Generation and reactivity of the 4-aminophenyl cation by photolysis of 4-chloroaniline

4-Chloroaniline and its N,N-dimethyl derivative are photostable in cyclohexane but undergo efficient photoheterolysis in polar media via the triplet state and give the corresponding triplet phenyl cations. CASSCF and UB3LYP calculations show that the 4-aminophenyl triplet cation has a planar geometry and is stabilized by >10 kcal mol(-1) with respect to the slightly bent singlet. The triplet has a mixed carbene-diradical character at the divalent carbon. This species either adds to the starting substrate forming 5-chloro-2,4'-diaminodiphenyls (via an intermediate cyclohexadienyl cation) or is reduced to the aniline (via the aniline radical cation) in a ratio depending on the hydrogen-donating properties of the solvent. Transients attributable to the triplet aminophenyl cation as well as to the ensuing intermediates are detected. Chemical evidence for the generation of the phenyl cation is given by trapping via electrophilic substitution with benzene, mesitylene, and hexamethylbenzene (in the last case the main product is a 6-aryl-3-methylene-1,4-cyclohexadiene). Relative rates of electrophilic attack to benzene and to some alkenes and five-membered heterocycles are measured and span over a factor of 15 or 30 for the two cations. The triplet cation formed under these conditions is trapped by iodide more efficiently than by the best pi nucleophiles. However, in contrast to the singlet cation, it does not form ethers with alcohols, by which it is rather reduced.     

Sulfur Monoxide Release and Capture: A Computational Study

Thermolysis of thiirane oxide leads to production of highly reactive sulfur monoxide. The liberated SO can in turn be trapped with a diene scavenger forming dihydrothiophene oxide. Since the intermediate diatomic possesses a triplet ground state, the SO transfer can proceed on two spin-state surfaces. Here, we study the competition between singlet concerted and stepwise triplet diradical mechanisms utilizing the M06-2X density functional as well as CCSD(T) and MRCI+Q wavefunction theories. We find that the decomposition of thiirane oxide prefers to pass through a triplet diradical intermediate that becomes accessible from a nearby minimum energy crossing point (MECP). Hence, the thermolysis of thiirane oxide is expected to predominantly release triplet ground state sulfur monoxide in agreement with previous experimental reports. The addition of ³SO to 1,3-butadiene initially generates an allylic diradical, that ring-closes to a thiirane oxide through another MECP, and a subsequent rearrangement gives access to the final product.     

Solvent dependence of the 2-naphthyl(carbomethoxy)carbene singlet-triplet energy gap

The solvent dependence of the 2-naphthyl(carbomethoxy)carbene (2) singlet-triplet energy gap has been examined by time-resolved infrared (TRIR) and computational methods. The ground state of 2 changes from the triplet state in hexane to the singlet state in acetonitrile. Preferential stabilization of the singlet carbene is the result of its increased dipole moment in polar solvents. Variable-temperature TRIR experiments provide measurements of the enthalpic and entropic differences between (1)2 and (3)2 and suggest that solvent and geometry effects on the entropy of singlet and triplet carbenes can offset differences arising from spin multiplicity. B3LYP calculations using the polarizable continuum solvation model (PCM) reproduce the general trends in enthalpic differences seen experimentally.     

2-苯基吲哚单重态氧反应机理

本文比较了在甲醇介质下, 2-苯基吲哚(1)单重态氧反应中的捕捉反应以及2-苯基-3H-吲哚-3-酮(4)的亲核加成反应特征。结果显示, 在1的单重态氧反应中, 捕捉反应主要发生于两性离子(2)阶段, 而4并非导致捕捉产物的重要中间体。根据上述事实, 结合有关反应溶剂极性效应的研究结果, 对反应机理进行了探讨。     

2-Silyl group effect on the reactivity of cyclopentane-1,3-diyls. Intramolecular ring-closure versus silyl migration

