Excited Singlet Electronic States Research Articles

The ground and excited states of HBrO2 [HOOBr, HOBrO, and HBr(O)O] and HBrO3 (HOOOBr and HOOBrO) isomers

The HBrO2 isomers have been analyzed computationally to confirm the previous experimental assignments for HOOBr and HOBrO and to assist in future identification of the as yet unobserved HBr(O)O isomer. Optimized geometries of the HOOBr, HOBrO, and HBr(O)O isomers and the transition states connecting them were obtained at the CCSD(T)/O, H: aug-cc-pVTZ, Br: aug-cc-pVTZ-PP level of theory. The corresponding harmonic vibrational frequencies for the HOOBr, HOBrO, and HBr(O)O isomers are reported for all isotopologues considered in the experimental measurements, i.e., those involving hydrogen, deuterium, $$^{79}$$ Br, $$^{81}$$ Br, $$^{16}$$ O, and $$^{18}$$ O. The relative energetics of the stationary point geometries were determined through extrapolation of energies to the complete basis set limit. To explain the photodestruction observed experimentally for HOOBr and HOBrO, the three lowest low-lying singlet and triplet excited electronic states for each of the three isomers were computed using the equation-of-motion coupled-cluster with inclusion of single and double excitations (EOM-CCSD) and time-dependent density functional theory (TD-B3LYP and TD-CAM-B3LYP) approaches; all utilizing the all-electron aug-cc-pVTZ basis sets for all atoms. Multi-reference configuration interaction (MRCI)/aug-cc-pVTZ computations were carried out for the lowest singlet and lowest two triplet excited states. The vertical excitation energies for the low-lying excited states of the most stable isomer (HOOBr) are reported for the first time. The vibrational frequencies for the $$\hbox{HBrO}_{2}$$ isomers are used along with new anharmonic vibrational frequency computations (at the PBE0/aug-cc-pVTZ level of theory) and vertical excitation energies (at the TD-B3LYP/aug-cc-pVTZ, TD-CAMB3LYP/aug-cc-pVTZ, and EOM-CCSD/aug-cc-pVTZ levels of theory) for the $$\hbox {HBrO}_{3}$$ isomers, HOOOBr and HOOBrO, to determine that previously unassigned peaks in the experimental spectrum generated from HBr/O $$_{2}$$ photolysis in a Ne matrix belong to HOOOBr.

Communication: An accurate calculation of the S1 C2H2 cis-trans isomerization barrier height.

A high level ab initio calculation of the cis-trans isomerization barrier height in the first excited singlet electronic state of acetylene is found to agree very well with a recent experimental determination.

Conformational analysis of N-methylformamide in ground S0 and excited S1 and T1 electronic states

For conformers of the N-methylformamide (HCONHCH3) molecule, calculations of equilibrium geometry parameters, harmonic vibration frequencies, energy differences and potential barriers to conformational transitions were performed in the ground (S0) and lowest excited singlet (S1) and triplet (T1) electronic states. In the S0 state, the molecule exists in trans and cis stable conformations (having Cs symmetry). Our calculations show that the electronic excitations T1←S0 and S1←S0 cause changes in the structure of conformers: both HCON and HNCC fragments become pyramidal and rotate around the CN bond. As a result, in each excited electronic state under consideration, there are 12 minima forming six pairs of equivalent conformers separated by relatively small potential barriers. One- and two-dimensional potential energy surface sections corresponding to different intramolecular large-amplitude motions were calculated using the MP2/aug-cc-pVTZ (S0) and CASPT2/cc-pVTZ (S1 and T1) methods. Anharmonic vibrational problems for large-amplitude motions were solved, and the corresponding frequencies were estimated.

Photodissociation dynamics of propyne at 193 nm: a trajectory surface hopping study.

Photodissociation dynamics of propyne at 193 nm are studied using the fewest switches nonadiabatic trajectory surface hopping method on its first excited singlet electronic state (1(1)A''). The trajectories are propagated based on potential energies, gradients and nonadiabatic couplings calculated at the MRCIS(6,7) level with the 6-31++G(d,p) basis set. Our trajectory calculations have revealed that H + H3CCC is the major dissociation channel, which has also been predicted experimentally. For the primary photodissociation channel H + H3CCC we demonstrate that nonadiabatic dynamics do not play a significant role. This observation is however contradictory to most of the previously reported experimental predictions. The calculated product translation energy distribution for the acetylenic H atom elimination peaked at ∼ 18 kcal mol(-1), indicating that the dissociation occurs adiabatically on a moderately repulsive excited surface that correlates with the ground state products (CH3C ≡ C + H). The H atom elimination process from the methyl fragment involving a transition state, which has to compete with the acetylenic H atom dissociation channel with no barrier in the excited singlet surface, was found to be too less probable to make a contribution to product branching. We observed that a fewer but significant number of trajectories led to CH3 + CCH product formation which has not been observed experimentally when propyne is excited at 193 nm.

