Determination of the excited-state structure of 7-azaindole-water cluster using a Franck-Condon analysis

The change of the 7-azaindole-water cluster structure upon electronic excitation was determined by a Franck-Condon analysis of the intensities in the fluorescence emission spectra obtained via excitation of five different vibronic bands. A total of 105 emission band intensities were fitted, together with the changes of rotational constants of one isotopomer. These rotational constants have been obtained from a fit to the rovibronic contour of the cluster. The geometry change upon electronic excitation to the pi pi* state can be described by a strong and asymmetric shortening of the hydrogen bonds and a deformation of both the pyridine and the pyrrole rings of 7-azaindole. The resulting geometry changes are interpreted on the basis of ab initio calculations.     

Photoelectron spectra and electronic structure of some 4-substituted 2-allylanisoles

The electronic structures of 4-substituted 2-allyl anisoles (1-9) have been investigated by ultraviolet photoelectron spectroscopy and quantum chemical methods. The ionisation potentials corresponding to the pi MOs pi(2) and pi(3) of the phenyl ring, and the n(O) orbital of the methoxy group as well as the allylic pi(C=C) orbital could be determined and assigned for 1-9. Linear regression analyses of the IPs related to these orbitals with different substituent constants indicated that Hammett sigma(p) values performed satisfactorily to fair for pi(2), pi(3) and n(O) but poor for allylic pi(C=C). Other substituent constants such as R and R(-) were fair only for pi(2) and pi(3), but poor for n(O) and pi(C=C).     

The kinetics of the reactions of C2 (a 3pi(u)) with alcohols

The reactions of C2 (a 3pi(u)) radicals with a series of alcohols have been studied at about 6.5 Torr total pressure and room temperature using the pulsed laser photolysis/laser-induced fluorescence technique. The relative concentration of C2 (a 3pi(u)) radicals, which are generated via the photolysis of C2Cl4 with the focused output from the fourth harmonic of a Nd:YAG laser (266 nm), was monitored by laser-induced fluorescence (LIF) in the (0, 0) band of the C2 (d 3pi(g)<--a 3pi(u)) transition at 516.5 nm. Under pseudo-first-order conditions, we measured the time evolution of C2 (a 3pi(u)) and determined the rate constants for reactions of C2 (a 3pi(u)) with alcohols. The rate constants increase linearly with the number of C atoms in the alcohols. All of them are larger than those for reactions of C2 (a 3pi(u)) with alkanes (C1-C5). Based on the bond dissociation energy and linear free energy correlations, we believe the reactions of C2 (a 3pi(u)) with alcohols proceed via the mechanism of hydrogen abstraction. The experimental results show that the H-atom on the C-H bonds is activated at the presence of the OH substituent group in the alcohol molecule. The theoretical calculations for the reaction of C2 (a 3pi(u)) with methanol also support these hypotheses.     

Vibrational mode selectivity in hyperfine interactions: polarization quantum beat spectroscopy of HCF(A1A")

We report on the vibrational mode dependence of the 19F and 1H hyperfine interaction constants in the A1A" state of HCF, determined using polarization quantum beat spectroscopy. The nuclear spin/overall rotation coupling constants display a pronounced energy dependence and mode selectivity which can be traced to variations in both the A rotational constant and nuclear spin/electron orbital coupling constant a. In particular, modes containing C-F stretching excitation display significantly larger 19F spin-rotation constants, which is explained in terms of a decrease in back donation of electron density into the C 2p(pi) orbitals.     

A laser spectroscopic study of the X2pi(g), A2pi(u), and B2sigma+ states of BS2: Renner-Teller, spin-orbit, and K-resonance effects

The lowest-lying vibronic levels of the X, A, and B states of BS2 have been investigated at high resolution using a combination of room-temperature absorption and supersonic jet data. In both cases, the BS2 radical was prepared in an electric discharge using a precursor gas mixture of BCl3,CS2, and either helium or argon. Extensive absorption spectra were obtained for the 0(0)0 and 2(1)1 bands of the A2pi(u)-X2pi(g) electronic transition in the visible. The A-X 2(1)1 and B2sigma(u)(+)-X2pi(g) 2(1) bands of jet-cooled BS2 were also studied with laser-induced fluorescence techniques. By fitting the 0(0) bands of both electronic transitions simultaneously, we were able to precisely determine the spin-orbit splittings in both the A and X states. Similarly, the 21 bands were fitted in a merged analysis in order to determine the relative separations of the vibronic components of the ground and first excited state bending levels as accurately as possible. Due to a large spin-orbit splitting and small Renner-Teller interaction, the A state bending level shows small but definite K-resonance effects, which were fitted using a full matrix for the four components of upsilon2' = 1. The resulting parameters were used along with previously published data to refine the Renner-Teller analyses in both the A2pi(u), and X2pi(g) electronic states. Where possible, the fitted constants and observed boron isotope splittings have been shown to be in accord with theoretical estimates of their sign and magnitude.     

