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Using the density functional (B3LYP), coupled cluster (CCSD(T)), and polarizable continuum model (PCM) methods the reaction mechanism of a model compound of ebselen, 2, [1,2-benziodelenazol-3(2H)-one] with peroxynitrous acid (HOONO) was investigated. The B3LYP and CCSD(T) calculations with a number of basis sets indicate that the B3LYP methods is not a reliable method for studying the energies and stability of the weakly bound .OH...ONO structure and could lead to wrong calculations.

Anomalous enhancement of stimulated Raman scattering (SRS) of the aromatic CH stretching band was observed when the laser-induced plasma was generated in benzene-toluene mixed solvents. Time-resolved spectra of the SRS revealed that the strong excitation condition that led to generation of ionized molecules was an important factor for anomalous enhancement.

The study shows how the proposed 'two-in-one' model di-enol intermediate successfully represents the 'less-substituted' and 'more substituted' enol intermediates generated from an unsymmetrical ketone. When 'local softness' based reactivity descriptors are used to evaluate the reactivities of the relevant atoms (the less-substituted and the more-substituted alpha-carbon atoms which take part in the aldol addition reaction) in the proposed model, it could successfully predict the predominant aldol product (the more substituted one).

High level ab initio calculations are performed to examine the mechanism of NO decomposition by NH3 (super +) and NH4 (super +) in a gas phase. Maximum activation energies for the various reactions are estimated at 55.3 and 77.3 kcal/mol.

Binding energies and electronic structures of Li+ and Cu+ in their interaction with small ligands of the type HX has been studied by using B3LYP and CCSD(T) calculations. It was established that the second ligand in Cu+ complexes has higher binding energies than the first one.

The structures, energetic, and reaction mechanisms of several H2 additions and H migrations in a series based on a model complex, (Cp2Zr)2(mu2,eta(super 2),eta(super 2)-N2H2) (Al) are studied. Calculations reveal that A1 has a twisted structure, resembling that of ([eta5-C5Me4H)2Zr]2(mu2,eta(super 2),eta(super 2)-N2)) (T1), which results in similar properties.

Density functional theory based on global and local electrophilicity descriptors are used to study the reliability of local electrophilicity values of the strongest electrophilic sites in generating global intermolecular electrophilicity trends. The values on 15 different organic chlorides show that, for systems having more than one comparatively strong electrophilic site, the local electrophilicity value of the strongest site does not produce a reliable global intermolecular electrophilicity trend.

The mechanism of the reaction of [1,2-benzisotillurazol-3(2H)-one], ebtellur derivative 2_Te, with perozynitrous acid, HOONO, was elucidated at the density functional (B3LYP) level. It was found that in the gas phase the reaction, proceeding via the concerted pathway, occurs with deltaH = 0.3 (deltaG = (3.4) kcal/mol HO-ONO bond cleavage barrier.

The mechanism of the reaction of an ebtellur derivative with peroxynitrite anion was studied at the density functional level. It was shown that the reaction proceeds via the pathway 2_Te + PN --> 2_Te-PN --> 2_Te-TS1 (O-O activ) --> 2_Te-O(NO(sub 2)(super -1)).

A theoretical study conducted on the methyl and ethyl derivatives of C, Si, Ge, Sn, and Pb is presented. A comprehensive study of fully optimized geometries and reaction energies are presented for both methyl and ethyl families.

The excited states (Q and B bands) of free-base porphyrins (FBP), Mg-porphyrin and Zn-porphyrin and their derivatives were investigated. The results of multireference Moller-Plesset perturbation studies with complete active space self-consistent field reference that the weakness of the Q band of FBP is due to pseudoparity-forbidden status that came from the pairing properties of alternant hydrocarbons. It was shown that breaking the pairing properties results in the strengthening of the Q band intensity, which can be accomplished by the substitution of C atoms with N atoms in the meso position.

In the present study, we investigate the use of the Δ S CF method and Slater's transition state (STS) theory to calculate the binding energies of the 2s and 2p electrons of third-period elements (P, S, and Cl). Both the Hartree-Fock (HF) and Kohn-Sham (KS) approximations are examined. The STS approximation performs well in reproducing the Δ S CF values. However, for the Δ S CF method itself, while the binding energy of the 2p electrons is accurately predicted, the results for 2s are fairly sensitive to the functional, exhibiting significant variations due to self-interaction errors (SIE). Nonetheless, the variations in chemical shifts between different species remain relatively small, and the values agree with experiments due to the cancellation of SIE. A notable observation is that the chemical shifts of the 2s and 2p electrons are similar, indicating a perturbation caused by the valence electrons. The error in the absolute binding energy of KS Δ S CF against the experiment is nearly constant for the same element in different molecules, and it depends largely on the functional owing to SIE. A shifting scheme previously developed can be employed to reproduce the experimental 2s and 2p binding energies, with dependence on the functional and atom but not on the molecule even for 2s KS Δ S CF binding energies. Upon obtaining the corrected binding energies, we find that the gap between 2s and 2p binding energy is nearly independent of chemical environment for a given element: 57.5, 63.9, and 70.9 eV for the elements P, S, and Cl, respectively.

