Scaled Quantum FTIR and FT-Raman Spectral Analysis of 1-Methoxynaphthalene

The structural and vibrational property of 1-methoxynaphthalene has been studied. The fundamental vibrational frequencies and intensity of vibrational bands were evaluated using B3LYP/6-31G (d, p) basis set and was scaled using various scale factors, which yielded a good agreement between observed and calculated frequencies. The vibrational spectra were interpreted with the aid of normal coordinate analysis. The results of the calculations were applied to simulated spectra of the title compound, which shows excellent agreement with observed spectra. The calculated force constants in vibrational internal coordinates are in closely coincides with the experimentally observed force constants.


Introduction
Vibrational analyses of molecules are more deep study in organic chemistry, for the confirmation of functional groups, molecular structure and reaction kinetics.The naphthalene and its derivatives 1,2 are the most important class of organic compounds containing two condensed rings.1-Methoxynaphthalene is used as an intermediate for the synthesis of nonsteroidal anti-inflammatory drugs.It is also used for soap perfumes and in the preparation of non-steroidal anti-inflammatory agents and bromination process.There are series of ladder-like compounds consisting of linearly fused benzene rings 1,2 , their technological potential [3][4][5] and their intrinsic value as models for more complex conjugated molecules have been the subjects of many theoretical and experimental investigations [6][7][8][9][10][11][12][13][14] .

Experimental
The compound under investigation namely 1-methoxynaphthene was purchased from M/S Aldrich chemicals, U.S.A 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 was recorded in Bruker IFS 66V spectrometer in the range of 4000 to 400 cm -1 .The spectral resolution is ±2 cm -1 .The FT-Raman spectra of this compound was 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 powers.The spectra were recorded 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 .

Computational methods
The quantum chemical calculations were performed for 1-methoxynaphthalene by applying B3LYP/6-31G (d,p) basis set method using the Gaussian 03 W program 15 .The Cartesian representation of the theoretical force constants has been computed at the fully optimized geometry by assuming C s point group symmetry.Scaling of the force field was performed according to the SQM procedure 16 using selective scaling in the natural internal coordinate representation 17 .The optimized bond lengths and bond angles were used for the calculation of vibrational frequencies at the B3LYP levels.The SCF converges to total energy about-500.430925363and zero point energy 113.19344 (kcal/mol).The rotational constants values are 1.64020, 0.82882 and 0.55253.GAUSSVIEW program 18 is very helpful for visual interpretation of vibrational frequencies assignments with normal mode of vibration.

Molecular geometry
The 1-methoxynaphthalene assumed as C s point group of symmetry and the optimized geometrical parameters of the title compound are calculated according to labeling of atoms as shown in the Figure 1.The most optimized bond lengths and bond angles of this compound were calculated and shown in Table 1.

Vibrational analysis and theoretical prediction of spectra
The sixty normal modes of vibrations of 1-methoxynaphthalene are distributed by symmetry species as Γvib = 41A′(in-plane) +19A″(out-of-plane) It is in agreement with C s point group symmetry, all vibrations are active both in FT-Raman and infrared absorption.The detailed vibrational assignments of fundamental modes of 1-methoxynaphthalene along with the calculated frequencies, IR intensities, Raman activities and polarization ratios are reported in the The RMS error of unscaled frequencies is obtained for 1-methoxy naphthalene and found to be 77.04 cm -1 .In order to reproduce the observed frequencies, the refinement through scaling factors was carried out and optimized RMS deviation was reduced to 12.1 cm -1.

C-H Vibrations
In aromatic molecules like 2 naphthalenol C-H vibrations appear 19 in the region 3085-3040 cm -1 and in 2,4 diisopropylnaphthalene 20 in the region 3070 -3030 cm -1 .In this compound the C-H stretch vibrations occur in the region 3065 -3000 cm -1 .The upper limit of frequency comparatively decreases may be due to the presence of oxygen and methyl group.There are seven aromatic C-H stretching vibrations, are observed at 3060, 3055, 3050, 3020 and 3010 cm -1 in FT-Raman and 3065 and 3005 cm -1 are in IR region.The maximum and minimum frequencies are lies in the IR region.The mean deviations between experimental and scaled calculated frequencies are nearly 8.57 cm -1 .The scaled vibrations are good in agreement with the experimentally reported values of naphthalene derivatives.Optimized C-H bond lengths are clearly shows that the C-H bonds nearer to the methoxy group C 5 -H 12 and C 15 -H 16 having maximum variation with others.This is undoubtedly due to the impact of methoxy group.It is also evident by the farthest bond C 2 -H 13 with maximum optimized bond length.

