PREPARATION, VIBRATIONAL-SPECTRA AND NORMAL-COORDINATE ANALYSIS OF THE D AND C-13 ISOTOPOMERES OF THE METHYLPENTAHYDRO-CLOSO-HEXABORATE [B6H5(CH3)](2-) AND THE CRYSTAL-STRUCTURE OF [P(C6H5)(4)][B6H6(CH3)]

The crystal structure of [P(C6H5)4][B6H6(CH3)] has been determined by single crystal X-ray diffraction analysis; triclinic space group P (1) over bar with a = 7.408(2), b = 11.887(3), c = 14.4486(9) Angstrom, alpha = 80.013(12), beta = 79.421(10), gamma = 88.85(2)degrees. The additional H atom could be refined with B-H distances between 1.21 and 1.31 Angstrom above one of the facettes of the B-6 octahedron adjacent to the methyl group. The IR and Raman spectra of the deprotonated Cs salts of the six isotopomers [B6H5(CH3)](2-), [B6H5(CH3)](2-), [B6H5((CH3)-C-13)](2-), [B6D5(CH3)](2-), [B6D5(CD3)](2-) and [B6D5((CH3)-C-13)](2-) exhibit characteristic shifts of their vibration bands. Using the crystallographic data a normal coordinate analysis has been performed, based on a general valence force field. With a set of 14 force constants (e. g. f(d)(BB) = 1.80, f(d)(BC) = 3.70, f(d)(BH) = 3.17, f(d)(CH) = 4.62 mdyn/Angstrom) good agreement between observed and calculated frequencies of all D and C-13 isotopically labeled derivatives has been achieved. The potential energy distribution on the force constants reveals strong vibrational couplings of the boron cage with the substituents. The B-C stretching vibration of the CH3 group is observed in the region 1142-1153 cm(-1), for the CD3 substituent at higher frequencies of 1168-1179 cm(-1).