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Details concerning the energetics and structure of the ion pair in gaseous chloroethane, obtained at the multi-reference configuration interaction with singles and doubles (MR-CISD) level, are given. It is formed in the third excited state (31A′), and it can be classified as a hydrogen-bonded contact ion pair, although from its total binding energy of 3.28 eV (including extensivity corrections, at the MR-CISD + Q level with the aug-cc-pVTZ basis set, and including zero-point energy corrections) only 0.14 eV is due to an underlying hydrogen bond. It is a highly polar structure, with a dipole moment of 9.56 D. As compared to previous systems for which the same type of bond has been observed, it has a much lower hydrogen-bond energy and a much larger distance between the charge centers. The three lowest frequency vibrational modes of the HBCIP correspond to intermolecular cation–anion modes. The structure here obtained brings important structural questions for HCFCs derived from chloroethane. [ABSTRACT FROM AUTHOR]

Two important aspects of the photophysics of dimethoxy curcumin in solvents are theoretically analysed. First, the large Stokes shift in cyclohexane and in acetonitrile is calculated using time-dependent density functional theory with large basis set. Second, a possible explanation is given for the considerable decrease of the nonradiative decay rate in changing from the nonpolar cyclohexane to polar acetonitrile. The solvent is treated using the polarizable continuum model. We analyse different isomers and obtain the absorption transition energies. Similarly, after obtaining the relaxed geometry of the excited state, the emission transition is obtained. In both cases, a red shift of the singlet transition is obtained with increasing solvent polarity. In contrast, the low-lying triplet states are essentially solvent independent. The calculated solvent shift in the emission is in very good agreement with experiment. The Stokes shifts are also in fair agreement with experiment when long-range or dispersion-corrected functionals are used. There are three calculated triplet states lying below the S state. As they are π → π* states, they cannot participate in the intersystem crossing by the El-Sayed rule. However, a fourth triplet state of n → π* nature is found only slightly above the Franck-Condon position of the S state, and the S → T separation increases with the solvent polarity, this being a possible explanation for the decrease of the nonradiative transition. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this work, we present a theoretical investigation on excitation energies of organic molecules embedded in a periodic monolayer. We use the self-consistent periodic-image-charges embedding approach, which takes into account all the electrostatic effects, to compute the perturbation on molecular orbitals and eigenvalues due to the presence of the surrounding periodic array of polar molecules. We considered vanadyl naphthalocyanine, mercaptobiphenyl, and tris-(8-hydroxyquinoline) aluminum (AlQ) at different coverages, and excitation energies computed using the time-dependent density-functional theory. We found a significant (0.1-0.2 eV) red- or blue-shift of the energies for different excited states, due to the different coupling of the molecule with the polarization field of the two-dimensional crystal. [ABSTRACT FROM AUTHOR]

The ground-state equilibrium geometries of the linear carbon chain cations NCN ( n = 1-7) have been investigated with B3LYP, CAM-B3LYP, and RCCSD(T) calculations. The ground state (XП) and excited state (1П) have been optimized by using the complete active space self-consistent field method. The present study reveals that these linear cations generally have the characteristic of bond length alternation in both electronic states. The vertical excited energies for the dipole-allowed (1, 2, 3)П ← XП transitions as well as the dipole-forbidden 1Φ ← XП transitions have been computed with the complete active space second-order perturbation theory. The calculated transition energies of 1П ← XП for NCN ( n = 1-6) in the gas phase are 2.26, 2.09, 1.91, 1.72, 1.56, and 1.39 eV, respectively, which mutually agree well with the available experimental values of 2.11, 2.07, 1.88, 1.67, 1.49, and 1.34 eV. Moreover, the corresponding absorption wavelengths are predicted to have the significant nonlinear size dependence, which is different from the bands origin in NCN ( n = 1-7). [ABSTRACT FROM AUTHOR]

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 symmetry. Our highly accurate calculation indicated that theoretically determined geometric parameters and harmonic vibrational frequencies for the ground-state XA′ are in good agreement with observed experimental data. The geometry of triplet HNCS is clearly favored C 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 XΠ (1A′, 1A″), 1Σ (1A″), and 1Σ (3A′) 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. [ABSTRACT FROM AUTHOR]

The complete active space self-consistent field and multiconfigurational second-order perturbation theory methods have been used to investigate the low-lying electronic states of the HCCO radical and its anion. The calculated geometrical structure and harmonic vibrational frequencies for the X2A″ state of HCCO are in good agreement with experimental values. The barrier to 12П (12A′) is estimated to be 0.069 eV (554 cm−1), which is also close to experimental result (540 cm−1). Moreover, we find that the 22A′ state is bent, while 22П (22A″) is linear. By comparing the oscillator strengths and adiabatic excitation energies, we show that the 22П (22A″) ← X2A″ transition is the most intense among these transitions. The unpaired electrons mainly locate on the C atoms in the 22A′ and 22П (22A″) states according to the Mulliken spin populations. On the other hand, for HCCO−, the first adiabatic and vertical detachment energies are 2.210 and 2.362 eV, respectively, which reasonably agree with experimental value of 2.338 ± 0.008 eV. Remarkably, we explore several higher excited states of the HCCO radical (14A′, 24A″ and cis-14A′) and its anion (13A″, 11A″ and 13A′), which have not been reported in previous studies. [ABSTRACT FROM AUTHOR]

