Halogenation of cubane under phase-transfer conditions: single and double C-H-bond substitution with conservation of the cage structure.

The first highly selective C-H chlorination, bromination, and iodination of cubane (1) utilizing polyhalomethanes as halogen sources under phase-transfer (PT) conditions is described. Isomeric dihalocubanes with all possible combinations of chlorine, bromine, and iodine in ortho, meta, and para positions were also prepared by this method; m-dihalo products form preferentially. Ab initio and density functional theory (DFT) computations were used to rationalize the pronounced differences in the reactions of 1 with halogen (Hal(*)) vs carbon-centered trihalomethyl (Hal(3)C(*)) radicals (Hal = Cl, Br). For Hal(3)C radicals the C-H abstraction pathway is less unfavorable (DeltaG(double dagger)(298) = 21.6 kcal/mol for Cl(3)C(*) and 19.4 kcal/mol for Br(3)C(*) at B3LYP/6-311+G//B3LYP/6-31G) than the fragmentation of the cubane skeleton via S(H)2-attack on one of the carbon atoms of 1 (DeltaG(double dagger)(298) = 33.8 and 35.1 kcal/mol, respectively). In stark contrast, the reaction of 1 with halogen atoms preferentially follows the fragmentation pathway (DeltaG(double dagger)(298) = 2.1 and 7.5 kcal/mol) and C-H abstraction is more unfavorable (DeltaG(double dagger)(298) = 4.6 and 12.0 kcal/mol). Our computational results nicely agree with the behavior of 1 under PT halogenation conditions (where Hal(3)C(*) is involved in the activation step) and under free-radical photohalogenation with Hal(2) (Della, E. W., et al. J. Am. Chem. Soc. 1992, 114, 10730). The incorporation of a second halogen atom preferentially in the meta position of halocubanes demonstrates the control of the regioselectivity by molecular orbital symmetry.     

Singlet hydrocarbon carbenes with high barriers toward isomerization: a computational investigation

A prerequisite for a stable singlet hydrocarbon carbene is the existence of high barriers toward isomerization. Four derivatives of cyclopentylidene (1-4) with rigid and varying carbon cages are examined computationally at the B3LYP/6-311+G(d,p) level of theory. Singlet ground states are predicted for carbenes 1-4, with DeltaE(ST)'s = 7-22 kcal/mol. The rearrangement paths considered are 1,3-hydrogen shift, 1,2-carbon shift and beta-CC bond-cleavage. Carbenes 3 and 4 lie in relatively shallow potential-energy wells (around 4 and 6 kcal/mol, respectively) and are expected to rearrange via 1,3-hydrogen shifts to cyclopropane derivatives. For 1 and 2, the lowest energy rearrangement path is beta-CC bond-cleavage requiring about 12 and 20 kcal/mol, respectively, placing 2 in the deepest potential-energy well among the four carbenes.     

NH tautomerism in the natural chlorin derivatives

The NH tautomerism of five Mg-free chlorophyll a and b derivatives 2-6 was studied utilizing NMR spectroscopy and molecular modeling. The results from the dynamic NMR measurements of the chlorins revealed that substituent effects contribute crucially to the free energy of activation (DeltaG(double dagger)) in the NH tautomeric processes. An intermediate tautomer for the total tautomeric NH exchange in a chlorin was observed for the first time, when the (1)H NMR spectra of chlorin e(6) TME (3) and rhodin g(7) TME (4) (TME = trimethyl ester) were measured at lower temperatures. The lower energy barriers (DeltaG(1)(double dagger)) obtained for the formation of the intermediate tautomers of 3 and 4, assigned to the N(22)-H, N(24)-H trans-tautomer, were 10.8 and 10.6 kcal/mol, respectively. The energy barrier (DeltaG(2)(double dagger) value) for the total tautomeric NH exchange in the five chlorins was found to vary from 13.6 kcal/mol to values higher than 18 kcal/mol. The lowest DeltaG(2)(double dagger) value (13.6 kcal/mol) was obtained for rhodochlorin XV dimethyl ester (2), which was the only chlorophyll derivative lacking the C(15) substituent. In the case of chlorins 4 and 5, the steric crowding around the methoxycarbonylmethyl group at C(15) raised the DeltaG(2)(double dagger) activation free-energy to 17.1 kcal/mol. However, the highest energy barrier with DeltaG(2)(double dagger) > 18 kcal/mol was observed for the NH exchange of pyropheophorbide a methyl ester (6), possessing the macrocycle rigidifying isocyclic ring E. Our results demonstrate that the steric strain, arising either from the steric crowding around the bulky substituent at C(15) or the macrocycle rigidifying isocyclic ring E, slows down the NH tautomeric process. We suggest that deformations in the chlorin skeleton are closely connected to the NH tautomeric exchange and that the exchange occurs by a stepwise proton-transfer mechanism via a hydrogen bridge.     

