Reductive cleavage mechanism of methylcobalamin: elementary steps of Co-C bond breaking

Density functional theory has been applied to the investigation of the reductive cleavage mechanism of methylcobalamin (MeCbl). In the reductive cleavage of MeCbl, the Co-C bond is cleaved homolytically, and formation of the anion radical ([MeCbl]*-) reduces the dissociation energy by approximately 50%. Such dissociation energy lowering in [MeCbl]*- arises from the involvement of two electronic states: the initial state, which is formed upon electron addition, has dominant pi*corrin character, but when the Co-C bond is stretched the unpaired electron moves to the sigma*Co-C state, and the final cleavage involves the three-electron (sigma)2(sigma*)1 bond. The pi*corrin-sigma*Co-C states crossing does not take place at the equilibrium geometry of [MeCbl]*- but only when the Co-C bond is stretched to 2.3 A. In contrast to the neutral cofactor, the most energetically efficient cleavage of the Co-C bond is from the base-off form. The analysis of thermodynamic and kinetic data provides a rationale as to why Co-C cleavage in reduced form requires prior departure of the axial base. Finally, the possible connection of present work to B12 enzymatic catalysis and the involvement of anion-radical-like [MeCbl]*- species in relevant methyl transfer reactions is discussed.     

Theoretical study of singlet and triplet excitation energies in oligothiophenes

We have analyzed singlet and triplet excitation energies in oligothiophenes (up to five rings) using time-dependent density-functional theory (TD-DFT) with different exchange-correlation functionals and compared them with results from the approximate coupled-cluster singles and doubles model (CC2) and experimental data. The excitation energies have been calculated in geometries obtained by TD-DFT optimization of the lowest excited singlet state and in the ground-state geometries of the neutral and anionic systems. TD-DFT methods underestimate photoluminescence energies but the energy difference between singlet and triplet states shows trends with the chain-length similar to CC2. We find that the second triplet excited state is below the first singlet excited state for long oligomers in contrast with the previous assignment of Rentsch et al. (Phys.Chem. Chem. Phys. 1999, 1, 1707). Their photodetachment photoelectron spectroscopy measurements are better described by considering higher triplet excited states.     

Carbon-cobalt bond distance and bond cleavage in one-electron reduced methylcobalamin: a failure of the conventional DFT method

Geometry optimizations at the HF, B3LYP, and CASSCF levels of electronic structure theory have been performed for methylcobalamin (MeCbl) model compounds in both the Co(III) (MeCbl(III)) and Co(II) (MeCbl(II)) formal oxidation states. Since the HOMO-LUMO and C-Co sigma-sigma MO gaps are significantly smaller in the MeCbl(II) compounds compared with MeCbl(III), a pseudo-Jahn Teller effect is possible. CASSCF calculations show that there is strong coupling between C-Co sigma-sigma MOs for the MeCbl(II) models leading to strong state mixing with significant total charge density transfer (approximately 0.4 e-), mainly from the C-Co sigma MO to C-Co sigma MO (approximately 0.3 e-). CASSCF(9:7) calculations show that the strong state mixing leads to an increase in the C-Co bond length for MeCbl(II) model compounds from 1.969 A (DFT and HF calculations) to 2.164 A in the base-on MeCbl(II) model and from 1.938 A to 2.144 A in the base-off MeCbl(II) model. Concomitantly, the Co-N axial bond length increases from 2.121 A (DFT) to 2.344 A in the CASSCF calculation. This coupling interaction between states can be used to explain the much lower Co-C bond dissociation enthalpy and much faster bond cleavage rate for the one-electron reduced methylcobalamin radical anion compared to MeCbl(III). It may also be important for axial bond distances in other Co(II) compounds.     

Characterization of transient species in laser photolysis of aromatic amino acids using acetone as photosensitizer

The photochemical processes of aromatic amino acids were investigated in aqueous solution using acetone as photosensitizer by KrF (248 nm) laser flash photolysis. Laser-induced transient species were characterized according to kinetic analysis and quenching experiments. The intermediates recorded were assigned to the excited triplet state of tryptophan, the radicals of tryptophan and tyrosine. The excited triplet state of tryptophan produced via a triplet-triplet excitation transfer and the radicals arising from electron transfer reaction has been identified. Neither electron transfer nor energy transfer between triplet acetone and phenylalanine can occur in photolysis of phenylalanine aqueous solution which contains acetone. Furthermore, triplet acetone-induced radical transformation: Trp/N-Tyr→Trp-Tyr/O was observed directly in photolysis of dipeptide (Trp-Tyr) aqueous solution containing acetone, and the transformation resulting from intramolecular electron transfer was suggested.     

