Accepted Manuscript Spectroscopic [FT-IR and FT-Raman] and theoretical [UV-Visible and NMR] analysis on α-Methylstyreneby DFT calculations N. Karthikeyan, J. Joseph Prince, S. Ramalingam, S. Periandy PII: DOI: Reference:

S1386-1425(15)00167-5 http://dx.doi.org/10.1016/j.saa.2015.02.015 SAA 13305

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received Date: Revised Date: Accepted Date:

8 August 2014 7 January 2015 4 February 2015

Please cite this article as: N. Karthikeyan, J. Joseph Prince, S. Ramalingam, S. Periandy, Spectroscopic [FT-IR and FT-Raman] and theoretical [UV-Visible and NMR] analysis on α-Methylstyreneby DFT calculations, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa. 2015.02.015

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Spectroscopic [FT-IR and FT-Raman] and theoretical [UV-Visible and NMR] analysis on α-Methylstyrene by DFT calculations N. Karthikeyan a, J. Joseph Prince b, S. Ramalingam* c, S. Periandy d a

Department of physics, Indra Ganesan College of Engineering, Trichy, Tamilnadu, India.

b

Department of Physics, Anna University, Bit Campus, Tiruchirappalli, Tamilnadu, India.

c

Department of Physics, A.V.C. College, Mayiladuthurai, Tamilnadu, India.

d

Department of Physics, Tagore Arts College, Puducherry, India. ABSTRACT In the present research work, the FT-IR, FT-Raman and 13C & 1H NMR spectra of the α-

Methylstyrene were recorded. The observed fundamental frequencies in finger print as well as functional group regions were assigned according to their uniqueness region. The Gaussian computational calculations are carried out by HF and DFT (B3LYP and B3PW91) methods with 6-31++G(d,p) and 6-311++G(d,p) basis sets and the corresponding results were tabulated. The impact of the presence of vinyl group in phenyl structure of the compound is investigated. The modified vibrational pattern of the molecule associated vinyl group was analyzed. Moreover, 13C NMR and 1H NMR were calculated by using the gauge independent atomic orbital (GIAO) method with B3LYP methods and the 6-311++G(d,p) basis set and their spectra were simulated and the chemical shifts linked to TMS were compared. A study on the electronic and optical properties; absorption wavelengths, excitation energy, dipole moment and frontier molecular orbital energies were carried out. The kubo gap of the present compound was calculated related to HOMO and LUMO energies which confirm the occurring of charge transformation between the base and ligand. Besides frontier molecular orbitals (FMO), molecular electrostatic potential (MEP) was performed. The NLO properties related to Polarizability and hyperpolarizability based on the finite-field approach were also discussed. Keywords: α-Methylstyrene; Vinyl group; Optical properties; GIAO; Chemical shifts; FMO. * Corresponding author. Tel.: +91 9003398477; fax: +04364 222264; e-mail: [email protected] 1

1. Introduction α -Methylstyrene has a methyl group and a vinyl group at α position on the ring and it can serve as a model system for studying the substitution effect on benzene. The methyl group can donate electron to the aromatic ring through σ bond, and the vinyl group can share the π electron with the ring [1]. It is also a good model system for studying the “through ring” interaction of the methyl group and vinyl group by their respective torsional potentials. The α-Methylstyrene (AMS) is a chemical intermediate used in the manufacture of plasticizers, resins and polymers. It is a co-product formed in a variation of the cumene process. The homopolymer obtained from this monomer, poly(α-Methylstyrene), is unstable, being characterized by a low ceiling. The polymerization and copolymerization reactions of AMS are very much influenced by its stereochemistry. Normally the alpha–Methylstyrene polymerization is slow because of the formation of a stable radical, which consequently generates polymers with low molecular weight [2]. Alpha methylstyrene is an intermediate that provides higher thermal performance and impact strength to resins-either directly or as an additive. It is used in acrylonitrile butadiene styrene (ABS), coatings, adhesives, acrylic resins, waxes, and various other applications. The AMS is used in coatings and adhesives as a plasticizer for paints and coatings. AMS modifies reaction rates and improves clarity in acrylic resins. In addition, it is used as a chain terminator in polycarbonate resins. It is a UV stabilizer and antioxidant intermediate. Other applications include perfumery chemicals, drying oils, lubricating oils, alkyd resins, and modified phenolic resins. Styrene and its derivatives are the constituents of many important functional polymeric materials. For example, styrene is one of the building units of poly(p-phenylene) [3] and poly(pphenylenevinylene) [4], which are known to have electroluminescent and semiconducting properties. So, studies on the structures, physico-chemical and electro-optical properties of this molecule are very imperative to bring it out industrially. The literature survey reveals that, to the best of our knowledge, no intensive observation of spectroscopic [FT-IR and FT-Raman] and theoretical [HF/DFT] investigation has been reported so far. Therefore, the present investigation was undertaken to investigate the vibrational spectra, geometrical frame work review, inter and intra molecular interaction between HOMO 2

and LUMO energy levels and first order hyperpolarizability of non linear optical (NLO) activity of the molecule.

