Abstract
The Raman spectra of <i>cis, trans,</i> and <i>vinyl</i> isomers of crotyl chloride (i.e., <i>cis</i> and <i>trans</i> 1-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 <i>cis</i> and <i>trans</i> isomers is very noticeable and nearly identical in energy to the peaks of <i>cis</i> and <i>trans</i> polybutadiene, which occur at 1655 and 1665 cm<sup>−1</sup>, respectively. However, with 231-nm excitation the vinyl-isomer ethylenic mode at 1633 cm<sup>−1</sup> is very strongly enhanced, while the <i>cis</i> and <i>trans</i> isomer 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 <i>cis</i> and <i>trans</i> crotyl 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<sup>−1</sup>.
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