Generation of singlet and triplet 2-silylcyclopentane-1,3-diyls and their reactivity have been investigated in the thermal and photochemical denitrogenation of 2,3-diaza-7-silylbicyclo[2.2.1]hept-2-ene. 5-Silylcyclopentene (silyl migration product) is quantitatively obtained, while 5-silylbicyclo[2.1.0]pentane (intramolecular ring-closure product) is not detected in the denitrogenation reactions. Deuterium labeling studies clarify that 5-silylcyclopentene is formed by a suprafacial [1,2] silyl migration in singlet 2-silylcyclopentane-1,3-diyl. UDFT calculations closely reproduce the observed reactivity of the singlet diradical: The enthalpic barriers of the intramolecular ring-closure are calculated to be DeltaH++exo468 = 5.8 kcal/mol and DeltaH++endo468 = 6.7 kcal/mol, which are much higher than the energy barrier for the [1,2] silyl migration, DeltaH++468 = 2.7 kcal/mol. The notable effect of the silyl group on raising the energy barrier of the intramolecular cyclization is rationalized by an electronic configuration of the lowest singlet state of 2-silylcyclopentane-1,3-diyls.     

The beta effect of silicon in phenyl cations

Irradiation of chloroanisoles, phenols, and N,N-dimethylanilines bearing a trimethylsilyl (TMS) group in the ortho position with respect to the chlorine atom caused photoheterolysis of the Ar-Cl bond and formation of the corresponding ortho-trimethylsilylphenyl cations in the triplet state. The beta effect of silicon on these intermediates has been studied by comparing the resulting chemistry in alcoholic solvents with that of the silicon-free analogues and by computational analysis (at the UB3LYP/6-311+G(2d,p) level in MeOH). TMS groups little affect the photophysics and the photocleavage of the starting phenyl chlorides, while stabilizing the phenyl cations, both in the triplet (ca. 4 kcal/mol per group) and, dramatically, in the singlet state (9 kcal/mol). As a result, although triplet phenyl cations are the first formed species, intersystem crossing to the more stable singlets is favored with chloroanisoles and phenols. Indeed, with these compounds, solvent addition to give aryl ethers (from the singlet) competed efficiently with reduction or arylation (from the triplet). In the case of the silylated 4-chloro-N,N-dimethylaniline, the triplet cation remained in the ground state and trapping by pi nucleophiles remained efficient, though slowed by the steric bulk of the TMS group. In alcohols, the silyl group was eliminated via a photoinduced protiodesilylation during the irradiation. Thus, the silyl group could be considered as a directing, photoremovable group that allowed shifting to the singlet phenyl cation chemistry and was smoothly eliminated in the same one-pot procedure.     

Theoretical mechanistic study on the radical-molecule reaction of CHCl2/CCl3 with NO2

The radical-molecule reaction mechanism of CHCl(2) and CCl(3) with NO(2) have been explored theoretically at the B3LYP/6-311G(d,p) and MC-QCISD (single-point) levels. For the singlet potential energy surface (PES) of CHCl(2) + NO(2) reaction, the association of CHCl(2) with NO(2) was found to be a barrierless carbon-to-nitrogen approach forming an energy-rich adduct a (HCl(2)CNO(2)) followed by isomerization to b(1) (trans-cis-HCl(2)CONO), which can easily interconvert to b(2), b(3), and b(4). Subsequently, the most feasible pathway is the 1,3-chlorine migration associated with N-O1 bond cleavage of b(1) leading to P(1) (CHClO + ClNO). The second competitive pathway is the 1,4-chlorine migration along with N-O1 bond rupture of b(4) giving rise to P(2) (CHClO + ClON). Moreover, some of P(1) and P(2) can further dissociate to give P(6) (CHClO + Cl + NO). The lesser followed competitive channel is the 1,3-H-shift from C to N atom along with N-O1 bond rupture of b(1) to form P(3) (CCl(2)O + HNO). The concerted 1,4-H-shift accompanied by N-O1 bond fission of b(3) to product P(4) (CCl(2)O + HON) is even much less feasible. For the singlet PES of CCl(3) + NO(2) reaction, the only primary product is found to be P(1) (CCl(2)O + ClNO), which can lead to P(2) (CCl(2)O + Cl + NO) via dissociation of ClNO. The obtained major products CHClO and CCl(2)O for CHCl(2) + NO(2) and CCl(3) + NO(2) reactions, respectively, are in good agreement with kinetic detection in experiment. Compared with the singlet pathways, the triplet pathways may have less contributions to both reactions. Because the rate-determining transition state involved in the feasible pathways lie above the reactants R, the title reactions may be important in high-temperature processes. The similarities and discrepancies among the CH(n)Cl(3-n) + NO(2) (n == 0-2) reactions are discussed in terms of the substitution effect. The present study may be helpful for further experimental investigation of the title reactions.     

