Two-photon spectroscopy of tungsten(0) arylisocyanides using nanosecond-pulsed excitation

The two-photon absorption (TPA) cross sections (δ) for tungsten(0) arylisocyanides (W(CNAr)6) were determined in the 800–1000 nm region using two-photon luminescence (TPL) spectroscopy. The complexes have high TPA cross sections, in the range 1000–2000 GM at 811.8 nm. In comparison, the cross section at 811.8 nm for tris-(2,2′-bipyridine)ruthenium(II), [Ru(bpy)_3]^(2+), is 7 GM. All measurements were performed using a nanosecond-pulsed laser system

Tryptophan-Accelerated Electron Flow Across a Protein−Protein Interface

We report a new metallolabeled blue copper protein, Re126W122Cu^I Pseudomonas aeruginosa azurin, which has three redox sites at well-defined distances in the protein fold: Re^I(CO)_3(4,7-dimethyl-1,10-phenanthroline) covalently bound at H126, a Cu center, and an indole side chain W122 situated between the Re and Cu sites (Re-W122(indole) = 13.1 Å, dmp-W122(indole) = 10.0 Å, Re-Cu = 25.6 Å). Near-UV excitation of the Re chromophore leads to prompt Cu^I oxidation (<50 ns), followed by slow back ET to regenerate Cu^I and ground-state Re^I with biexponential kinetics, 220 ns and 6 μs. From spectroscopic measurements of kinetics and relative ET yields at different concentrations, it is likely that the photoinduced ET reactions occur in protein dimers, (Re126W122CuI)2 and that the forward ET is accelerated by intermolecular electron hopping through the interfacial tryptophan: ^*Re//←W122←Cu^I, where // denotes a protein–protein interface. Solution mass spectrometry confirms a broad oligomer distribution with prevalent monomers and dimers, and the crystal structure of the Cu^(II) form shows two Re126W122Cu^(II) molecules oriented such that redox cofactors Re(dmp) and W122-indole on different protein molecules are located at the interface at much shorter intermolecular distances (Re-W122(indole) = 6.9 Å, dmp-W122(indole) = 3.5 Å, and Re-Cu = 14.0 Å) than within single protein folds. Whereas forward ET is accelerated by hopping through W122, BET is retarded by a space jump at the interface that lacks specific interactions or water molecules. These findings on interfacial electron hopping in (Re126W122Cu^I)^2 shed new light on optimal redox-unit placements required for functional long-range charge separation in protein complexes

Cavity Ringdown Spectroscopy of the Nitrate and Peroxy Radicals

The chemistry of the Earth's atmosphere consists of complex networks of reactions. Photooxidation of volatile organic compounds (VOCs) in the atmosphere initiates free radical formation. These radicals attack other VOCs to form pollutants and secondary organic aerosols. Quantitative understanding of the radicals and reactions is needed for accurate modeling of the atmosphere. Many species are difficult to study due to low concentrations and short lifetimes. Spectroscopic methods in the ultraviolet and visible regions either do not have the sensitivity or the specificity to characterize these reactions. The work here examines the chemistry and physics of atmospheric radicals by using the sensitive and fast spectroscopic technique cavity ringdown spectroscopy (CRDS), to detect transient species in the near-infrared (NIR) region. The nitrate radical NO₃ is a major nighttime oxidant in the troposphere. It is also a classic example of the breakdown of the Born-Oppenheimer approximation. The radical was first observed a century ago in atmospheric measurements. The structures of the three lowest electronic state however are still not well understood. Difficulties arise from the non-adiabatic Jahn Teller and Pseudo-Jahn-Teller effects. In Chapter 3, we examine the electronic-dipole forbidden A&#771; ← X&#771; transition of NO₃ in the NIR to elucidate the A&#771; state of NO₃. In Chapter 4, we examine the role of NO₃ in atmospheric reactions by detecting the peroxy radical intermediate of the oxidation of 2-butene by NO₃. The chlorine atom Cl is highly reactive and has been historically considered a coastal or marine layer oxidant. Studies now indicate that Cl atoms can play significant roles in urban mainland chemistry. Isoprene and 2-methyl-3-buten-2-ol (MBO232) are two important biogenic VOC emissions. Isoprene alone is responsible for emissions of 500 Tg C y⁻¹. The peroxy radical intermediates of the oxidation of isoprene and MBO232 by Cl have never been detected using absorption spectroscopy. Chapter 5 includes the first preliminary CRD spectra of the A&#771; ← X&#771; transition of Cl-isoprenyl and Cl-MBO232 peroxy radials in the NIR. We also outline kinetic experiments to measure the rates of reaction between the Cl-substituted peroxy radicals and nitric oxide (NO) and hydroperoxy radical (HO₂) under high and low NOₓ conditions in the troposphere.</p

The A~\widetilde{A}←\leftarrowX~\widetilde{X} ABSORPTION SPECTRUM OF 2-NITROOXYBUTYL PEROXY RADICAL

