Calculations on bound and autoionizing states,and continuum radiative properties for Li− and Na−

Semi-empirical model potential calculations have been performed for bound and continuum properties of Li^? and Na^?. The calculated electron affinities of the 2s²¹S state of Li^? and the 3s²¹S and 3p²³P^e states of Na^? are in agreement with the calculations of Norcross and with experimental data. Positions of possible autoionizing states are calculated using projection and root stabilization methods. The Stieltjes imaging method of Langhoff is employed to compute the photodetachment cross sections of Li^? and Na^? and the results are in excellent agreement with the close-coupling calculations of Moores and Norcross. A comparison of variational and numerical results for the coupled time-dependent Hartree-Fock photoionization of helium shows that good Stieltjes imaging results can be obtained with a very small basis set for the variational calculation. The continuous photoemission profile for the 3p²³P^e state of Na^? is also obtained.     

A theoretical study of photoinduced electron transfer between tetracyanoethylene and arene

 Ab initio calculations have been performed to investigate the state transition in photoinduced electron transfer reactions between tetracyanoethylene and biphenyl as well as naphthalene. Face-to-face conformations of electron donor–acceptor (EDA) complexes were selected for this purpose. The geometries of the EDA complexes were determined by using the isolated optimized geometries of the donor and the acceptor to search for the maximum stabilization energy along the center-to-center distance. The correction of interaction energies for basis set superposition error was considered by using counterpoise methods. The ground and excited states of the EDA complexes were optimized with complete-active-space self-consistent-field calculations. The theoretical study of the ground state and excited states of the EDA complex in this work reveals that the S₁ and S₂ states of the EDA complexes are charge–transfer (CT) excited states, and CT absorption which corresponds to the S₀→S₁ and S₀→S₂ transitions arise from π−π^* excitation. On the basis of an Onsager model, CT absorption in dichloromethane was investigated by considering the solvent reorganization energy. Detailed discussions on the excited state and on the CT absorptions were made. Received: 30 April 2001 / Accepted: 18 October 2001 / Published online: 9 January 2002     

On the crude adiabatic molecular vibronic states and their use in the interpretation of radiationless processes in molecules

In order to clarify the meaning of crude adiabatic vibronic states, the use of which has been repeatedly advocated as a possible starting point for the study of radiationless processes in polyatomic molecules, we explicitly carry out the determination of such states for two diatomics, H₂ and N₂. Ground-state crude adiabatic potential energies of the molecules H₂ and N₂ have been calculated using Hartree-Fock molecular wavefunctions. The calculated zero-point energies are 4179 cm^?1 for H₂ (exp: 2168 cm^?1) and 21826 cm^?1) for N₂ (exp: 1176 cm^?1). Semi-crude calculations for N₂ (the core orbitals are allowed to follow the nuclei) yield 2979 cm^?1 with a minimum basis set and 4320 cm^?1 with a double ζ basis set. These calculations indicate that vibronic crude adiabatic wavefunctions give a very poor representation of molecular states.     

A theoretical study on the ionization of OCS with an analysis of vibrational structures of the photoelectron spectrum

 Ab initio calculations have been performed to study the molecular structures and vibrational levels of the four low-lying ionic states (1, 2²Π, and 1, 2²Σ⁺) of carbonyl sulfide. The global regions of the potential-energy surfaces have been obtained by multireference single and double excitation configuration interaction calculations. Vibrational calculations using explicit vibrational Hamiltonians have been used for vibrational analysis. The equilibrium molecular structures and a vibrational analysis of the four ionic states are presented. The theoretical ionization intensity curves including the vibrational structures of the ionic states are also presented and are compared with the photoelectron spectrum. Received: 20 January 2001 / Accepted: 22 August 2001 / Published online: 30 October 2001     

Stabilization of a π double bond between two boron atoms: A theoretical approach

The low-lying states of HBBH, HBBNH₂ and H₂NBBNH₂ are investigated by means of ab initio CI calculations using a double-zeta + polarization basis set. Diborene is found to have a ³∑_g⁻ ground state. Replacement of hydrogen by amino groups on each side of the BB bond leads to an ethylene-like bond which corresponds to a ¹A_g state of D_2h symmetry. π back-donation by the amino lone pairs is responsible for the stabilization of this state.     

