Studies of intermolecular interactions in the gas phase

After 2000, Jackowski et ai18-19 94-96 have published the first papers concerning 33S NMR spectroscopy in the gas phase. The reported results can be considered preliminary, but they do show evidence for the potential utility of this technique in the study of the effect of intermolecular interactions on 33S NMR parameters. 

It is difficult to predict whether gas phase NMR applied to the 33S nucleus can have a future. It cannot be ignored that the two major inconveniences in the observation of the 33S resonances are amplified in gas phase NMR. Indeed, signals are weaker than in the condensed phases, due to the lower density of the samples (even at high pressure), and they are also broader. At the moment, it seems that the use of this technique is restricted only to the study of small molecules. 

In the case of gaseous samples with low density, the nuclear magnetic shielding of a nucleus X in a molecule A can be expressed by a virial expansion as a power series in p density 17 

For low-density systems, higher order terms can be neglected, and Equation (8) 

If the density of A is low, only A-B collisions are significant, and cr, (73 x tAb(D-According to Equation (8 ), for low-density samples, a linear dependence of the nuclear shielding constant on gas density is generally observed, and a0(T) can be determined by extrapolation to infinite diluition.94 

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