Knowledge Bank

We have modeled the intramolecular potential-energy function of the methyl halides with quadratic terms based on a general valence force $field1$ and linear, cubic and quartic terms based on an anharmonic Urey-Bradley force field. From this model we have calculated the cubic and quartic normal-coordinate force constants and have used these to estimate the anharmonic effects in the vibration-rotation spectra. The importance of this approach is that it predicts strong vibrational resonances, for example that between $\nu 1,2\nu 5,\nu 3+\nu 5+\nu 6$ and $2\nu 3+2\nu 6$ in $CH3Br$.