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Mechanism and dynamics of organic reactions: 1,2‐H shift in methylchlorocarbene
Author(s) -
Kraka Elfi,
Cremer Dieter
Publication year - 2002
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.495
Subject(s) - chemistry , arrhenius equation , quantum tunnelling , curvature , activation energy , adiabatic process , reaction mechanism , carbene , thermodynamics , exothermic reaction , computational chemistry , potential energy , isomerization , chemical physics , atomic physics , quantum mechanics , physics , organic chemistry , catalysis , geometry , mathematics
The unified reaction valley approach (URVA) was used to investigate the mechanism of the rearrangement of methylchlorocarbene to chloroethene [reaction(1)] in the gas phase with special emphasis on the role of H tunneling. The reaction valley of (1) was explored using different methods (HF, MP2 and DFT/B3LYP) and different basis sets [6–31G(d), 6–31G(d,p) and cc‐pVTZ]. Results were analyzed characterizing normal modes, reaction path vector and curvature vector in terms of generalized adiabatic modes associated with internal parameters that are used to describe the reaction complex. For reaction (1), H tunneling plays a significant role even at room temperature, but does not explain the strongly curved Arrhenius correlations observed experimentally. The probability of H tunneling can be directly related to the curvature of the reaction path and the associated curvature couplings. The reaction is preceeded in the forward and reverse direction by energy‐consuming conformational changes that prepare the reactant for the actual 1,2‐H shift, which requires only little energy. The effective energy needed for CH bond breaking is just 6 kcal mol −1 for (1). The gas‐phase and the solution‐phase mechanisms of (1) differ considerably, which is reflected by the activation enthalpies: 11.4 (gas, calculated) and 4.3 kcal mol −1 (solution, measured). Stabilizing interactions with solvent molecules take place in the latter case and reduce the importance of H tunneling. The non‐linearity of the measured Arrhenius correlations most likely results from bimolecular reactions of the carbene becoming more important at lower temperatures. Copyright © 2002 John Wiley & Sons, Ltd.