Conjugation in Phosphabutadienes: Ab initio Investigation and NMR Spectral Manifestation: II. Magnetic Shielding in Iminophosphines and Heterobutadienes Derived from Them

Abstract

The conjugation effects in iminophosphines H3C-N = P-R (R = H, Hlg, OH, NH2, PH2, SiH3, SH) and heterobutadienes H3C-N = P-X = CY2 (X = N, P; Y = NH2, H) and their manifestations in NMR spectra are analyzed by RHF and MP2 calculations using the 6-31+G and 6-311+G basis sets with different numbers of polarization functions. The contribution of π-mesomeric interactions to the electronic structure of these compounds is estimated from the theoretical and experimental structural parameters, calculated natural atomic charges and bond indices, surfaces of rotational coordinates of the potential energy, and energy balances of isodesmic reactions, and also from the experimental and theoretically calculated (GIAO) 15N and 31P NMR chemical shifts. The configuration of model heterobutadienes relative to the P = N double bond and the trans conformation of the N = P-X = C fragment, suggested by the calculation results, are consistent with the structural data for the compounds Mes*-N = P-X = C(NR2)2 (X = N, P; R = Alk). The character of π-meso-meric interactions in the examined series of iminophosphines and butadienes derived from them is discussed. The conjugation effects influence the stability of phosphabutadienes insignificantly.