Neopentane Vibrations: High Resolution Spectra and Anharmonic Calculations

High resolution infrared
absorption spectra of neopentane (2,2-dimethylpropane,
C5H12) have been recorded in the mid-infrared
region at room temperature and 232 K. Neopentane is a spherical top
with Td symmetry. The high symmetry and
low temperature allow for detailed comparison of theory and experiment
for the analysis of the fundamental vibrations. Four strong bands
with characteristic rotational structure of t2 modes were observed at 1257.6, 1369.4, 1472.5, and 1489.0
cm–1, and a fifth very weak band was found near
924.2 cm–1. Quantum chemical calculations (B3LYP/6-311++(d,p)/VPT2
and harmonic CCSD­(T)-pVTZ) were carried out in both normal and local
mode representations to help with the vibrational assignments and
elucidate the couplings. The local mode representation combined with
the experimental observations highlights the important potential couplings
between neighboring methyl groups and their influence on observed
transitions. We have analyzed spectral regions where the models failed
to provide agreement and have identified those couplings responsible
for the differences.