The S1 ← S0 fluorescence excitation spectrum and structure of propanal in the S1 excited electronic state.

We have obtained and analyzed the S1 ← S0 fluorescence excitation spectra of jet-cooled propanal-h1 (CH3CH2CHO) and -d1 (CH3CH2CDO). Using the results of theoretical studies of the structure of propanal molecule in the S1 lowest excited singlet electronic state, we have assigned the bands of both spectra to the vibronic transitions of the cis conformer (in the S0 ground electronic state) to the 1 and 3 conformers (in the S1 state) differed by the angle of the C2H5 ethyl group rotation around the central C-C bond. The origins of the 1 ← cis and 3 ← cis electronic transitions have been observed at 29 997 and 30 075 cm(-1) for propanal-h1 and at 30 040 and 30 115 cm(-1) for propanal-d1, respectively. The high activity of torsional (C2H5 ethyl groups) and inversional (CCHO/CCDO carbonyl fragments) vibrations and the intensity distribution of the bands in torsional sequences (passing through maximum) are in agreement with the theoretical prediction that the S1 ← S0 electronic excitation of the cis conformer causes (after geometrical relaxation) the pyramidalization of carbonyl fragments and the rotation of ethyl groups around the central C-C bond. A number of energy levels have been found for torsional and inversional vibrations, and also fundamentals of ν10 (CCO bend) and ν13 (CCC bend) for the both 1 and 3 conformers of propanal-h1 and -d1 have been found. Then the "experimental" potential functions of inversion for the pair of the 1 and 3 conformers have been determined. The heights of potential barriers to inversion and the angle values corresponding to the minima of potential functions of inversion are 900 cm(-1) and 35° for propanal-h1 and 820 cm(-1) and 34° for propanal-d1, respectively.

Classical blended quantum formulation of the parallel TDDVR method to the dynamics on furan

A major portion (∼98%) of the quantum-classical molecular dynamics approach, namely, Time-Dependent Discrete Variable Representation (TDDVR) method, is recently parallelized using shared-memory parallelization scheme with the aim of performing dynamics on relatively large molecular systems. Therefore, we have chosen the furan as a model system for dynamical simulation. Four lowest singlet excited electronic states 1A2(3s), 1B2(V), 1A1(V*), and 1B1(3p) of furan are vibronically coupled through several conical intersections in the vicinity of Frank-Condon region. The major focus of the present paper is to explore the efficiency of our newly implemented parallelized TDDVR algorithm to perform dynamics on large dimensional quantum systems in vibronically coupled electronic manifold. The present version of parallelized TDDVR algorithm show closely linear speed up with increasing number of computing processors. As a significant speed up is achieved by cycling in the correct way over arrays, all dynamical simulations are performed within a reasonable wall clock time. The photoelectron spectra calculated by the TDDVR method show peak by peak correspondence with the experimental spectra. The TDDVR calculated quantum-classical dynamical outcomes, viz., population and photoelectron spectra, etc. show good agreement with the findings of the well-established quantum dynamical method, i.e. Multi Configuration Time-Dependent Hartree (MCTDH) approach.

The low-lying singlet electronic excited states of ZrO2: A symmetry adapted cluster–configuration interaction (SAC–CI) study

The ground electronic state [X1A1(S0)] and nine low-lying singlet excited states [21A1(S2), 11A2(S3), 21A2(S6), 11B1(S4), 21B1(S7), 31B1(S11), 11B2(S1), 21B2(S5), and 31B2(S8) states] of ZrO2 were studied using SAC–CI theory. The geometries of the nine electronic excited states were optimized at the SAC–CI/def2-SVP, SAC–CI/def2-SVPD, and SAC–CI/LANL2TZ(f) levels. The vertical excitation energies and adiabatic excitation energies of the selected electronic excited states were calculated by single-point calculations at the SAC–CI/def2-TZVPPD and SAC–CI/LANL2TZ(f) levels. Stable C2v structures for the 11B2, 21B2, 31B2, and 11A2 states and stable Cs structures for the 11B1(A″), 31B1(A″), and 21A1(A′) states were determined. For comparison, the unrestricted density functional theory (UBPW91) was also applied to study the X1A1, 11A2, 11B1, and 11B2 states.

