Exploring the Reaction Paths on the Potential Energy Surfaces of the S1 and T1 States in Methylenecyclopropane

The reaction paths of methylenecyclopropane 1 on the potential energy surfaces (PESs) of the lowest triplet (T₁) state and the lowest excited singlet (S₁) state, as well as that of the ground state (S₀), were explored by using the nudged elastic band method at the MRMP2//MCSCF/6‐31++G(d,p) and DFT(B3LYP)/6‐31++G(d,p) levels of theory. After vertical excitation of 1, three transition states on the PES of the lowest triplet state and one transition state on the S₁ PES were found along the reaction path to produce a carbene, cyclobutylidene 2. All of these transition states are lower in energy than the S₁ state produced by vertical excitation at the S₀ energy minimum in 1. Fast transition is predicted to occur from the T₁ state or from the S₁ state to the S₀ state due to strong spin‐orbit coupling or nonadiabatic coupling in the geometrical vicinity of 2. On the MRMP2 S₀ PES, the energy barriers of 5.0, 10.3 and 13.5 kcal mol^?1 were obtained for C migration reaction (backward reaction), 1,2‐H migration reaction to cyclobutene 3, and 1,3‐H migration reaction to bicyclopropane 4, respectively, started at 2. The introduction of phenyl groups makes the energy barriers smaller due to the π conjugation between the carbene center and phenyl groups.     

TDDFT modeling of the CO‐photolysis of Fe2(S2C3H6)(CO)6, a model of the [FeFe]‐hydrogenase catalytic site

Upon irradiation with ultraviolet wavelengths, Fe₂(S₂C₃H₆)(CO)₆, a simple model of the [FeFe]‐hydrogenase active site, undergoes CO dissociation to form the unsaturated Fe₂(S₂C₃H₆)(CO)₅ species and successively a solvent adduct at the vacant coordination site. In the present work, the CO‐photolysis of Fe₂(S₂C₃H₆)(CO)₆ was investigated by density functional theory (DFT) and time‐dependent DFT (TDDFT). Trans Fe₂(S₂C₃H₆)(CO)₅ form and the corresponding trans heptane or acetonitrile solvent adducts are the lowest energy ground state forms. CO dissociation barriers computed for the lowest triplet state are roughly halved with respect to those for the ground state suggesting that some low‐lying excited potential energy surface (PES) could be loosely bound with respect to Fe? C bond cleavage. The TDDFT excited state PESs and geometry optimizations for the excited states likely involved in the CO‐photolysis suggest that the Fe? S bond elongation and the partial isomerization toward the rotated form could take place simultaneously, favoring the trans CO photodissociation. © 2014 Wiley Periodicals, Inc.     

Emission band shape probes of the mixed-valence excited state properties of polypyridyl-bridged bis-ruthenium(II) complexes

The absorption spectra and emission spectral band shapes of several polypyridine-ligand (PP) bridged bis-ruthenium(II) complexes imply that the Ru(II)/Ru(III) electronic coupling is weak in their lowest energy metal to ligand charge transfer (MLCT) excited states. Many of these PP-bridging ligands contain pyrazine moieties and the weak electronic coupling of the excited states contrasts to the strong electronic coupling inferred for the correlated mixed-valence ground states. Although the bimetallic complexes emit at significantly lower energy than their monometallic analogs, the vibronic contributions to their 77 K emission spectra are much stronger than expected based on comparison to the monometallic analogs (around twofold in some complexes) and this feature is characteristic of bimetallic complexes in which the mixed-valence excited states are electronically localized. The weaker excited state than ground state donor/acceptor electronic coupling in this class of complexes is attributed to PP-mediated super-exchange coupling in which the mediating orbital of the bridging ligand (PP-LUMO) is partly occupied in the MLCT excited states, but is unoccupied in the ground states; therefore, the vertical Ru(III)-PP⁻ (MLCT) energy is larger and the mixing coefficient smaller in these excited states than is found for Ru(II)-PP in the corresponding ground states.     

