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Cryptochrome 4a (Cry4a) has been proposed as the sensor at the heart of the magnetic compass in migratory songbirds. Blue-light excitation of this protein produces magnetically sensitive flavin–tryptophan radical pairs whose properties suggest that Cry4a could indeed be suitable as a magnetoreceptor. Here, we use cavity ring-down spectroscopy to measure magnetic field effects on the kinetics of these radical pairs in modified Cry4a proteins from the migratory European robin and from nonmigratory pigeon and chicken. B1/2, a parameter that characterizes the magnetic field-dependence of the reactions, was found to be larger than expected on the basis of hyperfine interactions and to increase with the delay between pump and probe laser pulses. Semiclassical spin dynamics simulations show that this behavior is consistent with a singlet–triplet dephasing (STD) relaxation mechanism. Analysis of the experimental data gives dephasing rate constants, rSTD, in the range 3–6 × 107 s−1. A simple "toy" model due to Maeda, Miura, and Arai [Mol. Phys. 104, 1779–1788 (2006)] is used to shed light on the origin of the time-dependence and the nature of the STD mechanism. Under the conditions of the experiments, STD results in an exponential approach to spin equilibrium at a rate considerably slower than rSTD. We attribute the loss of singlet–triplet coherence to electron hopping between the second and third tryptophans of the electron transfer chain and comment on whether this process could explain differences in the magnetic sensitivity of robin, chicken, and pigeon Cry4a's. [ABSTRACT FROM AUTHOR]

Isolated mixed-ligand complexes provide tractable model systems in which to study competitive and cooperative binding effects as well as controlled energy flow. Here, we report spectroscopic and isotopologue-selective infrared photofragmentation dynamics of mixed gas-phase Au(12/13CO)n(N2O)m+ complexes. The rich infrared action spectra, which are reproduced well using simulations of calculated lowest energy structures, clarify previous ambiguities in the assignment of vibrational bands, especially accidental coincidence of CO and N2O bands. The fragmentation dynamics exhibit the same unexpected behaviour as reported previously in which, once CO loss channels are energetically accessible, these dominate the fragmentation branching ratios, despite the much lower binding energy of N2O. We have investigated the dynamics computationally by considering anharmonic couplings between a relevant subset of normal modes involving both ligand stretch and intermolecular modes. Discrepancies between correlated and uncorrelated model fit to the ab initio potential energy curves are quantified using a Boltzmann sampled root mean squared deviation providing insight into efficiency of vibrational energy transfer between high frequency ligand stretches and the softer intermolecular modes which break during fragmentation. [ABSTRACT FROM AUTHOR]

We report a combined experimental and computational study of carbon dioxide activation at gas-phase Ho+ and HoO+ centres. Infrared action spectra of Ho(CO2)n+ and [HoO(CO2)n]+ ion–molecule complexes have been recorded in the spectral region 1700–2400 cm−1 and assigned by comparison with simulated spectra of energetically low-lying structures determined by density functional theory. Little by way of activation is observed in Ho(CO2)n+ complexes with CO2 binding end-on to the Ho+ ion. By contrast, all [HoO(CO2)n]+ complexes n ≥ 3 show unambiguous evidence for formation of a carbonate radical anion moiety,. The signature of this structure, a new vibrational band observed around 1840 cm−1 for n = 3, continues to red-shift monotonically with each successive CO2 ligand binding with net charge transfer from the ligand rather than the metal centre. [ABSTRACT FROM AUTHOR]

The application of evanescent wave cavity ring-down spectroscopy (EW-CRDS) in monitoring electrogenerated species within a thin-layer electrochemical cell is demonstrated. In the proof-of-concept experiments described, ferricyanide, Fe[(CN).sub.6.sup.3-], was produced by the transport-limited oxidation of ferrocyanide, Fe[(CN).sub.6.sup.4-], in a thin-layer solution cell (25-250 [micro]m) formed between an electrode and the hypotenuse of a fused-silica prism. The prism constituted one element of a high-finesse optical cavity arranged in a triangular ring geometry with light being totally internally reflected at the silica/solution interface. The cavity was pumped with the output (~417 nm) of a single-mode external cavity diode laser, which was continuously scanned across the cavity modes. The presence of electrogenerated ferricyanide within the resulting evanescent field, beyond the optical interface, was detected by the enhanced loss of light trapped within the cavity, as measured by the characteristic cavity ring down. In this way, the EW-CRDS technique is sensitive to absorption in only the first few hundred nanometers of solution above the silica surface. The cavity ring-down response accompanying both cyclic voltammetric and step potential chronoamperometry experiments at a variety of electrode-surface distances is presented, and the results are shown to be well reproduced in modeling by finite element methods. The studies herein thus provide a foundation for further applications of EW-CRDS combined with electrochemistry.

