Vertical Transition Energies Research Articles

Electronic spectra of nitroethylene

AbstractA systematic study of the electronic excited states of nitroethylene (C2H3NO2) was carried out using the approximate coupled‐cluster singles‐and‐doubles approach with the resolution of the identity (RI‐CC2), the time dependent density functional theory with the CAMB3LYP functional (TDDFT/CAMB3LYP) and the DFT multireference configuration interaction (DFT/MRCI) method. Vertical transition energies and optical oscillator strengths were computed for a maximum of 20 singlet transitions. Semiclassical simulations of the ultraviolet (UV) spectra were performed at the RI‐CC2 and DFT/MRCI levels. The main features in the UV spectrum were assigned to a weak n‐π* transition, and two higher energy πCC+O‐π* bands. These characteristics are common to molecules containing NO2 groups. Simulated spectra are in good agreement with the experimental spectrum. The energy of the bands in the DFT/MRCI simulation agrees quite well with the experiment, although it overestimates the band intensities. RI‐CC2 produced intensities comparable to the experiment, but the bands were blue shifted. A strong πCC+O‐π* band, not previously measured, was found in the 8–9 eV range. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Electronic structure calculations of low-lying electronic states of O3

Configuration-based multi-reference second order perturbation theory (CB-MRPT2) and multi-reference configuration interaction with single and double excitations (MRCISD) have been used to calculate the bending and dissociation potential energy curves (PECs) of ozone. Based on these PECs, equilibrium structures, vertical and adiabatic transition energies of the ground state and several low-lying excited states, as well as intersections and avoided crossings among the states displayed on the PECs are investigated. The energy separation of the open and ring structures and the dissociation energy of the ground state X(1)A(1) are determined by reference-selected MRCISD. Furthermore, one-dimensional cuts along the dissociation reaction coordinate for the lowest four electronic states of O(3) with (1)A' symmetry and possible pre-dissociations are studied. The Hartley band may be pre-dissociable, and the pre-dissociation limit is found to be 3871 cm(-1), which corresponds to symmetric stretching quanta n(ss) ≈ 6.

Photophysics and reductive quenching reactivity of gadusol in solution

The photostability and photophysics of gadusol in aqueous solution has been studied. The photodecomposition quantum yields (ca. 4 × 10(-2) and 1 × 10(-4) at acidic and neutral pH, respectively) confirm the high photostability of the metabolite, independently of the presence of oxygen, under physiological conditions. The nature of the electronic transition of gadusol has been assigned as π→π* on the basis of the solvatochromic shifts of the UV absorption spectrum and the time-dependent density functional theory calculation of the vertical transition energies. The results from the photoacoustic calorimetry point to the rapid non-radiative decay as the dominant relaxation pathway of the excited species at pH 7, which is consistent with the proposed UV-sunscreening role of the molecule in the early atmosphere. Laser flash photolysis experiments probed that the ground state of the enolate form (gadusolate) undergoes electron transfer reactions with some triplet sensitizers in water or methanol solution. A rate constant of 2 × 10(8) M(-1) s(-1) has been determined for the quenching of rose bengal triplet state in water at pH 7. This reductive quenching reactivity may be considered as one of the underlying mechanisms that support the antioxidant capacity of gadusol in biological environments.

Comprehensive evaluation of medium and long range correlated density functionals in TD-DFT investigation of DNA bases and base pairs: gas phase and water solution study

A comprehensive analysis of the performance of the TD-DFT method using different density functionals including recently developed medium and long-range correlation corrected density functionals have been carried out for lower-lying electronic singlet valence transitions of nucleic acid bases and the Watson–Crick base pairs in the gas phase and in the water solution. The standard 6-311++G(d,p) basis set was used. Ground state geometries of bases and base pairs were optimized at the M05-2X/6-311++G(d,p) level. The nature of potential energy surfaces (PES) was ascertained through the harmonic vibrational frequency analysis; all geometries were found to be minima at the respective PES. Electronic singlet vertical transition energies were also computed at the CC2/def2-TZVP level in the gas phase. The effect of state-specific water solvation on TD-DFT computed transition energies was considered using the PCM model. For the isolated bases the performance of the B3LYP functional was generally found to be superior among all functionals, but it measurably fails for charge-transfer states in the base pairs. The CC2/def2-TZVP computed transition energies were also revealed to be inferior compared with B3LYP results for the isolated bases. The performance of the ωB97XD, CAM-B3LYP and BMK functionals were found to be similar and comparable with the CC2 results for the isolated bases. However, for the Watson–Crick adenine–thymine and guanine–cytosine base pairs the performance of the ωB97XD functional was found to be the best among all the studied functionals in the present work in predicting the locally excited transitions as well as charge transfer states.

