The far-infrared and Raman spectra of solid (CH3)3CCl, (CD3)3CCl, (CH33SiCl, (CH3)3GeCl, (CH3)3CBr, (CH3)3SiBr, and (CH3)3GeBr have been obtained over a range of temperatures. For (CH3)3CCl, (CD3)3CCl, (CH3)3SiCl, and (CH3)3CBr torsional vibrations were observed, and from the frequencies, barriers to internal rotation of the methyl groups were calculated. The barrier of 4.51 kcal/mole for tertiary-butyl chloride (4.82 for the d9 compound) is consistent with that reported earlier for the fluoride. The 3.90-kcal/mole barrier calculated for the tertiary-butyl bromide is somewhat lower than expected. A barrier of 2.63 kcal/mole was found for the trimethylchlorosilane molecule. Intermolecular fundamentals were observed for all compounds in the series, and the higher-frequency modes observed in the infrared spectra were assigned as librational fundamentals whereas the lower-frequency Raman lines have been assigned as optical translations. The frequency shifts for these intermolecular fundamentals are discussed in terms of the increased intermolecular forces for the higher molecular weight compounds. Although the “heavy” members of the series are globular in shape, no phase transitions were observed and the lack of more than one crystalline form for these solids is believed to be the result of the relatively large intermolecular forces.

Articles you may be interested inLow frequency modes in molecular crystals. XIX. Methyl torsions and barriers to internal rotation of some three top molecules with C 3 symmetry

The far-infrared spectra of C(CH3)4, C(CD3)4, Si(CH3)4, Ge(CH3)4, and Sn(CH3)4 have been recorded from 300 to 33 cm−1. The f1 torsional modes were observed at 281, 206, 177.5, 134, and 101.5 cm−1 and the a2 torsional modes were assigned to bands at 221, 157, 163.5, 134, and 101.5 cm−1 for the C(CH3)4, and C(CD3)4, Si(CH3)4, Ge(CH3)4, and Sn(CH3)4 molecules, respectively. The corresponding CH3 torsional barriers were calculated to be 4.3, 4.6, 2.0, 1.3, and 0.8 kcal/mole. These barriers are shown to be consistent with those reported for the similar trimethyl molecules but are considerably higher than the barriers previously reported for these molecules from PMR studies.

The low frequency infrared spectra of (CHa)aCOH, (CDa)sCOH, (CHa)aCSH, (CHa)aCNCO, (CHa)a-CNCS, (CHa)aCCHO, (CH.)aCNH2, (CHa)sCOCHa, and (CH.)aCODC. have been recorded for these molecules in both the gaseous and solid states. Harmonic barriers have been calculated for t-butyl methyl group rotation from frequencies measured for the compounds in their solid state. The values are 4. 13, 4.52,4.41, 4.67, 4.86, 3.96, 4.28, 4.71, and 4.55 kcal/mole, respectively. Estimates of the periodic barriers to rotation of the tertiary butyl groups were also determined for (CH.)aCOH( ~1 kcal/mole), (CHa)aCSH (1.5 kcal/mole), (CH.)aCNCO (0.3 kcal/mole), (CHa)aCNCS (1.3 kcal/mole), (CHa)aCCHO (0.7 kcal/ mole), and (CHa).COCHa (3.57 kcal/mole) from the observed fundamentals in the infrared spectra of the gases.

The infrared spectra have been recorded from 33 to 4000 cm−1 for CH3CF2Br, CH3CBr2Cl, and CH3CBr3 and from 33 to 600 cm−1 for CH3CF2H and CH3CF2Cl for both the gaseous and solid states. The Raman spectra of CH3CF2Br and CH3CBr2Cl in the liquid state have also been recorded. The assignments of the 18 normal modes have been made on the basis of the band contours, Raman depolarization values, and characteristic ``group'' frequencies for the CH3CF2Br and CH3CBr2Cl molecules. The methyl torsional vibrations were observed for CH3CF2H, CH3CF2Cl, CH3CF2Br, CH3CBr2Cl, and CH3CBr3 at 248, 272, 275, 293, and 304 cm−1, respectively, in the solid state. From these assignments, barriers of 3.97, 4.80, 4.99, 5.66, and 6.08 kcal/mole were calculated, respectively. The effects of condensation on the torsional modes have been considered and the barrier values are compared with results obtained previously from similar molecules. Intermolecular fundamentals were observed for the CH3CF2H, CH3CF2Cl, and CH3CF2Br molecules in the far infrared spectra and possible assignments are discussed.