VIBRATIONAL BAND INTENSITIES \& MOLECULAR CONSTANTS PART 2 - RAMAN AND INTENSITY BOND POLARIZABILITY DERIVATIVES OF HYDROCARBONS
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Date
1981
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Ohio State University
Abstract
This work demonstrates the relative applicability of three available relations for obtaining absolute Raman intensities, viz. Long and Plane Method (L\&P), Lippincott \& Nagarajan Method (L\&N) and Fontal \& Spiro Method (F\&S) in the case of simple hydrocarbon systems and halogenated methanes. Thus bond polarizability derivatives, $\partial^{\alpha}/ _{\partial R} C-x;X=H, Cl, F $ and $\partial\alpha/ \partial R_{c-c}$ for $CH_{4}, C_{2}H_{4}, C_{2}H_{6}, C_{2}H_{2}, CCl_{4}, CF_{4} and C_{6}H_{6}$ have been computed and compared with experimental $^{1}$ and theoretical $^{2}$ Raman electro-optical parameters. The computed values of $\alpha^{\prime}(C-H)$ by all the three methods for C-H bond vary according to the trend; $\alpha^{\prime}_{C-H}(C_{2}H_{2})< C_{2}H_{4}< C_{6}H_{6} < CH_{4} < C_{2}H_{6}$ whereas observed values show a trend $\alpha_{C-H}^{\prime}(C_{6}H_{6})< C_{2}H_{2}< C_{2}H_{4}\approx CH_{4}< C_{2}H_{6}$. The $\alpha_{C-H}^{\prime}$ were also found to very according to the hybridization as $\alpha_{C-H}^{Sp^{1}}(C_{2}H_{6} \&CH_{4})>\alpha_{C-H}^{^{\prime} sp^{2}}(C_{6}H_{6} \&C_{2}H_{4}) >\alpha_{C-H}^{^{\prime} Sb}(C_{2}H_{2})$ contrary to the observation made by Yoshino \& $Bernstein^{1}$. The calculated values of $\alpha^{\prime}_(C-C)$ follow the trend; $\alpha^{\prime}_(c-c)(C_{2}H_{2})>(C_{2}H_{4})>(C_{2}H_{6})$ which shows that $\pi-bonds$ contribute more to the bond polarizabllity derivatives. The value of $\alpha^{\prime}_{c-c} by F\&S method (0.90{\AA}^{2})$ for $ C_{2}H_{6} $ coincides with that observed experimentally (0.92 $\AA^{2}$) while other systems have quite away. The L\&N method gives a value of 2.76 $\AA^{2}$ much closer to observed one $(2.92 \AA^{2})$ for $C_{2}H_{2}$ while L \& P method yields a value $2 .47 \AA^{2}$ more closer to the observed one $(1.89 \AA^{2}) ~for~ C_{2}H_{6}.$ The observed value for $ C_{2}H_{6} \alpha^{\prime} c-c (1.54) \AA^{2}$ which lies between thise for $C_{2}H_{6} (0.92 \AA^{2})~ and ~C_{2}H_{4} (1.89 \AA^{2}$ could not be reproduced by any of the three methods, may be due to over simplified picture of chemical binding adopted in the models and addition of $\pi$-bonds in case of $C_{6}H_{6}$ places a great strain on the model. 1. T. Yoshino \& H.J, Bernstein, Intensity in Raman Effect, Proceedings of the Institute of Petroleum, London (1958). 2. S. Abbate, M. Gussoni \& G. Zerbi, Indian J. Pure \& Applied Phys., 16, 119 (1978)
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