Knowledge Bank

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 $\partial \alpha /\partial Rc-c$ for $CH4,C2H4,C2H6,C2H2,CCl4,CF4andC6H6$ have been computed and compared with experimental $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ and theoretical $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ 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; whereas observed values show a trend . The $\alpha C-H\prime$ were also found to very according to the hybridization as contrary to the observation made by Yoshino & $Bernstein1$. The calculated values of $\alpha (\prime C-C)$ follow the trend; $\alpha (\prime c-c)(C2H2)>(C2H4)>(C2H6)$ which shows that $\pi -bonds$ contribute more to the bond polarizabllity derivatives. The value of $\alpha c-c\prime byF\&Smethod(0.90\backslash AA2)$ for $ C{2}H_{6} $ coincides with that observed experimentally (0.92 $\backslash AA2$) while other systems have quite away. The L&N method gives a value of 2.76 $\backslash AA2$ much closer to observed one $(2.92\backslash AA2)$ for $C2H2$ while L & P method yields a value $2.47\backslash AA2$ more closer to the observed one $(1.89\backslash AA2) for C2H6.$ The observed value for $ C_{2}H_{6} \alpha^{\prime} c-c (1.54) \AA^{2}$ which lies between thise for $C2H6(0.92\backslash AA2) and C2H4(1.89\backslash AA2$ 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 $C6H6$ 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)