Role of OH-stretch/torsion coupling and quantum yield effects in the first OH overtone spectrum of cis-cis HOONO

A joint theoretical and experimental investigation is undertaken to study the effects of OH-stretch/HOON torsion coupling and of quantum yield on the previously reported first overtone action spectrum of cis-cis HOONO (peroxynitrous acid). The minimum energy path along the HOON dihedral angle is computed at the coupled cluster singles and doubles with perturbative triples level with correlation consistent polarized quadruple zeta basis set, at the structure optimized using the triple zeta basis set (CCSD(T)/cc-pVQZ//CCSD(T)/cc-pVTZ). The two-dimensional ab initio potential energy and dipole moment surfaces for cis-cis HOONO are calculated as functions of the HOON torsion and OH bond length about the minimum energy path at the CCSD(T)/cc-pVTZ and QCISD/AUG-cc-pVTZ (QCISD—quadratic configuration interaction with single and double excitation and AUG-augmented with diffuse functions) level of theory/basis, respectively. The OH-stretch vibration depends strongly on the torsional angle, and the torsional potential possesses a broad shelf at ~90°, the cis-perp conformation. The calculated electronic energies and dipoles are fit to simple functional forms and absorption spectra in the region of the OH fundamental and first overtone are calculated from these surfaces. While the experimental and calculated spectra of the OH fundamental band are in good agreement, significant differences in the intensity patterns are observed between the calculated absorption spectrum and the measured action spectrum in the 2nuOH region. These differences are attributed to the fact that several of the experimentally accessible states do not have sufficient energy to dissociate to OH+NO2 and therefore are not detectable in an action spectrum. Scaling of the intensities of transitions to these states, assuming D0=82.0 kJ/mol, is shown to produce a spectrum that is in good agreement with the measured action spectrum. Based on this agreement, we assign two of the features in the spectrum to Delta n=0 transitions (where n is the HOON torsion quantum number) that are blue shifted relative to the origin band, while the large peak near 7000 cm^–1 is assigned to a series of Delta n=+1 transitions, with predominant contributions from torsionally excited states with substantial cis-perp character. The direct absorption spectrum of cis-cis HOONO (6300–6850 cm^–1) is recorded by cavity ringdown spectroscopy in a discharge flow cell. A single band of HOONO is observed at 6370 cm^–1 and is assigned as the origin of the first OH overtone of cis-cis HOONO. These results imply that the origin band is suppressed by over an order of magnitude in the action spectrum, due to a reduced quantum yield. The striking differences between absorption and action spectra are correctly predicted by the calculations

Post-translational insertion of boron in proteins to probe and modulate function

Boron is absent in proteins, yet is a micronutrient. It possesses unique bonding that could expand biological function including modes of Lewis acidity not available to typical elements of life. Here we show that post-translational Cβ–Bγ bond formation provides mild, direct, site-selective access to the minimally sized residue boronoalanine (Bal) in proteins. Precise anchoring of boron within complex biomolecular systems allows dative bond-mediated, site-dependent protein Lewis acid–base-pairing (LABP) by Bal. Dynamic protein-LABP creates tunable inter- and intramolecular ligand–host interactions, while reactive protein-LABP reveals reactively accessible sites through migratory boron-to-oxygen Cβ–Oγ covalent bond formation. These modes of dative bonding can also generate de novo function, such as control of thermo- and proteolytic stability in a target protein, or observation of transient structural features via chemical exchange. These results indicate that controlled insertion of boron facilitates stability modulation, structure determination, de novo binding activities and redox-responsive ‘mutation’

Post-translational insertion of boron in proteins to probe and modulate function

Boron is absent in proteins, yet is a micronutrient. It possesses unique bonding that could expand biological function including modes of Lewis acidity not available to typical elements of life. Here we show that post-translational Cβ–Bγ bond formation provides mild, direct, site-selective access to the minimally sized residue boronoalanine (Bal) in proteins. Precise anchoring of boron within complex biomolecular systems allows dative bond-mediated, site-dependent protein Lewis acid–base-pairing (LABP) by Bal. Dynamic protein-LABP creates tunable inter- and intramolecular ligand–host interactions, while reactive protein-LABP reveals reactively accessible sites through migratory boron-to-oxygen Cβ–Oγ covalent bond formation. These modes of dative bonding can also generate de novo function, such as control of thermo- and proteolytic stability in a target protein, or observation of transient structural features via chemical exchange. These results indicate that controlled insertion of boron facilitates stability modulation, structure determination, de novo binding activities and redox-responsive ‘mutation’

