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The mechanisms for the photochemical CO-dissociation and the oxidative addition reactions are studied theoretically using three model systems: M(CO)5 (M = Fe, Ru, and Os) and the CASSCF/Def2-SVP (fourteen-electron/ten-orbital active space) and MP2-CAS/Def2-SVP//CASSCF/Def2-SVP methods. The structures of the intersystem crossings and the conical intersections, which play a decisive role in these CO photo-extrusion reactions, are determined. The intermediates and the transition structures in either the singlet or triplet states are also computed, in order to explain the reaction routes. These model studies suggest that after the irradiation of Fe(CO)5 with UV light, it quickly loses one CO molecule to generate a 16-electron iron tetracarbonyl, in either the singlet or the triplet states. It is found that the triplet Fe(CO)4 plays a vital role in the formation of the final oxidative addition product, Fe(CO)4(H)(SiMe3), but the singlet Fe(CO)4 plays a relatively minor role in the formation of the final product. However, its vacant coordination site interacts weakly with solvent molecules ((Me3)SiH) to yield the alkyl-solvated iron complexes, which are detectable experimentally. The theoretical observations show that Ru(CO)5 and Os(CO)5 have similar photochemical and thermal potential energy profiles. In particular, this study demonstrates that the oxidative addition yield for Fe is much greater than those for its Ru and Os counterparts, under the same chemical conditions.

The group 9 organometallic complexes η5-CpM(CO)2 (M = Co, Rh, and Ir) and Si(CH3)3(H) have been considered as a model system to study their photochemical decarbonyl reactions as well as the Si–H bond activation reactions using the CASSCF and MP2-CAS computational methods. For the cobalt complex, three kinds of reaction pathways, which result in the same oxidative addition product, are investigated. Our theoretical finding demonstrated that after the photoirradiation, η5-CpCo(CO)2 loses one CO ligand without any difficulty to form either the triplet ([η5-CpCo(CO)]3) or singlet ([η5-CpCo(CO)]1) species. The former plays a decisive role in the formation of the final oxidative addition product. On the other hand, the latter plays no role in the production of the final product molecule, but its singlet cobalt center interacts weakly with solvent molecules ((Me3)SiH) to produce an alkyl-solvated organometallic complex, which is experimentally detectable. The present works reveal that both η5-CpRh(CO)2 and η5-CpIr(CO)2 should adopt the conical intersection mechanism after they are irradiated by light. Moreover, our theoretical examinations strongly suggest that for the 16-electron monocarbonyl η5-CpM(CO) (M = Rh and Ir) species, the insertion into a Si–H bond by the Ir system is much more facile and more exothermic than that for the Rh counterpart.

The mechanisms of photochemical rearrangement reactions are studied theoretically using a model system of bicyclic molecules that features a cyclopropane ring that is in proximity to a double bond moiety (such as CC or C O group), using the M06-L method and the 6-311G(d,p) basis set. The theoretical findings show that intersystem crossing is vital to successfully interpret the mechanisms. This theoretical research also shows that the photoisomerization mechanism should proceed as follows: Rea-S0 → FC-T1 → Min-T1 → TS1-T1 → T1/S0 → Int1-S0 → TS2-S0 → Int2-S0 → TS3-S0 → Pro-S0. The theoretical evidence in this work confirms the experimental results for both [3.1.0] and [5.1.0] bicyclic molecules. The theoretical observations also predict that for both [2.1.0] and [4.1.0] systems, the quantum yields of the tautomeric keto forms are greater than those of the corresponding enol isomers.

We used computational methods to explore the mechanisms of the photochemical decarbonylation and the Si–H bond activation reaction of the group 7 organometallic compounds, η5-CpM(CO)3 (M = Mn and Re). The energies of both conical intersections and the intersystem crossings, which play a decisive role in these photo-activation reactions, are determined. Both intermediates and transition states in either the singlet or triplet states are also computed to furnish a mechanistic interpretation of the whole reaction paths. In the case of Mn, four types of reaction pathways (path I–path IV) that lead to the final insertion product are examined. The theoretical findings suggest that at the higher-energy band (295 nm) the singlet-state channel is predominant. As a result, the conical intersection mechanism (i.e., path I) prevails. However, at the lower-energy band (325 nm) the triplet-state channel occurs. In such a situation, the intersystem crossing mechanism (i.e., path IV) can successfully explain its CO-photodissociation mechanism. In the case of Re, on the other hand, the theoretical evidence reveals that only the singlet state-channel is superior. In consequence, the conical intersection mechanism (i.e., path V) can more effectively explain its photochemical decarbonylation mechanism. These theoretical analyses agree well with the available experimental observations.

The mechanisms for the photochemical isomerization reactions are determined theoretically using the acrylonitrile model molecule. The CASSCF (twelve-electron/eleven-orbital active space) and MP2-CAS methods are respectively used with the 6-311G(d,p) and 6-311++G(3df,3pd) basis sets. The structure of the conical intersection that plays a prominent role in the photoisomerization of acrylonitrile is obtained. The intermediates and the transition structures of the ground states are also calculated, to allow a qualitative explanation of the reaction pathways. These model studies suggest that the preferred reaction route is: acrylonitrile → Franck-Condon region → conical intersection → isoacrylonitrile → transition state → intermediate complex → transition state → cyanoacetylene. The theoretical evidence suggests that conical intersections found in this paper can give a better understanding of the photochemical reactions of acrylonitrile and support the experimental observations.

