The Vaporization Enthalpies of 2- and 4-(N,N-Dimethylamino)pyridine, 1,5-Diazabicyclo[4.3.0]non-5-ene, 1,8-Diazabicyclo[5.4.0]undec-7-ene, Imidazo[1,2-a]pyridine and 1,2,4-Triazolo[1,5-a]pyrimidine by Correlation–Gas Chromatography

• Published in: The journal of physical chemistry. B 2011-07, Vol.115 (27), p.8785-8796
• Main Authors: Lipkind, Dmitry, Rath, Nigam, Chickos, James S, Pozdeev, Vasiliy A, Verevkin, Sergey P
• Format: Article
• Language: English
• Subjects: B: Statistical Mechanics, Thermodynamics, Medium Effects; Correlation; Derivatives; Enthalpy; Heterocyclic compounds; Pyridines; Stacking; Vaporization
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp2041857

The vaporization enthalpies of a series of heterocyclic compounds some suspected of showing evidence of a higher degree of self-association in comparison to many other nitrogen heterocycles have been measured both by correlation gas chromatography (CGC) and for comparison by transpiration. The compounds share some structural similarities. They include 2- and 4-(N,N-dimethylamino)pyridine, 1,5-diazabicyclo[4.3.0]non-5-ene, 1,8-diazabicyclo[5.4.0]undec-7-ene, 1,2,4-triazolo[1,5-a]pyrimidine and imidazo[1,2-a]pyridine. The vaporization enthalpies of the first three of the compounds are well reproduced by a standard series of reference compounds, mainly pyridine derivatives. The latter two compounds and 4-(N,N-dimethylamino)pyridine exhibit vaporization enthalpies approximately 7 kJ·mol–1 larger. Their values are reproduced by using standards previously shown to exhibit stronger but similar self-association properties. The crystal structure of 1,2,4-triazolo[1,5-a]pyrimidine has been determined in an effort to understand the nature of the self-association. The molecule crystallizes in planar stacks slightly offset with a stacking distance of 3.24 Å.