On the interpretation of H.F.S. in molecules in terms of molecular structure and nuclear moments

Conclusion

In cases where the effect of strongly penetratingp-valence electrons is important, such as in the halogens, it is possible to make a fairly good evaluation ofq. In the light elements, however, where the valence effects do not predominate, and in the elements with other than strongly penetratingp-valence electrons, such as the alkalis, effects such as close shell distortions become important and evaluation ofq is much more difficult.

We have seen that h.f.s. interaction in molecules can be used as a sensitive probe of molecular structure, but for gettingQ the problem is more difficult. In experiments with microwave spectroscopy, molecular beams, r.f. spectroscopy, etc., one obtains values foreqQ to very high accuracy, but values ofq are not known to better than 10 or 30% for molecules. In atomic spectroscopy one can evaluateq to a higher accuracy, but here the experimental hyperfine splitting is seldom known to better than 30%. Thus, except in the case of deuterium, where the coupling constanteqQ has been measured with greatprecision andq can be calculated fairly accurately because the molecular wave function of hydrogen is rather well known, measurements of nuclear quadrupole moments cannot be expected to be much more accurate than about 25%. The quadrupole moment of the deuteron is probably known within an error of 2%. There are in addition a few cases where atomic h.f.s. has been measured by r.f. spectroscopy. In these cases the accuracy of determination of quadrupole moments is limited to about 5 % by our lack of knowledge of the polarization of closed shells by the valence electrons.