Singlet O2 is a key reactive oxygen species responsible for photodynamic therapy and is generally recognized to be chemically reactive towards C=C double bonds. Herein, we report the hydroperoxidation/lactonization of α-ethereal C–H bonds by singlet O2 (1Δg) under exceptionally mild conditions, i.e., room temperature and ambient pressure, with modest to high yields (38~90%) and excellent site selectivity. Singlet O2 has been known for > 90 years, but was never reported to be able to react with weakly activated C–H bonds in saturated hydrocarbons. Theoretical calculations indicate that singlet O2 directly inserts into the α-ethereal C–H bond in one step with conservation of steric configuration in products. The current discovery of chemical reaction of singlet oxygen with weakly activated solvent C–H bonds, in addition to physical relaxation pathway, provides an important clue to a 35-year-old unresolved mystery regarding huge variations of solvent dependent lifetime of singlet O2., Singlet oxygen is known to react with carbon–carbon double bonds. Here, the authors show the unusual functionalization of α-ethereal C–H bonds mediated by singlet oxygen under mild conditions to afford lactones and hydroperoxide products.

The mechanisms for the photochemical isomerization reactions are determined theoretically using group 6 Fischer carbene complexes (CO)5M═C(Me)(OMe) (M = Cr, Mo, and W) and the complete-active-space self-consistent field (CASSCF) (10-orbital/8-electron active space) and second-order Moller–Plesset perturbation (MP2-CAS) methods with the Def2-SVPD basis set. The structures and energies of the singlet/singlet conical intersections and the triplet/singlet intersystem crossings, which play a decisive role in these photoisomerizations, are determined. The former is applied to the chromium and molybdenum systems because their photoproducts are essentially from the singlet excited states. The latter is applied to the tungsten complex because its photoproducts are formed from a low-lying triplet excited state. Two reaction pathways are examined in this work: photocarbonylation (path I) and CO-photoextrusion (path II). The model studies strongly indicate that in the photochemistry of Cr and Mo Fischer carbene syste...

The mechanisms for photochemical isomerization reactions are studied theoretically using the model systems, cyclotetrasilene and cyclobutene, both of which feature one four-membered-ring and one double bond, using the CAS(6,6)/6-311G(d) and CAS-MP2-(6,6)/6-311++G(3df,3pd) methods. The mechanisms, which are governed by the conical intersection concept, play a crucial role in these photorearrangement reactions, and explain the reaction pathways. The model computations demonstrate that the preferred reaction path for the photoisomerization reactions for both cyclotetrasilene and cyclobutene is as follows: four-membered-ring reactant → Franck–Condon region → conical intersection → butterfly-like photoproduct. These results allow a number of predictions to be made.

The mechanisms for photochemical carboxylation reactions are studied theoretically using two model systems: o-methylbenzophenone and o-methylacetophenone, with the M06-L and the 6-311G(d,p) basis set. Three reaction routes are used to ascertain the actual photochemical reaction mechanism for the o-alkylphenyl ketone molecules. The structures of the intersystem crossing, which play a central role in these photocarboxylations, are determined. The intermediates and transition structures for the ground singlet states and the excited triplet states are also computed to allow a qualitative interpretation of the entire reaction path. The theoretical findings show that when o-alkylphenyl ketone molecules are produced in the T1 state by photoexcitation at 365 nm, intersystem crossing to the S0 surface is the most probable pathway for deactivation. When they relax to the S0 state, the o-alkylphenyl ketone molecules can either react with CO2 to undergo the insertion reaction, which produces the carboxylic acid, or revert to the reactants. In particular, the theoretical investigations strongly suggest that the insertion of CO2 in these photocarboxylation reactions for o-alkylphenyl ketones occurs on the S0 surface, rather than on the excited T1 surface.

The mechanism for the photochemical rearrangement of a cyclic divinyldisilane (1-Si) in its first excited state ((1)π → (1)π*) is determined using the CAS/6-311G(d) and MP2-CAS/6-311++G(3df,3pd) levels of theory. The photoproduct, a cyclic silene, reacts with various alcohols to yield a mixture of cis- and trans- adducts. The two reaction pathways are denoted as the cis- addition path (path A) and the trans-addition path (path B). These model studies demonstrate that conical intersections play a crucial role in the photo-rearrangements of cyclic divinyldisilanes. The theoretical evidence also demonstrates that the addition of alcohol to a cyclic divinyldisilane follows the reaction path: cyclic divinyldisilane → Franck-Condon region → conical intersection → photoproduct (cyclic silene) → local intermediate (with alcohol) → transition state → cis- or trans-adduct. The theoretical studies demonstrate that the steric effects as well as the concentrations of CH3OH must have a dominant role in determining the yields of the final adducts by stereochemistry. The same mechanism for the carbon derivative (1-C) is also considered in this work. However, the theoretical results indicate that 1-C does not undergo a methanol addition reaction via the photochemical reaction pathway, since its energy of conical intersection (S1/S0-CI-C) is more than that of its FC (FC-C). The reason for these phenomena could be that the atomic radius of carbon is much smaller than that of silicon (77 and 117 pm, respectively). As a result, the conformation for 1-C is more sterically congested than that for 1-Si, along the 1,3-silyl-migration pathway.

The mechanism for the photochemical cyclization reaction is studied theoretically using a model system of α-N-methylamidoacetophenone with the M06-L/6-311G(d,p) method. These model studies indicate that the preferred reaction pathway for α-N-methylamino ketone, which leads to the photocyclization product, is as follows: reactant (singlet) → Franck–Condon region (triplet) → local minimum (triplet) → transition state (triplet) → intersystem crossing → intermediate (singlet) → transition state (singlet) → photoproduct (singlet). In other words, the intersystem crossing mechanism plays a decisive role in these photo-rearrangement reactions for α-N-alkylamino ketone systems. These computational observations are in agreement with the available experimental results.