The nature and extent of pi-stabilization within foiled carbenes

B3LYP/6-311+G(d,p) computations of the stabilization energies, singlet-triplet energy gaps, and lowest transition states for a set of cyclic alkenylidenes were performed in order to find the strongest interactions between the C-C double bond and the carbene center. The results suggest that among the alkenylidenes investigated in this study, those with a norbornenylidene structure represent strongly stabilized carbenes with a reduced reactivity toward intermolecular reactions. Further stabilization is found when the double bond is electron-rich or pyramidalized. Thus, for the rearrangement of syn-34 to take place, an activation barrier of about 22 kcal/mol needs to be overcome. The inclination to undergo a retro-Skatteb?l rearrangement, which to our knowledge has never been observed experimentally, is characteristic for highly stabilized foiled carbenes.     

Structure, strain, and degenerate rearrangement of tricyclo[2.1.0.0 1,3]pentasilane and related molecules

We theoretically investigate a highly strained tricyclic silane (tricyclo[2.1.0.0 1,3]pentasilane (4b), an isomer of pentasila[1.1.1]propellane (3b)) composed of three fused three-membered rings. The central ring is distorted. One of the fusion bonds in the central ring is shorter than the normal Si-Si single bond (2.350 A) whereas the other is as long as the fusion bonds in bicyclo[1.1.0]tetrasilane (2b) (2.860 A) and 3b (2.778 A). The tricyclic silane is less strained than the carbon congener and more strained than the isomer 3b. The electron delocalization between one of the fusion bonds and the geminal Si-Si ring bonds elongates the fusion bond and stabilizes the molecules to reduce the strain. The silanes composed of the fused three-membered rings are less strained than the carbon congener. A degenerate rearrangement of a three-membered ring is predicted. The enthalpy of activation of the rearrangement of the distorted central ring is low (7.2 kcal/mol) for 4b, but appreciable (22.3 kcal/mol) for the germanium congener, tricyclo[2.1.0.0(1,3)]pentagermane (4c). We investigate the effects of the substituents on the distortion of the central three-membered ring and the degenerate rearrangement.     

Accurate heats of formation of the "Arduengo-type" carbene and various adducts including H2 from ab initio molecular orbital theory

The heats of formation of saturated and unsaturated diaminocarbenes (imadazol(in)-2-ylidenes) have been calculated by using high levels of ab initio electronic structure theory. The calculations were done at the coupled cluster level through noniterative triple excitations with augmented correlation consistent basis sets up through quadruple. In addition, four other corrections were applied to the frozen core atomization energies: (1) a zero point vibrational correction; (2) a core/valence correlation correction; (3) a scalar relativistic correction; (4) a first-order atomic spin-orbit correction. The value of DeltaHf( 298) for the unsaturated carbene 1 is calculated to be 56.4 kcal/mol. The value of DeltaHf( 298) for the unsaturated triplet carbene (3)1 is calculated to be 142.8 kcal/mol, giving a singlet-triplet splitting of 86.4 kcal/mol. Addition of a proton to 1 forms 3 with DeltaHf( 298)(3) = 171.6 kcal/mol with a proton affinity for 1 of 250.5 kcal/mol at 298 K. Addition of a hydrogen atom to 1 forms 4 with DeltaHf( 298)(4) = 72.7 kcal/mol and a C-H bond energy of 35.8 kcal/mol at 298 K. Addition of H- to 1 gives 5 with DeltaHf( 298)(5) = 81.2 kcal/mol and 5 is not stable with respect to loss of an electron to form 4. Addition of H2 to the carbene center forms 6 with DeltaHf( 298)(6) = 41.5 kcal/mol and a heat of hydrogenation at 298 K of -14.9 kcal/mol. The value of DeltaHf( 298) for the saturated carbene 7 (obtained by adding H2 to the C=C bond of 1) is 47.4 kcal/mol. Hydrogenation of 7 to form the fully saturated imidazolidine, 8, gives DeltaHf( 298)(8) = 14.8 kcal/mol and a heat of hydrogenation at 298 K of -32.6 kcal/mol. The estimated error bars for the calculated heats of formation are +/-1.0 kcal/mol.     

