken, J. Chem. Phys. 23 (1955) 1833–1840. [30] M. Arivazhagan, D. Anitha Rexalin, Spectrochimica Acta Part A 83 (2011) 553– 560. [31] Y. Ataly, D. Avci, A. BaS， o˘glu, Struct. Chem. 19 (2008) 239. [32]T. Vijayakumar, I. HubertJoe, C.P.R.Nair, V.S. Jayakumar, Chem. Phys. 343 (2008) 83. [33] R. Ditchfield, J. Chem. Phys. 56 (1972) 5688. [34]K. Wolinski, J.F. Hilton, P. Pulay, J. Am. Chem. Soc. 112 (1990) 8251. [35] H.O. Kalinowski, S. Berger, S. Barun, C-13 NMR Spectroscopy, John Wiley & Sons, Chichester, 1988. [36]K. Pihlaja, E. Kleinpeter (Eds.), Carbon-13 Chemical Shifts in Structural and Stereo Chemical Analysis, VCH Publishers, Deerfield Beach, FL, 1994. [37] C. James, A. Amal Raj, R. Reghunathan, I.H. Joe, V.S. Jayakumar, J. Raman Spectrosc. 37 (2006) 1381–1392. [38] J.N. Liu, Z.R. Chen, S.F. Yuan, J. Zhejiang Univ. Sci. B6 (2005) 584–591. [39] A. Nataraj, V. Balachandran, T. Karthick, M. Karabacak, A. Atac, Journal of Molecular Structure 1027 (2012) 1–14. [40] M. Govindarajan, M. Karabacak, S. Periandy, D. Tanuja, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 231–245.

Table 1: Structural parameters calculated for 3-hydroxy-2-methyl-4pyrone obtained by B3LYP/6-31G* method. Bond length

Value (A◦)

Bond angle

Value (◦)

C2- O1

1.3757

C3- C2-O1

119.7234

C3- C2

1.3571

C4- C3-C2

122.0745

C4 -C3

1.4675

C5- C4-C3

114.5295

C5 -C4

1.4468

C5- C4-C3

119.5182

18

C6 -C5

1.3529

C7- C2- C3

126.9524

C7 -C2

1.4908

H8- C7-C2

109.64

H8 -C7

1.0916

H9- C7-C2

110.9868

H9 -C7

1.0969

H10- C7-C2

110.9868

H10- C7

1.0969

O11- C3-C4

114.9368

O11 -C3

1.3511

H12- O11-C3

102.9361

H12 -O11

0.9866

O13- C4-C5

126.9534

O13 -C4

1.2412

H14- C5-C6

120.5817

H14 -C5

1.0839

H15-C6-O1

111.1805

H15 -C6

1.0839 −1

Table 2: Calculated wavenumbers (cm ) of maltol by B3LYP/6-31G* method and vibrational assignment based on potential energy distribution (PED)

No

-

Observed

Calculated frequency(cm ¹) with

frequencies

B3LYP/6-31G*

Characterisation of

-1

(cm ) Symmetry

IR

normal modes with Raman

Unscaled

Scaled

species

1

A'

2

A'

3

A'

4

A'

5

A''

6

A'

7

A'

8

A'

9

A'

3261

IR

Raman

intensity

activity

PED (%)

3485

3261

96.681

64.472

OH(100)

3100

3251

3098

4.796

148.48

CH(99)

3080

3234

3082

1.277

79.729

CH(99)

3168

3063

3.524

58.551

CH3ips(97)

3102

2930

12.843

112.98

CH3ops(99)

2920

3051

2920

18.802

213.40

CH3ss(80),CH3ips(20)

1656

1737

1670

61.620

59.921

CC(75)

1650

1716

1649

351.167

45.042

C=O(59),CC(25)

1565

1634

1573

0.673

20.265

CC(55),bCH(16),

3063 2930

C=O(14),CO(10) 10

A"

1465

1515

1465

9.315

17.035

bCH3op(80),tCH3(14)

11

A'

1464

1507

1448

52.890

43.226

bCH3sb(34),CC(19), bCH3ip(15)

19

12

A'

1400

1486

1422

40.513

13.471

CC(22),bCH3ip(22), CO(15),bCH3sb(11), bCH(10)

13

A'

1394

1442

1387

1.931

7.596

bCH3ip(34), bCH3sb(34), CC(12)

14

A'

1371

1437

1379

49.204

3.453

bCH(48),CO(13), bCH3sb(11),bCOH(10)

15

A'

