Excited Singlet Electronic States Research Articles

Comment on “Excitations in photoactive molecules from quantum Monte Carlo” [J. Chem. Phys. 121, 5836 (2004)

It is shown that the qualitative differences between high-level ab initio calculations and restricted open-shell Kohn-Sham (ROKS) results for the lowest singlet excited electronic state of formaldimine along a particular isomerization path found by Schautz, Buda, and Filippi [J. Chem. Phys.121, 5836 (2004)] play a minor role in molecular dynamics simulations of photoisomerization at room temperature. In fact, ROKS yields, within its well-known limitations, a good representation of the physically relevant isomerization pathway.

Structure of acetyl chloride molecule isotopomers CH3COCl and CD3COCl 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.

One-dimensional models of internal rotation in CX 3NO molecules (X = H, D, F)

Various non-empirical methods for estimating the parameters of one-dimensional internal rotation potentials and energies of torsional transitions were compared for the CX 3NO molecules (X = H, D, F) in the ground ( S 0) and lowest excited singlet ( S 1) electronic states. The potential energy surfaces were studied by the ab initio MR-AQCC/cc-pVTZ, MR-AQCC/cc-pVTZ(- f), MP2/6-311G(2 d), and MP2/6-311G( d, p) methods. The one-dimensional internal rotation problem was solved using the following models: (1) geometry optimization at a given internal rotation coordinate; (2) intrinsic reaction path; (3) gradient extremal; and (4) use of only the data on potential energy surface stationary points. Special attention was paid to the problem of calculation of kinematic coefficient. In all cases, the calculated torsional energies for CX 3NO molecules (X = H, D, F) are in agreement with experiment. The results from different methods for constructing torsional cross-sections of the potential energy surface are virtually equivalent and differ insignificantly from the results of calculations within the framework of the simplest model, hence, estimates of the barrier to internal rotation are of most importance. It was found that a change in the zero-point energy could give a correction to the internal rotation potential as large as 15% of the potential barrier. However, in the case under consideration the calculations in the harmonic approximation taking into account this correction do not improve the agreement between the calculated torsional transitions and the experimental data.

Quantum Dynamics Study of the Excited‐State Double‐Proton Transfer in 2,2′‐Bipyridyl‐3,3′‐diol

Density functional theory and quantum dynamics simulations have been used to study the double-proton transfer reaction in 2,2'-bipyridyl-3,3'-diol in the first singlet excited electronic state. This process is experimentally known to be branched: It consists of a fast, concerted reaction mechanism (tau approximately 100 fs) and a stepwise reaction mechanism [with a fast initial step (tau approximately 100 fs) and a slower final step (tau approximately 10 ps)]. Quantum dynamics simulations on a two-dimensional model reveal that the concerted reaction occurs despite the nonexistence of a concerted reaction path, but they fail to explain the relative slowness of the stepwise mechanism. A qualitative simulation using a three-dimensional model suggests that internal vibrational relaxation (IVR) might be the reason why the second stage of the stepwise mechanism is so slow.

The (Lb)S1←S0 transition of phenylpropyne and phenylacetylene: an experimental and ab initio study

Abstract This work describes the first excited singlet electronic state, L b , of phenylpropyne (PPR) and phenylacetylene (PA). Ab initio calculations have been performed for the geometry and normal modes in the S 0 and S 1 states. One-photon and two-photon S 1 ← S 0 spectra of jet cooled samples have been examined (REMPI detection) and a detailed vibronic analysis has been carried out for PPR and PA. The origin band of PPR ( λ = 279.64 nm) is one order of magnitude weaker than the origin band ( λ = 278.63 nm) of PA and the acetylenic modes are greatly reduced by the H → CH 3 substitution. These changes and the parallel PPR-PA comparison provided a way for secure vibronic assignment in both molecules, which was also aided by theoretical predictions.

Does chlorine peroxide absorb below 250 nm?

