Investigation of Conformationally Rich Molecules: Rotational Spectra of Fifteen Conformational Isomers of 1-Octene

Abstract

The rotational spectra of 15 conformational isomers of 1-octene have been measured in a molecular beam at a rotational temperature of less than 2 K using a pulsed-nozzle Fourier transform microwave spectrometer. The transition assignments are guided by rotational constant calculations based on moderate-level ab initio theory and on the relative energy minima on the MM3 molecular-mechanics force field. The number of conformers identified is slightly more than 10% of the 131 predicted from the MM3 molecular-mechanics force field of Allinger et al. Fourteen of the observed conformers are identified with 14 of the 15 lowest energy minima predicted from the MM3 molecular-mechanics force field. The observation of such a large number of conformers, with a MM3 calculated energy spread of 365 cm-1 for this subset of 14, is a consequence of the minimal conformational cooling in the molecular-beam expansion. Limiting this cooling are the relatively high barriers separating the conformers compared to the thermal energy, kT, of the preexpansion gas. In some cases, primarily for the higher energy conformers, the need to cross multiple internal-rotation barriers provides an additional bottleneck for conformer relaxation. The rotational spectra furnish values for the principal moments of inertia, which are sensitive to the conformational geometry and can be compared with predictions from future high-level ab initio calculations. Assuming no conformer relaxation in the expansion allows the use of the transition-intensity data to estimate the energy ordering of the conformers.