Low-lying electronic states of HNCS and its ions: a CASSCF/CASPT2 study

Complete active space self-consistent-field (CASSCF) and multiconfigurational second-order perturbation theory (CASPT2) calculations in conjunction with the ANO-L basis set were performed to investigate systematically the low-lying electronic states of HNCS and its ions in C s symmetry. Our highly accurate calculation indicated that theoretically determined geometric parameters and harmonic vibrational frequencies for the ground-state X 1A′ are in good agreement with observed experimental data. The geometry of triplet HNCS is clearly favored C 1 symmetry, and the relative energy is predicted to be 3.000 eV (69.2 kcal/mol). The vertical transition energies for the selected excited states of HNCS were calculated at CASSCF/CASPT2/ANO-L level of theory based on CASSCF optimized geometry. Except for a few linear states of X 2Π (12A′, 12A″), 14Σ− (14A″), and 12Σ+ (32A′) states of HNCS+, our results confirmed that the majority of excited states are twisted trans-bend structures. The existence of bound excited anion states has been found for the first time in HNCS−. A more elaborate examination of ionization potential of HNCS (AIP, VIP) than previous reports has been presented.

Color centers in NaCl by hybrid functionals

We present in this work the electronic structure and transition energies (both thermodynamic and optical) of Cl vacancies in NaCl by hybrid density functionals. The underestimated transition energies by the semi-local functional inherited from the band gap problem are recovered by the PBE0 hybrid functional through the non-local exact exchange, whose amount is adjusted to reproduce the experimental band gap. The hybrid functional also gives a better account of the lattice relaxation for the defect systems arising from the reduced self-interaction. On the other hand, the quantitative agreement with experimental vertical transition energy cannot be achieved with hybrid functionals due to the inaccurate descriptions of the ionization energies of the localized defect and the positions of the band edges.

Evidence for excited state intramolecular charge transfer reaction in donor–acceptor molecule 5-(4-dimethylamino-phenyl)-penta-2,4-dienoic acid methyl ester: Experimental and quantum chemical approach

Intramolecular charge transfer (ICT) reaction has been investigated in 5-(4-dimethylamino-phenyl)-penta-2,4-dienoic acid methyl ester (DPDAME) using spectroscopic techniques. The molecule DPDAME shows local emission in non-polar solvent and dual emission in polar solvents. Solvatochromic effects on the Stokes shifted emission band clearly demonstrate the charge transfer character of the excited state. Quantum chemical calculations have been performed at Hartree–Fock (HF) and density functional theoretical (DFT) levels to correlate the experimental findings. Potential energy curves (PECs) for the ICT reaction have been evaluated along the donor twist angle at DFT and time dependent density functional theory (TDDFT) levels for the ground and excited states, respectively, using B3LYP hybrid functional and 6-31G ⁎ ⁎ basis set. The solvent effects on the spectral properties have been explored theoretically at the same level with time dependent density functional theory-polarized continuum model (TDDFT-PCM) and the theoretical results are found to well substantiate the solvent polarity dependent Stokes shifted emission of DPDAME. Huge enhancement of dipole moment (Δ μ=16.42 D) of the molecule following photoexcitation dictates the highly polar character of the excited state. Although elucidation of PECs does not exactly predict the operation of ICT according to twisted intramolecular charge transfer (TICT) model in DPDAME, lowering of vertical transition energy as a function of the donor twist coordinate scripts the occurrence of red shifted emission as observed experimentally.

Electronic Transition Energies: A Study of the Performance of a Large Range of Single Reference Density Functional and Wave Function Methods on Valence and Rydberg States Compared to Experiment.

