Blue-shifting intramolecular CH O(S) contacts in sterically strained systems

A group of model systems which may form chelate-type structures with intramolecular CH  Y (Y = O, S) contact is investigated computationally. The existence of several conformers permits to identify a reference molecule without the CH  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 CH  Y contact is interpreted in terms of the sterically enforced repulsion between the hydrogen atom in CH and the electron donor Y. This interpretation is supported by the negative (repulsive) estimates of the energy contribution due to CH  Y contacts.