2. Computational methods Generally, the methodological investigation of vibrational spectroscopy along with quantum computational calculations is a potent tool for the thoughtful of fundamental vibrational behavior of a molecule. In the present work, HF and some of the hybrid methods; B3LYP and B3PW91 were carried out using the basis sets 6-31++G(d,p) and 6-311++G(d,p). All these calculations were performed using GAUSSIAN 09W [5] program package on Pentium IV processor in personal computer. In DFT methods; Becke’s three parameter hybrids function combined with the Lee-Yang-Parr correlation function (B3LYP) [6-7], Becke’s three parameter exact exchangefunction (B3) [8] combined with gradient-corrected correlational functional of Lee, Yang and Parr (LYP) [9-10] and Perdew and Wang (PW91) [11-12] predict the best results for molecular geometry and vibrational frequencies for moderately larger molecules. The calculated frequencies are scaled down to yield the coherent with the observed frequencies. The scaling factors are 0.916, 0.904 and 0.890 for HF/6-311++G(d,p) method. For B3LYP/6-31++/6311++G(d,p) basis set, the scaling factors are 0.962, 0.945, 0.980 and 0.920 /0.975, 0.982, 0.958 and 0.905. For B3PW91/6-311+G(d,p) basis set, the scaling factors are 0.955,0.940, 0.970 and 0.840. The observed (FT-IR and FT-Raman) and calculated vibrational frequencies and vibrational assignments are submitted in Table 2. Experimental and simulated spectra of IR and Raman are presented in the Figures 2 and 3, respectively. The 1H and

13

C NMR isotropic shielding are calculated with the GIAO method [13] using

the optimized parameters obtained from B3LYP/6-311++G(d,p) method. 13C isotropic magnetic shielding (IMS) of any X carbon atoms is made according to value

13

C IMS of TMS,

CSX=IMSTMS-IMSx. The 1H and 13C isotropic chemical shifts of TMS at B3LYP methods with 6311++G(d,p) level using the IEFPCM method in DMSO and CCl4. The absolute chemical shift is found between isotropic peaks and the peaks of TMS[14]. The electronic properties; HOMO-LUMO energies, absorption wavelengths and oscillator strengths are calculated using B3LYP method of the time-dependent DFT (TD-DFT) [15-16], 3

basing on the optimized structure in gas phase and solvent [DMSO, chloroform and CCl4] mixed phase. Thermodynamic properties have been calculated at 298.15ºC in gas phase using B3LYP/6-311++G(d,p) method. Moreover, the dipole moment, nonlinear optical (NLO) properties, linear polarizabilities and first hyperpolarizabilities and chemical hardness have also been studied. 3. Experimental details The compound AMS is purchased from Sigma–Aldrich Chemicals, USA, which is of spectroscopic grade and hence used for recording the spectra as such without any further purification. The FT-IR spectrum of the compound is recorded in Bruker IFS 66V spectrometer in the range of 4000–400 cm−1. The spectral resolution is ±2 cm−1. The FT-Raman spectrum of AMS is also recorded in the same instrument with FRA 106 Raman module equipped with Nd:YAG laser source operating at 1.064 µm line widths with 200 mW power. The spectra are recorded in the range of 4000–100 cm−1 with scanning speed of 30 cm−1 min−1 of spectral width 2 cm−1. The frequencies of all sharp bands are accurate to ±1 cm−1. The 13C 1H NMR spectrum is recorded by Spin solve high resolution bench top FT-NMR Spectrometer. The operating frequency: 42.5 MHz Proton with Resolution: 50% line width, < 25 ppb (1 Hz) in Sensitivity is greater than 10000:1.

4.

