A comparison of acetyl- and methoxycarbonylnitrenes by computational methods and a laser flash photolysis study of benzoylnitrene

Density functional theory (DFT), CCSD(T), and CBS-QB3 calculations were performed to understand the chemical and reactivity differences between acetylnitrene (CH(3)C(=O)N) and methoxycarbonylnitrene (CH(3)OC(=O)N) and related compounds. CBS-QB3 theory alone correctly predicts that acetylnitrene has a singlet ground state. We agree with previous studies that there is a substantial N-O interaction in singlet acetylnitrene and find a corresponding but weaker interaction in methoxycarbonylnitrene. Methoxycarbonylnitrene has a triplet ground state because the oxygen atom stabilizes the triplet state of the carbonyl nitrene more than the corresponding singlet state. The oxygen atom also stabilizes the transition state of the Curtius rearrangement and accelerates the isomerization of methoxycarbonylnitrene relative to acetylnitrene. Acetyl azide is calculated to decompose by concerted migration of the methyl group along with nitrogen extrusion; the free energy of activation for this concerted process is only 27 kcal/mol, and a free nitrene is not produced upon pyrolysis of acetyl azide. Methoxycarbonyl azide, on the other hand, does have a preference for stepwise Curtius rearrangement via the free nitrene. The bimolecular reactions of acetylnitrene and methoxycarbonylnitrene with propane, ethylene, and methanol were calculated and found to have enthalpic barriers that are near zero and free energy barriers that are controlled by entropy. These predictions were tested by laser flash photolysis studies of benzoyl azide. The absolute bimolecular reaction rate constants of benzoylnitrene were measured with the following substrates: acetonitrile (k = 3.4 x 10(5) M(-1) (s-1)), methanol (6.5 x 10(6) M(-1) s(-1)), water (4.0 x 10(6) M(-1) s(-1)), cyclohexane (1.8 x 10(5) M(-1) s(-1)), and several representative alkenes. The activation energy for the reaction of benzoylnitrene with 1-hexene is -0.06 +/- 0.001 kcal/mol. The activation energy for the decay of benzoylnitrene in pentane is -3.20 +/- 0.02 kcal/mol. The latter results indicate that the rates of reactions of benzoylnitrene are controlled by entropic factors in a manner reminiscent of singlet carbene processes.     

THE PHOTOCHEMISTRY OF 3-NITROBENZOYL AND 4-NITROBENZOYL AZIDES: POSSIBLE REAGENTS FOR PHOTOAFFINITY LABELING

Abstract— The photochemistry of 3- and 4-nitrobenzoyl azides was studied to examine their suitability for use in photolabeling procedures. These aroyl azides absorb light in the near UV-spectral region. Their irradiation leads both to photo-Curtius rearrangement with formation of the analogous isocyan-ates and to generation of nitrenes by loss of nitrogen. The chemical and spectroscopic properties of the aroyl nitrenes indicate that they are singlets in their ground states. These singlet nitrenes insert rapidly into unactivated, aliphatic carbon-hydrogen bonds, trap nucleophiles, and add to olefins to form aziridines with retention of stereochemistry. Photolabeling agents based on these azides may offer some advantages over conventional agents for the labeling of lipophilic sites.     

Understanding the rate of spin-forbidden thermolysis of HN3 and CH3N3

The pyrolysis of the simplest azides HN(3) and CH(3)N(3) has been studied computationally. Nitrogen extrusion leads to the production of NH or CH(3)N. The azides have singlet ground states but the nitrenes CH(3)N and NH have triplet ground states. The competition between spin-allowed decomposition to the excited state singlet nitrenes and the spin-forbidden N(2) loss is explored using accurate electronic structure methods (CASSCF/cc-pVTZ and MR-AQCC/cc-pVTZ) as well as statistical rate theories. Nonadiabatic rate theories are used for the dissociation leading to the triplet nitrenes. For HN(3), (3)NH formation is predicted to dominate at low energy, and the calculated rate constant agrees very well with energy-resolved experimental measurements. Under thermal conditions, however, the singlet and triplet pathways are predicted to occur competitively, with the spin-allowed product increasingly favored at higher temperatures. For CH(3)N(3) thermolysis, spin-allowed dissociation to form (1)CH(3)N should largely dominate at all temperatures, with spin-forbidden formation of (3)CH(3)N almost negligible. Singlet methyl nitrene is very unstable and should rearrange to CH(2)NH immediately upon formation, and the latter species may lose H(2) competitively with vibrational cooling, depending on temperature and pressure.     

