The benefits of alternation and alkylation: large amplitude hydrogen bond librational modes of alcohol trimers and tetramers.

Intermolecular hydrogen bond librational modes in cyclic trimers and tetramers of methanol and t-butyl alcohol isolated at low temperature in pulsed supersonic jet expansions are observed by direct absorption spectroscopy in the far-infrared region. The large amplitude librational modes probe the strength and directionality of the intermolecular hydrogen bonds. In addition, their frequency and intensity is very sensitive to the angle which the alkyl groups form with the hydrogen bonded ring. Theoretical predictions which fail to describe the trends in cluster size, alkylation and symmetry splitting reported in this work are likely to miss important ingredients of the underlying intermolecular interaction. Analysis of the vibrational correlation diagram between planar and puckered tetramer structures circumvents some deficiencies of approximate treatments.