Electronic structures of permethyloligosilane radical cations at the ground and low-lying excited states

The electronic structures at the ground and low-lying excited states of permethyloligosilane radical cations, Si_n(CH₃)_2n+2⁺ (n = 4-7), have been investigated using DFT and ab initio calculations. The calculations showed that positive charge (hole) is delocalized along the Si-Si main chain at the ground and first excited states. On the other hand, the hole is transferred to the methyl side-chain at the second and higher excited states. From these results, it was concluded that hole can move along the Si-Si main chain at thermal conditions. Also, it was predicted that intermolecular hole hopping takes place by photo-irradiation to the permethyloligosilane radical cation. The mechanism of hole transfer was discussed on the basis of the results.     

Dipole moments and electronic charge distributions of the 1, 3(nπ*) excited states of formaldehyde,benzaldehyde and benzophenone. A theoretical study at the INDO level including the doubly- and triply-excited configurations

The dipole moments and charge distributions of the ^1,3(nπ*) excited states of formaldehyde, benzaldehyde and benzophenone have been investigated theoretically. This study was performed within the framework of the INDO approximations by introducing into the electronic wavefunctions doubly- and triply-excited configurations selected with respect to the (nπ*) configuration of suitable spin multiplicity. One basis set of MOs was used in the calculations on formaldehyde and on benzophenone, while two basis sets were used in the calculations on benzaldehyde. The results show, in particular, that the ^1,3(nπ*) excited states of benzaldehyde have a dipole moment lower than that of the corresponding ground state.     

Theory of single and double ionization of helium through two-photon excitation of the1 S 0 e (1) doubly excited resonance

We present a theory and calculations of two-photon-resonant three-photon ionization of He via the lowest even parity doubly excited state¹ S ₀ ^e (1). We assess the importance of double ionization relative to single ionization into excited ionic states. Although double ionization is found to be quite small in the present context, our calculations reveal the importance of autoionizing doubly excited states as virtual intermediate states and suggest contexts in which double ionization may be relatively more efficient.     

Azulene: polarized absorption in stretched polymers and magnetic circular dichroism

Linear dichroic and magnetic circular dichroic spectroscopy is used to characterize the excited states of azulene. Comparison with crystal spectra underlines the necessity of using two-parameter equations for evaluations of stretched sheet spectra despite some previous claims to the contrary. The MCD spectrum reveals strong vibronic interactions in the second transition.The results are interpreted on the basis of Pariser—Parr—Pople (PPP) type calculations including some with extensive configuration interaction (CI). Calculated signs of the first three B terms spectrum agree with experimental signs irrespective of the details of the PPP model, and even the numerical values are in an order-of-magnitude agreement. Only the calculation with extensive CI gives agreement for signs of higher B terms. It suggest the possible presence of two excited states, but the present experimental evidence for these is insufficient. One of these new states would be best described as a predominantly doubly excited state.The origin of the B terms is traced to contributions due to magnetic mixing of individual zero-order excited states and the results permit determination of the absolute sense of magnetic dipole transition moments between excited states with respect to the sense of the vector product of the electronic dipole transition moments from the ground state to these excited states. A comparison is performed with B terms calculated using finite perturbation technique and gauge-invariant orbitals.     

Excited states of indoloquinoxalines

Electronic transitions of low-symmetry molecules indolo[2,3-b]quinoxaline and its two methyl derivatives—were studied by linear dichroism, fluorescence polarization and magnetic circular dichroism methods. Transition moment directions were determined for low-lying excited states and found to be in good agreement with the results of INDO/S calculations.     

The excited states of cycloporane. MCD spectrum,and CD spectrum of an optically active derivative

The MCD spectrum of gas phase cyclopropane and the CD spectrum of trans 1,2-dimethyl cyclopropane were measured in the spectral region 210–140 nm. The absorption spectra are also reported. Comparison of CD and absorption spectra of dimethyl cyclopropane and consideration of the anisotropy factors g = Δε/ε as a guide to the assignment of magnetic dipole allowed transitions led to the ordering of the states of the first excited configuration as A′₁, A′₂, E′ in order of increasing energy. The magnetic moments of the two observed allowed excited states of cyclopropane were determined from the MCD spectrum. LCAO MO level calculations of the MCD parameters $\text{A}$₁/$\text{D}$₀ for the lowest three excited E′ states were carried out and the results were used to discuss the assignment of the allowed transitions. It is concluded that configuration interaction is of considerable importance in the low energy excited states of cyclopropane.     

Structures and binding energies of the (dibenzoylmethanato)boron difluoride complexes with aromatic hydrocarbons in the ground and excited states. Density functional theory calculations

Structures of the (dibenzoylmethanato)boron difluoride molecule (DBMBF2) and its complexes with a series of aromatic hydrocarbons (benzene; toluene; o-, m-, and p-xylenes, naphthalene; anthracene; and pyrene) in the ground and the first singlet excited states have been calculated. The calculations have been performed by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) for the ground and excited states, respectively, with the empirical dispersion correction. It has been shown that the complexes in the ground and excited states have similar stacking structures and are characterized by short contacts between the F atom of DBMBF2 and H atoms of the hydrocarbon molecule, which decrease on transition from the ground to the excited state. The calculated binding energies in the complexes in the excited state are two to three times higher than those in the ground state. The charge transfer in the ground state of the complexes is insignificant and directed from DBMBF2 to the ligand, while in the excited state it is 0.6–0.8 e and directed from the ligand to DBMBF2.     