Results and discussion

4.1. Molecular geometry The molecular structure of α-Methylstyrene belongs to CS point group symmetry. The molecular structure is optimized by Berny’s optimization algorithm using Gaussian 09 and Gauss view program and is shown in Figure 1. The comparative optimized structural parameters such as bond length, bond angle and dihedral angle are presented in Table 1. The present molecule contains methyl and vinyl groups. The structure optimization and zero point vibrational energy of the compound in HF and DFT(B3LYP/B3PW91) with 6-31++/6-311++G(d,p) are 107.41, 101.29, 100.88, 101.46 and 101.03 Kcal/Mol, respectively. The calculated energy of HF is greater than DFT method because the assumption of ground state energy in HF is greater than the true energy. Though, the benzene ring belongs to one plane, the substitutions belongs to multiple planes. The hexagonal structure 4

of the base molecule is broken due to the coupling of CH3-C=CH2 groups. The bond lengths of C1-C2 and C1-C6 are just pulled up by loading of substitution in the benzene ring. Thus the hexagonal ring is broken in favor of substitution. The other bond length in benzene ring is nearly equal. The bond angle of C2-C1-C6 is reduced by 0.010º than C3-C4-C5 due to the presence of CH3-C=CH2 group in the pedestal molecule. The entire C-H bonds in the ring having almost equal inter nuclear distance. Form the optimized molecular structure; it is observed that the functional group belongs to multiple planes. 4.2. Vibrational assignments In order to obtain the spectroscopic signature of the AMS, the computational calculations are carried out for frequency analysis. The molecule is indentified with CS point group symmetry, consists of 19 atoms, so it has 51 normal vibrational modes. On the basis of Cs symmetry, the 51 fundamental vibrations of the molecule can be distributed as 34 in-plane vibrations of A species and 17 out of plane vibrations of A species, i.e., vib = 34 A + 17 A. In the CS group symmetry of molecule is non-planar structure and has the 51 vibrational modes span in the irreducible representations. The harmonic vibrational frequencies (unscaled and scaled) calculated at HF, B3LYP and B3PW91 levels using the triple split valence basis set along with the diffuse and polarization functions; 6-31++ G(d,p) and 6-311++G(d,p) and observed FT-IR and FT-Raman frequencies for various modes of vibrations have been presented in Tables 2 and 3. The inclusion of electron correlation in the density functional theory to certain extends makes the frequency values better than the HF frequency data. 4.2.1. C-H vibrations The aromatic compounds, particularly, the benzene and its derivative compounds commonly exhibit multiple weak bands in the region 3000-3100 cm-1 due to the C-H bond stretching vibrations[17-19]. The present compound AMS, posses five C-H bonds and their stretching vibrations are observed with strong intensity at 3065, 3060, 3040, 3010 and 2980 cm-1 in IR and Raman spectra. Except one peak, the entire observed bands are distributed within the allowed region. This is because of the influence of vinyl group with dominant character. The C-H in plane ring bending vibrations normally occurred as a number of strong to weak intensity sharp bands in the region 1300-1000 cm-1 [20]. The bands for C-H in plane 5

bending vibrations are identified at 1190, 1160, 1110, 1070 and 1030 cm-1. All the bands found at the lower corner of the expected region with weak and medium intensity and also the peaks are present in IR and Raman. All the observed peaks are originated at tail end of the predictable region. Normally, the C-H out of plane bending vibrations are observed in the region 950-760 cm-1 [21-23]. In the present case, the out of plane bending bands are identified at 840, 780, 760, 750 and 740 cm-1. Unlike in plane bending, two bands are moved down from out of the allowed region. This is purely due to the effect of the substitutions in the ring. Except out of plane bending vibrations, the assigned frequencies for C-H vibrations are found to be well within their characteristic regions. As a result from above discussion, it is infer that C-H out of plane bending vibrations have been affected by the substitutions. 4.2.2. C-C vibrations The ring C=C and C-C stretching vibrations, known as semicircle stretching usually occur in the region 1400-1625 cm-1 [24-26]. The C=C stretching vibrations of the present compound are strongly observed at 1600, 1570 and 1495 cm-1. These assignments are in line with the literature [27-29]. The C-C stretching vibrations were observed at 1440, 1400 and 1370 cm-1. When compared to the literature range cited above, there is a considerable decrease in observed frequencies and one of them is moved down from the probable range which is also worsening with the increase of mass of substitutions held around the ring. Most of the CC stretching bands are observed with very strong and medium intensity and found in both IR and Raman. Apart from that, three crests present at 490, 370 and 365cm-1 are assigned to CCC in plane bending and three supplementary peaks are assigned at 200, 150 and 120 cm-1 to CCC out of plane bending. The entire bending bands are found away from the expected region which is probably due to the suppression of other vibrations in and around the ring. 4.2.3. Methyl group vibrations The title compound posses a methyl group along with vinyl group and their vibrational assignments ensure the place of methyl group within a molecule. The asymmetric and the symmetric C-H symmetric vibrations in methyl group usually observed between 2990-2920 cm-1, whereas the symmetric C-H vibrations for methyl group are observed at 2900 – 2840 cm-1 [3032]. Being in excellent agreement with the literature, in this study, for AMS, the symmetric C-H vibrations are occurring at 2940, 2915 and 2850 cm-1. There is no asymmetric C-H vibration and also such are not affected by other vibrations. The in plane and out of plane bending vibrations 6

