Symmetry-adapted Cluster-configuration Interaction Method Research Articles

The low-lying electronic states of HfO2− and HfO2: A symmetry adapted cluster–configuration interaction (SAC-CI) study

The low-lying doublet states of the HfO2− anion and low-lying triplet excited states of the HfO2 molecule were studied using the SAC-CI method. The vertical excitation spectrum of HfO2− was calculated. Four valence-bound states (X2A1, 12B1, 22A1, and 12B2(II)) and one dipole-bound state (2A1(DB)) of HfO2− were found. The vertical electron detachment energies (VDEs) from the ground electronic state of HfO2− to the triplet excited states of HfO2 were obtained. These VDEs were compared with previous photoelectron spectra of HfO2−, and possible candidate states for the unassigned bands were found. The equilibrium geometries of the valence-bound states of HfO2− and some selected triplet excited states of HfO2 were optimized. The adiabatic relative energies of these electronic states were obtained. Density functional theory (BPW91) was also applied to compute some electronic states of HfO2− and HfO2 for comparison.

Electronic Structure of Biotin Conformers Studied with SAC-CI and OVGF Methods.

In this work, the study was performed with 37 gas-phase conformers of biotin and two biologically active conformers of biotin in the ligand-receptor complexes with astavidin and streptavidin. The ionization energies and photoelectron spectra of conformers were calculated by two methods: the general-R symmetry-adapted cluster-configuration interaction (general-R-SAC-CI) method and the outer-valence Green's function (OVGF) method. The photoelectron spectrum of each conformer was calculated using basis set D95 (df,pd) for both methods. The simulated photoelectron spectra of free molecules and bioactive conformers calculated by the two methods were compared. Natural bonding orbital (NBO) calculations were also performed for the assignment of ionization bands of each conformer. NBO calculation indicated that the first to five ionization bands correspond to ionizations from orbitals localized in the two rings. The most important point about the ionization of all conformers is that the removal of an electron from the σ-bonding orbital (C-S) takes place above 10.0 eV.

Vibrational and Electronic Spectra of 2-Phenyl-2-Imidazoline: A Combined Experimental and Theoretical Study

The structural properties and vibrational frequencies of 2-phenyl-2-imidazoline have been investigated using theoretical and techniques by which a good correlation was observed. The assignments of the vibrational modes were performed with the help of normal co-ordinate analysis following the Scaled Quantum Mechanical Force Field methodology. Natural bond orbital analysis and the highest occupied molecular orbital-lowest unoccupied molecular orbital gap analysis have been carried out. UV-visible spectrum of the compound was recorded and compared with the theoretical UV-visible spectrum of the title molecule using Symmetry Adapted Cluster-Configuration Interaction method which yielded good agreement. Our results reflect that the title compound can be used as good source of UV light.

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

The low-lying doublet excited states of ZrO2− were studied using the SAC-CI method. The vertical excitation spectrum of ZrO2− was calculated and compared with that of TiO2−. Similar to that for TiO2−, ZrO2− also have a dipole-bound state. The ground electronic state (X2A1) and the excited states having bound characters (22A1, 12B1, and dipole-bound states) were optimized to C2v structures. The adiabatic electronic affinity of ZrO2 and the adiabatic excitation energies of the three bound excited states were obtained at the SAC-CI level.

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

The low-lying doublet excited states of the singly charged TiO2 anion (TiO2−) were investigated by the SAC-CI method. The vertical excitation spectrum of TiO2− was calculated at the SAC-CI/aug-cc-pVTZ levels. The ground electronic state (X2A1) and the lowest two valence excited states (22A1 and 12B1) having bound characters were optimized to stable C2v structures. A dipole-bound state (2A1) was found and optimized to a C2v structure. The adiabatic energies of the 22A1, 12B1, and dipole-bound states were obtained.

Low-lying π∗ resonances associated with cyano groups: A CAP/SAC-CI study

The complex absorbing potential (CAP)/symmetry-adapted cluster-configuration interaction (SAC-CI) method is applied to low-lying π∗ resonance states of molecules containing one or two cyano (CN) groups. Benchmark calculations are carried out comparing the non-variational and approximate variational approach of SAC-CI and studying the selection threshold of operators. Experimental resonance positions from electron transmission spectroscopy (ETS) are reproduced provided the anticipated deviations due to vibronic effects are taken into account. Moreover, the calculated positions and widths agree well with those obtained in previous electron scattering calculations for HCN, CH3CN and their isonitriles. Based on our results, we suggest a reassignment of the experimental ETS of fumaronitrile and malononitrile. Our present results demonstrate again that the CAP/SAC-CI method reliably predicts low-lying π∗ resonances, and regarding the total numbers of molecules and resonances investigated, it is fair to say that it is presently the most extensively used high-level method in the temporary anion field.

