Theoretical studies of structure, ground state vibrations and force field of methyldiborane

Abstract

Molecular geometry, force field and infrared absorption intensities of methyldiborane have been determined from ab initio calculations at the SCF and MP2 levels utilizing a DZP basis set. The theoretical geometry reveals slight distortion of the diborane ring, by 1°–2°, on methylation. The calculated value of the three-fold barrier for methyl torsion by both the methods is nearly 8.5 kJ mol −1 in satisfactory agreement with the experimental value of 10.5 kJ mol −1. The ground state effective force constants of methyldiborane have been obtained by the RECOVES procedure which we proposed recently. The RECOVES-SCF/DZP force constants of methyldiborane are very close to those of the RECOVES-MP2/DZP. Normal coordinate analyses of methyldiborane and its various 2H isotopic molecules and the predicted infrared intensities have provided reliable vibrational assignment for methyldiborane. The effects of methylation on the vibrational frequencies and force constants of diborane have been discussed.