The potential energy surfaces corresponding to the photolytic reactions of 1,2-dimethyl-cyclopentene, 3,4-dimethyl-silacyclopent-3-ene, and 3,4-dimethyl-germacyclopent-3-ene were investigated by employing the CAS(6,6)/6-311G(d) and MP2-CAS-(6,6)/6-311++G(3df,3pd)//CAS(6,6)/6-311G(d) methods. Also, six kinds of substituted germacyclopent-3-ene were used as model reactants by way of the CASSCF and MP2-CAS methods to study their photolytic mechanisms. The theoretical findings indicate that the photolysis of the above reactants all adopt the same reaction path as follows: reactant → Franck-Condon region → conical intersection → germylene and 1,3-butadiene. However, the theoretical results demonstrate that no photolysis ((1)(π →π*)) can be observed in the 1,2-dimethyl-cyclopentene system. Above all, the theoretical investigations strongly suggest that both steric effects, originating from the bulky substituents, and the atomic radius of the group 14 element (C, Si, and Ge) play a crucial role in determining the cis/trans selectivity of the conformation of 1,3-butadiene during their photolytic reactions.

The mechanisms for photochemical isomerization reactions were examined theoretically using a model system of a parent silabenzene with CAS(6,6)/6-311G(d) and MP2-CAS-(6,6)/6-311++G(3df,3pd)//CAS(6,6)/6-311G(d) methods. Five reaction pathways leading to five types of photoisomers have been investigated. The theoretical computations indicate that conical intersections play a prominent role in the photoisomerization of silabenzenes. The model investigations reveal that the preferred reaction route for silabenzene should result in the corresponding silabenzvalene, rather than the Dewar silabenzene isomer. Moreover, the theoretical computations suggest that all of the photochemical mechanisms of silabenzene should proceed as follows: reactant → Franck–Condon region → conical intersection → photoproduct. In other words, the photochemical mechanism for silabenzene should be a barrierless, single-step process. The computational results agree well with available experimental observations.

The mechanisms for photochemical isomerization reactions are studied theoretically using three model systems: 3-methylisothiazole, 4-methylisothiazole and 5-methylisothiazole. The CASSCF (ten-electron/seven-orbital active space) and MP2-CAS methods are used with the 6-311G(d) and 6-311++G(3df,3pd) basis sets, respectively. Three mechanisms, the internal cyclization-isomerization route (path A), the ring contraction-ring expansion route (path B) and the direct route (path C), are used to determine the actual photochemical reaction mechanism for these three model molecules. The structures of the conical intersections, which play a crucial role in these photo-transpositions, are determined. The intermediates and transition structures of the ground states are also calculated, to give a qualitative explanation of the reaction pathways. These model investigations suggest that the preferred reaction route is: reactant → Franck-Condon region → conical intersection → photoproduct. Particularly, the direct mechanism (path C) described in this work gives a better explanation than other pathways proposed before and is supported by the experimental observations. The results obtained allow a number of predictions to be made.

The mechanisms for the photoisomerization reactions are studied theoretically at the M06-2X/Def2-TZVPD level of theory, using the five-coordinated M(CO)4(CS) (M = Cr, Mo, and W) complexes as model systems. This study provides the first theoretical evidence for the mechanisms of these photorearrangements of the five coordinated metal complexes. That is, the photoisomerization process is primary the axial to basal movement of CS. The model study demonstrates that the preferred reaction route for the photorearrangement reactions is as follows: reactant → Franck-Condon region → minimum (triplet) → transition state (triplet) → triplet/singlet intersystem crossing → photoproduct. The theoretical results also show that the energy differences between the crucial points are quite small, which demonstrates that the CS group rotates easily to form the different conformations when the M(CO)4(CS) molecules have been photoirradiated. These photochemical mechanisms are consistent with the available experimental observations.

The mechanisms for the photochemical Si-H bond activation reaction are studied theoretically using a model system of the group 5 organometallic compounds, η(5)-CpM(CO)4 (M = V, Nb, and Ta), with the M06-2X method and the Def2-SVPD basis set. Three types of reaction pathways that lead to final insertion products are identified. The structures of the intersystem crossings, which play a central role in these photo-activation reactions, are determined. The intermediates and transitional structures in either the singlet or triplet states are also calculated to provide a mechanistic explanation of the reaction pathways. All of the potential energy surfaces for the group 5 η(5)-CpM(CO)4 complexes are quite similar. In particular, the theoretical evidence suggests that after irradiation using light, η(5)-CpM(CO)4 quickly loses one CO ligand to yield two tricarbonyls, in either the singlet or the triplet states. The triplet tricarbonyl 16-electron intermediates, ([η(5)-CpM(CO)3](3)), play a key role in the formation of the final oxidative addition product, η(5)-CpM(CO)3(H)(SiMe3). However, the singlet counterparts, ([η(5)-CpM(CO)3](1)), play no role in the formation of the final product molecule, but their singlet metal centers interact weakly with solvent molecules ((Me3)SiH) to produce alkyl-solvated organometallic complexes, which are observable experimentally. This theoretical evidence is in accordance with the available experimental observations.

The potential energy surfaces for the reactions of phosphino dimetalalkenes featuring an E═E double bond, Rea-E═E, where E = group 14 elements, were investigated using density functional theory (B3LYP/LANL2DZ). Three types of chemical reactions (i.e., the rearrangement reaction, the transition metal complexation reaction, and the [2 + 2] cycloaddition with a diazene) were used to study the reactivity of the Rea-E═E molecules. The theoretical findings reveal that the smaller the singlet-triplet splitting (ΔE(st)) of the Rea-E═E, the lower are its activation barriers and, in turn, the more rapid are its chemical reactions. Theoretical observations suggest that the relative reactivity increases in the order Rea-C═CRea-Si═SiRea-Ge═GeRea-Sn═SnRea-Pb═Pb. In other words, the smaller the atomic weight of the group 14 atom (E), the smaller is the atomic radius of E and the more stable is its phosphino Rea-E═E to chemical reaction. It is thus predicted that the phosphino Rea-C═C and Rea-Si═Si molecules should be stable and readily synthesized and isolated at room temperature, since they are quite inert to chemical reaction. The computational results are in good agreement with the available experimental observations. The theoretical results obtained in this work allow a number of predictions to be made.

