Out-of-Plane C—H Bending Vibrations

Assignments for C—H out-of-plane bending bands in the spectra of substituted benzenes appear in the chart of characteristic group absorptions (Appendix C). These assignments are usually reliable for alkyl-substituted benzenes, but caution must be observed in the interpretation of spectra when polar groups are attached directly to the ring, for example, in nitroben-zenes, aromatic acids, and esters or amides of aromatic acids. 

The absorption band that frequently appears in the spectra of substituted benzenes near 600-420 cm-1 is attributed to out-of-plane ring bending. Some spectra showing typical aromatic absorption appear in Appendix B benzene (No. 4), indene (No. 8), diethylphthalate (No. 21), and m-xylene (No. 6). 

The absorption band that frequently appears in the spectra of substituted benzenes near 600-420 cm-1 is attributed to out-of-plane ring bending. 

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The most characteristic vibrational modes of alkenes are the out-of-plane C—H bending vibrations between 1000 and 650 cm-1. These bands are usually the strongest in the spectra of alkenes. The most reliable bands are those of the vinyl group, the vinylidene group, and the trans-disubstituted alkene. Alkene absorption is summarized in Appendix Tables D-l and D-2. 

Box 3.4 Out-of-plane C-H Bending Vibration Frequencies of Mono- and Disubstituted Aromatic Rings... 

It is well known that the out-of-plane C—H bending vibration frequencies of olefins, and in particular ethylene derivatives, are indicative of the double-bond configuration of the olefin species [18]. These vibrational... 

Fig. 4.15. Out-of-plane C—H bending vibrations in some olefinic groups.

It is now appropriate to ask where the out-of-plane C H bending vibration appears There is a strong band at 940 cm and this is close to where the trans configuration out-of-plane mode should occur. However, the presence of two hands in the sp stretching region and the strength of the C=C stretch in the IR tend to discredit the presence of a trans configuration. 

Figure Bl.6.10 Energy-loss spectrum of 3.5 eV electrons specularly reflected from benzene absorbed on the rheniiun(l 11) surface [H]. Excitation of C-H vibrational modes appears at 100, 140 and 372 meV. Only modes with a changing electric dipole perpendicular to the surface are allowed for excitation in specular reflection. The great intensity of the out-of-plane C-H bending mode at 100 meV confimis that the plane of the molecule is parallel to the metal surface. Transitions at 43, 68 and 176 meV are associated with Rli-C and C-C vibrations.

Unfortnnately, no distinct LEED patterns could be generated from the adlayer of benzene chemisorbed on a Pd(lll) single-crystal electrode hence, meaningful results were obtained only from the HREELS experiments. Figure 10(A) shows the HREEL spectmm of benzene on Pd(lll) formed and emersed at 0.5 V based upon the above EC-STM results, a Pd(lll)-c(2V3x3)-rect-CeHe adlayer ( CgUg = 0.17) was assumed to be present on the surface. Except for the peak at 1717 cm, which is due to adventitious CO, all the peaks, when compared to published vibrational spectra of unadsorbed and adsorbed " benzene, are attributable to chemisorbed starting material. Unique to the surface-immobilized aromatic are the peaks labeled (a), 265 cm, and (b), 515 cm", which arise from direct metal-adsorbate (Pd-C) chemical bonds. Peaks (c) and (d) are out-of-plane C-H bends, y(C-H) peaks (e) and (i) are in-plane stretches, v(C-H), whereas peaks (f) and (g) are both inplane bends, 5(C-H). 

The band at 752 cm is assigned to the out-of-plane C—H bending for a mono-or meta substitution, while the 690 cm band is characteristic of mono-, meta-, or 1,3,5-trisubstitution. The sum tone pattern is consistent with monosubstitution. The Raman spectrum serves to confirm this conclusion since a band appears at 1000 cm due to an in-plane vibration for a mono-, meta-, or 1,3,5-tri-substitution. The 620 cm band is assigned to the veb vibration of a monosubstituted aromatic. 

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