Coherent quantum control of internal conversion: in pyrazine via weak field excitation

Coherent control of internal conversion (IC) between the first (S1) and second (S2) singlet excited electronic states in pyrazine, where the S2 state is populated from the ground singlet electronic state S0 by weak field excitation, is examined. Control is implemented by shaping the laser which excites S2. Excitation and IC are considered simultaneously, using the recently introduced resonance-based control approach. Highly successful control is achieved by optimizing both the amplitude and phase profiles of the laser spectrum. The dependence of control on the properties of resonances in S2 is demonstrated.

Novel 1- and 9-aminoanthracene/3-aminobenzanthrone bichromophores exhibiting through-bond excitation energy transfer

New 1/9- aminoanthracene-triazine-3-aminobenzanthrone bichromophores for the study of excitation energy transfer were synthesized and characterised. Absorption and emission spectra and fluorescence quantum yields were measured. Equilibrium ground state conformations and singlet electronic excited states characteristics were calculated at semiempirical level. A decrease of qF in polar solvents may be explained by the CT state connected with electron transfer from the HOMO localized on the donor to the LUMO spread over the acceptor. A highly efficient energy transfer mediated by through-bond mechanism was observed for all compounds.

The low-lying singlet electronic excited states of TiO2: A symmetry adapted cluster-configuration interaction (SAC-CI) study

The ground electronic state (X1A1) and nine low-lying singlet electronic excited states (21A1, 31A1, 11A2, 21A2, 11B1, 21B1, 31B1, 11B2, and 21B2) of TiO2 were studied using SAC-CI theory. The geometries of the nine electronic excited states were optimized at the SAC-CI/def2-SVP and SAC-CI/def2-SVPD levels. The adiabatic excitation energies of the nine electronic excited states were obtained from single-point calculations at the SAC-CI/def2-TZVPD//SAC-CI/def2-SVPD and SAC-CI/def2-TZVPPD//SAC-CI/def2-SVPD levels. Stable Cs structures for the 11B1 (A″) and 31B1 (A″) states were determined in this study. For comparison, the X1A1, 11A2, 11B1, and 11B2 states were also studied using the unrestricted DFT theory (UBPW91) with the def2-SVP, def2-SVPD, def2-TZVPD, def2-TZVPPD, and aug-cc-pVTZ basis sets.

An ab initio study of structure, stability, and spectroscopic parameters of 5-atomic [C, C, H, N, S] isomers

This theoretical study is focused on predicting structures, energetics, and selected spectroscopic constants for a range of 5-atomic chemical species sharing the [C, C, H, N, S] stoichiometry, including thioformyl cyanide and iminoethenethione. An in-depth study carried out on the four most stable isomers have yielded (i) structures and energy for the lowest singlet and triplet excited electronic states; (ii) anharmonic vibrational frequencies and IR absorption intensities for the fundamental, overtone and combination modes; (iii) molecular parameters of interest to microwave spectroscopy: rotational constants, electric dipole moments, and quadrupole coupling constants.

Electron Energy Loss of Terrylene Deposited on Au(111): Vibrational and Electronic Spectroscopy

We have investigated the vibrational and electronic properties of terrylene by high-resolution electron energy-loss spectroscopy (HREELS), Fourier-transform infrared spectroscopy, and low-temperature single-molecule fluorescence spectroscopy. Terrylene thin films were sublimated in an ultrahigh vacuum on the Au(111) surface in order to record the HREEL spectra. Polycrystalline p-dichlorobenzene was used as a matrix to isolate a single terrylene molecule at 1.5 K and record its fluorescence spectrum. The infrared spectrum, the vibrational components from the fluorescence spectrum, and density functional theory calculations were used for the assignment and identification of the active modes found in HREELS. Finally, we report a loss signal around 17 000 cm–1 (2.1 eV) for the first singlet electronic excited state in agreement with optical spectroscopy. The HREEL spectra show both IR- and Raman-active vibration modes because of specific surface selection rules. Energy-loss spectroscopy could be used as a complementary technique to explore some other degrees of freedom that are not accessible by optical means.

