Excited Singlet Electronic States Research Articles

Theoretical study of the structure of propanal in the first excited singlet and triplet electronic states: three-dimensional model for torsional and inversion vibrations

Potential energy surfaces of the conformationally nonrigid propanal molecule (CH 3CH 2CHO) in the lowest excited triplet (T 1) and singlet (S 1) electronic states were calculated by the multiconfigurational self-consistent field method CASSCF. Electronic excitations of the propanal conformers from the ground state (S 0) to the excited states (T 1 and S 1) were demonstrated to cause essential changes in the molecular structure, viz. the rotation of the ethyl tops and the out-of-plane distortion of carbonyl fragments. Nuclear vibrational motions in the propanal molecule in the excited electronic states were analyzed both in the harmonic approximation and by the use of the one-, two-, and three-dimensional variational calculations to describe correlated large-amplitude vibrations.

A theoretical study of the ground and first excited singlet state proton transfer reaction in isolated 7-azaindole–water complexes

Abstract A systematic study of the proton transfer in the 7-azaindole–water clusters ( 7 -AI ( H 2 O ) n ; n=1–4 ) in both the ground and first excited singlet electronic states is undertaken. DFT(B3LYP) calculations for the ground electronic state shows that the more stable geometry of the initial normal tautomer presents a cyclic set of hydrogen bonds that links the two nitrogen atoms of the base across the waters. For the n=4 cluster the water molecules adopt a double ring structure so that two cycles of hydrogen bonds are found there. From this structure full tautomerization implies only one transition state so that a concerted but non-synchronous process is predicted by our theoretical calculations. This behavior is found both in the ground and the excited states where CIS geometry optimizations and TD(B3LYP) energy calculations are performed. The difference between both states is the height of the energy barrier that is much lower in the excited state. Another clear difference between both electronic states is that full tautomerization is an endergonic process in the ground state whereas it is clearly exergonic (then favorable) in the excited state. This is so because electronic excitation implies a charge transfer from the five-member cycle to the six-member one of 7-azaindole so that the proton transfer from the pyrrolic side to the pyridinic one is favored. These results clearly indicate that full tautomerization will not likely occur in the ground state but it will be quite easy (and fast) in the excited state. Reaction is already feasible in the S 1 1:1 complex but it is faster in the 1:2 complex. However the reaction slows again for the 1:3 complex and, finally, reaches a new maximum for the largest cluster studied here, the n=4 case. These results, which are in agreement with experimental data, are explained in terms of the number of hydrogen bonds that are involved in the transfer. The proton transfer through a ring formed by the substrate and two water molecules is found to be the more efficient one, at least in this system.

Theoretical Study on the Molecular Mechanism for the Reaction of VO2+ with C2H4

The complex reaction between VO2+ (1A1/3A‘ ‘) and C2H4 (1Ag/3A1) to yield VO+ (1Δ/3Σ) and CH3CHO (1A‘/3A‘ ‘) has been studied by means of B3LYP/6-31G* and B3LYP/6-311G(2d,p) calculations. The structures of all reactants, products, intermediates, and transition structures of this reaction have been optimized and characterized at the fundamental singlet and first excited triplet electronic states. Crossing points are localized, and possible spin inversion processes are discussed by means of the intrinsic reaction coordinate approach. Relevant stationary points along the most favorable reaction pathways have been studied at the CCSD/6-311G(2d,p)//B3LYP/6-311G(2d,p) calculation level. The theoretical results allow the development of thermodynamic and kinetic arguments about the reaction pathways of the title process. In the singlet state, the first step is the barrierless obtention of a reactant complex associated with the formation of a V−C bond, while in the triplet state a three-membered ring addition comp...

