Modelling Photochemical Reactivity of Organic Systems - A New Challenge to Quantum Computational Chemistry

Mechanistic insight into the fluorescence activity of forensic fingerprinting reagents

Fingerprint detection is still the primary investigative technique for deciphering criminal inquiries and identifying individuals. The main forensic fingerprinting reagents (FFRs) currently in use can require multiple treatment steps to produce fingerprints of sufficient quality. Therefore, the development of new, more effective FFRs that require minimal chemical treatment is of great interest in forensic chemistry. In this work, prudently crafted density functional theory and time-dependent density functional theory calculations are utilized to derive mechanistic insight into the optical activity of the non-fluorescent product of ninhydrin, diketohydrindylidenediketohydrindamine (DYDA), and fluorescent product of DFO (1,8-diazafluoren-9-one). We investigate various protonation sites to gain an understanding of isomeric preference in the solid-state material. A relaxed scan of a single torsion angle rotation in the S₁ minimized geometry of the O-protonated DYDA isomer suggests a conical intersection upon ∼10° rotation. We show that the absence of a rigid hydrogen-bonded network in the crystal structure of DYDA supports the hypothesis of torsion rotation, which leads de-excitation to occur readily. Conversely, for the fluorescent DFO product, our calculations support an avoided crossing suggestive of a non-radiative mechanism when the torsion angle is rotated by about ∼100°. This mechanistic insight concurs with experimental observations of fluorescence activity in DFO and may aid the photophysical understanding of poorly visualized fingerprints due to weak fluorescence. We show that identifying suggestive avoided crossings via the method described here can be used to initialize thoughts toward the computational design of FFRs.

The mechanistic investigations of photochemical decarbonylations and oxidative addition reactions for M(CO)5 (M = Fe, Ru, Os) complexes

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.

Mechanistic Investigations on the Photoisomerization Reactions of Five-Membered Ring Heterocyclic Molecules Containing Sulfur and Selenium Atoms

The restricted active space self-consistent field method in the 26-electron/27-orbital active space and the 6-311(d) basis set has been used to investigate the mechanisms of the photochemical isomerization reactions concerning the model systems of 1,2,3-thiadiazole and 1,2,3-selenadiazole. The computational works suggest that the preferred reaction paths for both 1,2,3-thiadiazole and 1,2,3-selenadiazole are as follows: reactant → Franck-Condon region → conical intersection → intermediate → transition states → photoproducts. As a result, the structures of the conical intersections, which play a decisive role in these photoisomerization reactions, are obtained. In particular, the present theoretical evidences demonstrate that the potential energy surfaces for the formation of 1,3-diradicals are quite flat. This may explain why their experimental detections are so difficult.

Mechanistic Investigations of the Photochemical Isomerizations of [(CO)5MC(Me)(OMe)] (M = Cr, Mo, and W) Complexes

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 Møller-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 systems, the formation of metallaketene intermediates may occur at higher excitation wavenumbers, whereas the five-coordinated complexes that are attached by a solvent molecule are obtained at lower excitation wavenumbers. However, in the W analogue, because the activation barriers for path I are greater than that for path II and path I has more reaction steps than path II, the quantum yields for the metallaketene intermediate should be smaller than those for the five-coordinated species, which is also attached by a solvent molecule. These theoretical studies also suggest that the conical intersection and the spin crossover mechanisms that are identified in this work explain the process well and support the experimental observations.

Mechanistic Study for the Photochemical Reactions of d6 M(CO)5(CS) (M = Cr, Mo, and W) Complexes

The mechanisms of photoextrusion reactions are determined theoretically for the model system of six-coordinated M(CO)₅(CS) (M = Cr, Mo, and W), using both CASSCF and MP2-CAS methods and the Def2-SVPD basis set. Three types of elimination reaction pathways (i.e., path I, path II, and path III for axial CO extrusion, equatorial CO extrusion, and CS ligand extrusion, respectively) are considered in this study. Theoretical findings show that the photoextrusion mechanism for Cr and Mo complexes proceeds as follows: M-S₀-Rea + hν → M-S₁-FC → M-CI → M-Pro + CO. This study shows that when the reactant, M(CO)₅(CS) (M-S₀-Rea), is photoirradiated by UV light, it is excited vertically to many low-lying singlet excited states. It then relaxes to the first singlet excited state from the Franck-Condon point (M-S₁-FC). After passing through a conical intersection point (M-CI), this species eliminates a CO group to yield a five-coordinated product, M(CO)₄(CS) (M-S₀-Pro). However, for the W analogue, the photolysis mechanism is represented as W-S₀-Rea + hν → W-T₁-Min → W-T₁-TS → W-T₁/S₀ → W-S₀-Pro + CO. That is to say, when the reactant, W(CO)₅(CS) (W-S₀-Rea), absorbs UV light, it is excited to its several low-lying excited states by a vertical excitation. This species may then return to an intermediate at the first triplet excited state (W-T₁-Min) by means of intersystem crossings or conical intersections. After passing through a triplet transition state (W-T₁-TS) and a subsequent intersystem crossing (W-T₁/S₀), this molecule finally loses a CO ligand to produce a photoproduct at the ground singlet state (W-S₀-Pro). In other words, conical intersections and intersystem crossings play a decisive role in these photoextrusion reactions for M(CO)₅(CS). Theoretical evidence from a kinetic viewpoint strongly supports the theory that the photolysis of M(CO)₅(CS) only produces CO-loss photoproducts rather than the CS-loss photoproduct. Theoretical analysis using the results of this study allows a good interpretation of the available experimental observations.

