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

Kinetics of n-Butoxy and 2-Pentoxy Isomerization and Detection of Primary Products by Infrared Cavity Ringdown Spectroscopy

The primary products of n-butoxy and 2-pentoxy isomerization in the presence and absence of O_2 have been detected using pulsed laser photolysis-cavity ringdown spectroscopy (PLP-CRDS). Alkoxy radicals n-butoxy and 2-pentoxy were generated by photolysis of alkyl nitrite precursors (n-butyl nitrite or 2-pentyl nitrite, respectively), and the isomerization products with and without O_2 were detected by infrared cavity ringdown spectroscopy 20 μs after the photolysis. We report the mid-IR OH stretch (ν_1) absorption spectra for δ-HO-1-C_4H_8•, δ-HO-1-C_4H_8OO•, δ-HO-1-C_5H_(10)•, and δ-HO-1-C_5H_(10)OO•. The observed ν_1 bands are similar in position and shape to the related alcohols (n-butanol and 2-pentanol), although the HOROO• absorption is slightly stronger than the HOR• absorption. We determined the rate of isomerization relative to reaction with O_2 for the n-butoxy and 2-pentoxy radicals by measuring the relative ν_1 absorbance of HOROO• as a function of [O_2]. At 295 K and 670 Torr of N_2 or N_2/O_2, we found rate constant ratios of k_(isom)/k_(O2) = 1.7 (±0.1) × 10^(19) cm^(–3) for n-butoxy and k_(isom)/k_(O2) = 3.4(±0.4) × 10^(19) cm^(–3) for 2-pentoxy (2σ uncertainty). Using currently known rate constants k_(O2), we estimate isomerization rates of k_(isom) = 2.4 (±1.2) × 10^5 s^(–1) and k_(isom) ≈ 3 × 10^5 s^(–1) for n-butoxy and 2-pentoxy radicals, respectively, where the uncertainties are primarily due to uncertainties in k_(O2). Because isomerization is predicted to be in the high pressure limit at 670 Torr, these relative rates are expected to be the same at atmospheric pressure. Our results include corrections for prompt isomerization of hot nascent alkoxy radicals as well as reaction with background NO and unimolecular alkoxy decomposition. We estimate prompt isomerization yields under our conditions of 4 ± 2% and 5 ± 2% for n-butoxy and 2-pentoxy formed from photolysis of the alkyl nitrites at 351 nm. Our measured relative rate values are in good agreement with and more precise than previous end-product analysis studies conducted on the n-butoxy and 2-pentoxy systems. We show that reactions typically neglected in the analysis of alkoxy relative kinetics (decomposition, recombination with NO, and prompt isomerization) may need to be included to obtain accurate values of k_(isom)/k_(O2)

Development of Level 2 Calibration and Validation Plans for GOES-R; What is a RIMP?

Calibration and Validation (CalVal) plans for Geostationary Operational Environmental Satellite version R (GOES-R) Level 2 (L2) products were documented via Resource, Implementation, and Management Plans (RIMPs) for all of the official L2 products required from the GOES-R Advanced Baseline Imager (ABI). In 2015 the GOES-R program decided to replace the typical CalVal plans with RIMPs that covered, for a given L2 product, what was required from that product, how it would be validated, and what tools would be used to do so. Similar to Level 1b products, the intent was to cover the full spectrum of planning required for the CalVal of L2 ABI products. Instead of focusing on step-by-step procedures, the RIMPs concentrated on the criteria for each stage of the validation process (Beta, Provisional, and Full Validation) and the many elements required to prove when each stage was reached

Imaging the State-Specific Vibrational Predissociation of the Hydrogen Chloride-Water Hydrogen-Bonded Dimer †

The state-to-state vibrational predissociation dynamics of the hydrogen-bonded HCl-H 2 O dimer were studied following excitation of the HCl stretch of the dimer. Velocity-map imaging and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the HCl stretch of the dimer, HCl fragments were detected by 2 + 1 REMPI via the f 3 ∆ 2 r X REMPI spectra clearly show HCl from dissociation produced in the ground vibrational state with J′′ up to 11. The fragments' center-of-mass translational energy distributions were determined from images of selected rotational states of HCl and were converted to rotational state distributions of the water cofragment. All the distributions could be fit well when using a dimer dissociation energy of D 0 ) 1334 ( 10 cm -1 . The rotational distributions in the water cofragment pair-correlated with specific rotational states of HCl appear nonstatistical when compared to predictions of the statistical phase space theory. A detailed analysis of pair-correlated state distributions was complicated by the large number of water rotational states available, but the data show that the water rotational populations increase with decreasing translational energy