The singlet electronic excited states of the F2 molecule

By using multireference single excitation configuration interaction calculations and multireference single and double excitation CI calculations, we consider the Σu+1, Πg1, and Πu1 excited states of the F2 molecule which lie between 4.3 and 14.1 eV above the ground state. The basis set is composed of 13s, 10p, 7d, and 2 f contracted Gaussian-type functions, and covers molecular orbitals spanned by 4s, 4p, and 3d Rydberg orbitals. Of the Σu+1 states, G 1Σu+ is sometimes disregarded, presumably because it is not directly observed by optical measurements, but is inferred from perturbations in the visible and ultraviolet spectra. We find that G 1Σu+ arises from the shallow local minimum in the lowest Σu+1 potential curve, which also has a stable minimum corresponding to the state designated C 1Σu+. The experimental excitation energies (T0 values) for G 1Σu+ are 12.81–12.87 eV according to electron energy loss spectroscopy, and our theoretical value is 13.06 eV. Agreement between the experiment and the calculation is quite close. The state has a mixed ionic-Rydberg character with an interesting Rydberg portion. The experimental and calculated T0 values for C 1Σu+ are, respectively, 11.57 and 11.59 eV, suggesting that the present calculation for the state is reliable. Ambiguity found in experimental assignments of the vibrational levels for C 1Σu+ is settled here. The Πg1 and Πu1 states are also discussed.

Theoretical study of the structure of the CClF2NO and CCl2FNO molecules in the ground and lowest excited singlet and triplet electronic states

The potential energy surfaces of the nitroso compounds CClF2NO and CCl2FNO in the ground and lowest excited singlet and triplet electronic states were studied by various ab initio methods (including multiconfigurational methods). The equilibrium geometric parameters, vibrational frequencies, internal rotation potential functions, and rotational contours of bands in the S1 ← S0 vibronic spectrum of the CClF2NO molecule were calculated. For the molecules under consideration, the quantum-mechanical problem on torsional motion was solved. The results of calculations are, on the whole, in good agreement with experiment.

Structurally nonrigid molecular complexes (HF)n(H2O)m (n + m ≥ 2) and their spectroscopic features

The problem of the formation and stability of structurally nonrigid, donor—acceptor molecular complexes is considered. These complexes are formed between the molecules of water and hydrogen fluoride, the latter being the hydrolysis product of the chemical, radiochemical, and metallurgical industrial wastes. Based on the results of ab initio quantum-chemical calculations of the potential energy surfaces by the Hartree—Fock—Roothaan method, the equilibrium configurations of the (H2O)n(HF)m complexes (n : m = 1 : 1; 1 : 2; 2 : 1; 2 : 2; and 3 : 3) were determined. These configurations are necessary for correct interpretation of the IR absorption spectra and for routine remote monitoring of such environmentally hazardous complexes in the Earth atmosphere. The harmonic vibrational frequencies of the (H2O)n(HF)m complexes (n + m ≥ 2) were calculated and the interaction energies between the monomers were found. The influence of UV radiation on the (H2O)n(HF)n complexes (n = 1—3) upon the transition from the ground to the lowest excited singlet electronic state was studied. Characteristic spectroscopic features of the (H2O)n(HF)m complexes were established.

The electronic spectroscopy of transition metal di-hydrides H2M(CO)4(M = Fe,Os): a theoretical study based on CASSCF/MS-CASPT2 and TD-DFT