Intra- and intermolecular electronic relaxation of the second excited singlet and the lowest excited triplet states of 1,3-dimethyl-4-thiouracil in solution

Intramolecular processes of deactivation of 1,3-dimethyl-4-thiouracil (DMTU) from the second excited singlet (S2) (pi, pi*) and the lowest excited triplet (T1) (pi, pi*) states have been studied using perfluoro-1,3-dimethylcyclohexane (PFDMCH) as a solvent. The spectral and photophysical (PP) properties of DMTU in CCl4, hexane and water have also been described. For the first time, the fluorescence from S2 state DMTU has been observed. The picosecond lifetime of DMTU in the S2 state (tau(S2)) in PFDMCH has been proposed to be determined by a very fast intramolecular reversible process of hydrogen abstraction from the ortho methyl group by the thiocarbonyl group. The shortening of tau(S2) in CCl4 is interpreted to be caused by the intermolecular interactions between DMTU (S2) and the solvent. Results of the phosphorescence decay as a function of DMTU concentration were analyzed using the Stern-Volmer formalism, which enabled determination of the intrinsic lifetime of the T1 state (tau0(T1)) and rate constants of self-quenching (k(sq)). The lifetimes, tau0(T1), of DMTU in PFDMCH and CCl4 are much longer than the values hitherto obtained in more reactive solvents. The PP properties of DMTU both in the S2 and T1 states have been shown to be determined by the thiocarbonyl group.     

Ab initio evaluation of intramolecular electron transfer reactions in halobenzenes and stabilized derivatives

The potential energy surfaces for the fragmentation of the radical anions of p-nitrochlorobenzene and p- and m-chloroacetophenones were explored using first principle methods. The behavior of these compounds, stabilized by pi acceptors, is compared to that shown by the unsubstituted halobenzenes (PhX, X = F, Cl, Br, I). The presence of pi and sigma radical anions was inspected as well as the intramolecular electron transfer (intra-ET) from the pi to the sigma surface, responsible for the dissociation of these intermediates. The profiles obtained with the B3LYP functional in the gas phase and in the presence of a polar solvent are in agreement with the spectroscopic evidence and with the experimentally observed reactivity of the compounds under study. The stability of the radical anion of p-nitrochlorobenzene and the adiabatic and endothermic nature of its dissociation are explained. The order of the rate constants for dissociation m-chloroacetophenone < p-chloroacetophenone is interpreted on the basis of the differences in the adiabatic character of the intra-ET of both isomers which is ascribed to the nodal properties of their SOMOs. In the halobenzene family, the electronic factors responsible for the intra-ET are analyzed. The stabilization of the sigma surface exerted by the different halogens and its effect on the rate constants for dissociation are explained.     

Polarizability effects and dispersion interactions in alkene-Br2 pi-complexes

Weakly bound molecular complexes play an important role in chemistry, physics, and biodisciplines. The preequilibrium pi-complexes of various alkenes with bromine have been examined quantitatively, and a direct relationship between association constants (KF) of these pi-complexes and polarizability of the olefins was found. The stability of the Br2-olefin pi complexes is affected by both the donor ionization potential and the polarizability of the olefin, and an equation able to take into account both effects is proposed.     

Substituent effect on exo stereoselectivity in the 1,3-Dipolar cycloaddition reaction of tulipalin A with nitrile ylides

1,3-Dipolar cycloaddition reactions of dihydro-3-methylene-2(3H)-furanone (tulipalin A) with various benzonitrile(p-X-benzylide) ylides prefer formation of exo-cycloadducts in the extent corresponding to an increasing electron donor character of the substituent X in the para-position of the benzylide phenyl ring of the 1,3-dipolar reagent. The substituent effect on diastereoselectivity of the 1,3-DC reaction is rationalized in terms of CH/pi interaction between the dipole and the dipolarophile in an exo-transition state. The determining role of such an interaction is demonstrated by the correlation of the observed diastereoselectivities with substituent Hammett sigma constants, which shows a small negative rho value. A certain contribution of CO/pi interaction between the lactone carbonyl and the substituted phenyl ring to mediation of the substituent effect is also discussed. The energy profiles of both reaction pathways were analyzed using DFT and RI-MP2 theoretical approaches. Calculated energy and structural differences between located transition states are consistent with reaction diastereoselectivities.     