Low-lying singlet and triplet electronic excited states of ClOOCl are presented. Calculations of the excitation energies and oscillator strengths are reported using excited state coupled cluster response methods, as well as the complete active space self-consistent field method with the full Breit-Pauli spin-orbit operator. These calculations predict that for ClOOCl there should be a weakly absorbing triplet state lying below the lowest absorbing singlet excited state. This state is predicted to have an absorption maximum at about 385 +/- 25 nm. This lowest triplet state is calculated to be dissociative and leads to ClOO+Cl.

Near-infrared diode laser spectroscopy of free radicals

Spectroscopic results on the radicals HCSi, CCO, and FeC obtained by studying in detail energy level structures using 0.8 μm diode laser system are reported. Of these radicals, the CCO radical was investigated mainly using Fabry–Perot type diode lasers with inconvenient mode gaps in the early stage of our near-infrared diode laser spectroscopic study of free radicals, and on the other hand, the FeC and HCSi radicals were studied using an external cavity diode laser. For the FeC radical, which is an interesting radical composed of an iron atom having 3d electrons, information on spin–orbit interaction between the triplet electronic ground state and a low-lying singlet electronic excited state is reported somewhat in detail. For the HCSi and CCO radicals, spectral particularities produced by a Renner–Teller interaction and a spin–orbit interaction are described for their high-resolution spectroscopic interest.

Coherent spin control of matrix isolated molecules by IR+UV laser pulses: quantum simulations for ClF in Ar.

Two coherent sequential IR+UV laser pulses may be used to generate two time-dependent nuclear wave functions in electronic excited triplet and singlet states via single (UV) and two photon (IR+UV) excitation pathways, exploiting spin-orbit coupling and vibrational pre-excitation, respectively. These wave functions evolve from different Franck-Condon domains until they overlap in a domain of bond stretching with efficient intersystem crossing. Here, the coherence of the laser pulses is turned into optimal interferences of the wave packets, yielding the total wave packet at the target place, time, and with dominant target spin. The time resolution of spin control is few femtoseconds. The mechanism is demonstrated by means of quantum model simulations for ClF in an Ar matrix.

Multireference CI Study of Excitation Energies and Potential Energy Surfaces of CH3F

Vertical excitation energies for the eight singlet-excited electronic states 1 1 E (2e → 3s), 2 1 E (2e → 3p a 1 ), 3 1 E (2e → 3p e ), 2 1 A 1 (2e → 3p e ), 1 1 A 2 (2e → 3p e ), 3 1 A 1 (2p a 1 → 3s), 4 1 A 1 (2p a 1 → 3p a 1 ), and 4 1 E (2p a 1 → 3p e of CH 3 F were investigated using the SA-MCSCF, MR-CISD, and MRCISD+Q approaches. Our results mostly confirm the experimental assignments but suggest some modifications for the main contribution to the maximum observed in the range from 12.5-14 eV. The dissociation channels for the production of fluorine atoms have been characterized. Potential energy curves for the dissociation of the CF bond under C 3 v symmetry restrictions were computed for all states mentioned above leading to the ground-state dissociation channel CH 3 (X 2 A 2 ) + F( 2 P), the excited-state channels CH 3 (3s 2 A 1 ') + F( 2 P) and CH 3 (3p 2 A 2 ) + F( 2 P) and also to the ionic limit CH 3 +( 1 A 1 ') + F - ( 1 S). All curves except the one for the ionic state show repulsive behavior. The search for a global minimum for the ionic state led to the structure H 2 CH + F - in the 3 1 A' state. It is strongly bound by 5.67 eV with respect to the ionic dissociation limit of F - + CH 3 +.

Statisticodynamical approach of state distributions in the products of four-atom planar unimolecular reactions. II. Validation and distribution shape analysis in the barrier case.

In the first part of this series, we proposed a statisticodynamical approach of state distributions in the products of four-atom planar unimolecular reactions governed by short-range forces. In this second part, the approach is tested against quasiclassical trajectory calculations on an ab initio potential energy surface. The process considered is the fragmentation of isocyanic acid in the first excited singlet electronic state. The study leads to a very good agreement between both methods. In addition to that, we pinpoint in the barrier case the main mechanical parameters governing the shape of rotational state distributions. It appears that these parameters are related to two distinct physical effects. The first one is of the impulsive type. The second, already observed in triatomic processes, is the so-called bending effect.