Transition energies to the low-lying singlet electronic excited states of H2M(CO)4 (M = Fe,Os) are calculated at the CASSCF/MS-CASPT2 level of theory using relativistic effective core potentials in the ab initio model potential (AIMP) approach. The main features of the absorption spectra of both molecules differ significantly. The spectrum of H2Fe(CO)4 is dominated by low-lying excited states corresponding mainly to 3dFe → σ*u and 3dFe → σ*g excitations calculated between 39 310 cm−1 (4.9 eV) and 43 210 cm−1 (5.4 eV). These allowed transitions contribute to the shoulder observed in the experimental spectrum around 270 nm (37 040 cm−1) and are responsible for the photoreactivity of H2Fe(CO)4 at 254 nm (39 370 cm−1). In contrast the lowest part of the absorption spectrum of the osmium analog H2Os(CO)4 is characterized by a high density of metal–ligand-charge-transfer (MLCT) states between 47 220 cm−1 (5.9 eV) and 52 430 cm−1 (6.55 eV) and corresponding to 5dOs → π*CO excitations. The transitions corresponding to 5dOs → σ*u and 5dOs → σ*g excitations are displaced to the upper part of the absorption spectrum of H2Os(CO)4 (beyond 60 000 cm−1 or 166 nm) and cannot induce H2 elimination under irradiation at 254 nm like in the iron complex. A series of sigma-bond–ligand-charge-transfer (SBLCT) states corresponding to σg → π*CO and σu → π*CO excitations not present in H2Fe(CO)4 is found exclusively in the spectrum of H2Os(CO)4 beyond 55 000 cm−1. The upper part of the spectrum of H2Fe(CO)4 (below 220 nm) is assigned to intense MLCT transitions corresponding to 3dFe → π*CO excitations. The MS-CASPT2 and TD-DFT methods agree qualitatively as far as the assignment and transition energies of the low-lying states are concerned in both molecules whereas a comparison of the calculated oscillator strengths is more problematic. The transitions are generally underestimated at the TD-DFT level, an effect that is even more pronounced for the charge transfer excitations to the carbonyl ligands. The high density of singlet electronic states in these simple transition metal hydrides carbonyls illustrates the complexity of the electronic spectroscopy in this class of molecules. This explains the poor resolution of these overcrowded spectra and the difficulty in understanding the photoreactivity of these molecules.

Quantum-mechanical description of the intensity distribution in two-photon absorption spectra of dihalogenated benzenes

Using molecules of dihalogenated benzenes as an example, the verification of an earlier proposed method of direct quantum-mechanical calculation of the relative intensities of lines in the two-photon absorption spectra of polyatomic molecules is continued. The method of calculation is based on the adiabatic model in the context of the Herzberg-Teller approximation. A comparative analysis of the intensity distribution in the two-photon absorption spectra of dihalogenated and monosubstituted benzenes is carried out, and the vibrational characteristics of these molecules in the lowest singlet excited electronic states are determined. A satisfactory agreement with experimental values, along with the analysis of the possibilities of the method, confirms the expediency of its use for gaining information about the intensity distribution in two-photon absorption spectra of polyatomic molecules.

Fast hydrogen elimination from the [Ru(PH3)3(CO)(H)2] complex in the first singlet excited states. A quantum dynamics study

Abstract The photodissociation dynamics of [Ru(PH 3 ) 3 (CO)(H) 2 ] complex in the lowest two singlet excited electronic states has been theoretically analyzed. Reduced two-dimensional potential energy surfaces (PES) are built up by combining a time-dependent method to calculate the excited states energy with DFT (B3LYP) electronic calculations of the ground state. By means of a Fast Fourier Transform (FFT) algorithm the time evolution of the wavefunction upon vertical transition from the minimum of the ground state to both diabatic states has been followed. The propagation in S 1 , the lowest in energy at the vertical transition point and the one with a larger transition probability from the ground state, discloses that the system is not evolving from the initial position at least in the time spanned by the calculations. Conversely the H 2 elimination is very fast (about 37 fs) in the S 2 state. In this state the vertical transition puts the system in a purely dissociative zone of the PES. In that state FFT results indicate that the lengthening of the Ru–H 2 distance and the shortening of the H–H one are taking place almost simultaneously.

Spectroscopic data on nuclear configuration of dibenzo- p-dioxin in S 0, S 1, and T 1 electronic states

There is no common opinion about the nuclear configuration pattern of the dibenzo- p-dioxin (DD) molecule in the ground electronic state S 0–whether its configuration is planar or non-planar, as in the “butterfly” form. In this work, we have analyzed several spectroscopic data (on infrared and Raman spectra, vibronic structure of the singlet electronic excited state S 1 and the phosphorescence spectra) considering the selection rules. These data point to the plane configuration of DD in all (S 0, S 1, and triplet T 1) electronic states. The S 1 state is found to be due to g-type symmetry.