We present an approximate approach for the calculation of ionization potential (IP) and electron affinity (EA) by exploiting the complementary energy non-linearity errors for a species M and its one-electron-ionized counterpart (M + ). Reasonable IPs and EAs are thus obtained by averaging the orbital energies of M and M + , even with a low-level method such as BLYP/6-31G(d). By combining the corrected IPs and EAs, we can further obtain reasonable excitation energies. The errors in uncorrected valence IPs and uncorrected virtual-orbital energies show systematic trends. These characteristics provide a convenient and computationally efficient avenue for qualitative estimation of these properties with single corrections for multiple IPs and excitation energies.

In the present study, we have investigated the applicability of long-range-corrected (LC) functionals to a Kohn-Sham (KS) Koopmans'-type theorem. Specifically, we have examined the performance of optimally tuned LCgau-core functionals (in combination with BOP and PW86-PW91 exchange-correlation functionals) by calculating the ionization potential (IP) within the context of Koopmans' prediction. In the LC scheme, the electron repulsion operator, 1/ r 12 , is divided into short-range and long-range components using a standard error function, with a range separation parameter μ determining the weight of the two ranges. For each system that we have examined (H 2 O, CO, benzene, N 2 , HF, H 2 CO, C 2 H 4 , and five-membered ring compounds cyclo-C 4 H 4 X, with X = CH 2 , NH, O, and S, and pyridine), the value of μ is optimized to minimize the deviation of the negative HOMO energy from the experimental IP. Our results demonstrate the utility of optimally tuned LC functionals in predicting the IP of outer valence levels. The accuracy is comparable to that of highly accurate ab initio theory. However, our Koopmans' method is less accurate for the inner valence and core levels. Overall, our results support the notion that orbitals in KS-DFT, when obtained with the LC functional, provide an accurate one-electron energy spectrum. This method represents a one-electron orbital theory that is attractive in its simple formulation and effective in its practical application.

The core electron binding energies (CEBEs) and core-level excitation energies of thymine, adenine, cytosine, and uracil are studied by the Kohn-Sham (KS) method with long-range corrected (LC) functionals. The CEBEs are estimated according to the Koopmans-type theorem for density functional theory. The excitation energies from the core to the valence π* and Rydberg states are calculated as the orbital energy differences between core-level orbitals of a neutral parent/cation and unoccupied π* or Rydberg orbitals of its cation. The model is intuitive, and the spectra can easily be assigned. Core excitation energies from oxygen 1s, nitrogen 1s, and carbon 1s to π* and Rydberg states, and the chemical shifts, agree well with previously reported theoretical and experimental data. The straightforward use of KS orbitals in this scheme carries the advantage that it can be applied efficiently to large systems such as biomolecules and nanomaterials.

Previously proposed theoretical schemes for estimating one-electron excitation energies using Kohn-Sham (KS) solutions with long-range corrected (LC) functionals are applied to the charge-transfer (CT) excitations of the ethylene···tetrafluoroethylene (C 2 H 4 -C 2 F 4 ) system, and the CT complex between an aromatic donor (Ar = benzene, toluene, o -xylene, naphthalene, anthracene, and various meso-substituted anthracenes) and the tetracyanoethylene (TCNE) acceptor. The CT excited state is described well as a single-electron excitation between specific orbitals of donor and acceptor. Thus, CT excitation energies are well approximated by the orbital energies because of the satisfaction of the Koopmans-type theorem and the asymptotic behavior of the LC functional. We have examined three computational schemes: scheme 1 employs the orbital energies for the neutral and cationic systems, scheme 2 utilizes orbital energies of just the cation, and in scheme 3, because the electron affinity of a molecule is the ionization energy of its anion, a scale factor is applied to enforce this identity. The present schemes reproduce the correct asymptotic behavior of CT excitation energy of C 2 H 4 ···C 2 F 4 for the long intermolecular distances and give good agreement with accurate ab initio results. Calculated CT excitation energies for Ar-TCNE are compared with those of TD-DFT and ΔSCF methods. Scheme 1 with the optimal range-separation parameter μ accurately reproduces CT excitation energies for all Ar-TCNE systems and gives good agreement with the best TD-DFT calculations and experiment. Scheme 1, scheme 3, and TD-DFT show similar tendencies with respect to the variation in μ. Scheme 2 and ΔSCF approaches are rather insensitive to changes in μ, but both considerably underestimate the CT excitation energies for these systems. KS orbital energies are physically meaningful and they are practically useful; if the range-separation parameter is tuned, then good results can be obtained.

