Chapter 2. FTZR and FT Raman Spectra, Vibrational Assignment and Analysis of l,2-cyclohexanedion

Transcription

1 Chapter 2 FTZR and FT Raman Spectra, Vibrational Assignment and Analysis of l,2-cyclohexanedion

2 CHAPTER - 2 INTRODUCTION Cyclohexane is a cycloalkane which was Baeyer in 1893 and discovered in caucasian crude oil by Markovnikov soon aftward. Cyclohexane was first synthes~zed by hydrogenation of benzene in 1898 [I]. Its presence in U.S. crude oils was reported in 1931 [2]. The predominant use of cyclohexane is in the manufacture of nylon. 1,2,cyclohexanedione, the derivative of cyclohexane appears in pale yellow solid. This compound has melting point C and is moderately soluable in water. This chemlcal compound is used as analytical reagent for Ni and its applications were studled by the various scientists The reflection-absorpt~on ~nfrared spectrum-of cyclohexane absorbed on Pt(II1) has been recorded using a Fourier Transform spectrometer. The spectrum at 150K showed the broad, low-frequency CH stretch band previously revealed by electron energy loss spectroscopy and also three sharper bands attributed to vibrational modes ~nvolving the relatively unperturbed hydrogens [S]. The vibrational spectra and ring-puckering potential energy functions of 1,4,cyclohexadiene, 4H-pyran and 1,4dioxin have been examined using a density functional theory (DFT) method as well as the HarteeFock (HF) and second-order Moiler-Plesset methods. The calculated vibrational Nuencies and

3 potential energy Wens of those molecules have been compad with previously reported e-ental data [6]. The spectra of the diketone has been investigated by the technique of lowenergy variable-angle electron energy-loss spectroscopy [7]. Galisteo and othen reported empirical relationship between structure and mokcular optical rotation in cyclohexane derivatives [8]. Microwave spectrum and conformation of 3 methylene - 1,4 cyclohexadiene has been investigated by Hutter and others [9]. Sciesinska and others recorded for i&ad spectra of cyclohexane C&,, in the fiquency range of cm-' for the phases I and I1 at various temperatures [lo]. Ravindranath and others [ll] reported the Raman intensity analysis of cyclic molecules-cyclohexane. However, there is no report in the literature about the vibrational spectra and analysis of 1,2, Cyclohexanedione. Hence, the present investigation has been undertaken to provide complete spectroscopic information and vibrational analysis of 1,2, Cyclohexanedione through FTIR and FT Raman spectroscopy. The normal co-ordinate calculation have also been performed to check the validity of the present assignment. 2.1 EXPERIMENTAL DETAILS The FTIR spectrum of 1,2,cyclohexanedione was recorded on Bruker IFS 66V FTIR spectrometer in the region cm-i. The FT Raman spectrum of the same compound was also recorded on the same instrument with FRA 106 Raman module equipped with Nd:YAG laser source operating at 1.06 pm line with a scanning speed of 30 cm -' min " of spectral width 20 cm-i. The frequencies for all sharp bands were accurate to rt 1 cm". 48

4 The molecular structure of this compound is given in Fig.2.1. The recorded spectrum of 1,2,cyclohexanedione is shown in the Fig THEORETICAL CONSIDERATIONS The symmetry possessed by a 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, point group symmetry. The 42 fundamental vibrations are distributed as l?, = 29 a' + 13 a". All the vibrations are both hfked and Raman active. The observed frequencies have been assigned to various modes of vibration on the basis of intensity, fkquency and from allied molecules. The normal coordinate calculations have also been employed for this purpose. 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 modes are calculated and thereby the magnitude of mixing up of various skeletal frequencies are investigated. 2.3 NORMAL CO-ORDINATE ANALYSIS With the modified computer program developed in this laboratory [12] the normal coordinate analysis for this compound was carried out using Wilson's F-G mahix method. The simple general valence force field was adopted for both in plane and out of plane vjbrations. The structural parameters are taken from related molecules and Sutton's table [13]. The initial set of force constants were

5 Fig. 2.1 Structure of 1,2 CY&LOHEXANEDIONE

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7 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. 2.4 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,,LZ* PED = - where F,, are the force constants defined by damped least square technique, L,, the normalised amplitude of the associated element (i,k) and hk the eigen value correspondmg to the vibrational fkquency of the element k. The PED contribution corresponding to each of the observed frequencies over 10% are alone listed in the present work. 2.3 RESULTS AND DISCUSSION The observed ffequencies along with their relative intensities of 1,2,cyclohexanedione and probable assignments are presented in table 2.1. The assignment of frequencies has been made as follows.

8 Stretching C-H stretching The very strong IR band at 2942 cm", weak Raman at 2931 cm'l and 2914 cm", medium IR band at 2900 cm'l, weak Raman band at 2882 cm-', medium IR band medium at 2869 cm-i, weak IR and Raman at 2850 cm-i and medium IR band at 2838 cal have been assigned to C-H stretching vibrations. The calculated wave numbers 2936, 2920, 2909, 2902, 2881, 2858, 2844 and 2828 cm-i agree quite well with the observed frequencies and literature values [14]. The PED calculation for C-H stretching mode reveals that all are pure modes except at the assigned frequency of 2869 cm-i which is a mixed mode with 78% contribution due to GH stretching and a little contribution of about 10% due to C-C stretching. CEO stretching The carbonyl group (C=O) is contained in a large number of different classes of compound e.g. aldehydes, ketones, carboxylic acid, esters and amides etc for which a strong absorption band due to the C=O stretching vibration is observed in the region cm-i. Because of its intensity in the inhred and relatively interference free region in which it occurs, this band is reasonably easy to recognize. If the double-bond character of the carbonyl group is increased (ie., the force constant of the bondis i ncd) by its neighbowing groups, then the fwiency of the stretching vibration is increased.

