Singlet Reaction Research Articles

Ground and excited state reaction of N + with ammonia

A theoretical study of the ground and excited state reactions of N + with ammonia has been carried out. Ab initio calculations demonstrate that the reaction N +( 3P) + NH 3 → NH 2 + + NH is a triplet reaction and has negative activation energy (−68.3 kcal mol −1) and enthalpy (−96.8 kcal mol −1). The ion-molecule complex H 3N-N + is the global minimum on the triplet potential energy surface of [N 2, H 3] +. The activation energies of migration and fragmentation in this reaction pathway are 49.4 and 131.8 kcal mol −1 respectively. The reaction of N +( 1D) + NH 3 → N 2H + + H 2 is a singlet reaction; the 1,1 elimination of H 2 from the singlet collision complex H 3N-N + forms the observed products N 2H + + H 2; the smaller barrier of 1,1 elimination of H 2 is 25.5 kcal mol −1. This singlet reaction has negative activation energy (−203.2 kcal mol 1) and enthalpy (−296.6kcalmol −1). The crossing point of the singlet and triplet surfaces of [N 2,H 3] + system is discussed.

A mechanistic study of the ring-opening reaction of singlet oxirane

The C-C ring opening reaction of the oxirane molecule on the singlet potential energy surface has been investigated within the context of ab initio electronic structure theory. Self-consistent-field (SCF), two-configuration SCF (TCSCF), and configuration interaction with single and double excitations (CISD) levels of theory are employed in conjunction with two basis sets, double-[zeta] plus polarization (DZP) and triple-[zeta] plus double polarization (TZ2P). For the SCF and TCSCF methods, the TZ2P basis set supplemented with higher angular momentum functions, TZ(2df,-2pd), is also used. Ten stationary points are located on the singlet reaction surface and unambiguously characterized by analyzing the energy second derivatives (Hessian). Three minima are located on the surface corresponding to the closed-shell ring structure (as reactant) and two closed-shell open structures (as products) of C[sub 2] and C[sub s] symmetry. These minima are shown to be connected via four genuine transition states along the corresponding reaction coordinates. Planar open oxirane with C[sub 2v] symmetry (edge-to-edge, E-E isomer) is characterized as neither a minimum nor a transition state but as a higher order stationary point with Hessian index 2. The electrocyclic ring opening of the C-C bond in oxirane is shown to occur preferably through a conrotatory double methylene rotationmore » pathway leading to the C[sub 2] open structure with an activation energy of 52.0 kcal/mol with the zero-point vibrational energy (ZPVE) correction. 57 refs., 7 figs., 3 tabs.« less

Thiols as potential UV radiation protectors: An in vitro study

The following thiols were investigated with regard to their possible UV-radiation protective properties: captopril, cysteamine, ergothioneine, mesna, mercaptopropionylglycine, N-acetylcysteine, and penicillamine. As a measure for protection, the inhibition of in vitro irreversible photobinding of the labeled phototoxic drugs chlorpromazine (CPZ) and 8-methoxypsoralen (8-MOP) to protein and DNA was used. Besides photobinding to biomacromolecules, the photodegradation of CPZ and the formation of promazine (PZH) and hydroxypromazine (PZOH) were measured as well. Because of the H-atom and electron donating capacity of the thiols, the ratio [PZOH]/[PZH] was expected to be decreased and the photodegradation of CPZ was expected to be higher in the presence of thiols. Maximum inhibition of CPZ photobinding ranged for the different thiols between 21–100% (DNA) and 17–87% (human serum albumin). All thiols enhanced the photodegradation of CPZ (19–84%) and inhibited the ratio [PZOH]/[PZH] (90–97%). 8-MOP photobinding to human serum albumin was also clearly inhibited (75–96%), but remarkably less to DNA (2–41%). This study indicates that thiols are able to cope with a variety of reactive species. Scavenging of radicals, quenching of singlet molecular oxygen species and reaction with excited states seem to be essential mechanisms involved with this process.

