A theoretical investigation of the structure and vibrational frequencies of CH3CN-BF3 and CH3CN-BCl3

Ab initio calculations have been performed at MP2/6-31 + G(2d,p) level to study the structures, vibrational frequencies, binding energies, and barriers to internal rotation of the complexes, CH3CN-BF3 and CH3CN-BCl3. Internal rotation of the complexes is predicted to be essentially barrierless. The vibrational frequencies and force constants of the complexes have been calculated and compared with experimental values, including those of CH3CN. The force constants are later used to calculate the frequencies of other isotopomers. The large frequency changes of the C drop N and C-C stretches upon complexation are well reproduced. The distinctive hypsochromic shift of the C drop N stretch of the complex is originated from the increase in force constant, whereas the frequency shift of the C-C stretch from the change in vibrational coupling upon complexation. The C drop N stretch also shows a dramatic change in absorption intensity due to higher charge separation in the complex. The shorter N-B bond length and higher stretch frequency of CH3CN-BCl3 along with the higher binding energy indicate that BCl3 is a more effective electron pair acceptor than BF3. (C) 2000 Elsevier Science B.V. All rights reserved.