The proton magnetic resonance spectra of some alkylberyllium hydrides. Evidence for cis–trans isomerism about the BeH2Be bridge

Abstract

The proton magnetic resonance (p.m.r.) spectra of the dimers (Me₃NEtBeH)₂, (Et₂OMeBeH)₂, and (py MeBeH)₂ are discussed and assigned, including the broad resonances due to the BeH₂Be bridges. Neither in (Me₃NMeBeH)₂ nor in [(CD₃)₃NMeBeH]₂ could a BeH₂Be resonance be detected, but the trimethylamine resonance in the former is partially split at 25°. On the basis of the variation with temperature of the p.m.r. spectrum of (Me₃NMeBeH)₂, the splitting of resonances is attributed to the presence of cis- and trans-isomers, and this interpretation is supported by the relatively large entropy change (ΔS= 13 ± 2 cal. deg. mole^–1) associated with the isomerisation.