1. INTRODUCTION:

1, 2, dimethyl cyclohexene, derivative of cyclohexene is a liquid with boiling point 135 - 6°. This alicyclic organic chemical compound was studied and analyzed by various scientists and its applications were also discussed by these workers [1‑6]. A consistent force field for the simultaneous analysis of vibrational frequencies, conformations and thermodynamic properties of alkenes is applied to the normal mode analysis of cyclohexene [7]. Hutter, Bodensel and Koch reported the microwave spectrum and ring planarity of 3 methylene-1,4, cyclohexadiene [8]. However, there is no report in the literature on the vibrational spectra and analysis of 1, 2, Dimethyl cyclohexene. Hence the present investigation has been undertaken to provide complete spectroscopic information and vibrational analysis of 1,2, Dimethyl cyclohexene through FTIR and FT Raman spectroscopy. The normal co-ordinate calculations have also been performed to check the validity of the present assignment.

2. EXPERIMENTAL DETAILS:

The FTIR spectrum of 1, 2-Dimethylcyclochexene was recorded on Bruker IFS 66V FTIR spectrometer in the frequency region 4000 - 200 cm^-1. The FT Raman spectrum of the same compound was also recorded by the same instrument with FRA 106 Raman module equipped with Nd: YAG laser source operating at 1.06 mm line with a scanning speed of 30 cm^-1 min^-1 of spectral width 20 cm¹. The frequencies for all sharp bands were accurate to ± 1 cm^-1. The molecular structure of this compound is given in Fig.1. The recorded spectrum of 1, 2-Dimethyl cyclohexene is shown in Fig.2.

Fig.1 Structure of 1, 2-dimethyl cyclohexene

Fig.2 FTIR and FTR spectra of 1, 2-dimethylcyclohexene

3. THEORETICAL CONSIDERATIONS:

The symmetry possessed by the molecule helps to determine and classify the actual number of fundamental vibrations of the system. The observed spectrum is explained on the basis of C_s point group symmetry. The 60 fundamental vibrations are distributed as G_vib = 41a¢ + 19a¢¢. All the vibrations are both infrared and Raman active. The observed frequencies have been assigned to various modes of vibration on the basis of intensity, frequency from allied molecules and the normal coordinate calculations. The initial set of force constants are taken from allied molecules and small alterations are made in few constants to obtain a close fit between the observed and calculated frequencies. The potential energy distribution associated with each vibrational mode is calculated and thereby the magnitude of mixing up of various skeletal frequencies was investigated.

4. NORMAL CO-ORDINATE ANALYSIS:

With the modified computer program developed in this laboratory for Fuhrer et al., program [9] the normal coordinate analysis for this compound is carried out using Wilson's F-G matrix method. The simple general valence force field was adopted for both in plane and out of plane vibrations. The structural parameters are taken from related molecules and Sutton's table [10]. The initial set of force constants were refined by keeping few interaction constants fixed throughout the refinement process. The assignment to all the in plane and out of plane fundamentals are made on the basis of intensities of Raman and IR bands, normal coordinate analysis and on comparison with those of similar molecules.

5. POTENTIAL ENERGY DISTRIBUTION

To check whether the chosen set of assignments contribute maximum to the potential energy associated with normal coordinates of the molecules, the potential energy distribution (PED) has been calculated using the relation

F_iiL²_ik

PED= ------------------------------- (1)

l_k

where F_ii are the force constants defined by damped least square technique, L_ik the normalized amplitude of the associated element (i,k) and l_k the eigne value corresponding to the vibrational frequency of the element k. Only the PED contribution corresponding to each of the observed frequencies over 10% are alone listed in the present work.

6. RESULTS AND DISCUSSION:

The observed frequencies along with their relative intensities of 1, 2-Dimethyl cyclohexene and probable assignments are presented in Table 1.