Even though the interaction of a <1 mT magnetic field with an electron spin is less than a millionth of the thermal energy at room temperature (kBT), it still can have a profound effect on the quantum yields of radical pair reactions. We present a study of the effects of sub-millitesla magnetic fields on the photoreaction of flavin mononucleotide with ascorbic acid. Direct control of the reaction pathway is achieved by varying the rate of electron transfer from ascorbic acid to the photo-excited flavin. At pH 7.0, we verify the theoretical prediction that, apart from a sign change, the form of the magnetic field effect is independent of the initial spin configuration of the radical pair. The data agree well with model calculations based on a Green's function approach that allows multinuclear spin systems to be treated including the diffusive motion of the radicals, their spin-selective recombination reactions, and the effects of the inter-radical exchange interaction. The protonation states of the radicals are uniquely determined from the form of the magnetic field-dependence. At pH 3.0, the effects of two chemically distinct radical pair complexes combine to produce a pronounced response to ~500 μT magnetic fields. These findings are relevant to the magnetic responses of cryptochromes (flavin-containing proteins proposed as magnetoreceptors in birds) and may aid the evaluation of effects of weak magnetic fields on other biologically relevant electron transfer processes. [ABSTRACT FROM AUTHOR]

The near ultraviolet photodissociation dynamics of silver atom-rare gas dimers have been studied by velocity map imaging. Ag-RG (RG = Ar, Kr, Xe) species generated by laser ablation are excited in the region of the C ((²Σ+)←X (²Σ+) continuum leading to direct, near-threshold dissociation generating Ag* (²P3/2) + RG¹S0) products. Images recorded at excitation wavelengths throughout the C (²Σ+)←X (²Σ+) continuum, coupled with known atomic energy levels, permit determination of the ground X (²Σ+) state dissociation energies of 85.9 ± 23.4 cm-1 (Ag-Ar), 149.3 ± 22.4 cm-1 (Ag-Kr), and 256.3 ± 16.0 cm-1 (Ag-Xe). Three additional photolysis processes, each yielding Ag atom photoproducts, are observed in the same spectral region. Two of these are markedly enhanced in intensity upon seeding the molecular beam with nitrous oxide, and are assigned to photodissociation of AgO at the two-photon level. These features yield an improved ground state dissociation energy for AgO of 15 965 ± 81 cm-1, which is in good agreement with high level calculations. The third process results in Ag atom fragments whose kinetic energy shows anomalously weak photon energy dependence and is assigned tentatively to dissociative ionization of the silver dimer Ag2. [ABSTRACT FROM AUTHOR]

Infrared multiple-photon dissociation spectroscopy has been applied to study Ptn(N2O)+ (n = 1–8) clusters which represent entrance-channel complexes on the reactive potential energy surface for nitrous oxide decomposition on platinum. Comparison of spectra recorded in the spectral region 950 cm−1 to 2400 cm−1 with those simulated for energetically low-lying structures from density functional theory shows a clear preference for molecular binding via the terminal N atom, though evidence of O-binding is observed for some cluster sizes. Enhanced reactivity of Ptn+n ≥ 6 clusters towards N2O is reflected in the calculated reactive potential energy surfaces and, uniquely in the size range studied, Pt6(N2O)+ proved impossible to form in significant number density even with cryogenic cooling of the cluster source. Infrared-driven N2O decomposition, resulting in the formation of cluster oxides, PtnO+, is observed following vibrational excitation of several Ptn(N2O)+ complexes. [ABSTRACT FROM AUTHOR]