Ab initio study of the reaction mechanism of CO2 with Ti atom in the ground and excited electronic states

Density functional B3LYP/6-311+G(3df )//B3LYP/6-31G* calculations of potential energy surfaces (PES) have been performed for the Ti+CO2→TiO+CO reaction in the triplet, quintet, and singlet electronic states. The results indicate that in the ground triplet state the most favorable reaction mechanism involves insertion of the Ti atom into a CO bond [via a η2-C,O coordinated t-(TiOC)O complex] to produce a triplet t-OTiCO molecule with the energy gain of 43.9 kcal/mol and the latter can further dissociate to TiO(3Δ)+CO with the total reaction exothermicity of ∼30 kcal/mol. The addition mechanism leading to the same TiO(3Δ)+CO products via a metastable η2-O,O complex t-cyc-TiCO2 is also feasible at ambient temperatures since the highest barrier on the reaction pathway is only 4.7 kcal/mol. The reaction mechanisms in excited singlet and quintet electronic states have many similar features with the ground state reaction but also exhibit some differences. In the singlet state, the reaction can follow A″1 and A′1 pathways, of those the insertion via a s-(TiOC)O (1A′) complex leading to s-OTiCO (1A′) and then to TiO(1Δ)+CO does not have an activation barrier. The insertion mechanism on the A″1 PES depicts a low barrier of 1.8 kcal/mol and leads to s-OTiCO (1A″), which dissociates into TiO(1Δ)+CO. The addition pathways via η2-O,O coordinated complexes require to overcome significant barriers, 7.8 and 34.9 kcal/mol for the A″1 and A′1 states, respectively. In the quintet state, the reaction at low and ambient temperatures can proceed only by coordination of Ti(5F) toward CO2 with formation of η2-C,O q-(TiOC)O, η2-O,O q-cyc-TiCO2, and η1-O q-TiOCO bound by 9.7, 6.1, and 4.6 kcal/mol, respectively, relative to the reactants. The η2-C,O and η1-O coordinations occur without barriers, while the η2-O,O coordination has an entrance barrier of 4.2 kcal/mol. The calculated PESs show that the carbon dioxide reforming into CO in the presence of Ti atoms should take place spontaneously.

Vibrational Analysis of trans-Stilbene in the Ground and Excited Singlet Electronic States Revisited

The Raman and infrared bands of trans-stilbene in the ground electronic state (S0) and an excited singlet state (S1) are assigned on the basis of density functional calculations at the 6-311+G** level for the S0 state and configuration interaction single calculations with the 6-311+G** basis set for the S1 state. Not only the wavenumbers of normal modes but also Raman activities, Raman depolarization ratios, and infrared intensities are calculated and used for band assignments. The vibrational patterns of some characteristic modes are discussed. It is found that, with respect to a few low-wavenumber modes in both the S0 and S1 states, the results of the present calculations are inconsistent with those derived from an analysis of the fluorescence excitation spectra and dispersed fluorescence spectra of trans-stilbene in a supersonic jet.

Nor-Harmane (9H-Pyrido[3,4-b]indole) as Outstanding Matrix for UV-Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Analysis of Synthetic and Bio-Polymers.

In an effort to gain an understanding of the processes governing matrix-assisted ultraviolet laser desorption/ionization (UV-MALDI), we made a direct comparison of the mass spectra of synthetic (PEG, PMMA, PS, PDMS) and bio-polymers (neutral cyclic and acyclic, sulfated and basic mono- and polysaccharides) by using nor-harmane and gentisic acid (GA) as UV (337 nm) -MALDI-TOF-MS matrices in positive and negative ion modes in linear, reflectron and post-source decay (PSD) modes. The majority of the examples discussed shows nor-harmane working better as matrix than GA. This peculiarity is quite noticeable in negative ion mode. Because the initiation of the UV-MALDI process is a photochemical reaction (matrix+hν (337 nm) → matrix), the acid-base character of nor-harmane in the ground and the lower electronic excited singlet state, together with other photophysical properties are discussed in brief. Excellent results obtained with commercial synthetic polymers and commercial cyclodextrins with nor-harmane allow us to propose these analyte-matrix systems for UV-MALDI-TOF-MS calibration in positive and negative ion modes in linear, reflectron and PSD modes.