Theoretical study of the vertical excited states of benzene, pyrimidine, and pyrazine by the symmetry adapted cluster--configuration interaction method

The ground state and the excited states of benzene, pyrimidine, and pyrazine have been examined by using the symmetry adapted cluster-configuration interaction (SAC-CI) method. Detailed characterizations and the structures of the absorption peaks in the vacuum ultraviolet (VUV), low energy electron impact (LEEI), and electron energy loss (EEL) spectra were theoretically clarified by calculating the excitation energy and the oscillator strength for each excited state. We show that SAC-CI has the power to well reproduce the electronic excitation spectra (VUV, LEEI, and EEL) simultaneously to an accuracy for both the singlet and the triplet excited states originated from the low-lying pi --> pi*, n --> pi*, pi --> sigma* and n --> sigma* excited states of the titled compounds. The present results are compared with those of the previous theoretical studies by methods, such as EOM-CCSD(T), STEOM-CCSD, CASPT2 and TD-B3LYP, etc.     

DFT study of Co-C bond cleavage in the neutral and one-electron-reduced alkyl-cobalt(III) phthalocyanines

Density functional theory (DFT) has been applied to the analysis of the structural and electronic properties of the alkyl-cobalt(III) phthalocyanine complexes, [CoIIIPc]-R (Pc = phthalocyanine, R = Me or Et), and their pyridine adducts. The BP86/6-31G(d) level of theory shows good reliability for the optimized axial bond lengths and bond dissociation energies (BDEs). The mechanism of the reductive cleavage was probed for the [CoIIIPc]-Me complex which is known as a highly effective methyl group donor. In the present analysis, which follows a recent study on the reductive Co-C bond cleavage in methylcobalamin (J. Phys. Chem. B 2007, 111, 7638-7645), it is demonstrated that addition of an electron and formation of the pi-anion radical [CoIII(Pc*)]-Me- significantly lowers the energetic barrier required for homolytic Co-C bond dissociation. Such BDE lowering in [CoIII(Pc*)]-Me- arises from the involvement of two electronic states: upon electron addition, a quasi-degenerate pi*Pc state is initially formed, but when the cobalt-carbon bond is stretched, the unpaired electron moves to a sigma*Co-C state and the final cleavage involves the three-electron (sigma)2(sigma*)1 bond. As in corrin complexes, the pi*Pc-sigma*Co-C states crossing does not take place at the equilibrium geometry of [CoIII(Pc*)]-Me- but only when the Co-C bond is stretched to approximately 2.3 A. The DFT computed Co-C BDE of 23.3 kcal/mol in the one-electron-reduced phthalocyanine species, [CoIII(Pc*)]-Me-, is lowered by approximately 37% compared to the neutral Py-[CoIIIPc]-Me complex where BDE = 36.8 kcal/mol. A similar comparison for the corrin-containing complexes shows that a DFT computed BDE of 20.4 kcal/mol for [CoIII(corrin*)]-Me leads to approximately 45% bond strength reduction, in comparison to 37.0 kcal/mol for Im-[CoIII(corrin)]-Me+. These results suggest some preference by the alkylcorrinoids for the reductive cleavage mechanism.     

Theoretical investigation of anthracene-9,10-endoperoxide vertical singlet and triplet excitation spectra