In Search of Singlet Phosphinidenes

We have examined singlet-triplet energy separations in different phosphinidenes (RP) substituted by first- and second-row elements, making use of ab initio molecular orbital theory. Our main purpose is to find out the substituents that particularly favor the singlet electronic state. The QCISD(T)/6-311++G(3df,2p) + ZPE level has been applied to small molecules and the CISD(Q) and QCISD(T) with the 6-311G(d,p) basis set for all species considered. We have identified few factors that come into play rendering the singlet phosphinidene more stable than the triplet. The parent phosphinidene, PH, has a triplet ground state lying 28 kcal/mol below the closed-shell singlet excited state. The triplet ground state is mainly favored when negative hyperconjugation is involved. In the boryl-, alkyl-, and silyl-substituted phosphinidenes, the triplet state remains by far the ground state. When the substituents have pi-type lone pair electrons (i.e., -NX(2), -PX(2), -OX, -SX), the singlet state becomes stabilized by such an amount that both states have similar energies or even a change in ground state occurs. The most stabilized singlet ground states are attributed to PSF and PSCl. P and S have similar p-orbital sizes, making pi-delocalization easier. Implantation of alkyl and/or amino groups in the beta-position of amino- and phosphinophosphinidenes also contributes to a singlet stabilization. Bulky beta-groups also destabilize the triplet state by a steric effect. From a practical viewpoint, amino (P-NR(2)) and phosphino (P-PR(2)) derivatives bearing large alkyl groups (R) are the most plausible and feasible targets for preparing phosphinidenes possessing a closed-shell singlet ground state.     

Bonding and structure of copper nitrenes

Copper nitrenes are of interest as intermediates in the catalytic aziridination of olefins and the amination of C-H bonds. However, despite advances in the isolation and study of late-transition-metal multiply bonded complexes, a bona fide structurally characterized example of a terminal copper nitrene has, to our knowledge, not been reported. In anticipation of such a report, terminal copper nitrenes are studied from a computational perspective. The nitrene complexes studied here are of the form (beta-diketiminate)Cu(NPh). Density functional theory (DFT), complete active space self-consistent-field (CASSCF) electronic structure techniques, and hybrid quantum mechanical/molecular mechanical (QM/MM) methods are employed to study such species. While DFT methods indicate that a triplet (S = 1) is the ground state, CASSCF calculations indicate that a singlet (S = 0) is the ground state, with only a small energy gap between the singlet and triplet. Moreover, the ground-state (open-shell) singlet copper nitrene is found to be highly multiconfigurational (i.e., biradical) and to possess a bent geometry about the nitrene nitrogen, contrasting with the linear nitrene geometry of the triplet copper nitrenes. CASSCF calculations also reveal the existence of a closed-shell singlet state with some degree of multiple bonding character for the copper-nitrene bond.     

Photochemistry of ortho, ortho' dialkyl phenyl azides

Phenyl azide, 2,6-diethylphenyl azide, 2,6-diisopropylphenyl azide, and 2,4,6-tri-tert-butylphenyl azide were studied by laser flash photolysis (LFP) methods. LFP (266 nm) of the azides in glassy 3-methylpentane at 77 K produces the transient UV-vis absorption spectra of the corresponding singlet nitrenes. At 77 K, the singlet nitrenes relax to the corresponding triplet nitrenes. The triplet nitrenes are persistent at 77 K and their spectra were recorded. The rate constants of singlet to triplet intersystem crossing were determined at this temperature. LFP of 2,4,6-tri-tert-butyl phenyl azide in pentane at ambient temperature again produces a singlet nitrene, which is too short-lived to detect by nanosecond spectroscopy under these conditions. Unlike the other azides, the first detectable intermediate produced upon LFP of 2,4,6-tri-tert-butyl phenyl azide at ambient temperature is the benzazirine (285 nm) which has a lifetime of 62 ns controlled by ring opening to a didehydroazepine. The results are interpreted with the aid of Density Functional Theoretical and Molecular Orbital Calculations.     

