On the lower excited electronic states of glyoxal

The polarization of both nπ* absorption bands of glyoxal has been measured in a glass matrix of 2-methyltetrahydrofuran by the photoselection method. The second absorption band in the 30 000 cm^?1 region has been assigned to a ¹A_g → ¹B_g nπ* transition. Its intensity is mainly induced by interaction with the solvent. An absorption band at about 43 000 cm^?1 has been ascribed to a charge transfer transition in complexes of glyoxal and 2-MTHF.     

On the lower excited electronic states of biacetyl

An ab initio SCF and CI study has been carried out for the ground and electronically excited states of biacetyl (CH₃COCOCH₃). The second absorption band in the 4.40 eV region has been assigned to a ¹A_g → ¹B_g nπ^* transition The character of the lower-lying states has been analyzed in terms of the CI wavefunctions.     

On the electronically excited states of uracil

Vertical excitation energies in uracil in the gas phase and in water solution are investigated by the equation-of-motion coupled-cluster and multireference configuration interaction methods. Basis set effects are found to be important for converged results. The analysis of electronic wave functions reveals that the lowest singlet states are predominantly of a singly excited character and are therefore well described by single-reference equation-of-motion methods augmented by a perturbative triples correction to account for dynamical correlation.Our best estimates for the vertical excitation energies for the lowest singlet n --> pi* and pi --> pi* are 5.0 +/- 0.1 eV and 5.3 +/- 0.1 eV, respectively. The solvent effects for these states are estimated to be +0.5 eV and +/- 0.1 eV, respectively. We attribute the difference between the computed vertical excitations and the maximum of the experimental absorption to strong vibronic interaction between the lowest A" and A' states leading to intensity borrowing by the forbidden transition.     

On the calculation of energies of excited states

We reformulate a scheme for calculation of the energies of excited states which, unlike the variation method, does not require that the trial function be orthogonalized to the wavefunctions for lower excited states. The possibility of obtaining wavefunctions as well as energies is discussed, and an example of the scheme's application is made to the harmonic oscillator.

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Zusammenfassung Es wird eine neue Form für eine Methode zur Berechnung von angeregten Zuständen gegeben. Die Probefunktion braucht nicht wie bei der Variationsmethode orthogonal zu den Wellenfunktionen der tiefer liegenden Zuständen zu sein. Es wird die Möglichkeit diskutiert, Wellenfunktionen ebenso wie Energien zu erhalten. Das Schema wird auf den harmonischen Oszillator angewandt.

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Resume On donne une nouvelle forme pour une methode de calcul des niveaux excités. La methode ne demande pas l'orthogonalisation de la fonction d'essai par rapport aux fonctions d'onde correspondant aux niveaux inférieurs, ce qui montre un avantage auprès de la methode variationnelle. La possibilité d'obtenir les fonctions d'onde elles-mêmes est discutée, et on donne un exemple de l'application de cette nouvelle methode.

     

Calculation of the energies of the lower excited states of CH3

Ab initio LCAO-MO-SCF calculations have been performed on assorted low-lying configurations of the CH₃ radical to assess the adequacy of the Hartree-Fock treatment of this species. The ² E configuration of lowest energy in the planar ground state equilibrium geometry is shown not to be the valence configuration. Near its (pyramidal) equilibrium geometry the ² E valence configuration is well below the other ² E configuration. The two ² E configurations must, then, cross as the molecule is distorted. In this region the Hartree-Fock formation is unable to describe ² E states. The other low-lying states [² A'₂ (ground), ² A₁ (3s Rydberg), and ¹ A₁ (ion)] and the ² E states in the pyramidal geometry are satisfactorily determined within the Hartree-Fock formalism.

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Zusammenfassung Die Resultate von ab initio-Rechnungen für das CH₃-Radical für die tiefliegenden Konfigurationen werden angegeben. Die Konfiguration des tiefsten ² E-Zustandes resultiert allerdings bei planarer Geometrie nicht aus der Elektronen-Grundkonfiguration. Letztere ist zwar bei fast pyramidaler Struktur wesentlich tiefer, schneidet aber erstere im Verlauf der Verzerrung des Moleküls. In dieser Region ist die HF-Näherung nicht mehr gültig. — Die anderen tiefliegenden Konfigurationen [²A₂ (Grund-Konfiguration), ² A₁ (3s Rydberg) und ¹ A₁ (Ion)] verhalten sich normal.

     

On the valence excited states of conjugated dienes

Electron impact spectroscopic results are reported for the lowest-energy A_g and B_u valence excited states of several conjugated dienes. In all cases, the B_u state was found to lie below the A_g state. The B_u state of the s-cis compounds was found to be ≈ 0.5 eV lower in energy than that of the s-trans compounds. Correlations of these results with those obtained for several s-trans and s-cis polyenes leads to a new possible interpretation of the state observed below the B_u state in intermediate length polynes.     