Transient absorption spectroscopy in biology using the Super-ACO storage ring FEL and the synchrotron radiation combination

The Super-ACO storage ring FEL, covering the UV range down to 300 nm with a high average power (300 mW at 350 nm) together with a high stability and long lifetime, is a unique tool for the performance of users applications. We present here the first pump-probe two color experiments on biological species using a storage ring FEL coupled to the synchrotron radiation. The intense UV pulse of the Super-ACO FEL is used to prepare a high initial concentration of chromophores in their first singlet electronic excited state. The nearby bending magnet synchrotron radiation provides, on the other hand a pulsed, white light continuum (UV–IR), naturally synchronized with the FEL pulses and used to probe the photochemical subsequent events and the associated transient species. We have demonstrated the feasibility with a dye molecule (POPOP) observing a two-color effect, signature of excited state absorption and a temporal signature with Acridine. Applications on various chromophores of biological interest are carried out, such as the time-resolved absorption study of the first excited state of Acridine.

The MRSDCI/CIS study of excited electronic states of the SF 2 radical

The vertical ( T v) and adiabatic ( T 0) excitation energies for singlet electronic excited states of the SF 2 radical have been calculated by using the multireference single and double excitation configuration interaction (MRSDCI) method and aug-cc-pVTZ basis sets augmented by Rydberg functions. The MRSDCI T v calculations indicate that the X 1 A 1 , 1 1 A 2 , 1 1 B 1 , 2 1 B 1 , 2 1 A 2 , 2 1 A 1 , 3 1 B 1 , 4 1 B 1 , 3 1 A 1 , and 1 1 B 2 states are the 10 lowest-lying singlet states. Based on the MRSDCI//CIS T 0 calculations (using CIS optimized geometries for excited states), the A, B, C, E, F, G, H, and I states of SF 2 are assigned to 1 1 B 1 , 2 1 B 1 , 3 1 B 1 , 2 1 A 2 , 2 1 A 1 , 3 1 A 1 , 4 1 B 1 , and 1 1 B 2 , respectively.

Understanding the solvatochromism of 10-hydroxybenzo[h]quinoline. An appraisal of a polarity calibrator

The solvatochromism of the 10-hydroxybenzo[h]quinoline molecule (HBQ) has been investigated for the S 0→S 1 absorption of the normal tautomer, and for the S 1 ′→S 0 ′ fluorescence emission of the proton-transfer (PT) tautomer. The wavelength shifts of the first absorption band depend principally on polarity of the solvent used with a minor contribution of acidity, contrary to the fluorescence band wavelength shifts, which exhibit a greater dependence on acidity than on polarity. These observations preclude the use of HBQ for probing polarity in cavities of biopolymers. Theoretical calculations show that the local minimum of electrostatic potential ( V min) which is found at the carbonyl oxygen center in the S 1 ′ state of the PT tautomer is much deeper than that V min located at the hydroxy oxygen center in the S 1 state of the normal tautomer. This observation is interpreted as a significant increase in basicity of the carbonyl oxygen center, this being approximately twice more basic than the hydroxy oxygen center for the first singlet excited electronic state. Therefore, the oxygen center is the most subjected to be perturbed by solvent acidity. The steps followed in this paper for analyzing the response of HBQ to solvent effects can be applied to any molecular system to be used for probing polarity of the environment (e.g., in biopolymers).

Electronic States of a Novel Smaragdyrin Isomer: Polarized Spectroscopy and Theoretical Studies

Spectral and theoretical techniques were applied to investigate the electronic structure and spectra of two recently synthesized pentapyrrolic macrocycles, isomers of smaragdyrin: 16,20-dibutyl-2,3,6,7,10,11,15,21-octamethyl-[22]pentaphyrin-(1.1.1.0.0) (1) and 16,20-dibutyl-2,3,6,7,10,11,15,21-octamethyl-5-oxa-[22]pentaphyrin-(1.1.1.0.0) (2). Combined use of linear dichroism, magnetic circular dichroism, fluorescence anisotropy and INDO/S calculations resulted in the location and assignments of a number of electronic transitions (eleven for 1 and seven for 2). Even though the spectral pattern differs somewhat from that characteristic of most porphyrins, the results show that the four lowest excited electronic singlet states of both compounds are very well described by a four-orbital model, widely used for the interpretation of spectra in this type of compounds. Fluorescence and transient absorption/bleaching measurements enabled the determination of the rate constants of the radiative and nonradiative S1 depopulation processes. These photophysical properties are consistent with a rigid, nearly planar excited state geometry that is essentially unchanged with respect to the ground-state structure.

