Computational Chemistry-Based Enthalpy-of-Formation, Enthalpy-of-Vaporization, and Enthalpy-of-Sublimation Predictions for Azide-Functionalized Compounds

The applicability of semi-empirical models for estimating the gas-phase enthalpies-of-formation [deltafHog (298)], enthalpies-of-vaporization [ deltaHv (298)], and enthalpies-of-sublimation [deltaHs, (298)] of azide-functionalized compounds was evaluated. The models, which rely on B3LYP/6-31G(d) characterizations of the electronic properties of an isolated molecule of a compound, include (1) an atom-equivalent (AE) approach for estimating deltafHog (298) and (2) correlations for estimating deltaHv(298) and deltaH (298) from properties of the electrostatic potential on an electron isodensity surface. Based on the validation effort, the need to add an equivalent to the AE model that is tailored for azide groups was identified and addressed. However, the AE model's estimates are still prone to systematic error, and deltafHoe (298) estimates derived from B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) calculations are recommended. DeltaHv (298) estimates for azide-functionalized compounds proved to be in reasonable agreement with values derived from experiments and from molecular dynamics simulations. Direct validation of deltaHs (298) estimates was not obtained, but deltafHos (298) estimates derived from deltafHog (298) and deltaHs (298) estimates were found to be in reasonable agreement with deltafHos (298) values derived from experiments. With the validity of the models for azide-functionalized compounds so established, best estimates were obtained for a set of compounds with multiple azide groups that was synthesized by the U.S. Army Armament Research, Development, and Engineering Center.