THE DIPOLE CHANGE VECTOR FOR ISOLATED C-H STRETCHES .1. PARTIALLY DEUTERATED METHYL-BROMIDE

The vapour phase infrared contour of the isolated C-H stretch fundamental of CHD2Br, obtained at 0.5 cm-1 resolution, has been simulated using anharmonic rigid asymmetric top calculations. Anharmonic rotational constants were obtained (i) from a simple Morse model, (ii) from fitting the constants to observed rotational fine structure recorded at 0.1 cm-1 resolution, and (iii) from ab initio anharmonic force fields. The Morse model was found to overestimate the anharmonicity; the other two methods were found to give similar results. The simulations indicate that the dipole change vector, (partial derivative mu over arrow pointing right/partial derivative Q)0, shows an angle of 44 +/- 2-degrees with the C-H bond, while the same angle obtained from atomic polar tensors is equal to 43 +/- 2-degrees.