SAC-CI Calculations Research Articles

The direct observation of the doorway 1nπ* state of methylcinnamate and hydrogen-bonding effects on the photochemistry of cinnamate-based sunscreens.

The electronic states and photochemistry including nonradiative decay (NRD) and trans(E) → cis(Z) isomerization of methylcinnamate (MC) and its hydrogen-bonded complex with methanol have been investigated under jet-cooled conditions. S1(1nπ*) and S2(1ππ*) are directly observed in MC. This is the first direct observation of S1(1nπ*) in cinnamate derivatives. Surprisingly, the order of the energies between the nπ* and ππ* states is opposite to substituted cinnamates. TD-DFT and SAC-CI calculations support the observed result and show that the substitution to the benzene ring largely lowers the 1ππ* energy while the effect on 1nπ* is rather small. The S2(ππ*) state lifetime of MC is determined to be equal to or shorter than 10 ps, and the production of the transient T1 state is observed. The T1(ππ*) state is calculated to have a structure in which propenyl C[double bond, length as m-dash]C is twisted by 90°, suggesting the trans → cis isomerization proceeds via T1. The production of the cis isomer is confirmed by low-temperature matrix-isolated FTIR spectroscopy. The effect of H-bonding is examined for the MC-methanol complex. The S2 lifetime of MC-methanol is determined to be 180 ps, indicating that the H-bonding to the C[double bond, length as m-dash]O group largely prohibits the 1ππ* → 1nπ* internal conversion. This lifetime elongation in the methanol complex also describes well a higher fluorescence quantum yield of MC in methanol solution than in cyclohexane, while such a solvent dependence is not observed in para-substituted MC. Determination of the photochemical reaction pathways of MC and MC-methanol will help us to design photofunctional cinnamate derivatives.

Photophysical properties of fluorescent imaging biological probes of nucleic acids: SAC-CI and TD-DFT Study.

Recently, exciton-controlled hybridization-sensitive fluorescent oligonucleotide (ECHO) probe, which shows strong emission in the near-infrared region via hybridization to the target DNA and/or RNA strand, has been developed. In this work, photophysical properties of the chromophores of these probes and the fluorescent mechanism have been investigated by the SAC-CI and TD-DFT calculations. Three fluorescent cyanine chromophores whose excitation is challenging for TD-DFT methods, have been examined regarding the photo-absorption and emission spectra. The SAC-CI method well reproduces the experimental values with respect to transition energies, while the quantitative prediction by TD-DFT calculations is difficult for these chromophores. Some stable structures of H-aggregate system were computationally located and two of the configurations were examined for the photo-absorption. The present results support for the assumption based on experimental measurement in which strong fluorescence is due to the monomer unit in nearly planar structure and its suppression of probes is to the H-aggregates of two exciton units. Stokes shifts of these three chromophores were qualitatively reproduced by the theoretical calculations, while the energy splitting due to H-aggregate in the hybridized probe was slightly overestimated. © 2018 Wiley Periodicals, Inc.

Ionization of vitamin C in gas phase: Theoretical study

In this work, the gas phase ionization energies and photoelectron spectra of four important conformers of vitamin C were calculated. Symmetry adapted cluster/configuration interaction methodology employing the single and double excitation operators (SAC-CI SD-R) along with D95++(d,p) basis set were used for the calculations. Thermochemistry calculations were also performed on all possible conformers of vitamin C to find the relative stability of conformers in the gas phase. The calculated ionization bands of each conformer were assigned by calculating the contribution of natural bonding orbital (NBO) in the calculated canonical molecular orbitals involved in the ionization. SAC-CI calculations showed that the first ionization band of vitamin C is related to the π electrons of CC bond of the ring of molecule although, there is the lone electron pairs of oxygen atoms and π electrons of CO bond in the molecule.

