The far infrared spectrum, ab initio calculations and conformational energy differences of 1-butene

The far infrared spectra of 1-butene. CH2=CHCH2-CH3, and the d(3) species CH2=CHCH2CD3 have been recorded from 50 to 360 cm(-1) at a resolution of 0.10 cm(-1). The geometrical structures and vibrational frequencies of the cis(synplanar) and gauche(anticlinal) conformers have been determined by ab initio calculations in order to calculate torsional constants and to aid in assignments of the far infrared bands. A wide range of basis sets and ab initio theoretical methods up to MP4, have been used in order to predict the cis-gauche conformational energy difference. The fundamental and a few other methyl torsional transitions have been observed for both the cis and gauche conformers for both isotopic species and threefold barriers to internal rotation of 1680 +/- 15 cm(-1) (4.80 +/- 0.04 kcal/mol) for the: cis rotamer and 1150 +/- 50 cm(-1) (3.29 +/- 0.14 kcal/mol) for the gauche conformer have been determined. The methyl barrier for the cis conformer is much higher than that derived from the microwave splittings. The potential energy function for asymmetric torsion has been redetermined from which values of the cis to gauche barrier of 835 +/- 48 cm(-1) (2.39 +/- 0.14 kcal/mol), and the gauche to gauche barrier of 615 +/- 51 cm(-1) (1.76 +/- 0.15 kcal/mol) have been obtained. The cis-gauche enthalpy difference has been determined to be 73 +/- 6 cm(-1) (209 +/- 17 cal/mol) in krypton and 64 +/- 10 cm(-1) (183 +/- 29 cal/mol) in xenon with the cis conformer as the more stable form from the variable temperature studies of the infrared spectra (3500-400 cm(-1)). These experimental and theoretical results are compared with the corresponding quantities of some similar molecules. (C) 2000 Elsevier Science B.V. All rights reserved.