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.

Quantum–chemical study of the structure of the acetyl fluoride molecule in the ground and lowest excited singlet and triplet electronic states

The structure of the conformationally flexible acetyl fluoride molecule (CH 3CFO and CD 3CFO) in the ground (S 0) and lowest excited triplet (T 1) and singlet (S 1) electronic states was calculated by different quantum–chemical methods (RHF, UHF, MP2, CASSCF). The equilibrium geometric parameters and harmonic vibrational frequencies of the molecules in these electronic states were estimated. The calculations demonstrated that the electronic excitation causes considerable conformational changes involving the rotation of the CH 3(CD 3) top and a substantial deviation of the CCFO carbonyl fragment from planarity. For large-amplitude vibrations, namely, for the torsional vibration in the S 0 state and the torsional and inversion (nonplanar carbonyl fragment) vibrations in the T 1 and S 1 states, the quantum–mechanical problems were solved in one-dimensional (1D) and two-dimensional (2D) approximations. The results of calculations are in good agreement with experimental data.

Nonradiative decay of photoexcited methylated guanine

Radiationless decay of the two most stable methylated guanine tautomers after photoexcitation to the first excited singlet electronic state has been investigated using an “on the fly” nonadiabatic surface hopping approach based on density functional theory. The biologically relevant 9Me-keto guanine exhibits large changes in geometric and electronic structure in the S1 state giving rise to an increased nonadiabatic transition probability and thus a shorter lifetime compared to the 7Me-keto tautomer. A detailed analysis in terms of the excited state topology and individual vibrational modes is presented.

Structure and vibrations of the CF3NO molecule in the ground and lowest excited electronic states: A test of ab initio methods

AbstractEquilibrium geometries, harmonic vibrational frequencies, and barriers to internal rotation of the CF3NO molecule in the ground singlet (S0) and lowest excited triplet (T1) and singlet (S1) electronic states are calculated using various ab initio techniques and compared with the experimental data. The sensitivity of the results to the choice of atomic orbital basis set, in particular, to the presence of f‐symmetry functions, and to the dynamical correlation treatment is analyzed. Anharmonic vibrational frequencies and dipole moment of the CF3NO molecule in the ground state are computed. The results of all approaches employed are in good agreement with each other and with available experimental data. Conclusions concerning the accuracy of different methods are drawn. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

The lowest-lying excited singlet and triplet electronic states of propanal: an ab initio molecular orbital investigation of the potential energy surfaces

This study explores the potential energy surfaces of the S0, S1 and T1 states using ab initio theory to provide insight into the spectroscopy, photochemistry and reaction dynamics of propanal. Minima associated with the formyl potential energy coordinate in the S1 and T1 states are found to be ∼60° out-of-phase with the S0 state. Furthermore, the excited states possess a pyramidal formyl carbon atom that leads to a double minimum at ±33° and ±49° for the S1 and T1 states, respectively. An exploration of the C–C dissociation coordinate on the T1 surface, yielding the products CH3CH2+CHO, shows that a three-fold potential due to the formyl torsion is still operational, resulting in three unique transition states that lead to dissociation. The lowest energy pathway to dissociation occurs at a barrier height of 4766 cm−1 and the energy of the products is found to be 1017 cm−1, relative to the T1 global minimum. Consequently, a reverse barrier of 3749 cm−1 is calculated. Parameters calculated for the lowest energy transition state geometry are distinctly different from structures inferred from experiment, which assumed an isotropic dissociation channel.

H-bonded N-heterocyclic base-pair phototautomerizational potential barrier and mechanism: The 7-azaindole dimer.

