Selective UV Resonance Raman Excitation of Vinyl Group Vibrations in an Isomer of Crotyl Chloride

The Raman spectra of cis, trans, and vinyl isomers of crotyl chloride (i.e., cisand trans1-chloro-2-butene, and 3-chloro-1-butene) have been excited at 218 nm and at 231 nm. With 218-nm excitation the ethylenic mode belonging to the vinyl isomer is only modestly preresonance enhanced. On the other hand, the preresonance enhancement of the peaks belonging to the cisand transisomers is very noticeable and nearly identical in energy to the peaks of cisand transpolybutadiene, which occur at 1655 and 1665 cm−1, respectively. However, with 231-nm excitation the vinyl-isomer ethylenic mode at 1633 cm−1is very strongly enhanced, while the cisand transisomer modes show no corresponding enhancement, but appear to decrease in relative intensity. Responsibility for the strong vinyl group enhancement is assigned to an electronic transition observed as a broad shoulder near 230 nm. By exciting at 231 nm it is possible to detect small amounts of the vinyl isomer in the presence of cisand transcrotyl chloride. Remarkably strong enhancement under 231-nm excitation has been noted as well for the first overtone of the vinyl ethylenic mode observed at 3263 cm−1.