Molecular rotational resonance effective structure

The methyl resonances of several oleanene derivatives have been assigned using lanthanide shifts and homonuclear INDOR techniques to measure the nuclear Overhauser effects between methyl signals. The assignments of the resonances of a series of 18jSH- and 18aH-oleanenes have been published. The potential use of molecular rotation differences in the identification of triterpenoid structures has been discussed. ... 

This spectrum is called a Raman spectrum and corresponds to the vibrational or rotational changes in the molecule. The selection rules for Raman activity are different from those for i.r. activity and the two types of spectroscopy are complementary in the study of molecular structure. Modern Raman spectrometers use lasers for excitation. In the resonance Raman effect excitation at a frequency corresponding to electronic absorption causes great enhancement of the Raman spectrum. 

Two important applications of radiation to determine molecular structure—X-ray crystallography and magnetic resonance—were discussed in Chapters 3 and 5. In this chapter we will discuss a variety of other techniques. Microwave absorption usually forces molecules to rotate more rapidly, and the frequencies of these absorptions provide a direct measure of bond distances. Individual bonds in a molecule can vibrate, as discussed classically in Chapter 3. Here we will do the quantum description, which explains why the greenhouse effect, which overheats the atmosphere of Venus and may be starting to affect the Earth s climate, is a direct result of infrared radiation inducing vibrations in molecules such as carbon dioxide. 

---