Group frequencies trans

The absorption frequency of the alkene bond in conjugation with a carbonyl group is lowered by about 30 cm-1 the intensity of absorption is increased. In s-cis structures, the alkene absorption may be as intense as that of the carbonyl group. s-Trans structures absorb more weakly than s-cis structures. 

Consider the cw-disubstituted ethylene performing the in-phase, out-of-plane hydrogen bending vibration. In Fig. 4.15(c) we can see that this vibration rotates the H—C=C—H plane relative to the X—C=C—X plane, so in reality this vibration is an X—C—H bending vibration and the X atom is mechanically involved. This makes this cis vibration mechanically sensitive to the nature of X and makes this vibration a poorer group frequency than the trans or vinylidine vibrations discussed above. 

The 970-660 cm region contains a strong group frequency vibration at a position determined by the structure around the double bond. The sensitivity of this group frequency to structure is exemplified by the positions of the band in cis and trans alkenes. In cw-2-butene the band is near 675 cm" while in the trans compound it is near 960 cm The olefinic group frequencies in the 970-660 cm region can be found in the spectra shown in Figures 5.7, 5-8, 5-9, 5-16, 5-17, 5-18, and 5-19. 

Because of the difference in energy of these two groups, temperature changes alter the population of the two states. Ab initio calculations indicated [57,58] that the high-frequency peak is due to the trans conformation and the lower frequency peak to the cis conformation. Analysis of the relative band area gives the population of the two states as a function of temperature and hence allows... 

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