Structural basis for the discrepancy of spectral behavior in C-H stretching band between steroids and long chain hydrocarbon compounds

The discrepancies of the spectral behavior for the C-H stretching band betw een some long chain hydrocarbon compounds and steroids were investigated. A t low temperature, the C-H stretching bands exhibit complex fine structure in steroids but remain simple in long chain hydrocarbon compounds. MM3 mole cular mechanics calculation indicates that, for long chain hydrocarbon comp ounds, the C-H groups vibrate with large scale coupling. There exist a few bands where the C-H groups vibrate in synchronous and inphase mode. Thus th e variations of dipole moment for these bands are enhanced and the intensit ies are obviously stronger than others and ax er other band in the spectra. This is just the reason why the C-H stretching bands are simple even at lo w temperature environment. Nevertheless, for the steroids, the C-H stretchi ng bands vibrate with local coupling mode. The synchronous enhancement effe ct does not occur, the differences of intensities for various modes are not as large as those in long chain hydrocarbon compounds. Hence the fine spec tral structure could be observed in cryogenic FT-IR spectra. The structural basis for the above phenomenon is that the C-H groups in long chain hydroc arbon compound exist in the similar microenvironment, while the C-H group o ccurred in different microenvironment for steroids.