Effects of lactam ring size on the thermodynamics of hydrogen bonding in CCl4 solutions: experimental and ab initio studies

The thermodynamic properties of hydrogen bonding among lactams with ring si zes n = 4, 5 and 6 were investigated using infrared spectroscopy of the fun damental N-H stretching frequencies of both the monomer and dimer species. The results are consistent with a predominantly monomer-dimer equilibrium w ith the dimer having C-2 symmetry. The thermodynamic properties (in CCl4) f or the four-, five- and six-membered lactam rings are K-d (at 25 degrees C) = 12.4(9), 24(6) and 25(3), Delta H-d (kJ mol(-1)) = -30(2), -30(1) and -2 8(2) and Delta S-d (J mol(-1) K-1) = -79(4), -74(3) and -67(5), respectivel y (precisions in parentheses). The experimentally determined thermodynamic properties are compared with ab initio calculations (6-31G** basis set) whi ch represent gas-phase results. The comparison of these two approaches yiel ds a picture which is consistent with the notion that the primary effect of ring size is an entropy of solvation effect and not the enthalpy of hydrog en bonding between the monomer units.