Spectroscopic Probes of Nonbonded Interactions

The spectroscopic probes to be examined in this section involve (a) ultraviolet spectroscopy and (b) vibrational spectroscopy. 

We first consider the relative electronic transition energies in cis and trans 1,2 disubstituted ethylenes. From Fig. 28 we can clearly see that the pi HOMO LUMO energy gap is larger for the case of the cis isomer relative to the trans isomer. Hence, the fl Tr transition is expected to occur at shorter wavelengths in cis 1,2-disubsti-tuted ethylenes. 

UV data shown in Table 26 clearly support our expectations. For example, the mt transition energy for cis 1,2-dichloroethylene is 42300 cm while the same quantity is 41700 cm in the corresponding trans molecule. The same trend is observed in the 1,2-difluoroethyIenes, as well as in the 1,2-dialkylethylenes, where a distinct hypsodiromic shift is observed in the comparison of trans and cis isomers. 

An interesting example is found in the nir transition of ortho and para benzoquinone. The two molecules are dissected as shown below  

The relative energies of the ir LUMO s in the two cases can be deduced by evaluating the overlap int ral as shown schematically below  

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