The potential energy surfaces for the intramolecular reactions of heavy 1,3-butadiene have been explored using DFT. All the stationary points, which include the heavy 1,3-butadienes (R2E=ER–ER=ER2, E = group 14 element), intramolecular fragments, transition states, and the products, were completely optimized at the B3LYP/LANL2DZ level of theory. Five 1,3-butadiene species, which include carbon, silicon, germanium, tin, and lead, were chosen as model reactants. Our theoretical findings suggest the following: (1) both sterically bulky substituents attached to the heavy butadiene and the weakness of the E=E double bond lead to the easy cleavage of one E=E double bond in heavy 1,3-butadiene and (2) for two intramolecular reactions (cycloaddition and 1,2-migration) of the heavy 1,3-butadienes with sterically overcrowded substituents, the lighter the E atoms involved in the 1,3-butadiene molecule, the smaller the intramolecular barrier, the lower the reaction enthalpy, and the more facile its intramolecular reaction at room temperature. Furthermore, a configuration-mixing model has been used to rationalize the computational results, and the results obtained allow a number of predictions to be made.

The potential energy surfaces of the cycloaddition reactions M(CO)(4) + C(60) → (CO)(4)M(C(60)) (M = Fe, Ru, and Os) have been studied at the B3LYP/LANL2DZ level of theory. It has been found that these reactions have two competing pathways, which can be classified as a [6,5]-attack (path A) and a [6,6]-attack (path B). Our B3LYP results suggest that, given the same reaction conditions, the [6,6]-attack is more favorable than the [6,5]-attack both kinetically and thermodynamically. A qualitative model based on the theory of Pross and Shaik has been used to develop an explanation for the barrier heights. As a consequence, the theoretical findings indicate that the singlet-triplet splitting ΔE(st) (=E(triplet) - E(singlet)) of the 16-electron d(8) M(CO)(4) and C(60) species can be used as a guide to predict their reactivity toward cycloaddition. Our computational results reveal that the reactivity of d(8) M(CO)(4) cycloaddition to C(60) decreases in the order Fe(CO)(4)Os(CO)(4)Ru(CO)(4). Accordingly, we demonstrate that both electronic and geometric effects play a crucial role in determining the energy barriers as well as the reaction enthalpy.

The mechanisms of the photochemical isomerization reactions were investigated theoretically using a model system of cyclohexene (1), cycloheptene (2), norbornene (3), and two bicyclic olefins (4 and 5) using the CASSCF (six-electron/six-orbital active space) and MP2-CAS methods with the 6-311(d,p) basis set. The structures of the conical intersections, which play a decisive role in such photoisomerizations, were obtained. The intermediates and transition structures of the ground state were also calculated to assist in providing a qualitative explanation of the reaction pathways. Two photoreaction pathways were examined in the present work. The first can produce a photoproduct with an extra ring. The other can yield a photoproduct with a smaller ring with an external double bond. Both pathways involve cyclic carbene intermediates. Also, our model investigations suggest that both reaction pathways follow a similar photochemical pattern as follows: reactant → Franck-Condon region → conical intersection → cyclic carbene intermediate → transition state → photoproduct. Moreover, these two reaction pathways can compete with each other since the energetics of their conical intersection points are quite similar. Our present theoretical results agree with the available experimental observations.

The mechanisms of the photochemical isomerization reactions were investigated theoretically using three model systems; 2-methylthiophene, 2-cyanothiophene, and 2-phenylthiophene. The CASSCF (10-electron/eight-orbital active space) and MP2-CAS methods were employed with the 6-311(d) basis set. Three mechanisms, i.e., the internal cyclization-isomerization route (path A), the zwitterion-tricyclic route (path B), and the direct route (path C), have been used to explore the real photochemical reaction mechanism of these three model molecules. The structures of the conical intersections, which play a key role in such phototranspositions, were obtained. The intermediates and transition structures of the ground states were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred reaction route is as follows: reactant → Franck-Condon region → conical intersection → photoproduct. In particular, the conical intersection mechanism described in this work gives a better explanation than either the previously proposed internal cyclization-isomerization (path A) or the zwitterion-tricyclic pathway (path B) mechanisms, and is supported by the experimental observations. The results obtained allow a number of predictions to be made. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010

The potential energy surfaces for the formation, addition, and cycloaddition reactions of dialkylsilanechalcogenone (2) species have been studied using three levels of theories, i.e., B3LYP/LANL2DZdp, CCSD theory, and Gibbs free energy (at the B3LYP level). Four dialkylsilanechalcogenone species with a Si═X bond, where X = O, S, Se, and Te, have been chosen as model reactants in this work. Also, both MeOH addition and isoprene cycloaddition have been used to study the chemical reactivities of these species (2). The present theoretical investigations suggest that the relative reactivity of 2 increases in the order X = O < S < Se ≪ Te. That is, the species with a less electronegative and a heavier chalcogen atom will have a smaller ΔEst, which facilitates its addition with MeOH and its cycloaddition reaction to isoprene. Furthermore, the singlet−triplet energy splitting, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict the ...