Methyl group vibrations
The methyl group stretching vibrations are in the region 3000-2980 cm -1 .In this compound, there two asymmetric and one symmetric C-H stretching vibrations are appeared at 2990, 2940 and 2860 cm -1 respectively.These assignments are also evidenced by literature values 21 .The calculated frequencies are at 2992, 2940 and 2865 cm -1 .Usually the symmetrical bands are sharper than the asymmetrical bands.The same trend is appeared here also.
2-methoxy-1-naphthaldhyde shows methyl vibrations are in the range 2885-2803 cm -1 .This may be indicates that the additional substitutions are pushes down the range of vibration.The asymmetric and symmetric stretching modes of methyl group attached the benzene ring are usually downshifted due to electronic effects and are expected near 2925 and 2865 cm -1 asymmetric and symmetric stretching vibration.This evident that the same electronic effect is also applicable in double ring compound like 1-methoxynaphthalene and the vibrations are observed 22 at 2940 and 2865 cm -1 .Here also the above experimental values coincide with the scaled B3LYP/6-31G (d, p) basis set values.There is an appreciable change in one calculated C 19 -H 21 bond due to its orientation and very near to the oxygen atom.
The bands due to C-H in-plane bending vibration interact some what with C-C stretching vibrations, are observed as number of bands in the region 1300-1000 cm -1 .The C-H out-of-plane bending vibrations occur 23 in the region 900-667 cm -1 .

C-O vibrations
The compounds contain a carbonyl group; the absorption caused by the C-O stretching is generally very strong 24 .In p-anisaldehyde, the C-O vibration with B3LYP/6-31G(d,p) predicated frequencies at 1332 and 1305 cm -1 are agreement with experimental frequencies 25 at 1322 and 1291 cm -1 .The characteristic infrared absorption frequencies of carbonyl group in cyclic ketones are normally strong in intensity and found in the region 1685-1660 cm −1 .Carboxylic acids, the C=O stretching observation 26

C-C vibrations
Generally the C-C stretching vibrations in aromatic compounds form 27 the band in the region of 1430 to 1650 cm -1 .In the present case investigation the bands are observed at 1640,1595,1580,1505, 1440, 1325 and 1310 cm -1 .The last two bands lie below the expected range, this may be due to the substitution of heavy elements oxygen and methyl group in the present case.The in-plane bending C-C vibrations generally occur at higher frequencies than of out-of-plane bending 28 .In the present study the bands observed at 760, 720, 580, 500, 465 and 430 are assigned to C-C in plane bending modes.The out of plane bending modes of frequencies are attributed to 554, 460 and 420 cm -1 in Raman and 175, 140 and 85 cm -1 in IR.

Conclusions
A complete vibrational analysis of 1-methoxynaphthalene was performed on the basis of DFT functional calculations at B3LYP/6-31G (d, p) basis level.The influences of carbonoxygen, and methyl group in the vibrational frequencies of the title compound were discussed.The observed and stimulated spectra are in agreement and show a good frequency fit.The difference between theoretical and experimental wave numbers within 10 cm -1 shows that the assignments of the fundamentals are valid, by the qualitative agreement between the calculated and observed frequencies.

Figure 3 .
Figure 3.Comparison of observed and calculated FT-Raman 1-methoxynaphthalene (a) Observed (b) calculated with B3LYP/6-31G (d,p).Root mean square (RMS) values of frequencies were obtained using the relation observed at 1725±65.In this compound C=O and C-O stretching vibrations are observed at 1640 and 1325 cm −1 .The bands are strong and medium appeared in Raman and IR.The scaled calculated values obtained from B3LYP/6-31G (d,p )l are 1631 and 1325 cm −1 .The in plane bending vibration is observed at 280 cm −1.

Table 2 .
The observed and simulated FT-IR and laser Raman spectra are presented in Figures 2 & 3 respectively.