Nitrenes and the Decomposition of Carbonylazides

The decomposition of organic carbonylazides can lead to the formation of nitrenes. Ethoxycarbonylnitrene is formed in the photolytic and thermal decomposition of ethyl azidoformate and by α-elimination from N-(p-nitrobenzenesulfonyloxy)urethan. Both of the possible electronic states of this nitrene take part in intermolecular reactions. Pure singlet nitrene is formed by α-elimination from the urethan and on thermal decomposition of ethyl azidoformate, but changes so rapidly into the triplet form that the reactions of both forms are observed. Singlet ethoxycarbonylnitrene undergoes selective and stereospecific insertion into C? H bonds and adds stereospecifically to olefins. Triplet ethoxycarbonylnitrene, however, does not undergo insertion into C? H bonds, and adds to olefins with complete loss of the geometric configuration. By following quantitatively the stereospecificity of the addition reaction and by selective interception of the triplet and singlet forms of the nitrene, it can be shown that the photolysis of ethyl azidoformate leads directly to nitrene of which one third is in the triplet state. In the decomposition of aryl- and alkylcarbonylazides (acid azides), the removal of nitrogen is accompanied by a synchronous rearrangement to isocyanates (Curtius rearrangement). In this system, nitrenes are obtained only by photolysis. They add to double bonds and undergo very selective insertion into C? H bonds, but do not rearrange at a measurable rate to isocyanates. The photolytic Curtius rearrangement is also a concerted reaction.     

Carbenes and Nitrenes: Recent Developments in Fundamental Chemistry

Carbenes and nitrenes can exist in both singlet and triplet states, sometimes equally stable and interconverting either thermally or photochemically. Many carbene and nitrene reactions proceed via tunneling at low temperatures. Numerous singlet and triplet states have been characterized spectroscopically, and a detailed understanding of the chemical and physical properties of carbenes and nitrenes is emerging. There has been significant progress in the direct observation of carbenes, nitrenes, and many other reactive intermediates in recent years through the application of matrix photolysis and flash vacuum pyrolysis linked with matrix isolation at cryogenic temperatures. Our understanding of singlet and triplet states has improved through the interplay of spectroscopy and computations. Bistable carbenes and nitrenes as well as many examples of tunneling have been discovered and numerous rearrangements and fragmentations have been documented. The correlation of the zero‐field splitting parameter D with calculated spin densities on nitrenes and carbenes is discussed. This Minireview gives an overview of some of these developments.     

The Curtius rearrangement of cyclopropyl and cyclopropenoyl azides. A combined theoretical and experimental mechanistic study

A combined experimental and theoretical study addresses the concertedness of the thermal Curtius rearrangement. The kinetics of the Curtius rearrangements of methyl 1-azidocarbonyl cycloprop-2-ene-1-carboxylate and methyl 1-azidocarbonyl cyclopropane-1-carboxylate were studied by (1)H NMR spectroscopy, and there is close agreement between calculated and experimental enthalpies and entropies of activation. Density functional theory (DFT) calculations (B3LYP/6-311+G(d,p)) on these same acyl azides suggest gas phase barriers of 27.8 and 25.1 kcal/mol. By comparison, gas phase activation barriers for the rearrangement of acetyl, pivaloyl, and phenyl azides are 27.6, 27.4, and 30.0 kcal/mol, respectively. The barrier for the concerted Curtius reaction of acetyl azide at the CCSD(T)/6-311+G(d,p) level exhibited a comparable activation energy of 26.3 kcal/mol. Intrinsic reaction coordinate (IRC) analyses suggest that all of the rearrangements occur by a concerted pathway with the concomitant loss of N2. The lower activation energy for the rearrangement of methyl 1-azidocarbonyl cycloprop-2-ene-1-carboxylate relative to methyl 1-azidocarbonyl cyclopropane-1-carboxylate was attributed to a weaker bond between the carbonyl carbon and the three-membered ring in the former compound. Calculations on the rearrangement of cycloprop-2-ene-1-oyl azides do not support pi-stabilization of the transition state by the cyclopropene double bond. A comparison of reaction pathways at the CBS-QB3 level for the Curtius rearrangement versus the loss of N2 to form a nitrene intermediate provides strong evidence that the concerted Curtius rearrangement is the dominant process.     

p-Phenylenecarbenonitrene and its halogen derivatives: how does resonance interaction between a nitrene and a carbene center affect the overall electronic configuration?

