Knowledge Bank

This survey was undertaken (a) to test the suitability of the program Gaussian 03 (G03) for anharmonicity calculations: (b) to compare results for different models (B3LYP, MP2) and basis sets (6-311++G**, cc-pVTZ): (c) to see if anharmonicity corrections to harmonic frequencies were identical for different types of C-H stretching vibration. Results to be presented show that the version of G03 available was inadequate to detect familiar instances of Fermi resonance. For {\em C}-type molecules G03 yielded anharmonicity constants {\em x} unusable by the experimentalist. In CH$2$Cl$2$ the antisymmetric C-H stretching frequency is associated with a larger anharmonicity correction than is the case for the C-H stretch, in a situation where Fermi resonance is negligible. Calculated values of {\em x}$\mathrm{<mrow\; data-mjx-texclass="ORD">1,1</mrow>}$, {\em x}$\mathrm{<mrow\; data-mjx-texclass="ORD">1,6</mrow>}$ and {\em x}$\mathrm{<mrow\; data-mjx-texclass="ORD">6,6</mrow>}$ for CH$2$Cl$2$ agree excellently with experimental data. In ethylene a marked effect of basis set was found in MP2 calculations for the out-of-plane bending modes, linked to the presence or absence of {\em f} functions.