The mechanisms of the photochemical isomerization reactions were investigated using two model systems of indoxazene (1) and isoazole-3-carboxylate (5) with the CASSCF and MP2-CAS methods using the 6-311G(d,p) basis set. The active space of the former consists of 14 electrons in 11 orbitals, while that of the latter consists of 10 electrons in seven orbitals. Two reaction pathways were examined in the present work. They are referred to as the internal cyclization-isomerization path (paths I and III) and the direct path (paths II and IV). Our model investigations suggest that the preferred reaction route for these species is as follows: reactant --> Franck-Condon region --> conical intersection --> photoproduct. In particular, the direct (conical intersection) mechanism found in this work gives a better explanation than the previous proposed mechanism and also supports the available experimental observations. Additionally, a simple p-pi orbital topology model is proposed that can be used as a diagnostic tool to predict the location of the conical intersections as well as the geometries of the phototransposition products of various heterocycles.

The potential energy surfaces for the chemical reactions of group 14 carbenes have been studied using density functional theory (B3LYP/LANL2DZ). Five group 14 carbene species containing three six-membered rings, perimidine-E, where E = C, Si, Ge, Sn, and Pb, have been chosen as model reactants in this work. Also, four kinds of chemical reaction, C−H bond insertion, alkene cycloaddition, O−H bond insertion, and dimerization, have been used to study the chemical reactivities of these group 14 carbenoids. The present theoretical investigations suggest that the relative carbenic reactivity decreases in the order C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the less reactive are its carbene compounds with respect to chemical reaction. Also, our theoretical findings suggest that all the perimidine-E carbenes are isolable at room temperature, since they are quite inert to chemical reactions, except for reaction with water. Furthermore, the group 14 carbene singlet−triplet energy splitting, ...

The mechanisms of the photochemical isomerization reactions were investigated theoretically using a model system of bicyclo[4,1,0]hept-2-ene (2-norcarene) 1 with the CASSCF (eight-electron/eight-orbital active space) and MP2-CAS methods and the 6-311(d) basis set. The structures of the conical intersections and intersystem crossings, which play a crucial role in such photoisomerization reactions, were obtained. The intermediates and transition structures of the ground-state were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred singlet photoreaction route for 1 is as follows: singlet reactant --> Franck-Condon region --> conical intersection --> intermediate --> transition state --> photoproduct. On the other hand, our theoretical findings indicate that the preferred triplet photoreaction route for 1 is as follows: singlet reactant --> Franck-Condon region --> triplet minimum --> triplet transition state --> intersystem crossing --> intermediate --> singlet transition state --> photoproduct. In particular, the intersystem crossing mechanism found in this work gives a better explanation and supports the available experimental observations. Two kinds of reaction pathways, which can lead to final photoproducts, have been identified: (paths I or III) ring-expansion to form a cycloheptene ring and (paths II or IV) ring-closure to form a methylcyclohexene structure. Both exhibit biradical character. Also, our theoretical investigations strongly indicate that substantial interaction occurs between the cyclopropane moiety and the isolated carbon-carbon double bond in the excited state of (1).

The ground-, (1)(pipi*)-, and (3)(pipi*)-state potential-energy surfaces of 1,2-cyclononadiene and isomeric C(9)H(14) species, as well as 1-methyl-1,2-cyclononadiene and isomeric C(10)H(16) species were all mapped using CASSCF and the 6-31G(d) basis set. Theoretical results were found to be in good agreement with the available experimental observations for both 1,2-cyclononadiene and 1-methyl-1,2-cyclononadiene isomerization reactions under singlet and triplet direct or sensitized irradiation. Extremely efficient decay occurs from the first singlet excited state to the ground state through at least three different conical intersections (surface crossings). The first of these crossing points is accessed by a one-bond ring closure. From this conical intersection point (CI-A or CI-C), some possible subsequent ground-state reaction paths have been identified: 1) intramolecular C--H bond insertion to form the bicyclic photoproduct and 2) intramolecular C--H bond insertion to form tricyclic photoproducts. An excited state [1,3]-sigmatropic shift leads to the second conical intersection (CI-B or CI-E), which can give a three-bond cyclononyne species. Besides these, in the singlet photochemical reactions of 1-methyl-1,2-cyclononadiene, excited-state, one allenic C--H bond insertion leads to a third conical intersection (CI-D). Possible ground-state reaction pathways from this structure lead to the formation of a diene photoproduct or to transannular insertion photoproducts. Moreover, in the case of triplet 1,2-cyclononadiene and 1-methyl-1,2-cyclononadiene photoisomerization reactions, both chemical reactions will adopt a 1,3-biradical (T(1)/S(0)-1, T(1)/S(0)-2, and T(1)/S(0)-3), which may undergo intersystem crossings leading to the formation of tricyclic or bicyclic photoproducts. The results obtained allow a number of predictions to be made.

The potential energy surfaces for the reactions of stable silylenes with carbon tetrachloride have been characterized in detail using density functional theory [B3LYP/6-311G(d)], including zero-point corrections. Five stable silylene species (1-5) have been chosen in this work as model reactants. The activation barriers and enthalpies of the reactions are compared to determine the relative reactivity of the stable silylenes on the reaction potential energy surface. Our theoretical findings suggest that stable silylene 5, which has two carbon atoms bonded to the silicon center and does not contain a resonance structure, is relatively unstable with respect to the reaction with haloalkanes, in comparison with the other stable silylenes (1-4). Of the three possible reaction paths, Cl abstraction (path 1), CCl3 abstraction (path 2), and CCl4 insertion (path 3), path 1 is found to be most favorable, with a very low activation energy and a large exothermicity. In short, electronic as well as steric factors play a dominant role in determining the chemical reactivity of the stable silylene species kinetically as well as thermodynamically. Furthermore, a configuration mixing model based on the work of Pross and Shaik is used to rationalize the computational results. The results obtained allow a number of predictions to be made.