Bimolecular hydrogen abstraction from phenols by aromatic ketone triplets

Absolute rate constants for hydrogen abstraction from 4-methylphenol (para-cresol) by the lowest triplet states of 24 aromatic ketones have been determined in acetonitrile solution at 23 degrees C, and the results combined with previously reported data for roughly a dozen other compounds under identical conditions. The ketones studied include various ring-substituted benzophenones and acetophenones, alpha,alpha,alpha-trifluoroacetophenone and its 4-methoxy analog, 2-benzoylthiophene, 2-acetonaphthone, and various other polycyclic aromatic ketones such as fluorenone, xanthone and thioxanthone, and encompass n,pi*, pi,pi*(CT) and arenoid pi,pi* lowest triplets with (triplet) reduction potentials (E(red)*) varying from about -10 to -38 kcal mol(-1). The 4-methylphenoxyl radical is observed as the product of triplet quenching in almost every case, along with the corresponding hemipinacol radical in most instances. Hammett plots for the acetophenones and benzophenones are quite different, but plots of log k(Q) vs E(red)* reveal a common behavior for most of the compounds studied. The results are consistent with reaction via two mechanisms: a simple electron-transfer mechanism, which applies to the n,pi* triplet ketones and those pi,pi* triplets that possess particularly low reduction potentials, and a coupled electron-/proton-transfer mechanism involving the intermediacy of a hydrogen-bonded exciplex, which applies to the pi,pi* ketone triplets. Ketones with lowest charge-transfer pi,pi* states exhibit rate constants that vary only slightly with triplet reduction potential over the full range investigated; this is due to the compensating effect of substituents on triplet state basicity and reduction potential, which both play a role in quenching by the hydrogen-bonded exciplex mechanism. Ketones with arenoid pi,pi* states exhibit the fall-off in rate constant that is typical of photoinduced electron transfer reactions, but it occurs at a much higher potential than would be normally expected due to the effects of hydrogen-bonding on the rate of electron-transfer within the exciplex.     

Photochemistry of flavothione and hydroxyflavothiones: mechanisms and kinetics

In this work we present a detailed study of the mechanism of photochemistry and thermal reactions, as well as of the kinetics of flavothione (FLT) in ethanol. Furthermore, we analyzed how the hydroxysubstitution pattern of FLT influenced both the kinetics and the mechanism relative to the parent FLT. We show that the primary photochemical reaction of FLT in the absence of oxygen is hydrogen (H)-atom abstraction from the solvent by way of the excited triplet state of FLT. Several products result from thermal reactions of the resulting semireduced FLTH* radical, including more than one dimer. A full mechanism is proposed, and the relevant rate constants are evaluated. On the other hand, in the presence of oxygen and a low concentration of FLT, we found that the principal photoproduct is the parent flavone (FL). The reaction leading to photoxidation is not via 1O2 attacking a thione, but instead, it is via a reaction of the FLTH* radical with ground state oxygen. The kinetic data also demonstrate that the relative values of concentrations of reactants and the rate constants of the reactions can control the dominance of one mechanism over others. We also have examined the photochemical mechanisms and kinetics for several hydroxyflavothiones (n-OHFLT) and compared them with FLT itself. We have found that the photochemical mechanism radically changes depending on the positions of substitution. These differences are directly related to the ordering of the excited states of the n-OHFLT. Specifically, FLT with lowest 3n,pi* states (FLT, 6-hydroxyflavothione, 7-hydroxyflavothione and 7,8-dihydroxyflavothione) efficiently abstract H atoms to give the semireduced radical of the thione. The radical can (1) dimerize to form two different dimers; (2) react with oxygen to produce the parent FL; and (3) recombine with the solvent radical to yield the original FLT. In contrast, FLT with lowest 3pi,pi* states (3-hydroxyflavothione, 3,6-dihydroxyflavothione and 3,7-dihydroxyflavothione) behave as photosensitizers of oxygen to form singlet oxygen, which then reacts with the ground state of the substituted FLT. Finally, when T2(pi,pi*) is above S1(n,ppi*), as for 5-hydroxyflavothione and 5,7-dihydroxyflavothione, both the S1(n,pi*) --> T1(n,pi*) intersystem crossing and photodegradation are inefficient.     

