General Papers ARKIVOC 2014 (v) 376-383
Singlet carbene stability: linear free-energy analysis of substituent effects
Ronald R. Sauers
Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, NJ 08903, United States E-mail:
[email protected]
Abstract
The energies of a series of substituted methylene and methylcarbene singlet states were computed by quantum mechanical calculations at the CCSD(T)/cc-pVDZ//M06-2X/cc-pVDZ level. Stabilization energies relative to singlet methylene were correlated with polarizability, polar and resonance Hammett sigma constants using multiple regression analysis. The results revealed that most of the carbenes gave rise to inverse polar effects. It is concluded that separation of polar and resonance effects via linear free energy methods (LFE) fails for singlet carbenes with substituents directly attached to carbene carbons.
Keywords: Singlet carbene stability, linear free energy correlations, quantum computations
Introduction
The unique ability of carbenes to both donate and accept electron pairs has long been appreciated. Thus, singlet aminocarbene is stabilized by resonance donation of an electron pair from nitrogen into the empty carbon p-orbital (Eq. 1). By the same token, singlet cyanocarbene can be stabilized by donation of electron density from the filled p-orbital of the carbenic carbon into the π orbital network of the CN triple bond (Eq. 2).
The relative importance of the inductive properties of the substituents has received less attention. Since both the amino group and the cyano group are characterized as electron-
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