Structure and conformational dynamics of the cyclopropanecarbaldehyde molecule in the ground (S<Superscript>b0</Superscript>) and lowest excited (T<Superscript>b1</Superscript> and S<Superscript>b1</Superscript>) electronic states

The structure and conformational dynamics of nonrigid cyclopropanecarbaldehyde (CPCA) molecule in the ground (S^b0) and lowest excited triplet (T^b1) and singlet (S^b1) electronic states were calculated using the MP2, DFT, CASSCF, CASPT2, and CCSD quantum chemical methods. According to ab initio calculations, in the S^b0 electronic state there are two symmetrical (cis and trans) conformers of the CPCA molecule. Excitation of the CPCA molecule to the ?1 and S1 electronic states causes significant structural changes: carbonyl CCHO fragment becomes nonplanar, cyclopropane fragment rotates around the C–C bond, thus changing the relative positions of the formyl and cyclopropane fragments. Using sections of the potential energy surfaces (PES) of the CPCA molecule in the T^b1 and S^b1 states, we calculated the torsion and inversion wave functions and vibrational transition energies. The results obtained suggest a strong coupling of the torsion and inversion motions in the T^b1 and S^b1 excited states.     

Singlet and Triplet Excited States and Intersystem Crossing in Free‐Base Porphyrin: TDDFT and DFT/MRCI Study

Extensive time-dependent DFT (TDDFT) and DFT/multireference configuration interaction (MRCI) calculations are performed on the singlet and triplet excited states of free-base porphyrin, with emphasis on intersystem crossing processes. The equilibrium geometries, as well as the vertical and adiabatic excitation energies of the lowest singlet and triplet excited states are determined. Single and double proton-transfer reactions in the first excited singlet state are explored. Harmonic vibrational frequencies are calculated at the equilibrium geometries of the ground state and of the lowest singlet and triplet excited states. Furthermore, spin–orbit coupling matrix elements of the lowest singlet and triplet states and their numerical derivatives with respect to nuclear displacements are computed. It is shown that opening of an unprotonated pyrrole ring as well as excited-state single and double proton transfer inside the porphyrin cavity lead to crossings of the potential energy curves of the lowest singlet and triplet excited states. It is also found that displacements along out-of-plane normal modes of the first excited singlet state cause a significant increase of the 2|H_so|S₁>, 1|H_so|S₁>, and 1|H_so|S₀> spin–orbit coupling matrix elements. These phenomena lead to efficient radiationless deactivation of the lowest excited states of free-base porphyrin via intercombination conversion. In particular, the S₁→T₁ population transfer is found to proceed at a rate of ≈10⁷ s⁻¹ in the isolated molecule.     

One- and Two-Dimensional Torsion-Inversion Models for the Fluoral Molecule (CF3CHO) in Excited Electronic States

The structure of the conformationally nonrigid fluoral molecule (CF₃CHO) in the ground (S₀) and lowest excited triplet (T₁) and singlet (S₁) electronic states was studied by ab initio quantum-chemical methods. The equilibrium geometric parameters and harmonic vibrational frequencies of the molecule in these electronic states were determined. The calculations demonstrated that the electronic excitation causes substantial changes in the molecular structure involving the rotation of the CF₃ top and the deviation of the CCHO carbonyl fragment from planarity. The quantum-mechanical problems for large-amplitude vibrations, namely, for the torsional vibration in the S₀ state and the torsional and inversion vibrations (nonplanar carbonyl fragment) in the T₁ and S₁ states, were solved in the one- and two-dimensional approximations. A comparison of the results of calculations revealed the correlation between the torsional and inversion motions.     

AsH3 ultraviolet photochemistry: an ab initio view

Multireference configuration interaction calculations have been carried out for low-lying electronic states of AsH(3). Bending potentials for the nine lowest states of AsH(3) are obtained in C(3v) symmetry for As-H distances fixed at the ground state equilibrium value of 2.850 a(0), as well as for the minimum energy path constrained to R(1) = R(2) = R(3). The calculated equilibrium geometry and bond energy for the X (1)A(1) ground state agree very well with the previous experimental and theoretical data. It is shown that the lowest excited singlet state belongs to the (1)A(1) symmetry (in C(3v)), in contradiction to the previous calculations. This state is characterized by a planar equilibrium geometry. Asymmetric stretch potential energy surface (PES) cuts along the H(2)As-H recoil coordinate (at R(1) = R(2) = 2.850 a(0), θ = 123.9° and 90°) for numerous excited states and two-dimensional PESs for the X and ? states up to the dissociation limits are obtained for the first time. The ? (1)A(1), B(1)E-X (1)A(1) transition moments are calculated as well and used together with the PES data for the analysis of possible photodecay channels of arsine in its first absorption band.     