The electronic absorption spectrum of anthracene-9,10-endoperoxide (APO) has been investigated by means of multiconfigurational multi-state second order perturbation theory on complete active space self-consistent field wavefunctions (MS-CASPT2/CASSCF) and two single reference methods: time-dependent density functional theory (TD-DFT) and coupled cluster of second order (CC2). After testing several active spaces and basis sets, a CAS (14,12) active space together with an ANO-S basis set was found an appropriate choice to describe the vertical singlet and triplet electronic states of APO. Unfortunately, TD-DFT and CC2 methods cannot reproduce the MS-CASPT2 and experimental spectrum. Our MS-CASPT2//CASSCF(14,12)/ANO-S calculations predict a predominant pi*(OO)sigma*(OO) character for the lowest singlet excited state S(1) at 3.85 eV. Accordingly, the lowest singlet state of APO should be responsible for homolysis of the endoperoxide group. The next two absorbing excited states, experimentally proposed to be responsible for singlet oxygen production and therefore connected to the biological interest of APO, have been computed vertically at 4.34 and 4.59 eV and assigned to pi(CC)pi*(CC) and pi*(OO)pi*(CC) transitions, respectively. The vertical triplet electronic spectrum follows the singlet vertical spectrum ordering. The high density of triplet and singlet excited states of different nature within few eV points to the possibility of intersystem crossings between potential energy surfaces of different multiplicity.     

Stepwise laser photolysis studies of beta-bond cleavage in highly excited triplet states of biphenyl derivatives having C-O bonds

Photochemical profiles of beta-bond dissociation in highly excited triplet states (Tn) of biphenyl derivatives having C-O bonds were investigated in solution, using stepwise laser photolysis techniques. The lowest triplet states (T1) were produced by triplet sensitization of acetone (Ac) upon 308-nm laser photolysis. The molar absorption coefficients of the T1 states were determined using triplet sensitization techniques. Any photochemical reactions were absent in the T1 states. Upon 355-nm laser flash photolysis of the T1 states, they underwent fragmentation, because of homolysis of the C-O bond in the Tn states from the observations of the transient absorption of the corresponding radicals. The quantum yields (Phidec) for the decomposition of the T1 states upon the second 355-nm laser excitation were determined. Based on the Phidec values and the bond dissociation energies (BDEs) for the C-O bond fission, the state energies (ERT) of the reactive highly excited triplet states (TR) were determined. It was revealed that (i) the Phidec was related to the energy difference (DeltaE) between the BDE and the ERT, and (ii) the rate (kdis) of beta-cleavage in the TR state was formulated as being simply proportional to DeltaE. The reaction mechanism for beta-bond cleavage in the TR states was discussed.     

Photochemistry and photophysics of thienocarbazoles

Two methylated thienocarbazoles and two of their synthetic nitro-precursors have been examined by absorption, luminescence, laser flash photolysis and photoacoustic techniques. Their spectroscopic and photophysical characterization involves fluorescence spectra, fluorescence quantum yields and lifetimes, and phosphorescence spectra and phosphorescence lifetimes for all the compounds. Triplet-singlet difference absorption spectra, triplet molar absorption coefficients, triplet lifetimes, intersystem crossing S1 --> T1 and singlet molecular oxygen yields were obtained for the thienocarbazoles. In the case of the thienocarbazoles it was found that the lowest-lying singlet and triplet excited states, S1 and T1, are of pi,pi* origin, whereas for their precursors S1 is n,pi*, and T1 is pi,pi*. In both thienocarbazoles it appears that the thianaphthene ring dictates the S1 --> T1 yield, albeit there is less predominance of that ring in the triplet state of the linear thienocarbazole, which leads to a decrease in the observed phiT value.     

Theoretical studies on metal-metal interaction and spectroscopic properties of a series of hetero-binuclear d(8)-d(10) complexes containing iridium(i) and gold(i)

