Torsional modes, methyl

Many molecules are composed of functional groups (hat can rotate with respect to the rest of the molecule. The classical example is ethane, as the possibility of rotation of one methyl group against the other was recognized long ego. Because the torsional mode does not result in infrared activity, its frequency was estimated from thermodynamic data. 

The inelastic neutron TOF spectrum of a butane monolayer adsorbed on Carbopack B at 80 K (10) is shown at the top of Fig. 6. The background inelastic scattering from the substrate has been subtracted. Typical counting time for such a spectrum is -100 hours. The intramolecular torsional modes of the CH, and CH groups observed in the bulk liquid and solid (23) are also found in the monolayer spectrum. In the two methyl torsionsal modes the CH, groups rotate either in the same or opposite sense about the terminal C-C bonds, and in the CH2-CH torsion the two halves of the molecule rotate in the opposite sense about the internal C-C bond. In addition to the intramolecular torsional modes, new features appear in the monolayer spectrum which are not observed in bulk samples an intense peak at -112 cm"-1 and a broader band centered at 50 cm-1. 

An alkyne which has been studied is 2-butyne (dimethylacetylene, CH3-CSC-CH3) [28]. The crystal contains two molecules in the unit cell and the focus of the work was on the interactions between the methyl groups and how their influence on the methyl rotational tunnelling spectrum and the librational and torsional modes. 

The most obvious discrepancies are the methyl torsions. These are always observed at about 250 10 cm, irrespective of the alkane chain length or the crystallographic enviromnent of the end group. This is considerably above the maximum value for other out-of-plane torsional modes and shows them to be imlike the other TAMs. 

We shall now consider a few examples of the application of neutron spectroscopy (primarily the small k method) to the study of methyl torsional modes in two or more top molecules. The measurement of differences in the frequency of in-phase and out-of-phase torsional modes can in principle yield independent evidence concerning the effects of top-top interaction on the potential governing internal rotation, thus providing an excellent complement to microwave results. 

For rate coefficients, the importance of treating the low frequency torsional modes in radical addition reactions as hindered internal rotations has been investigated in a number of assessment studies (37,79,93). For small systems such as methyl addition to ethylene and propylene, the errors are relatively minor (less than a factor of 2) (37). However, for reactions of substituted radicals (such as n-alkyl radicals (93) and the ethyl benzyl radical (79)), the errors are somewhat larger (as much as a factor of 6), as there are additional low frequency torsional modes to consider. Nonetheless, the errors are still relatively small, and the harmonic oscillator approximation might be expected to provide reasonable order-of-magnitude estimates of rate coefficients. 

Ratcliffe, C. I., Waddington, T. C., Internal Torsional Modes in Mixed Methyl Halo-genocompounds of Group IV Elements Studied by Inelastic Neutron Scattering, J. Chem. Soc., Faraday Trans. II 72 [1976] 1840/50. 

The methyl group bonded to a metal (M-CH3) exhibits the six normal vibrations expected for tetrahedral ZXY3 molecules. In addition, CMC bending and CH3 torsional modes are expected for M(CH3) (n > 2) compounds. In M(CHs)4 derivatives (M = Si or Sn) the CH3 rocking, MC stretching and CMC... 

The preference for endo attack in 7,7-dimethylnorbomene is certainly steric in origin, with the 7-methyl substituent shielding the exo direction of approach. The origin of the preferred exo-attack in norbomene is more subject to discussion. A purely steric explanation views the endo hydrogens at C—5 and C—6 as sterically shieldihg the endo approach. There probably is also a major torsional effect Comparison of the exo and endo modes of reproach shows that greater torsional strain develops in the endo mode of... 

The structure of a second polymorph of 4,5-diphenyl- lH-imidazole has been discussed, with the new form exhibiting significantly different phenyl/imidazole dihedral angles and mode of crystal packing relative to the known form [53], A new triclinic polymorph of 1,4-dibenzoyl-butane was found, differing from the monoclinic form in the torsional angles of the central chain [54], Two polymorphs of diphenyl-(4-pyridyl)methyl methacrylate have been found, where the molecules in the two forms contain weak C—H— n and C—H O/N contacts that lead to the existence of different conformations [55]. 

Figure 3. Librational OH modes in hydrogen bonded alcohol clusters may be correlated with overall rotation (bottom left) and torsion (top left) of the monomer (illustrated for methanol), but methyl rotation is actually decoupled from OH torsion by hydrogen bonding. Note that the wavenumbers of monomer rotation (fa 4 cm-1) and torsion (fa 280 cm-1) are much lower than that of the cluster libration (fa 600cm ) [93].

Fig, 17. Band strvcturea corresponding to the methyl torsion and aldehy-die hydrogen wagging modes in the a A" — spectrum of thioacetalde ... 

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