An MCSCF/MP2 study of methylene insertion-abstraction reactions with C-H and C-Cl bonds

The reaction of singlet and triplet methylene with H3CCl has been investigated using ab initio MCSCF and multi-reference MP2 methods (MCSCF/MP2) with a 6–31G* basis set. In both singlet and triplet cases two transition states (one for the C-H and one for the C-Cl attack) have been located. For the triplet state the two transition structures correspond to H and Cl abstraction, respectively, with the H abstraction favoured at the MCSCF/MP2 level. For the singlet state the transition structures do not allow the type of path followed by the reaction to be assessed. The problem has been solved by computing the intrinsic reaction coordinate (IRC). The IRC results have demonstrated that the two transition states correspond to a methylene insertion into the C-H bond and to a Cl abstraction. The Cl abstraction is favoured with respect to the C-H insertion at the MCSCF/MP2 level in agreement with the experimental evidences. A qualitative diabatic analysis has been used to rationalize these results.

Triplet photochemistry of acyl and imino cyclopropenes. 164. A rearrangement to afford furans and pyrroles: reaction and mechanisms

Syntheses of 3-substituted 3-acylcyclopropenes and 3-benzoylcyclopropene imines were devised. A triplet rearrangement of acylcyclopropenes having C-3 aryl substitution to afford furans has been uncovered and studied with several examples. Unlike the singlet reaction, the triplet process does not afford multiple products such as indenones and cyclopentenones. Two examples of an imino counterpart to afford a pyrrole were found. The acylcyclopropene rearrangement contrasts with the triplet dimerization encountered when there is no C-3 aryl substitution

Reaction of singlet and triplet methylene with ethene. A multireference configuration interaction study

Large-scale multireference configuration interaction (MRD-CI) calculations in a flexible atomic orbital (AO) basis are employed to study the reaction of C 2 H 4 with CH 2 in its first triplet and singlet state.The minimum energy path (MEP) of both reactions is calculated, a different mechanism are discussed. To examine the possible participation of the singlet state in the overall reaction starting from the triplet channel and terminating in the singlet-state c-C 3 H 6 , various cuts through both hypersurfaces are calculated

Singlet- and triplet-state photodissociation of CO and CN bonds in aromatic acetones studied by 1H-CIDNP spectroscopy

The photolysis of (4-methoxyphenoxy)acetone ( 1a) and ( N-methylanilino)acetone ( 1b) in fluid solution was investigated by photo 1H chemically induced dynamic nuclear polarization (CIDNP) spectroscopy. The primary step is a dissociation of the CO or CN bond to give acetonyl and 4-methoxyphenoxy or N-methylanilino radicals respectively. For 1a the dissociative state is the singlet state on direct excitation while by triplet state in a separate reaction channel, acetone is formed. On direct excitation, 1b decomposes via both the singlet and the triplet states to the same primary radical pair. At temperatures below 290 K the triplet reaction dominates. With increasing temperature the singlet reaction becomes favoured and thus an unusual inversion of the CIDNP polarization sign is observed. These quite unexpected observations suggest that the photochemistry of these acetone derivatives should not be classified as those of a typical carbonyl compound. Rather, they show strong resemblance to the photo-Claisen reaction of unsatured ethers or the photo-Fries rearrangement of phenyl esters and anilides.

The effect of hydrogen bonding on the electron transfer reactions of the excited singlet and triplet states of flavins

The effects of hydrogen bonding of riboflavin tetrabutyrate to acetic and trichloroacetic acids in 1,2-dichloroethane were studied. The reactivity of the excited singlet and triplet states shows dramatic increases in the presence of trichloroacetic acid and to a lesser extent acetic acid. However, it is shown that the effects of trichloroacetic acid on the excited triplet state are partly due to protonation in addition to hydrogen bonding. The effects of hydrogen bonding confirm earlier molecular orbital calculations predicting an increase in electrophilicity of flavin excited states upon hydrogen bonding.

Singlet and triplet reactivity in the photoinduced electron transfer from aromatic amines to chlorobenzenes

The electron transfer quenching from the excited singlet state of several aromatic amines and 9-methylcarbazole to chlorobenzenes and chlorobiphenyls was studied by fluorescence quenching in methanolic solutions. The triplet quenching of N, N-dimethylaniline and 9-methylcarbazole was measured by laser flash photolysis. It was found that the singlet rate constants do not follow the same free-energy correlation as the triplet state. The reactivity of the excited singlet is lower than that of the triplet for a given overall free-energy change. It is proposed that this effect is due to a contribution from an inner-sphere reorganization to the intrinsic energy barrier, which would be higher in the case of the singlet reaction.

