| Abstract
| - Two model systems, 3-methylacroleine and 3-(difluoromethyl)acroleine, are investigated computationally withrespect to the character of the C−H···O interaction in their chelate-type (ZZ) conformers. By selecting theappropriate reference conformers, the C−H···O interaction is shown to result in the increase of the C−Hstretching frequency (i.e., in the blue shift of the C−H stretching band). This is accompanied by the shorteningof the C−H bond distance as compared to its values in reference molecules. Parallel to calculations of theC−H bond distance and stretching frequency, the energy contribution of the C−H···O interaction to the totalenergy (i.e., the energy associated with the C−H···O contact) is evaluated by using the methods proposedrecently for the estimation of the energies of intramolecular hydrogen bonds. It is found that the C−H···Ocontact in the chelate-type forms of 3-methylacroleine and 3-(difluoromethyl)acroleine corresponds to thenegative energy contribution and is repulsive. It is concluded that, despite the stability of the ZZ conformersof the two molecules and their deceptive structural shape, no hydrogen bond in the usual sense is formedbetween the C−H bond and the lone pair donor. The results are interpreted in terms of the steric compression,which leads to the dominance of the valence repulsion contribution in the C−H···O contact. This mechanismsuggests that blue-shifting intramolecular interactions should not be that uncommon, although their recognitionrequires a careful consideration of the reference system.
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