Substituent effects on the blue-shifted hydrogen bonds

For the blue-shifted hydrogen bonds between CF3H and phenols, fluorobenzenes, and anilines, a donor para-substituent increases the interaction energy causing larger blue shift. For the red-shifted hydrogen bond between CF3H and phenolate, a donor para-substituent also increases the interaction energy but causing larger red shift. These behaviors indicate that the blue shift is attributable to the attraction between the positively charged carbon of CF3H and the hydrogen bond acceptor. This attraction forces the hydrogen-bond donor and acceptor to come so close to each other that the hydrogen of CF3H feels considerable Pauli repulsion from the acceptor, resulting in C-H contraction. (C) 2002 Elsevier Science B.V. All rights reserved.