The ground and triplet excited state geometries, metal-metal (Ir-Au) attractive interaction, electronic structures, absorptions, and phosphorescence of three d(8)-d(10) Ir(i)-Au(i) complexes [Ir(CO)ClAu(mu-dpm)(2)](-) (1), [Ir(CNCH(3))(2)Au(mu-dpm)(2)](2-) (2), and [Ir(CNCH(3))(3)Au(mu-dpm)(2)](2-) (3) [dpm = bis(diphosphino)methane] were investigated theoretically. Their ground and triplet excited states geometries were fully optimized at the MP2 and UMP2 (6-31G for H/C/N/O atoms, LANL2DZ for Ir/Au/P/Cl) levels, respectively, and the calculated geometries are well consistent with the X-ray results. The calculated results indicated that a weak Ir-Au interaction exists in the ground state of , moreover the interaction of and is strengthened by excitation, on contrast, the Ir-Au attractive interaction of in the excited state becomes little lower than that in the ground state. By adding one more CNMe group on complex , the bond type of HOMO can be changed from sigma*[d(z(2))(Ir/Au)] to sigma[d(z(2))(Ir/Au)]. Under the TD-DFT level with PCM model, the absorptions and phosphorescence of were calculated based on the optimized ground and excited states geometries, respectively. The lowest-lying absorptions of 1 and 2 are all attributed to sigma*[d(z(2))] --> sigma[p(z)] and that of 3 is assigned to sigma[d(z(2))] --> pi[p(z)] with MC/MMLCT transition characters. The phosphorescence of 1, 2 and 3 and are assigned to sigma[p(z)] --> sigma*[d], sigma[p(z)] --> sigma*[d], and pi[p(z)] --> sigma[d] transitions, respectively. The calculated results also indicated that with the increase of the Ir-Au bond distance both in the ground and the excited state, the absorptions and the emissions are red-shifted correspondingly.     

Photodecomposition profiles of beta-bond cleavage of phenylphenacyl derivatives in the higher triplet excited states during stepwise two-color two-laser flash photolysis

Photochemical properties of p-phenylphenacyl derivatives (PP-X) having C-halide, C-S, and C-O bonds in the lowest (T 1) and higher (T n ) triplet excited states were investigated in solution by using single-color and stepwise two-color two-laser flash photolysis techniques. PP-Xs (X = Br, SH, and SPh) undergo beta-bond dissociation in the lowest singlet excited states (S 1) while the C-X bonds of other PP-Xs are stable upon 266-nm laser photolysis. The T 1(pi,pi*) states of PP-X were efficiently produced during 355-nm laser photolysis of benzophenone as a triplet sensitizer. Triplet PP-Xs deactivate to the ground state without photochemical reactions. Upon 430-nm laser photolysis of the T 1 states of PP-X (X = Br, Cl, SH, SPh, OH, OMe, and OPh), decomposition of PP-X in the T n states was found. On the basis of the changes in the transient absorption, quantum yields (Phi dec) of the decomposition of PP-X in the T n states were determined, while bond dissociation energies (BDE) of the C-X bonds were calculated by computations. According to the relationship between the Phi dec and BDE values, it was shown that the decomposition of PP-X in the T n state is due to beta-cleavage of the corresponding C-X bond, and that the state energy of the reactive T n for the C-O bond cleavage differs from that for the C-halide and C-S bond cleavage. The reaction profiles of the C-X bond cleavage of PP-X in the T n states were discussed.     

Involvement of excited triplet state in the photodissociation of cyclobutane

The potential energy curves (PECs) of the ground state and the low‐lying excited states for the photodissociation of cyclobutane have been calculated at the multi‐reference configuration interaction with singlet and doublet excitation (MRCISD) and the multi‐reference second order perturbation theory (MRPT2). Firstly, the PECs are constructed following a reaction path determined by semiclassical dynamics simulation, which suggests that the lowest triplet state of tetramethylene is involved in the photodissociation of cyclobutane. Then, the adiabatic PECs are calculated for the breaking processes of C1? C3 and C2? C4 bond respectively. The singlet‐triplet PECs' intersections have been found in the two breaking C? C bond processes. During the breaking process of the second C2? C4 bond, a local minimum has been found on the PEC of the lowest triplet state, which gives us some insight to reinterpret the experimental observed diradical intermediate as being trapped in its triplet state. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Ab initio DFT study of Z–E isomerization pathways of <Emphasis Type="Italic">N</Emphasis>–benzylideneaniline

The ground state properties and absorption spectra of N-benzylideneaniline (NBA) have been studied at the density functional (DFT) and at the time-dependent density functional (TD-DFT) level of the theory. The equilibrium geometries of the E and Z isomers in the ground state and their vibrational frequencies have been computed. Furthermore, the excitation energies of the lowest excited singlet and triplet states and the potential energy curves along the torsion and the inversion isomerization coordinates were evaluated. The results are discussed in light of the available experimental data.     

