Blue-shifting hydrogen bonds—are they improper or proper?

In a review published in 2000 Hobza and Havlas coined the expression ‘improper, blue-shifting’ to distinguish a type of hydrogen bond which displays characteristics opposite to that expected for a conventional X–H· · ·Y hydrogen bond, i.e. the X–H bond length contracts rather than elongates and the X–H stretching mode shifts to higher frequency, often with reduced intensity, rather than shifting to lower frequency with enhanced intensity. In the present paper, the experimental evidence relating to blue-shifting hydrogen bonding, and the theoretical interpretations which have been advanced, are critically reviewed with the aim of answering the question posed in the title. q 2004 Elsevier B.V. All rights reserved.