FT-IR, FT-Raman, UV/Vis spectra and fluorescence imaging studies on 2-(bromoacetyl)benzo(b)furan by ab initio DFT calculations

The vibrational and electronic properties of 2-(bromoacetyl)benzo(b)furan have been studied in the ground state using experimental techniques (FT-IR, FT-Raman and UV) and density functional theory (DFT) employing B3LYP exchange correlation with the 6-31G(d,p) basis set. The theoretically calculated optimized parameters, vibrational frequencies etc., were compared with the experimental values, which yield good agreement between the observed and calculated values. The complete assignments of fundamental modes were performed on the basis of the potential energy distribution (PED). UV–visible spectrum of the compound was recorded in the region 300–600nm and compared with the theoretical spectrum obtained from SAC–CI calculations. A good agreement is observed between the experimental and theoretical spectra. Fluorescence microscopic imaging studies proved that the compound can be used as one of the potential light sources in the yellow region with suitable excitation. Further, the predicted electronic transitions between the MOs 47→64, 52→62, 56→65, 56→72, 56→77 of the compound show a strong line at 569.8nm.

Electronic transitions in conformationally controlled tetrasilanes with a wide range of SiSiSiSi dihedral angles.

Unlike π-electron chromophores, the peralkylated n-tetrasilane σ-electron chromophore resembles a chameleon in that its electronic spectrum changes dramatically as its silicon backbone is twisted almost effortlessly from the syn to the anti conformation (changing the SiSiSiSi dihedral angle ω from 0 to 180°). A combination of UV absorption, magnetic circular dichroism (MCD), and linear dichroism (LD) spectroscopy on conformationally controlled tetrasilanes 1-9, which cover fairly evenly the full range of angles ω, permitted a construction of an experimental correlation diagram for three to four lowest valence electronic states. The free chain tetrasilane n-Si4 Me10 (10), normally present as a mixture of three enantiomeric conformer pairs of widely different angles ω, has also been included in our study. The spectral trends are interpreted in terms of avoided crossings of 1B with 2B and 2A with 3A states, in agreement with SAC-CI calculations on the excited states of 1-7 and conformers of 10.

Meso-Arylporpholactones and their Reduction Products

The rational syntheses of meso-tetraaryl-3-oxo-2-oxaporphyrins 5, known as porpholactones, via MnO(4)(-)-mediated oxidations of the corresponding meso-tetraaryl-2,3-dihydroxychlorins (7) is detailed. Since chlorin 7 is prepared from the parent porphyrin 1, this amounts to a 2-step replacement of a pyrrole moiety in 1 by an oxazolone moiety. The stepwise reduction of the porpholactone 5 results in the formation of chlorin analogues, meso-tetraaryl-3-hydroxy-2-oxachlorin (11) and meso-tetraaryl-2-oxachlorins (12). The reactivity of 11 with respect to nucleophilic substitution by O-, N-, and S-nucleophiles is described. The profound photophysical consequences of the formal replacement of a pyrrole with an oxazolone (porphyrin-like chromophore) or (substituted) oxazole moiety (chlorin-like chromophore with, for the parent oxazolochlorin 12, red-shifted Q(x) band with enhanced oscillator strengths) are detailed and rationalized on the basis of SAC-CI and MNDO-PSDCI molecular orbital theory calculations. The single crystal X-ray structures of the porpholactones point at a minor steric interaction between the carbonyl oxygen and the flanking phenyl group. The essentially planar structures of all chromophores in all oxidation states prove that the observed optical properties originate from the intrinsic electronic properties of the chromophores and are not subject to conformational modulation.

Color Tuning Mechanism of Human Red, Green, and Blue Cone Pigments: SAC-CI Theoretical Study

Abstract Human red (HR), green (HG), and blue (HB) cone pigments are responsible for human color vision, and their photoabsorption wavelengths spread uniquely over the three primary colors. These pigments, however, include only one common chromophore, retinal. Here, we report physical basis of the color tuning in human vision on the basis of SAC-CI calculations for excited states of the cone pigments. The dominant origin of the red–green–blue distinction lies in differences in electrostatic interactions between retinal and its surrounding proteins. Structural distortion effect of the retinal chromophore is important in human blue pigment. Detailed analysis on individual roles of amino acids within these proteins has clarified elaborate mechanisms of the color tuning, in accordance with previous mutagenesis experiments. Furthermore, the color tuning is regulated by amino acids at specific positions in the proteins, suggesting some genetic origins for the color tuning.

