The thermal conductivities of H2O and D2O vapors were measured in a modified thick hot wire cell between 358 and 386 K at pressures ranging from 100 to 1000 Torr. Analysis of the data indicates that molecular association to form a dimeric species is the main source of enhancement of the thermal conductivity of both vapors. The enthalpy and entropy of association of the H2O dimer are −3.59 kcal mol−1 and −18.59 cal deg−1 mol−1, respectively. The enthalpy and entropy of association of the D2O dimer are −3.66 kcal mol−1 and −18.67 cal deg−1 mol−1, respectively. The measured enthalpy of association of the H2O dimer is in agreement with recently reported ab initio molecular orbital calculations on the H2O dimer. The entropies of association of the H2O and D2O dimers are calculated theoretically and are found to be in agreement with the measured values.

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Thermal conductivity measurements are reported for the vapors of acetic acid and trifluoroacetic acid in the temperature range 350-415 K over the pressure range Torr. Very large enhancements of the thermal conductivity were found and analysis of the data is consistent with a monomer-dimer equilibrium for both vapors. The standard thermodynamic parameters for the dimerization reactions are -AH2 = 14.64 kcal mol-1 and -AS2 = 35.50 cal mol-1 K_1 for acetic acid, and -AH2 = 13.66 kcal mol™"1 and -AS2 = 36.15 cal mol-1 K_1 for trifluoroacetic acid. No evidence was found for the presence of significant amounts of associated species larger than the dimer. Ab initio molecular orbital calculations were performed on the monomers and dimers of both acids to determine the lowest energy structures. Both dimers are found to contain two nearly parallel C=0-H-O hydrogen bonds.

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