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

Abstract

The thermodynamic properties of hydrogen bonding among lactams with ring sizes n=4, 5 and 6 were investigated using infrared spectroscopy of the fundamental N–H stretching frequencies of both the monomer and dimer species. The results are consistent with a predominantly monomer–dimer equilibrium with the dimer having C₂ symmetry. The thermodynamic properties (in CCl₄) for the four-, five- and six-membered lactam rings are K_d (at 25°C)=12.4(9), 24(6) and 25(3), ΔH_d (kJ mol^-1)=-30(2), -30(1) and -28(2) and ΔS_d (J mol^-1 K^-1)=-79(4), -74(3) and -67(5), respectively (precisions in parentheses). The experimentally determined thermodynamic properties are compared with abinitio calculations (6–31G** basis set) which represent gas-phase results. The comparison of these two approaches yields 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 hydrogen bonding between the monomer units.