Lowest electronic states of neutral and ionic LiN

We have investigated the potential energy curves (PECs) of the LiN heteronuclear diatomic molecule, including its ionic species LiN⁺ and LiN⁻, using explicitly correlated multi-reference configuration interaction (MRCI-F12) calculations in conjunction with the correlation consistent quintuple-𝜁 basis set. The effect of core–valence correlation, scalar relativistic effects, and the size of the basis sets has been investigated. A comprehensive set of spectroscopic constants determined based on the above-mentioned calculations are also reported for the lowest electronic states and all systems, including dissociation energies, harmonic and anharmonic vibrational frequencies, and rotational constants. Additional parameters, such as the dipole moments, equilibrium spin-orbit constants, excitation energies, and rovibrational energy levels, are also documented. We found that the three triplet states of LiN, namely, X ³∑⁻, A ³Π, and 2 ³∑⁻, exhibit substantial potential wells in the PEC diagrams, while the quintet states are repulsive in nature. The ground state of the anion also shows a deep potential well in the vicinity of its equilibrium geometry. In contrast, the ground and excited states of the cation are very loosely bound. Charge transfer properties of each of these states are also analyzed to obtain an in-depth understanding of the interatomic interactions. We found that the core–valence correlation has a substantial effect on the calculated spectroscopic constants.     

Theoretical study on the kinetics and branching ratios of the gas phase reactions of 1, 1-Dichlorodimethylether (DCDME) with Cl atom

Quantum mechanical calculations are carried out on the reactions of CH₃OCHCl₂ (DCDME) with Cl atom by means of DFT and couple cluster methods. The geometries of the reactants, products, and transition states involved in the reaction pathways are optimized at BHandHLYP level of theory using 6-311G(d,p) basis set. Transition states are searched on the potential energy surface involved during the reaction channels, and each of the transition states is characterized by the presence of only one imaginary frequency. The existence of transition states on the corresponding potential energy surface is ascertained by performing intrinsic reaction coordinate calculation. Single point energy calculations are performed at CCSD(T) level using the same basis set. The hydrogen abstraction rate constant for the title reaction is calculated at 298 K and atmospheric pressure using the canonical transition state theory including tunneling correction. The calculated value for rate constant as 1.204 × 10^?12 cm³ molecule^?1 s^?1 is found to be in very good agreement with the recent experimental data. The percentage contributions of both reaction channels are also reported at 298 K.     

Pyramidal distortions in the ethylene triplet π,π* state

Pyramidal distortions which may stabilize the triplet π,π* state of ethylene are investigated using ab initio molecular orbital calculations. Using a minimal basis set, the calculated stabilization energies are 5.1 kcal mole^?1 for a cis flapped conformation, 3.6 kcal mole^?1 for a trans flapped conformation, and 0.8 kcal mole^?1 for a twisted flapped conformation. With a double zeta basis set the predicted stabilization energies are much smaller: 1.3, 0.7, and 0.0 kcal mole^?1, respectively.     

Energies of Resonances in Atoms and Ions with Three Electrons

The energies of resonances of ² Ssymmetry were calculated for atoms and ions with three electrons, H^2?, He^?, and Li, using the stabilization technique. The variational wave function was expanded over a large number (up to 2030) of normalized three-electron basis functions. The basis functions that made significant contributions to the energies were selected using a special procedure. Many high-energy resonances were found with energies of H^2? resonances ranging from 1 to 12 eV. The calculated energies of some He^? and Li resonances were compared with the experimental data and calculations of other authors. The selected basis set of three-electron wave functions was found to be efficient in calculations of high-energy resonances.     

