Moving toward Ylide‐Stabilized Carbenes

The effect of ylide substitution at the α position to the carbene carbon (C_c) atom on the stability and σ‐donating ability of a number of cyclic carbenes has been studied theoretically. The stabilities of all of the carbenes were investigated from an evaluation of their singlet–triplet energy gaps and stabilization energies. All carbenes were found to have a stable singlet state. The energy of the σ‐symmetric lone‐pair orbital at the C_c atom increases as a result of the introduction of ylide centers near to the C_c atom. This indicates an enhanced σ‐donating ability of the ylide‐containing carbenes. The calculated carbonyl‐stretching frequencies of the corresponding rhodium complexes, proton affinities, and nucleophilicity index values correlate well with the σ basicity of the carbenes.     

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.     

Resonance stabilization energy of 1,2-azaborines: a quantitative experimental study by reaction calorimetry

Aromatic and single-olefin six-membered BN heterocycles were synthesized, and the heats of hydrogenation were measured calorimetrically. A comparison of the hydrogenation enthalpies of these compounds revealed that 1,2-azaborines have a resonance stabilization energy of 16.6 ± 1.3 kcal/mol, in good agreement with calculated values.     

Small ScCn cyclic clusters: a density functional study of their structure and stability

A theoretical study of the ScCn, ScCn+, and ScCn- (n = 1-10) cyclic clusters has been carried out employing the B3LYP density functional method. Predictions for several molecular properties that could help in their possible experimental characterization, such as equilibrium geometries, electronic structures, dipole moments, and vibrational frequencies, are reported. All ScCn cyclic clusters are predicted to have doublet ground states. For cationic clusters the ground state is alternate between singlets (n-even species) and triplets (n-odd members). In the case of anionic clusters the singlet-triplet separation is relatively small, with the singlets being favored in most cases. In general, even-odd parity effects are also observed for different properties, such as incremental binding energies, ionization energies, and electron affinities. For all neutral, cationic, and anionic clusters it is found that cyclic species are more stable than their open-chain counterparts. Therefore, cyclic structures are the most interesting possible targets for an experimental search of scandium-doped carbon clusters.     

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.     

A mndo study of 3-, 5-, 7- and 9-membered carbocyclic,completely conjugated,planar carbenes and their nonplanar isomers

Molecular orbital calculations have been conducted on a series of planar, cyclic, completely conjugated carbenes and their nonplanar isomers. Energy surfaces have been probed using reaction coordinate diagrams and vibrational force constant calculations. The importance of the nonplanar allenic and carbenic isomers is emphasized. In most cases, planar, cyclic, completely conjugated carbenes are not found to be energy minima.     

Geometry and energy of substituted phenyl cations

The geometry and the energy of a number of substituted phenyl cations have been calculated for both spin states at the UB3LYP/6-31G(d) level (o-, m-, p-Me, OMe, NH(2), CN, NO(2)) or at the UB3LYP/6-311++G(2d,p) level (o-, m-, p-SiMe(3), SMe). The geometric differences were assessed by means of a self-organizing neural network. The triplets maintain a regular hexagonal structure that is minimally affected by substituents, while in the singlets C1 puckers inward and, when an electron-donating group is present, shifts out of the plane. The triplets have the character of aromatic radical ions and are strongly stabilized by electron-donating substituents, independently of the position of the latter. In the case of singlets, the effect of substituents on the energy is weaker and depends on the position (the largest effect is exerted when the group is in meta). A two-parameter correlation of all of the triplet energies shows the predominant mesomeric effect of the substituents. In the case of singlets, linear correlations are obtained only when each position is treated separately and when the predominant effect is inductive for the ortho and, less markedly, the para position, whereas at the meta position, mesomeric and inductive effects are comparable. The ground state is determined to be the singlet for the parent cation and for electron-withdrawing substituted ions. With electron-donating substituents, the triplet is the ground state for ortho and para derivatives, while the two spin states are roughly isoenergetic when the donating group is in the meta position. These data allow predicting the reactivity of each cation.     

