On Differences between Hydrogen Bonding and Improper Blue‐Shifting Hydrogen Bonding

Publisher Website
Google Scholar
Abstract
Twenty two hydrogen‐bonded and improper blue‐shifting hydrogen‐bonded complexes were studied by means of the HF, MP2 and B3LYP methods 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 or electrostatics—makes an important contribution as well. The symmetry‐adapted perturbation theory (SAPT) has been utilized to decompose the total interaction energy into physically meaningful contributions. In the red‐shifting 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 increases their red shift.