We present the results of a combined experimental and computational study of the structures of gas-phase M+(N2O)n (M = Li, Al) complexes. Infrared spectra were recorded in the region of the N2O asymmetric (N = N) stretch using photodissociation spectroscopy employing the inert messenger technique. Unlike in our previous studies on M+(N2O)n (M = Cu, Ag, Au and M = Co, Rh, Ir) complexes, N– and O–bound isomers in this case are near isoenergetic and are not distinguished spectroscopically at this resolution. In the case of Li+ complexes, there is, however, evidence for the presence of bound N2 moieties, indicating the presence of inserted, OLi+N2(N2O)n–type structures. The weak N2 band lies to the blue of the signature of molecularly N– and O–bound ligands and is well–reproduced in the simulated spectra of energetically low-lying structures computed from density functional theory. No such inserted isomers are observed in the case of Al+(N2O)n complexes whose infrared spectra can be understood on the basis of molecularly-bound N2O ligands. The differences in M+(N2O)n structures observed for these closed–shell, ns2, metal centres relative to other metal cations are discussed in terms of the likely bonding motifs. [ABSTRACT FROM AUTHOR]

The dissociative ionization dynamics of excited electronic states of the xenon dimer, Xe2, have been studied using velocity map ion imaging (VMI). A one-colour, (2+1) resonant excitation scheme was employed to first excite and then ionize selected vibrational levels of the Xe2 6p 2[1/2]0 0g+ Rydberg state. Cationic fragments were then detected by the VMI. The data provide an outstanding example of the reflection principle in photodissociation with the full nodal structure of the Rydberg state wavefunctions clearly observed in the final Xe+ kinetic energy distributions without the need for scanning the excitation energy. Fitting of the observed distributions provides detailed and precise information on the form of the Xe2+ I(1/2g) potential energy curve involved which is in excellent agreement with the results of photoelectron imaging studies [Shubert and Pratt, J. Chem. Phys. 134, 044315 (2011) ]. Furthermore, the anisotropy of the product angular distributions yields information on the evolution of the electronic character of the ionic state with internuclear separation, R. The combination of the nature of dissociative ionization and the extent of the bound state wavefunctions provide information over an unusually wide range of internuclear separation R (ΔR > 0.75 Å). This would normally require scanning over a considerable energy region but is obtained in these studies at a fixed excitation energy. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of Au-Xe leading to Xe+ formation via the Ξ1/2-X2Σ+ (v′, 0) band system (41 500-41 800 cm-1) have been investigated by velocity map imaging. Five product channels have been indentified, which can be assigned to photoinduced charge transfer followed by photodissociation in either the neutral or the [Au-Xe]+ species. For the neutral species, charge transfer occurs via a superexcited Rydberg state prior to dissociative ionization, while single-photon excitation of the gold atom in Au+-Xe accesses an (Au+)*-Xe excited state that couples to a dissociative continuum in Au-Xe+. Mechanisms by which charge transfer occurs are proposed, and branching ratios for Xe+ formation via the superexcited Rydberg state are reported. The bond dissociation energy for the first excited state of Au+-Xe is determined to be ∼9720 ± 110 cm-1. [ABSTRACT FROM AUTHOR]

The ultraviolet photodissociation dynamics of the gold-rare gas atom van der Waals complexes (Au–RG, RG=Ar, Kr, and Xe) have been studied by velocity map imaging. Photofragmentation of Au–Ar and Au–Kr at several wavelengths permits extrapolation to zero of the total kinetic energy release (TKER) spectra as monitored in the Au(2P3/2°[5d106p]) fragment channel, facilitating the determination of ground state dissociation energies of D0″(Au–Ar)=149±13 cm-1 and D0″(Au–Kr)=240±19 cm-1, respectively. In the same spectral region, transitions to vibrational levels of an Ω′=1/2 state of the Au–Xe complex result in predissociation to the lower Au(2P1/2°[5d106p])+Xe(1S0[5p6]) fragment channel for which TKER extrapolation yields a value of D0″(Au–Xe)=636±27 cm-1. Asymmetric line shapes for transitions to the v′=14 level of this state indicate coupling to the Au(2P3/2°[5d106p])+Xe(1S0[5p6]) continuum, which allows us to refine this value to D0″(Au–Xe)=607±5 cm-1. The dissociation dynamics of this vibrational level have been studied at the level of individual isotopologues by fitting the observed excitation spectra to Fano profiles. These fits reveal a remarkable variation in the predissociation dynamics for different Au–Xe isotopologues. For Au–Ar and Au–Xe, the determined ground state dissociation energies are in good agreement with recent theoretical calculations; the agreement of the Au–Kr value with theory is less satisfactory. [ABSTRACT FROM AUTHOR]