A new-type photoreaction of a carbonyl compound: Part 2. Photoinduced ω-bond dissociation in p-halomethylbenzophenone studied by time-resolved EPR technique and laser flash photolysis

Photodissociation of the carbon-X (X = Br and Cl) bonds in p-bromo- and p-chloromethylbenzophenone (BMBP and CMBP) in solution were investigated by time-resolved EPR and laser flash photolysis techniques. BMBP and CMBP were found to undergo ω-bond cleavage to yield the p-benzoylbenzyl radical (BBR) at 295 K, and the quantum yields (Φ_BBR) were determined. The CIDEP signal originated from BBR formed upon decomposition of CMBP was obtained while that for BMBP was absent. By using triplet sensitization of acetone, the efficiencies (_BBR) of the CX bond fission in the triplet states of BMBP and CMBP were determined. The agreement between the Φ_BBR and _BBR values for CMBP indicates that the CCl bond dissociation occurs only in the triplet state. In contrast to CMBP, the cleavage of the CBr bond in BMBP upon direct excitation was concluded to be the event only in the excited singlet state without triplet formation, whereas the triplet state was also reactive for ω-bond dissociation. The rate of CBr bond dissociation seemed to be greater than that of intersystem crossing from the S₁ to the T₁ state. Schematic energy diagrams of the excited states of BMBP and CMBP were shown, and the reaction profiles were discussed from the viewpoint of the CX bond enthalpies.     

Excited state dynamics of Michler's ketone: a laser flash photolysis study

Steady state absorption and fluorescence as well as the time resolved absorption studies in the pico and subpicosecond time domain have been performed to characterize the excited singlet and triplet states of Michler's ketone (MK). The nature of the lowest excited singlet (S₁) and triplet (T₁) states depends on the polarity of the solvent - in nonpolar solvents they have either pure nπ * character or mixed character of nπ * and ππ * states but in more polar solvents the states have CT character. Concentration dependence of the shapes of the fluorescence as well the excited singlet and triplet absorption spectra provide the evidence for the association of the MK molecules in the ground state.     

Spectroscopic and computational studies of Co2+corrinoids: spectral and electronic properties of the biologically relevant base-on and base-off forms of Co2+cobalamin

Co(2+)cobalmain (Co(2+)Cbl) is implicated in the catalytic cycles of all adenosylcobalamin (AdoCbl)-dependent enzymes, as in each case catalysis is initiated through homolytic cleavage of the cofactor's Co-C bond. The rate of Co-C bond homolysis, while slow for the free cofactor, is accelerated by 12 orders of magnitude when AdoCbl is bound to the protein active site, possibly through enzyme-mediated stabilization of the post-homolysis products. As an essential step toward the elucidation of the mechanism of enzymatic Co-C bond activation, we employed electronic absorption (Abs), magnetic circular dichroism (MCD), and resonance Raman spectroscopies to characterize the electronic excited states of Co(2+)Cbl and Co(2+)cobinamide (Co(2+)Cbi(+), a cobalamin derivative that lacks the nucleotide loop and 5,6-dimethylbenzimazole (DMB) base and instead binds a water molecule in the lower axial position). Although relatively modest differences exist between the Abs spectra of these two Co(2+)corrinoid species, MCD data reveal that substitution of the lower axial ligand gives rise to dramatic changes in the low-energy region where Co(2+)-centered ligand field transitions are expected to occur. Our quantitative analysis of these spectral changes within the framework of time-dependent density functional theory (TD-DFT) calculations indicates that corrin-based pi --> pi transitions, which dominate the Co(2+)corrinoid Abs spectra, are essentially insulated from perturbations of the lower ligand environment. Contrastingly, the Co(2+)-centered ligand field transitions, which are observed here for the first time using MCD spectroscopy, are extremely sensitive to alterations in the Co(2+) ligand environment and thus may serve as excellent reporters of enzyme-induced perturbations of the Co(2+) state. The power of this combined spectroscopic/computational methodology for studying Co(2+)corrinoid/enzyme active site interactions is demonstrated by the dramatic changes in the MCD spectrum as Co(2+)Cbi(+) binds to the adenosyltransferase CobA.     