Ab initio Quantum-Mechanical Calculations of Molecular Structure and Conformations of 2,2-Dichloroethanal in the Ground and Excited Lowest Triplet States

Ab initio calculations were carried out to investigate the molecular structure of 2,2-dichloroethanal (DCE, CHCl₂CHO) in the ground (S ₀) and excited lowest triplet (₁) states. It is found that electronic excitation of DCE from the S ₀ to T ₁ state occurs with top rotations and a loss of planarity of the carbonyl fragments. Six minima corresponding to three pairs of enantiomers were found on the potential energy surface (PES) of the DCE molecule in the ₁ state. Based on the PES calculated (by the UHF and CASSCF methods in a 6-31G** basis) for DCE in the ₁ state, the one-dimensional torsional and inversion problems and the two-dimensional torsional-inversion problems are solved. A comparison of the results has revealed a relationship between the torsional and inversion motions.     

Substituent effects on the photophysical properties of 2,9-substituted phenanthroline copper(I) complexes: a theoretical investigation

The electronic and nuclear structures of a series of [Cu(2,9-(X)₂-phen)₂]⁺ copper(I) complexes (phen=1,10-phenanthroline; X=H, F, Cl, Br, I, Me, CN) in their ground and excited states are investigated by means of density functional theory (DFT) and time-dependent (TD-DFT) methods. Subsequent Born-Oppenheimer molecular dynamics is used for exploring the T₁ potential energy surface (PES). The T₁ and S₁ energy profiles, which connect the degenerate minima induced by ligand flattening and Cu−N bond symmetry breaking when exciting the molecule are calculated as well as transition state (TS) structures and related energy barriers. Three nuclear motions drive the photophysics, namely the coordination sphere asymmetric breathing, the well-documented pseudo Jahn-Teller (PJT) distortion and the bending of the phen ligands. This theoretical study reveals the limit of the static picture based on potential energy surfaces minima and transition states for interpreting the luminescent and TADF properties of this class of molecules. Whereas minor asymmetric Cu−N bonds breathing accompanies the metal-to-ligand-charge-transfer re-localization over one or the other phen ligand, the three nuclear movements participate to the flattening of the electronically excited complexes. This leads to negligible energy barriers whatever the ligand X for the first process and significant ligand dependent energy barriers for the formation of the flattened conformers. Born-Oppenheimer (BO) dynamics simulation of the structural evolution on the T₁ PES over 11 ps at 300 K confirms the fast backwards and forwards motion of the phenanthroline within 200–300 fs period and corroborates the presence of metastable C₂ structures.     

Rotamerisation and intramolecular proton transfer of 2-(2′-hydroxyphenyl)oxazole, 2-(2′-hydroxyphenyl)imidazole and 2-(2′-hydroxyphenyl)thiazole: a theoretical study

Semi-empirical (AM1-SCI) calculations have been performed on 2-(2′-hydroxyphenyl)oxazole (HPO), 2-(2′-hydroxyphenyl)imidazole (HPI) and 2-(2′-hydroxyphenyl)thiazole (HPT) to rationalise the photophysical behaviour of the compounds exhibiting intramolecular rotation as well as excited state intramolecular proton transfer (ESIPT). The calculations reveal that there is a gradual variation in the properties from HPO to HPT through HPI so far as the existence of the rotational isomers in the ground state is concerned. While HPO gives rise to two stable rotamers (I and II) in all the common solvents, there is only one stable species for HPT in the S₀ state. For HPI, rotamer II is possible only in the isolated state and/or in solvents of low polarity, but in high polar solvents it gives rise to the normal form (I) only. For all the molecules in the series, however, intramolecular proton transfer (IPT) takes place in the lowest excited singlet (S₁) and the triplet (T₁) states. Combination of the rotamerism and ESIPT gives rise to multiple fluorescence bands for the fluorophores. Theoretical assignments have been made for the excitation, fluorescence and phosphorescence bands. Simulated potential energy curves (PEC) in different electronic states reveal that the IPT process is feasible in either of the S₁ and T₁ states but not in the ground state. The ESIPT reaction has been found to be favoured both thermodynamically and kinetically in these electronic states compared to the ground state. However, quantum mechanical tunnelling has been proposed for the prototropic reaction to proceed in the excited states.     

