Hydrogen bonded rings, chains and lassos: the case of t-butyl alcohol clusters

Infrared OH stretching spectra of hydrogen bonded 2-methyl-propan-2-ol (t-b utyl alcohol) clusters are investigated by ragout-jet FTIR spectroscopy. A spectral difference technique is used to discriminate approximately between neighbouring cluster sizes. Dimers, trimers and cyclic tetramers can be de tected along with larger clusters, which exhibit a surprisingly structured vibrational fingerprint. Comparison is made to the spectra of related alcoh ols and to energetic and harmonic vibrational predictions from electronic s tructure calculations. The experimentally observed 32% increase in OH stret ching wavenumber shift from methanol dimer to t-butyl alcohol dimer is repr oduced at the HF/3-21G level (+33%). It is also qualitatively correct at th e MP2/6-311G* level (+15%), whereas it has the wrong sign at the B3LYP/6-31 1G* level (-5%) and is negligible at the HF/6-311G* level, disregarding anh armonic effects. The cyclic tetramer of t-butyl alcohol is found to be part icularly stable due to a favourable up-down alternation of the bulky t-buty l groups. Beyond the t-butyl alcohol tetramer, lasso structures are found t o be energetically competitive with simple ring structures. A many-body dec omposition shows that this is due to a reduced cooperativity in the sterica lly hindered pentamer ring. The resulting thermodynamic and kinetic relevan ce of cyclic tetramers is discussed.