Electronic excitation and ionization behavior of N-hydroxypyridine-2(1H)-thione and its deprotonated anion in a polarizable medium studied using quantum chemical computations

N-Hydroxypyridine-2(1H)-thione (N-HPT) is an important photochemical generator of hydroxyl radicals; however, it has been pointed out that N-HPT is not a specific precursor of hydroxyl radical. Photoionization of N-HPT competes with photochemical N–O bond cleavage in neutral aqueous solution. The possibility of a competitive reaction could be critical for studies using N-HPT as the radical precursor; therefore, the detailed behaviors of electronic excitation and ionization of N-HPT and its deprotonated anion, which is the dominant tautomer under neutral pH conditions, are studied using quantum chemical methods with the symmetry-adapted cluster-configuration interaction (SAC-CI) method and the polarizable continuum model (PCM). The detailed assignment of the UV–Vis spectra of N-HPT is provided, and the origin of the observed negative solvatochromism is found to be the charge transfer excitation between the sulfur and the pyridine ring. The photochemical N–O bond cleavage occurs via the conical intersections between the lowest π → π* and π → σ* states and between the π → σ* and ground state, when N-HPT dissociates into PyS· and ·OH radicals. The calculated ionization potentials of N-HPT and the deprotonated N-HPT anion are 5.75 and 4.67 eV in PCM water. This demonstrates that the charge transfer excitation energy between N-HPT and liquid water becomes significantly lower for the deprotonated anion in comparison with the neutral molecule. Even under mild photochemical conditions, photoinduced ionization of N-HPT may occur in neutral aqueous solution.

On the performance of time-dependent double-hybrid density functionals for description of absorption and emission spectra of heteroaromatic compounds

Theoretical chemistry provides a panel of powerful tools to investigate the geometries and electronic properties of excited states. However, it is necessary to benchmark the accuracy of existing models to determine the range of their applicability and accountability. In the present work, the performance of two related methodologies in this context, symmetry-adapted cluster-configuration interaction (SAC-CI) and time-dependent double-hybrid density functional theory (TD-DHDFT), is compared in detail for the calculation of absorption and fluorescence energies of small organic molecules and π-conjugated heteroaromatic compounds. Pragmatically, for the SAC-CI calculations the singles and doubles linked excitation operators are considered in the wave functions. On the other hand, for the TD-DHDFT calculations the linear-response Tamm–Dancoff formalism has been imposed. The considered DH density functionals include the approximations from both families of adjusted and non-adjusted models. Our numerical data reveal that of the tested adjusted DHs, B2-PLYP performs the best, while among the category of non-adjusted approximations the PBE0-DH functional outperforms others. Furthermore, the DH density functionals show deviations less than those obtained from SAC-CI method. Putting all the results together, the B2-PLYP and B2GP-PLYP functionals are found to be superior for overall performance. Altogether, the present contribution and recent efforts in this arena point out that besides the ground-state properties the DH approximations have also unquestionably entered into the field of excited-state calculations, where the DHDFT again comes into play and further evidence on the quality of the corresponding models is pronounced.

Low-Lying π* Resonances of Standard and Rare DNA and RNA Bases Studied by the Projected CAP/SAC-CI Method.

Low-lying π* resonance states of DNA and RNA bases have been investigated by the recently developed projected complex absorbing potential (CAP)/symmetry-adapted cluster-configuration interaction (SAC-CI) method using a smooth Voronoi potential as CAP. In spite of the challenging CAP applications to higher resonance states of molecules of this size, the present calculations reproduce resonance positions observed by electron transmission spectra (ETS) provided the anticipated deviations due to vibronic effects and limited basis sets are taken into account. Moreover, for the standard nucleobases, the calculated positions and widths qualitatively agree with those obtained in previous electron scattering calculations. For guanine, both keto and enol forms were examined, and the calculated values of the keto form agree clearly better with the experimental findings. In addition to these standard bases, three modified forms of cytosine, which serve as epigenetic or biomarkers, were investigated: formylcytosine, methylcytosine, and chlorocytosine. Last, a strong correlation between the computed positions and the observed ETS values is demonstrated, clearly suggesting that the present computational protocol should be useful for predicting the π* resonances of congeners of DNA and RNA bases.