Ring inversion dynamics of derivatives of thianthrene di- and tetraoxide

The ring inversion barrier for thianthrene tetraoxide was determined by making use of the variable temperature 13C NMR spectra of the 2,7-diisopropyl derivative (DeltaG(double dagger) = 6.5 kcal mol(-1)). The barrier is lower than that measured for a trans thianthrene dioxide derivative (DeltaG(double dagger) = 9.35 kcal mol(-1)). These results agree well with ab initio theoretical predictions.     

Formation and stability of N-heterocyclic carbenes in water: the carbon acid pKa of imidazolium cations in aqueous solution

We report second-order rate constants kDO (M-1 s-1) for exchange for deuterium of the C(2)-proton of a series of simple imidazolium cations to give the corresponding singlet imidazol-2-yl carbenes in D2O at 25 degrees C and I = 1.0 (KCl). Evidence is presented that the reverse protonation of imidazol-2-yl carbenes by solvent water is limited by solvent reorganization and occurs with a rate constant of kHOH = kreorg = 10(11) s-1. The data were used to calculate reliable carbon acid pK(a)s for ionization of imidazolium cations at C(2) to give the corresponding singlet imidazol-2-yl carbenes in water: pKa = 23.8 for the imidazolium cation, pKa = 23.0 for the 1,3-dimethylimidazolium cation, pKa = 21.6 for the 1,3-dimethylbenzimidazolium cation, and pKa = 21.2 for the 1,3-bis-((S)-1-phenylethyl)benzimidazolium cation. The data also provide the thermodynamic driving force for a 1,2-hydrogen shift at a singlet carbene: K12 = 5 x 10(16) for rearrangement of the parent imidazol-2-yl carbene to give neutral imidazole in water at 298 K, which corresponds to a favorable Gibbs free energy change of 23 kcal/mol. We present a simple rationale for the observed substituent effects on the thermodynamic stability of N-heterocyclic carbenes relative to a variety of neutral and cationic derivatives that emphasizes the importance of the choice of reference reaction when assessing the stability of N-heterocyclic carbenes.     

Conformational analysis of alkylated biuret and triuret: evidence for helicity and helical inversion in oligoisocyanates

The conformations of several oligoisocyanates have been investigated by NMR in order to study the onset and dynamics of helicity in polyisocyanates. Pentaethylbiuret and hexaethyltriuret were found to adopt turns and helices in solution. For hexaethyltriuret, symmetric and asymmetric helices are present. Not only is an interconversion of these forms observed (DeltaG(SA)(double dagger) = 9.3 +/- 0.4 kcal/mol) but also a reversal of helicity (DeltaG(PM)(double dagger) = 9.0 +/- 0.4 kcal/mol). The coalescence pattern for the latter process provides direct evidence for a concerted, conrotatory helical inversion.     

The ozonolysis of acetylene--a quantum chemical investigation.

The ozonolysis of acetylene was investigated using CCSD(T), CASPT2, and B3LYP-DFT in connection with a 6-311+G(2d,2p) basis set. The reaction is initiated by the formation of a van der Waals complex followed by a [4pi + 2pi] cycloaddition between ozone and acetylene (activation enthalpy DeltaH(a)(298) = 9.6 kcal/mol; experiment, 10.2 kcal/mol), yielding 1,2,3-trioxolene, which rapidly opens to alpha-ketocarbonyl oxide 5. Alternatively, an O atom can be transferred from ozone to acetylene (DeltaH(a)(298) = 15.6 kcal/mol), thus leading to formyl carbene, which can rearrange to oxirene or ketene. The key compound in the ozonolysis of acetylene is 5 because it is the starting point for the isomerization to the corresponding dioxirane 19 (DeltaH(a)(298) = 16.9 kcal/mol), for the cyclization to trioxabicyclo[2.1.0]pentane 10 (DeltaH(a)(298) = 19.5 kcal/mol), for the formation of hydroperoxy ketene 15 (DeltaH(a)(298) = 20.6 kcal/mol), and for the rearrangement to dioxetanone 9 (DeltaH(a)(298) = 23.6 kcal/mol). Compounds 19, 10, 15, and 9 rearrange or decompose with barriers between 13 and 16 kcal/mol to yield as major products formanhydride, glyoxal, formaldehyde, formic acid, and (to a minor extent) glyoxylic acid. Hence, the ozonolysis of acetylene possesses a very complicated reaction mechanism that deserves intensive experimental studies.     