A series of para-conjugatively coupled phenylenecarbenonitrenes [(4-nitrenophenyl)methylene (3a), (4-nitrenophenyl)fluoromethylene (3b), (4-nitrenophenyl)chloromethylene (3c), and (4-nitrenophenyl)bromomethylene (3d)] were generated in argon matrix at low temperature (10 or 13 K) and characterized by IR and UV/vis spectroscopy. Density functional theory (B3LYP/6-31G(d)) and ab initio (MCSCF, CASPT2) methods were used to study the ground- and some low-lying excited states of 3a-d. The experimental and computational data suggest that 3a-d have singlet ground states (S0) and can be thought of as quinonoidal biradicals. In all cases, the lowest triplet (T1) and quintet (Q1) states lie about 2 kcal mol(-1) and 28-29 kcal mol(-1), respectively, higher in energy than S0. On the other hand the substituent is found to have a significant effect on the relative energy of the second excited triplet (T2) state. This state tends to become relatively more stable as the ability of the substituent to enforce a closed-shell configuration at the carbene subunit increases. Interestingly, the energy difference between the T2 and S0 states in 3a-d is found to depend linearly on the S-T gap of the corresponding phenylcarbenes 7a-d. This relationship is helpful in predicting when a substituted p-phenylenecarbenonitrene may have a triplet ground state instead of a singlet one.     

Phenyloxenium ions: more like phenylnitrenium ions than isoelectronic phenylnitrenes?

The geometries and energies of the electronic states of phenyloxenium ion 1 (Ph-O(+)) were computed at the multireference CASPT2/pVTZ level of theory. Despite being isoelectronic to phenylnitrene 4, the phenyloxenium ion 1 has remarkably different energetic orderings of its electronic states. The closed-shell singlet configuration ((1)A(1)) is the ground state of the phenyloxenium ion 1, with a computed adiabatic energy gap of 22.1 kcal/mol to the lowest-energy triplet state ((3)A(2)). Open-shell singlet configurations ((1)A(2), (1)B(1), (1)B(2), 2(1)A(1)) are significantly higher in energy (>30 kcal/mol) than the closed-shell singlet configuration. These values suggest a revision to the current assignments of the ultraviolet photoelectron spectroscopy bands for the phenoxy radical to generate the phenyloxenium ion 1. For para-substituted phenyloxenium ions, the adiabatic singlet-triplet energy gap (ΔE(ST)) is found to have a positive linear free energy relationship with the Hammett-like σ(+)(R)/σ(+) substituent parameters; for meta substituents, the relationship is nonlinear and negatively correlated. CASPT2 analyses of the excited states of p-aminophenyloxenium ion 5 and p-cyanophenyloxenium ion 10 indicate that the relative orderings of the electronic states remain largely unperturbed for these para substitutions. In contrast, meta-donor-substituted phenyloxenium ions have low-energy open-shell states (open-shell singlet, triplet) due to stabilization of a π,π* diradical state by the donor substituent. However, all of the other phenyloxenium ions and larger aryloxenium ions (naphthyl, anthryl) included in this study have closed-shell singlet ground states. Consequently, ground-state reactions of phenyloxenium ions are anticipated to be more closely related to closed-shell singlet arylnitrenium ions (Ar-NH(+)) than their isoelectronic arylnitrene (Ar-N) counterparts.     

Diarylphosphinic azides: Photochemical reactions including rearrangement in methanol

On photolysis in methanol the diarylphosphinic azides AR₂P(O)N₃ (Ar = phenyl,p-tolyl,p-anisyl,p-chlorophenyl) rearragement with loss of nitrogen to form (monemeric) metaphosphonimidates which are trapped by the solvent to give methyl NP-diarylphosphonamidates (7) (41–53%). Diarylphosphinic amides (18–42%) are also usually formed, presumably from (triplet) nitrenes. The limited evidence available suggests that the rearrangements take place directly from the photo-excited azides rather than via (singlet) nitrene intermediates. One of the products of rearrangement, methyl NP-di(p-chlorophenyl)phosphonamidate, suffers extensive photochemical dechlorination giving methyl N-phenyl-P-p-chlorophenylphosphonamidate.     

Photochemistry of N-acetyl-, N-trifluoroacetyl-, N- mesyl-, and N-tosyldibenzothiophene sulfilimines

Time-resolved infrared (TRIR) spectroscopy, product studies, and computational methods were applied to the photolysis of sulfilimines derived from dibenzothiophene that were expected to release acetylnitrene, trifluoroacetylnitrene, mesylnitrene, and tosylnitrene. All three methods provided results for acetylnitrene consistent with literature precedent and analogous experiments with the benzoylnitrene precursor, i.e., that the ground-state multiplicity is singlet. In contrast, product studies clearly indicate triplet reactivity for trifluoroacetylnitrene, though TRIR experiments were more ambiguous. Product studies suggest that these sulfilimines are superior sources for sulfonylnitrenes, which have triplet grounds states, to the corresponding azides, and computational studies shed light on the electronic structure of the nitrenes.     