Table: Observed, calculated frequencies (cm^-1), vibrational assignments and potential energy distribution of 1,2, Dimethyl cyclohexene

Species	Observed frequency / Intensity		Calculated wavenumber	Assignment	% PED
Species	Infrared	Raman	Calculated wavenumber	Assignment	% PED
a¢	2988 w	2990 s	2988	CH asymmetric stretching in CH₃	89 n_asCH
a¢	2981s	-	2980	CH asymmetric stretching in CH₃	91 n_asCH
a¢	-	2938 s	2935	CH asymmetric stretching in CH₃	72 n_asCH + 23 n_CC
a¢	2925 s	-	2921	CH asymmetric stretching in CH₃	74 n_asCH + 19 n_CC
a¢	-	2900 vs	2989	CH symmetric stretching in CH₃	86 n_sCH
a¢	2887 m	-	2884	CH symmetric stretching in CH₃	94 n_sCH
a¢	2850 s	-	2842	C-H stretching	87 n_CH
a¢	-	2838 s	2836	C-H stretching	94 n_CH
a¢	2825 m	-	2819	C-H stretching	81 n_CH
a¢	-	2808 m	2801	C-H stretching	89 n_CH
a¢	-	2763 vw	2758	C-H stretching	90 n_CH
a¢	-	2738 w	2731	C-H stretching	77 n_CH + 16 n_CC
a¢	2713 w	2713 w	2707	C-H stretching	86 n_CH
a¢	2700 w	2700 w	2702	C-H stretching	70 n_CH + 21 n_CC
-	2600 vw	-	-	1275 + 1238	-
-	2250 vw	-	-	1144 + 1125	-
a¢	1681 vw	1682 vs	1674	C=C stretching	87 n_C=C
a¢	1463 s	-	1458	C-H in plane bending	88 b_CH
a¢	1450 vs	1444 s	1445	C-H in plane bending	91 b_CH
a¢	-	1425 vs	1422	C-H in plane bending	85 b_CH
a¢	1381 s	1382 s	1380	C-H in plane bending	70 b_CH + 19 b_CC
a¢	1350 w	1356 w	1344	C-H in plane bending	69 b_CH + 28 b_CC
a¢	1338 w	1338 w	1331	CH₃ deformation	70 d_CH₃ + 24 r_CH₃
a¢	-	1294 w	1286	CH₃ rocking	65 d_CH₃ + 31 d_CH₃
a¢	1275 m	1275 m	1270	CH₃ deformation	69 d_CH₃ + 30 r_CH₃
a¢	-	1250 w	1239	CH₃ rocking	72 r_CH₃ + 19 d_CH₃
a¢	1233 s	-	1229	C-H in plane bending	67 b_CH + 19 b_CC
a¢	1225 vw	-	1221	C-H in plane bending	88 b_CH

a¢	-	1212 w	1206	C-H in plane bending	61 b_CH + 24 b_CC
a¢	-	1182 s	1174	C-C stretching	78 n_CC + 10 n_CH
a¢	-	1168 s	1159	C-C stretching	81 n_CC
a¢	1144 vs	1144 vw	1141	C-C stretching	76 n_CC + 17 n_CH
a¢	1125 vs	-	1120	C-C stretching	85 n_CC
a¢	1094 m	1094 m	1088	C-C stretching	89 n_CC
a¢¢	-	1068 w	1062	C-H out of plane bending	65 h_CH + 20 h_CH₃ + 14 h_CC
a¢¢	1056 m	-	1048	C-H out of plane bending	82 h_CH
a¢	1006 vs	1006 w	1001	CCC trigonal bending	93 n_CC
a¢	987 s	987 vw	979	C-CH₃ stretching	68 n_CH₃ + 26 n_CC
a¢	956 m	-	949	C-CH₃ stretching	61 n_C-CH₃ + 23 n_CC + 10 n_CH
a¢¢	919 vs	-	911	C-H out of plane bending	72 h_CH + 16 h_CC + 10 h_CH₃
a¢¢	881 m	-	872	C-H out of plane bending	78 h_CH
a¢¢	869 s	869 s	863	C-H out of plane bending	65 h_CH + 25 h_CC
a¢	850 s	850 vw	842	CCC ring breathing	90 b_CC
a¢¢	819 s	813 w	817	C-H out of plane bending	72 h_CH
a¢¢	763 vw	-	758	C-H out of plane bending	66 h_CH + 21 h_CC
a¢¢	738 vw	-	728	C-H out of plane bending	73 h_CH
a¢	675 vs	675 vs	677	CCC in plane bending	92 b_CC
a¢	600 vw	600vw	590	CCC in plane bending	74 b_CCC + 19 b_CH
a¢	563 vw	-	554	CCC in plane bending	81 b_CC
a¢¢	-	538 w	533	CCC out of plane bending	67 h_CC + 21 h_CH₃ + 14 h_CH
a¢¢	513 m	513 w	509	CCC out of plane bending	61 h_CC + 19 h_CH
a¢¢	-	463 w	459	CCC out of plane bending	74 h_CC + 16 h_CH
a¢¢	444 w	-	434	CCC out of plane bending	65 h_CCC + 20h_CC + 13 h_CH
a¢¢	400 w	-	386	CH₃ wagging	64 w_CH₃ + 24 t_CH₃
a¢¢	369 w	369 w	360	CH₃ torsion	58 t_CH₃ + 32 w_CH₃
a¢¢	-	356 vw	349	CH₃ wagging	70 w_CH₃ + 18 t_CH₃

