Spectroscopic studies of lewis acid—base complexes: IV. An infrared study of oriented single crystals and normal coordinate analyses of trimethylamine—boron halide adducts

A total of 19 isotopic species of various (CH₃)₃NBX₃ complexes have been prepared and their IR spectra studied in the solid state in the region 40–4000 cm^?1. The isotopes involved are deuterium, boron-10, boron-11, and nitrogen-15 with X = F, CI, Br and I. C_3v symmetry has been preserved in all cases. Symmetry classifications of the fundamental molecular frequencies have been derived from polarized IR measurements on oriented single crystals in the range 250–4000 cm^?1. Errors and inconsistencies in previous studies have been resolved and revised vibrational assignments are proposed on the basis of the new data. Normal coordinate analyses have been carried out utilizing symmetry compliance constants without the assumption of point mass methyl groups. A common potential function containing 22 parameters for the A₁ class and 26 for the E class was utilized. The results support the proposed frequency assignments. B-N force constants obtained by inversion of the compliance matrices vary systematically from the chloro to the iodo complex. However, the B-N force constant for the fluoro complex falls between that for the chloro and bromo species; no systematic trend appears for the B-N compliance constants.