Corrigendum to: Quantum-chemical Ab Initio Calculations on the Donor–Acceptor Complex Pyridine–Borabenzene (C5H5N–BC5H5)

The adduct of borabenzene (C5H5B) and pyridine (C5H5N) was studied by means of quantum-chemical ab initio and time-dependent density functional theory calculations at different levels of theory. In the fully optimized structure (MP2/6-311++G**) of the free donor–acceptor complex (C2), the C–B–C angle amounts to 120.6°. The planes of the two aromatic rings enclose a torsion angle of ~40° with a barrier to rotation about the B–N bond of less than 3kcalmol–1 (1kcalmol–1=4.186kJmol–1). The highest computational level applied in this study (complete basis set limit, coupled cluster with single and double excitations (CCSD)) results in an energy associated with the reaction of borabenzene with pyridine of –52.2kcalmol–1. Natural bond orbital analyses were performed to study the bond between the borabenzene and the pyridine unit of the adduct. The UV-vis spectrum of the adduct was calculated employing time-dependent density functional theory methods and the symmetry-adapted cluster-configuration interaction method. Our calculated electronic excitation spectrum of the pyridine adduct as well as its spectrum of the normal modes qualitatively reproduce the characteristic features of the IR and the UV-vis spectra described by experimentalists and thus allows assignment of the observed absorption bands, which in part agree with those by other authors.

Coumarin-based donor–π–acceptor organic dyes for a dye-sensitized solar cell: photophysical properties and electron injection mechanism

The electronic structure and photophysical properties of five coumarin-based donor–π–acceptor (D–π–A)-type organic dyes for a dye-sensitized solar cell (DSSC) which were recently developed have been investigated using the time-dependent density functional theory and the symmetry-adapted cluster configuration interaction method. Theoretical calculations including the solvent effect in state-specific and linear-response scheme reproduced the experimental UV–Vis absorption spectra of these dyes satisfactorily. The π-spacers, thiophene and thiophene–phenylene mixed units, affect the planarity of the molecular structures which is relevant to the photophysical properties and charge polarization. Energy levels of the frontier orbitals and charge separation were analyzed, and the thiophene linker was found to be effective for the electron injection in DSSC. The adsorption of these dyes on the TiO2 anatase (101) surface and the electron injection mechanism were also investigated using a dye–(TiO2)38 cluster model employing PBE and TD-CAM-B3LYP calculations, respectively. The adsorption energies of these dyes were estimated to be ~14 kcal/mol, indicating strong adsorption of dye to a TiO2 surface by carboxylate group. The possible direct electron injection mechanism was suggested in the present coumarin-based D–π–A dyes in a dye–TiO2 interacting system.

Projected CAP/SAC‐CI method with smooth Voronoi potential for calculating resonance states

The complex absorbing potential (CAP)/symmetry-adapted cluster-configuration interaction (SAC-CI) method has been combined with a smooth Voronoi potential, which was recently introduced in the extrapolation procedure, to locate π* resonance states of small- to medium-size molecules. Here, the projected CAP/SAC-CI method is combined with this potential and used to calculate the double-bond and heteroaromatic π* resonances of acetaldehyde, butadiene, glyoxal, pyridine, pyrazine, and furan. As observed in the pilot applications, the corrected η-trajectories provide a stable resonance energy and width or lifetime regardless of the size parameter (rcut ) of the smooth Voronoi potential. However, in general, the stabilization behavior of the trajectories is clearer for larger rcut values, which implies that the interaction of the CAP with the valence electrons is more advantageously addressed by a larger "cavity" size.

Synthesis and Optical Properties of Imidazole- and Benzimidazole-Based Fused π-Conjugated Compounds: Influence of Substituent, Counteranion, and π-Conjugated System.

Fused π-conjugated imidazolium chlorides having hydrogen (1-Cl), octyloxy (2-Cl), N,N-dibutylamino (3-Cl), trifluoromethyl (4-Cl), and cyano (5-Cl) groups substituted on the benzene ring at the 2-position of imidazole were prepared. Counteranion exchanges from chloride to bis(trifluoromethanesulfonyl)imidate (2-TFSI) and tetrafluoroborate (2-BF4) were performed. The optical properties of these compounds (absorption and emission wavelengths, fluorescence quantum yield, and solvatochromism) were influenced by both the substituent and anion character, which was investigated by theoretical calculations using the density functional theory (DFT) and symmetry-adapted cluster-configuration interaction (SAC-CI) methods. Fused π-conjugated benzimidazolium chlorides having N,N-dibutylamino (6-Cl) and cyano (7-Cl) groups were also prepared to observe the different solvatochromic shifts.