A density functional study of triplet ground-state carbenes was carried out using the B3LYP/6-311G(d) and B3LYP/cc-PVTZ_DK methods. The structures of 10 model carbenes in their closed-shell singlet and open-shell triplet states were obtained. Both a valence electron model and our quantum calculations strongly suggest that a persistent triplet ground-state carbene occurs when the carbenic center is attached to two bulky substituents arranged in a perpendicular conformation. If the attached substituents bear aromatic rings, then their ortho positions must be occupied by other protecting groups. Moreover, if the attached atoms are less electronegative than the carbon atom, this can also lead to an increase in the singlet-triplet energy gap, and thus to further stabilization of the triplet state. The results obtained allow a number of predictions to be made.

The potential energy surfaces for the abstraction reactions of heavy cyclopropenes (X-Y-Z) with carbon tetrachloride have been characterized in detail using density functional theory (B3LYP), including zero-point corrections. Five cyclopropene analogues including A(C-C-C), B(Ge-Si-Si), C(Si-Si-Si), D(Si-Si-Ge), and E(Ge-Ge-Ge), have been chosen in this work as model reactants. Two reaction paths, the Cl abstraction I and the CCl3 abstraction II, have been considered in the present study. Our theoretical findings strongly suggest that the former is more favorable, with a very low activation energy and a large exothermicity. This is in accordance with available experimental observations. Moreover, our theoretical investigations also indicate that the more electropositive the elements making up the double bond of a heavy cyclopropene, the lower its activation barrier and the more exothermic the haloalkane abstraction. That is, electronic factors play a dominant role in determining the chemical reactivity of the heavy cyclopropene species kinetically as well as thermodynamically. Furthermore, a configuration mixing model based on the work of Pross and Shaik is used to rationalize the computational results. The results obtained allow a number of predictions to be made.

The potential energy surfaces for the cycloaddition reactions of formally double-bonded molecules containing group 13 elements have been studied using density functional theory (B3LYP/LANL2DZ). Five group 13 alkene analogues, ArX=XAr, where X = B, Al, Ga, In, and Tl, have been chosen as model reactants in this work. Our present theoretical work predicts that the smaller the singlet-triplet splitting in ArX=XAr, the lower the activation barrier and, in turn, the more rapid are its [4 + 2] cycloaddition reactions. Moreover, the theoretical investigations suggest that the relative dimeric reactivity decreases in the order BAlGaInTl. That is, the heavier the group 13 atom (X), the more stable is its dimetallene toward chemical reactions. In consequence, our results predict that the dimetallenes containing heavier group 13 elements (in particular, X = Ga, In, and Tl) should be stable and should be readily synthesized and isolated at room temperature. This is in good agreement with available experimental observations. Besides this, the singlet-triplet energy splitting of a dimetallene, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict its reactivity. The results obtained allow a number of predictions to be made.

The potential energy surfaces for the unimolecular rearrangement reactions of beta-dicarbonyl ylides and beta-carbonylimidoyl ylides have been studied using the density functional method. All of the stationary points were determined at the B3LYP/LANL2DZdp level of theory. Four kinds of beta-dicarbonyl ylide species containing fluorine, chlorine, bromine, and iodine have been chosen in this work as model reactants. Also, five beta-carbonylimidoyl ylide molecules bearing nitrogen, phosphorus, arsenic, stibium, and bismuth have been used in the present study. In the latter reactions, two different reaction pathways have been proposed: (1) a 1,2-aryl shift to the pnicogen element and (2) A 1,2-aryl shift to the oxygen atom. That is, path 1 is reactant-->TS-1-->Pro-1 and path 2 is reactant-->TS-2-->Pro-2. Our theoretical findings strongly suggest that all intramolecular aryl migration reactions proceed via a one-step (concerted) reaction path. For the beta-dicarbonyl ylide species, the smaller the atomic number of the halogen atom, the lower the barrier height, the larger the reaction enthalpy, and, in turn, the easier it is to undergo the intramolecular aryl migration under thermal conditions. Alternatively, the heavier the pnicogen element in the beta-carbonylimidoyl ylides, the smaller the barrier height, and the larger the migration reaction enthalpy, even under thermal conditions. The results obtained allow a number of predictions to be made.

The mechanisms of the photochemical isomerization reactions were investigated theoretically using a model system of 2-cyanopyrrole and 2-cyano-5-methylpyrrole with the CASSCF (eight-electron/seven-orbital active space) and MP2-CAS methods and the 6-311(d,p) basis set. The structures of the conical intersections, which play a decisive role in such phototranspositions, were obtained. The intermediates and transition structures of the ground state were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred reaction route for the cyanopyrroles is as follows: reactant --> Franck-Condon region --> conical intersection --> photoproduct. In particular, the conical intersection mechanism found in this work gives a better explanation than the previously proposed internal cyclization-isomerization mechanism and supports the experimental observations. In addition, we suggest a simple p-pi orbital topology model, which can be used as a guide tool to predict the location at which conical intersections are likely to occur, as well as the conformations of the phototransposition products of various heterocycles.

