Blue-shifting intramolecular C-H center dot center dot center dot O(S) contacts in sterically strained systems

摘要

A group of model systems which may form chelate-type structures with intramolecular C-H... Y (Y = O, S) contact is investigated computation ally. The existence of several conformers permits to identify a reference molecule without the C-H... Y intramolecular contact and to establish the blue-shifting character of this interaction. The CH stretching frequency in chelate forms is found to increase with respect to its value in the reference system. A parallel decrease of the CH bond distance is also established. The blue-shifting character of the intramolecular C-H... Y contact is interpreted in terms of the sterically enforced repulsion between the hydrogen atom in C-H and the electron donor Y. This interpretation is supported by the negative (repulsive) estimates of the energy contribution due to C-H... Y contacts. (C) 2007 Elsevier B.V. All rights reserved.