Nanosecond CO photodissociation and excited-state character of [Ru(X)(X')(CO)2(N,N'-diisopropyl-1,4-diazabutadiene)] (X=X'=Cl or I; X=Me, X'=I; X=SnPh3, X'=Cl) studied by time-resolved infrared spectroscopy and DFT calculations

The character and dynamics of the low-lying excited states of [Ru(X)(X')(CO)2(iPr-dab)] (X=X'=Cl or I; X=Me, X'=I; X=SnPh3, X'=Cl; iPr-dab=N, N'-diisopropyl-1,4-diazabutadiene) were studied experimentally by pico- and nanosecond time-resolved IR spectroscopy (TRIR) and (for X=X'=Cl or I) computationally using density functional theory (DFT) and time-dependent DFT (TD-DFT) techniques. The lowest allowed electronic transition occurs between 390 and 460 nm and involves charge transfer from the Ru(halide)(CO) 2 unit to iPr-dab, denoted (1)MLCT/XLCT (metal-to-ligand/halide-to-ligand charge transfer). The lowest triplet state is well modeled by UKS-DFT-CPCM calculations, which quite accurately reproduce the excited-state IR spectrum in the nu(CO) region. It has a (3)MLCT/XLCT character with an intraligand (iPr-dab) (3)pipi* admixture. TRIR spectra of the lowest triplet excited state show two nu(CO) bands that are shifted to higher energies from their corresponding ground-state positions. The magnitude of this upward shift increases as a function of the ligands X and X' [(I)2 < (Sn)(Cl) < (Me)(I) < (Cl)2] and reveals increasing contribution of the Ru(CO)2-->dab MLCT character to the excited state. The lowest triplet state of [Ru(Cl)2(CO)2(iPr-dab)] undergoes a approximately 10 ps relaxation that is followed by CO dissociation, producing cis(CO,CH 3CN),trans(Cl,Cl)-[Ru(Cl)2(CH 3CN)(CO)(iPr-dab)] with a unity quantum yield and 7.2 ns lifetime and without any observable intermediate. To our knowledge, this is the first example of a "slow" CO dissociation from a thermally equilibrated triplet charge-transfer excited state.     

Role of the low-energy excited states in the radiolysis of aromatic liquids

The contribution of the low-energy excited states to the overall product formation in the radiolysis of simple aromatic liquids--benzene, pyridine, toluene, and aniline--has been examined by comparison of product yields obtained in UV-photolysis and in γ-radiolysis. In photolysis, these electronic excited states were selectively populated using UV-light excitation sources with various energies. Yields of molecular hydrogen and of "dimers" (biphenyl, bibenzyl, dipyridyl for benzene, toluene, pyridine, respectively, and of ammonia and diphenylamine for aniline) have been determined, since they are the most abundant radiolytic products. Negligibly small production of molecular hydrogen in the UV-photolysis of aromatic liquids with excitation to energies of 4.88, 5.41, 5.79, and 6.70 eV and the lack of a scavenger effect suggest that this product originates from short-lived high-energy singlet states. A significant reduction in "dimer" radiation-chemical yields in the presence of scavengers such as anthracene or naphthalene indicates that the triplet excited states are important precursors to these products. The results for toluene and aniline suggest that efficient dissociation from the lowest-energy excited triplet state leads to noticeable "dimer" production. For benzene and pyridine, the lowest-energy triplet excited states are not likely to fragment into radicals because of the relatively large energy gap between the excited state level and corresponding bond dissociation energy. The "dimer" formation in the radiolysis of benzene and pyridine is likely to involve short-lived high-energy triplet states.     

LUMINESCENCE SPECTRA AND PHOTOCYCLOADDITION OF THE EXCITED COUMARINS TO DNA BASES

Abstract— The lowest excited singlet and triplet states of coumarin, psoralen, and 4-hydroxy-coumarin have been assigned to the (π,π * ) type on the basis of the luminescence spectroscopy and MO calculations. The mechanism of photocycloaddition of courmarin and psoralen to thymine has been described in terms of the perturbational MO model and MO reactivity indices. All possible cycloaddition patterns have been examined. Results suggest that the 3,4-bond of coumarin in the excited state is somewhat more reactive than the same bond of psoralen in the excited state. It is also predicted that the 3,4-bond of psoralen in the triplet state is more reactive than the 4', 5'-bond. The results have been favorably correlated with the electronic characteristics of excited coumarin molecules and with available experimental data on the relative yields of photoadducts.