Modeling Molecular Systems at Extreme Pressure by an Extension of the Polarizable Continuum Model (PCM) Based on the Symmetry-Adapted Cluster-Configuration Interaction (SAC-CI) Method: Confined Electronic Excited States of Furan as a Test Case.

Novel molecular photochemistry can be developed by combining high pressure and laser irradiation. For studying such high-pressure effects on the confined electronic ground and excited states, we extend the PCM (polarizable continuum model) SAC (symmetry-adapted cluster) and SAC-CI (SAC-configuration interaction) methods to the PCM-XP (extreme pressure) framework. By using the PCM-XP SAC/SAC-CI method, molecular systems in various electronic states can be confined by polarizable media in a smooth and flexible way. The PCM-XP SAC/SAC-CI method is applied to a furan (C4H4O) molecule in cyclohexane at high pressure (1-60 GPa). The relationship between the calculated free-energy and cavity volume can be approximately represented with the Murnaghan equation of state. The excitation energies of furan in cyclohexane show blueshifts with increasing pressure, and the extents of the blueshifts significantly depend on the character of the excitations. Particularly large confinement effects are found in the Rydberg states. The energy ordering of the lowest Rydberg and valence states alters under high-pressure. The pressure effects on the electronic structure may be classified into two contributions: a confinement of the molecular orbital and a suppression of the mixing between the valence and Rydberg configurations. The valence or Rydberg character in an excited state is, therefore, enhanced under high pressure.

Electronic excitation spectra of doublet anion radicals of cyanobenzene and nitrobenzene derivatives: SAC-CI theoretical studies

Electronic ground and excited states of anion radicals of cyanobenzene derivatives: 1,3,5-tricyanobenzene, 1,2,4,5-tetracyanobenzene, and tetracyanoquinodimethane (TCNQ) and nitrobenzene derivatives: nitrobenzene, p-nitroaniline, m-nitroaniline, and o-nitroaniline were theoretically investigated by the symmetry adapted cluster-configuration interaction (SAC-CI) method, which is able to produce accurate theoretical electronic excitation spectra even for radical doublet states. For all the target molecules, the present calculations reproduced the positive electron affinities, which were mostly in good agreement with the experimental values, and their features, especially for TCNQ, were characterised by singly occupied molecular orbitals as well as the number of the electron-withdrawing terminal groups. The excitation energies and their oscillator strengths by the SAC-CI method were also in good agreement with the corresponding experimental UV/VIS/NIR spectra observed by one of the authors and other experimental evidences. Except for TCNQ, the present theoretical calculations were successful to first predict the existences of the forbidden (or very low intense) pure valence excited states in near-infrared region. The physical natures of the observed intense spectral bands were clarified and some new assignments to their electronic states were provided. By extending the present work, photo-related molecular designs of new functional electron acceptors may be challenged.

An efficient computational scheme for electronic excitation spectra of molecules in solution using the symmetry-adapted cluster-configuration interaction method: the accuracy of excitation energies and intuitive charge-transfer indices.

Solvent effects on electronic excitation spectra are considerable in many situations; therefore, we propose an efficient and reliable computational scheme that is based on the symmetry-adapted cluster-configuration interaction (SAC-CI) method and the polarizable continuum model (PCM) for describing electronic excitations in solution. The new scheme combines the recently proposed first-order PCM SAC-CI method with the PTE (perturbation theory at the energy level) PCM SAC scheme. This is essentially equivalent to the usual SAC and SAC-CI computations with using the PCM Hartree-Fock orbital and integrals, except for the additional correction terms that represent solute-solvent interactions. The test calculations demonstrate that the present method is a very good approximation of the more costly iterative PCM SAC-CI method for excitation energies of closed-shell molecules in their equilibrium geometry. This method provides very accurate values of electric dipole moments but is insufficient for describing the charge-transfer (CT) indices in polar solvent. The present method accurately reproduces the absorption spectra and their solvatochromism of push-pull type 2,2'-bithiophene molecules. Significant solvent and substituent effects on these molecules are intuitively visualized using the CT indices. The present method is the simplest and theoretically consistent extension of SAC-CI method for including PCM environment, and therefore, it is useful for theoretical and computational spectroscopy.