1280

1362

1286

219.835

13.102

bCOH(55),CO(12), CC(10)

16

A'

1260

1308

1269

60.832

2.624

CO(48),bring(21), bCH(11),CC(11)

17

A'

1225

1255

1211

83.577

0.675

bCH(30),CC(26), CO(19),bCOH(10), bCO(10)

18

A'

1201

1216

1180

95.295

0.705

CO(41),CC(23), bCH3ip(14)

19

A'

1080

1104

1079

5.329

7.195

bCH3ipr(30),bCH(21), CC(18),CO(14)

20

A"

21

A'

1032 1029

1076

1033

0.214

1.344

bCH3opr(57),tCH3(30)

1052

1019

4.218

12.675

CC(32),CO(27), bCH(18), bCO(10)

22

A"

23

A'

940 922

947

936

0.196

0.120

gCH(91)

938

924

35.103

0.188

CO(34),CC(22), bCH3ip(19)

24

A'

853

857

850

36.951

2.603

bring(68),CC(14),CO(1 2)

20

25

A"

820

836

825

46.504

2.620

gCH(67),gCO(23), tring(10)

26

A"

711

735

722

0.199

2.772

gCO(51),tring(38), gCH(10)

27

A'

690

697

673

2.616

17.050

CC(62),CO(25)

28

A"

565

657

573

93.148

3.996

tOH(74),gCO(12), tring(10)

29

A'

559

571

560

0.699

3.519

bCO(39)bCO(38), CC(13)

30

A'

31

A"

520 513

521

506

0.980

0.070

bring(52),CC(38)

514

507

3.745

6.888

gCO(45),tring(34), tCH3(10)

32

A"

497

513

494

11.194

1.808

tring(65),tCH3(13)

33

A'

480

492

481

1.206

0.928

bring(58),bCO(20)

34

A'

327

337

328

20.67

0.531

bCO(72),bring(10)

35

A"

306

312

303

0.160

0.426

gCO(31),tring(28), tCH3(24),gCH(16)

36

A'

215

275

234

2.162

1.126

bCOC

37

A"

180

193

183

4.551

1.275

tring(76),tCH3(12)

38

A"

115

132

124

0.020

0.112

tring(72),gCO(18)

39

A"

100

106

105

0.053

0.472

tCH3(100)

( 74)

Table 3: Calculated hyperpolarizability β, polarizability α of 3-hydroxy-2-methyl-4-pyrone. β

–30

Values(10

α

esu)

Values(10

–30

esu)

βxxx

55.582

αxx

64.855

βxxy

0.0000

αxy

0.0000

βxyy

-10.076

αyy

11.809

βyyy

0.0000

αyz

10.961

21

βxxz

-90.584

αzz

0.0000

βxyz

0.0000

αxz

48.603

βyyz

-3.9268

βxzz

11.927

βyzz

0.0000

βzzz

14.421

Table 4: Mulliken atomic charges of maltol and 4-pyrone at B3LYP with 6-31G (d) basis set. Atoms

Mulliken atomic charges maltol

4-pyrone

O1

-0.448705

-0.412392

C2

0.302266

0.112974

C3

0.232317

-0.214757

C4

0.435193

0.450130

C5

-0.221729

-0.214778

C6

0.115935

0.112990

C7(H7)

-0.517154

0.177039

H8

0.197358

0.163777

H9(O9)

0.168890

-0.515796

H10

0.183830

0.163776

O11(H11)

-0.652651

0.177038

H12

0.425462

O13

-0.559880

H14

0.163666

H15

0.175204

Table 5: HOMO – LUMO energy value calculated by B3LYP/6-31G* Parameters (eV)

maltol

HOMO

-6.14977

LUMO

-1.19730

HOMO -LUMO

-4.95247

Table 6: Theoretically computed energies (a.u), zero-point vibrational energies (kcal/mol), rotational constants (GHz), entropies (cal/mol-Kelvin) and dipole moment (Debye). Parameters

B3LYP 6-31G*

Total energy

-457.89900

22

Zero-point energy

71.14807

Rotational constant 2.39457 1.83972 1.04720 Entropy Total

86.617

Translational

40.408

Rotational

28.634

Vibrational

17.575

Dipole moment

3.3637

1

Table 7: Theoretical and experimental H NMR and

13

C NMR spectra of maltol (with respect to

TMS, all values in ppm) for B3LYP/6-31G* Atoms

maltol

Atoms

Exp

Cal

C2

149.54

149.02

C3

143.41

C4

maltol Exp

Cal

H8

2.377

2.880

149.63

H9

2.377

2.880

173.22

172.04

H10

2.377

2.426

C5

113.33

117.79

H12

7.400

6.544

C6

154.17

157.49

H14

6.441

6.299

C7

14.33

24.94

H15

7.721

7.867

Table8: Significant donor–acceptor interactions of maltol and their second order perturbation energies. Donor

