T. S. Little

Spectra and structure of small ring compounds. LIX: Raman and far infrared spectra, conformational stability, vibrational assignments, normal coordinate analysis, and ab initio calculations of fluorocyclobutane

The Raman (3200 to 10 cm−1) and far infrared (380 to 60 cm−1) spectra of gaseous fluorocyclobutane,c-C4H7F, have been recorded. A series of Q-branches observed in both of these spectra beginning at 166 cm−1 with successive transitions falling to lower frequencies have been assigned to the ring puckering vibrations of both the low energy equatorial and high energy axial conformers. These data have been fit to an asymmetric potential function of the form:V (cm−1)=(1.76±0.05)103X+(4.88±0.28)104X2+(2.12 ±0.16)103 exp(−5.66±0.41)10X2 with a reduced mass function ofg44= 0.00386−0.00295X+0.03485X2+0.1228X3 +0.3459X4, whereX is the ring puckering coordinate. Utilizing this potential, the difference between the puckering angles for the two conformers was calculated to be 4° with the equatorial conformer having the larger value of 28°. This potential function is consistent with an energy difference between the equatorial and axial forms of 447 cm−1 (1.28 kcal/mol) and a barrier to ring inversion from the equatorial to the axial conformation of 713 cm−1 (2.04 kcal/mol). Experimental values for the enthalpy difference between the two conformers have been determined for both the liquid (400±30 c−1) and gas (413±43 cm−1) from investigations of the Raman spectra at variable temperatures. The conformational stability, enthalpy difference, structural parameters, and fundamental vibrational frequencies, which have been determined experimentally, are compared to those obtained from ab initio Hartree-Fock calculations employing the 3-21G, 6-31 G*, and 6-31 G** basis sets.