Investigation of conformationally rich molecules: Rotational spectra of fifteen conformational isomers of 1-octene

The rotational spectra of 15 conformational isomers of 1-octene have been m easured in a molecular beam at a rotational temperature of less than 2 K us ing a pulsed-nozzle Fourier transform microwave spectrometer. The transitio n assignments are guided by rotational constant calculations based on moder ate-level ab initio theory and on the relative energy minima on the MM3. mo lecular-mechanics force field. The number of conformers identified is sligh tly more than 10% of the 131 predicted from the MM3 molecular-mechanics for ce field of Allinger et al. Fourteen of the observed conformers are identif ied with 14 of the 15 lowest energy minima predicted from the MM3 molecular -mechanics force field. The observation of such a large number of conformer s, with a MM3 calculated energy spread of 365 cm(-1) for this subset of 14, is a consequence of the minimal conformational cooling ih the molecular-be am expansion. Limiting this cooling are the relatively high barriers separa ting the conformers compared to the thermal energy, kT, of the preexpansion gas. In some cases, primarily for the higher energy conformers, the need t o cross multiple internal-rotation barriers provides an additional bottlene ck for conformer relaxation. The rotational spectra furnish values for the principal moments of inertia, which are sensitive to the conformational geo metry 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 th e conformers.