Type

NBO(i)

of

Occupancy

Accepter

Type

NBO(j)

of

23

Occupancy

E(2)

a

(Kcal/mol)

E j- E b i

F(I,j) a.u.

c

bond O1 - C 2

O1 - C6

C2 - C3

C2 - C3

C2 - C7

C3 - C4

C3 -O11

C4 - C5

C4 -O13

C4 -O13

σ

σ

σ

π

σ

σ

σ

σ

σ

π

a.u.

bond 1.98763

1.99079

1.98227

1.80847

1.98299

1.97761

1.99171

1.97877

1.99501

1.97150

C2 - C3

σ*

0.03121

0.65

1.53

0.028

C3 -O11

σ*

0.07535

2.81

1.30

0.054

C6 - H15

σ*

0.01565

1.34

1.37

0.038

C7 - H8

σ*

0.00452

0.72

1.37

0.028

C2 - C7

σ*

0.01656

1.90

1.37

0.046

C5 - C6

σ*

0.01678

0.60

1.55

0.027

C5 - H14

σ*

0.01200

1.50

1.42

0.041

C2 - C7

σ*

0.01656

2.84

1.18

0.052

C3 - C4

σ*

0.07535

2.42

1.22

0.049

C3 - O11

σ*

0.01688

0.97

1.15

0.030

C4 - O13

σ*

0.01033

1.44

1.34

0.039

O11 -H12

σ*

0.03826

1.58

1.17

0.039

C4 - O13

π*

0.34155

20.54

0.30

0.073

C7 - H9

σ*

0.00955

2.51

0.73

0.040

C7 - H10

σ*

0.00955

2.51

0.73

0.040

O1 - C6

σ*

0.02143

2.93

1.02

0.049

C2 - C3

σ*

0.03121

3.17

1.26

0.057

C3 - C4

σ*

0.07535

3.06

1.11

0.053

C7 - H9

σ*

0.00955

0.55

1.09

0.022

C7 - H10

σ*

0.00955

0.55

1.09

0.022

C2 - C3

σ*

0.03121

3.21

1.28

0.057

C2 - C7

σ*

0.01656

4.06

1.09

0.060

C4 - C5

σ*

0.05261

0.86

1.16

0.028

C5 - H14

σ*

0.01565

2.24

1.14

0.045

O1 - C2

σ*

0.03274

2.56

1.24

0.051

C2 - C3

σ*

0.03121

1.63

1.52

0.045

C4 - C5

σ*

0.05261

1.27

1.40

0.038

C3 - C4

σ*

0.07535

0.86

1.14

0.028

C3 - O11

σ*

0.01688

2.52

1.07

0.046

C4 - O13

σ*

0.01033

1.18

1.26

0.034

C5 - C6

σ*

0.01678

2.03

1.29

0.046

C5 - H14

σ*

0.01200

0.85

1.16

0.028

C6 - H15

σ*

0.01565

3.03

1.14

0.053

C2 - C3

σ*

0.03121

1.38

1.64

0.043

C3 - C4

σ*

0.07535

0.98

1.48

0.035

C4 - C5

σ*

0.05261

1.75

1.52

0.047

C5 - C6

σ*

0.01678

0.73

1.63

0.031

C2 - C3

π*

0.26297

6.01

0.38

0.046

C5 - C6

π*

0.19629

5.47

0.37

0.042

24

C5 - C6

C5 - C6

C5 -H14

C6 -H15

σ

π σ

σ

1.98826

1.84150

1.97432

1.98118

C4 - C5

σ*

0.05261

1.65

1.25

0.041

C4 - O13

σ*

0.01033

3.34

1.33

0.060

C5 - H14

σ*

0.01200

1.46

1.23

0.038

C6 - H15

σ*

0.01565

1.51

1.22

0.038

C4 - O13

π*

0.34155

23.10

0.31

0.079

C5 - C6

π*

0.19629

2.21

0.31

0.024

O1 - C6

σ*

0.02143

6.65

0.90

0.069

C3 - C4

σ*

0.07535

3.03

0.99

0.050

C4 - C5

σ*

0.05261

0.53

1.03

0.021

C5 - C6

σ*

0.01678

1.40

1.14

0.036

O1 - C2

σ*

0.03274

4.40

0.90

0.056

C4 - C5

σ*

0.05261

4.02

1.06

0.059

C5 - C6

σ*

0.01678

1.35

1.17

0.036

C7 – H8

σ

1.98172

O1 - C2

σ*

0.03274

6.09

0.84

0.064

C7 - H9

σ

1.97812

C2 - C3

σ*

0.03121

2.22

1.12

0.045

C2 - C3

π*

0.26297

3.43

0.54

0.041

C2 - C3

σ*

0.03121

2.22

1.12

0.045

C2 - C3

π*

0.26297

3.43

0.54

0.041

C2 - C3

σ*

0.03121