Fast hydrogen elimination from the [Ru(PH3)3(CO)(H)2] and [Ru(PH3)4(H)2] complexes in the first singlet excited states: a diabatic quantum dynamics study

The photodissociation dynamics of [Ru(PH3)3(CO)(H)2] and cis-[Ru(PH3)4(H)2] is theoretically analyzed in the lowest two excited singlet states. Energies obtained through electronic density functional theory calculations that use the time-dependent formalism are fitted to analytical reduced two-dimensional potential energy surfaces (2D-PES). The metal-H2 (R) and H-H (r) distances are the variables of these 2D-PES, the rest of the parameters being kept frozen at the values of the minimum energy structure in the ground electronic state. The time evolution in these 2D-PES is exactly followed by means of a fast Fourier transform algorithm applied to solve the time-dependent Schr?dinger equation. A simple diabatization scheme is devised to take into account the probability of transitions between both excited states. The quantum dynamics results point out that photoelimination is almost inexistent if the H2 fragment is to be expelled without further rearrangement of the rest of the complex. Conversely, when the geometries of the complex are optimized by keeping r and R frozen at the hydrogen elimination barrier coordinates, the new 2D-PES so obtained are highly dissociative, the H2 fragment being expelled in less than 100 fs. Finally the picture of the whole reaction that emerges from our theoretical results is described and the main differences between both complexes are examined.     

Multireference Calculation of the Photodissociation of Benzyl Chloride

The photodissociation mechanism of benzyl chloride （BzCl） under 248 nm has been investigated by the complete active space SCF （CASSCF） method by calculating the geometries of the ground （S0） and lower excited states, the vertical （Tv） and adiabatic （T0） excitation energies of the lower states, and the dissociation reaction pathways on the potential energy surfaces （PES） of SI, TI and T2 states. The calculated results clearly elucidated the photodissociation mechanism of BzCl, and indicated that the photodissociation on the PES of T1 state is the most favorable.     

Channels to Singlet and Triplet Phenylcarbenes in Phenyldiazomethane: A CASSCF and MRCI Study

Diazo compounds such as phenyldiazomethane (C₆H₅C(H)N₂) exhibit intriguing phenomena including the ultrafast formation of singlet carbene and the excited‐state rearrangement reaction (RIES). In this work, we have used multi‐reference configuration interaction with single and double excitations (MRCI‐SD) and complete active space self‐consistent field (CASSCF) methods to study the photodissociation dynamics of C₆H₅C(H)N₂. The equilibrium structures, transition states in the lowest three electronic states (S₁, T₁, and S₀), and S₁/S₀ and T₁/S₀ minimum‐energy crossing points both in the Franck–Condon region and on the pathway of the CN bond dissociation have been optimized. On the basis of the calculated S₁, T₁, and S₀ potential energy surfaces, we have uncovered the most efficient pathways to the lowest singlet and triplet phenylcarbenes (C₆H₅CH) in irradiated C₆H₅C(H)N₂.     

Structure of acetyl chloride molecule isotopomers CH<Subscript>3</Subscript>COCl and CD<Subscript>3</Subscript>COCl in the ground and lowest excited singlet and triplet electronic states: a quantum mechanical study

The structures of isotopomers of conformationally flexible acetyl chloride molecule, CH₃COCl and CD₃COCl, in the ground (S₀ and lowest excited singlet (S₁) and triplet (T₁) electronic states were calculated by the RHF, MP2, and CASSCF methods. The equilibrium geometric parameters and harmonic vibrational frequencies of the molecules in these electronic states were estimated. According to calculations, electronic excitation causes considerable conformational changes involving rotation of the CH₃ (CD₃) top and a substantial deviation of the CCOCl fragment from planarity. The results of calculations agree with experimental data. Two dimensional torsional inversion sections of the potential energy surface were calculated and analyzed. Vibrational problems for large amplitude vibrations (torsional vibration in the S₀ state and both torsional and inversion vibrations in the T₁ and S₁ states) were solved in one- and two-dimensional approximations.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 62–70, January, 2005.     