are observed at 1020, 1000 and 950 cm-1 and 620, 545 and 540 cm-1 respectively. From the bending vibrations, it is observed that, except two, the entire vibrations found behind the expected region due to the interaction of vinyl group. Predicted by the DFT calculations, the compounds containing CH3 group, the series of the bands appearing as asymmetric and symmetric deformation modes in the region 1400-1500 cm-1 [33-34] are mainly due to methyl deformation, coupling with the C-H and C-C stretching frequencies, two different extends and in different way. In the present study, the Raman bands at 1460 cm−1 (very strong) and 1450 cm−1 (strong) are attributed to the asymmetric deformation modes of isopropyl group. Appearance of these bands is due to presence of two independent CH3 groups in the amino acid residues in different environments. 4.2.4. Vinyl group vibrations The present compound has one vinyl group along with the methyl group, so the interaction is possible within the molecule. The asymmetric =CH2 stretching of vinyl and vinylidine group (3092-3077cm-1) absorb at a higher frequency than =CH vibration (30503000cm-1) in hydrocarbons [19]. Accordingly, the asymmetric C–H stretching vibrations are located at higher frequency region than those of the aromatic C–H ring stretching which are observed with strong intensity at 3090 and 3070 cm−1. In spite of the presence of CH2 in void position, these stretching peaks have not affected much. Normally, the C-H scissoring mode is very active in ethylene substituted molecules [35]. In the present assignment, for CH2, the C-H scissoring bending modes are found with very strong intensity at 900 and 895 cm-1 and the wagging modes traced at 520 and 495 cm-1. The assignments for bending vibrations are illogical with the literature report [36-38]. This is mainly due to the interaction effect of carbon in the ring. Normally, the C=C stretching frequency is observed near 1640 cm-1 in vinyl group with medium intensity and which becomes interactive in the IR region in a cis-trans or symmetrical tetra substituted double compound[39], both of which have centers of symmetry. The double bond vibrations are usually appeared strongly in the Raman Effect, however, the trans- tri- and tetra alkyl substituted olefin have somewhat higher C=C stretching frequency than cis, vinylidine or vinyl groups[40]. Conjugation, which weakens the C=C force constant, lower the frequency about 10-50 cm-1. Therefore, in the present case, the C=C stretching for vinyl is absorbed with 7

very strong intensity at 1635 cm-1. This observation is in line with the literature. The corresponding C-C stretching vibrations for CH3-C-CH2 group are found consistently at 1305 and 1220 cm-1 which are identified in the spectra without disturbance. The C-C in plane and out of plane bending vibrational peaks established at 710 and 690 cm-1 and 250 and 230 cm-1 respectively. The in plane bending bands are positioned correctly within the expected region whereas the out of plane bending pushed down to the lower end of the spectra. 4.3. NMR analysis NMR spectroscopy technique is throwing new light on organic structure elucidation of much difficult complex molecules. The combined use of experimental and computational tools offers a powerful gadget to interpret and predict the structure of bulky molecules. In this way, the optimized structure of AMS is used to calculate the NMR spectra by B3LYP method with 6311++G(d,p) level using the GIAO method and the chemical shifts of the compound are reported in ppm relative to TMS for 1H and

13

C NMR spectra which are presented in Tables 4. The

corresponding spectra are shown in Figure 5. Normally, the range of 13C NMR chemical shifts for is greater than 100 ppm [41-43] and the accuracy ensure that the reliable interpretation of spectroscopic parameters. In the present work, 13

C NMR chemical shifts of entire carbons in and out of the ring are greater than 100 ppm, as in

the expected regions. The present molecule posses hexagonal ring, in which the chemical shift of six carbons are (C1, C2, C3, C4, C5 and C6) are 111.10, 73.62, 85.96, 82.59, 85.03 and 72.18 ppm respectively. Except C1, the chemical shift entire carbons in the ring are less than 100 ppm. The chemical shift of C1 is more than rest of others. This is mainly due to the breaking of paramagnetic shield of proton by the substitutions of CH3-C=CH2. This view ensured that, the existence of substitutional groups in the molecule and thus chemical property of the ring is changed on par with the substitutions. The C12 and C16 in the chain have more shifted than C13 due to the delocalization of σ and π electrons by CH3 group. The shift of the carbons of C3, C4 and C5 of the ring is found nearly equal when compared with C2 and C6. The shift of H in the benzene ring is less than the H of methyl group due to the further substitutions in the chain. From the observation, it is clear that the change of chemical property of benzene is only in favor of H2C=C-CH3 groups. In addition to that, due to the accessibility of this group, the property of the entire molecule is depends upon 8