Electronic structure and optical properties of chelating heteroatomic conjugated molecules: a SAC-CI study

The electronic structure and optical properties of 13 chelating heteroatomic conjugated molecules such as pyridine, benzoxazole, and benzothiazole derivatives, which are used as C–N ligands in organometallic compounds, have been investigated. The geometries of the ground and first excited states were obtained by the DFT and CIS methods, respectively, followed by the SAC-CI calculations of the transition energies for absorption and emission. For six compounds whose experimental data are available, the SAC-CI calculations reproduced the experimental values satisfactorily with deviations of less than 0.3 eV for absorption and 0.1 eV for emission except for benzoxazoles. For other molecules, the theoretical absorption and emission spectra were predicted. The lowest ππ* excited-state geometries was calculated to be planar for most of the molecules with two or three conjugated rings connected by single bond. The geometry change due to the ππ* excitation was qualitatively interpreted by electrostatic force theory based on SAC/SAC-CI electron density difference. The excitations are relatively localized in the central region and in the lowest ππ* excited state, the inter-ring single bond shows large change, with a contraction of 0.05–0.09 A. The present calculations provide reliable information regarding the energy levels of these chelating heteroatomic conjugated compounds.

Density Functional Theory for Charge Transfer: The Nature of the N-Bands of Porphyrins and Chlorophylls Revealed through CAM-B3LYP, CASPT2, and SAC-CI Calculations

While density functional theory (DFT) has been proven to be extremely useful for the prediction of thermodynamic and spectroscopic properties of molecules, to date most functionals used in common implementations of DFT display a systematic failure to predict the properties of charge-transfer processes. While this is explicitly manifest in Rydberg transitions of atoms and molecules and in molecular charge-transfer spectroscopy, it also becomes critical for systems containing extended conjugation such as polyenes and other conducting polymers, porphyrins, chlorophylls, etc. A new density functional, a Coulomb-attenuated hybrid exchange-correlation functional (CAM-B3LYP), has recently been developed specifically to overcome these limitations, and it has been shown to properly predict molecular charge-transfer spectra. Here, we demonstrate that it predicts qualitatively reasonable spectra for porphyrin, some oligoporphyrins, and chlorophyll. However, alternate density functionals developed to overcome the same limitations such as current-density functional theory are shown, in their present implementation, to remain inadequate. The CAM-B3LYP results are shown to be in excellent agreement with complete-active-space plus second-order Møller-Plesset perturbation theory and symmetry-adapted cluster configuration interaction calculations: These depict the N and higher bands of porphyrins and chlorophylls as being charge-transfer bands associated with localization of molecular orbitals on individual pyrrole rings. The validity of the basic Gouterman model for the spectra of porphyrins and chlorophylls is confirmed, rejecting modern suggestions that non-Gouterman transitions lie close in energy to the Q-bands of chlorophylls. As porphyrins and chlorophylls provide useful paradigms for problems involving extended conjugation, the results obtained suggest that many significant areas of nanotechnology and biotechnology may now be realistically treated by cost-effective density-functional-based computational methods.

Characterization of Cationic Diarylethene by Electron Spin Resonance and Absorption SpectraRatio of Open/Closed-Ring Isomers

Electrochemical cyclization/cycloreversion reactions of a diarylethene, 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene, are examined experimentally by electron spin resonance (ESR) and absorption spectra. To understand the ESR spectrum, the hyperfine coupling constants are calculated by the density functional theory (DFT) with the B3LYP exchange-correlation functional. The averaged values of the hyperfine coupling constants are approximated by imposing the C(2) symmetry on the structure of the diarylethene. We found that the spectral width of the ESR is significantly different between the open- and closed-ring isomers. This is due to the difference in the pi-conjugation between two isomers. The ESR spectral width analysis could, thus, be used to identify the isomerization of the radical species, which involve the change of the pi-conjugation. The experimentally observed spectrum is found to be the mixture of the open- and closed-ring isomers of the diarylethene. The excitation energies of the cationic diarylethenes are further identified by the SAC-CI calculations.

Ab initio study of molecular structures and excited states in anthocyanidins

The structural and electronic characters of four types of hydroxyl group-substituted anthocyanidins (pelargonidin, cyanidin, delphinidin, and aurantinidin) were examined using quantum chemical calculations. For these cationic molecules, both the planar and non-planar structures in the electronic ground state were determined at the B3LYP/D95 level of theory. We revealed that the planar structure is slightly more stable than the non-planar structure for each molecule. For the optimized planar structures, single excitation–configuration interaction (SE–CI) based on the restricted Hartree–Fock (RHF) wave function was evaluated and the electronic character in the low-excited states was discussed in terms of the MO theory. Symmetry adapted cluster (SAC)/SAC-CI calculations were also carried out to estimate the excitation energies precisely. The results showed that hydroxylation of the phenyl group causes a change in the excitation energies without taking the solvent effects into account. The results are in agreement with spectral experiments and previous MO calculations.