[18F]Difluorocarbene for Positron Emission Tomography

The advent of total-body Positron Emission Tomography (PET) has vastly broadened the range of research and clinical applications of this powerful molecular imaging technology1. Such possibilities have accelerated progress in 18F-radiochemistry with numerous methods available to 18F-label (hetero)arenes and alkanes2. However, access to 18F-difluoromethylated molecules in high molar activity (Am) is largely an unsolved problem, despite the indispensability of the difluoromethyl group for pharmaceutical drug discovery3. We report herein a general solution by introducing carbene chemistry to the field of nuclear imaging with a [18F]difluorocarbene reagent capable of a myriad of 18F-difluoromethylation processes. In contrast to the tens of known difluorocarbene reagents, this 18F-reagent is carefully designed for facile accessibility, high molar activity and versatility. The issue of Am is solved using an assay examining the likelihood of isotopic dilution upon variation of the electronics of the difluorocarbene precursor. Versatility is demonstrated with multiple [18F]difluorocarbene based reactions including O–H, S–H and N–H insertions, and cross-couplings that harness the reactivity of ubiquitous functional groups such as (thio)phenols, N-heteroarenes, and aryl boronic acids that are easy to install. Impact is illustrated with the labelling of highly complex and functionalised biologically relevant molecules and radiotracers

IR CAVITY RINGDOWN STUDY OF HOONO FORMATION FROM OH+NO2OH + NO_{2}

Author Institution: Chemistry Dept., California Institute of Technology; Jet Propulsion Laboratory, California Institute of TechnologyThe reaction of $OH + NO_{2} \rightarrow HNO_{3}$ is a critical sink of $HO_{x}$ and $NO_{x}$ radicals in the atmosphere. The unstable intermediate HOONO has long been proposed as a minor channel. Because HOONO is predicted to be bound by $<20$ kcal/mol, it would thermally decompose to reactants and would not serve as a sink for OH radicals; hence, its formation would reduce the rate of stable nitric acid formation. We report direct detection of this adduct at $3280cm^{-1}$ by IR cavity ring-down spectroscopy in a discharge flow cell, and present a study of its relative yield and significance in the troposphere and stratosphere

OH-STRETCH/TORSION COUPLING AND QUANTUM YIELD EFFECTS IN THE 2νOH2\nu_{\rm OH} REGION OF THE CIS-CIS HOONO SPECTRUM

{J. L. Fry, S. A. Nizkorodov, M. Okumura, C. M. Roehl, J. S. Francisco and P. O. Wennberg, J. Chem. Phys. {\bf 121{J. Matthews, A. Sinha and J. S. Francisco, J. Chem. Phys. {\bf 120Author Institution: Arthur Amos Noyes Laboratory of Chemical Physics,; California Institute of Technology, Pasadena, CA 91125; Department of Chemistry, Purdue University, West Lafayette, IN 47907; Department of Chemistry, The Ohio State University, Columbus, OH 43210Peroxynitrous acid (HOONO) is a weakly bound isomer of nitric acid that is formed as a secondary product of the reaction of OH and NO$_2$. As the formation of nitric acid provides a sink for HO$_x$ and NO$_x$ in the atmosphere, the possibility of forming the less stable HOONO reduces the efficacy of this process. Recent spectroscopic studies of the intramolecularlly hydrogen-bonded {\it cis-cis} conformer of HOONO in the $2\nu_{\rm OH}$ region by Fry {\it et. al.}}, 1432 (2004).} and independently by Sinha and co-workers}, 10543 (2004).} indicated that the most intense feature in this region was 600 cm$^{-1}$ to the blue of the feature that was assigned as the overtone. This unusual intensity pattern led us to investigate a two-dimensional model of the potential and dipole moment surface for {\it cis-cis} HOONO. The potential was obtained at the CCSD(T)/cc-pVTZ level of theory/basis, while the dipole surface was evaluated at the QCISD/AUG-cc-pVTZ level of theory/basis. They were evaluated on a grid of 169 geometries with the four heavy atoms in a plane and the values of the six remaining coordinates chosen to minimize the electronic energy. The potential has a global minimum in the {\it cis-cis} conformation, but shows a broad shelf when the HOON torsion angle approaches 90$^irc$, the {\it cis-perp} conformation. Using this potential surface, the energies and wave functions were obtained and the wave functions were used, along with the dipole moment surface, to evaluate the intensities of the transitions. Excellent agreement between the experimental and calculated spectra are obtained when quantum yield effects are taken into account. It is found that the intense feature in the calculated spectrum reflect perpendicular transitions among the states that are just above the {\it cis-perp} shelf on the potential. In addition to a large increase in the density of states that are near-by in energy, the oscillator strength for these perpendicular transitions are larger than that for the overtone