New selective haloform-type reaction yielding 3-hydroxy-2,2-difluoroacids: theoretical study of the mechanism

Experimental results of an unprecedented haloform-type reaction in which 4-alkyl-4-hydroxy-3,3-difluoromethyl trifluoromethyl ketones undergo base-promoted selective cleavage of the CO-CF(3) bond, yielding 3-hydroxy-2,2-difluoroacids and fluoroform, are rationalized using DFT (B3LYP) calculations. The gas-phase addition of hydroxide ion to 1,1,1,3,3-pentafluoro-4-hydroxypentan-2-one (R) is found to be a barrierless process, yielding a tetrahedral intermediate (INT), involving a DeltaG(r)(298 K) of -61.4 kcal/mol. The CO-CF(3) bond cleavage in INT leads to a hydrogen-bonded [CH(3)CHOHCF(2)CO(2)H...CF(3)](-) complex by passage through a transition structure (TS1) with a DeltaG()(298 K) of 20.8 kcal/mol and a DeltaG(r)(298 K) of 9.8 kcal/mol. This complex undergoes a proton transfer between its components, yielding a hydrogen-bonded [CH(3)CHOHCF(2)CO(2)...CHF(3)](-) complex. This process has associated with it a DeltaG()(298 K) of only 3.1 kcal/mol and a DeltaG(r)(298 K) of -43.3 kcal/mol. The CO-CF(2) bond cleavage in INT leads to a hydrogen-bonded [CH(3)CHOHCF(2)...CF(3)CO(2)H](-) complex by passage through a transition structure (TS3) with a DeltaG()(298 K) of 29.2 kcal/mol and a DeltaG(r)(298 K) of 25.1 kcal/mol. The lower energy barrier found for CO-CF(3) bond cleavage in INT is ascribed to the larger number of fluorine atoms stabilizing the negative charge accumulated on the CF(3) moiety of TS1, as compared to the number of fluorine atoms stabilizing the negative charge on the CH(3)CHOHCF(2) moiety of TS3. The solvent-induced effects on the two pathways, introduced within the SCRF formalism through PCM calculations, do not reverse the predicted preference of the CO-CF(3) over the CO-CF(2) bond cleavage of R in the gas phase.     

Thermal behavior of [2.1.1]propellane: a DFT/ab initio study

Density functional and ab initio molecular orbital calculations have been used to search for the low energy path of the thermal isomerization of [2.1.1]propellane 1. Three reaction modes were considered: ring opening of the bicyclo[1.1.0]butane unit in 1 to give 1,2-dimethylenecyclobutane 21, opening of the four-membered ring of 1 to afford 1,3-dimethylenecyclobutane 20, and breaking of the [2.1.1]propellane central bond and one of the bicyclo[1.1.0]butane side bonds to form carbene 17. At the CAS(12,12)PT2N/6-31G(d) level of theory, the activation barrier of the latter route was lowest in energy. Further investigation of this process at the QCISD(T)/6-311G(d,p)//QCISD/6-31G(d) and B3PW91/6-311G(d,p)// B3PW91/6-311G(d,p) level of theory indicated that the barrier of isomerization of 1 --> 17 amounts to 29 kcal/mol and that 17 is stabilized by hydrogen migration to give dienes 18 and 19.     

Foiled carbenes revisited: When sigma-stabilization surpasses pi-stabilization

The bicyclic alkenylidenes 9 (bicyclo[3.2.1]oct-2-en-8-ylidene) and 17 (bicyclo[3.3.1]non-2-en-9-ylidene) were claimed to be stabilized foiled carbenes. Our B3LYP and MP2 computations confirm previous experimental data. Moreover, they show that these carbenes are very reactive and rearrange rapidly, mainly through a 1,2-vinyl shift by overcoming a low barrier (1.2 to 5.4 kcal/mol). This is in contrast to the high barriers (up to 30 kcal/mol) predicted for the same type of rearrangements in norborn-2-en-7-ylidene derivatives. In 17 and bicyclo[4.1.1]oct-2-en-7-ylidene (23), the divalent carbon atom is even bent away from the double bond!     

