Competition between tunnel- and viscosity-effects on bimolecular hydrogen-transfer reaction

The photocyclization of a methylsubstituted diphenylamine results in the formation of a transient zwitterionic dihydrocarbazole. The transient converts into a stable dihydrocarbazole by an intramolecular, sigmatropic hydrogen shift and by an intermolecular hydrogen exchange reaction. The rates of both reactions are governed by hydrogen tunnel effects. The intermolecular hydrogen exchange is the first example for a bimolecular reaction where tunnel effects have been observed. The role of solvent viscosity is discussedS

A search for radical intermediates in the photocycle of LOV domains

LOV domains are the light sensitive parts of phototropins and many other light-activated enzymes that regulate the response to blue light in plants and algae as well as some fungi and bacteria. Unlike all other biological photoreceptors known so far, the photocycle of LOV domains involves the excited triplet state of the chromophore. This chromophore is flavin mononucleotide (FMN) which forms a covalent adduct with a cysteine residue in the signaling state. Since the formation of this adduct from the triplet state involves breaking and forming of two bonds as well as a change from the triplet to the singlet spin state, various intermediates have been proposed, e.g. a protonated triplet state 3FMNH+, the radical anion 2FMN˙−, or the neutral semiquinone radical 2FMNH˙. We performed an extensive search for these intermediates by two-dimensional transient absorption (2D-TA) with a streak camera. However, no transient with a rate constant between the decay of fluorescence and the decay of the triplet state could be detected. Analysis of the decay associated difference spectra results in quantum yields for the formation of the adduct from the triplet of ΦA(LOV1) ≈ 0.75 and ΦA(LOV2) ≈ 0.80. This is lower than the values ΦA(LOV1) ≈ 0.95 and ΦA(LOV2) ≈ 0.99 calculated from the rate constants, giving indirect evidence of an intermediate that reacts either to form the adduct or to decay back to the ground state. Since there is no measurable delay between the decay of the triplet and the formation of the adduct, we conclude that this intermediate reacts much faster than it is formed. The LOV1-C57S mutant shows a weak and slowly decaying (τ > 100 μs) transient whose decay associated spectrum has bands at 375 and 500 nm, with a shoulder at 400 nm. This transient is insensitive to the pH change in the range 6.5–10.0 but increases on addition of β-mercaptoethanol as the reducing agent. We assign this intermediate to the radical anion which is protected from protonation by the protein. We propose that the adduct is formed via the same intermediate by combination of the radical ion pair

Photodissociation dynamics of tert-butylnitrite following excitation to the S1 and S2 states. A study by velocity-map ion-imaging and 3D-REMPI spectroscopy

Excitation of tert-butylnitrite into the first and second UV absorption bands leads to efficient dissociation into the fragment radicals NO and tert-butoxy in their electronic ground states 2Π and 2E, respectively. Velocity distributions and angular anisotropies for the NO fragment in several hundred rotational and vibrational quantum states were obtained by velocity-map imaging and the recently developed 3D-REMPI method. Excitation into the well resolved vibronic progression bands (k = 0, 1, 2) of the NO stretch mode in the S1 ← S0 transition produces NO fragments mostly in the vibrational state with v = k, with smaller fractions in v = k − 1 and v = k − 2. It is concluded that dissociation occurs on the purely repulsive PES of S1 without barrier. All velocity distributions from photolysis via the S1(nπ*) state are monomodal and show high negative anisotropy (β ≈ −1). The rotational distributions peak near j = 30.5 irrespective of the vibronic state S1(k) excited and the vibrational state v of the NO fragment. On average 46% of the excess energy is converted to kinetic energy, 23% and 31% remain as internal energy in the NO fragment and the t-BuO radical, respectively. Photolysis via excitation into the S2 ← S0 transition at 227 nm yields NO fragments with about equal populations in v = 0 and v = 1. The rotational distributions have a single maximum near j = 59.5. The velocity distributions are monomodal with positive anisotropy β ≈ 0.8. The average fractions of the excess energy distributed into translation, internal energy of NO, and internal energy of t-BuO are 39%, 23%, and 38%, respectively. In all cases ∼8500 cm−1 of energy remain in the internal degrees of freedom of the t-BuO fragment. This is mostly assigned to rotational energy. An ab initio calculation of the dynamic reaction path shows that not only the NO fragment but also the t-BuO fragment gain large angular momentum during dissociation on the purely repulsive potential energy surface of S2

