Theoretical study of the molecular force field and vibration analysis of the hydrogen cyanide addition compound with boron tribromide.

An extensive HF, MP2, B3LYP and CCSD study of the molecular structure, force field and normal vibrations has been carried out for the hydrogen cyanide compound with boron tribromide. Most of the calculations agree that the HCN-BBr(3) molecule belongs to C(3v) point group and has a N-B length of 1.55-1.70 A and a N-B-Br angle of 103-105 degrees. These calculations also have estimated the missing low-wavenumber fundamentals in the 190 (nu(5)), 150 (nu(9)) and 105 cm(-1) (nu(10)) regions, instead of the supposed fundamentals at 207, 188 and 150 cm(-1), respectively, based on the combination bands. The quantum chemical force constants, by the B3LYP/6-31G and CCSD/3-21G calculations, have been adjusted by the scaling factors to reproduce the fundamentals in the literature [3] to include the bands below 200 cm(-1) proposed in this study. Normal coordinate analysis using the scaled force constants has been performed to interpret the molecular vibrations of four isotopic molecules, HCN-(10)BBr(3), HCN-(11)BBr(3), DCN-(10)BBr(3) and DCN-(11)BBr(3).