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.     

Vibronic coupling in the benzyl radical

The vibronic perturbations among the lowest excited states of benzyl, computed by a CNDO/S program in the floating-orbital scheme, are presented. The ν_8b (ν_18b) modes are the vibrations that most strongly (weakly) couple the quasidegenerate 1A₂ and 2B₂ states. The results are relevant to the interpretation of benzyl emission and absorption spectra.     

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.     

Ultraviolet photodissociation dynamics of the benzyl radical

Ultraviolet (UV) photodissociation dynamics of jet-cooled benzyl radical via the 4(2)B(2) electronically excited state is studied in the photolysis wavelength region of 228 to 270 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance enhanced multiphoton ionization (REMPI) techniques. In this wavelength region, H-atom photofragment yield (PFY) spectra are obtained using ethylbenzene and benzyl chloride as the precursors of benzyl radical, and they have a broad peak centered around 254 nm and are in a good agreement with the previous UV absorption spectra of benzyl. The H + C(7)H(6) product translational energy distributions, P(E(T))s, are derived from the H-atom TOF spectra. The P(E(T)) distributions peak near 5.5 kcal mol(-1), and the fraction of average translational energy in the total excess energy, , is ～0.3. The P(E(T))s indicate the production of fulvenallene + H, which was suggested by recent theoretical studies. The H-atom product angular distribution is isotropic, with the anisotropy parameter β ≈ 0. The H/D product ratios from isotope labeling studies using C(6)H(5)CD(2) and C(6)D(5)CH(2) are reasonably close to the statistical H/D ratios, suggesting that the H/D atoms are scrambled in the photodissociation of benzyl. The dissociation mechanism is consistent with internal conversion of the electronically excited benzyl followed by unimolecular decomposition of the hot benzyl radical on the ground state.     

The flourescence lifetime of the benzyl radical

Time resolved fluorescence of the benzyl, monomethylbenzyl and dimethylbenzyl radicals strapped in rigid solvents at low temperatures has been observed using the second harmonic of the ruby laser at the exciting source. The fluorescence lifetimes of these radicals are very long (10^?7–10^?6 sec), which are influenced considerably by the methyl substituents. The long fluorescence lifetimes of the benzyl radical and its methyl derivatives are interpreted in terms of the forbidden character of the first doublet-doublet electronic transition.     

Full configuration interaction for the benzyl radical

The electronic structure of the benzyl radical in its ground state has been computed using a model Hamiltonian due to Pariser–Parr with full configuration interaction as well as with different truncated configurational sets built on SCF open-shell orbitals. The correlation energy corresponding to this model was found to be equal to –0.929722 eV. With the singly excited configurations only 18% of this energy is taken into account. By extending the basis to include the doubly excited configurations one can account for 94% of the correlation energy. An analysis of the accuracy of the proton hyperfine splitting calculation caused by inaccurate computation of the wave function is given. If only singly and even doubly excited configurations are taken into account one cannot hope to obtain splittings with an accuracy of more than 0.5 g. Inclusion of triply excited configurations lowers this error by one order. In addition, the use of the simple McConnell relation may lead to an error in splitting calculations of no less than 1.5 g.     

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.     

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.     

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.     

Excited states in electron-transfer reaction products: ultrafast relaxation dynamics of an isolated acceptor radical anion

The spectroscopy and ultrafast relaxation dynamics of excited states of the radical anion of a representative charge-transfer acceptor molecule, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, have been studied in the gas phase using time-resolved photoelectron spectroscopy. The photoelectron spectra reveal that at least two anion excited states are bound. Time-resolved studies show that both excited states are very short-lived and internally convert to the anion ground state, with the lower energy state relaxing within 200 fs and a near-threshold valence-excited state relaxing on a 60 fs time scale. These excited states, and in particular the valence-excited state, present efficient pathways for electron-transfer reactions in the highly exergonic inverted region which commonly displays rates exceeding predictions from electron-transfer theory.     

Cluster expansion of the wavefunction. Excited states

A method for excited states is given on the basis of the symmetry-adapted-cluster (SAC) expansion method. It is based on the fact that the SAC expansion method gives incidentally a set of excited functions which satisfies the Brillouin theorem with the ground state.     

Kinetics of nanoconfined benzyl methacrylate radical polymerization

The effect of nanoconfinement on the kinetics of benzyl methacrylate radical polymerization is investigated using differential scanning calorimetry. Controlled pore glass (CPG), ordered mesoporous carbons, and mesoporous silica are used as confinement media with pore sizes from 2 to 8 nm. The initial polymerization rate in CPG and mesoporous silica increases relative to the bulk and increases linearly with reciprocal pore size; whereas, the rate in the carbon mesopores decreases linearly with reciprocal pore size; the changes are consistent with the rate being related to the ratio of the pore surface area to pore volume. Induction times are longer for nanoconfined polymerizations, and in the case of CPG and carbon mesopores, autoacceleration occurs earlier, presumably due to the limited diffusivity and lower termination rates for the confined polymer chains. The molecular weight of the polymer synthesized in the nanopores is generally higher than that obtained in the bulk except at the lowest temperatures investigated. The equilibrium conversion under nanoconfinement decreases with decreasing temperature and with confinement size, exhibiting what appears to be a floor temperature at low temperatures.     

Electronic structure of the ground state of the benzyl radical

Results from the -electron approximation are given for the total energy, spin-density distribution, and electron density, which have been derived by configuration interaction via self-consistent orbitals for closed and open shells and with allowance for all singly excited configurations and some doubly excited ones. Some singly excited configurations do not mix with the ground-state configuration in the first order of perturbation theory but make a contribution to the latter greater than do some of the configurations that do mix. Incorporation of all singly excited configurations results in a lower ground-state energy if orbitals for a closed shell are used instead of those for an open one. Calculations via closed-shell orbitals lead to an inhomogeneous electron-density distribution, which is gradually smoothed out as the set of configurations is expanded. The spin-density distribution is very much dependent on the number of configurations used and on the orbitals employed in them.     

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.     

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.