Molecular geometry and excited electronic states : Part VII. Theoretical study on conformational geometries and the dual fluorescence of styrene

Equilibrium molecular geometries of styrene in the electronic S₀, S₁, and S₂, states have been calculated within the scope of the adiabatic approximation by minimization of the total energy with respect to all molecular coordinates. Besides the most stable planar structures, some further conformers twisted about the single and/or double bonds have been found. An interpretation of the styrene dual fluorescence is given using these results.     

Molecular geometry and excited electronic states: Part IV. Theoretical study on molecular geometries and spectral properties of bipyrimidine compounds

The equilibrium geometry of two bipyrimidine photoproducts of nucleic acids in the electronic ground and first excited singlet states has been calculated by minimization of the total energy with respect to all molecular coordinates. Using these results and the method of configuration analysis, this paper discusses whether or not the model of individual chromophores can be invoked to rationalize the spectral properties.     

Coincidence studies of fluorescence and dissociation processes in electronic excited states of I2+, Br2+, IBr+ and ICl+

The PIFCO technique in which mass-selected photoion—fluorescence photon coincidences are counted, was used to investigate whether I₂⁺, IBr⁺ and ICl⁺ fluoresce. Measurements were made of lifetimes and fluorescence quantum yields of electronic excited states of these ions. Emission was discovered for I₂⁺ and IBr⁺, but ICl⁺ apparently does not fluoresce. Information on the radiative properties of Br₂⁺ was obtained as a by-product of the work on IBr⁺. Fragment ion kinetic energy releases were determined and provide information on dissociative ionization processes in the halogen and interhalogen ions studied.     

Molecular geometry and excited electronic states : Part XIII. Conformational geometries of 9,10-diphenyl anthracene in different electronic states and its electronic spectral behaviour

The conformational geometries of 9,10-diphenyl anthracene in the S₀, S₁, and T₁ electronic states determined by means of the Warshel—Karplus method are presented. Based on the theoretical equilibrium geometries of the con- and dis-twisted conformations the calculated electronic transition energies and oscillator strengths are used for an interpretation of the electronic spectral behaviour in absorption and fluorescence. The corresponding theory-experiment comparison performed is satisfactory.     

The first two excited 1Σg+ states of Cs2

Laser-induced fluorescence Of Cs₂ molecules in the infrared region (4000–9000 cm^?1) has been observed using several exciting wavelengths from an argon-ion laser and from a ring dye laser. Accurate molecular constants for the first two excited ¹Σ_g⁺ electronic states are derived from spectra recorded at high resolution by Fourier transform spectroscopy. Main molecular constants are: (2)¹Σ_g⁺: T_c = 12114.090 cm^?1, ω_e = 23.350 cm^?1, B_c = 7.4.5 × 10^?3 cm^?1, R_c = 5.8316 Å; (3)¹Σ_g⁺: T_e = 15975.450 cm^?1, ω_e = 22.423 cm^?1 , B_e = 8.23 × 10^?3 cm^?1, R_c = 5.5569 Å.     

Nanoseconds UV laser study of radiationless deactivation from upper electronic excited states in solution: Tb3+

Using 25 ns half width pulses of a frequency-quadrupled Nd laser at 265 nm Tb³⁺ perchlorate in D₂O solution, or in borate glass, was excited to high electronic states and the fluorescence on transition from the upper ⁵D₃ and lower ⁵D₄ excited states to various sub-levels of the lowest ⁷F state were measured. The appropriate energy gap spacing of this rare earth ion results in lifetimes which enabled us to observe the decay of ³D₃ as well as the growing in and decay of ⁵D₄. Radiationless processes from ⁵D₃ direct to ⁷F or cascading to ⁵D₄ are discussed and correlated with previous theoretical and experimental results on related systems involving the interaction of the non-complexed solvated ion with the medium, and compared with related phenomena in glassy solids.     