Calculating quasi-bound rotation-vibrational states of HOCl using massively parallel computers

We calculate positions and predissociation widths for quasi-bound states of HOCl with total angular momentum of J=0 and J=3. An ab initio potential energy surface is used in conjunction with a complex absorbing potential (CAP). These calculations are performed by diagonalising a complex symmetric Hamiltonian using our discrete variable representation (DVR) based parallel code, PDVR3D, and a truncation and diagonalisation algorithm. The resonances are identified as those states in the continuum, which are stable with respect to CAP and basis set parameters. Test on the resonances are carried out using over 90 different absorbing potential heights. Resonances of both Feshbach (vibrational trapping) and shape (rotational trapping) are identified.     

Analytical gradients for Singer's multicenter n‐electron explicitly correlated Gaussians

Analytical gradients for Singer's basis of n‐electron multicenter explicitly correlated Gaussian functions are derived and implemented to variationally optimize the energy and wave function of molecular systems within the Born–Oppenheimer approximation. Wave functions are optimized with respect to (½n(n+1)+3n) nonlinear variational parameters and one linear coefficient per term in the basis set. Preliminary results for the ground states of H₃⁺ and H₃ suggest that the method can be more flexible and can achieve lower energies than previously reported calculations. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 151–159, 2001     

Ab initio potential curves for the lowest 1Σ+ and 3Π states of the ion CN+

Ab initio configuration-interaction calculations using a 9s5p|3s2p gaussian basis supplemented with polarization functions are presented for potential curves for the a ¹Σ⁺ and the lowest ³Π state of CN⁺. The two states are very close in energy and calculations at this level do not give an unequivocal indication of the identity of the ground state.     

Steric and electronic structure of 2-alkoxyphenyltrichlorostannanes according to results of ab initio calculations

Steric and electronic structure of 2-methoxy- and 2-ethoxyphenyltrichlorostannanes, as well as of 2-methoxyphenyltrichlorostannanes substituted in the ring, was studied using the RHF and B3LYP levels with the 3?C21G* basis set. The results of calculations were compared with experimental ³⁵Cl NQR data. In all studied molecules the Sn atom is pentacoordinated. The structure of the coordination polyhedron is a highly distorted trigonal bipyramid. Replacing methyl group in the alkoxy substituent involved in the Sn??O coordinating interaction by a more electron-donor ethyl group increases the strength of the Sn??O coordination bond. The same occurs also at the introduction of an electron-releasing substituent in the aromatic ring.     

Rydberg character of the higher excited states of free-base porphin

 The Rydberg character of the excited states of free-base porphin (FBP) has been investigated by the ab initio configuration interaction singles (CIS) method and the state-averaged complete-active-space self-consistent-field method. Double-zeta basis sets augmented with s, p, and d Rydberg functions and d polarization functions have been employed. Two types of molecular orbitals sets, the restricted Hartree–Fock molecular orbitals obtained for the ground state (¹A _g ) and for the cation state (²A _u ), have been used in the CIS calculations. All the calculations show that Rydberg-type excitations play important roles especially in the N bands. In this article we propose applying the model of a perturbed Rydberg series to interpret the excited states of FBP. By using this model, we have succeeded in analyzing the characteristics of the excited states as well as the experimental oscillator strengths, which have considerable magnitude even in the higher excited states. Received: 27 November 2000 / Accepted: 11 April 2001 / Published online: 27 June 2001     

A comparative basis-set study of NeH+ using coupled-cluster techniques

Unrestricted Hartree-Fock, coupled-cluster calculations are reported for the ground state of NeH⁺ using atomic basis sets of increasing size and accuracy for both Ne and H. The goal is to determine the basis set and coupled-cluster level of calculation needed to obtain a NeH⁺ potential energy curve of known accuracy. Here, it is shown that calculations using a quintuple zeta basis at the coupled-cluster singles and doubles level with noniterative triples, CCSD(T) , predict a Ne—H bond dissociation energy that is within about 0.01 eV of the exact Born–Oppenheimer molecular electronic structure result. Spectroscopic constants determined using the Simons–Parr–Finlan procedure are found to be in very good agreement with the experimental results. Calculations at the augmented quadruple zeta level for the two lowest triplet excited states of the NeH⁺ species are presented. Both of these states separate into ground-state Ne⁺ and H(1s). The resulting potential curves predict stable minima at the SCF, CCSD, and CCSD(T) levels with dissociation energies of about 0.07 eV. Spectroscopic constants from the potential curves and dissociation constants are reported. © 1994 John Wiley & Sons, Inc.     

