Infrared Spectroscopy at Subambient Temperatures

The development of low-temperature IR spectroscopy has been rather sporadic. The first published investigations appear to be those of Hettner [1] in 1932. Little further effort was devoted to the area until the 1940s when several investigators became interested in the various spectral effects which took place with both simple and complex molecules on lowering the temperature. This work in the 1940s and early 1950s has been reviewed by Sheppard [2] and by Hornig [3]. Most of the theoretical principles involved were elucidated in this period and a good deal of the potential of the technique was envisioned. Nevertheless, low-temperature IR spectroscopic investigations during the next 10 to 15 years were limited primarily to applications in the area of conformational isomerism. Workers in this area normally are interested only in gross spectral changes occurring during liquid-solid phase changes. In the late 1960s there occurred a general revival of interest in low-temperature IR spectroscopy in another context. Various investigators began to develop the potential of the technique for structural and molecular dynamic studies and to define its widespread applicability to chemical problems. A part of this recent work, as well as the earlier work, has been reviewed by Hermann and co-workers [4–6]. These review articles are not extensive but do have a very complete bibliography up to 1968. In addition, a later review [7] briefly illustrates some of the effects of low temperature on the IR spectra of complex molecules and how these effects can be informative with regard to details of molecular structure and dynamics.