Azo dicarboxylates are not conjugated: X-ray crystal structure and theoretical calculations on di-t-butylazodicarboxylate
| Autor: | Mean See Goh, Dennis P. Arnold, Llew Rintoul, John C. McMurtrie, Michael C. Pfrunder |
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| Rok vydání: | 2015 |
| Předmět: |
chemistry.chemical_classification
Double bond 010405 organic chemistry Organic Chemistry Crystal structure Dihedral angle 010402 general chemistry 01 natural sciences 0104 chemical sciences Analytical Chemistry Inorganic Chemistry Diethyl azodicarboxylate Crystallography chemistry.chemical_compound chemistry Computational chemistry Molecule Density functional theory Spectroscopy Cis–trans isomerism Natural bond orbital |
| Zdroj: | Journal of Molecular Structure. 1098:298-305 |
| ISSN: | 0022-2860 |
| DOI: | 10.1016/j.molstruc.2015.06.027 |
| Popis: | The X-ray crystal structure of trans-di-t-butyl azodicarboxylate (DTBAD, 2) was determined and this revealed that the torsion angle between the N N and C O double bonds is 84.0(2)°, and that between the anti-disposed C O vectors is 180°. This is the first report of the solid state structure of an azodicarboxylate ester. The molecule was subjected to Density Functional Theory geometry optimization at the B3LYP/6-31G(d) level in cyclohexane medium, and the global minimum structure agreed in principle with that determined in the solid state by crystallography. The N–C(O) torsion angle in the optimized structure is 107.7°, and the C O vectors lie in an anti relationship. Similar calculations on the unknown cis-N N isomer revealed an optimum geometry whose energy is predicted to lie only 11.9 kJ/mol higher than that of the trans isomer. M062X/6-311+G(d) model chemistry was used to determine relative electronic energies and to conduct Natural Bond Orbital (NBO) calculations. Exploration of the energetics of rotations about the N–C(O) bonds revealed a clear preference for near-orthogonality in azodicarboxylates, and suggests almost complete absence of classical conjugation between the neighbouring π bonds. Electronic transitions were simulated using the time-dependent DFT (TD-DFT) approach at the B3LYP/6-311+G(d) level, and the weak band in the near-UV for 2 in cyclohexane was reproduced in the calculations. The electronic isolation of the N N bond may be important in the numerous applications of azodicarboxylates in organic synthesis, and the small energy difference between the trans and cis isomers implies the likely involvement of the latter in the successful photochemical diaza-Diels–Alder reaction of diethyl azodicarboxylate with 1,3-cyclohexadiene. |
| Databáze: | OpenAIRE |
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