Shpolskii Effect

the appearance of quasi-line spectra of complex organic compounds in specially selected solvents at low temperatures. The Shpol’skii effect was first observed in 1952 by E. V. Shpol’skii and his co-workers L. A. Klimova and A. A. II’-ina.

The solvent used should be chemically neutral with respect to the solute molecules, should not interact with the solute molecules, and should be optically transparent in the spectral region where the solute molecules emit or absorb light. Liquid n-paraffin, for example, has such properties. The substance in question is dissolved in the solvent in low concentrations (10^–5–10^–7 g/cm³). The solution is then cooled to a temperature below the freezing point of the solvent; in the case of n-paraffin, the solution is cooled to –196°C or lower. Under such conditions, the molecules in question are isolated from one another and are fixed in the solvent. The electronic-vibrational emission and absorption spectra (seeMOLECULAR SPECTRA) of the molecules in question consist of series of narrow spectral lines and resemble atomic spectra. Hence, they are referred to as quasi-line spectra. Under ordinary conditions, the spectra of complex organic compounds are continuous overlapping bands.

Quasi-line spectra have a well-developed vibrational structure, making it possible to determine the vibrational frequencies of molecules not only in the ground state but also in excited states. Such spectra have a markedly individual character. They differ for molecules that are similar in structure and even for isomeric molecules. Quasi-line spectra make it possible to investigate the following: the structure of individual complex organic compounds; the physical and chemical properties of free complex organic radicals; photochemical processes for organic compounds; compounds that are important for life, such as chlorophyll and its analogs, the porphyrins; carcinogenic compounds; and individual organic compounds in the earth’s crust, in order to study the formation of hydrocarbons in the crust and to develop new methods of prospecting for petroleum and other minerals.

The spectrochemical analysis of mixtures on the basis of the Shpol’skii effect makes possible the simultaneous determination of several individual compounds in a mixture and has an absolute sensitivity of up to 10^–11 g.