A THEORETICAL TREATMENT OF THE ELECTRONIC STATES OF THIONINE AND RELATED MOLECULES

Abstract— –Problems associated with the protolytic equilibria of thionine and related molecules in their lowest excited electronic states were investigated. The theoretical arguments are based on semi-empirical SCF MO (CI) calculations for the π-electronic system of these molecules; all singly excited configurations were included in the CI. The results indicate that the basic form of thionine in its ground, first excited singlet and lowest triplet state is protonated at the heterocyclic N atom. The difference of the p K values of these three states can be explained in terms of the calculated charge densities. The photochemical reactivity of the lowest triplet of the acidic form of thionine (³TH₂²⁺) differs greatly from that of the lowest triplet of the basic form (³TH⁺). Some arguments for the assignment of nπ* character to ³TH₂²⁺ and ππ* character to ³TH⁺ are advanced.     

π-Electron ring currents and magnetic properties of porphyrin molecules in the MO LCAO SCF method

The coupled variant of double-parameter perturbation theory in the MO LCAO SCF method in the London approximation has been used for the calculation of π-electron current distributions in the molecules of porphin and its derivatives. The chemical shifts of¹H-NMR have been computed on the basis of calculations of ring currents and charge distributions. It is shown that π-electron ring currents are responsible for the dominant contribution to the shielding of protons. The theoretical and experimental values of proton chemical shifts are in a good agreement. Chemical shifts of the¹³C and¹⁵N nuclei have also been estimated. Two aromaticity scales are proposed for the compounds under study based on the calculations of the π-electron contribution to the diamagnetic susceptibility and of π-electron currents, respectively.     

Struktur und photochemische Reaktivität: Photohydrolyse von Trifluormethylsubstituierten Phenolen und Naphtholen

Irradiation of trifluoromethylphenols (three isomers) and -naphthols (eight isomers) in water solution yields the corresponding hydroxybenzoic and naphthoic acids, respectively. The reaction mechanism has been investigated by means of flash photolysis and quenching experiments. It is proposed that the photohydrolysis is initiated in the lowest excited singlet state by a heterolytic C? F bond cleavage. The observed photochemical and thermal reaction rates as well as the shifts of the lowest electronic transitions upon CF₃-substitution compare well with predictions from semiempirical π-electron calculations (PPP SCF CI). A rationale for the correlation between excited state charge densities and photochemical reactivity is given. pK values for the ground state and first excited singlet state are reported.     

SCF?CI and SCF open-shell studies of the base components of the nucleic acids

The π-electron structure of adenine, guanine, cytosine, thymine and uracil in their ground, ionized, singlet and triplet excited states are investigated by means of the SCF ? CI and SCF open-shell methods. The calculations for singlets fit the maxima of the absorption bands well. The energy difference between the first and the second singlet states of adenine is found to be very small. The open-shell method leads to the same relative ionization potential as does the SCF (with the integrals empirically corrected). The calculated energies of the triplet states almost coincide in the SCF open-shell and the SCF ? CI approximation. The calculated transition energies to the first triplet state of the pyrimidines are higher than in the case of the purines. The value of the singlet–triplet separation energy of purines is in agreement with experimental data.     

x-RAY PHOTOELECTRON VALENCE BAND STUDIES ON FIREFLY D-(-)-LUCIFERIN

Abstract— The valence band spectrum of firefly D-(-)-luciferin (Ln) was measured for the first time by x-ray photoelectron spectroscopy (XPS). Since the conversion of Ln to decarboxydehydroluciferin (dcrL) by x-ray irradiation occurred even within ∼30 min, the effect of the irradiation on the valence band spectrum of Ln was investigated quantitatively. We found that the XPS valence band spectrum of Ln during irradiation for 3 h corresponds to the state density of Ln calculated by the LCAO-ASMO-SCF-CI method for π-electrons. Moreover, the weak coupling model for π-electron transfer between Ln and oxygen was proposed for the primary electronic process in Ln chemiluminescence, assuming that the calculated π-electron state densities of Ln in acidic and basic solutions reflect the characteristics of their XPS valence band spectra.     

Chemically Initiated Electron Exchange Luminescence of Silyloxyaryl-Substituted Spiroadamantyl Dioxetanes: Kinetics and Excited State Yields