Photochemical transformations. 49. Solvent effects on singlet ionic and triplet photoreactions of some bridged polycyclic chlorides: Comparison with ground-state reactions

Direct irradiations of 2-chloro-6,7 : 8,9-dibenzotricyclo [3.2.2.02,4] nona-6,8-diene (5-Cl) were conducted in cyclohexane, acetic acid and wet acetonitrile. The products are 1-methylfluoranthene (8) in all three solvents, the allylic chlorides 8-chloro-7-methylene-2,3 : 5,6-dibenzobicyclo [2.2.2] octa-2,5-diene (3-Cl) and 7-chloromethyl-2,3 : 5,6-dibenzobicyclo [2.2.2] octa-2,5,7-triene (2-Cl) in acetic acid and acetonitrile and solvolysis product amides in wet acetonitrile. Compound 5-Cl had previously been shown to be the product of triplet sensitization of 2-Cl and 3-Cl, so that the singlet reaction reverses that of the triplet. The formation of 8 from 5-Cl was quenched with piperylene, whereas that of the allylic chlorides was not. Quantum yields of products and singlet lifetimes in the three solvents were measured. The solvent effects are discussed. Deuterium-labeling results on the formation of 5-Cl from 2-Cl and of 8 from 5-Cl are reported.

Chemistry with Red and Near Infrared Light

Opportunities are explored to initiate controlled chemistry by photolysis with long-wavelength visible and near infrared light. In one direction, bimolecular reactions are induced by exciting collisional pairs in a solid matrix to energy surfaces well below reactant dissociation limits. Examples discussed include product specific (including stereospecific) photo-oxidation of small alkenes and alkynes by NO2, and cycloaddition reactions of singlet SO and singlet O2. Conducting the chemistry in rare gas matrices allows us to elucidate elementary reaction steps by FT-IR spectroscopy of trapped intermediates, and to gain insight into the dynamics of transients by wavelength-dependent laser photochemistry. In the case of olefin epoxidations, stereochemical details of reaction paths are uncovered by chemical trapping of transient oxirane biradicals in their nascent conformation. State-specific reactions of singlet excited SO and singlet O2 with hydrocarbons illustrate how photons deep in the near infrared can be used for controlled chemical synthesis. In parallel work, excited state redox reactions in homogeneous and colloidal semiconductor solutions are explored that pertain to chemical storage of near infrared quanta and conversion into electrical energy. Examples are studies on chemical storage of singlet delta molecular oxygen (O2(1Δ)), a metastable near infrared energy carrier, its excited state redox chemistry in aqueous solution, and direct reduction of O2(1Δ), at a semiconductor electrode. Using very sensitive time resolved optical techniques, new mechanistic insight is gained on singlet O2 reactions, and on photo-oxidation of halide at the semiconductor-solution interface.

MECHANISMS OF PHOTOELIMINATION REACTIONS OF ALKYL BENZOHYDROXAMATES

Abstract— Photolysis at 254 nm of alkyl benzohydroxamates [C, H, CONHOR: R = CH3 H2CH3 CH(CH3)2, CH2C6H5 CH(CH3)C2H5 CH(CH3)‐n‐C6H13] in acetonitrile or hydrocarbon solvents gives benzamide. These reactions can be sensitized by benzophenone (at ca. 350 nm) and are quenched by cis‐piperylene. Racemization occurred when 2‐octyl (+)‐benzohydroxamate was irradiated in cyclohexane. These results are consistent with a mechanism involving a triplet biradical. Photolysis of phenyl benzohydroxamate [C6H5CONHOC6H5] and benzyl N‐methylbenzohydroxamate [C6H5CON‐(CH3)OCH2Q6H5] cannot be quenched with ris‐piperylene and appear to be singlet reactions.

Isomer formation selectivity in the reactions of the excited singlet and triplet states of pyruvates: A CIDEP study

On irradiation in solution in hydrogen-donating solvents, methyl and ethyl pyruvate react through their excited singlet and triplet states to yield a pair of isomeric radicals. Each isomer is formed via both pathways, but in some solvents one is produced predominantly from the singlet state and the other from the triplet state. The behaviour is rationalised to a large extent by consideration of the overall symmetry of the excited states of the parent molecule, in its two isomeric forms, leading to the concept of triplet state spin symmetry control of the reaction.

ChemInform Abstract: Photodimerization of Singlet trans‐ and cis‐Anethole. Concerted or Stepwise?

AbstractThe products (III) and (IV) are formed from the reaction of singlet (II) with ground state (I) and (II), respectively.