Quantum chemical modeling of photoadsorption properties of the nitrogen-vacancy point defect in diamond

Quantum chemical calculations of geometric and electronic structure and vertical transition energies for several low-lying excited states of the neutral and negatively charged nitrogen-vacancy point defect in diamond (NV(0) and NV(-)) have been performed employing various theoretical methods and basis sets and using finite model NC(n)H(m) clusters. Unpaired electrons in the ground doublet state of NV(0) and triplet state of NV(-) are found to be localized mainly on three carbon atoms around the vacancy and the electronic density on the nitrogen and rest of C atoms is only weakly disturbed. The lowest excited states involve different electronic distributions on molecular orbitals localized close to the vacancy and their wave functions exhibit a strong multireference character with significant contributions from diffuse functions. CASSCF calculations underestimate excitation energies for the anionic defect and overestimate those for the neutral system. The inclusion of dynamic electronic correlation at the CASPT2 level leads to a reasonable agreement (within 0.25 eV) of the calculated transition energy to the lowest excited state with experiment for both systems. Several excited states for NV(-) are found in the energy range of 2-3 eV, but only for the 1(3)E and 5(3)E states the excitation probabilities from the ground state are significant, with the first absorption band calculated at approximately 1.9 eV and the second lying 0.8-1 eV higher in energy than the first one. For NV(0), we predict the following order of electronic states: 1(2)E (0.0), 1(2)A(2) (approximately 2.4 eV), 2(2)E (2.7-2.8 eV), 1(2)A(1), 3(2)E (approximately 3.2 eV and higher).     

The UV absorption spectrum and geometry of the HS2 radical

The ultraviolet absorption spectrum of the transient HS₂ radical produced in the flash photolysis of H₂S₂ was recorded. The spectrum observed in the λ = 3070–3800 Å region is assigned to the òA′ ← X²A″ electronic transition, and three progressions to the SS stretching (ν′₁), SSz.sbnd;H bending (ν′₂) and SH stretching (ν′₃) vibrations of the ²A′ upper electronic state. Open-shell CNDO/2 calculations were carried out to optimize the ground state (²A″) molecular geometry. For comparison, similar calculations were also performed on the HO₂ radical. HS₂ is bent in the ground state and nearly linear in the (²A′) and (⁴A″) excited states.     

An ab initio study of electronic spectra and excited-state properties of 7-azaindole in vapour phase and aqueous solution

The geometries of 7-azaindole (7AI), its tautomer (7AT), and 7AI–H₂O and 7AT–H₂O complexes were optimised in the ground state and some low-lying singlet excited states using the 3-21G basis set. Differences of total energies of the optimised ground and excited states and the vertical excitation energies of these systems were used to explain the observed electronic spectra. Effect of solvation of these systems in bulk water was studied using the polarized continuum model (PCM). The mode of binding of a water molecule in the S₂(n–π^*) excited state of 7AI was found to be quite different from those in its ground and π–π^* excited states. It is shown that crossing of the lowest two singlet excited-state potential surfaces S₁(π–π^*) and S₂(n–π^*) of 7AI occurs in the vapour phase under geometry relaxation while on interaction with water, the S₂(n–π^*) excited state is raised up appreciably going even above the S₃(π–π^*) excited state. Ground- and excited-state molecular electrostatic potential mapping was carried out, which led to valuable information regarding the nature of excited states of the above-mentioned systems.     

Theoretical studies on Ru(fppz)2(CO)L (L = N-heterocyclic ligand): Electronic structure, absorption, phosphorescence, and solvatochromism