2-(3,5-Dinitrophenyl)-1,3-dithiane carbanion: a benzylic anion with a low energy triplet state

Calculations at the DFT level predict that benzyl anions with strong π-electron-withdrawing groups in the meta position(s) have low energy diradical or triplet electronic states. Specifically, the 2-(3,5-dinitrophenyl)-1,3-dithiane carbanion is predicted to have nearly degenerate singlet and triplet states at the (U)B3LYP level as a free anion. Its lithium ion pair is predicted to be a ground-state triplet with a substantial (26 kcal/mol) singlet-triplet energy gap. Experiments on this anion using chemical trapping, NMR, and the Evans method strongly suggest that this anion is either a triplet or a ground-state singlet with a very low energy triplet state.     

On the electronic structures of the 1,3-diboracyclobutane-1,3-diyls and their valence isomers with a B2E2 skeleton (E=N,P, As)

The concept of through-space versus through-bond interactions on the stabilization of biradical structures with a singlet or triplet ground state is evaluated for the 1,3-diboracyclobutane-1,3-diyls and related congeners. Singlet biradicals are favored when the intermediate units E feature singlet character (PH(2) (+), AsH(2) (+)), while E fragments with triplet character (NH(2) (+)) induce small energy separations between the lowest singlet and triplet states. These considerations are supported by quantum chemical calculations with energy optimization at 1) MCSCF level plus MR-MP2 correction, 2) MR-MP2 level, and 3) two different types of density functional levels for the planar (D(2h)) geometries. The singlet-triplet energy separations in the planar compounds increase with increasing singlet stability of the corresponding E fragments. In addition to this newly developed principal features for singlet stabilization, which primarily occurs in bonded structures with higher main-group elements, the corresponding valence isomers with bicyclobutane, cyclobutene and cis-butadiene structures are investigated.     

(Sensitized) photolysis of diazonium salts as a mild general method for the generation of aryl cations. Chemoselectivity of the singlet and triplet 4-substituted phenyl cations

The photolysis of a series of 4-X-benzenediazonium tetrafluoroborates is studied in MeCN. Loss of nitrogen occurs from the singlet excited state with X=H, t-Bu, and NMe2 and leads to the singlet aryl cation. This adds to the solvent yielding the corresponding acetanilides. With other substituents, ISC competes with (X=Br, CN) or overcomes (X=COMe, NO2) fragmentation and the aryl cation is formed in part or completely in the triplet state. In neat MeCN, this either abstracts hydrogen from the solvent (in most cases inefficiently) or undergoes intersystem crossing to the more stable singlet that reacts as above. In the presence of pi nucleophiles (allyltrimethylsilane or benzene), the triplet aryl cation is efficiently trapped giving substituted allylbenzenes and biphenyls, respectively. By triplet sensitization by xanthone, the triplet cation and the products from it are obtained from the whole series considered. The direct or sensitized photodecomposition of diazonium fluoroborates, substituted with both electron-donating and -withdrawing substituents, in the presence of alkenes and arenes offers an access to an alternative arylation procedure.     

