Knowledge Bank

A technique to calculate the rotational constants of molecules such as $CH3CCH$ in various levels of the $\nu 10$ vibration and the $\nu 3$ vibrations of $CH3CN$ for several isotopic species is presented. The moments of inertia and a computer iteration technique over experimental data for each isotopic species were employed to evaluate the constant A, in an excited vibrational state for these symmetric top molecule. Systematic differences are observed in the experimental and calculated values of the rotational constant A, for $CH3CN$. The bending and stretch modes of the -CEX bond are explored to characterize this trend. Tam and Roberts made some tests on the $n\nu 10,n=1,2,3,4$ vibrations of $CH3CCH$ which showed some minor departures in that molecule as well. The scheme is applied to $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$ isotopic species of $CH3CN$ to obtain a value of $Au$ using both a quadratic and linear extrapolation. Linear and quadratic extrapolation of A, over vibrational levels indicate that the quadratic dependence is more favored.