This work reports a comparison among wave function and DFT single reference methods for vertical electronic transition energy calculations toward singlet states, valence and Rydberg in nature. A series of 11 small organic molecules are used as test cases, where accurate experimental data in gas phase are available. We compared CIS, RPA, CIS(D), EOM-CCSD, and 28 multipurpose density functionals of the type LSDA, GGA, M-GGA, H-GGA, HM-GGA and with separated short and long-range exchange. The list of functionals is obviously not complete, but it spans more than 20 years of DFT development and includes functionals which are commonly used in the computation of a variety of molecular properties. Large differences in the results were found between the various functionals. The aim of this work is therefore to shed some light on the performance of the plethora of functionals available and compare them with some traditional wave function based methods on a molecular property of large interest as the transition energy.

Intensity of d–s symmetry-forbidden electronic transition for Cu + impurity in sodium chloride

Embedded cluster model is used to describe electronic structure of Cu + ion in NaCl host. Transition energies and oscillator strengths are calculated for the 3d 10 → 3d 94s 1 Cu + ligand field transitions. These transitions are not allowed by dipole selection rules, which can, however, be broken through by vibronic coupling mechanism. A method is proposed to take into account the effect of vibronic coupling in the value of the oscillator strengths. Results for vertical transition energy are in much better agreement with experiments than previous calculations and optical oscillator strengths are reported for the first time.

DFT STUDY ON POLYDIACETYLENES AND THEIR DERIVATIVES

Optimized geometries and vibrational frequencies of polydiacetylenes (PDAs) and their derivatives were studied by density functional calculations at the B3LYP/6-31G* level. The time-dependent density functional theory was used to determine their vertical transition energies and corresponding oscillator strengths. Calculations show that different side groups in these linear carbon chains can significantly modify their structural and electronic properties, whereas the effect of terminal substitution is negligible. Predicted equilibrium geometries indicate that the single, double, and triple bonds of PDAs and their derivatives are almost unchanged as the chain increases, showing a remarkable character of localized bond. The periodic boundary condition calculations reveal that the strongest adsorption for the infinite chain of PDA appears at 723 nm, and the HOMO → LUMO excitation is responsible for this strong electronic transition.

Quantum Chemical Modeling of Photoabsorption Properties of Two- and Three-Nitrogen Vacancy Point Defects in Diamond

Quantum chemical calculations of the geometric and electronic structures and vertical transition energies for several low-lying excited states of the neutral and negatively charged vacancy-related point defects in diamond containing two and three nitrogen atoms (N2V0, N2V−, and N3V0) have been performed employing various theoretical methods (time-dependent density functional theory, equation-of-motion coupled cluster, and multireference perturbation theory) and different basis sets and using C21H28, C35H36, and C51H52 finite model clusters. In the ground states, the vacancy-related atoms are found to be shifted away from the vacancy center by ∼0.1 Å, whereas the positions of atoms from the second layer around the vacancy remain nearly unchanged, indicating a local character of geometry relaxation due to defects. The lowest excited states are formed with participation of the stretched (N2V) or broken (N3V) C−C bond and nonbonding combinations of nitrogen lone pairs as donors, with the C−C antibonding molecular orbital (MO) in N2V0, broken C−C bond in N3V0, and diffuse vacancy-related MOs serving as acceptors. Normally, the first excited states have a valence character, but the diffuse states are rather close in energy, especially for N3V0 (22A1 and 12E excited states). The first optically active excitation in the N2V0 defect with the calculated energy of ∼2.6 eV (in close agreement with the experiment) is formed by the electronic transition from the stretched C−C bond to the antibonding C−C MO, with an additional contribution from the combination of nitrogen lone pairs. For the negatively charged N2V− system, the lowest excitation to the 12A1 state is predicted to occur from the singly occupied antibonding b1 MO to the empty diffuse a1 orbital, but the CASPT2 calculated excitation energy, ∼0.9 eV, underestimates the experimental zero phonon line observed at 1.26 eV. The lowest excited states of N3V0, 22A1, and 12E correspond to transitions from the singly occupied MO (SOMO) to the diffuse lowest vacant orbital and from the nonbonding combination of nitrogen lone pairs to SOMO, respectively, and have similar energies of about 3.1−3.3 eV, in agreement with the experimental photoabsorption band maximum at ∼3 eV.

