MICROWAVE AND FAR-IR SPECTRA, CONFORMATIONAL STABILITY, MOLECULAR-STRUCTURE, BARRIERS TO INTERNAL-ROTATION AND AB-INITIO CALCULATIONS FOR THE ANTICLINAL CONFORMER OF TRANS-1-FLUORO-2-BUTENE

The microwave spectrum of trans-1-fluoro-2-butene, trans-(CH3)HC=CH(CH 2F), has been recorded in the region of 18.0-39.0 GHz. Both a-type R- and b-type Q-branch assignments have been made for the ground and firs t two vibrationally excited states of the asymmetric torsion for the g auche (anticlinal) conformer. The ground state rotational constants fo r this conformer are found to have the following values: A = 19,938.33 +/- 0.48, B = 2071.37 +/- 0.01, C = 2022.17 +/- 0.01 MHz. From an ana lysis of the internal rotational splittings of the Q-branches, the thr ee-fold rotational barrier for the methyl group is determined to be 59 6 +/- 7 cm-1 (1.70 +/- 0.02 kcal/mol). From the Stark effect the dipol e moment components for the gauche conformer were determined to be \mu (a)\=1.86 +/- 0.01, \mu(b)\=1.16 +/- 0.01, \mu(c)\=0.31 +/- 0.05, and \mu(t)\=2.21 +/- 0.01 D. The fundamental asymmetric torsion for the ci s (synclinal) conformer has been observed in the far-IR spectrum of th e vapor at 123.95 cm-1 whereas that for the gauche conformer has been determined to occur at 82.8 +/- 5 cm-1 based on relative intensity mea surements obtained from the microwave spectrum. From these data the po tential function which governs the internal rotation of the asymmetric top has been determined and the following potential constants have be en evaluated: V1 = -191 +/- 10, V2 = 598 +/- 10, V3 = 786 +/- 13, V4 = 59 +/- 5, and V6 = 79 +/- 5 cm-1. These data are consistent with the more stable conformer having the fluorine atom cis (synclinal) to the double bond and lying 300 +/- 33 cm -1 (858 +/- 94 cal/mol) lower in e nergy than the gauche rotamer. Utilizing ab initio calculations with t he MP2/6-31G basis set and the three rotational constants, r0 structu ral parameters are estimated. Also, the barriers to conformer intercon version have been calculated with the RHF/3-21G, RHF/6-31G, and MP2/6 -31G basis sets. All of these results have been compared to the simil ar quantities of some corresponding molecules.