THEORETICAL-STUDIES OF STRUCTURE, GROUND-STATE VIBRATIONS AND FORCE-FIELD OF METHYLDIBORANE

Molecular geometry, force field and infrared absorption intensities of methyldiborane have been determined from ab initio calculations at th e SCF and MP2 levels utilizing a DZP basis set. The theoretical geomet ry reveals slight distortion of the diborane ring, by 1 degrees-2 degr ees, on methylation. The calculated value of the three-fold barrier fo r methyl torsion by both the methods is nearly 8.5 kJ mol(-1) in satis factory agreement with the experimental value of 10.5 kJ mol(-1). The ground state effective force constants of methyldiborane have been obt ained by the RECOVES procedure which we proposed recently. The RECOVES SCF/DZP force constants of methyldiborane are very close to those of t he RECOVES-MP2/DZP. Normal coordinate analyses of methyldiborane and i ts various H-2 isotopic molecules and the predicted infrared intensiti es have provided reliable vibrational assignment for methyldiborane. T he effects of methylation on the vibrational frequencies and force con stants of diborane have been discussed.