Spin-state dependent radical stabilization in nitrenes: the unusually small singlet-triplet splitting in 2-furanylnitrene

Geometries and energies of the triplet and singlet states of 2-furanylnitrene and 3-furanylnitrene have been calculated by using spin-flip coupled-cluster methods. Calculations with triple-ζ basis sets predict a singlet-triplet splitting of 10.9 kcal/mol for 2-furanylnitrene, 4.5 kcal/mol smaller than that in phenylnitrene. In contrast, the singlet-triplet splitting in 3-furanylnitrene is computed to be 1.9 kcal/mol larger than that in phenylnitrene. The differences in the singlet-triplet splittings for the furanylnitrenes are attributed to the differences in the radical stabilizing abilities of the 2-furanyl- and 3-furanyl-groups compared to a phenyl ring. Comparison of the singlet-triplet splittings of more than 20 substituted aromatic nitrenes and the radical stabilizing ability of the aromatic systems reveals a high degree of correlation between the singlet-triplet splitting and the radical stabilizing ability, indicating that singlet states of aromatic nitrenes are preferentially stabilized by radical stabilizing substituents. The preferential stabilization of the singlet states is attributed to the decrease in electron pair repulsion resulting from increased delocalization of the radical electron.     

Rearrangements of Aromatic Nitrile Oxides and Nitrile Ylides: Potential Ring Expansion to Cycloheptatetraene Derivatives Mimicking Arylcarbenes

Nitrile imines, nitrile oxides and nitrile ylides are widely used in 1,3-dipolar cycloaddition reactions. They also undergo thermal and photochemical rearrangements to carbodiimides, isocyanates, and ketenimines, respectively. Calculations at DFT and CASPT2 levels of theory reveal novel, potential rearrangements, in which the aromatic 1,3-dipoles mimic phenylcarbene and undergo ring expansion to cycloheptatetraene derivatives. These rearrangements can potentially take place in both the singlet ground states and the triplet excited states, and they are accelerated by m,m’-bis(dimethylamino) substitution on the phenyl moieties. The new rearrangement becomes the energetically preferred path for m,m’-bis(dimethylamino)benzonitrile oxide in the triplet state. In the m,m’-bis(dimethylamino)benzo nitrile ylide, the cyclization to the 2-phenyl-1-azirine is favored over the ring expansion to a cycloheptatetraene by ca. 5 kcal mol⁻¹ in the singlet state. In the bent triplet states, 1,3-hydrogen shifts interconverting nitrile ylides are potentially possible.     

Photoinduced C-N bond cleavage in 2-azido-1,3-diphenyl-propan-1-one derivatives: photorelease of hydrazoic acid

Photolysis of 3-azido-1,3-diphenyl-propan-1-one (1a) in toluene yields 1,3-diphenyl-propen-1-one (2), whereas irradiation of 3-azido-2,2-dimethyl-1,3-diphenyl-propan-1-one (1b) results in the formation of mainly 2,2-dimethyl-1,3-diphenyl-propan-1-one. Laser flash photolysis (308 nm) of 1a,b in acetonitrile reveals a transient absorption (lambda max = approximately 310 nm) due to the formation of radicals 4a and 4b, respectively, which have lifetimes of approximately 14 micros at ambient temperature. TD-DFT calculations (B3LYP/6-31+G(d)) reveal that the first and second excited states of the triplet ketone (T1K (n,pi*) and T2K (pi,pi*)) in azide 1a are almost degenerate, at approximately 74 and 76 kcal/mol above the ground state (S0), respectively. Similarly, azide 1b has T1K and T2K 75 and 82 kcal/mol above S0, respectively. The calculated transition state for cleaving the C-N bond is located 71 and 74 kcal/mol above S0 in azides 1a and 1b, respectively. The calculated bond dissociation energies for breaking the C-N bond are 55 and 58 kcal/mol for azides 1a and 1b, respectively, making C-N bond breakage accessible from T1K in azides 1 at ambient temperature. In comparison, the irradiation of azides 1 in argon matrices at 14 K lead to the formation of the corresponding triplet alkyl nitrenes (1-n), via intramolecular energy transfer from T2K. The characterization of 1-n was supported by isotope labeling, IR spectroscopy, and molecular modeling.     