Thermochemistry of N‐heterocyclic carbenes with 5‐, 4‐, 3‐, and 2‐membered rings

N‐heterocyclic carbenes (NHCs) based on imidazole‐2‐ylidene ( 1 ) or the saturated imidazolidine‐2‐ylidene ( 2 ) scaffolds are long‐lived singlet carbenes. Both benefit from inductive stabilization of the sigma lone pair on carbon by neighboring N atoms and delocalization of the N pi lone pairs into the nominally vacant p‐pi atomic orbital at the carbene carbon. With thermochemical schemes G4 and CBS‐QB3, we estimate the relative thermodynamic stabilization of smaller ring carbenes and acyclic species which may share the keys to NHC stability. These include four‐membered ring systems incorporating the carbene center, two trivalent N centers, and either a boron or a phosphorus atom to complete the ring. Amino‐substituted cyclopropenylidenes have been reported but three‐membered rings containing the carbene center and two N atoms are not known. Our calculations suggest that amino‐substituted cyclopropenylidenes are comparable in stability to the four‐membered NHCs but that diazacyclopropanylidenes would be substantially less effectively stabilized. Concluding the series are acyclic carbenes with and without neighboring N atoms and a series of “two‐membered ring” azapropadienenylidene cations of form :C?N?W with W = an electron‐withdrawing agent. We have studied W = NO₂, CH₂(+), CF₂(+), and (CN)₂C(+). Although these systems display a degree of stabilization and carbene‐like electronic structure, the stability of the NHCs is unsurpassed. © 2014 Wiley Periodicals, Inc.     

The carbene reactivity surface: a classification

A new two-dimensional classification of singlet carbenes based on the difference in reactivity of their insertion reactions into the C-H bonds of acetonitrile and isobutane is presented. This classification combines the stability and the philicity of divalent species. Until now all of the experimentally based philicity scales are based on the addition to alkenes. Moreover, a new terminology for describing the reactivity of carbenes is introduced. Among the alkyl carbenes, acetyl carbene (2) and cyclopentadienylidene are shown as highly reactive electrophilic carbenes, whereas the other alkylidenes and alkenylidenes investigated are all less active than 2 and more nucleophilic. The stabilized-nucleophilic bicyclo[2.1.1]hex-2-en-5-ylidene (13) possesses a stability similar to that of cyclic alkyl amino carbene (CAAC) 18 and aminophosphoniocarbene 7. Strong hydrogen bridging is found between a C-H bond of acetonitrile and the nucleophilic carbenes 13 and 14.     

Experimentally Observed Reverse Intersystem Crossing-Boosted Lasing

Thermally activated delayed-fluorescent (TADF) materials are anticipated to overcome triplet-related losses towards electrically driven organic lasers. Thus far, contributions from triplets to lasing have not yet been experimentally demonstrated owing to the limited knowledge about the excited-state processes. Herein, we experimentally achieve reverse intersystem crossing (RISC)-boosted lasing in organic microspheres with uniformly dispersed TADF emitters. In these materials, triplets are continuously converted to radiative singlets through RISC, giving rise to reduced losses in stimulated emission. The involvement of regenerated singlets in population inversion results in a thermally activated lasing; that is, the lasing intensity increases with increasing temperature, accompanied by accelerated depletion of the excited-state population. Benefiting from the suppression of triplet accumulations by RISC processes, a high-repetition-rate microlaser was achieved.     

Correlation of singlet-triplet gaps for aryl carbenes calculated by MINDO/3, MNDO,AM1, and PM3 with Hammett-type substituent constants

Heats of formation, atomic charges, and geometries of some 110 structures involving substituted singlet and triplet phenyl and 4,4-dimethyl-1,4-dihydronaphthalene carbenes and the corresponding diazomethanes were calculated by MINDO/3, MNDO, AM1, and PM3 semiempirical molecular orbital methods. The singlet-triplet gaps for AM1 and PM3 calculations for the para derivatives in both systems have been successfully correlated with Brown σ⁺ constants. Good correlations with σ⁺ were found for the charges on the carbenic centers of the singlets as well as with the energy barrier for rotation of the aryl group about the C-C single bond in substituted singlet phenylcarbenes. Comparisons of these results with experimental data indicate that AM1 and PM3 are much better than MNDO and MINDO/3 in predicting the intrinsic substituent effects in singlet carbenes.     

