IJCRR - 5(4), February, 2013
Pages: 10-22
Date of Publication: 28-Feb-2013
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SCALED QUANTUM CHEMICAL STUDIES OF THE STRUCTURE AND VIBRATIONAL SPECTRA OF 1, 5- DIMETHYLNAPHTHALENE
Author: N. Jayamani , N. Geetha
Category: General Sciences
Abstract:The scaled phase FTIR and FT-Raman spectra of 1,5-dimethylnaphthalene(DMN) has been recorded in the region 4000-400 cm-1 and 4000-50 cm-1, respectively. The fundamental vibrational wavenumbers and intensities of vibrational bands are evaluated with the aid of normal coordinate analysis based on density functional theory (DFT) using the standard B3LYP/6-31G** method and basis set combination with scaled quantum mechanical force field. The Infrared and Raman spectra are also predicted from the calculated intensities. Comparison of simulated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibrational modes.
Keywords: Vibrational spectra, 1, 5-dimethylnaphthalene, DFT calculations, Vibrational analysis
Full Text:
INTRODUCTION
The structure of naphthalene is benzene like, having two six membered rings fused together. Naphthalene resembles benzene in many of its reaction altogether, it is mild aromatic but more reactive than benzene. DMN is a relatively new sprout inhibiter for use a maincrop and seed potatoes. No harm is expected to the public, the environment. DMN is used as an intermediate for synthesis of pharmaceuticals, photochemical, plant growth hormones, insecticides dyes and other organic compounds. It is also used as a surfactant; water reducing and dispersant.1 The department of Health and Human Services [DHS] concluded that naphthalene is reasonably anticipated to be a human carcinogen.2 The vibrational assignments of the compound can be proposed on the basis of wavenumber agreement between the computed harmonics and the observed fundamentals. Quantum chemical computational methods have proven to be an essential tool for interpretations and prediction of vibrational spectra.3,4 A significant advances in this area was made by scaled quantum mechanical (SQM) force field method5-8 . In the SQM approach, the systematic errors of the computed harmonic force field are corrected by a few scale factors which were found to be well transferable between chemically related molecules.4, 9-11 In the present study, we extend a probing with the application of the B3LYP/6-3IG** (basis set) based on the SQM method12 to vibrational analysis and deformational stability of DMN. The geometrical parameters of the most optimized geometry obtained were used for the DFT calculations. The Infrared and Raman intensities were also predicted. Based on these calculations, the simulated FTIR and FTRaman spectra were obtained.
EXPERIMENTAL
The pure crystalline samples of DMN was obtained from Lancaster chemical company, UK, and used without further purification for the spectral measurements. The room temperature Fourier transform infrared spectra of the title compounds were measured in the region 4000-400 cm-1 at a resolution of ±1 cm?¹, using BRUKER IFS 66V Fourier transform spectrometer, equipped with MCT detector, a KBr beam splitter and globar source. The FT-Raman spectra were recorded on the same instrument with FRA-106 Raman accessories in the region 4000-50 cm-1 . The 1064 nm Nd: YAG laser was used as excitation source, and the laser power was set to 200mw.
COMPUTATIONAL DETAILS
The molecular geometry optimization was carried out with the Gaussian 03W program package13 at the B3LYP level, 14,15 supplemented with the standard 6-31G** basis set. The Cartesian representation of the theoretical force constants have been computed at the fully optimized geometry by assuming Cs point group symmetry. Scaling of the force fields were performed by the scaled quantum mechanical procedures with the selective scaling in the local symmetry coordinates representation16 using transferable scale factors available in the literature.12 Transformations of the force field and the subsequent normal coordinates including the refinement of the scaling factors calculation of potential energy distribution (PED), and the prediction of IR and Raman intensities were done on a PC with the MOLVIB program (version V 7.0-G77) written by Sundius. 17, 18, 19 For the plots of simulated IR and Raman spectra, pure Lorenzian band shapes were used with a band width (FWHM) of 10 cm-1 .The vibrational modes were assigned by means of visual inspection using the GAUSSVIEW program.20The symmetry of the molecule was also helpful in making vibrational assignments. The symmetries of the vibrational modes were determined by using standard procedure4, 12 of decomposing the traces of the symmetry operations into the irreducible representations. The analysis for the vibrational modes of DMN is presented in some details in order to better describe the basis for the assignments.
Prediction of Raman intensities:
The Raman activities (Si) calculated with the GAUSSIAN98 program and adjusted during the scaling procedure with MOLVIB were converted to relative Raman intensities (Ii) using the following relationship derived from the basic theory of Raman scattering. 21, 22, 23
Where ?o is the exciting wavenumber (in cm-1 units), ?i the vibrational wavenumber of the ith normal mode; h, c and k are fundamental constants; and ƒ is a suitably chosen scale factor for all peak intensities.
