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.