Does diphenylacetylene (tolan) fluoresce from its second excited singlet state? Semiempirical MO calculations and fluorescence quantum yield measurements

It is confirmed by measurements of fluorescence spectra and quantum yields that the fluorescence in tolan originates from the same state that causes the absorption band at lowest energy. The temp. dependence of the fluorescence quantum yield shows that this state is thermally deactivated with an activation energy of EA = 14.0 kJ/mol. Geometry optimizations of the states S0, S1, and T1 of tolane with the semiempirical AM1 method lead to planar structures with D2h symmetry. Potential energy curves along the triple-bond stretching coordinates have been calcd. for several low-lying excited states with a combination of the AM1 and the INDO/S methods. It is found that for large triple-bond lengths, the 1Au-state with sp* character becomes the lowest excited singlet state. It is proposed that thermal deactivation of S1(11Blu) leads to this state. Nonvertical excitation of 11Au could explain the weak lines found in supersonic jet expts. below the onset of the 11Ag -> 11Blu transition

NO product yield excitation spectrum of the S0 -> S2 transition of nitrosobenzene in a supersonic jet

The absorption spectrum of the S0 -> S2 transition of ultracold PhNO in a supersonic jet was measured indirectly by monitoring the product yield of the NO fragment. The vibrational structure and relative line intensities agree well with the corresponding spectrum in an Ar matrix. The photofragmentation yield is const. for all vibronic S2-states up to an excess energy of 3500 cm-1. The electronic origin line has a Lorentzian lineshape with a homogeneous width of 90 +- 5 cm-1. This corresponds to a lifetime of t(S2) = 60 +- 3 fs

Fluorescence excitation and UV-UV double-resonance spectroscopy of the S0 -> S1(Lb) transition of 1,6-methano[10]annulene cooled in a supersonic jet - Dedicated to Professor F. Doerr on the occasion of his 80th birthday

The 1st electronic singlet transition S0 -> S1 of the 10p-arom. compd. 1,6-methano[10]annulene (MA) cooled in a supersonic jet was studied up to an excess energy of 4000 cm-1. The strongest line at 25,154 cm-1 is assigned as the electronic origin. Anal. of the rotational envelope of this line proved that the transition dipole is parallel to the long axis of the mol. Optical-optical double resonance was used to identify the lines which share the same ground state with the origin transition. These lines occur all at higher energies. A few weaker lines which are always present but do not lead to double resonances are tentatively attributed to a van-der-Waals dimer of MA. The rich vibrational structure is interpreted in terms of 13 fundamental vibrations of a1 symmetry and 11 of a2 symmetry, based on the anal. of the rotational contours. The fundamental vibrational frequencies of the excited state are in very good agreement with ab initio calcns. Based on these calcns. 8 further lines which are not combination bands are tentatively assigned to double quantum transitions in b1 and b2 modes. These results strongly support the assignment of a delocalized structure without bond length alternation to the electronic ground state as well as to the 1st electronically excited singlet state

Velocity-map ion-imaging of the NO fragment from the UV-photodissociation of nitrosobenzene

The velocity and angular distribution of NO fragments produced by UV photodissocn. of nitrosobenzene have been detd. by velocity-map ion-imaging. Excitation of the S2-state by irradn. into the peak of the first UV absorption band at 290.5 nm leads to a completely isotropic velocity distribution with Gaussian shape. The av. kinetic energy in both fragments correlates with the rotational energy of the NO fragment and increases from 6% of the excess energy for j = 6.5 to 11% for j = 29.5. A similar isotropic distribution albeit with larger av. velocity is obsd. when the ionization laser at 226 nm is also used for photodissocn., corresponding to excitation into a higher electronic state Sn (n >= 3) of nitrosobenzene. It is concluded that photodissocn. occurs on a timescale much slower than rotation of the parent mol., and after redistribution of the excess energy into the vibrational degrees of freedom