Spectroscopy in inert gas cryogenic matrixes: Unusual Structures and Chemistry

Study of molecules trapped in inert gas cryogenic matrixes,
popularly referred to as matrix isolation, has been a popular technique
for many decades.  The solid inert gas matrix affords small linewidths for
the trapped molecules, enabling one to study conformations and weak
intermolecular interactions.  Matrix isolation has also been popularly
used to study intermediates in many photochemical reactions.  While the
idea of using inert gas matrixes is to trap molecules in an inert
environment, very often, the matrix, far from being inert, actually plays
a very subtle role in modifying the chemistry and spectroscopy of the
molecules trapped in such media.  While gas phase studies very often
provide information on the structure corresponding to the global minimum,
the matrix isolation experiment traps local minima, in addition to the
global minimum.  This aspect has been used to advantage to study
conformations and complexes that correspond to higher energy local minima.
Furthermore, the matrix, oftentimes, through their interactions, stabilize
complexes that are otherwise weakly bound. A classic example is the H-?
complex of the methanol-acetylene, which was observed only in a nitrogen
matrix.  Yet another interesting possibility is the role of the matrix in
frustrating certain reactions, which otherwise occur in a rather facile
manner in the gas phase; thus resulting in new reaction channels in the
matrix.  We will discuss such unusual structures and reactions occurring
in the inert gas matrix.