Excited States and Electron-transfer Mechanism in the Photosynthetic Reaction Center of Rhodobactor sphaeroides: SAC-CI Theoretical Study

Abstract The SAC-CI calculations clarify the natures of the excited states and the electron-transfer (ET) processes in the photosynthetic reaction center (PSRC) of Rhodobactor (Rb.) sphaeroides. The absorption spectrum was assigned with the averaged error of 0.11 eV. The electronic factors calculated from the SAC-CI wave functions clarified the mechanism of the unidirectionality of the ET in Rb. sphaeroides. It is controlled by the ET step from bacteriochlorophyll (B) to bacteriopheophytin (H), not from the special pair (P) to B as in the Rhodopseudomonas (Rps.) viridis reported previously: the electronic factor of the A-branch ET is 20 times larger than that of the B-branch. An analysis clarified that the unidirectionality originates from the inter-chromophore distances, and further that the hyperconjugations of the methyl groups with the π electrons of the chromophores have primary contributions to the electronic factor.

Aza-substitution effect on the Q-band excitations of free-base porphin, chlorin, and bacteriochlorin: SAC-CI theoretical study

Electronic structure of the excited states and absorption spectra of free-base azaporphins, azachlorin, and azabacteriochlorin were systematically investigated by SAC-CI calculations. Aza-substitution at the meso position affects the orbital energy of the next-HOMO, and the transition dipole moment enlarges as the number of the substitution increases. Some of the aza-substitutions dramatically affect the direction of the transition dipole, due to reduction of the molecular symmetry, which was studied in detail by a decomposition analysis of the transition dipole moment. Tetraza-substitution in chlorin and bacteriochlorin increases the oscillator strength more than that of tetrazaporphin. The mechanism underlying these changes originates mainly from the relaxation of near-degeneracy in the main configurations. These findings would be useful for the application to the molecular design of the excited states.

Excited states of four hemes in a c-type cytochrome subunit of the photosynthetic reaction center ofRhodopseudomonas viridis: SAC-CI calculations

The excited states of four hemes, c-552, c-554, c-556, and c-559, in a c-type cytochrome subunit of the photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps.) viridis were studied using the symmetry-adapted cluster (SAC)-configuration interaction (CI) method. c-552 has two imidazole ligands, while the other three hemes have imidazole and methionine ligands. The electronic states of these four hemes are affected significantly by both the ligands and protein and water environments. The ligand effect classifies the four hemes into two groups, while the environmental effect is unique in each heme. The HOMO–LUMO energy separation of c-552 is much smaller than those of the other hemes, and therefore, the low-lying excited states of c-552 are more stable than those of the other hemes. The low-lying excited states and their oscillator strengths of c-559 are significantly modified by the environment when compared with c-556 and c-554. Some observed UV peaks of the PSRC of Rps. viridis were found to have been assigned reasonably to the excited states of these four hemes in a previous report. The calculated linear dichroism angles also supported our assignment.

DFT/MRCI calculations on the excited states of porphyrin, hydroporphyrins, tetrazaporphyrins and metalloporphyrins

