Spectroscopic studies of Lewis acid‐base complexes. III: Vibrational frequencies, assignments and normal coordinate analyses for isotopic varieties of phosphine borane and trifluorophosphine borane

Abstract

Vibrational spectra of the two Lewis cxnplexes, phosphine borane, PH3BH3, and trifluorophosphine borane, PF3BH3, have been investigated for several isotopic species preserving C3u symmetry. New assignments are proposed and sutstantiated by normal coordinate analyses formulated in the compliance constant basis.

Spectroscopic studies of Lewis acid-base complexes

“…Experimentally, the interchange was established by the observation that, although the two P-F stretches were not well resolved in the Raman spectrum of the liquid, the low-frequency side of the composite band was clearly polarized in the spectrum of the hydrogen compounds while the high-frequency side was polarized in the spectra of the BD3 adducts. A similar interchange of the relative position of the P-F stretches has been observed in the case of PF3BH3 [4]. It is perhaps worth mentioning that the potential energy distribution from the normal coordinate analysis shows the band assigned to the A' P-F stretch in the BD3 adducts to be much more strongly mixed than are any of the bands assigned to P-F motions in the other isotopic species.…”

“…These can be calculated straightforwardly in the case of bond stretches using equations in Table 6 defining the symmetry coordinates. The principal force and compliance constants of interest are given in Table 8 together with corresponding literature values for HPF, and the borane complexes of PH3 and PF3 [4,7].…”

“…The normal coordinate anaiysis of the assigned vibrational frequencies was carried out in terms of symmetry compliance constants to facilitate compari- [9, lo] , this approach has seen limited application to date and additional treatments are much to be desired. Details of the computational procedures have been described previously [4] and will not be repeated here. Figure 4 displays the atom numbering scheme and the structural parameters taken from the microwave work [ 31.…”

“…To provide additional information which may contribute to understanding the factors involved, HPF2BH3 and several of its isotopic species have been studied by infrared and Raman spectroscopy. The assigned fundamentals have been subjected to a normal coordinate analysis using the compliance constant basis for comparison with earlier results on PHJBH, and PF3BH3 [4].…”

Mean Amplitudes of Vibration of Phosphine Borane and Trifluorophosphine Borane

“…In the case of PF 3 BH 3 , the necessary vibrational frequencies and structural parameters were taken from the recent paper of Jensen [3]. For PH 3 BH 3 the used vibrational data were those reported by Taylor et al [10] whereas the molecular parameters were obtained from the microwave study of Durig et al [11]. A set of force constants for both species, derived from slightly different frequency data as those employed here, but with the same general assignment scheme, are also known [12] and may be useful for comparative purposes.…”