THE VIBRATIONAL FREQUENCIES OF BORANE (BH3) - A COMPARISON OF HIGH-LEVEL THEORETICAL RESULTS

Using the borane molecule, BH3, as an example, a detailed comparison is made among the results achieved using many levels of theory for the treatment of electron correlation. In particular, with three different basis sets ranging from a simple double-zeta plus polarization (DZP) to the very flexible triple-zeta plus two sets of polarization functions plus higher angular momentum functions (TZ2P(fd)), total energetic, geometrical and harmonic vibrational frequency results from several of the more complete single reference methods of electron correlation are compared to the full configuration interaction (FCI) results. Emphasis is placed on comparison of augmented coupled cluster (CCSD(T)) theory and the CI method including all singles, doubles, triples and quadruples (CISDTQ) methods with FCI. These high-level results may also provide some additional insight into the ongoing controversy surrounding the experimentally assigned fundamental vibrational frequencies of BH3. The TZ2P(f,d) CCSD(T) harmonic vibrational frequencies predicted in this work were omega1 = 2567, omega2 = 1163, omega3 = 2696 and omega4 = 1223 cm-1 implying experimental misassignments by Kaldor and Porter [J. Am. Chem. Soc., 93 (1971) 2140] of the third and fourth vibrational frequencies, in agreement with previous theoretical works.