Study on excited states of hexamethoxybenzene-NO+ charge-transfer complex: incorporation of excited radical cation

A charge-transfer (CT) complex of NOBF₄ and hexamethoxybenzene (HMB), which gives out HMB^?+ as a “fluorescent radical cation probe,” upon one-electron oxidation, has been designed to explore the excited state dynamics of contact radical ion pairs by laser-induced fluorescence and femtosecond transient absorption spectroscopic techniques. The acetonitrile solution of the CT complex showed weak fluorescence with a similar spectrum to that observed for free excited HMB radical cation (HMB^?+*), suggesting the formation of HMB^?+* upon the one-photonic excitation of the CT complex. The laser-power dependence of the fluorescence intensity supported the one-photonic excitation event. We have also observed a short-lived transient species but no long-lived species by femtosecond laser flash photolysis of the CT complex. The lifetime (6.5 ps) was in good accordance with its fluorescence quantum yield (2.5 × 10^?5) and was able to assign the transient species to the fluorescent state, an excited radical ion pair [HMB ^?+*/NO^?]. All the events were completed within the inner sphere and the short lifetime of the transient species could be attributed to rapid back-electron transfer. It is concluded that the excited radical cation character in the excited state of the CT complex originates from the radical ion character in the CT complex in the ground state and that a relatively long lifetime of HMB^?+* facilitates its observation even in the contact ion pair.     

Yields of excited states from geminate recombination of hydrocarbon radical ions

A theory similar to the radical pair model for CIDNP, etc., is used to describe the rate of loss of spin correlation in geminate pairs of radical ions produced by radiolysis. The ratio of the yields of triplet and singlet excited states and the effects of magnetic fields are discussed.     

On the low-lying excited states of sym-triazine-based herbicides.

We report a joint computational and luminescence study on the low-lying excited states of sym-triazines, namely, 1,3,5-triazine (1) and the ubiquitous herbicides atrazine [6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (2)] and ametryn [6-methylthio-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (3)]. Geometrical structures, energetics, and transition and state properties of I and 2 were computed at the TD-DFT, CASSCF, and CASPT2 levels of theory. The fluorescence and phosphorescence emission spectra, lifetimes, and fluorescence quantum yields were measured for the three compounds, and from these, the energies of the lowest excited states and their corresponding radiative rates were determined. The predictions from CASPT2 calculations are in good agreement with the experimental results obtained from the luminescence studies and allow the interpretation of different absorption and emission features.     

On the computation of excited states with MCSCF methods

We discuss the theoretical and practical problems arising when trying to compute excited states of nonrelativistic electrons in a molecular system, by multiconfiguration (MCSCF) methods. These nonlinear models approximate the linear Schrödinger theory and are a generalization of the well-known Hartree–Fock approach. Due to the MCSCF nonlinearity, a theoretical definition of what should be a MCSCF excited state is not clear at all, contrarily to the ground state case. We compare various definitions used in Quantum Chemistry. We in particular stress that some defects may lead to important computational problems, already observed in Quantum Chemistry (root flipping). We then present a definition of MCSCF excited states based on a solid mathematical ground and compare it with the most used methods. This new definition leads to a completely new algorithm for computing the first excited state, which was proposed and tested in a collaboration with Cancès and Galicher. Numerical results are provided for the simple case of two-electron systems, as an illustration of the possible issues which can arise as consequences of the nonlinearity of the MCSCF method.     

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.     

On the calculation of the excited states of small molecules

The excited states of CO, H₂O and NH₃ have been calculated by the singly excited configuration interaction method using two large basis sets, one of which contained diffuse functions in order to describe Rydberg states. Results are found to be in good agreement with experiment for both excitation energies and oscillator strengths for the transitions from the ground state.     

All-valence-electron CI study of the ground and lowest excited states of the hydroperoxyl radical

Ab initio calculations using a double-zeta plus polarization AO basis are used to study the lowest-lying (²A′ ← ²A″) transition of the HO₂ radical. A large-scale CI treatment is found to result in a transition energy for this system which agrees to within less than 0.1 eV with its experimental value. The life-time of the process is predicted to be in the order of 3 × 10^?3 s using the dipole length formula for this quantity, while the generally less reliable dipole velocity result is found to be several orders of magnitude larger.     