9 The C-O stretching bands of 1,2,cyclohexanedione have been assigned at and 1675 cm-' which agree with the calculated values 1726 cm-' and 1669 cm-i. The PED obtained for C=O stretching at 1669 cmt1 (calculated) shows that there is 81% contribution h m C=O stretching mode along with 15% of contribution from C-C stretching mode. C-C stretching The six C-C stretching vibrations of 1,2,cyclohexanedione have been assigned at strong IR band at 1250 cm-i, strong IR at 1225 cm-i, strong IR at 1188 cm-' and weak Raman at 1081 cm-i, strong IR at 1144 cm-' and strong IR at 1122 cm-'. They all agree with the calculated frequencies 1251, 1220, 1176, 1138, and 1069 cm-i. The PED for C - C stretching at 1069 cm*' shows that it is a mixed mode with a little contribution from C-H stretching (12%). In-plane and out of plane bendings The eight C-H in-plane bending5 are assigned to medium IR at 1468 cm", strong IR 1449 cm-', weak Raman 1438 cm", very strong IR 1400 cm-i, medium IR 1363 cm-i, medium IR 1338 cmh', medium IR 1325 cn-' and strong IR 1275 cm'l which agree closely with the calculated wave numbem 1460, 1438, 1426, 1406, , 1320 and 1276 cm". These vibrations appear in the expected range and find support from literature values [15].

10 The PED calahtian for GH in plaoe bending shows that all art pun modes except at 1460 cm-' and 1276 cm'l where there are contributions from CC in plane bending (16%) and C=O in plane bending (19%) also. At lower hquencies the assignments become more complex since the bands am no longer correspond to pure motions. They rtre complex mixtures of different internal coordinates or mixing of low-lying deformations with lattice modes. In the case of vibrational analysis of a rather complex vibmting molecular system, the lower the symmetry, the greater is the attention it deserves. The Raman spectrum is pasticularly rich in the lower frequency range, where there is little absorbance in the infrared spectrum. The C-H out of plane motions give rise to bands in the region between cm-i. The C-H out of plane vibrations are known to produce strong bands in the ~nfrared spectrum in general. Hence eight C-H out of plane vibrations are asslgned to 1035 cm'l (very weak Raman), 1018 cm-i (very weak Raman), 980 cm'l (medium IR), 900 cm" (strong IR), 882 cm" (strong IR), 869 cm" (very weak Raman), 856 cm" (very weak Raman), 819 cm-' (weak IR), which agree with calculated wave numbers 1031, 1008,985,928,891,878,861,842 and 820 cm". The PED for C-H out of plane bendings indicates all are pure mode except at 891 cm-i (calculated) where there is a little contribution h m CC out of plane bending (1 8%). C=O in plane bendings have been assigned to medium Raman at 713 cm" and medium IR at 700 cm" which agree quite well with the calculated values 702 cm-' and 6% cm-'. Similarly C=O out of plane bending5 are assigned to weak Raman at 638 cm-i and medium at IR 618 cm" which agree with the calculated values 631 cm-i and 608 cm".

11 CCC in plane bending vibration is assigned to medium IR at 1068 cm" which agree with calculated value 1054 cm-i. CCC out of plane vibrahons are assigned to strong IR band at 544 cm", medium LR band at 475 cm-' and weak Raman at 419 cm'l which agree well with calculated values 538, 471 and 419 cm-i. CCC ring breathing and trigonal bending CCC ring breathing and trigonal bending are assigned to IR band at 980 cm" and at 1046 cm-i which agrees well with calculated values 985 cm'l and cm". CONCLUSION A complete vibrational spectra and analysis has been reported in the present work for the first time for 1,2-cyclohexanedione. The close agreement beween the observed and calculated frequencies confirm the validlty of the present assignment.

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15 P.Sabatier, Ind. Eng. Chem (1926). J.H.Bmun, M.M.Hick-Brunn, J. Res. Natl. Bur. Stand Set A7 607, (1931). Butz, Davis, Guddis, J Org. Chem 12, 122, (1947). Rauh et al, J. Org. Chem 10,199, (1945). M.A. Chesters and P. Gardner Spectrochim Acta, 46 A, , (1 990). Seongho Moon J. Mol. Str 470,265, (1998). KN Walzl; 1M-Jr Xavier; A Kuppemann. J. Chem. Phy, 86,6701 (1987). D Galisteo. J Mol. Str. 326,239, (1994). W.Hutter. H.K. Bodensel and A. Kwh. J Mol. Str, , (1994).. Sciesinska. J. Mol. Str. 267,235, (1992). K.Ravindranath, G.Swarnakurnari and N.Rajeswararao. J. Mol. Spectres, 114,336, (1985). H.Fuhrer, V.B.Kartha, K.L.Kidd, P.J.Krudge1 and H.H.Manstch computer program for infrared and spectrometry, normal cordinate analysis, Volume 5, National Research Council, Ottawa, Canada (1976). L.E.Sutton, The interatormc bond distances and bond angles in molecules and ions, London Chern., Sw., London, (1983). G.Varsangi, Assignments for vibrational spectra of seven hundred benzene derivates, Volume No.7, Adam Hilger London, (1974). George Swrates-infrared and Raman characteristic group frequencies - Table and Charts - Third Edition John Wiley & Sons Ltd. (2001 ).