Arylpentazoles revisited: experimental and theoretical studies of 4-hydroxyphenylpentazole and 4-oxophenylpentazole anion

Kinetic measurements for the degradation of 4-hydroxyphenylpentazole (2a) and its salt 2b-NBu4 in CD3OD and in CD2Cl2 provided a set of activation parameters. The resulting free energies of activation in methanol (DeltaG298 = 19.7 kcal/mol for 2a and DeltaG298 = 20.6 kcal/mol for 2b-NBu4) were compared with previous results for the 4-chloro derivative, 2c, and collectively correlated with results of gas-phase calculations at the B3LYP/6-31+G(d,p) level of theory. This, and another linear correlation of the seven computed DeltaG298 values with the previously reported kinetic data of Ugi and Huisgen, gave the basis for the estimation of the stability of pentazole anion (1) and its derivatives in solutions. Thus, N5(-) is predicted to have t(1/2) = 2.2 d, while the half-lifetime for HN5 is expected to be only about 10 min in methanol at 0 degrees C. Controlled ozonolysis of 2b-NBu4 followed by 1H and 15N NMR spectroscopy shows a preferential destruction of the N5 ring, which excludes it from possible methods for preparation of the parent pentazole.     

Developing a new class of axial chiral phosphorus ligands: preparation and characterization of enantiopure atropisomeric phosphinines

Both enantiomers of the first atropisomeric phosphinine (1) have been isolated by using analytical HPLC on a chiral stationary phase. The enrichment of one enantiomer and a subsequent investigation into its racemization kinetics revealed a barrier for internal rotation of DeltaG(298)(double dagger) = (109.5+/-0.5) kJ mol(-1), which is in excellent agreement with the theoretically predicted value of DeltaG(298)(double dagger) =116 kJ mol(-1). Further analysis with UV and circular dichroism spectroscopies and density functional theory calculations led to the determination and assignment of the absolute configurations of both enantiomers. These results are the basis for future investigations into this new class of axially chiral phosphinine-based ligands and their possible applications in asymmetric homogeneous catalysis.     

Carboranes and baskets from reaction of B4H10 with allene

The reaction of the boron hydride B4H10 with allene was studied at the CCSD(T)/6-311+G(d)//MP2/6-31G(d) level. The mechanism is surprisingly complex with 44 transition states and several branching points located. The four carboranes and one basket that have been observed experimentally are all connected by pathways that have very similar free energies of activation. In addition, two new structures, a basket (2,4-(CH2CH2CH2)B4H8, 5a) and a "classical" structure (1,4-(Me2C)bisdiborane, 7), which might be obtained from the B4H10 + C3H4 reaction under the right conditions (hot/cold, quenched, etc.) have been identified. The first branch point in the reaction is the competition between H2 elimination from B4H10 (DeltaG(298 K) = 32.2 kcal/mol) and the hydroboration of allene by B4H10 (DeltaG(298 K) = 31.3 kcal/mol). The next branch point in the hydroboration mechanism controls the formation of 2,4-(MeCHCH2)B4H8 (1) (DeltaG(298 K) = 31.5 kcal/mol) and arachno-1,2/arachno-1,3-Me2-1-CB4H7 (8 and 8a) (DeltaG(298 K) = 34.3 kcal/mol). Another branch point in the H2-elimination mechanism controls the formation of 1-Me-2,5-micro-CH2-1-CB4H7 (29) (DeltaG(298 K) = 0.1 kcal/mol) and 2,5-micro-CHMe-1-CB4H7 (25/26) (DeltaG(298 K) = 7.3 kcal/mol). Formation of 2-Me-2,3-C2B4H7, a carborane observed in the reaction of methylacetylene with B4H10, is calculated to be blocked by a high barrier for H2 elimination. All free energies are relative to B4H10 + allene. An interesting reaction step discovered is the "reverse hydroboration step" in which a hydrogen atom is transferred from carbon back to boron, which allows a CH hydrogen to shuttle between the terminal and central carbon of allene.     

