Rotational spectra of four of the five conformers of 1-pentene

The rotational spectra of four of the five expected conformers of 1-pentene , together with their monosubstituted C-13 isotopic forms, have been measur ed in a molecular beam using a pulsed-nozzle Fourier-transform microwave sp ectrometer. One of the conformers has C-s point-group symmetry while the ot her three conformers have C-1 point-group symmetry. The measurements are co mpared to results from molecular modeling calculations using the MM3 molecu lar-mechanics force field of Allinger and to ab initio electronic structure calculations (MP2/6-31G*, MP2/6-311G*, MP4/6-31G*, MP4/6-311G*). Both type s of calculations suggest the existence of five distinct conformers of 1-pe ntene, four of C-1 symmetry and one of C-s symmetry. Both the MM3 and ab in itio rotational constants deviate from the measured values by less than or equal to 5%. The relatively high barriers between the four conformers limit the conformational cooling in the expansion, allowing all four conformers to be observed at the < 2 K rotational temperature of the molecular beam. E fforts to identify the fifth conformer were unsuccessful, presumably due to its reduced intensity, which makes it difficult to identify its spectral p attern from among the plethora of weak unassigned lines due to impurities, complexes, and possible vibrationally excited conformers. The fifth conform er is predicted to have the highest energy of the five conformers of 1-pent ene, as well as a low-energy barrier (109 cm(-1) at MP2/6-311G* level) for conformational isomerization. (C) 2000 American Institute of Physics. [S002 1-9606(00)00514-6].