Isotopomeric Conformational Change in Anisole-Water

摘要

Isotopic substitution is generally considered not to perturb the structure of a molecular system. It affects, however, the molecular spectroscopy of the system, especially the frequencies of rotational transitions, which depend on moments of inertia. Differences in moments of inertia among isotopomers represent, in turn, the best tool for the determination of molecular structure.[1] Furthermore, changes in chemical properties, such as kinetics[2] and equilibrium constants,[3] are well known, and it has been observed that the temperature of spontaneous phase transitions can vary,[4] sometimes by as much as 25 K.[5] A smaller modification (relative to that described herein) of the structure of a system through the so-called geometric isotope effect was outlined by Ichikawa.[6] However, when isotopically labeled substances are used, the usually justified assumption is made that they do not alter the fundamental nature of the material under study. Reasonable information on the geometry of molecular complexes, for example of complexes of water with ethers, has been obtained upon deuteration of the water moiety. Limitations to this assumption are due to a remarkable shrinking (Ubbelohde effect[7]) of the distance between the two heavy atoms involved in the hydrogen bond upon deuteration. Such an effect has been observed both in crystals[7] and in the gas phase.[8] Zhou et al. showed very recently that this assumption is unjustified for the complex of pentachlorophenol with 4-methylpyridine, because the substitution of a single hydrogen atom for a deuterium atom leads to the thermodynamic stability of an entirely different crystal polymorph. [9] We show herein that a similar large effect is observed for the gas-phase configuration of the molecular complex anisole-water (ANIwater; see Figure 1).