Microwave spectrum, barrier to internal rotation of CF3, structure, and ab initio calculation of 1,1,1,2,2-pentafluoropropane

The microwave spectrum of 1, 1, 1,2,2-pentafluoropropane CF3CF2CH3 (HFC 245cb) has been studied for the first time using both a Stark modulation and Fourier transform microwave spectrometer. A least-squares analysis of the observed a- and b-type transition frequencies gave rotational and centrifugal distortion constants for the vibrational ground state as follows:A = 2715.9979(2) MHz, B = 1831.3817(2) MHz, C = 1810.6342(2) MHz, Delta(J) = 0.112(3) kHz, Delta(JK) = 0.033(3) kHz, Delta(K) = -0.0066(19) kHz delta(J) = 0.0030(2) kHz, delta(K) = -0.73(9) kHz. The barrier height to internal rotation of the CF3 group has been determined to be V-3 = 2.86 +/- 0.72 kcal/mol. A reasonable molecular structure has been derived from the observed rotational constants combining with ab initio calculation. These barrier height and structural parameters are compared with those of other related molecules. (C) 2001 Elsevier Science B.V. All rights reserved.