Vibrational spectroscopic, NMR parameters and electronic properties of three 3-phenylthiophene derivatives via density functional theory.

Quantum chemistry calculations have been performed to compute the optimized geometries, vibrational frequencies, and Mulliken Charges at B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) levels for 3-(4-fluorophenyl)thiophene (FPT), 3-(4-nitrophenyl)thiophene (NPT) and 3-(4-cyanophenyl) thiophene (CPT) in the ground state. In addition, the 13C and 1H NMR are calculated by B3LYP/6-311++G(d,p) and B3LYP/6-311++G(2d,2p) methods. The singlet electronic excited state properties of the three compounds were investigated using the time-dependent density functional method (TD-DFT) at the B3LYP/6-311++G(d,p)//TD- B3LYP/6-311++G(d,p) level of theory. The influence of the substituted groups on C9 atom is discussed.

BF3 valence and Rydberg states as probed by electron energy loss spectroscopy and ab initio calculations.

In this contribution we probe BF3 low-lying excited singlet states measured at 100 eV, 2.8° scattering angle and triplet states at 40 eV, 40° scattering angle, while sweeping the energy loss over the range 10.0-20.0 eV. The electronic state spectroscopy has been investigated and the assignments supported by quantum chemical calculations. This provides the first comprehensive investigation of all singlet and triplet excited electronic states of boron trifluoride up to the first ionization potential. A generalized oscillator strength analysis is employed to derive oscillator strength f0 value and integral cross sections (ICSs) from the corresponding differential cross sections (DCSs). The f0 value is compared with the optical oscillator strength (OOS) from photoabsorption, and the unscaled Born ICSs are then compared with relevant energy and binary-encounter and f-scaled Born cross section (BEf-scaling) results determined as a part of this investigation. The lowest n members of the Rydberg series have been assigned as converging to the lowest ionization energy limits of boron trifluoride and classified according to the magnitude of the quantum defects (δ).

An experimental and theoretical investigation into the excited electronic states of phenol.

We present experimental electron-energy loss spectra (EELS) that were measured at impact energies of 20 and 30 eV and at angles of 90° and 10°, respectively, with energy resolution ∼70 meV. EELS for 250 eV incident electron energy over a range of angles between 3° and 50° have also been measured at a moderate energy resolution (∼0.9 eV). The latter spectra were used to derive differential cross sections and generalised oscillator strengths (GOS) for the dipole-allowed electronic transitions, through normalization to data for elastic electron scattering from benzene. Theoretical calculations were performed using time-dependent density functional theory and single-excitation configuration interaction methods. These calculations were used to assign the experimentally measured spectra. Calculated optical oscillator strengths were also compared to those derived from the GOS data. This provides the first investigation of all singlet and triplet excited electronic states of phenol up to the first ionization potential.

Quantum Dynamics Simulations Reveal Vibronic Effects on the Optical Properties of [n]Cycloparaphenylenes

The size-dependent ultraviolet/visible photophysical property trends of [n]cycloparaphenylenes ([n]CPPs, n = 6, 8, and 10) are theoretically investigated using quantum dynamics simulations. For geometry optimizations on the ground- and excited-state Born-Oppenheimer potential energy surfaces (PESs), we employ density functional theory (DFT) and time-dependent DFT calculations. Harmonic normal-mode analyses are carried out for the electronic ground state at Franck-Condon geometries. A diabatic Hamiltonian, comprising four low-lying singlet excited electronic states and 26 vibrational degrees of freedom of CPP, is constructed within the linear vibronic coupling (VC) model to elucidate the absorption spectral features in the range of 300-500 nm. Quantum nuclear dynamics is simulated within the multiconfiguration time-dependent Hartree approach to calculate the vibronic structure of the excited electronic states. The symmetry-forbidden S0 → S1 transition appears in the longer wavelength region of the spectrum with weak intensity due to VC. It is found that the Jahn-Teller and pseudo-Jahn-Teller effects in the doubly degenerate S2 and S3 electronic states are essential in the quantitative interpretation of the experimental observation of a broad absorption peak around 340 nm. The vibronic mixing of the S1 state with higher electronic states is responsible for the efficient photoluminescence from the S1 state. The fluorescence properties are characterized on the basis of the stationary points of the excited-state PESs. The findings reveal that vibronic effects become important in determining the photophysical properties of CPPs with increased ring size.