A series of ruthenium(II) complexes Ru(fppz)₂(CO)L [fppz = 3-trifluoromethyl-5(2-pyridyl)pyrazole; L = pyridine (1), 4-dimethylaminopyridine (2), 4-cyanopyridine (3)] were designed and investigated theoretically to explore their electronic structures, absorption, and emissions as well as the solvatochromism. The singlet ground state and triplet excited state geometries were fully optimized at the B3LYP/LANL2DZ and CIS/LANL2DZ level, respectively. The HOMO of 1–3 is composed of d_yz(Ru) atom and π(fppz). The LUMO of 1 and 2 is dominantly contributed by π*(fppz) orbital, but that of 3 is contribute by π*(L). Absorption and phosphorescence in vacuo, C₆H₁₂, and CH₃CN media were calculated using the TD-DFT level of theory with the PCM model based on the optimized ground and excited state geometries, respectively. The lowest-lying absorption of 1 and 2 at 387 and 391 nm is attributed to {[d_yz(Ru) + π(fppz)] → [π*(fppz)]} transition, but that of 3 at 479 nm is assigned to {[d_yz(Ru) + π(fppz)] → [π*(L)]} transition. The phosphorescence of 1 and 2 at 436 and 438 nm originates from ³{[d_yz(Ru) + π(fppz)] [π*(fppz)]} excited state, while that of 3 at 606 nm is from ³{[d_yz(Ru) + π(fppz)] [π*(L)]} excited state. The calculation results showed that the absorption and emission transition character can be changed from MLCT/ILCT to MLCT/LLCT transition by altering the substituent on the L ligand. The phosphorescence of 1 and 2 does not have solvatochromism, but that of 3 at 606 nm (vacuo), 584 nm (C₆H₁₂), and 541 nm (CH₃CN) is strongly dependent on the solvent polarity, so introducing electron-withdrawing group on ligand L will induce remarkable solvatochromism.     

A CASSCF study on the lowest π→π* excitation of urocanic acid

The photochemical cis/trans isomerization of urocanic acid (UCA, (E)‐3‐(1′H‐imidazol‐4′‐yl)propenoic acid) was investigated using complete active space SCF (CASSCF) ab initio calculations. The singlet ground state and the triplet and the singlet manifolds of the lowest‐lying π→π* (HOMO→LUMO) excitation of the neutral and the anionic UCA were calculated using the 6‐31G* and the 6‐31+G* basis sets, respectively. The torsional barrier of the double bond of the propenoic acid moiety in UCA is observed to be considerably lower in the T₁ and S₁ excited states of the neutral UCA and in the T₁ but not in the S₁ excited state of the anionic UCA, as compared to the S₀ state of the respective protonation form. The cis‐isomer of both the neutral and the anionic UCA is lower in energy than the trans‐isomer in the S₀, T₁, and S₁ states. This energy difference is larger in the excited states than in the ground state, probably due to strengthening of the intramolecular hydrogen bond of cis‐UCA as the molecule is excited. The results of the calculations, interpreted in terms of the idea that UCA is deprotonated upon electronic excitation, led to construction of a new model for the photoisomerization mechanisms of UCA. According to this model, the trans‐to‐cis isomerization proceeds via both the triplet and the singlet manifolds in the deprotonated form of UCA. This isomerization may occur in the S₀ state of the neutral UCA as well. The cis‐to‐trans isomerization is suggested to proceed only in the S₀ state of the neutral UCA. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 25–37, 1999     

Molecular Structure,Thermodynamic and Spectral Characteristics of Metal-Free and Nickel Complex of Tetrakis(1,2,5-thiadiazolo)porphyrazine

The Knudsen effusion method with mass spectrometric control of the vapor composition was used to study the possibility of a congruent transition to the gas phase and to estimate the enthalpy of sublimation of metal-free tetrakis(1,2,5-thiadiazolo)porphyrazine and its nickel complex (H₂TTDPz and NiTTDPz, respectively). The geometrical and electronic structure of H₂TTDPz and NiTTDPz in ground and low-lying excited electronic states were determined by DFT calculations. The electronic structure of NiTTDPz was studied by the complete active space (CASSCF) method, following accounting dynamic correlation by multiconfigurational quasi-degenerate second-order perturbation theory (MCQDPT2). A geometrical structure of D_2h and D_4h symmetry was obtained for H₂TTDPz and NiTTDPz, respectively. According to data obtained by the MCQDPT2 method, the nickel complex possesses the ground state ¹A_1g, and the wave function of the ground state has the form of a single determinant. Electronic absorption and vibrational (IR and resonance Raman) spectra of H₂TTDPz and NiTTDPz were studied experimentally and simulated theoretically.