HSO自由基电子基态激发态的CAS计算研究

使用CASSCF方法和ANO-L基组优化了HSO自由基的基态和3个低占据激发态的结构, 并采用包括更多电子动态相关能的CASPT2方法进行了单点能校正. 频率计算结果表明, 优化的4个几何为势能面上的稳定点. 通过电子结构的研究合理地解释了各个激发态相对于电子基态的结构变化.     

Zero-field splitting of Mn2+ ions in tetragonal and rhombic symmetry. The effect of excited state contributions

Zero-field splitting parameters D and E are derived by a complete crystal field and spin-orbit interaction calculation for the d⁵ configuration in fields of tetragonal and rhombic symmetry. The differences as compared to the results of approximate calculations are due to the combined effect of excited states on the splitting of the ground state.     

Analytical ab initio potential-energy surfaces for the ground and the first singlet excited states of HeH2

Analytical potential-energy surfaces have been constructed for the ground and the first excited states of HeH₂. The functions fit ab initio MRD CI calculations with standard deviations of 0.05 and 0.13 eV for the ground and the excited surface respectively. Classical trajectory calculations for collisions of ⁴Hc with HD(B ¹Σ_u⁺, υ = 3, J = 2) at the temperature T = 297 K yields the electronic quenching cross section σ_Q = 6.5 A² and the vibrational cross section σ_3→2 = 3.8 A². The results are in qualitative agreement with the experimental values of Fink, Akins and Moore.     

Linear response calculation of potential energy curves of BH

The multiconfiguration time-dependent Hartree–Fock (MC TDHF ) approximation is applied to study the excited states of the BH molecule. The potential energy curves and spectroscopic properties of the excited states are analyzed. The MC TDHF results are compared to similar MC SCF calculations for individual excited states. © 1994 John Wiley & Sons, Inc.     

Magnetic circular dichroism of some cyano-substituted cyclopentadienide anions

MCD spectra of pentacyanocyclopentadienide anion, tetracyanocyclopentadienide anion and diazotetracyanocyclopentadiene were measured. They all exhibited the Faraday A term or B terms which are caused by π → π^* electonic transitions of the compounds. Transition energies and symmetries of the excited states were calculated by use of the Pariser-Parr-Pople method. The results of the calculations gave a qualitative interpretation of the observed MCD spectra.     

INDO-SCF and CI calculations on the trans-cis isomerization of azomethane in the ground state and in excited states

INDO-SCF calculations followed by CI calculations with inclusion of multiply, excited configurations were carried out to obtain potential energy curves for isomerization in the ground state and in some low-lying excited states of azomethane. The SCF wavefunctions are analyzed with the aid of newly defined bond characters providing a connection between the chemical concepts of bonds, lone-pairs, etc. and molecular orbital theory. Two different pathways for isomerization are considered and by comparison of the calculated results with experimental data it is concluded that this reaction proceeds in the ^1,3 (nπ*) states via rotation of both methyl groups around the NN double bond.     

Study of 3d – 4f and 3d – 5f quartet and doublet transitions of doubly excited three-electron ions

We present the first observation of optical transitions between doubly excited doublet states in the term systems N V, O VI and F VII. The spectra were produced by foil excitation of fast ion beams. The assignment of the spectral lines was made by comparison with the results of MCDF calculations along the isoelectronic sequence. The same method also led to the identification of two 3d – 4f quartet transitions in Mg X.     

A theoretical description of thetrans-cis conversion in the lowest excited states of diimide

The lowest excited states of the diimide molecule have been calculated by three different SCF-type methods, the performances of which are analyzed in regard to the prediction of geometries and energies. The calculations have been performed with two basis sets (STO-3G and 4-31G) both supplemented with more diffuse functions on the N atoms. Lowest energy pathways for thetrans-cis interconversion in the various states are presented for the three calculation methods and for the two basis sets.     

Structural changes accompanying excited-state electron transfer in meta- and para-dialkylaminopyridines

The electronic structure of the lowest excited singlet states and molecular geometries of a series of dialkylaminopyridines (DAAPs) representing electron donor–acceptor systems were studied by photostationary and time-resolved UV–vis spectroscopic methods and quantum chemical calculations. The comparative studies allow us to rationalize dual luminescence of 4-DAAPs in terms of the TICT state model—the analysis of the electronic transition dipole moments indicates a nearly orthogonal conformation of the fluorescent ICT states. Introduction of the amino group at meta position as in 3-diisopropylaminopyridine completely changes photophysics of these pyridine derivatives: (i) the Franck-Condon excited state initially reached upon excitation and the solvent equilibrated fluorescent state are most probably of the same nature (both excited states do not correspond to a full separation of charges) and (ii) the electronic structure and geometry of the fluorescent CT states of m-DIAP are solvent dependent.     

Coupling between autoionizing states in atomic strontium

I present a theory and calculations of transitions between autoionizing states in Sr. Such transitions are found to become significant at photon fluxes above 10²⁹ photons/cm² s and tend to leave the ion in excited states. Examples of transitions to the excited ionic states 4d and 5p are presented. Their dependence on laser intensity is found to be quite intricate.     

Semiempirical NDDO/MC calculations of the excited states of the chromate ion

A new semiempirical method of calculating the excited states of transition metal complexes is developed. This technique uses the configuration interaction and semiempirical NDDO/MC methods to obtain the ground state of a set of Slater type valence spd-orbitals chosen from the optical spectra of transition metals together with the corresponding core integrals. The method is tested in calculations of the electronically excited states of the chromate ion. Good agreement with the experimental energies of vertical transitions and the results of ab initio calculations is achieved.