Recent advances in infrared matrix isolation spectroscopy. Invited lecture

Abstract

The matrix isolation technique has been employed for producing and trapping chemical species in inert matrices, typically in the ratio 1 : 1000 at 5–20 K. The matrices can be studied by a variety of spectroscopic techniques, including infrared (IR), Raman, visible, ultraviolet, fluorescence, Mössbauer, magnetic circular dichroism and electron spin resonance. This paper will be restricted to IR studies, as this is by far the most common technique for identifying the matrix isolated species. Convenient closed cycle He-cooled cryostats have made matrix isolation spectroscopy a common technique for various analytical applications. Combining these cryostats with Fourier transform IR spectrometers, matrix isolation spectroscopy is no longer a speciality employed by only a few specialists, but is now within the reach of many molecular spectroscopists. Some applications take advantage of the stabilization of free radicals and reactive species in the matrices; in others the reduced intermolecular interactions and absence of rotational fine structure make the matrices; suitable for high resolution spectra, frequently showing isotopic splitting. Weak H-bonds and charge transfer complexes can conveniently be studied in matrices. Various applications of IR isolation spectroscopy will be reviewed including studies of: (i) molecular ions; (ii) free radicals; (iii) complexes; (iv) clusters; (v) photochemical reactions with ozone; (vi) photoselection and infrared dichroism; and (vii) conformational equilibria. Studies of conformational equilibria will be discussed in some detail. Here the solute–matrix gas mixture is heated in a Knudsen cell or in a quartz nozzle to temperatures as high as 900 K before being quenched on the cold window. The high temperature thermodynamic equilibrium can be trapped and maintained in the matrix and evaluated from the spectra. By annealing (heating) the matrices, new thermodynamic equilibria can be achieved.