CASSCF and CASMP2 study on the photoisomerization mechanisms of [tris(trialkylsilyl)silyl]cyclotetrasilene and related cyclobutene molecules

The photo-isomerization reaction mechanisms for both cyclotetrasilene and cyclobutene are theoretically studied. The model computations show that the conical intersection mechanism plays a key role in these photochemical reactions and that there are no radicals.

A mechanistic study of the addition of alcohol to a five-membered ring silene via a photochemical reaction

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.

Excited-State Photolytic Mechanism of Cyclopentene Containing a Group 14 Element: An MP2-CAS//CASSCF Study

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.

A model study on the photochemical isomerization of cyclic silenes

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.

A multi-reference study of the byproduct formation for a ring-closed dithienylethene photoswitch

Description of a ground state reaction path for the formation of a detrimental byproduct in the photo reaction of dithienylethene.

A theoretical investigation of photochemical reactions of an isolable silylene with benzene

The mechanisms of photochemical insertion reactions are investigated theoretically using the model system, an isolable dialkylsilylene and benzene, using the CAS(10,10)/6-31G(d) and MP2-CAS-(10,10)/6-311++G(df,pd)//CAS(10,10)/6-31G(d) methods. The structures of the conical intersections, which play a key role in such photoinsertion reactions, are determined to provide a qualitative explanation of the reaction pathways. The model investigation demonstrates that the preferred reaction route for the isolable dialkylsilylene with benzene is as follows: reactants → Franck-Condon region → conical intersection → seven-membered-ring photoproduct. The theoretical findings suggest that the singlet excited dialkylsilylene should attack benzene in the perpendicular conformation, and that no silyl radicals should exist during these photoinsertion reactions. The results obtained allow a number of predictions to be made.

A model study on the photochemical isomerization of isothiazoles and thiazoles

The mechanisms for photochemical isomerization reactions are studied theoretically using three model systems: 3-methylisothiazole, 4-methylisothiazole and 5-methylisothiazole.

A computational study of the mechanisms of the photoisomerization reactions of monocyclic and bicyclic olefins

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.

A theoretical characterization of the photoisomerization channels of 1,2-cyclononadienes on both singlet and triplet potential-energy surfaces

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.

CASCSF study on the photochemical transposition reactions of pyrazines

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.

Toward a Mechanistic Understanding of the Photochromism of Dimethyldihydropyrenes

A mechanism accounting for the photochromic properties of dimethyldihydropyrene (DHP)/metacyclophanediene (CPD) is proposed based on high-level CASSCF/CASPT2 ab initio calculations. The very low ring-opening quantum yield observed upon irradiation of DHP is explained by the quenching of the photoisomerization channel by internal conversion of the initially excited zwitterionic state to the lowest excited state minimum, where the system can fluoresce or decay back to the ground state via another internal conversion. On the other hand, irradiation of CPD leads to the formation of a biradical excited state minimum along the ring-closure reaction path. Internal conversion back to the ground state can then occur at a well-characterized conical intersection, leading to the DHP photoproduct. Thus, our results point unambiguously toward a stepwise mechanism involving the formation of a biradical intermediate on a singlet excited state. The fact that this intermediate does not correspond to the lowest excited state minimum is mainly responsible for the inefficiency of this system.

The Photochemical Rearrangement Pathways of Imidazoles: A Theoretical Study

The mechanisms of the two reaction pathways for the photochemical transformations of methyl substituted imidazoles (i.e., 1,4-dimethyl-imidazole and 1,4,5-trimethyl-imidazole) in their first excited state (1pi --> 1pi*) have been determined using the CASSCF (10-electron/8-orbital active space) and MP2-CAS methods with the 6-311(d) basis set. These two reaction pathways are denoted as the conical intersection path (path 1) and the internal cyclization-isomerization path (path 2). Our model investigations suggest that conical intersections play a crucial role in the photorearrangements of imidazoles. Additionally, the present theoretical findings suggest that photoisomerizations of imidazoles via path 1 should adopt the reaction path as follows: imidazole --> Franck-Condon region --> conical intersection --> photoproduct. Moreover, we have examined the alternative mechanism, the internal cyclization-isomerization path (path 2), which consists of a sequence of small geometric rearrangements. Our theoretical investigations suggest that for the photorearrangement of 1,4-dimethyl-imidazole both mechanisms are comparable. On the other hand, for the photorearrangement of 1,4,5-trimethyl-imidazole path 1 should be favored over path 2. Our present theoretical results agree with the available experimental observations.