Abstract— The kinetics of the fluoride-induced decomposition of the thermally stable silyloxyaryl-substituted spiroadamantyl dioxetanes 1a,b and the excited state formation of this chemically initiated electron exchange luminescence (CIEEL) have been investigated. Two limiting kinetic regimes flash and glow have been identified, which depend on the fluoride concentration, the first at high, the second at low [F^-] triggering, whose detailed kinetic analysis affords the rate constants for the deprotected dioxetanes 2a,b cleavage in acetonitrile and dimethyl sulfoxide and chemiluminescence measurements the CIEEL and phen-olate 4 (CIEEL emitter) excitation yields. Chloro-substi-tution in the spiroadamantyl dioxetane does not affect the deprotection step k ₂ but leads to a ca five-fold faster cleavage of the deprotected dioxetane 2, while the chemiexcitation yield is the same for both dioxetanes. The energies of the first excited singlet and triplet states of the emitting phenolate 4 were estimated by AM1 configuration interaction calculations with explicit consideration of acetonitrile as solvent (self-consistent reaction field approach). The first excited singlet and triplet state of the CIEEL emitter phenolate 4 possess π,π* character, as suggested by the π-type molecular orbitals and the large singlet-triplet energy gap. The chemiexcitation of both singlet and triplet states of the excited phenolate 4 is feasible during the dioxetanes 1a,b cleavage, but the experimentally determined high singlet excitation yields suggest that preferentially the phenolate 4 singlet state is populated in the fluoride ion-triggered CIEEL process.     

An attempt to approach the solutions of the N-electron CNDO hamiltonian for conformational studies of conjugated systems

While dealing with the same N-electron hamiltonian, the SCF approximation and the third order PCILO results completely disagree about the conformation of conjugated systems. An attempt is made, using the CIPSI algorithm, to approach the exact solution of the N-electron CNDO hamiltonian for the butadiene molecule starting from either a priori localized, SCF localized or SCF delocalized MO's. The partial CI's performed from SCF delocalized MO's give arbitrary results when increasing the number of doubly excited determinants and Pancir?'s recent results is fortuitous; on the contrary the results obtained from a priori and SCF localized MO's have better convergence and consistence; from our best calculations the final solution seems to be a rather flat potential curve, the stable minimum being the trans planar minimum, with a second gauche minimum for θ = 120-150° (ΔH ≈ 1 kcal/mole), separated by a weak barrier (ΔH ≈ 2 kcal/mole). The third order PCILO solution is in much better agreement with this estimate of the exact solution than the SCF one.     

LUMINESCENCE SPECTRA AND PHOTOCYCLOADDITION OF THE EXCITED COUMARINS TO DNA BASES

Abstract— The lowest excited singlet and triplet states of coumarin, psoralen, and 4-hydroxy-coumarin have been assigned to the (π,π * ) type on the basis of the luminescence spectroscopy and MO calculations. The mechanism of photocycloaddition of courmarin and psoralen to thymine has been described in terms of the perturbational MO model and MO reactivity indices. All possible cycloaddition patterns have been examined. Results suggest that the 3,4-bond of coumarin in the excited state is somewhat more reactive than the same bond of psoralen in the excited state. It is also predicted that the 3,4-bond of psoralen in the triplet state is more reactive than the 4', 5'-bond. The results have been favorably correlated with the electronic characteristics of excited coumarin molecules and with available experimental data on the relative yields of photoadducts.     

An ab initio study of the ground and excited states of p-benzoquinone

The results of anab initio SCF calculation for the ground state and CI calculations for the excited states of p-benzoquinone are presented and discussed. A minimum basis set of Slater type orbitals was employed and the CI calculations were performed by considering single excitations from valence to virtual SCF molecular orbitals. The convergence of the calculated excitation energies is studied as a function of the number of orbitals used in the CI calculations. These calculations explain quite well the experimental results.     

Effect of methyl and hydroxyl substituents on the π-electronic spectra and ionization potential of p-benzoquinone

SCF -LCAO -MO -CI semi-empirical π-electron calculations have been made of the first two singlet transitions and ionization potentials of hydroxy-, methyl- and hydroxy-methyl substituted p-benzoquinone, using a modified Pariser-Parr-Pople method. The various molecules could be divided into two classes on the basis of their first π-electron transition, namely, those in which the 3-position was occupied and others where this position was vacant.     

Structures and spectra of Na(NH3) n=1,2

Geometric structures and the energies for the ground and several excited electronic states of a sodium atom bound with one or two ammonia molecules are presented. All self consistent field (SCF) calculations are performed with extended basis sets. Geometry optimization and one electron properties have been performed within the SCF approximation. Excited states have been calculated with the multi-configuration SCF (MCSCF) technique. This system may be viewed as a precursor to solvation in a macroscopic system. The excited state calculations provide important information for spectroscopic studies of these complexes.     

Ab initio calculations on the three lowest states of HO+2

Ab initio calculations were performed for the three lowest lying states of HO⁺₂. The ground state was found to be a bend ³A″ state. The first excited ¹A′ state cannot appropriately be described by a single determinant, therefore a MC SCF calculation was employed.     