Conjugated Biradical Intermediates: Spectroscopic,Kinetic, and Trapping Studies of 2,2-Dimethyl-1,3-perinaphthadiyl

The biradical 2,2-dimethyl-1,3-perinaphthadiyl ( a ) was generated from two different precursors, the naphthocyclopropane 1 and the azo compound 2 , and from each by three different pathways (pyrolysis, direct photolysis, and triplet sensitization, Scheme 1). The combined evidence from flash photolysis, low-temperature spectroscopy, and product analyses provides a detailed mechanistic picture of the formation and decay of this reactive intermediate which is persistent at 77 K in the triplet ground state (³ a ) and rather long-lived (400 μs) at room temperature. When formed in its lowest singlet state (¹ a ), the biradical is too short-lived to undergo intersystem crossing to ³ a or bimolecular reactions. Thus, ³ a is formed exclusively from the excited triplet state of the precursor compounds, ³ 1 * or ³ 3 *. The monomolecular decay of ³ a is retarded by the spin barrier; ³ a initiates the polymerization of acrylonitrile and is trapped by ³O₂.     

Hydrogen Bonding Switches the Spin State of Diphenylcarbene from Triplet to Singlet

Spin specificity is one of the most important properties of carbenes in their reactions. Alcohols are typically used to probe the reactive spin states of carbenes: O? H insertions are assumed to be characteristic of singlet states, whereas C? H insertions are typical for the triplets. Surprisingly, the experiments presented here suggest that the spin ground state of diphenylcarbene 1 switches from triplet to singlet if the carbene is allowed to interact with methanol. Carbene 1 and methanol form a strongly hydrogen‐bonded singlet ground state complex that was synthesized in low‐temperature matrices and characterized by IR spectroscopy. This methanol complex is only metastable, and even at 3 K slowly rearranges to form the product of O? H insertion through quantum chemical tunneling. Thus, the ground state triplet (in the gas phase) carbene 1 forms exclusively the products expected from a singlet carbene. Whereas the assumption of spin specific reactions of carbenes is correct, the spin state itself can be changed by solvent interactions, and therefore widely accepted conclusions drawn from earlier experiments have to be revisited.     

Structure and Stability of Interstellar Molecule C_3S

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Cobalt-catalyzed cyclotrimerization of alkynes: the answer to the puzzle of parallel reaction pathways

To understand some experimental data at odds with the computed mechanism of the CpCo(L2)-catalyzed [2 + 2 + 2] cyclotrimerization of ethyne, DFT computations were carried out following the fate of methyl- and hydroxycarbonyl-substituted alkynes to give the corresponding arenes. The key intermediate in all cases is a triplet cobaltacyclopentadiene obtained by oxidative coupling of the corresponding CpCo(bisalkyne) complex and subsequent spin change via a minimum energy crossing point (MECP). From that species, two different catalytic cycles lead to an arene product, depending on the nature of the alkyne and other ligands present: either alkyne ligation to furnish a cobaltacyclopentadiene(alkyne) intermediate or trapping by a sigma-donor ligand to generate a coordinatively saturated cobaltacyclopentadiene(PR3) complex. The former leads to the CpCo-complexed arene product via intramolecular cobalt-assisted [4 + 2] cycloaddition, whereas the latter may, in the case of a reactive dienophile (butynedioic acid), undergo direct intermolecular [4 + 2] cycloaddition to generate a cobaltanorbornene. The bridgehead cobalt atom is then reductively eliminated after another change in spin state from singlet to triplet. The necessary conditions for one or the other mechanistic pathway are elaborated.     

