On differences between hydrogen bonding and improper blue-shifting hydrogen bonding

Abstract

Twenty two hudrogen-bonded and improper blue-shifting hydrogen-bonded complexes were studied by means of the HF, MP2 and B3LYP mehtod using the 6-31G(d,p) and 6-311++G(d,p) basis sets. In contrast to the standard H bonding, the origin of the improper blue-shifting H bonding is still not fully understood. Contrary to a frequently presented idea, the electric field of the proton acceptor cannot solely explain the different behavior of the H-bonded and improper blue-shifting H-bonded complexes. Compression of the hydrogen bond due to different attractive forces-dispersion of electrostatics-makes an important contribution as well. The symmetry-adapted perturbation theory (SAPT) has been utilizd to decompose the total interaction energy into physically meaningful contributions. In the red-sgifting complexes, the induction energy is mostly larger than the dispersion energy while, in the case of blue-shifting complexes, the situation is opposite. Dispersion as an attractive force increases the blue-shift in the blue-shifting complexes as it compresses the H bond and, therefore, it increases the Pauli repulsion. On the other hand, dispersion in the red-shifting complexes their red shift. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.