MICROWAVE, INFRARED, AND RAMAN-SPECTRA, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, ABINITIO CALCULATIONS, AND VIBRATIONAL ASSIGNMENT OF 3-FLUORO-2-METHYLPROPENE

The microwave spectrum of 3-fluoro-2-methylpropene (methallyl fluoride), CH2C(CH3)CH2F, has been recorded from 18.0 to 39.0 GHz. The a-type R branches have been observed and assigned in the ground and first two vibrationally excited states of the asymmetric torsion for both the low-energy cis and high-energy gauche conformers. Additionally, the c-type Q branches for the gauche conformer have been assigned. The ground-state rotational constants for the gauche conformer are A = 7691.75 +/- 0.02, B = 3900.77 +/- 0.01, and C = 2876.85 +/- 0.01 MHz, and those for the cis are A = 9035.14 +/- 0.01, B = 3655.63 +/- 0.01, and C = 2690.16 +/- 0.01. From the Stark effect the dipole moment components were determined for the cis conformer to be (mu-a) = 1.65 +/- 0.02, (mu-b) = 1.13 +/- 0.01, and (mu-t) = 2.00 +/- 0.01 D, and those for the gauche conformer to be (mu-a) = 1.36 +/- 0.03, (mu-b) = 0.26 +/- 0.07, (mu-c) = 0.89 +/- 0.07, and (mu-t) = 1.65 +/- 0.01 D. The infrared (3500 to 40 cm-1) and Raman (3500 to 20 cm-1) spectra of gaseous and solid 2-methyl-3-fluoropropene as well as the Raman spectrum of the liquid have also been recorded and qualitative depolarization values obtained. From variable-temperature Raman studies the enthalpy differences between the more stable cis and high-energy gauche forms have been found to be 54 +/- 16 cm-1 (154 cal/mol) and 104 +/- 30 cm-1) (297 cal/mol) for the liquid and vapor phases, respectively. The fundamental asymmetric torsion for the cis conformer has been observed at 109.57 cm-1 with seven excited states falling to lower frequencies whereas this fundamental for the gauche has been observed at 92.47 cm-1 with six excited states falling to lower frequency. From these data along with the gauche dihedral angle, the potential function for internal rotation of the asymmetric top has been determined and the following potential constants have been evaluated: V1 = 61 +/- 36, V2 = 62 +/- 42, V3 = 1045 +/- 13, V4 = -42 +/- 11, V5 = 85 +/- 17, and V6 = -56 +/- 6 cm-1 with a DELTA-H of 104 +/- 91 cm-1 (297 +/- 260 cal/mol), with the cis conformer more stable. The calculated cis to gauche, gauche to gauche, and gauche to cis barriers are 1096, 1078, and 983 cm-1, respectively. The barriers to internal rotation of the methyl group have also been determined from the far-infrared spectral data of the vapor to be 609 cm-1 (1.74 kcal/mol) and 662 cm-1 (1.89 kcal/mol) for the cis and gauche conformers, respectively. The asymmetric potential function, conformational energy difference, and optimized geometries of both conformers have also been obtained from ab initio calculations with both the 3-21G and 6-31G* basis sets. A normal-coordinate analysis has also been performed with a force field determined from the 3-21G basis set. Combining the ab initio data with the microwave rotational constants, the heavy-atom structural parameters are estimated. These data are compared to the corresponding data for some similar molecules.