the substitutions in the compound. There is no considerable difference chemical shift between gas and solvent phases. 4.4. Electronic properties (frontier molecular analysis) The frontier molecular orbitals are very much useful for studying the electric and optical properties of the organic molecules. The stabilization of the bonding molecular orbital and destabilization of the antibonding can be made by the overlapping of molecular orbitals. The stabilization of the bonding molecular orbital and destabilization of the antibonding can increase when the overlap of two orbitals increases [44]. In the molecular interaction, there are the two important orbitals that interact with each other. One is the highest energy occupied molecular orbital is called HOMO represents the ability to donate an electron. The other one is the lowest energy unoccupied molecular orbital is called LUMO as an electron acceptor. These orbitals are sometimes called the frontier orbitals. The interaction between them is much stable and is called filled empty interaction. When the two same sign orbitals overlap to form a molecular orbital, the electron density will occupy at the region between two nuclei. The molecular orbital resulting from in-phase interaction is defined as the bonding orbital which has lower energy than the original atomic orbital. The out of phase interaction forms the anti bonding molecular orbital with the higher energy than the initial atomic orbital. The orbital interactions are depending on their symmetry. It is stated that, the orbital interactions are allowed if the symmetries of the atomic orbitals are compatible with one another. Under the symmetry’s affect, the result of orbital interaction from anti bonding is nonbonding to bonding. Based on the symmetry, the orbital interactions from bonding are classified as σ, π and δ bonding. The 3D plots of the frontier orbitals; HOMO and LUMO of AMS molecule for are in gas, shown in Figure 4. According to Figures 4, In the present compound, the HOMO and HOMO+1 are shaped by the π orbital bonding interaction and the HOMO is mainly localized over the C=C of the upper and lower moieties of benzene ring and the vinyl group. The HOMO+1 is mainly localized over the C=C of the left and right moieties of benzene ring itself. The LUMO and LUMO-1 are wrought by the σ orbital bonding interaction and the LUMO is mainly localized over the C-C and C-H of the hexagonal ring and the vinyl group. The LUMO-1 is mainly localized over the C-H of the left and right moieties of benzene ring itself. 9

The antibonding orbital lobes are also taking place over some part of the molecule. From this observation, it is clear that, the in and out of phase interaction, π orbital and σ orbital bonding interaction are present in HOMO and LUMO respectively. The HOMO→LUMO transition implies an electron density transferred among ring and vinyl group. The HOMO and LUMO energy are 9.3261 eV and 5.0724 eV in gas phase (figure 4). Energy difference between HOMO and LUMO orbital is called as energy gap (kubo gap) that is an important stability for structures. The DFT level calculated energy gap is 4.2537 eV, show the large energy gap and reflect the dull electrical activity of the molecule. 4.5. Optical properties (HOMO-LUMO analysis) The UV and visible spectroscopy is used to detect the presence of chromophores in the molecule and whether the compound has NLO properties or not. The electronic structure calculations of AMS are optimized in singlet state. The low energy electronic excited states of the molecule are calculated at the B3LYP/6-311++G(d,p) level using the TD-DFT approach on the previously optimized ground-state geometry of the molecule. The calculations are performed in gas phase and with the solvent of DMSO, CCl3 and CCl4. The calculated excitation energies, oscillator strength (f) and wavelength (λ) and spectral assignments are given in Table 5 and the corresponding 3D plots of the frontier orbitals is outlined in figure 7. The major contributions of the transitions are designated with the aid of SWizard program [45]. The aromatic system contains p electrons, absorb strongly in the ultraviolet. In general, the greater the length of a conjugated system in a molecule, the nearer the λmax comes to the visible region. Thus, the characteristic energy of a transition and hence the wavelength of absorption is a property of a group of atoms rather than the electrons themselves. The TD-DFT calculations predict that, irrespective of the gas and solvent phase, the entire transitions belong to quartz ultraviolet region. In the case of gas phase, the strong transition is at 266.07, 257.37 and 220.26 nm with an oscillator strength f=0.14, 0.09 and 0.20 with 4.65, 4.81 and 5.62 eV energy gap. The transition is n→π* in quartz ultraviolet region. The designation of the band is R-band (German, radikalartig) which is attributed to above said transition due to the addition of auxochromes with chromophoric group, such as H3C-C=CH2 group. They are characterized by low molar absorptivities (ξmax