a¢¢	-	356 vw		CCC out of plane bending
a¢¢	310 m	-	302	CH₃ torsion	64 t_CH₃ + 22 w_CH₃
a¢	250 w	-	241	C-CH₃ in plane bending	44 b_C-CH₃ + 22 b_CC + 16 b_CH
a¢	225 w	-	218	C-CH₃ in plane bending	51 b_C-CH₃ + 28 b_CC + 20 b_CH
a¢¢	200 w	-	192	C-CH₃ out of plane bending	39 h_C-CH₃ + 24 h_CC + 26 h_CH
a¢¢	175 w	-	164	C-CH₃ out of plane bending	42 h_C-CH₃ + 26h_CC + 18 h_CH

a¢ - in-plane vibrations, a¢¢ - out-of plane vibrations.

abbreviations used : w - weak; m - medium; s - strong; vw - very weak; vs - very strong.

The assignment of frequencies is made as follows.

Stretching vibration:

C-H stretching:

The carbon-hydrogen stretching modes can be assigned on the basis that the two CH₃ methyl groups have four asymmetric stretching modes which will be close in frequency and higher than the C-H symmetric stretching. The C-H stretching fundamentals in the infrared and Raman spectra are expected in the region 3100-2800 cm^-1. The bands at 2925, 2938, 2981 and 2990 cm^-1 have been assigned to C-H asymmetric stretching in CH₃[11]. The calculated wave numbers 2921, 2935, 2980, and 2988 cm^-1 agree quite well with the observed frequencies. The PED shows that C-H asymmetric stretching in CH₃ at calculated frequencies 2921 and 2935 cm^-1 are not pure C-H stretching characters whereas C-C stretching characters contribute to some extent. The very strong Raman band at 2900 cm^-1 and medium infrared at 2887 cm^-1 have been assigned to CH symmetric stretching in CH₃ for each of the two methyl groups. The eight C-H stretching vibration bands of cyclohexene are assigned at 2700, 2713, 2738, 2763, 2808, 2825, 2838 and 2850 cm^-1 which agree with the calculated frequencies.

C=C Stretching:

The most important C=C stretching band in the compound can be readily assigned to the very strong Raman band at 1682 cm^-1 (calculated - 1674 cm^-1).

CH₃ deformation and rocking:

The weak infrared at 1338 cm^-1 and medium Raman at 1275 cm^-1 (calculated 1331 and 1270cm^-1) have been assigned to the methyl deformation modes. The weak Raman bands at 1294cm^-1 and 1250 cm^-1 (calculated 1286 cm^-1 and 1239 cm^-1) have been assigned to two methyl rocking bands [12].