Ab Initio Based Exploration of the Potential Energy Surface for the Double Proton Transfer in the First Excited Singlet Electronic State of the 7-Azaindole Dimer

In this paper ab initio full geometry optimizations are carried out for the ground and first excited singlet electronic states of the 7-azaindole dimer, a well-known prototype of the DNA base pairs. Results indicate that the C2h symmetry of the ground-state minimum energy is not maintained in the excited state that has to be described as a dimer between an excited base unit and another one in the ground state. Given this asymmetry, the double proton transfer in the excited state is found to be stepwise in nature, passing through a very shallow zwitterionic intermediate. Inclusion of the zero point energy and the rest of the thermodynamic corrections points to the nonexistence of bound states for the intermediate well. Our theoretical calculations have also confirmed the presence of another intermediate where the transfer of a single proton is compensated by a charge-transfer electronic excitation. This neutral intermediate is found lower in energy than the zwitterionic one and could be responsible for the stepwise reaction observed in several recent experiments done at the femtosecond time scale. A nonadiabatic transition should occur between the initially accessed electronic state (involving an excitation localized in one base unit) and the one possessing the neutral intermediate that involves a charge-transfer transition. When including the bulk effect of a polar solvent it is observed that the energies of the intermediates are lowered so that the zwitterionic intermediate may exist in solution. The transition state energies are also lower, a result that points to an increase in the rate constant of the process in condensed phase.

Ab initio study of the structure and conformations of 2-fluoroethanal in the ground and lowest excited electronic states

The structure of the conformationally flexible 2-fluoroethanal molecule (CH2FCHO, FE) in the ground (S0) and lowest excited triplet (T1) and singlet (S1) electronic states was investigated by ab initio quantum-chemical methods. The FE molecule in the S0 state was found to exist as two conformers, viz., as cis (the F—C—C—O angle is 0°) and trans (the F—C—C—O angle is 180°) conformers. On going both to the T1 and S1 states, the FE molecule undergoes substantial structural changes, in particular, the CH2F top is rotated with respect to the core and the carbonyl CCHO fragment becomes nonplanar. The potential energy surfaces for the T1 and S1 states are qualitatively similar, viz., six minima in each of the excited states of FE correspond to three pairs of mirror-symmetrical conformers. Based on the potential energy surfaces calculated for the FE molecule in the T1 and S1 states, the one-dimensional problems on the torsion and inversion nuclear motions as well as the two-dimensional torsion-inversion problems were solved.

Photodissociation of DOBr—Time-dependent wavepacket calculations

DOBr photoabsorption cross-sections and product rotational state distributions of the OD fragment resulting from excitation to the two lowest-lying excited singlet electronic states, 11A″ and 21A′, are computed using time-dependent wavepacket dynamics. The dynamical calculations are based on two-dimensional ab initio potential energy and transition dipole moment surfaces, in which the OD bond length is held fixed. The computed absorption band for DOBr is very similar to that of HOBr for excitation from the ground vibrational state. For vibrationally mediated photofragmentation spectra in which the initial state is a vibrationally excited state, the absorption line shape for DOBr differs markedly from the corresponding HOBr absorption. For all initial vibrational states, the resulting OD fragments are produced more rotationally “hot” than their OH counterparts. The resulting OD and OH rotational distributions agree qualitatively with experimental measurements at 266 nm, where the excitation is dominated by the parallel 2 1A′←1A′ transition. Predictions are also made for the rotational distributions at 355 nm, where the perpendicular transition 11A″←1A′ is dominant and no experimental product state distributions are as yet available.