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

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(dp) and 6-311++G(dp) 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 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.