Comment on “Theoretical Investigation of Perylene Dimers and Excimers and Their Signatures in X-Ray Diffraction”

The structures of the ground and excimer states of perylene pairs are calculated [using density functional theory (DFT) and time-dependent DFT techniques] in a free as well as a crystal environment, and their spectroscopic properties are studied for the most stable configurations. The vertical transition energies for the absorption and emission bands are obtained, and they are in good agreement with experimental data. In these calculations, up to six excited states are considered. With the calculated structures of the ground and excimer states, the scattering factors are analyzed as a function of the concentration of excimers in a crystal. The intensity of the 110, 005, and 0 10 0 reflections are found to be fairly sensitive to the presence of excimers in the crystal. The finite (nanosecond) lifetime of the excimer may make it possible to observe this state using time-resolved X-ray diffraction techniques.

Quantum-chemical study of the photochemical addition reaction of dialkyl disulfides with olefins

Dimethyl disulfide (Me-SS-Me) and methyl ethyl disulfide (Me-SS-Et) in ground (S0) and electronically excited (S1) states were studied by configuration interaction methods (RCIS and CASSCF). In both molecules, the nonplanar geometric structure with a C-S-S-C dihedral angle of about 88° transforms into the planar structure upon the adiabatic transition S0 → S1. For Me-SS-Me, the vertical transition energy is 5.28 eV and the energy of the adiabatic transition is 2.44 eV. It was shown that the SS bond is elongated by 0.402 A in Me-SS-Me and 0.453 A in Me-SS-Et. Calculations showed that the photolysis of both molecules brings the system via the vertical transition to a state approaching the dissociation threshold of the electronically excited molecules into two thiyl radicals. The structure of the thiyl radicals produced was established. The fragments of the potential energy surface for the interaction of the methylthiyl radical with the ethylene molecule were calculated. The activation energy of this process was found to be 3.8 kcal/mol. After passing through the saddle point, the stable radical H3C-S-CH2-CH2 is formed which has a transoid configuration.

Investigation of the Ligand-Field States of the Hexaammine Cobalt(III) Ion with Quantum Chemical Methods

The ligand-field (LF) transition energies of the Co(NH3)6(3+) ion have been computed with multiconfiguration quasidegenerate second-order perturbation theory (MCQDPT2). The water solvent was treated with the polarizable continuum model (PCM), and the environment in crystals was modeled by the Co(NH3)6·Cl4(-) complex. The Co-N bond lengths, calculated for the hydrated cation and the Co(NH3)6·Cl4(-) model compound, agree with those in the crystal structures. The vertical transition energies agree with experiment, whereby those based on Co(NH3)6·Cl4(-) are more accurate than those for the hydrated ion. The 0-0 transitions were based on the OPBE geometries of ground and excited (1)T1g, (3)T1g, (5)T2g states of the hydrated ion. The (3)T1g state is the lowest excited state; the (5)T2g state lies higher by >0.6 eV.

Formation and localization of a solvated electron in ground and low-lying excited states of Li(NH3)n and Li(H2O)n clusters: a comparison with Na(NH3)n and Na(H2O)n

A theoretical study of the ground and low-lying excited states of Li(NH(3))(n) and Li(H(2)O)(n) (n = 1-8) clusters is presented. Their structures, binding energies, vertical ionization energies and vertical transition energies were calculated using ab initio molecular orbital methods at correlated levels. Compared with Na(NH(3))(n) and Na(H(2)O)(n), the incremental binding energies and the spectroscopic energies are found to be almost metal-independent, but solvent-dependent after first-shell closure in both M(NH(3))(n) and M(H(2)O)(n) (M = Li and Na) clusters(.) Autoionization of the alkali atoms occurs via a spatial expansion of unpaired electron distribution extending to outside of the first solvation-shell, irrespective of the combinations of the metal and the solvent. The localization mode of the wave function of the solvated electron was investigated in both the ground and excited states. A change from a one-center diffuse state to a two-center localized state proceeds more quickly against n in M(H(2)O)(n) than in M(NH(3))(n), which is behind the solvent-dependence of the evaluated quantities.

Theoretical study on the structure，spectra and thermodynamic property of 2-（toluene-4-sulfonylamino）-benzoic

The structure optimization and frequency calculation have been carried out at B3LYP/6-31++G** theoretical level，and IR spectrum，Raman spectrum and thermodynamic property at different temperatures have been obtained. The results indicate that the carbon and the oxygen in the —COOH group，and the nitron in —（OS）2—NH group come into formation of different large π bonds with the benzene rings，and there is a dihedral angle of 63.2° because of place blocking and conjugate effect. The first vertical excited state electronic transition energy was calculated by time-dependent density functional theory，and the maximal absorption wavelength was obtained at 312.7?nm，belonging to near UV，which is in good agreement with experimental value of 307?nm in dichloromethane solvent.

