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Download fileCarbene Stabilization by Aryl Substituents. Is Bigger Better?
journal contribution
posted on 2007-03-28, 00:00 authored by H. Lee Woodcock, Damian Moran, Bernard R. Brooks, Paul v. R. Schleyer, Henry F. SchaeferThe geometries and relative stabilities of the singlet and triplet states of phenyl- (Cs), diphenyl-
(C2), 1-naphthyl- (Cs), di(1-naphthyl)- (C2), and 9-anthryl-substituted (Cs) carbenes were investigated at
the B3LYP/6-311+G(d,p) + ZPVE level of density functional theory. The singlet−triplet energy separations
(ΔEST), 2.7, 2.9, 3.4, 3.7, and 5.7 kcal/mol, respectively, after including an empirical correction (2.8 kcal/mol) based on the error in the computed singlet−triplet gap for methylene versus experiment, are in good
agreement with available experimental values. Consistent with literature reports, triplet di(9-anthryl)carbene
has a linear, D2d symmetrical, allene structure with 1.336 Å CC bond lengths and considerable biradical
character. B3LYP favors such cumulene biradical structures and triplet spin states and predicts a large
(>15 kcal/mol) “di(9-anthryl)carbene” singlet−triplet (biradical) energy gap. The resonance stabilization of
both singlet and triplet carbenes increases modestly with the size of the arene substituent and overall,
(di)arylcarbenes, both singlet and triplet, are better stabilized by bigger substituents. For example, methylene
is stabilized more by a naphthyl than a phenyl group (singlets, 26.6 versus 24.4; and triplets, 20.9 versus
18.1 kcal/mol, respectively). The carbene geometries are affected by both steric effects and arene−carbene
orbital interactions (σ−p and p−π). For instance, the central angles at the carbene are widened by a second
arene group, which leads to increased s-character and shorter carbene bond lengths (i.e., C̈-C, C̈-H). In
general, the aromaticity of the substituted rings in triplet carbenes is most affected by the presence of the
unpaired electrons.
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D 2dtriplet carbenesarene groupliterature reportscarbene bond lengthssteric effectsCarbene StabilizationZPVE levelcarbene geometrieskcalAryl Substituentsb 3LYPcumulene biradical structuresarene substituentresonance stabilizationbiradical characterESTtriplet carbenes increasesB 3LYPsinglettriplet statesenergy gapallene structurephenyl group