Enthalpies of combustion, formation, vaporization and sublimation of organics

Abstract

R e p le 2 octobre 1990 R. SABBAH et E. N. L. E. BULUKU. Can. J. Chem. 69,481 (1991). Le prtsent travail porte sur 1'Ctude thermodynamique des trois isomeres du dihydroxybenzene. Cette ttude a permis la dCtermination des enthalpies de combustion, de sublimation, de fusion et de transition des 1,2-, 1,3-et 1,4-dihydroxybenzenes. Les techniques utilisCes ont Ct C la calorimCtrie de combustion de faibles quantitCs de substances, la calorimCtrie de changement d'Ctat (sublimation), l'analyse thermique differentielle et la mesure des capacitts calorifiques. L'ensemble des rCsultats expCrimentaux a permis de : mettre en Cvidence l'existence d'une liaison hydrogkne intramolCculaire dans l'isomkre ortho; prkciser la stabilitk relative des trois molCcules de dihydrobenzenes; et determiner : i partir de l'enthalpie de sublimation, une valeur CnergCtique pour les liaisons intermolCculaires; 1'Cnergie de conjugaison expkrimentale et de la comparer i la valeur thtorique; l'enthalpie d'atomisation 3 partir de laquelle a Ct C calculCe I'enthalpie de la liaison intramolCculaire Cb-OH. Mots clts : thermodynamique, calorimCtrie, analyse thermique differentielle, 1,2-dihydroxybenzkne ou catCcho1; 1,3-di-hydroxybenzkne ou rCsorcino1; 1,4-dihydroxybenzene ou hydroquinone ou quinol; enthalpies de combustion, de sublimation, de fusion, de transition; Cnergie de conjugaison; enthalpies d'atomisation, de liaisons inter-et intra-molCculaires; temperature du point triple et de transition. R. SABBAH and E. N. L. E. BULUKU. Can. J. Chem. 69,481 (1991). The present work is concerned with a thermodynamic study of the three isomers of dihydroxybenzene. By combustion calorimetry of small amounts of substance, sublimation calorimetry, differential thermal analysis, and heat capacity measurements, it was possible to determine the enthalpies of formation of 1,2-, 1 3 , and 1,4-dihydroxybenzene in the condensed and gaseous phases, their enthalpies of fusion and transition, and the temperature of their triple point and transition. The experimental results are used to discuss the relative stability and determine the conjugation energy of the three compounds. The values are in good agreement with the theoretical values obtained from a quantum chemical calculation. The enthalpies of atomization enabled us to determine an energy value for the intramolecular Cb-OH bond in dihydroxybenzenes and to correlate it with previous results obtained from a study of alkane-diols. The enthalpies of sublimation were discussed. An intramolecular hydrogen bond was displayed in the ortho isomer. Intermolecular hydrogen bonds associated with van der Waals interactions exist in dihydroxybenzenes. Their energy contributions were determined.

Evaluation of Force-Field Calculations of Lattice Energies on a Large Public Dataset, Assessment of Pharmaceutical Relevance, and Comparison to Density Functional Theory

“…In principle, calculations based upon crystal structures offer the ability to rank polymorph stability and support crystal structure prediction, albeit the SUB-BIG dataset does not offer insights into this (see SUB-BIG Dataset under Methods and Data). However, the level of predictive performance obtained with the recommended force-field protocol, as estimated using the SUB-BIG dataset (Table ), indicates it is far too inaccurate to capture the typically small energetic differences between polymorphs. , Indeed, it would be interesting to see how well methods based purely on molecular structure ,, compare to the force-field protocol recommended herein based upon the SUB-BIG dataset. For example, Cardozo reported a group contribution method with a standard deviation of 15 kJ/mol (experimental vs. calculated sublimation enthalpies) on an, unidentified, set of 115 organic compounds .…”

“…However, the level of predictive performance obtained with the recommended force-field protocol, as estimated using the SUB-BIG dataset (Table ), indicates it is far too inaccurate to capture the typically small energetic differences between polymorphs. , Indeed, it would be interesting to see how well methods based purely on molecular structure ,, compare to the force-field protocol recommended herein based upon the SUB-BIG dataset. For example, Cardozo reported a group contribution method with a standard deviation of 15 kJ/mol (experimental vs. calculated sublimation enthalpies) on an, unidentified, set of 115 organic compounds . This is comparable to the value obtained (17 kJ/mol) using the recommended force-field protocol (estimated experimental vs. calculated lattice energies), coupled with the heuristic filtering criteria, on all applicable 226 SUB-BIG entries.…”

“…For example, Cardozo reported a group contribution method with a standard deviation of 15 kJ/mol (experimental vs. calculated sublimation enthalpies) on an, unidentified, set of 115 organic compounds. 84 This is comparable to the value obtained (17 kJ/mol) using the recommended force-field protocol (estimated experimental vs. calculated lattice energies), coupled with the heuristic filtering criteria, on all applicable 226 SUB-BIG entries. However, whether these statistics are directly comparable in light of possible differences in data distributions 2,65 or whether the apparent performance would be comparable in terms of other statistics (Table 2) is an open question.…”