Spin-orbit coupling effects in the gas-phase reaction Se + O2

The potential energy surface sections of the ground and excited states of the SeOO (C _s ) superoxo complex have been constructed on the basis of spin-orbit coupling calculations. The activation barriers to the forward and reverse reactions??superoxide SeOO ? peroxide Se(O₂) ? dioxide SeO₂??have been calculated. Changes in the spectral and spin properties of oxygen (zero-field splitting) and possible channels of formation of active forms of ¹O₂ oxygen (¹?? _g , ¹?? _g ⁺ ) upon dissociation of excited states of selenium oxo complexes have been scrutinized.     

Correlated and gauge origin independent calculations of magnetic properties

Summary We have applied a gauge origin invariant method for calculations of nuclear magnetic shielding constants to the singly bonded molecules BF, F₂, BH₃, CH₄, NH₃, H₂O, and HF as well as to the¹H shielding constants of HCN and C₂H₂. The calculations were performed at the RPA and second order polarization propagator (SOPPA) level of theory. For most molecules the correlation contribution in SOPPA is less diamagnetic than in the comparable MP2 calculations. For F₂, SOPPA gives a large paramagnetic correlation correction whereas the MP2 method gives a very small correlation contribution. For all molecules agreement with experimental results is generally improved at the SOPPA level compared to RPA. We have also demonstrated that second order gauge origin invariant, common and local origin (SOLO) methods do not necessarily give the same shielding even in the limit of a converged basis set.     

Theoretical analysis of counterfoil influence over the physicochemical properties of aniline tetramers in several oxidation states

We present semiempirical quantum chemical calculations of geometric structures, charge distributions, energy levels, ionization potentials, and enthalpies of formation for aniline tetramers in different oxidation states using the semiempirical AM1 method. For tetraaniline radical cation the effect of these three counterions on the above-mentioned physicochemical parameters was analyzed. The ions studied included Cl^–, HSQ₄ ^–, and CH₃COO^–. Chloride counterion showed a large charge transfer to the chain, as was shown in a preceding work [4]. HSO₄ ^– showed a strong charge stabilization without transfer. CH₃COO^– exhibits hydrogen bond formation and also displays a strong charge stabilization in a fragment of the chain, but does not transfer any charge. Also, Cl^– was able to form a hydrogen bond, depending on the initial position that it occupies in relation to the tetrameric chain, which is then optimized. Charge transfer is present for the Cl^– cases. For dication structures the effect of SO ₄ ^2– was analyzed. Our calculations showed that the distribution of the energy levels of tetraaniline radical cation can be comparable to the polyaniline ones by adding a counterion to the tetramer.     

The least-squares stabilization method for resonance eigenvalues

A variation on the stabilization method of Hazi and Taylor is proposed. The expectation value of H² is known to be sensitive to changes in the wavefunction and can be used to detect the presence and the position of the resonances. By systematically increasing the size of the basis and analyzing the convergence behaviour, it is possible to obtain accurate information about the resonance states. For a model problem with a single resonance, the results are in good agreement with complex coordinate calculations.     

Association of metal cations with alkanes: Na(CH4)+ versus Cu(CH4)+ as molecular models

Summary Ab initio molecular orbital calculations give small stabilization energies for the various Na(CH₄)⁺ adducts (less than 4 kcal mol^–1), but predict a stronger binding for the copper compounds (about 13 kcal mol^–1). The different behaviour of Na⁺ and Cu⁺, already present at the SCF level, is reinforced by electron correlation. This can be attributed to an important contribution of the dispersion energy to the binding energy of the copper ion: about 40% of the total, including basis set superposition corrections.Dedicated to Mrs A. Pullman