Dispersed fluorescence spectroscopy and fluorescence lifetime measurements of excited vibrational levels of CdCH3

Laser-induced excitation and dispersed fluorescence spectra (originating from the 0⁰, 2¹ and 3¹ vibrational levels of the electronic state) of jet-cooled CdCH₃ are presented and assigned. The observation of emission from these levels is a result having important implications for the dynamics of photodissociation of Cd(CH₃)₂. Fluorescence lifetimes for emission from the 0⁰, 2¹ and 3¹ vibrational levels are also presented.     

Laser excited fluorescence of the Π vibronic states of BO2

The Π vibronic states of the $\text{X}$²Π_g electronic state of BO₂ have been studied by laser induced fluorescence. The (0, 0, 0) Π — (1, 0, 0) Π and the (1, 0, 0) Π — (2, 0, 0) Π vibronic bands were pumped with a cw dye laser. The results were used to obtain a new set of parameters for the $\text{X}$²Π_g electronic state. Also, the effect of the Fermi interaction on the (1, 0, 0), (2, 0, 0) and (3, 0, 0) fermiads was investigated.     

Theoretical study on geometry and spectroscopic properties of 1,1′-binaphthyl in the electronic ground and first excited singlet states

Equilibrium geometries for the electronic ground and first excited singlet states of 1,1'-binaphthyl have been calculated by minimization of the total energy with respect to all internal coordinates. Using these results, an interpretation of the fluorescence S₁→ S₀ and absorption spectra S_m ← S₀ and S_n ← S₁ in rigid and fluid solutions is given.For the first time the equilibrium geometry of the first excited singlet state of 1, 1′-binaphthyl has been calculated. On excitation to the S₁ state the dihedral angle θ between the two naphthalene moieties is de- creased from 61 ° to 41 °. A detailed survey of CH bond lengths in the S₀ and S₁ states has been given. This result should be of particular importance for the theoretical treatment of radiationless transitions.Using equilibrium geometries for the S₀ and S₁ states a satisfactory interpretation of the S_m ← S₀ and S_n ← S₁ absorption spectra as well as of the fluorescence spectra in fluid and rigid solutions can be given. Concerning the S_n ← S₁ absorption spectrum in fluid solution, the calculations predict a strong absorption (A ← B transition) in the still uninvestigated region of energies lower than 11000 cm^?1.From the results of this paper and of other calculations it can be concluded that the Warshel-Karplus method yields reliable equilibrium geometries for electronic ground and excited states of unsaturated hydrocarbons [22,23].     

Molecular electric properties in electronic excited states: multipole moments and polarizabilities of H2O in the lowest1 B 1 and3 B 1 excited states

Summary The dipole and quadrupole moments and the dipole polarizability tensor components are calculated for the¹ B ₁ and³ B ₁ excited states of the water molecule by using the complete active space (CAS) SCF method and an extended basis set of atomic natural orbitals. The dipole moment in the lowest¹ B ₁ (0.640 a.u.) and³ B ₁ (0.416 a.u.) states is found to be antiparallel to that in the ground electronic state of H₂O. The shape of the quadrupole moment ellipsoid is significantly modified by the electronic excitation to both states investigated in this paper. All components of the excited state dipole polarizability tensor increase by about an order of magnitude compared to their values in the ground electronic state. The present results are used to discuss some aspects of intermolecular interactions involving molecules in their excited electronic states.     

AB initio study of the HN2+ molecule ion and its dissociation products in ground and excited states

Non-empirical SCF and Cl calculations for the HN₂⁺ molecule ion are reported. Potential curves for ground and excited states of this system are obtained, with particular emphasis on the manner in which these species dissociate upon NH stretch. The electronic spectrum of HN₂⁺ is also calculated and compared with that of an unperturbed N₂ molecule.     

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.     

