Hydrogen bonds between pyrazine and HX linear acids (X = F, NC, CN and CCH): A theoretical study

Theoretical studies using the B3LYP and MP2 levels of calculation, with the 6-31++G** basis set, have been used to characterize hydrogen bonded complexes between pyrazine and HX linear acids with X = F, NC, CN and CCH. The molecular properties of these complexes and, the structural, electronic and vibrational changes that occur in the isolated species due to H-bond formation have been calculated and then, compared with those previously obtained in T-shaped complexes, involving simple unsaturated hydrocarbons as proton acceptors. Our calculations have shown that these changes in Pyz...HX complexes are much more pronounced than those observed in the pi...HX complexes, as well as their binding energies. BSSE corrections to the energy of binding-H estimated by the B3LYP results are approximately three times smaller than those obtained through MP2 results. Our calculations have also shown that stronger hydrogen bonds in the complexes Pyz...HX are associated with smaller H-bond length values, larger intermolecular charge-transfer and greater downward displacement to the stretching frequency H-X. The H-bond strength for these complexes follows the order: X = F > X = NC > X = CN > X = CCH. As expected, the H-X stretching intensity was enhanced substantially after the H-bond formation. Finally, the new vibrational modes that appear due to the complexation are also analyzed.