We have previously reported that, whereas conventional density functional theory (DFT) functionals have provided poor calculations on the alkane isodesmic reaction energy and isomerization reaction energy of organic molecules that include C, N, and O atoms, our developed long-range corrected (LC)- and LC including Gaussian attenuation (LCgau)-DFT + local response dispersion (LRD) functionals, which can accurately calculate inter- and intramolecular weak interactions, give accurate isomerization energies on these reactions. In this work, we found that B3LYP-D3, LC-ωPBE-D3, and ωB97XD, known for their good descriptions of weak interaction calculations, fail to reproduce the isomerization reaction energies of the molecules that include the S atom, such as methyl-thiourea, ethyl-thiourea, and propyl-thiourea. In contrast, LC- and LCgau-BOP+LRD functionals provide isomerization reaction energies that are very close to those produced by highly accurate wave function methods. These results show that an accurate description of the intramolecular weak interaction between the alkyl group and the S atom, unlike in the case of urea, is significant to reproduce the correct energy of the molecules with an alkyl group and S atom.

We introduced two methods to correct the singularity in the calculation of long-range Hartree-Fock (HF) exchange for long-range-corrected density functional theory (LC-DFT) calculations in plane-wave basis sets. The first method introduces an auxiliary function to cancel out the singularity. The second method introduces a truncated long-range Coulomb potential, which has no singularity. We assessed the introduced methods using the LC-BLYP functional by applying it to isolated systems of naphthalene and pyridine. We first compared the total energies and the HOMO energies of the singularity-corrected and uncorrected calculations and confirmed that singularity correction is essential for LC-DFT calculations using plane-wave basis sets. The LC-DFT calculation results converged rapidly with respect to the cell size as the other functionals, and their results were in good agreement with the calculated results obtained using Gaussian basis sets. LC-DFT succeeded in obtaining accurate orbital energies and excitation energies. We next applied LC-DFT with singularity correction methods to the electronic structure calculations of the extended systems, Si and SiC. We confirmed that singularity correction is important for calculations of extended systems as well. The calculation results of the valence and conduction bands by LC-BLYP showed good convergence with respect to the number of k points sampled. The introduced methods succeeded in overcoming the singularity problem in HF exchange calculation. We investigated the effect of the singularity correction on the excitation state calculation and found that careful treatment of the singularities is required compared to ground-state calculations. We finally examined the excitonic effect on the band gap of the extended systems. We calculated the excitation energies to the first excited state of the extended systems using a supercell model at the Γ point and found that the excitonic binding energy, supposed to be small for inorganic semiconductors, was quite large. Our findings suggest that more investigation on the effect of the excitonic binding energy on band gaps is necessary.

Using time-dependent density functional theory (TD-DFT), configuration interaction single (CIS) method, and approximate coupled cluster singles and doubles (CC2) method, we investigated the absorption spectra of coumarin derivative dyes (C343, NKX-2388, NKX-2311, NKX-2586, and NKX-2677), which have been synthesized for efficient dye-sensitized solar cells. The CC2 calculations are found in good agreement with the experimental results except for the smallest coumarin dye (C343). TD-DFT underestimates the vertical excitation energy of the larger coumarin dyes (NKX-2586 and -2677). Solvents (methanol) are found to induce a red shift of the vertical excitation energies, and their effects on the molecular geometry and the electronic structure are examined in detail. The deprotonated form of coumarin is also investigated, where a blue shift of the vertical excitation energies is observed.

The reaction mechanism of the reduction of dinitrogen coordinated side-on to a binuclear Zr complex, [(eta(5)-C(5)Me(4)H)(2)Zr](2)(mu(2),eta(2),eta(2)-N(2)) (T1), was investigated theoretically using a model complex, [(eta(5)-C(5)H(5))(2)Zr](2)(mu(2),eta(2),eta(2)-N(2)) (A1), employing density functional theory calculations. The effectiveness of A1 in describing T1 was confirmed by comparing the structures, distributions of charge, and frontier molecular orbitals. Our calculations showed that A1 has a twisted structure, resembling that of T1, which results in similar properties. The calculations for A1 and its derivatives on H(2) addition clearly explain the reaction mechanism and the reaction path that T1 follows, as well as the experimentally required reaction conditions. The immediate reaction of the first and second H(2) additions produces [(eta(5)-C(5)Me(4)H)(2)ZrH](2)(mu(2),eta(2),eta(2)-N(2)H(2)) (T2), and this is explained by the barrier heights of the reaction, which were calculated to be 20.4 and 10.9 kcal/mol, respectively. The latter barrier was below that of A1 + 2H(2). Complex T2 may be the initial complex for further H(2) addition under proper conditions, or it could lose one H(2) molecule followed by H migration from the Zr site to the N site. Both reactions are expected to occur, because of the closeness of the barrier heights (25.1 and 36.5 kcal/mol, respectively). Gentle warming is required for further H(2) additions, which can be understood from the energetics as well. The high reactivity of T1 with H(2) has been discussed by the comparison of the calculation of A1 and another complex with different ligands, presenting an interesting indication on the effects of the ligands. These theoretical results and discussion explaining the experiment should provide insight into the nature of the hydrogenation mechanism.