Laser-induced fluorescence in supersonic nozzle beams predissociation in the Rb2 C1πu and D 1πu states

By exciting Rb₂ in a supersonic nozzle beam with a pulsed dye laser in the C ¹Π_u-X ¹Σ⁺_g and the D ¹Π_u-X ¹Σ⁺_g band system, we find evidence tor different predissociation processes The products appear as follows from the C state, Rb^* (5 ²P_{$\text{3}\text{2}$}) exclusively, and from the D state Rb*(4²D_{$\text{3}\text{2}$}) predominantly, followcd by Rb^*(5 ²P_i-5²S) cascade radiation In addition, a lower bound of D_e(Rb₂X¹Σ⁺_g)? 3939± 10 cm^?1 is obtained.     

Quantum chemical study of the electronic structure of NaX and NaY zeolites1

The CNDO/2 method is employed to determine S_III type positions for the location of Na cations in the faujasite zeolites. The minimum energy path of Na migration between these positions exhibiting an energy banier of 20 kJ/mol has been calculated.     

Molecular-beam-laser studies of the states X1Σ+ and A0+ of PbS

By using a molecular-beam apparatus, laser excitation spectra and microwave-optical double-resonance spectra were observed on the electronic states X¹Σ⁺ and A0⁺ of PbS. For the microwave spectra a resolution up to 10 kHz was achieved and microwave two-photon transitions could be detected. All information was combined into single fitting routine including data from other sources. Besides the usual Dunham parameters the fit model takes account of the breakdown of the Born-Oppenheimer approximation and the isotopic field shift. From the field shift of the electronic energy, the vibrational motion and the rotational motion the electron density as a function of the internuclear distance at the Pb nucleus is derived, which allows a more detailed discussion of the electronic structure of the molecule in combination with future quantum-chemical calculations. Additionally it is shown that the variation of the electron density along the vibrational motion at low vibrational levels can be as large as the variation for the electronic transition A0⁺ -X¹Σ⁺ in the case of PbS. The magnetic hyperfine structure of ²⁰⁷Pb is resolved in a molecule for the first time. The coupling parameter of PbS is compared with that of the isovalent molecule TlCl.     

Selected fluorescence and dissociation lifetimes on N2O+ in its Ã(2Σ+),000 and 100 states

Radiative and radiationless lifetimes together with the quantum yield towards fluorescence of individual vibronic states of molecular ions can be determined by threshold electron—fluorescence photon coincidence measurements. This method is here applied to the X?²Π_{$\text{3}\text{2,1,2}$} and òΣ⁺ states of N₂O⁺ which were also studied by high resolution photoionization resonance spectroscopy.     

The fragmentation of formaldehyde molecular cations: the lifetime of CD2O+(Ã 2B1)

He Iα photoelectron-photoion coincidence spectroscopy was employed to identify CD₂O⁺(Ã ²B₁) as precursor of the unusual metastable signal in the mass spectrum of dideuteroformaldehyde. The lifetimes of this electronically excited state of the molecular cation are reported.     

Potential-energy curves of some electronic states of Na2 and Na2+

Two different compounds were simultaneously adsorbed on a silver colloid, and Raman spectra were taken. One, which does not fulfil the resonance condition (in solution), showed the usual large (SERS) enhancement. The other compound, which (in solution) absorbs the light used, showed no net enhancement compared to the first. The extent of surface electric field amplification is discussed.     

Calculations on the electronic excited states of ureas and oligoureas

We report CASPT2 calculations on the electronic excited states of several ureas. For monoureas, we find an electric dipole forbidden n --> pi* transition between 180 and 210 nm, dependent on the geometry and substituents of the urea. We find two intense pinb --> pi* transitions between 150 and 210 nm, which account for the absorptions seen in the experimental spectra. The n' --> pi* and pib --> pi* transitions are at wavelengths below 125 nm, which is below the lower limit of the experimental spectra. Parameter sets modeling the charge densities of the electronic transitions have been derived and permit calculations on larger oligoureas, using the exciton matrix method. For glycouril, a urea dimer, both the CASPT2 method and the matrix method yield similar results. Calculations of the electronic circular dichroism spectrum of an oligourea containing eight urea groups indicate that the experimental spectrum cannot be reproduced without the inclusion of electronic excitations involving the side chains. These calculations are one of the first attempts to understand the relationship between the structure and excited states of this class of macromolecule.