Ab initio calculation of the lowest singlet and triplet states in CH2, CHF,CF2, and CHCH3

The lowest singlet and triplet states of the radicals CH₂, CHF, CF₂, and CHCH₃ have been investigated both in SCF and IEPA approximation (“independent electron pair approach” to account for electron correlation). The SCF calculations yield triplet ground states for CH₂, CHF, and CHCH₃, and a singlet ground state for CF₂. Electron correlation stabilizes the singlet state by about 14 kcal/mole with respect to the triplet for all four radicals leading to a singlet ground state also for CHF. The final triplet-singlet energy separations are 10, 6, ?11, ?47 kcal/mole for CH₂, CHCH₃, CHF, CF₂, respectively. Values for equilibrium bond angles, ionization potentials and bond energies are also given.     

The rearrangements of naphthylnitrenes: UV/Vis and IR spectra of azirines, cyclic ketenimines, and cyclic nitrile ylides

Ar matrix photolysis of 1- and 2-naphthyl azides 3 and 4 at 313 nm initially affords the singlet naphthyl nitrenes, (1)()1 and (1)()2. Relaxation to the corresponding lower energy, persistent triplet nitrenes (3)()1 and (3)()2 competes with cyclization to the azirines 15 and 18, which can also be formed photochemically from the triplet nitrenes. On prolonged irradiation, the azirines can be converted to the seven-membered cyclic ketenimines 10 and 13, respectively, as described earlier by Dunkin and Thomson. However, instead of the o-quinoid ketenimines 16 and 19, which are the expected primary ring-opening products of azirines 15 and 18, respectively, we observed their novel bond-shift isomers 17 and 20, which may be formally regarded as cyclic nitrile ylides. The existence of such ylidic heterocumulenes has been predicted previously, but this work provides the first experimental observation of such species. The factors which are responsible for the special stability of the ylidic species 17 and 20 are discussed.     

理论研究V2O6-的结构和电子性质

The geometrical and electronic properties of the anionic and neutral V2O6 clusters were studied with the spin unrestricted hybrid density functional B3LYP method. The calculated ground states of both clusters are different from the previous theoretical results. The ground state of V2O6- is found to be a doublet with C2v symmetry, while a doublet with D2h symmetry was previously obtained by Vyboishchikov and Sauer. For neutral V2O6, the ground state is an open-shell singlet with D2h symmetry whose energy is very close to that of the triplet state. In contrast, a closed-shell singlet with D2h symmetry was obtained by Vyboishchikov and Sauer, and Calatayud et al. found a triplet ground state with Cs symmetry. Moreover,the calculated adiabatic and vertical detachment energies of the anion cluster are in much better agreement with the experimental results of photoelectron spectroscopy than previous theoretical values.     

The almost bottleable triplet carbene: 2,6-dibromo-4-tert-butyl-2',6'-bis(trifluoromethyl)-4'-isopropyldiphenylcarbene.

Computations on 2,6-dibromo-4-tert-butyl-2',6'-bis(trifluoromethyl)-4'-isopropyldiphenylcarbene (1) using ab initio and density functional theory methods underscore the unusual stability of the triplet over the singlet state. At the B3LYP/6-311G(d,p) level, the triplet state had a slightly bent central C-C-C bond angle of 167 degrees, whereas this angle in the singlet was 134 degrees. The B3LYP singlet-triplet splitting (12.2 kcal/mol) was larger than that of the parent molecule (5.8 kcal/mol), diphenylcarbene (2), which also has a triplet ground state. The energy of a suitable isodesmic reaction showed the triplet and singlet states of (1) to be destabilized, by 6.3 and 12.5 kcal/mol, respectively, due to the combined effects of the CF3, Br, and alkyl substituents. The linear-coplanar form of (3)(1), which might facilitate dimerization or electrophilic attack at the more exposed diradical center, was prohibitively (35.9 kcal/mol) higher in energy. Our results confirm Tomioka's conclusion that the triplet diarylcarbene, ortho-substituted with bulky CF3 and Br substituents, is persistent due to steric protection of the diradical center. Dimerization and other possible reaction pathways are inhibited, not only by the bulky ortho substituents but also by the para alkyl groups. The increase in stability of the triplet ((3)(1)) state relative to the singlet ((1)(1)) state does not influence the reactivity directly.