Theoretical study of long oligothiophene dications: bipolaron vs polaron pair vs triplet state

A series of oligothiophene dications (from the sexithiophene dication to the 50-mer oligothiophene dication, nT2+, n = 6-50) were studied. Density functional theory (DFT) at the B3LYP/6-31G(d) level and, in some cases, also at BLYP/6-31Gd, was applied to study the singlet and triplet states of the whole series. We found that the singlet state is the ground state for all oligothiophene dications up to the 20-mer, and that the singlet and triplet states are degenerate for longer oligomers. Thus, the triplet state is never a pure ground state for these dications. We found that, for short oligothiophenes dication (e.g., 6T2+), the bipolaron state is the more important state, with only a small contribution made by the polaron pair state. For medium size oligothiophene dications (e.g., 14T2+), both the bipolaron state and the polaron-pair state contribute to the electronic structure. Finally, in long oligothiophene dications, such as 30T2+ and 50T2+, the contribution from the polaron pair state becomes dominant, and these molecules can be considered as consisting of two independent cation radicals or a polaron pair. Results from isodesmic reactions show that the stability of oligothiophene cation radicals over dications is inversely proportional to chain length. Small oligothiophene dications (n = 6-12) were studied at the CASSCF(m,m)/6-31G(d) (m = 4, 6, and 10) level. The major conclusions of this paper regarding the relative energy of the singlet state versus the triplet state and regarding the relative stability of the bipolaron versus the polaron pair were also supported by CASSCF calculations.     

Reaction mechanism of NO with hydrolysates of NAMI-A: an MD simulation by combining the QM/MM(ABEEM) with the MD-FEP method

Nitrosylation reaction mechanisms of the hydrolysates of NAMI-A and hydrolysis reactions of ruthenium nitrosyl complexes were investigated in the triplet state and the singlet state. Activation free energies were calculated by combining the QM/MM(ABEEM) method with free energy perturbation theory, and the explicit solvent environment was simulated by an ABEEMσπ polarizable force field. Our results demonstrate that nitrosylation reactions of the hydrolysates of NAMI-A occur in both the triplet and the singlet states. The Ru-N-O angle of the triplet ruthenium nitrosyl complexes is in the range of 132.0°–138.2°. However, all the ruthenium nitrosyl complexes at the singlet state show an almost linear Ru-N-O angle. The nitrosylation reaction happens prior to the hydrolysis reaction for the first-step hydrolysates. The activation free energies of the nitrosylation reactions show that the H₂O-NO exchange reaction of [RuCl₄(Im)(H₂O)] in the singlet spin sate is the most likely one. Comparing with the activation free energies of the hydrolysis reactions of the ruthenium nitrosyl complexes, the results indicate that the rate of the DMSO–H₂O exchange reaction of [RuCl₃(NO)(Im)(DMSO)] is faster than that of [RuCl₃(H₂O)(Im)(DMSO)] in both the triplet spin state and the singlet spin state. © 2018 Wiley Periodicals, Inc.     

Dynamical calculations of charge-transfer-to-solvent excited states of small I- (CH3CN)n clusters

Relaxation dynamics of photoexcited charge-transfer-to-solvent (CTTS) states for the I(-)(CH(3)CN)(n) (n = 2 and 3) clusters has been theoretically studied using electronic structure methods. First, we have calculated several lowest singlet and triplet potential energy surfaces using the multireference configuration interaction method. It was found that the character of the singlet CTTS excited-state potential surfaces is very similar to that of the triplet CTTS states. Due to a small singlet-triplet splitting, the lowest triplet potential energy surface was used as a good model to understand the dynamics of the photoexcited singlet CTTS states. We have carried out direct molecular dynamics simulations on the lowest triplet surface at the B3LYP level. When an I(-) anion is exteriorly solvated by CH(3)CN molecules, we found that the (CH(3)CN)(n)(-) anion cluster is effectively produced. In addition, when the I(-) anion is placed in the interior in I(-)(CH(3)CN)(n) clusters, photoexcitation gives an acetonitrile monomer anion plus neutral monomers. However, if the initial geometric configuration is distorted from the minimum structure, we also found that the (CH(3)CN)(2)(-) anion cluster, where an excess electron is internally trapped, is formed via I(-)(CH(3)CN)(2) + hnu --> I + (CH(3)CN)(2)(-) process.     