Beyond the Roger Brown rearrangement: long-range atom topomerization in conjugated polyynes

Long-range carbon atom topomerization in a 1,3-diyne has been demonstrated for the first time. 1-Phenyl-4-p-tolyl-1,3-butadiyne, (13)C-enriched at C-1, was synthesized and subjected to flash vacuum pyrolysis. At 800 degrees C and 0.01 Torr, this resulted in nearly complete (13)C label equilibration between C-1 and C-2, as seen by NMR analysis. Pyrolysis at 900 degrees C further led to ca. 35% of the label migrating about equally to C-3 and C-4. These results demonstrate that both intrabond and interbond atom exchange processes are operative, with the former having a lower activation barrier. DFT and Moller-Plesset calculations support a mechanism that passes through Brown rearrangement (1,2-shift), closure to trialene (bicyclo[1.1.0]-1,3-butadiene), bond-shift isomerization to exchange C-2 and C-3, and ring opening. The resulting vinylidene can rearrange to a butadiyne with the isotopic label at C-3 or C-4. Consistent with earlier calculations, trialene is predicted to have alternating peripheral bonds, with a weak central sigma bond and significant diradical character. Trialene is predicted [(B3LYP/6-311+G(2d,p)] to lie 64.6 kcal/mol above butadiyne, with barriers of 2.2 and 4.4 kcal/mol, respectively, for ring opening or bond-shift isomerization. Other potential rearrangement mechanisms which pass through tetrahedrene (E(rel) = 167.2 kcal/mol) or 1,2,3-cyclobutatriene (E(rel) = 161.1 kcal/mol) lie at much higher energies.     

Heats of formation and singlet-triplet separations of hydroxymethylene and 1-hydroxyethylidene

Thermochemical parameters of hydroxymethylene (HC:OH) and 1-hydroxyethylidene (CH3C:OH) were evaluated by using coupled-cluster, CCSD(T), theory, in conjunction with the augmented correlation consistent, aug-cc-pVnZ, basis sets, with n = D, T, Q, and 5, extrapolated to the complete basis set limit. The predicted value at 298 K for Delta Hf(CH2O) is -26.0 +/- 1 kcal/mol, as compared to an experimental value of -25.98 +/- 0.01 kcal/mol, and for Delta Hf(CH:OH) it is 26.1 +/- 1 kcal/mol. The hydroxymethylene-formaldehyde energy gap is 52.1 +/- 0.5 kcal/mol, the singlet-triplet separation of hydroxymethylene is Delta E(ST)(HC:OH) = 25.3 +/- 0.5 kcal/mol, the proton affinity is PA(HC:OH) = 222.5 +/- 0.5 kcal/mol, and the ionization energy is IEa(HC:OH) = 8.91 +/- 0.04 eV. The predicted value at 298 K for Delta Hf(CH3CHO) is -39.1 +/- 1 kcal/mol as compared to an experimental value of -40.80 +/- 0.35 kcal/mol, and for Delta Hf(CH3C:OH) it is 11.2 +/- 1 kcal/mol. The hydroxyethylidene-acetaldehyde energy gap is 50.6 +/- 0.5 kcal/mol, the singlet-triplet separation of 1-hydroxyethylidene is Delta E(ST)(CH3C:OH) = 30.5 +/- 0.5 kcal/mol, the proton affinity is PA(CH3C:OH) = 234.7 +/- 0.5 kcal/mol, and the ionization energy is IEa(CH3C:OH) = 8.18 +/- 0.04 eV. The calculated energy differences between the carbene and aldehyde isomers, and, thus, the heats of formation of the carbenes, differ from the experimental values by 2.5 kcal/mol.     

Activation parameters for additions of ambiphilic methoxychlorocarbene to alkenes

Activation parameters are determined for the additions of methoxychlorocarbene (MeOCCl) to four alkenes, including tetramethylethylene (TME). MeOCCl is the first carbene to exhibit appreciable activation energy in addition to TME (5.8 kcal/mol), although this reaction is still "dominated" by entropy (-TDeltaS(double dagger) approximately 7 kcal/mol).     

Carbene stabilization by aryl substituents. Is bigger better?

The 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 (DeltaEST), 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 A C=C 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 (sigma-p and p-pi). 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., C-C, C-H). In general, the aromaticity of the substituted rings in triplet carbenes is most affected by the presence of the unpaired electrons.