Low-Frequency Modes in Molecular Crystals. VI. Methyl Torsions and Barriers to Internal Rotation of C(CH3)4, C(CD3)4, Si(CH3)4, Ge(CH3)4, and Sn(CH3)4

Abstract

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.

Absolute configuration of chirally deuterated neopentane

“…Thermodynamic data 17 , the frequency of the methyl torsional mode 18,19 , and cold neutron scattering 20,21 yield values between 18 and 21 kJ per mole. This amounts to about seven times the value of kT at room temperature.…”

The decomposition of chemically activated n–butane, isopentane, neohexane, and n–pentane and the correlation of their decomposition rates with radical recombination rates.

“…The validity of these results can be questioned, since measurements of the barriers to internal rotation in C(CHJ4, Ge(CH3)4, and Sn(CH3)4 by NMR are too low by a factor of 2 [38]. Also, other experimental measurements [41] and theoretical studies [42] suggest that there may be barriers to internal rotation about the bonds adjacent to the unpaired electron in alkyl radicals, due to a combination of effects related to hyperconjugation, spin polarization, and spin delocalization.…”

“…Vibrational entropies and enthalpy functions were taken from tables of harmonic oscillator thermodynamic functions [61]. The internal rotational barriers for the radicals were derived from the internal rotational barriers of the related alkanes [36][37][38]47,48,62].…”

Synthesis and Stereochemical Assignment of Crypto‐Optically Active 2H6‐Neopentane

“…Thee xperimental infrared (IR) spectrum of 2 H 6 -1 agrees well with the spectrum computed at the CCSD(T)/-cc-pVTZ level of theory (unscaled, Figure 2A;s ee Figure S5 for enlarged details), which is considered the "gold standard" [24] of currently available ab initio methods.N ote that the nine rotamers have approximately equal statistical weight [5] as their energy difference at this level amounts to less than 10 À4 kcal mol À1 only due zero-point vibrational energy contributions.A st he rotational barrier in 1 derived from IR measurements in the crystalline state [25] or in the gas phase [26] only amounts to 4.3 kcal mol À1 ,t he nine conformers of 2 H 6 -1 must be essentially equally populated in the gas phase at room temperature.T he computed rotational barrier only amounts to 3.7 kcal mol À1 at CCSD(T)/cc-pVTZ//B3LYP/augcc-pVTZ + ZPVE. Such ar elatively small rotational barrier implies that at 0-25 8 8Ct here is essentially free internal rotation.…”