MICROWAVE, INFRARED AND RAMAN-SPECTRA, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, VIBRATIONAL ASSIGNMENT, AND AB-INITIO CALCULATIONS FOR 1-FLUORO-2-METHYLPROPANE

The microwave spectrum of 1-fluoro-2-methylpropane (isobutyl fluoride), (CH3)(2)CHCH2F, has been investigated in the region from 18.0 to 38.5 GHz. The a-type R-branch transitions have been assigned on the basis of the rigid rotor model. For the ground vibrational state the rotational constants were found to be A = 7631.03 +/- 0.14, B = 3480.96 +/- 0.01, and C = 2633.38 +/- 0.01 MHz. The dipole moment components were determined from the Stark effect to be \mu(a)\ = 1.702 +/- 0.002, \mu(b)\ = 0.540 +/- 0.045, \mu(c)\ = 0.296 +/- 0.007, and \mu(t)\ = 1.810 +/- 0.003 D. These data clearly indicate that the assigned microwave spectrum is that of the gauche conformer. The IR (3500-40 cm(-1)) and Raman (3500-20 cm(-1)) spectra of the gas and solid have been recorded. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. Both the trans and gauche conformers have been identified in the vibrational spectrum of the fluid phases. From temperature studies of the Raman spectra of the gas and Liquid, the enthalpy differences between the trans and gauche conformers were determined to be 154 +/- 38 cm(-1) (440 +/- 109 cal/mol) and 157 +/- 19 cm(-1) (449 +/- 54 cal/mol), respectively, with the gauche conformer being thermodynamically preferred in both phases. Upon crystallization only the gauche conformer remains in the solid state. Complete equilibrium geometries have been determined for both rotamers by ab initio Hartree-Fock gradient calculations employing RHF/3-21G, RHF/6-31G* and MP2/6-31G* basis sets. The normal modes and potential surface calculations have been carried out employing the RHF/6-31G* basis set. These results are discussed and compared to those obtained for some similar molecules.