Experimental and computational study of the thermochemistry of the three iodonitrobenzene isomers

Abstract

Thermochemical and thermodynamic properties of 2-, 3- and 4-iodonitrobenzene have been determined using a combination of calorimetric and effusion techniques as well as computational calculations. The standard (p°=0.1MPa) molar enthalpies of formation, in the crystalline state, ΔfHm∘(cr), at T=298.15K, were derived from the standard molar enthalpies of combustion, ΔcHm∘(cr), in oxygen, to yield CO2(g), N2(g), and I2(cr), at T=298.15K, measured by rotating bomb combustion calorimetry. The standard molar enthalpies of sublimation of these compounds, ΔcrgHm∘, at T=298.15K, were determined using high temperature Calvet microcalorimetry. The Knudsen mass-loss effusion technique was used to determine the standard molar enthalpies, entropies and Gibbs free energies of sublimation, at T=298.15K, of the three studied compounds. The combination of some of the referred thermodynamic parameters yielded the standard (po=0.1MPa) molar enthalpies of formation in the gaseous phase, at T=298.15K, of the three isomers: ΔfHm∘(2-iodonitrobenzene,g) = (178.8±1.5) kJ·mol-1, ΔfHm∘(3-iodonitrobenzene,g)=(155.4±1.8) kJ·mol-1 and ΔfHm∘(4-iodonitrobenzene,g)=(151.8±1.6) kJ·mol-1. The results were analyzed and interpreted in terms of enthalpic increments and molecular structure. Using the empirical scheme developed by Cox, the values of the standard molar enthalpies of formation in the gaseous phase were estimated and afterwards compared with the ones obtained experimentally, being both interpreted in terms of the molecular structure of the compounds. For comparison purposes, standard molecular calculations at the B3LYP6-311++G(d, p) level were performed, and the gas-phase enthalpies of formation of the three compounds were estimated; the results are in good agreement with experimental data. Furthermore, the molecular structures of the three molecules were established and the structural parameters were determined at the B3LYP/6-311++G(d,p) level of theory. The computational study was also extended to the determination of proton and electron affinities, basicities, and adiabatic ionization enthalpies.