A theoretical analysis of the double proton transfer (PT) in a hydrogen-bonded N-heterocyclic base pair is presented. The calculated (time-dependent density functional theory) double PT barrier calculated for the concerted process of the 7-azaindole C(2h) dimer in the first excited singlet electronic state S(1) conforms well to the kinetic data and the photophysical evidence reported in this article. The calculated PT energy barrier of 4.8 kcal/mol height, and the corresponding zero point energy value, yield for the S(1) state an activation energy barrier of 0.3 kcal/mol. This finding implies that the double PT concerted process is almost barrierless, confirming previous experiments. Upon N-H deuteration of the 7-azaindole dimer, the theoretical excited-state activation energy for the double deuterium transfer is determined to be 1.4 kcal/mol, in agreement with experiment, which in low-temperature spectroscopy is shown to negate excited-state double-deuteron transfer.

Optical enantioselection in a random ensemble of unidirectionally oriented chiral olefins

A laser control scenario for selective excitation of chiral molecules in a random ensemble of unidirectionally oriented racemic mixtures is presented. This selection is carried out using two linearly polarized phase-locked pulses. The two pulses are propagating perpendicular to each other, and polarizations are such that for defined molecular orientations the interaction with one selected enantiomer vanishes, while that with the other is maximized. The method is demonstrated by numerical simulations in all possible orientations around a fixed axis of the axial chiral olefin (4-methyl-cyclohexylidene)fluoromethane. After the pulse sequence, ca. 10% enantiomeric excess is created in the excited singlet electronic state on a picosecond time scale.

UV-Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of synthetic polymers by using nor-harmane as matrix

The matrix-assisted ultraviolet laser desorption/ionization time-of-flight mass spectra (UVMALDI-TOF-MS) analysis of some synthetic polymers was studied. The results obtained by using nor-harmane, gentisic acid (GA) and other UV-MALDI matrices, in positive and negative ion modes, were compared. Nor-harmane worked as an efficient matrix for the synthetic polymers belonging to the families of polyethylenglycol (PEG), polymethylmethacrylate (PMMA), polystyrene (PS) and poly(dimethyl)siloxane (PDMS). This behaviour was quite noticeable in negative ion mode for polar-protic polymers (PEG) and in both ion modes for non-polar polymers (PS). For the latter analysis the addition of Ag + salts was not necessary for the ionization process. The formation of Ag clusters when Ag + salts are used as the cationizing agent is discussed. 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 briefly discussed. The excellent results obtained in the analysis of commercial synthetic polymers with norharmane allow us to propose these analyte-matrix systems for UV-MALDI-TOF-MS calibration.

Ab initio description of the structure and dynamics of the nitrosomethane molecule in the first excited singlet and triplet electronic states

AbstractElectronic structure of the low‐lying excited states of the CH3NO molecule is considered. The detailed analysis of the first excited triplet and singlet states of this molecule is performed by different ab initio methods to estimate equilibrium geometry, barriers to internal rotation, harmonic frequencies, and adiabatic transition energies. Anharmonic vibrational approximations are also considered. The multidimensional VibSCF scheme and the 1‐D variational method for the section of the potential energy surface along the torsion coordinate are used. Theoretical results are found to be in good agreement with experimental data. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Photophysical properties of methyl β-carboline-3-carboxylate mediated by hydrogen-bonded complexes—a comparative study in different solvents

The hydrogen bonding interactions of methyl β-carboline-3-carboxylate (BCCM) in both ground and first singlet excited electronic states have been studied in solvents with different properties in the presence of acetic acid, a hydrogen-bonding donor/acceptor. The methyl ester substituent reduces the pyridinic nitrogen basicity of this β-carboline derivative. This fact has let us study the hydrogen bonding interactions in a higher range of acetic acid concentrations than for other β-carboline derivatives previously studied. Steady and non-steady photophysical studies have been carried out in two non-polar solvents, benzene and p-dioxane; and in two polar solvents, acetonitrile and dichloromethane. Six different fluorescence emissions have been isolated corresponding to the uncomplexed BCCM, the protonated species and four different complexes between BCCM and acetic acid whose structures we have tried to elucidate.