Equilibrium geometries of low-lying electronic states of cyanosilylene (HSiCN), isocyanosilylene (HSiNC), and their ions have been investigated using the complete active space self-consistent field (CASSCF) approach. The harmonic vibrational frequencies on the optimized geometries were calculated using the multiconfiguration linear response (MCLR) method. Taking the further correlation effects into account, the complete active space perturbation theory of second-order (CASPT2) was carried out for the energetic correction. The CASPT2 calculations have been performed to obtain the vertical excitation energies of selected low-lying excited states of HSiCN and HSiNC. Computed results show that the singlet-triplet splittings are calculated to be 0.99 and 1.30 eV for HSiCN and HSiNC, respectively. The vertical excitation energies of the lowest singlet and triplet excited states in HSiCN are lower than those in HSiNC. The first vertical ionization energy of HSiCN (10.04 eV) is higher than that of HSiNC (9.97 eV). The ground-state adiabatic electron affinities are found to be rather high, and the value of HSiCN (1.85 eV) higher than that of HSiNC (1.52 eV). The existences of dipole-bound excited negative ion states have been discovered within HSiCN and HSiNC. [ABSTRACT FROM AUTHOR]

An economic basis set for ab initio calculations of systems in excited states has been studied. The economic basis set, in which the polarization functions are applied only to oxygen (O), while the 6-31G basis set is used for carbon (C) and hydrogen (H), is shown to save considerable computing time and to give reliable geometric and frequency measurements. In addition, the economic basis set has been considered for obtaining reliable excitation energies, with the correction factor, using high-level single-point energy calculations. This study is expected to shed light on basis set selection for computations of large systems in excited states. [ABSTRACT FROM AUTHOR]

A simulation of the $$n\rightarrow\pi^*$$ absorption and the $$\pi^*\rightarrow n$$ fluorescence of acetone in aqueous solution is reported. The model has an explicit solvent representation with an effective ab initio treatment of the solute. The model attempts to balance quantum chemistry, intermolecular interactions and statistical thermodynamics. It includes a non-electrostatic perturbation on the solute which models the solute–solvent exchange repulsion and the restriction put on the electronic structure of the solute by the antisymmetry to the solvent. The solvent shift to the absorption transition is found to be between 0.16 and 0.21 eV; the shift to the fluorescence transition is found to be between 0.02 and 0.05 eV. The simulation supports the conclusion that the first peak in the fluorescence spectrum of acetone is from a single molecule in equilibrium with the solvent, not from an excimer. [ABSTRACT FROM AUTHOR]

The energy eigenvalues for confined H 2 + molecule are computed by using the variational method. The approach proposed here uses a trial molecular function for the ground state composed of a linear combination of atomic orbitals for confined hydrogen atom; for other states, we build the trial molecular eigenfunctions inspired in atomic orbitals and by using the orthogonality of the wave functions. The molecule is confined in an impenetrable prolate spheroidal box. The atomic orbital for 1s state is built from a previous suggestion inspired by the factorization of the Schrödinger equation, and for 2s state, we used the Gram–Schmidt process to build a trial atomic function orthogonal with 1s trial function. The main contribution of this work is to propose new wave functions to be used for the confined hydrogen ion molecule. The results obtained are in agreement with other results present in the literature, and the trial functions proposed here can be used to study other confined molecules. [ABSTRACT FROM AUTHOR]

A series of cyclometalated platinum(II) complexes with similar molecular structures but distinct phosphorescence quantum yields were investigated. To explore the intersystem crossing (ISC) and radiative decay processes, we investigated the absorption and phosphorescence properties, spin–orbit coupling matrix elements, major ISC channels and phosphorescent transition rates. For the temperature-dependent nonradiative decay processes, the metal-centered excited states (3MC), the transition states, and the minimum energy crossing points were investigated with theoretical calculations. The way of temperature-independent nonradiative decay which has weak influence on nonradiative decay rate was also compared through the calculation of reorganization energy (λ). [ABSTRACT FROM AUTHOR]

The photoisomerization process between tetrahedrane (THD) and cyclobutadiene (CBD) has attracted tremendous attention during the past two decades due to its potential application toward designing new materials. Recently, an experimental study found that perfluoroaryl-substituted tetrahedrane could isomerize to its cyclobutadiene derivative under irradiation > 420 nm. In this paper, the detailed mechanism of this photoisomerization process will be investigated using density functional theory and time-dependent density functional theory. These results suggest that the chromophore 9,10-dicyanoanthracene is first activated under light irradiation, and then, electron transfer occurs from perfluoroaryltetrahedrane to the chromophore to form its cation derivative. The electron transfer process has been theoretically confirmed by the location of the crossing point between two excited states of the perfluoroaryltetrahedrane and 9,10-dicyanoanthracene complex in which the higher excited state can be denoted as the local excitation of 9,10-dicyanoanthracene and the lower excited state is characterized as the charge transfer state. After that, the cation of perfluoroaryltetrahedrane can easily isomerize to the perfluoroarylcyclobutadiene cation with a barrier of 6.2 kcal/mol. After isomerization, the electron could transfer back from perfluoroarylcyclobutadiene to 9,10-dicyanoanthracene, and thus, 9,10-dicyanoanthracene can be considered as the photocatalyst of the isomerization process. The studied mechanism in this work could be helpful to reveal the nature of the isomerization process between THD and CBD and instructive for the design of new light control materials. [ABSTRACT FROM AUTHOR]