Excited states of antiaromatic systems1

SINDO1 studies were performed to optimize the geometry of excited states of some antiaromatic molecules. It is discussed how such states can exhibit aromatic character upon suitable electronic excitation. The nodal pattern of the molecular orbitals involved in the electronic excitation are used to invoke bond equilization in excited states. We have investigated singlet and triplet excited states of five-membered rings C₄H₅B, C₅H₅⁺ and C₅H₄O containing four π electrons and bicyclic systems bicyclo-(1,1,0)-butadiene, bicyclo-(2,2,0)-hexatriene and benzocylobutadiene. It is seen that in bicyclo-(2,2,0)-hexatriene, both the bicylic structure and the 1,4-diradical structure determine the equilibrium geometry.     

Optical transitions of symmetrical mixed-valence systems in the Class II-III transition regime

In the Robin and Day classification, mixed-valence systems are characterized as Class I, II or III depending on the strength of the electronic interaction between the oxidized and reduced sites, ranging from essentially zero (Class I), to moderate (Class II), to very strong electronic coupling (Class III). The properties of Class I systems are essentially those of the separate sites. Class II systems possess new optical and electronic properties in addition to those of the separate sites. However, the interaction between the sites is sufficiently weak that Class II systems are valence trapped or charge localized and can the be described by a double-well potential. In Class III systems the interaction of the donor and acceptor sites is so great that two separate minima are no longer discernible and the energy surface features a single minimum. The electron is delocalized and the system has its own unique properties. The Robin and Day classification has enjoyed considerable success and most of the redox systems studied to date are readily assigned to Class II. However the situation becomes much more complicated when the system shows borderline Class II/III behavior. Such "almost delocalized" mixed-valence systems are difficult to characterize. In this article spectral band shapes and intensities are calculated utilizing increasingly complex models including two to four states. Free-energy surfaces are constructed for harmonic diabetic surfaces and characterized as a function of increasing electronic coupling to simulate the Class II to III transition. The properties of the charge-transfer absorption bands predicted for borderline mixed-valence systems are compared with experimental data. The treatment is restricted to symmetrical (delta G0 = 0) systems.     

Excited vibrational states near dissociation in weakly bound triatomic systems

The vibrational motion of a weakly bound M₃ complex is investigated using hyperspherical coordinates and an adiabatic separation of the radial and angular motions within the molecule. It is shown that the vibrational energy levels near dissociation correlate to an M atom orbiting about a rotating M₂ core. Application is made to the neon trimer assuming an approximate pairwise additive potential.     

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.     

Inequivalent XPS binding energies in symmetrical delocalized mixed-valence complexes

It has previously been assumed that an ESCA measurement on a mixed-valence-e.g., M(II)-M(III) - compound which yields two peaks for an inner-shell M ionization at energies close to those measured for separate M(II) and M(III) ions provides direct evidence that the complex has an unsymmetrical (trapped-valence) ground state rather than one in which the electrons are symmetrically delocalized. This assumption is incorrect. A complex which has a symmetrical ground state will have two accessible unsymmetrical photoionized states owing to electron relaxation in the strong field of the core hole. For a range of values of binding energy differences and electron coupling parameters, the photoionized states will be very nearly localized and the peak separation for a complex with delocalized ground state will be close to that for isolated M(II) and M(III) ions. The appearance of two M binding energies is thus not in itself evidence for electronic ground-state asymmetry in a mixed-valence compound. A model is proposed from which quantitative predictions are made.     

Excited electronic states of NiCO

The results of ab initio SCF and CI calculations on the electronic states of NiCO are reported. The ¹Σ⁺ ground state is a mixture of two primary configurations associated with the Ni 3d¹⁰ and 3d⁹4s states, and is bound by 18 kcal mol⁻¹ with respect to Ni and CO at r_nic =1.77 Å. The excited states (within 22000 cm⁻¹ of the ground state) can be divided into a lower manifold, principally involving the Ni(3d⁹4s) electronic configuration, and a higher manifold, formally associated with the charge transfer configuration Ni⁺ (3d⁹)CO⁻ (π1).     

Excited states of porphyrin macrocycles

S1 --> S(n) spectra of porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication have been measured in the energy range 2-3 eV above S1 at room temperature in solution by means of transient absorption spectroscopy exciting with femtosecond pulses. Highly excited pi pi* states not active in the conventional S0 --> S(n) spectrum have been observed. The experimental data are discussed on the basis of the time dependent density functional theory taking advantage of large scale calculations of configuration interaction between singly excited configurations (DF/SCI). The DF/SCI calculation on porphyrin has allowed to assign g states active in the S1 --> S(n) spectrum. Applying the same calculation method to tetraoxaporphyrin dication the S0 --> S(n) spectrum is reproduced relatively to the Q and B (Soret) bands as well as to the weaker E(u) bands at higher energy. According to our calculation the S1 --> S(n) transient spectrum is related to states of g symmetry mainly arising from excitations between doubly degenerate pi and pi* orbitals such as 2e(g) --> 4e(g). In the case of diprotonated porphyrin it is shown that the complex of the macrocycle with two trifluoroacetate anions plays a significant role for absorption. Charge transfer excitations from the anions to the macrocycle contribute to absorption above the Soret band, justifying the intensity enhancement of the S0 --> S(n) spectrum with respect to the other two macrocyclic systems.     