A combination of density functional theory and multi-reference configuration interaction methods (DFT/MRCI) has been applied to the calculation of electronic absorption spectra in a series of porphyrin-type molecules. The calculated excitation energies and oscillator strengths for free-base porphyrin ( PH 2) are in excellent agreement with experiment for both lower and higher excited states which are characterized by a significant contribution of double excitations (>20%). The 41 B 2 u , 41 B 3 u , and 51 B 2 u states are assigned to the L-band and the 71 B 3 u state to the M-band. The results for the hydroporphyrins chlorin ( CH 2) and bacteriochlorin ( BH 2) are in agreement with the experimentally observed increase in intensity for the Q-bands relative to PH 2. For BH 2 we predict a red shift of the Q x -band (0.2 eV) and a blue shift of the B-band (0.5–0.7 eV) in comparison to both PH 2 and CH 2. For porphyrazine ( PzH 2) and the commercial pigment phthalocyanine ( PcH 2) the calculated oscillator strengths of the Q- and B-bands are of comparable size explaining the intense color of PcH 2. For the metalloporphyrins with magnesium ( PMg ) and zinc ( PZn ), the x- and y-polarized components of the Q- and B-bands collapse, due to the higher D4 h symmetry of the molecules. The calculations reproduce the slight, experimentally observed increase in the oscillator strength of the Q-band and the decrease for the B-band. These effects are ascribed to the electropositive nature of the metals relative to hydrogen. Except for the Q-bands, which are adequately described by the 'four-orbital model,' it is essential to account for excitations outside the four frontier orbitals as well as double and triple excitations for accurate reproduction of experimental data. We compare our results both with experiment and, where available, recent first-principle SAC-CI, MRMP, and TDDFT calculations.

An ab initio study of the valence excitation of methyl methacrylate as observed by EELS

Ab initio calculations have been carried out in order to assign the main bands observed in a recent electron energy-loss study of the methyl methacrylate molecule. We employed the Dunning-Huzinaga double-ζ basis set for the early steps of geometry optimization but for the excited states Rydberg p functions were added to the conjugated heavy atoms. Both isomers, the s- cis and s- trans , had its vertical spectrum calculated in order to evaluate possible conformation effects on the VUV spectrum. SAC (Symmetry Adapted Cluster)-CI calculations pointed to the ethylenic π orbital (8.99 eV) as the HOMO for s- cis while the n σ orbital is predicted around 1 eV more stable. For the n π orbital, although it was not possible to obtain a SAC-CI estimative, Koopmans’ ionization potentials place it 1.68 eV more stable than the π orbital. The lowest observable transitions have been assigned by SAC-CI calculations as to ethylenic π-π* (7.10 eV) and n π-π* (7.97 eV) for the cis isomer. For the trans isomer similar values were obtained (7.37 eV and 7.93 eV, respectively). Oscillator strengths have been calculated showing acceptable agreement with the experimental results. Previous assignments have been revised.

Sac ci calculations of the electron affinity of HCO

Ab initio calculations of HCO and HCO − were carried out by using a cluster expansion of the wavefunction theory. The adiabatic electron affinity of HCO was calculated to be +0.005 eV.

Geometries and energies of the excited states of pyridazine studied by sac and sac CI calculations

The geometries and energies of the ground and excited states of pyridazine are calculated by an ab initio calculation which includes electron correlation. It is found that electron correlation plays an essential role in determining the geometries of the excited states. The optimized geometry of the T 1 state as well as that of the ground state are planar. On the other hand, the force constant along the twisted mode of the NN bond in the T 1 state is much smaller than that in the ground state. It is suggested that this distorted potential produces a large Franck-Condon factor between the T 1 and S 0 states, which leads to a very large radiationless decay rate constant of the T 1 state. The location of the S 2 origin is discussed on the basis of the present results.

SAC-CI calculations of the electron affinity of SO2

First Page

Valence-shell momentum distributions and ionization energies of carbon disulfide

Valence-shell binding energy spectra and momentum distributions of CS 2 have been measured using non-coplanar symmetric binary (e,2e) spectroscopy. The present measurements are compared with previously published binding energy spectra calculated using the many body 2ph-TDA Green's function (GF) method and the symmetry-adapted cluster configuration-interaction (SAC CI) method. The measured and the calculated binding energy spectra both show extensive population splittings particularly above 20 eV, confirming a significant breakdown of independent particle ionization picture. A relatively strong-outer valence many-body state at 17.0 eV is shown to be satellite of the (2π 0) −1 state, in accord with earlier conclusions of photoelectron studies. Momentum distributions measured at several carefully chosen binding energies are compared with the corresponding molecular orbital momentum distributions calculated using small and extended gaussian basis sets. The good qualitative agreement between momentum distributions measured in the inner-valence region wth theoretical 4σ m and 5σ g orbital momentum distributions confirms the qualitative predictions of satellite parentages by GF and SAC CI calculations. Momentum and position density contour maps of individual orbitals are used to interpret the shapes and atomic characters of the experimental momentum distributions. Momentum densities of the valence orbitals of CS 2 are compared with those of the respective valence isoelectronic species CO 2