Energies of low-lying excited states of linear polyenes.

Room temperature absorption and emission spectra of the all-trans isomers of decatetraene, dodecapentaene, tetradecahexaene, and hexadecaheptaene have been obtained in a series of nonpolar solvents. The resolved vibronic features in the optical spectra of these model systems allow the accurate determination of S(0) (1(1)A(g)(-)) --> S(2) (1(1)B(u)(+)) and S(1) (2(1)A(g)(-)) --> S(0) (1(1)A(g)(-)) electronic origins as a function of solvent polarizability. These data can be extrapolated to predict the transition energies in the absence of solvent perturbations. The effects of the terminal methyl substituents on the transition energies also can be estimated. Franck-Condon maxima in the absorption and emission spectra were used to estimate differences between S(0) (1(1)A(g)(-)) --> S(1) (2(1)A(g)(-)) and S(0) (1(1)A(g)(-)) --> S(2) (1(1)B(u)(+)) electronic origins and "vertical" transition energies. Experimental estimates of the vertical transition energies of unsubstituted, all-trans polyenes in vacuum as a function of conjugation length are compared with long-standing multireference configuration interaction (MRCI) treatments and with more recent ab initio calculations of the energies of the 2(1)A(g)(-) (S(1)) and 1(1)B(u)(+) (S(2)) states.

Theoretical study of molecular properties of low-lying electronic excited states of H2O and H2S

Geometries, excitation energies, dipole moments and dipole polarisability tensor components of the ground and four lowest excited states 3 B 1, 1 B 1, 3 A 2, 1 A 2 of the H2O and H2S molecules were calculated at the CASSCF, CASPT2, CCSD and CCSD(T) level of approximation. Vertical excitation and equilibrium transition energies of these states, having the Rydberg character, are reported too. Properties of both molecules in the ground and in low lying excited states are compared and discussed from the point of view of their molecular electronic structure. Upon excitation we observe dramatic changes of dipole moments and polarisabilities with respect to the ground state. We stress the change of the polarity of H2O in all excited states accompanied by the enhancement of the dipole polarisability by an order of magnitude. Large, even if less pronounced, are changes of electric properties of H2S in its excited states. Dipole moments and dipole polarisabilities of 3 B 1, 1 B 1 states of H2S and H2O behave quite analogously in comparison to their respective ground state. The general pattern of properties for both molecules in their 3 A 2 and 1 A 2 excited states is more different due to a pronounced participation of the sulphur d-orbitals in these states of the H2S molecule.

Photoelectron spectroscopy and theoretical studies of [Com(pyrene)n]− (m=1,2 and n=1,2) complexes

Anion photoelectron spectroscopic experiments and density functional theory based calculations have been used to investigate the structural, electronic, and magnetic properties of neutral and anionic [Co(m)(pyrene)(n)] (m,n=1-2) complexes. The calculated electron affinities and vertical transition energies of Co(m)(pyrene)(n) are in good agreement with the measured values. Our results provide clear evidence for dimerization of Co atoms and formation of sandwich structures in these complexes. While the calculated spin magnetic moments of neutral Co(2)(pyrene)(n) complexes suggest a preference for ferromagnetic coupling between Co atoms, the spin magnetic moment of Co atom in Co(pyrene) and Co(pyrene)(2) complexes was reduced to 1mu(B).

Modeling of the structure and electronic spectra of green fluorescent protein chromophore

The structure and electronic absorption spectra of a model green fluorescent protein chromophore were studied in the neutral, cationic, and anionic forms in the isolated state. The energies of S0-S1 vertical transitions were calculated using a new method based on a modified multiconfiguration quasi-degenerate perturbation theory (aug-MCQDPT2). This method was used to obtain quantitative estimates of S0-S1 vertical transition energies for chromophores in the isolated state, 2.54, 3.12, and 3.11 eV (the experimental values were 2.59, 3.05, and 2.99 eV) for the anionic, cationic, and neutral forms, respectively.