The delocalisation energy of benzene and the non-empirical MO theory

Algebraic expressions for the vertical Delocalisation Energy (DE) of benzene are derived from non-empirical MO theory. For comparison with early work in the π-electron approximation, and ultimately with Hückel theory, the results are formulated in terms of a core resonance integral,β, and π-electronic repulsion integrals. All integral values are inferred from the results ofab initio SCF calculations. Two expressions are derived, which refer to two ways of forming the localised π MOs: one where three pairs of adjacent atomic orbitals are selected from a set of six orthogonalised orbitals; and another where a non-orthogonal set of atomic orbitals is used. The first expression is formally similar to an expression originally derived by Pople from a different point of view and with many approximations. This expression gives too large a magnitude for DE when used with anab initio value ofβ. The second expression gives a result much closer to an empirical value of DE and shows that the main reason for DE being about 50% of 2β rather than 2β is the stabilising effect of overlap in the localised structure, and that the less important factor is the inclusion of electronic repulsion.     

PHOTO-DIMERIZATION AND PHTO-REDUCTION OF ALPHA-NAPTHOQUINONE IN DIFFERENT SOVENT MEDIA

Abstract -Violet and ultravioet radiation are equally efficient in phot-dimerizing aplhanathoquinone in benzene. The reactants yielding the dimeric photo-product are moelcules in the ground state and excited ones. The photo-reduction in 2-propanol occurs with the same effciency for excitation in the n→* and →π* absorption bands. It is concluded that either reaction occurs via an n,π* excited state molecule, by direct excitation or after 100 per cent effucient internal conversion from the π,π* to the n,π* state.     

Assignment of electronic transitions by geometry optimization

Adiabatic excitation energies, excited state geometries, excited state charges, bond orders and dipole moments have been obtained for HCN, CO₂,H₂CO, HFCO, F₂CO, ethylene, trans-butadiene, furan, pyrrole and uracil using the SINDO1 semi-empirical method with configuration interaction. Our results generally agree with those ofab initio calculations and experiment satisfactorily. Geometry optimization is found to mix configurations differing in their allowedness in vertical excitation from the ground state, which in turn helps in the assignment of spectral transitions. TheV excited singlet state of trans-butadiene and various excited states of furan, pyrrole and uracil have been found to be appreciably non-planar. The single and double CC bonds are found to exchange positions due to the lowest triplet and singlet transitions of furan and pyrrole. The first triplet and first singlet transitions of uracil have been found to be of π-π* and π-σ* types respectively in agreement with recent experimental findings. On leave of absence from the Department of Physics, Banaras Hindu University, Varanasi-221005, India     

ABSORPTION CHANGES IN THE REACTION CENTER OF PHOTOSYNTHETIC BACTERIA AND n-ELECTRON CALCULATIONS ON BACTERIOCHLOROPHYLL, ITS MONO CATION AND ANION

Abstract— π-Electron calculations of the MO-SCF-CI type in the PPP approximations are carried out for the tetra pyrrole ring of the bacteriochlorophyll molecule, its mono cation and anion.
According to the calculations the formation of the positive ion will be accompanied by the disappearance of the longest wavelength absorption band of the neutral system and the appearance of a new band near 1200 nm. Absorption changes of the same kind are found in the reaction center of photosynthetic bacteria upon illumination, supporting the hypothesis that the primary reaction is an expulsion of an electron from the π-electron system of the reaction center bacteriochlorophyll.     

Molecular geometries from spin Hamiltonian calculations through simultaneous optimization of geometry and wave function

The recently developed spin Hamiltonian approach to conjugated-electron molecules is reexamined. A simultaneous optimization of the geometry and wave functions, achieved by the use of the conjugate gradient method, facilitates calculations of the molecular geometries in the ground and excited electronic states. The computation time increases approximately linearly with the number of basis functions, making calculations for molecules having up to 18 carbon atoms (48 620 basis functions) readily available. Geometries of several benzenoid hydrocarbons are optimized and the results are discussed.     

Theoretical study on the vertical electronic spectrum of carbonyl fluoride,F2CO

Ab initio self-consistent-field (SCF ) and configuration interaction (CI ) calculations on the ground and excited states of carbonyl fluoride (F₂CO) were carried out at its experimental ground-state equilibrium geometry. Vertical transition energies deduced from the CI results provide assignments for the electronic systems I–IV, experimentally observed by Workman and Duncan. The singlet excited state, ¹A₁ (π→π*), is found to be a mixed valence–Rydberg state and to he 1 to 1.2 eV above the suggested experimental value, irrespective of the choice of the basis used for the CI calculations.     

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.     

Study of the electronic structure and the spectra of methyl-substituted carbanions of nitrogen heterocycles by the selfconsistent field MO LCAO method

The -electron density distribution in the ground state and the energies of the spectral transitions were calculated by a semi-empirical version of the Pople-Pariser-Parr method for a number of carbanions derived from the removal of a proton from the methyl groups of toluene, isomeric methylnaphthalenes, methylpyridines, and benzoquinones. The results obtained are compared with the results obtained in a Hückel calculation of the same anions and with experimental data from the electronic spectra of methyl-substituted nitrogen heterocycles in a solution of potassium amide in liquid ammonia.The calculations reflect satisfactorily the characteristic features of the observed spectra. The effect of the choice of parameters for the self-consistent calculation is discussed and the contribution of singly or doubly excited configurations is explained.