Aromaticity and stability going in opposite directions: An energetic,structural, magnetic and electronic study of aminopyrimidines |
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| Institution: | 1. Department of Physics, Faculty of Science, University of Yüzüncü Yıl, 65080 Van, Turkey;2. Department of Chemistry, Faculty of Science, University of Yüzüncü Yıl, 65080 Van, Turkey;3. Department of Chemistry, Faculty of Science and Arts, University of Gaziantep, 27310 Gaziantep, Turkey;1. Pharmaceutical Chemistry, School of Pharmacy, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa;2. Centre of Excellence for Pharmaceutical Sciences, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa;3. Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Private Bag 3, PO WITS 2050, South Africa;1. Department of Food Processing, Vocational High School, Kilis 7 Aralik University, Kilis, Turkey;2. Department of Chemistry, Faculty of Arts & Sciences, Gaziantep University, Gaziantep, Turkey;3. Department of Opticianry, Vocational High School of Health Services, Kilis 7 Aralik University, Kilis, Turkey;4. Department of Physics, Faculty of Arts & Sciences, Ondokuz Mayıs University, Samsun, Turkey;5. Department of Secondary School Science and Mathematics Education, Faculty of Education, Ondokuz Mayıs University, Samsun, Turkey;1. Key Laboratory of Structure-Based Drug Design and Discovery (Shenyang Pharmaceutical University), Ministry of Education, Shenyang 110016, China;2. School of Biomedical Sciences, Huaqiao University, Quanzhou 362021, China |
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| Abstract: | The relation between molecular energetics and aromaticity was investigated for the interaction between the amino functional group and the nitrogen atoms of the pyridine and pyrimidine rings, using experimental thermodynamic techniques and computational geometries, enthalpies, chemical shifts, atomic charges and the Quantum Theory of Atoms in Molecules. 2,4-diaminopyrimidine and 2,4,6-triaminopyrimidine were studied by static bomb combustion calorimetry and Knudsen effusion technique. The derived gaseous-phase enthalpies of formation together with the enthalpies of formation of the three isomers of aminopyridine reported in the literature, were compared with the calculated computationally ones and extended to other diamino- and triaminopyrimidine isomers using the MP2/6-311++G(d,p) level of theory.The results were analyzed in terms of enthalpy of interaction between substituents and, due to the absence of meaningful stereochemical hindrance, strong inductive effects, or intramolecular hydrogen bonds according to QTAIM results, the resonance electron delocalization plays an almost exclusive role in the very exothermic enthalpies obtained. Therefore, this enthalpy of interaction was used as an experimental energetic measure of resonance effects and analyzed in terms of aromaticity. It was found that more conjugation between substituents means less aromaticity according to the magnetic (NICS) and electronic (Shannon) criteria, but more aromaticity according to the geometric (HOMA) criterion. |
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