RESULT AND DISCUSSION
Molecular geometry: The optimized molecular structure of DMN having Cs symmetry is shown in fig.1. The global minimum energy obtained by DFT structure optimization for DMN is calculated as 464.5443 Hartrees. The optimized geometrical parameters obtained in this study for DMN is presented in Table 1. Detailed description of vibrational modes can be given by means of normal coordinate analysis (NCA). For this purpose the full set of 89 standard internal coordinates containing 23
redundancies for DMN was defined in Table 2. From this, a nonredundant set of local symmetry coordinates were constructed by suitable linear combinations of internal coordinates following the recommendations of Fogarasi and Pulay7, 16 is summarized in Table 3.
Vibrational spectra
The 66 normal modes of DMN is distributed amongst the symmetry species as Γvib = 45A’ (in-plane) +21A” (out-of-plane) iIn agreement with Cs symmetry. 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” to depolarized bands. The detailed vibrational assignments of fundamental modes of DMN along with observed and calculated wavenumbers, IR and Raman intensities and normal mode dichiptions (characterized by PED) is reported in Table 4. For visual comparison, the observed and simulated FTIR and FT-Raman spectra of the title compound is presented in Figs.2-3 which helps to understand the observed spectral features. They are convenient to discuss the vibrational spectra of DMN in terms of characteristic spectral regions as described below. Methyl Group vibrations: For the assignments of CH3 group wavenumbers, basically nini fundamentals can be associated to each CH3 group namely, CH3 ss – symmetric stretch; CH3 ips – in-plane stretch (i.e. in-plane hydrogen stretching modes); CH3 ipb – inplane bending (i.e. in-plane hydrogen deformation modes); CH3 sb – symmetric bending; CH3 ipr – in-plane rocking; CH3 opr – out-of-plane rocking; tCH3 – twisting hydrogen bending modes. In addition to that, CH3 ops – out-of-plane-stretch and CH3 opb – out-ofplane bending modes of CH3 group would be expected to be depolarized for A’’ symmetry species. The CH3 ss wavenumbers are established at 2897 cm-1 and 2923 cm-1 in IR and CH3 ips are assigned at 2945 cm-1 and 2965 cm-1 in IR for DMN. These assignments are also supported by literature 24 in addition to PED output. The four in-plane methyl hydrogen deformation modes are also well established. We have observed the symmetrical methyl deformation modes CH3 sb at 1447 cm-1 and 1438 cm-1 in IR and in-plane methyl deformation modes CH3 ipb at 1471 cm-1 and 1453 cm-1 in IR. The bands at 2863 cm-1 and 2845 cm-1 in IR and 1401 cm-1 and 1382 cm-1 in IR are attributed to CH3 ops and CH3 ipb respectively in the A’’ species. The methyl deformation modes mainly coupled with inplane bending vibrations. The bands obtained at 1066 cm-1 , 1036 cm-1 and 1016 cm-1 , 977 cm-1 in IR are assigned to CH3 in-plane and out-of-plane rocking modes. The assignment of the bands at 115 cm-1 and 119 cm-1 in Raman are attributed to methyl twisting modes C-H vibrations: Aromatic compounds commonly exhibit multiple week bands in the region 3100-3000 cm-1 due to C-H stretching vibrations. The bands due to C-H in-plane bending vibrations interact some what with CC stretching vibrations, are observed as a number of bands in the region 1500-1100 cm-1 . The C-H out-of-plane bending vibrations occur in the region 1000-700 cm-1 25, 26. Hence, in the present investigation, the FTIR bands identified at 3081 cm-1 , 3069 cm-1 , 3066 cm-1 , 3035 cm-1 , 3026 cm-1 , and 3011 cm-1 are assigned to C-H stretching vibrations of DMN. The FTIR bands at 1423 cm-1 , 1409 cm-1 , 1261 cm-1 , 1202 cm-1 , 1170 cm-1 and FT-Raman band at 1262 cm-1 are assigned to C-H in-plane bending vibrations of DMN. The C-H out-ofplane bending vibrations of the title compound are well identified in the recorded spectra, within their characteristic region.
Ring vibrations: .In the present study, the bands appeared at 864, 863, 803, 586, 564, 528, 512, 440, 410, 380, 332, 270, 219, 208, 195 and 63 cm-1 are assigned to ring in-plane and out-of-plane bending modes, respectively by careful considerations of their quantitative descriptions. Small changes in wave numbers observed for these modes are due to changes in the force constant/reduced mass ratio, resulting mainly due to addition of two methyl groups to DMN and from different extents of mixing between ring and substituent group vibrations.
CONCLUSION
The FTIR and FT-Raman spectra have been recorded and the detailed vibrational assignment is presented for DMN. The equilibrium geometries, harmonic vibrational frequencies, IR and Raman spectra was performed according to the SQM force field method based on DFT calculations at the B3LYP/6-31G** level. The difference between corresponding wavenumbers (observed and calculated) is very small for most of fundamentals. Therefore, the results presented in this work for DMN indicate that the level of theory is reliable for prediction of Infrared and Raman spectra of the title compound (DMN).
ACKNOWLEDGEMENT
The authors are thankful to Sophisticated Analytical Instrumentation Facility (SAIF), IIT Madras, Chennai, and Nehru Memorial College, Puthanampatti, Trichirappalli, India for providing spectral measurements.
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