Excited states of the benzyl radical

The CI method is used in the -electron approximation with orbitals for closed and open shells to calculate the properties of excited doublet states with allowance for all singly excited configurations and some doubly excited ones, and also for the first quartet and sextet states, which are calculated in the one-configuration approximation via the open-shell theory. The energies and transition moments agree satisfactorily with the available experimental evidence. A classification and assignment is given for the excited terms. Truncation of the complete set of singly excited configurations greatly distorts the calculated spectrum. Inclusion of doubly excited configurations in the CI also produces a substantial change in the spectrum; in some cases it alters the order of adjacent terms. Conversion in CI from basis closed-shell orbitals to open-shell ones produces a considerable lowering of all terms in the spectrum. As in the case of triplet terms for molecules, weakening of electron interaction brings the lowest excited term of the radical closer to the ground-state term. The electron-density and spin-density distributions are calculated for the excited states.     

Excited states of phenyl carbonyl compounds

In an attempt to clarify the origin of the dual phosphorescence in phenyl alkyl ketones, we have made some calculation (within the C.I.P.S.I. method in an excitonic scheme) to elucidate the conformation of both ground states and excited states of propiophenone. Our calculations have shown the presence of two stable isomers in the ground state, first n ^* state, and first ^* singlet and triplet states. So our work suggests that the origin of the dual phosphorescence of propiophenone could be related to the conformational change of the molecule in the n ^* state, because the most stable conformations in the n ^* state and in the ground state are different.     

Group-theoretical classification of the resonance states of mixed-valence clusters

Moldova State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 4, pp. 743–747, July–August, 1995.     

Excited electronic states of 1,3,5-cycloheptatriene

The electron-impact energy-loss spectrum of 1,3,5-cycloheptatriene has been measured at impact energies of 30,50, and 75 eV, and scattering angles varying from 5° to 80 °. Singlet → triplet transitions were observed at 3.05 eV and 3.95 eV. No evidence for the weak transition at 2.1 eV previously reported on the basis of threshold electron-impact studies was found. Single → singlet transitions were observed at 4.85 eV and 6.40 eV in good agreement with the optical spectrum and semi-empirical calculations.     

Adiabatic potentials of linear three-site mixed-valence systems

We propose a method for calculating adiabatic potentials of linear or quasi-linear threesite systems of mixed valence. The method is based on transformation of the potential energy matrix to two effective interacting modes. The adiabatic potential is calculated and its extrema are found. The electronic wave functions at the minima of the adiabatic potential correspond to complex charge distributions in a cluster. The barrier to transition of the extra electron between the terminal atoms is lower than for the transition from the terminal atom to the central atom.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 2, pp. 129–134, March–April, 1989.     

Excited states and photodissociation of hydroxymethyl hydroperoxide

The structure of hydroxymethyl hydroperoxide (HOCH(2)OOH) (HMHP) has been examined using coupled cluster and multireference configuration interaction methods to study the excited states and probable photodissociation products. The results are compared to experiments. The vertical excitation energies for several excited states of HOCH(2)OOH are presented as well as the excited state energies along the O-O, O-H, C-O, and C-H dissociation pathways. The results help in the interpretation of experimental UV absorption spectra and elucidate the photodissociation mechanism of HMHP under tropospheric conditions.     

Excited electronic states of small water clusters

The lowest electronic states that are initially formed upon excitation of small water clusters having a central water molecule with one stretched OH bond are studied with electronic structure methods. It is found that in water dimer, trimer, and pentamer the lowest excited singlet and triplet states are each nondissociative for stretching of an OH bond that is hydrogen bonded in an icelike configuration to a neighboring water molecule. This is in marked contrast to the behavior of an isolated gas phase water monomer, where it is well known that the lowest excited state is strongly dissociative upon OH stretching. The conclusions of this study may serve as a basis to interpret recent experimental evidence that suggests a significant lifetime for excited water in irradiated thin ice films, and may also have important implications for the behavior of excitation of liquid water.     

Excited states and electronic spectrum of ferrocene

The excited states of ferrocene have been calculated using the ab-initio singly excited configuration interaction and self-consistent field methods. The results obtained are used to assign the bands of the electronic absorption spectrum of the molecule.     

Electronic and vibronic states of an asymmetric trimeric mixed-valence cluster

Electronic and adiabatic vibronic states of d¹-d¹-d² trimeric clusters are considered in terms of the many-electron theory of mixed-valence states. The initial model includes two d-orbitals on each center and allows for Heisenberg and double exchange, asymmetry of the system, and vibronic coupling with local shifts of ligands. Influence of distortions in the trimers on the structure of tunnel-exchange states, on ferro-or antiferromagnetism of the clusters, and on the form of the adiabatic potential is examined. State University, Moldova Republic. Valencia University, Spain. Institute of Chemistry, Academy of Sciences, Moldova Republic. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 4, pp. 35–45, July–August, 1994. Translated by L. Smolina     

Excited biradical states in oxirane ring opening

The mechanism of oxirane ring opening was studied by ab initio [RHF, ROHF, GVB/DH, RHF/SBK(p, d)] calculations. The strained structure of oxiranes and their complexes with aliphatic alcohols and amines is characterized by low-lying biradical states whose thermal population leads to the ring opening. The examined oxirane ring opening reacitons have low activation energy (<10 kcal mol-¹) and are catalyzed by labile hydrogen atoms in hydroxy and amino groups of the reaction complex.     

Projected electron density method of π-eletron systems II. Excited states

A perturbation theory based on the time-dependent Schrödinger equation is presented; Coulombic interactions are taken into account and spin properties are neglected. Using wave functions given by the projected electron density method described in Part I as a basis set the energies of excited π-electron states are calculated. For a series of porphyrin compounds the electronic spectra are calculated and are found to be in good agreement with experiment.