Knowledge Bank

The vibrational frequencies of isotopically substituted planar $XY3$ molecules have been obtained using the Green's function and partioning techniques. Taking $HBCl2$ as an ``isotopic'' species of $BCl3$, we have calculated its spectrum, without invoking a force constant model. The excellent agreement between predicted and observed spectra indicates that even the force constant changes may be neglected as a first approximation. Using the information provided by the calculations, we have observed and identified five of the six $HBCl2$ fundamentals: $\nu 1=762$ and $740cm-1,B10-$ and $B11-Cl$ symmetric stretches; $\nu 2=2617cm-1$, B-H stretch (no splitting); $\nu 4=1100$ and $1089cm-1,B10-$ and $B11-Cl$ asymmetric stretch; $\nu 5=892cm-1(B11$ only), in-plane H deformation; and $\nu 4=795$ and $784cm-1$, out-of-plane deformation. Frequencies observed and identified for the deuterated compound are: $\nu 2=1969cm-1,B-D$ stretch; and $\nu 6=661$ and $645cm-1,B10-B11$ splitting. The calculations also indicate that the other frequencies we have not observed are hidden by $BCl3$ bands, and that $\nu 3$, a $B-Cl$ bending mode is at $\approx 285-290cm-1$, outside the range of our instrument.