Computational thermochemistry of C-nitroso compounds

The molecular structure and thermodynamic stability of C-nitroso and dinitroso (azodioxide) compounds (CNC) have been studied by using high-level, composite, ab initio method (G4MP2) via the series of appropriate homodesmotic, dimerization reactions. The calculated reaction enthalpies and Gibbs free energies allowed us to estimate relative stability of dinitroso moiety and the role of substituents in thermodynamic stabilization of dinitroso group versus its monomeric precursor. The stability of dinitroso compounds is generally low as is indicated by large positive values for standard enthalpies of formation. The stabilization of the dinitroso group is much more pronounced in alkyl, alkenyl, or alkynyl derivatives than in the aromatic derivatives. The thermodynamic stability of E-stereoisomers of dinitroso compounds is generally larger than their Z-stereoisomer analogues. However, the difference in E/Z stereoisomer stability is quite small in aromatic dinitroso compounds. We have discussed the influence of substituents on the molecular geometry of the nitroso and dinitroso groups. We have also discussed the nature and strengths of solid-state forces pertaining to CNC.     

1,3-Heterocumulene-to-alkyne

Transition structures and energy barriers of the concerted prototypical cycloaddition reaction of 1,3-heterocumulenes (S=C=S, S=C=NR, RN=C=NR, and heteroanalogs) to acetylene resulting in nucleophilic carbenes were calculated by G2(MP2) and CBS-Q ab initio quantum chemical and by density functional theory (DFT) methods. According to the calculations the activation energies (activation enthalpies) of the homoheteroatomic cumulenes decrease in the order O > S > Se and NH > PH and the reaction energies in the order O > S approximately Se and PH > NH. The reaction of carbon disulfide and acetylene has a lower reaction barrier than that of carbodiimide and acetylene although the first reaction is less exothermic than the second one. The stronger cyclic stabilization of the 1, 3-dithiol-2-ylidene in the transition state is discussed in terms of deformation and stabilization energies and of bond indices. The known reactions of carbon disulfide with ring-strained cycloheptynes were examined by DFT and by DFT:PM3 two-layered hybrid ONIOM methods. In agreement with qualitative experimental findings the activation energy increases and the reaction energy decreases in the sequence S, SO(2), and SiMe(2) if CH(2) in the 5-position of 3,3,7, 7-tetramethyl-1-cycloheptyne is replaced by a heteroatom or heteroatomic group, respectively. The results of these calculations were corroborated by experimental studies with carbon diselenide and isothiocyanates as 1,3-heterocumulenes. The cycloaddition of carbon diselenide to cyclooctyne proceeded faster than with carbon disulfide, the main product being the 1,3-diselenol-2-selone. Under more drastic conditions it was possible to add methyl and phenyl isothiocyanate, respectively, to 3,3,6, 6-tetramethyl-1-thia-4-cycloheptyne. The products are 1:3 adducts (cycloalkyne:isothiocyanate) whose formation is explained by a trapping reaction of the first formed 1,3-thiazol-2-ylidenes.     

Molecular fragmentations: Part VI. Structures and energies of the valence isomers of benzene,and their molecular cations and anions

Molecular structures and energies have been calculated, in the MINDO/3 approximation, for neutral singlets and triplets, and for molecular cations and anions of benzene and seven of its valence isomers.     

Stable Cyclic Carbenes and Related Species beyond Diaminocarbenes

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ‐donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus‐based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon‐based ligands (which are not NHCs), and which feature even stronger σ‐donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.     