Potential energy surfaces for the chemical reactions of neutral five-membered group 13 carbenoids have been studied using density functional theory (B3LYP/LANL2DZ). Five five-membered group 13 carbenoid species, HCMeP(PhN)2X, where X = B, Al, Ga, In, and Tl, have been chosen as model reactants in this work. Also, three kinds of chemical reaction, C-H bond insertion, alkene cycloaddition, and dimerization, have been used to study the chemical reactivities of these group 13 carbenoids. Our present theoretical work predicts that the larger the angleNXN bond angle in the neutral five-membered group 13 carbenoid, the smaller the singlet-triplet splitting, the lower the activation barrier, and, in turn, the more rapid are its various chemical reactions. Moreover, the theoretical investigations suggest that the relative carbenoidic reactivity decreases in the order B > Al > Ga > In > Tl. That is, the heavier the group 13 atom (X), the more stable is its carbenoid with respect to chemical reactions. As a result, we predict that the neutral five-membered group 13 carbenoids (X = Al, Ga, In, and Tl) should be stable, readily synthesized, and isolated at room temperature. Furthermore, the neutral five-membered group 13 carbenoid singlet-triplet energy splitting, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made.

Several reaction pathways for the photochemical transformations of methyl-substituted pyrazine in its first excited state 1(pi --pi*) have been determined using the CASSCF (six-orbital/six-electron active space) and MP2-CAS methods with the 6-311G(d) basis set. Our model investigations suggest that conical intersections play a crucial role in the photoisomerization of pyrazines. Moreover, the present theoretical findings indicate that all of the photoisomerizations of pyrazines adopt the same reaction path as follows: pyrazine --Franck-Condon region --conical intersection --pyrimidine. That is, although an excited-state pyrazine molecule can initiate a phototransposition process easily, this process can be completed on the ground-state potential energy surface after passage through a conical intersection where a fast, radiationless decay is possible. The existence of these nonadiabatic reaction pathways is consistent with the available experimental observations of the photochemistry and photophysics of pyrazine and its methyl derivatives. In the present work, we propose a simple p-pi orbital topology model, which can be used as a diagnostic tool to predict the location of the conical intersections, as well as the geometries of the phototransposition products of various heterocycles.

A theoretical study of the mechanism of the reaction of a single-walled carbon nanotube (SWCNT) with carbene (H 2 C), silylene (H 2 Si), and germylene (H 2 Ge) has been carried out using a two-layered ONIOM(B3LYP/6-311G ∗ :PM3) approach. The main findings are as follows: (1) The computational results based on the method used in this work are in good agreement with recent theoretical findings [Angew. Chem. Int. Ed. 41 (2002) 1853]. That is, SWCNTs with H 2 C, H 2 Si, and H 2 Ge addends favor opened structures rather than three-membered rings. (2) The greater the atomic number of the carbene center, the larger the activation energy and the less exothermic (or the more endothermic) the cycloaddition reaction becomes. Therefore, addition to the C C bond of a SWCNT is more difficult the heavier the carbene center. (3) The theoretical observations suggest that the singlet–triplet splitting of a carbene can be used as a guide to its reactivity during the SWCNT cycloaddition process.

The potential energy surfaces for the abstraction reactions of disilenes and digermenes with carbon tetrachloride have been characterized in detail using density functional theory (B3LYP/6-311G(d)), including zero-point corrections. Ten substituted disilene and digermene species, R 2 X=XR 2 (X = Si, Ge; R = H, F, Cl, CH 3 , and SiH 3 ), have been chosen in this work as model reactants. Of the two possible reaction paths, the Cl abstraction (path 1) and the CCl 3 abstraction (path 2), the former is found to be more favorable, with a very low activation energy and a larger exothermicity. This is in accordance with available experimental observations. The activation barriers and enthalpies of the reactions are compared to determine the relative reactivity of disilene and digermene as well as the influence of substitution on the reaction potential energy surface. Our theoretical investigations indicate that, irrespective of disilene and digermene species, the more electropositive and/or the more bulky the substituents, the lower the activation barrier and the more exothermic the haloalkane ion. In short, electronic as well as steric factors play a dominant role in determining the chemical reactivity of the disilene or digermene species kinetically as well as thermodynamically. Furthermore, a configuration mixing model based on the work of Pross and Shaik is used to rationalize the computational results. The results obtained allow a number of predictions to be made.

The potential energy surface for the unimolecular rearrangement XSn⋮As → TS → SnAsX was investigated using the B3LYP and QCISD methods. To explore electronic effects on the relative stability of XSn⋮As and SnAsX, first-row substituents (X = H, Li, BeH, BH2, CH3, NH2, OH, and F) have been used. Our theoretical findings suggest that the double bonded SnAsX species are always kinetically and thermodynamically more stable than the triply bonded XSn⋮As isomers, regardless of the electronegativity of substituent X. Nevertheless, our model calculations based on the ONIOM(B3LYP/LANL2DZ:PM3) method indicate that an aryl group can, if sufficiently bulky, stabilize triply bonded XSn⋮As molecules with respect to both isomerization and polymerization. That is to say, it is not electronic effects but steric effects that play a decisive role in stabilizing the Sn⋮As triple bond.

The lowest singlet and triplet potential energy surfaces for all group 15 HNXH (X = N, P, As, Sb, and Bi) systems have been explored through ab initio calculations. The geometries of the various isomers were determined at the QCISD/LANL2DZdp level and confirmed to be minima by vibrational analysis. In the case of nitrogen and phosphorus, the global minimum is found to be a singlet trans-HNXH structure. Moreover, both singlet and triplet H2NX (X = N and P) species are also found to be local minima surrounded by large activation barriers, so that they should be observable. For arsenic, theoretical investigations demonstrate that the stability of HNAsH isomers decreases in the order triplet H2NAs > singlet trans-HNAsH ≈ singlet cis-HNAsH > singlet H2NAs > triplet HNAsH > triplet H2AsN > singlet H2AsN. For antimony and bismuth, the theoretical findings suggest that the stability of HNXH (X = Sb and Bi) systems decreases in the order triplet H2NX > singlet H2NX > singlet trans-HNXH ≈ singlet cis-HN=XH > triple...