Experimental studies of the spin-correlated radical pair in micellar and microemulsion media; MARY, RYDMRB0and RYDMRB1spectra

The effects of static magnetic fields and resonant microwave fields on the reaction of spin-correlated radical pairs are reported in well-characterized micellar and microemulsion media, using MARY, RYDMR B 0 and RYDMR B 1 techniques in a new apparatus. Large field effects were observed and direct evidence was obtained for the influence of the electron exchange interaction on the results. The results of a theoretical analysis of the RYDMR lineshapes resulting from reaction of an initial triplet radical pair through a singlet reaction channel are presented.

ChemInform Abstract: Photocyclization of N‐Alkyl‐N‐(3‐aryl‐3‐butenyl)ureas. Hydrogen Transfer via Charge Transfer Exciplexes in Alkene Photochemistry.

Photolysis of N-alkyl-N-(3-aryl-3-butenyl) ureas (1) in acetonitrile gave cyclization products, 3-aryl-3-methyl-pyrrolidines, in good yields, whereas irradiation of 1 in methanol afforded methanol adducts as well as the cyclization products. Both the reactions are singlet reactions, and the cyclization is presumed to proceed via 1,6-hydrogen transfer from exciplexes with charge transfer character.

Pressure effects on the photocycloaddition of 2-adamantanone with fumaronitrile

The photochemical [2+2] cycloaddition of 2-adamantanone (AD) with trans-dicyanoethylene ( t-DCE) was studied in acetonitrile as a function of pressure, and the activation volumes for both singlet and triplet reactions were determined. The competing reactions between excited singlet and triplet states of AD were found to be pressure dependent. At high pressure, the intersystem crossing rate, k st, from singlet to triplet increased, and as a result the singlet lifetime of AD was shortened and the triplet reaction which leads to cis—trans isomerization of the starting olefin became favored as pressure was increased. The Stern—Volmer slopes were found to correlate linearly with solvent viscosity; however, the triplet reaction ( k q t/ k t) correlated linearly only with the solvent dielectric constant.

Reactions of singlet and triplet methylene with a C-H bond of ethylene. An ab initio study

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTReactions of singlet and triplet methylene with a C-H bond of ethylene. An ab initio studyMiquel Moreno, Jose M. Lluch, Antonio Oliva, and Juan BertranCite this: J. Phys. Chem. 1988, 92, 14, 4180–4184Publication Date (Print):July 1, 1988Publication History Published online1 May 2002Published inissue 1 July 1988https://doi.org/10.1021/j100325a037RIGHTS & PERMISSIONSArticle Views48Altmetric-Citations11LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (694 KB) Get e-Alerts

Photocyclization of N‐Alkyl‐N‐(3‐aryl‐3‐butenyl) ureas. Hydrogen transfer via charge transfer exciplexes in alkene photochemistry

AbstractPhotolysis of N‐alkyl‐N‐(3‐aryl‐3‐butenyl) ureas (1) in acetonitrile gave cyclization products, 3‐aryl‐3‐methyl‐pyrrolidines, in good yields, whereas irradiation of 1 in methanol afforded methanol adducts as well as the cyclization products. Both the reactions are singlet reactions, and the cyclization is presumed to proceed via 1,6‐hydrogen transfer from exciplexes with charge transfer character.

Photochemical reactions of c i s- and t r a n s-1,2-dichloroethene adsorbed on Pd(111) and Pt(111)

The photochemical behaviors of cis- and trans-1,2-dichloroethene (DCE) adsorbed on Pd(111) and Pt(111) surfaces have been studied using electron energy loss spectroscopy (EELS). For multilayer coverage on either metal surface, irradiation of physisorbed DCE at 110 K with broad band irradiation (λ>200 nm) results in photoisomerization, cis–trans. For monolayer coverage on Pt(111) at 110 K, photolysis of chemisorbed DCE causes loss of the two chlorine atoms to form a single hydrocarbon product, chemisorbed acetylene. Apparently, for λ>237 nm, the chlorine atoms remain bound to the platinum surface whereas for shorter wavelengths, λ>200 nm, the chlorine atoms leave the surface. These results are interpretable in terms of singlet excitation of the chemisorbed alkane followed by chlorine elimination on an excited singlet reaction surface. This study indicates that photochemistry of molecules chemisorbed on a metal surface is possible despite the proximity of the conducting surface. It shows that energy relaxation processes connected with this proximity are not prohibitively fast.