Quantitative determination of 1sigma(g)+ and 1delta(g) singlet oxygen in solvents of very different polarity. General energy gap law for rate constants of electronic energy transfer to and from O2 in the absence of charge transfer interactions

The quenching of excited triplet states of sufficient energy by O2 leads to O2(1sigma(g)+) and O2(1delta(g)) singlet oxygen and O2(3sigma(g)-) ground-state oxygen as well. The present work investigates the question whether in the absence of charge transfer (CT) interactions between triplet sensitizer and O2 the rate constants of formation of the three different O2 product states follow a generally valid energy gap law. For that purpose, lifetimes of the upper excited O2(1sigma(g)+) have been determined in a mixture of 7 vol % benzene in carbon tetrachloride, in chloroform, and in perdeuterated acetonitrile. They amount to 1.86, 1.40, and 0.58 ns, respectively. Furthermore, rate constants of O2(1sigma(g)+), O2(1delta(g)), and O2(3sigma(g)-) formation have been measured in these three solvents for five pi pi* triplet sensitizers with negligible CT interactions. The rate constants are independent of solvent polarity. After normalization for the multiplicity of the respective O2 product state, the rate constants follow a common dependence on the excess energies of the respective product channels. This empirical energy gap relation describes also quantitatively the rate constants of quenching of O2(1delta(g)) by 28 carotenoids. Therefore, it represents in the absence of CT interactions a generally valid energy gap law for the rate constants of electronic energy transfer to and from O2.     

The C(2)1pi(u) state of Na2 molecule studied by polarization labelling spectroscopy method

The C1pi(u) <-- X1sigma(g)+ system of Na2 is studied by the polarization labelling spectroscopy technique. Accurate molecular constants are derived for the observed levels nu = 0-12, J = 12-100 in the C1pi(u) state.     

Photophysical properties of ruthenium(II) tris(2,2'-bipyridine) complexes bearing conjugated thiophene appendages

A small series of ruthenium(II) tris(2,2'-bipyridine) complexes has been synthesized in which ethynylated thiophene residues are attached to one of the 2,2'-bipyridine ligands. The photophysical properties depend on the conjugation length of the thiophene-based ligand, and in each case, dual emission is observed. The two emitting states reside in thermal equilibrium at ambient temperature and can be resolved by emission spectral curve-fitting routines. This allows the properties of the two states to be evaluated in both fluid butyronitrile solution and a transparent KBr disk. It is concluded that both emitting states are of metal-to-ligand charge-transfer (MLCT) character, and despite the presence of conjugated thiophene residues, there is no indication for a low-lying pi,pi*-triplet state that promotes nonradiative decay of the excited-state manifold. A key feature of these systems is that the conjugation length imposed by the thiophene-based ligand helps to control the rate constants for both radiative and nonradiative decay from the two MLCT triplet states.     

Coupled-states statistical investigation of vibrational and rotational relaxation of OH(2pi) by collisions with atomic hydrogen

We report state-to-state cross sections and thermal rate constants for vibrational and rotational relaxation of OH(2pi) by collision with H atoms. The cross sections are calculated by the coupled-states (CS) statistical method including the full open-shell character of the OH + H system. Four potential energy surfaces (PESs) ((1,3)A' and (1,3)A') describe the interaction of OH(X2pi) with H atoms. Of these, three are repulsive, and one (1A') correlates with the deep H2O well. Consequently, rotationally and ro-vibrationally inelastic scattering of OH in collisions with H can occur by scattering on the repulsive PESs, in a manner similar to the inelastic scattering of OH by noble gas atoms, or by collisions which enter the H2O well and then reemerge. At 300 K, we predict large (approximately 1 x 10(-10) cm3 molecule(-1) s(-1)) vibrational relaxation rates out of both v = 2 and v = 1, comparable to earlier experimental observations. This anomalously fast relaxation results from capture into the H2O complex. There exists a significant propensity toward formation of OH in the pi(A') lambda-doublet level. We also report state-resolved cross sections and rate constants for rotational excitation within the OH v = 0 manifold. Collisional excitation from the F1 to the F2 spin-orbit manifold leads to an inverted lambda-doublet population.     

