Far infrared and nuclear magnetic resonance spectra of molecules with internal rotor groups

Abstract

The torsional frequencies for 1,2-butadiene, methyl phosphine and 1-chIoro-1, 1-difIuoroethane were observed in the far infrared at 153.9, 207 and 246 cm⁻¹ respectively. The corresponding barriers to internal rotation were calculated and found to be 1509, 1703 and 3942 cal/mole respectively. The torsional bond of methyl amine was also resolved and the complicated band is discussed in the light of previously reported results. Far infrared spectra are given for the following 1,3 dienes: 1,3 butadiene (V), isoprene (VI), 2,3 dimethylbutadiene (VII), 1-trans-3 pentadiene (VIII) and 1-cis-3 pentadiene (IX). The torsional frequency about the C-C single bond of the 1,3 diene structure was observed for V at 163 cm⁻¹ in the vapor. Results on the liquid for VI through IX place this torsional frequency at 171, 97, 146 and 136 cm⁻¹ respectively. The barrier heights, V*, where V*= V₁+4V₂ +9V₃, were calculated from a harmonic oscillator approximation to be 15,595, 15,630, 9,490 and 9,810 cm⁻¹ for VI through IX. Other low skeletal modes are also assigned. The ethane derivative, CF₂BrCHFCl, shows over 20 bands below 660 cm⁻¹, indicating that several rotomers make significant contributions to the spectrum. A torsional mode was assigned to a very weak band in the vapor spectrum at ~ 75 cm⁻¹, while the liquid value is ~ 80 cm⁻¹. The far infrared spectrum agrees well with the reported Raman spectrum. Variable temperature NMR studies on this ethane derivative tend to verify the greater stability of one rotational isomer, namely the one in which the bromine is gauche to both hydrogen and chlorine. The NMR spectrum of diazomethane, recorded at -50°C, shows only a single line at 3.34 ppm from tetramethyIsilane. There is no indication of the presence of the tautomer of diazomethane, isodiazomethene, although a rapid equilibrium between the two forms cannot be ruled out. The NMR spectrum of the reactive compound, N-methyl methylenimine (CH₃-N=CH₂ shows a large geminal coupling of 16.3 cps between the two methylene protons. Quadrupolar effects from the nitrogen tend to broaden all resonances and the broadening of one methylene proton is noticeably greater. The chemical shifts for the methyl and methylene protons are 3.20, 6.95 and 7.24 ppm (from TMS), respectively.