Nuclear Spin Conversion in Molecules

摘要

Molecules with identical nuclei having nonzero spin can exist in different states called nuclear spin modifications by most researchers and nuclear spin isomers by some. Once prepared in a particular state, the arrangement of spins can in principle convert to another arrangement. The concept of nuclear spin modifications in molecules is intriguing to chemists because interconversion between different spin states is often slow enough to be considered negligible for many purposes. This in turn allows one to treat different spin modifications as though they were different molecules both in terms of spectroscopy and collision dynamics. In addition, these spin modifications are associated one-to-one with different rotational levels in the molecule. In fact, however, the interconversion rates are not zero, so the scientific questions become: How can these extremely slow rates be measured, and how can these rates be explained by theory? On page 1938 of this issue, Sun et al. present answers to both of these questions in the case of the ethylene molecule.