Photochemical isomerization reactions of cyanopyrroles: a theoretical study

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.

Photochemical Reactivity of 2-Vinylbiphenyl and 2-Vinyl-1,3-terphenyl: The Balance between Nonadiabatic and Adiabatic Photocyclization

A mechanism for the photochemical conversion of 2-vinyl-1,3-terphenyl to 8,9a-dihydrophenanthrene (Lewis, F. D.; Zuo, X.; Gevorgyan, V.; Rubin, M. J. Am. Chem. Soc. 2002, 124, 13664-13665) is presented in this study, based on ab initio restricted active space self-consistent field calculations and a molecular mechanics-valence bond dynamics simulation of a model system: the syn isomer of 2-vinylbiphenyl. An extended crossing seam between the ground and first excited electronic states was found to be largely responsible for the efficient photocyclization of the photochemically active syn isomer. This mechanism is nonadiabatic in nature, with an excited-state reaction pathway approaching the crossing region during the initial stage of cyclization. Dynamics simulation shows that this seam is easily accessible by vibrational motion in the branching space, once a small barrier is passed on the S1 excited-state potential energy surface. Ultrafast radiationless decay to the ground state then follows, and the cyclization is completed on this surface. A second possible mechanism was identified, which involves complete adiabatic cyclization on the S1 surface, with decay to the ground state (at a different conical intersection) only taking place once the product is formed. Thus, there is a competition between these two mechanisms-nonadiabatic and adiabatic-governed by the dynamics of the system. A large quantum yield is predicted for the photocyclization of the syn isomer of 2-vinylbiphenyl and 2-vinyl-1,3-terphenyl, in agreement with experimental observations.

A survey of recent developments in ab initio valence bond theory

Starting from the 1980s and onwards, Valence Bond theory has been enjoying renaissance that is characterized by the development of a growing number of ab initio methods, and by many applications to chemical reactivity and to the central paradigms of chemistry. Owing the increase of computational power of modern computers and to significant advances in the methodology, valence bond theory begins to offer a sound and attractive alternative to Molecular Orbital theory. This review aims at summarizing the most important developments of ab initio valence bond methods during the last two or three decades, and is primarily devoted to a description of what the various methods can actually achieve within their specific scopes and limitations. Key available softwares are surveyed.

Electronically nonadiabatic thermal reactions of organic molecules

This critical review seeks to bring together organic reactions in which thermal generation of electronic excited states plays an important role. The best known such reactions are those producing chemiluminescent products. However, it appears that there may exist at least as many nonadiabatic reactions in which the excited molecules react before they luminesce. An effort is made to understand the efficiency of excited state production. The crucial roles played by reactive intermediates are highlighted.

Electrostatic Control of the Regioselectivity in the Photoisomerization of trans,trans-1-Fluoro-2,4-hexadiene: Evidence for Competing Conical Intersections

trans,trans-1-Fluoro-2,4-hexadiene (EE-FHD) was used to explore the excited potential energy surfaces of acyclic 1,3-dienes. Our investigations show that the regioselectivity of double-bond photoisomerization is primarily controlled by the charge-stabilizing characteristic of substituents on the double bonds of the diene. Furthermore, changes in the photoregioselectivity with solvent polarity suggest that the excited unsymmetrical diene returns to the ground state by competing pathways which traverse polarized 1Bu states and drop into two different conical intersections

Nonadiabatic interactions in wave packet dynamics of the bromoacetyl chloride photodissociation

The competitive photodissociation of bromoacetyl chloride BrCH2COCl in the first 1A" state (S1) by 248 nm photons is investigated by nonadiabatic wave packet simulations. We show that the preferential breaking of the stronger C-Cl bond (alpha to the excited carbonyl) over the weaker C-Br bond (beta) could be explained by a diabatic trapping or nonadiabatic recrossing as previously proposed. Our energy resolved flux analysis agrees fairly well with the experimental branching ratio (C-Cl:C-Br=1.0:0.4). Even if this does not prove the mechanism, this at least prevents to discard it. A reduced dimensionality approach based on constrained Hamiltonian is used. The nonadiabatic dissociation is studied in the two C-O/C-X (X=Br, Cl) subspaces to emphasize the role of the C-O vibration upon [nO-->piCO*] excitation. The internal torsion and wagging dihedral angles are frozen at their Franck-Condon value, according to preliminary dynamical tests. The other inactive coordinates are optimized at the trans and Cs constrained geometry in the first excited state. Corresponding 2D cuts in the potential energy surfaces have been computed at the CASSCF level. The nonadiabatic kinetic couplings are highly peaked along an avoided crossing seam in both cases. A two-state diabatic model with a constant potential coupling is proposed in the two C-O/C-X subspaces. The inclusion of the C-O stretching in the active coordinates improves the value of the branching ratio over our previous 1D computation.