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A Valence Bond Study of the Bergman Cyclization: Geometric Features, Resonance Energy, and Nucleus‐Independent Chemical Shift (NICS) Values
Author(s) -
Galbraith John Morrison,
Schreiner Peter R.,
Harris Nathan,
Wei Wu,
Wittkopp Alexander,
Shaik Sason
Publication year - 2000
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/(sici)1521-3765(20000417)6:8<1446::aid-chem1446>3.0.co;2-i
Subject(s) - chemistry , valence (chemistry) , wave function , valence bond theory , density functional theory , computational chemistry , resonance (particle physics) , transition state , atomic physics , chemical shift , bond length , aryne , crystallography , molecule , physics , molecular orbital , organic chemistry , crystal structure , biochemistry , catalysis
The Bergman cyclization of ( Z )‐hex‐3‐ene‐1,5‐diynes ( 1 , enediynes), which produces pharmacologically important DNA‐cleaving biradicals (1,4‐benzyne, 2 ), was studied by using Hartree‐Fock (HF) and density‐functional theory (DFT) based valence bond (VB) methods (VB‐HF and VB‐DFT respectively). We found that only three VB configurations are needed to arrive at results not too far from complete active space {CASSCF(6×6)} computations, while the quality of VB‐DTF utilizing the same three configurations improves upon CASSCF(6×6) analogous to CASPT2. The dominant VB configuration in 1 contributes little to 2 , while the most important biradical configuration in 2 plays a negligible role in 1 . The avoided crossing of the energy curves of these two configurations along the reaction coordinate leads to the transition state ( TS ). As a consequence of the shape and position of the crossing section, the changes in geometry and in the electronic wavefunction along the reaction coordinate are non‐synchronous; the TS is geometrically ≈80 % product‐like and electronically ≈70 % reactant‐like. While the π resonance in the TS is very small, it is large (64.4 kcal mol −1 ) for 2 (cf. benzene=61.5 kcal mol −1 ). As a consequence, substituents operating on the σ electrons should be much more effective in changing the Bergman reaction cyclization barrier. Furthermore, additional σ resonance in 2 results in unusually high values for the nucleus‐independent chemical shift (NICS, a direct measure for aromaticity). Similarly, the high NICS value of the TS is due mostly to σ resonance to which the NICS procedure is relatively sensitive.