Cryospectroscopic and ab initio studies of haloform–trimethylamine H-bonded complexes

Abstract

The FTIR spectra of F₂ClCH and Cl₃CH(D) in mixtures with trimethylamine (TMA) have been studied in liquefied Kr in ∼800–4000 cm⁻¹ frequency range. Spectroscopic evidence of a medium strength H-bond formation with conventional features has been found between these weak CH “proton donors” and TMA. The relative stability of the complexes has been determined in a series of temperature (T = 118–157 K) measurements of integrated intensities of vibrational bands ascribed to monomer and complex species. The complexes found are characterized by the red shift of the ν₁(CH) stretching vibration of the haloforms studied. In the case of chloroform, the substantial red shift is accompanied by a noticeable broadening effect and by a marked growth of the intensity of this band. The spectroscopic changes caused by the complex formation have been also registered for vibrations of TMA. Specifically, the bands situated in the frequency domain of CH stretching vibrations of TMA show a weak blue shift, whereas the NC stretching vibrations are red-shifted. Ab initio MP2/6-311++G(2d,2p)a priori counterpoise corrected calculations, made for a series of haloforms and TMA, reproduce the main spectroscopic observations. The results obtained also suggest that the vibrational mode coupling between vicinal C–H and C–B (B = Hal, N) bonds is to a great extent responsible for the sign of these bond length changes and for the sign of the frequency shifts of the respective stretching vibrations upon complex formation.