The potential energy surfaces for the addition reactions of N-heterocyclic carbene with C60 and C70 are characterized in detail, using density functional theory (M06-2X). These theoretical investigations strongly suggest that the one-point attack in formation of the single-bonded adduct is the most favorable reaction pathway at room temperature, rather than the production of the cycloadduct with a three-membered ring, from both kinetic and thermodynamic viewpoints. Also, this theoretical work indicates that the dispersion interactions cannot make the contributions to determine the regioselective formations for these NHC–fullerene Lewis acid–base adducts. These theoretical conclusions are consistent with the available experimental observations.

The mechanism for the photochemical isomerization reaction is investigated theoretically using a model system of a five-membered-ring silene and the CAS(8,8)/6-311G(d) and MP2-CAS-(8,8)/6-311++G(3df,3pd)//CAS(8,8)/6-311G(d) methods. These model investigations indicate that the preferred reaction route for a five-membered-ring silene, which leads to the photorearrangement product, is as follows: reactant → Franck–Condon region → conical intersection→ photoproduct. In other words, the direct mechanism is a one-step process that has no barrier. These theoretical results agree with the available experimental observations.

The potential energy surfaces for the abstraction reactions of heavy carbenes with oxirane have been studied using density functional theory (B3LYP). Five carbene species, including methylene, CH 2 ; silylene, SiH 2 ; germylene, GeH 2 ; stannylene, SnH 2 ; and plumbylene, PbH 2 ; have been chosen in this work as model reactants. The present theoretical investigations suggest that the relative carbenic reactivity decreases in the order: CH 2 > SiH 2 > GeH 2 » SnH 2 > PbH 2 . That is to say, for oxirane deoxygenations, there is a very clear trend toward higher activation barriers and more endothermic (or less exothermic) reactions on going from C to Pb. More specifically, the theoretical findings strongly suggest that CH 2 , SiH 2 , and GeH 2 should readily oxygen atoms from oxiranes, while SnH 2 and PbH 2 are unreactive with oxiranes. Furthermore, a configuration mixing model based on the work of Pross and Shaik is used to rationalize the computational results. The results obtained allow a number of predictions to be made.

The mechanisms for the photochemical isomerization reactions are investigated theoretically using a model system of (+)-2-carene-4α-methanol (2-carene) 1, using the CASSCF (eight-electron/six-orbital active space) and MP2-CAS methods and the 6-311(d) and 6-311++G(3df, 3pd) basis sets, respectively. The structures of the intersystem crossings, which play a crucial role in these photo-isomerization reactions, are identified. The intermediates and transition structures of the ground state are also calculated to provide a qualitative explanation of the reaction pathways. The model investigations suggest that the preferred reaction route for 1 is as follows: singlet reactant → Franck–Condon region → triplet reactant → triplet transition state → intersystem crossing → intermediate → singlet transition state → photoproduct. In particular, the intersystem-crossing mechanism in this work gives a better explanation and supports the available experimental observations. Two types of reaction pathways that lead to final photoproducts are identified: (path I) ring expansion to form a cycloheptene ring and (path II) ring closure to form a methylcyclohexene structure. Both exhibit a biradical character. These theoretical results agree well with the available experimental observations.

To extend our knowledge of the germanium-phosphorus triple bond, the isomerization reactions of singlet XGeP (i.e., XGe≡P → TS → Ge=PX; X = H, Li, BeH, BH 2 , CH 3 , SiH 3 , NH 2 , PH 2 , OH, SH, F, and Cl) were investigated using B3LYP and CCSD(T) calculations. It is found that electronegative and π-electron-donating substituents (X) can stabilize the XGe≡P triply bonded species in a kinetic sense, although germanium is less reluctant to form Ge=P doubly bonded than Ge≡P triply bonded compounds.

We have chosen eight [n]-metallocenophane complexes to investigate the oxidative additions with methane using the B3LYP/LANL2DZ level of theory. Considering the effects from the geometry and the nature of the metal center, the following conclusions emerge: the less bent 16-electron [n]-metallocenophane complex with the transition metal center W should be a potential model for the oxidative addition of saturated C−H bonds. Our model calculations suggest that a methane σ-complex intermediate formed between the [n]-metallocenophane and methane plays a major role in the oxidative additions. Moreover, we show that both electronic and geometric effects play a significant role in determining the energy barriers.

Substituent effects on the disubstituted heavier analogues of olefin A2XYH2 (X, Y = Si, Ge; A = H, CH3, NH2, OH, F, and Cl) were investigated using density functional theory (B3LYP/6-311+G*). Our theoretical findings suggest that the strength of the π-bond and the tendency to trans-bending are strongly related to the singlet−triplet energy gap (ΔEst) of the corresponding disubstituted divalent species A2X:. Namely, as ΔEst becomes larger, the π-bond is weakened, the distortion increases, and the thermodynamic stability of the doubly bonded molecule is reduced. In this regard, heavier analogues of olefins with more electropositive substituents (or more bulky groups) will give rise to less distortion and possess greater thermodynamic stability, making them interesting goals for synthesis and isolation.

Density functional theory was used to perform a theoretical evaluation of (E)-1,2-disubstituted ethylenes as dipolarophiles for the 1,3-dipolar cycloaddition reaction. The reactivities of electron-withdrawing and -donating substituted ethylenes were examined by estimating their activation energies. The calculated activation energies predicted that the most reactive species is (E)-1,2-C2H2(NO)2, whereas the least reactive is (E)-2-butene. Namely, it was demonstrated that 16-electron 1,3-dipole reactants with more electropositive substituents in terminal positions and ethylenes that possess more strongly electron-withdrawing substituents facilitate 1,3-dipolar cycloaddition reactions. All of the theoretical results can be rationalized using the configuration mixing model. c