Electron momentum spectroscopy of NF3

The electronic structure of nitrogen trifluoride was investigated by combining the high-resolution electron momentum spectroscopy with the high-level calculations. The experimental binding energy spectra and the momentum distributions of each orbital were compared with the results of Hartree—Fock, density functional theory (DFT), and symmetry-adapted-cluster configuration-interaction (SAC-CI) methods. SAC-CI and DFT-B3LYP with the aug-cc-pVTZ basis set can well reproduce the binding energy spectra and the observed momentum distributions of the valence orbitals except 1a2 and 4e orbitals. It was found that the calculated momentum distributions using DFT-B3LYP are even better than those using the high-level SAC-CI method.

Benchmark Study on the Triplet Excited-State Geometries and Phosphorescence Energies of Heterocyclic Compounds: Comparison Between TD-PBE0 and SAC-CI.

In this work, we investigated the properties of the triplet excited states of heterocyclic compounds including their geometries, electronic properties, and phosphorescence energies by using both the direct symmetry-adapted cluster-configuration interaction (SAC-CI) method and the TD-DFT approach with the PBE0 exchange-correlation functional (TD-PBE0). The target states are the ππ* and nπ* triplet states of furan, pyrrole, pyridine, p-benzoquinone, uracil, adenine, 9,10-anthraquinone, coumarin, and 1,8-naphthalimide as well as the Rydberg states. The present benchmark demonstrates that these two methods provide reasonably accurate geometries for the excited states of these heterocyclic compounds. The calculated Stokes shifts, which reflect geometry changes, were consistent for both these methods. The trends of agreement with experimental or reference values obtained for a panel of exchange-correlation functionals used to compute the absolute emission energies from the triplet states, differ from those found for the singlet excited states. Some of the low-lying triplet excited states were examined in detail for the first time, including vibrational analysis.

Experimental and theoretical investigation on the outer valence electronic structure of cyclopropylamine by (e, 2e) electron momentum spectroscopy.

The binding energy spectra and electron momentum distributions for the outer-valence molecular orbitals of gaseous cyclopropylamine (CPA) have been measured by (e, 2e) electron momentum spectrometer employing noncoplanar asymmetric geometry at the impact energy of 2500 eV. The experimental results are interpreted on the basis of the quantitative calculations of the ionization energies and the relevant molecular orbitals at benchmark theoretical levels using the outer-valence Green's function method, the symmetry-adapted cluster configuration interaction method, and the density functional theory with B3LYP hybrid functional. The total energies of the trans and gauche conformers of CPA are also calculated by the second-order Møller-Plesset perturbation theory with large basis sets and the derived enthalpy differences (2.02-2.12 kcal/mol) are consistent with the previous experimental data (2.19 kcal/mol). The theoretical binding energy spectra and electron momentum distributions, in which the relative abundances of trans and gauche are taken into account, are generally in accordance with the experimental results except for the ionization band from the trans 8a' and gauche 11a orbitals. The discrepancy is explained qualitatively in view of the picture of molecular geometry change at the instant of ionization.

How Does LCDFT Compare to SAC-CI for the Treatment of Valence and Rydberg Excited States of Organic Compounds?

The computation of excitation energies for electronically excited states poses a challenge in quantum chemistry. In the present work, the performance of two related methodologies in this context, symmetry-adapted cluster-configuration interaction (SAC-CI) and time-dependent long-range corrected density functional theory (TDLCDFT), is compared in detail for the calculation of valence and Rydberg excitation energies against an experimental benchmark set comprising some organic compounds from different categories. Practically, the single- and double-linked excitation operators are considered in the SAC-CI wave functions. The considered LC density functionals include the combination forms of exchange and correlation functionals (BLYP, PBE, TPSS), pure functionals (tHCTH and B97-D), exchange-only functionals (HFS, HFB, and XAlpha), hybrid functionals (CAM-B3LYP, LC-ωPBE, ωB97, ωB97X), and dispersion-corrected hybrid functional ωB97X-D. Our results reveal that the SAC-CI gives the best performance for Rydberg excited states. However, the values of mean absolute deviation show that the applicability of some LC functionals is comparable to SAC-CI. For valence excited states, the functionals ωB97X-D, ωB97X, and LC-ωPBE outperform the other tested methods. Overall, the ωB97X-D functional is found to offer the best performance, and its validity compared with SAC-CI has also been verified by computing low-lying excited states of a few molecules as representative examples. Lastly, it is shown that not only is there a reasonable agreement between TDLCDFT and SAC-CI methods for the calculation of excitation energies but also the LC density functionals have quantitatively better overall performance for some excited states than the SAC-CI approach.