The Na2 2 3pi(g) state: new observations and hyperfine structure

Many more Na2 2 3pi(g) v = 0-43, omega = 0, 1, 2 levels have been observed by sub-Doppler continuous wave perturbation facilitated optical-optical double resonance fluorescence excitation spectroscopy and the hyperfine structure of the omega = 0 and 2 levels has been resolved. New molecular constants for the less perturbed v = 0-43 levels have been obtained with these new and the previously reported data. The hyperfine coupling scheme of the observed 2 3pi(g) levels is close to Hund's case a(beta) with a Fermi contact constant b(F) = 160+/-5 MHz, which is smaller than the Fermi contact constants of other Na2 triplet Rydberg states, b(F) = 200-245 MHz.     

Ultrafast dynamics and coherent oscillations in ethylene and ethylene-d4 excited at 162 nm

The fifth harmonic (162 nm, 11 fs), generated in a short argon cell from 12 fs Ti-sapphire laser pulses, was used to excite C2H4 and C2D4 in the maximum of the first pi pi* transition. Around 10% of the molecules were excited to the pi3s Rydberg state instead. The subsequent motion of the wave packet, moving over the potentials from the Franck-Condon region down to the ground state, was monitored by nonresonant ionization at 810 nm with mass-selective detection of the ion yield. Five time constants (from approximately 20 fs in excited states to 0.6-11 ps in the hot ground state) and four coherent oscillations (CC stretch and torsion vibrations or hindered free rotation) were determined for each isotopomer. The initial relaxation follows a superposition of CC twist and stretch coordinates; this explains a surprisingly small deuterium isotope effect of the initial time constant (21 versus 24 fs). Also the vibrations in the Franck-Condon region have such a mixed character and a correspondingly small isotope shift. From the perpendicular minimum the wave packet reaches (within 17 or 21 fs for the two isotopomers) a conical intersection via a direction that also involves partial hydrogen migration. This is concluded from the detection of ethylidene (CH3CH), formed simultaneously with ground-state ethylene. This carbene isomerizes in the ground state within 0.6 ps (1.6 ps for CD3CD) to ethylene. Two time constants for dissociation (4.5 and 11 ps) in the hot ground state were also identified. The small yields of bimolecular reactions (photodimerization, addition reactions involving a "suddenly polarized" excited state, carbene reactions) are interpreted in terms of the short lifetimes. It is pointed out that the relaxation path starting from the Rydberg state merges into that from the pi pi* state; nevertheless, there is a wavelength dependence in the photochemistry of olefins, because due to a momentum effect the wave packet remembers from which state it came.     

Jet-cooled phosphorescence excitation spectrum of the T1(n,pi*) <-- S0 transition of 2-cyclopenten-1-one

The T1(n,pi*) <-- S0 transition of 2-cyclopenten-1-one (2CP) was investigated by using phosphorescence excitation (PE) spectroscopy in a free-jet expansion. The origin band, near 385 nm, is the most intense feature in the T1(n,pi*) <-- S0 PE spectrum. A short progression in the ring-bending mode (nu'(30)) is also observed. The effective vibrational temperature in the jet is estimated at 50 K. The spectral simplification arising from jet cooling helps confirm assignments made previously in the room-temperature cavity ringdown (CRD) absorption spectrum, which is congested by vibrational hot bands. In addition to the origin and nu'(30) assignments, the jet-cooled PE spectrum also confirms the 28(0)(1) (C=O out-of-plane wag), 29(0)(1) (C=C twist), and 19(0)(1) (C=O in-plane wag) band assignments that were made in the T1(n,pi*) <-- S0 room-temperature CRD spectrum. The temporal decay of the T1 state of 2CP was investigated as a function of vibronic excitation. Phosphorescence from the v' = 0 level persists the entire time the molecules traverse the emission detection zone. Thus the phosphorescence lifetime of the v' = 0 level is significantly longer than the 2 micros transit time through the viewing zone. Higher vibrational levels in the T1 state have shorter phosphorescence lifetimes, on the order of 2 micros or less. The concomitant reduction in emission quantum yield causes the higher vibronic bands (above 200 cm(-1)) in the PE spectrum to be weak. It is proposed that intersystem crossing to highly vibrationally excited levels of the ground state is responsible for the faster decay and diminished quantum yield. The jet cooling affords partial rotational resolution in the T1(n,pi*) <-- S0 spectrum of 2CP. The rotational structure of the origin band was simulated by using inertial constants available from a previously reported density functional (DFT) calculation of the T1(n,pi*) state, along with spin constants obtained via a fitting procedure. Intensity parameters were also systematically varied. The optimized intensity factors support a model that identifies the S2(pi,pi*) <-- S0 transition in 2CP as the sole source of oscillator strength for the T1(n,pi*) <-- S0 transition.