Exponential transformation of molecular orbitals. II. General formulation for UHF calculations and application to diatomics and molecular fragments

The exponential transformation of the molecular orbitals, that has been previously used to achieve a process with a convergence of quadratic quality in SCF closed-shell calculations [J. Chem. Phys. 72 , 1452 (1980)] has been extended to UHF determinantal wave functions built from different orbitals for different spins. Explicit formulas are given for the first and second derivatives of the energy to be varied. The method is illustrated by UHF calculations for systems described as standard singlets (Li₂ and F₂) or triplets (NH) at the RHF approximation level, as well as for CH, CH₂, CH₃ molecular fragments in their valence states.     

Hole-induced quenching of triplet and singlet excitons in conjugated polymers

Quantitative information on the mechanisms and rates of hole (radical cation)-induced quenching of triplet and singlet excitons in the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] has been acquired by a new technique, fluorescence-voltage time-resolved single molecule spectroscopy (FV-TR-SMS). FV-TR-SMS measures the fluorescence intensity of a single conjugated polymer molecule that is embedded in a capacitor-like device while simultaneously modulating the bias on the device and the irradiation intensity. The results demonstrate that triplet excitons are efficiently quenched by holes in conjugated polymers for hole densities >10(16) charges/cm(3), while singlet excitons are quenched with a much lower efficiency. Detailed kinetic analysis shows that the greater efficiency for quenching of triplets by holes (compared to that for singlets) is due to a >10(6) times longer exciton lifetime for triplets. In fact, the results suggest that while singlet quenching is less efficient due to a much shorter singlet lifetime, the rate constant for the quenching of singlets by holes actually exceeds that for triplets by several orders of magnitude.     

水溶性氮杂环卡宾金属配合物的研究进展

氮杂环卡宾(NHCs)金属配合物作为一类重要的催化剂一直是有机合成领域研究的热点。近年来,通过引入水溶性配体而得到的水溶性氮杂环卡宾过渡金属配合物受到广大科研工作者的青睐。本文主要总结了水溶性NHCs的分类、合成及其在C-C偶联反应、复分解反应以及催化加氢反应中的应用,并对水溶性NHCs金属配合物的发展趋势进行了展望。     

Reliable determination of amidicity in acyclic amides and lactams

Two independent computational methods have been used for determination of amide resonance stabilization and amidicities relative to N,N-dimethylacetamide for a wide range of acyclic and cyclic amides. The first method utilizes carbonyl substitution nitrogen atom replacement (COSNAR). The second, new approach involves determination of the difference in amide resonance between N,N-dimethylacetamide and the target amide using an isodesmic trans-amidation process and is calibrated relative to 1-aza-2-adamantanone with zero amidicity and N,N-dimethylacetamide with 100% amidicity. Results indicate excellent coherence between the methods, which must be regarded as more reliable than a recently reported approach to amidicities based upon enthalpies of hydrogenation. Data for acyclic planar and twisted amides are predictable on the basis of the degrees of pyramidalization at nitrogen and twisting about the C-N bonds. Monocyclic lactams are predicted to have amidicities at least as high as N,N-dimethylacetamide, and the β-lactam system is planar with greater amide resonance than that of N,N-dimethylacetamide. Bicyclic penam/em and cepham/em scaffolds lose some amidicity in line with the degree of strain-induced pyramidalization at the bridgehead nitrogen and twist about the amide bond, but the most puckered penem system still retains substantial amidicity equivalent to 73% that of N,N-dimethylacetamide.     

ENERGY TRANSFER FROM CHEMIEXCITED ACETONE TO SUBSTANCES THAT DISPLAY ANOMALOUS FLUORESCENCE

Abstract— Excited acetone generated in the thermolysis of tetramethyldioxetane elicits the anomalous S₂→ S₀ fluorescence from azulene and from xanthione. In the case of azulene it could be demonstrated that (i) only the acetone singlets transfer energy to the S ₂ state and (ii) the acetone triplets are quenched. These energy transfer processes are diffusion-controlled.