On the SCF calculation of excited states: Singlet states in the two-electron problem

The problem of determining SCF wave functions for excited electronic states is examined for singlet states of two-electron systems using a Lowdin natural orbital transformation of the full CI wave function. This analysis facilitates the comparison of various SCF methods with one another. The distribution of the full CI states among the natural orbital MCSCF states is obtained for the S states of helium using a modest Gaussian basis set. For SCF methods that are not equivalent to the full CI wave functions, it is shown that the Hartree-Fock plus all single excitation wave functions are equivalent to that of Hartree-Fock plus one single excitation. It is further shown that these wave functions are equivalent to the perfect pair or TCSCF wave functions in which the CI expansion coefficients are restricted to have opposite signs. The case of the natural orbital MCSCF wave function for two orbitals is examined in greater detail. It is shown that the first excited state must always be found on the lower natural orbital MCSCF CI root, thus precluding the use of the Hylleras-Undeim-MacDonald (HUM) theorem in locating this state. It is finally demonstrated that the solution obtained by applying the HUM theorem (minimizing the upper MCSCF CI root with respect to orbital mixing parameters) is an artifact of the MCSCF method and does not correspond to any of the full CI states.     

Theoretical study of the electronic excited states of tetracyanoethylene and its radical anion.

The low-lying electronic states of tetracyanoethylene (TCNE) and its radical anion were studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The results obtained yield a full interpretation of the electronic absorption spectra, explain the spectral changes undergone upon reduction, give support to the occurrence of a bound excited state for the anionic species, and provide valuable information for the rationalization of the experimental data obtained with electron transmission spectroscopy.     

Mechanism of radical cation formation from the excited states of zeaxanthin and astaxanthin in chloroform

The C-40 xanthophylls zeaxanthin and astaxanthin were confirmed to form radical cations, Car.+, in the electron-accepting solvent chloroform by direct excitation using subpicosecond time-resolved absorption spectroscopy in combination with spectroelectrochemical determination of the near-infrared absorption of Car.+. For the singlets, the S2(1B(u+) state and most likely the S(x)(3A(g)-) state directly eject electrons to chloroform leading to the rapid formation of Car.+ on a timescale of approximately 100 fs; the lowest-lying S1(2A(g)-) state, however, remains inactive. Standard reduction potential for Car.+ was determined by cyclic voltametry to have the value 0.63 V for zeaxanthin and 0.75 V for astaxanthin from which excited state potentials were calculated, which confirmed the reactivity toward radical cation formation. On the other hand, Car.+ formation from the lowest triplet excited state T1 populated through anthracene sensitization is mediated by a precursor suggested to be a solute-solvent complex detected with broad near-infrared absorption to the shorter wavelength side of the characteristic Car.+ absorption. However, ground state carotenoids are able to react with a secondary solvent radical to yield Car.+, a process occurring within 16 micros for zeaxanthin and within 21 mus for astaxanthin. Among the two xanthophylls together with lycopene and beta-carotene, all having 11 conjugated double bonds, zeaxanthin ranks with the highest reactivity in forming Car.+ from either the S2(1B(u+)) or the ground state. The effects of substituent groups on the reactivity are discussed.     

Two-photon excitation of alkyl-substituted magnesium phthalocyanine: radical formation via higher excited states

The photophysical properties of tetra-(tert-butyl)-phthalocyanato-magnesium (t₄-PcMg) in solution and microheterogeneous systems (liposomes and micelles) were investigated. Radical cation formation occurs in chloroform during UV excitation in the presence of an electron acceptor (CBr₄). The same result is achieved by two-step absorption in the singlet manifold using pulsed excitation at λ_exc=670 nm, which is of interest from the viewpoint of photon delivery through the therapeutic window of tissues. To obtain a deeper insight into the photophysics leading to radical cation formation via the higher excited singlet state, the transient spectra and singlet—singlet absorption cross-sections were determined. In addition to strong excited state absorption within the spectral range of the Q_x-band, relatively large absorption cross-sections were also found in regions with low ground state absorption. The importance of these transitions for an effective two-colour excitation regime is discussed with regard to new start mechanisms for photodynamic laser tumour theraphy.     

On the nature of the first excited states of the uranyl ion

Results of calculations on the uranylion using the LCAO MO Hartree—Fock—Slater method including relativistic effects are reported. The highest occupied molecular orbital is calculated to be σ_u, consisting predominantly of U 5f character. The σ_u orbital is the HOMO partly because of “pushing-from-below” by the U 6p orbital, but also as a result of the change in potential of the U 5f electrons with the uranium core elections brought about by relativistic contraction of the core electrons. This effect also determines the character of the first virtual levels (δ_u and Φ_u, respectively) in equatorial ligand fields.     

High-level ab initio studies of the electronic excited states of the hydroxyl radical and water-hydroxyl complex

The lowest-energy electronic transitions in the hydroxyl radical and the hydrogen bound complex H(2)O.HO have been studied using ab initio methods. We have used the complete active-space self-consistent field and multireference configuration interaction (MRCI) methods to calculate vertical excitation energies and oscillator strengths. At the MRCI level the lowest-lying (2)Sigma(+)<--(2)Pi electronic transition is redshifted by about 2500 cm(-1) upon formation of the H(2)O.HO complex. We propose that this transition could be used to identify the complex in the gas phase, which in turn could be used to examine the role of H(2)O.HO in atmospheric reactions.