Hydrogen bonding in the gas phase. Part 4.—Infrared spectroscopic investigation of O—H ⋯ O and C—H ⋯ N complexes : alcohol + ether and trichloromethane + amine systems

Abstract

An infrared investigation has been made of relatively weak hydrogen bonded complexes in the gas phase. Wavenumbers and band contours for hydrogen stretching modes have been investigated for O—H ⋯ O complexes formed by methanol, ethanol, propan-2-ol and 2-methylpropan-2-ol with some simple ethers, and also with tetrahydrofuran and 1,4-dioxan. The wavenumber displacements provide a measure of the electron donating abilities of the free ethers, leading to the following series according to increasing basicity : Me₂O ≈ 1,4-dioxan < Et₂O < THF < Pr¹₂O. The bands for the O—H stretching vibration of the alcohol + ether systems are found to be broad but with little or no structure, and this is discussed in terms of the sum-and-difference band theory of broadening. Examples of C—H ⋯ N hydrogen bonding in the gas phase have been detected for trichloromethane + amine systems and frequency displacements are listed. The order of increasing basicity of amines as indicated by these frequency displacements is : NH₃ < MeNH₂ < Me₂N < Me₃N.