Abstract
The cooperativity between hydrogen bond and halogen bond is investigated by analyzing the bond energy and bond length of three sets of nearly linear trimer systems: X─HCN─HF, X─HCN─BrF, and X─HCN─Br2. The symbol X refers to X-ligand and includes C2H2, H2O, NH3, HCl, HCN, HF. Three computational levels, MP2, mPW1PW91, and B3LYP are used to carry out the theoretical computations with the 6-311++G(d, p) basis set in the Gaussian 98 package. Different sequences are observed in bond lengths and bond energies of the hydrogen bonds and halogen bonds in trimers. The concept of the Hammett equation is applied to examine the sequence of the hydrogen-bond and halogen-bond energies in trimers. Linear relationships between the cooperativity and the bond energies of the bonding between the nitrogen of HCN and hydrogen of HF in trimers are observed. The intercepts are found to be the corresponding dimer bond energies without cooperativity effects. The deviation of the intercepts from calculated dimer bond energies is less than 2.7 %. The BSSE effect on the linear correlations is examined, and it does not change the chemical properties of trimer systems with a variation less than 0.103 for the slopes of the linear correlation. The slopes of these linear correlations show the reaction sensitivity to the change of X-ligands in the interactions between hydrogen bond and halogen bond. By using the slopes, the cooperativity in the three trimer groups can be estimated with acceptable accuracy.