OH-stretching dynamics in trimethylamine monohydrate: what can we learn from three different direct absorption spectra?

Abstract

The hydrogen-bonded binary complex between water and trimethylamine is characterised in jet expansions, in the room temperature gas phase and in frozen argon matrices via its OH-stretching fundamental. The spectral comparison reveals two bracketing resonance partners with weak environmental dependence of their wavenumber. They correspond to the water bending overtone alone and in combination with a stretching motion of the monomers relative to each other. The environment-sensitive OH-stretching mode moves from the combination band location at room temperature all the way down to the pure water bending overtone in cryomatrix isolation, sharing its infrared intensity in proportion to the spectral vicinity and coupling strength. The intermediate jet-cooled spectrum largely removes thermal excitation and embedding effects. It thus provides the easiest entry point for theoretical modelling. Chemical and isotope substitutions at the amine support the robustness of the assignment, whereas a switch from water to methanol removes the bending-based resonance opportunities. After correction for the resonances, the OH-stretching positions of the water complex follow those of the methanol complex quite closely. This shows that methanol and water undergo similar hydrogen bond interactions with the amine. Previous contradicting spectral interpretations of water trimethylamine mixtures in supersonic jets and in argon matrices are discussed and discouraged. For the dihydrate of trimethylamine, both hydrogen-bonded OH stretching vibrations are characterised in the jet expansion.