Author Institution: Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125The nitrate radical is an important atmospheric oxidant in the nighttime sky. Nitrate radicals react by addition to alkenes, and in the presence of oxygen form nitrooxyalkyl peroxy radicals. The peroxy radical formed from the reaction of 2-butene, nitrate radical, and oxygen was detected by cavity ringdown spectroscopy (CRDS) via its $\widetilde{A}$$\leftarrow$$\widetilde{X}$ electronic absorption spectrum. The $\widetilde{A}$$\leftarrow$$\widetilde{X}$ electronic transition is a bound-bound transition with enough structure to distinguish between different peroxy radicals as well as different conformers of the same peroxy radical. Two conformers of the nitrooxybutyl peroxy radical have been observed; the absorption features are red shifted from the same absorption features of sec-butyl peroxy radical. Calculations on the structure of nitrooxyalkyl peroxy radicals and general trends of the position of the $\widetilde{A}$$\leftarrow$$\widetilde{X}$ absorption transitions have also been performed and compared to those of unsubstituted peroxy radicals

THE JAHN-TELLER (JT) EFFECT IN THE A~\widetilde{A} STATE OF THE NITRATE RADICAL NO3_3

Author Institution: Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125; Department of Chemistry, University of Texas at Austin, Austin, TX 78712The JT effect in the $\widetilde{A}$$^2$E$^{\prime \prime}$ of NO$_3$ is poorly understood. A preliminary spectrum of the vibronically-allowed $\widetilde{A}$$\leftarrow$$\widetilde{X}$ transition, coupled with ab initio calculations, shows moderate JT activity in the $\widetilde{A}$ state. Vibronic bands exhibit either static or dynamic JT distortions depending on the vibrational level of the upper $\widetilde{A}$ state. The picture of the $\widetilde{A}$ state is however incomplete. For example, in the E$^{\prime \prime}$$\otimes$e$^{\prime}$=a$_1$$^{\prime \prime}$$\oplus$a$_2$$^{\prime \prime}$$\oplus$e$^{\prime \prime}$ manifold, while the splitting would provide a direct measure of the JT strength, only the a$_1$$^{\prime \prime}$ levels have been observed. We have gained new insight into the $\widetilde{A}$ state by examining the hot bands of NO$_3$ which access previously unobserved dark levels of the $\widetilde{A}$ state

Spectroscopic studies of the Jahn-Teller effect in the Ã^2E" state of the nitrate radical NO^3

We report spectra of the forbidden origin and several hot bands of the Ã^2E" ← X^2A'_2 transition of NO_3 using cavity ringdown and enhanced absorption spectroscopies. We directly observed the origin at T_0(Ã) = 7062.25(50) cm^(−1), which arises from a magnetic-dipole or rotationally-forbidden transition. We used this assignment to estimate fundamental frequencies in the à state. The Jahn Teller (JT) effect splits the vibronic levels E(Γ_(ev)) of the V'_4 = 1, E″⊗e′ manifold. We measured E(E″)–E(A"_1) to be 1.6(1.9) cm^(−1) implying weak JT effect in the Q_4 mode. We discuss the possible assignment of a band observed near the origin to either spin-orbit splitting or an A"_2 level

JET-COOLED LASER SPECTROSCOPY OF A JAHN-TELLER AND PSEUDO JAHN-TELLER ACTIVE MOLECULE: THE NITRATE RADICAL

J. F. Stanton, {\textit{J. Chem. Phys.K. Kawaguchi, E. Hirota, T. Ishiwata, and I. Tanaka, {\textit{J. Chem. Phys.K. Kawaguchi, T. Ishiwata, E. Hirota, and I. Tanaka, {\textit{Chem. Phys.A. Deev, J. Sommar, and M. Okumura, {\textit{J. Chem. Phys.S. Wu, P. Dupr\acute{eAuthor Institution: Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, 120 W. 18th Avenue, Columbus, Ohio 43210; Arthor Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125; Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, 120 W. 18th Avenue, Columbus, Ohio 43210Well-known as an important intermediate in atmospheric chemistry, the nitrate radical (NO_3$) has been extensively studied both experimentally and theoretically. The three energetically lowest electronic states ($\tilde{X}$$^{2}A_{2}^{\prime}$,$\tilde{A}$$^{2}E^{\prime\prime}$, and$\tilde{B}$$^{2}E^{\prime}) are strongly coupled by vibronic interactions and hence it is a textbook molecule for understanding the coupling between nearby potential energy surfaces. Such coupling has been treated in considerable detail theoretically., \underline{\textbf{126}}, 134309 (2007)}} However, corresponding experimental characterization of the interaction is much less detailed. The experimental results primarily consist of IR measurements of vibrational transitions in the ground state., \underline{\textbf{93}}, 951 (1990)}}, \underline{\textbf{231}}, 193 (1998)}} In addition, the electronically forbidden \tilde{A}$-$\tilde{X} transition has been observed in ambient temperature CRDS studies., \underline{\textbf{122}}, 224305 (2005)}} To understand both the Jahn-Teller and pseudo Jahn-Teller coupling in the molecule, further measurements are required with different selection rules and/or higher resolution to resolve the rotational structures of different transitions. In our group, a high-resolution (source \Delta\nu\approx$100 MHz in NIR region), jet-cooled CRDS system$, and T. A. Miller, {\textit{Phys. Chem. Chem. Phys.}, \underline{\textbf{8}}, 1682, (2006)}} can be applied to rotationally resolve the electronically forbidden $\tilde{A}$-$\tilde{X}$ transition. Furthermore, our high-resolution LIF/SEP system (source $\Delta\nu\approx$ 100 MHz) can provide the direct, rotationally resolved measurements of the $\tilde{B}$-$\tilde{X}$ and $\tilde{B}$-$\tilde{A}$ transitions by operating in the LIF and SEP modes respectively. Such data can provide unambiguous spectral assignments in the $\tilde{X}$, $\tilde{A}$ and $\tilde{B}$ states