MODELING THE TORSION-STRETCH COUPLING IN THE OH SPECTRUM OF cis-cis HOONO USING A THREE DIMENSIONAL POTENTIAL SURFACE

Author Institution: Arthur Amos Noyes Laboratory for Chemical Physics, California Institute; of Technology, Pasadena, CA 91125; Department of Chemistry, The Ohio State University, Columbus, OH 43210; Arthur Amos Noyes Laboratory for Chemical Physics, California Institute; of Technology, Pasadena, CA 91125\maketitle The lowest energy conformer of HOONO, the \textit{cis-cis} isomer, forms a ring structure with an internal hydrogen bond. Because of this hydrogen bond, the OH stretch frequency and band intensity are dependent on the dihedral angles HOON and OONO. The torsional motions about these angles have comparable frequencies and are coupled. Previous calculations used two dimensional models that explicitly treated the OH stretch and the HOON torsion (rotation of the OH out of the ring plane)} \textbf{122}, 104311 (2005).}$^{,}$} \textbf{123}, 134318 (2005).}$^{,}$} \textbf{123}, 134318 (2005).}, or three dimensional models that also explicitly treat the OOH bond angle} \textbf{109}, 1810 (2005).}. These models were used to compute the 2D or 3D energies, and $\Delta$ v$_{\rm OH}$ transition intensities. In this work, we extended the previous 2D treatments in order to examine the importance of the remaining OONO torsional mode. The minimum energy path along the two dihedral angles of HOONO was computed at the CCSD(T)/cc-pVTZ level. The three dimensional potential energy surface was then computed as a function of the HOON and OONO dihedral angles, and of the OH bond length. Vibrational levels were obtained by solving the three dimensional Hamiltonian in the vibrationally adiabatic approximation. The potential surface shows coupling of the two torsional angles and the OH stretch. This coupling is harmonic at small torsional angles, and anharmonic at large torsional angles. The effects of the coupling on the torsional energy levels are compared to previous coupling treatments

MULTIPHOTON IONIZATION AND DISSOCIATION OF DIAZIRINE

Author Institution: Department of Chemistry, University of Southern California, Los Angeles, CA 90089Multiphoton ionization and dissociation processes in diazirine have been studied experimentally via 304-325 nm two-photon absorption, and theoretically by using the EOM-CCSD and B3LYP methods. The electronic structure calculations indicate the strongest one-photon absorption is to the $2^{1}A_{1}$(3$p_{x}$$\leftarrow$n) Rydberg state. However, in two-photon absorption at comparable energies the first photon excites the low-lying $1^{1}B_{2} (\Pi^{*}$$\leftarrow$n) valence state, from which the strongest absorption is to the dissociative valence $1^{1}A_{2} (\Pi^{*}$$\leftarrow$$\sigma_{NN}$) state. In the experimental studies, resonance enhanced multiphoton ionization (REMPI) experiments show no ions at the parent diazirine mass but only CH$_{2}^{+}$ ions from dissociative photoionization. It is proposed that weak one-photon absorption to the $1^{1}B_{2}$ state is immediately followed by more efficient absorption of another photon to reach the $1^{1}A_{2}$ state from which competition between ionization and fast dissociation takes place. Strong signals of CH$^{+}$ ions are also detected and assigned to 2+1 of CH fragments. Velocity map CH$^{+}$ images show that CH fragments are born with substantial translational energy indicating that they arise from absorption of two photons in diazirine. It is argued that two-photon processes via the $1^{1}B_{2}$ intermediate state are very efficient in this wavelength range, leading predominantly to dissociation of diazirine from the $1^{1}A_{2}$ state. The most likely route to CH(X) formation is isomerization to isodiazirine followed by dissociation to CH + HN$_{2}$