Ground and excited state hydrogen bonding effects of 6-aminocoumarin in water: An ab initio study

The hydrogen bonded complexes of 6-aminocoumarin (6AC) with several water molecules in their ground and lowest excited singlet electronic state have been investigated using the ab initio methods. It was shown that the complex containing five water molecules in the surrounding of the amino group of the 6AC molecule has the lowest stabilization energy in the ground electronic state. This complex can serve as a model of the first solvation shell of the 6AC molecule in bulk water. The energies of hydrogen bonds formed by the carbonyl group with up to two water molecules were also determined. The calculated change in the total hydrogen bond energy due to S0→S1 excitation is in good agreement with the experimental data [E. Krystkowiak, A. Maciejewski, Phys. Chem. Chem. Phys. 2011;13:11317–11324]. The maxima of absorption spectra of the hydrogen-bonded complexes, calculated taking into account nonspecific solute–solvent interactions, are in reasonable agreement with the experimental data.

A TD-DFT study of the absorption spectra of mono-nitrated fluoranthenes

The singlet electronic excited state properties of 1-, 2-, 3-, 7-, and 8-nitrofluoranthene (1-, 2-, 3-, 7-, 8-NF) were investigated using the time-dependent density functional method (TD-DFT) at the B3LYP/6-311+G(d,p)//TD-B3LYP/6-311(d,p) level of theory. The simulated and experimental UV/Vis spectra are in good agreement, thus enabling assignments and interpretation of the observed absorption bands. The influence of the nitro group upon electronic spectra, and structure–spectroscopic–mutagenicity correlations are made. The absorption λmax (S1 excitation energy) for the nitrated fluoranthenes are red-shifted by ∼10–30nm vis-à-vis fluoranthene: with wavelength shifts dependent on the planarity of the nitro group relative to the aryl moiety. The chemical hardness and HOMO–LUMO energy gaps for the nitro-fluoranthenes are predicted in the range 3.5–3.8eV and follow the order: fluoranthene (3.99eV)>7-NF (3.81eV)>2-NF (3.72eV)≈8-NF (3.71eV)>1-NF (3.52eV)>3-NF (3.50eV).

Tautomerism in substituted cyanopyridone: Ultrafast dynamics and TDDFT studies in water

Tautomerism of cyanopyridone (CPy) has been studied in both the ground (S0) and first singlet excited (S1) electronic states by means of steady-state and time resolved optical spectroscopy as well as time-dependent density functional theory (TDDFT) calculations. The enol (E)-to-keto (K) tautomerization reactions occur in both the ground and excited states of the neutral structures in water. Gas-phase calculations for the K and E tautomers support that both have similar energies in the ground state. Calculations of the S1 states of each tautomer suggest the energy difference between the tautomers is increased in the excited state, favoring the K tautomer.

Molecular switch properties of 7‐hydroxyquinoline compounds

The present study is concerned with the theoretical study of possible molecular switch systems. The 7‐hydroxyquinoline‐8‐carboxamide molecule and its single‐, double‐, and triple‐substituted derivatives are investigated with the aim of revealing characteristic switch features. Molecular switches can be considered as composed of a frame and a crane component. According to a recent study, the 7‐hydroxyquinoline double‐ring system constitutes the frame moiety, while a carboxamide group at position 8 plays the role of the crane part (Csehi et al., Phys. Chem. Chem. Phys. 2013, 15, 18048). The effect of single 2‐,4‐,6‐methyl, double 2,4‐, 2,6‐diamino, and triple 2,4,6‐triamino substitutions to the molecular frame has been investigated using high level ab initio techniques. As a possible reaction mechanism, excited state intramolecular hydrogen transfer mediated by the frame‐crane torsion has been considered. At the terminal structures of this pathway, second‐order approximate coupled‐cluster (CC2) quality vertical excitation energies and oscillator strengths have been calculated for the three lowest‐lying singlet electronic excited states of all the studied systems. Single point calculations at selected geometries of the reaction path were carried out at the CC2 level as well, while conical intersections (CIs) between the ground and first excited states near perpendicular twisted geometries were optimized using the complete active space self‐consistent field method. To confirm the presence of CIs, nonadiabatic coupling terms have been derived and applying the topological line integral technique, the topological (or Berry) phase has been calculated surrounding the point of CI. The results of this work clearly demonstrate the fulfillment of several molecular switch properties by the investigated quinoline derivatives. An extensive comparison between the different compounds is presented as well. © 2014 Wiley Periodicals, Inc.