6.84

1.32

0.085

C3 - C4

σ*

0.07535

0.68

1.17

0.026

C2 - C3

σ*

0.03121

5.68

1.17

0.073

C2 - C7

σ*

0.01656

0.98

0.98

0.028

C3 - O11

σ*

0.01688

0.53

0.94

0.020

C5 - C6

σ*

0.01678

5.96

1.17

0.075

C6 – H15

σ*

0.01565

1.61

1.01

0.036

C2 - C3

π*

0.26297

26.59

0.38

0.090

C5 - C6

π*

0.01200

33.14

0.37

0.101

C7 -H10

O11 -H12

σ σ

LP(1)O1

1.97812

1.98465

1.96335

LP(2)O1

1.71586

LP(1)O11

1.97747

C3 - C4

σ*

0.03121

5.50

1.05

0.069

LP(2)O11

1.85730

C2 - C3

π*

0.26297

34.91

0.34

0.100

LP(1)O13

1.97724

C3 - C4

σ*

0.07535

2.78

1.13

0.051

C4 - C5

σ*

0.05261

1.59

1.16

0.039

O11 - H12

σ*

0.03826

1.15

1.08

0.032

C3 - C4

σ*

0.07535

17.44

0.71

0.101

C4 - C5

σ*

0.05261

18.78

0.74

0.108

O11 - H12

σ*

0.03826

6.90

0.66

0.062

C2 - C3

π*

0.26297

131.53

0.02

0.079

LP(2)O13

C4 - O13

1.86616

π*

0.01033

a

E(2) means energy of hyper conjugative 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.

25

Figure captions. Fig. 1 (a) and 1(b): Two different conformers of 3-hydroxy-2-methyl-4pyrone. Fig. 2: Molecular structure of 3-hydroxy-2-methyl-4pyrone. Fig.3: Comparison of observed and calculated FTIR spectra of 3-hydroxy2-methyl-4pyrone. Fig.4: Comparison of observed and calculated FT-Raman spectra of3hydroxy-2- methyl-4pyrone. Fig. 5: Mulliken charge distribution of maltol and 4-pyrone. Fig.6: Frontier molecular orbitals of 3-hydroxy-2-methyl-4pyrone. Fig.7: The linear regression between the experimental and theoretical 1H and 13

C NMR Chemical shifts of 3-hydroxy-2-methyl-4pyrone.

26

Fig.1 (a) and 1(b): Two different Conformers of 3-hydroxy-2-methyl-4pyrone. .

Fig. 2: Optimised structure of 3-hydroxy-2-methyl-4pyrone.

27

Fig.3: Comparison of observed and calculated FTIR spectra of 3-hydroxy2-methyl-4pyrone (a) calculated with B3 LYP/6-31G* (b) observed with KBr disc

28

Fig.4: Comparison of observed and calculated FT-Raman spectra of 3hydroxy-2-methyl-4-pyrone (a) calculated with B3 LYP/6-31G* (b) observed with KBr disc

29

Fig. 5: Mulliken charge distribution of maltol and 4-pyrone.

30

Fig.6: Frontier molecular orbitals of 3-hydroxy-2-methyl-4pyrone

Fig.7: The linear regression between the experimental and theoretical 1H and 13

C NMR Chemical shifts of 3-hydroxy-2-methyl-4pyrone.

31

Highlights 

The experimental and theoretical study on the vibrations of maltol is

presented. 

The energies of it obtained for most stable conformer.



NLO and NBO analysis of the molecule were studied.



The electronic properties were measured.

GRAPHICAL ABSTRACT The conformational analyses of maltol were performed. The complete vibrational assignment and analysis of the fundamental modes of the most stable conformer was carried out using the experimental FTIR and FT-Raman data and quantum mechanical studies.