Experimental and theoretical studies of the C2F4 + O reaction: nonadiabatic reaction mechanism

In this work, the C(2)F(4)(X(1)A(g)) + O((3)P) reaction was investigated experimentally using molecular beam-threshold ionization mass spectrometry (MB-TIMS). The major primary products were observed to be CF(2)O (+ CF(2)) and CF(3) (+ CFO), with measured approximate yields of % versus %, respectively, neglecting minor products. Furthermore, the lowest-lying triplet and singlet potential energy surfaces for this reaction were constructed theoretically using B3LYP, G2M(UCC, MP2), CBS-QB3, and G3 methods in combination with various basis sets such as 6-31G(d), 6-311+G(3df), and cc-pVDZ. The primary product distribution for the multiwell multichannel reaction was then determined by RRKM statistical rate theory and weak-collision master equation analysis. It was found that the observed production of CF(3) (+ CFO) can only occur on the singlet surface, in parallel with formation of ca. 5 times more CF(2)O(X) + CF(2)(X(1)A(1)). This requires fast intersystem crossing (ISC) from the triplet to the singlet surface at a rate of ca. 4 x 10(12) s(-1). The theoretical calculations combined with the experimental results thus indicate that the yield of triplet CF(2)(?(3)B(1)) + CF(2)O formed on the triplet surface prior to ISC is < or =35%, whereas singlet CF(2)(X(1)A(1)) + CF(2)O is produced with yield > or =60%, after ISC. In addition, the thermal rate coefficients k(O + C(2)F(4)) in the T = 150-1500 K range were computed using multistate transition state theory and can be expressed as k(T) = 1.67 x 10(-16) x T(1.48) cm(3) molecule(-1) s(-1); they are in agreement with the available experimental results in the T = 298-500 K range.     

Thermodynamic Control of Intramolecular Singlet Fission and Exciton Transport in Linear Tetracene Oligomers

We newly synthesized a series of homo- and hetero-tetracene (Tc) oligomers to propose a molecular design strategy for the efficient exciton transport in linear oligomers by promoting correlated triplet pair (TT) dissociation and controlling sequential exciton trapping process of individual doubled triplet excitons (T+T) by intramolecular singlet fission. First, entropic gain effects on the number of Tc units are examined by comparing Tc-homo-oligomers [(Tc)_n: n=2, 4, 6]. Then, a comparison of (Tc)_n and Tc-hetero-oligomer [TcF₃-(Tc)₄-TcF₃] reveals the vibronic coupling effect for entropic gain. Observed entropic effects on the T+T formation indicated that the exciton migration is rationalized by number of possible TT states increased both by increasing the number of Tc units and by the vibronic levels at the terminal TcF₃ units. Finally, we successfully observed high-yield exciton trapping process (trapped triplet yield: Φ_TrT=176 %).     

Solvent dependence of singlet oxygen / substrate interactions in ene-reactions, (4+2)- and (2+2)-cycloaddition reactions

Product formation of singlet oxygen reactions with simple olefins occurring as ene-reactions, (4+2)- and (2+2)-cycloaddition reactions is independent on solvent polarity. Thus, 2,3-dimethyl-2-butene (1) and 2-methy]-2-butene (3), 1,3-cyclohexadiene (6), and benzvalene (8) yield allylic hydroperoxides (2) and(4) (54%) + (5) (46%), endoperoxide (7), and dioxetane (9), respectively. The rates of the ene-reactions and (4+2)-cycloaddition reactions are only slightly dependent, those of the (2+2)-cycloaddition reaction, however,are clearly dependent on solvent polarity. “Physical” quenching of singlet oxygen by the olefins is negligible, but substantial by the sensitizer tetraphenylporphin (TPP) in chlorinated solvents.