C-C Stretching:

The five C - C stretching vibrations are assigned to 1094, 1125, 1144, 1168, and 1182 cm^-1which agrees with the calculated frequencies. The PED for C - C stretchings at 1144 and 1182 cm^-1 indicates that they are combination of C-H stretching modes.

C-CH₃ Stretching:

The two C - CH₃ stretchings are assigned at 956 and 987 cm^-1. The PED for the C - CH₃ stretching at 979 cm^-1 (calculated) shows that it is a combination of 68% of C - CH₃ stretching and 26% of C-C stretching. Similarly C - CH₃ stretching at 949 cm^-1 (calculated) is a combination of 61% of C-CH₃ stretching, 23% of C-C stretching and 10% of C-H stretching [13].

In plane and out of plane bendings:

The C-H in plane bending vibrations are assigned at 1212, 1225, 1233, 1356, 1382, 1425, 1450 and 1463 cm^-1 which agree with the calculated frequencies. The C - CH₃ in plane bendings have been assigned to low frequency band at 225 and 250 cm^-1.Three CCC in plane bending vibrations are assigned at 563, 600 and 675 cm^-1. The eight C-H out of plane bending vibrations have been assigned to 738, 763, 819, 869, 881, 919, 1056 and 1068 cm^-1 which are in close agreement with the calculated values. The four CCC out of plane bendings are assigned at 356, 444, 463, 513, and 538 cm^-1. The C - CH₃ out of plane bendings are assigned at 200 and 175 cm^-1.

The weak infrared bands at 400 and 356 cm^-1 (calculated at 386 cm^-1and 349 cm^-1) have been assigned to CH₃ wagging modes. The infrared bands at 369 and 310 cm^-1 (calculated 360 and 302 cm^-1) are assigned to CH₃ torsion modes.

CCC ring breathing and trigonal bending:

The CCC ring breathing mode and CCC trigonal bending are assigned to the bands at 850 and 1006 cm^-1which agree with the calculated values.

7. CONCLUSION:

A complete vibrational spectra and analysis is reported in the present work for the first time for 1, 2-Dimethyl Cyclohexene. The close agreement between the observed and calculated frequencies confirms the validity of the present assignment.

8. REFERENCES:

1. P. Mani. and Vadamalar, International Journal of Physics, 2 (2009) 95-100

2. P. Mani and S. Suresh, Asian Journal of Chemistry, 22(2010) 2648-2652

3. P. Mani, S. Suresh and C.Revathi, Asian Journal of Chemistry, 22(2010)2653-2658

4. P. Mani and S. Suresh. Scholars Research Library, 1(2010) 211-216.

5. Mani. P, and Suresh.S, International Journal of Materials Science, 5 (2010) 271-277.

6. Mani. P and S. Suresh, Asian Journal of Chemistry, 23(2011)4299-4302

7. O. Ermer, S. Lifson. J. Mol. Spectr. 51(1974)261

8. W. Hutter. H.K.Bodensel, and A.Koch J. Mol. Str. 319 (1994)73.

9. H.Fuhrer, V. B.Kartha, K.L.Kidd, P.J.Krudgel and H.H.Manstch computer program for infrared and spectrometry, normal cordinate analysis, Volume 5, National Research Council, Ottawa, Canada, (1976).

10. L.E.Sutton, The interatomic bond distances and bond angles in molecules and ions, London Chem., Soc., London, (1983).

11. G.Varsanyi, Assignments for vibrational spectra of seven hundred Benzene Derivations Volume No.7, Adam Hilger, London, (1974).

12. George socrates, Infrared and Raman characteristic Group Frequencies Tables and charts (third - edition) - John Wiley & Sons Ltd (2001).

13. G.A.Growder and Pat Blkankenship, J. Mol. Str., 156 (1987)147.

Received on 03.06.2018 Modified on 17.07.2018

Research J. Pharm. and Tech 2019; 12(8):3731-3734.

DOI: 10.5958/0974-360X.2019.00638.3

A Study of Vibrational Spectroscopic of 1, 2-Dimethyl Cyclohexene