Hydrogen wagging

The more complex a-amino acids additionally show characteristic bands which aid in their identification. Thus in the spectrum of L-tryptophan (Fig. 3.37) the N—H stretching vibration, and the out-of-plane hydrogen wag deformation which appears at 742 cm-1 (four adjacent hydrogens), readily allow it to be distinguished from other a-amino acids. 

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

The indole molecule is a planar asymmetric rotor with symmetry and 29 planar and 13 nonplanar fundamentals. Selected fundamental vibrations of indole in the gas and liquid phase have been re-examined < 1995SAA1291 >. The IR overtone/combination region from 1600 to 2000 cm was used to establish the wave numbers of nonplanar, hydrogenic-wagging modes for which the active IR and Raman fundamental is weak. A complete assignment of vibrational modes for indole by application of DFT and a hybrid Hartree-Fock/ DFT method has been provided <1996J(P2)2653>. 

Methyl Torsion and Amine Hydrogen Wagging in Methylamine... 

Fig. 7.1. Olefinic vibrations. The left-hand column illustrates the in-plane vibrations of the vinyl group. The right-hand column illustrates the out-of-plane vibrations (+ and —) of the vinyl group and for comparison the in-phase, out-of-plane hydrogen wagging vibrations of trans-, cis, and 1,1-disubstituted olefins and trisubstituted olefins. The approximate frequencies are given for hydrocarbon-substituted olefins.

In both the hydrogen wag vibrations discussed previously, the motions of the hydrogens are partially balanced by countermotions of the substituents (See Fig. 4.15). This makes the substituents more mechanically involved in the vibrations which makes these vibrations more variable in frequency than those discussed. When rotational isomers are possible in the cw-olefins, the wag band can be broadened. Cyclohexenes absorb at the low frequency end of the cis region. 

The in-phase, out-of-plane aromatic CH wag vibrations give rise to strong infrared bands which are arranged according to the number of adjacent hydrogens in Table 8.2. The general usefulness of the adjacent hydrogen wag correlation is illustrated by the fact that it can be extended to include... 

Examples of these infrared bands are seen in Fig. 8.7. The first two rows of spectra illustrate benzene rings with alkane substituents. For these, the adjacent hydrogen wag bands tend to increase in frequency somewhat as the a carbon of the substituent is more highly substituted. We can note absorptions for toluene at 728 cm ethylbenzene at 745 cm isopropylbenzene at 759 cm and /ert-butylbenzene at 763 cm ... 

In-Phase, Out-of-Plane Hydrogen Wagging and Out-of-Plane Sextant Ring Bending Frequencies (in cm )... 

It turns out that the electron donating or withdrawing characteristic of the substituent X correlates best with the weak CH wag band wavenumber near 900 cm It does not correlate well with the 750 cm 5 adjacent in-phase wag band which is more sensitive to mechanical interaction effects of the substituents (note, for example, the first column of Fig. 8.7). In the 900 cm" monosubstituted benzene band, the para hydrogen wags in the opposite direction from the two ortho hydrogens as seen in Fig. 8.1 and 8.6, and the ring carbon with the substituent is nearly motionless. The relationship of die i>(900 cm" ) wavenumber for X-phenyl with the wavenumbers of the previously discussed CH2 wag for substituted ethylene XHC=CH2, i>(E), and the CH wag for substituted acetylene XC=CH, is as follows ... 

Most of these five-membered heteroaromatics with a CH=CH group unsubstituted have strong hydrogen wag absorption at 800-700 cm. Hy-... 

The out-of-plane C—OH deformation in the bonded state absorbs very broadly and diffusely near 650 cm" . The in-plane C—OH deformation is complicated by interaction with hydrogen wagging vibrations in primary and secondary alcohols. The hydrogens on the oxygen and on the carbon move more or less parallel to the C—O bond in the same direction in one case and the opposite direction in the other case, giving rise to more than one band that involves OH deformation. 

Smeyers et al. have performed an ab initio study of the joint large amplitude methyl rotation and hydrogen wagging vibrations in the X A and a A" states of CHjCHS. They found the eclipsed and antieclipsed conformers to be the preferred structures for the lower and upper electronic states, respectively. The calculated barrier heights to methyl rotation of 118.3/455.6 cm" for the respective states are in good agreement with the experimental values of 94.2/534.3 cm". The barrier to inversion of the aldehyde hydrogen in the a A" state was calculated to be very low, 67.4 cm". As a consequence, the potential function of Fig. 27 is flat and resembles a square well. 

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