Knowledge Bank

THE RAMAN effect was discovered in 1928 and since that time an average of 125 papers per year on this subject have appeared. Our Raman spectrograms are obtained by use of two cylindrical, horizontal, low-pressure, Pyrex Hg arcs. Raman-displacement spectrograms are obtained with a 2-prism spectrograph having a dispersion of 33 \AA/mm and depolarization-factor spectrograms with a hilger E518 spectrograph having a dispersion of 63 \AA/mm, both at 4500 \AA. Relative intensities are obtained with a microdensitometer and a microphotometer. A reliable single-exposure method (J. Chem. Phys. 13, 101 (1945) is used for the depolarization factors because short-cut methods can lead to erroneous conclusions about molecular structure. Since some vibrations are not observable in Raman spectra, an infrared spectrometer is a necessary supplement. Infrared data are especially needed from 100 to $400cm-1$ and with greater resolution above $2000cm-1$. Group theory selection rules predict the spectra that should result for various assumed structures of the molecule (Am. J. Phys. 11, 239 (1943)). Comparison with experimental data may thus enable one to determine the structure. Dangerous assumptions-frequently made-are that all of the observed Raman lines are fundamentals, and that fundamentals correspond only to strong bands. Hence, a reliable assignment of the fundamentals is necessary. Dr. Meister suggests that an anharmonicity treatment, even if incomplete, may provide useful additional checks upon the assignments. Another important check is a normal coordinate treatment, using the most general potential-energy expression possible (Am. J. Phys. 14, 13 (1946)). Even this, however, does not always lead to an unambiguous assignment for all lines, especially when two fundamentals fall close together. The final decision on the assignment of the fundamentals must be a matter of considered judgment in which all the possible tests are taken into account. When the fundamentals have been reliably assigned, and their degeneracies determined, one can then calculate thermodynamic properties for the ideal gaseous state, provide the product of the three principal moments of inertia, or the bond distances and interbond angles, are known (Chem. Rev. 27, 17 (1940)).