Nonstatistical Unimolecular Decay of the CHOO Criegee Intermediate in the Tunneling Regime.

Unimolecular decay of the formaldehyde oxide (CHOO) Criegee intermediate proceeds via a 1,3 ring-closure pathway to dioxirane and subsequent rearrangement and/or dissociation to many products including hydroxyl (OH) radicals that are detected. Vibrational activation of jet-cooled CHOO with two quanta of CH stretch (17-18 kcal mol) leads to unimolecular decay at an energy significantly below the transition state barrier of 19.46 ± 0.

On the performance of composite schemes in determining equilibrium molecular structures.

Determination of equilibrium molecular structures is an essential ingredient in predicting spectroscopic parameters that help in identifying molecular carriers of microwave transitions. Here, the performance of two different ab initio composite approaches for obtaining equilibrium structures, "energy scheme" and "geometry scheme," is explored and compared to semi-experimental equilibrium structures. This study is performed for a set of 11 molecules which includes diatomics, linear triatomics, and a few non-linear molecules.

Influence of fourth-order vibrational corrections on semi-experimental (reSE) structures of linear molecules.

Semi-experimental structures (reSE) are derived from experimental ground state rotational constants combined with theoretical vibrational corrections. They permit a meaningful comparison with equilibrium structures based on high-level ab initio calculations. Typically, the vibrational corrections are evaluated with second-order vibrational perturbation theory (VPT2).

Direct Probes of π-Delocalization in Prototypical Resonance-Stabilized Radicals: Hyperfine-Resolved Microwave Spectroscopy of Isotopic Propargyl and Cyanomethyl.

Delocalization of the unpaired electron in π-conjugated radicals has profound implications for their chemistry, but direct and quantitative characterization of this electronic structure in isolated molecules remains challenging. We apply hyperfine-resolved microwave rotational spectroscopy to rigorously probe π-delocalization in propargyl, CHCCH, a prototypical resonance-stabilized radical and key reactive intermediate. Using the spectroscopic constants derived from the high-resolution cavity Fourier transform microwave measurements of an exhaustive set of C- and H-substituted isotopologues, together with high-level ab initio calculations of zero-point vibrational effects, we derive its precise semiexperimental equilibrium geometry and quantitatively characterize the spatial distribution of its unpaired electron.

Vibrationally excited states of 1H- and 2H-1,2,3-triazole isotopologues analyzed by millimeter-wave and high-resolution infrared spectroscopy with approximate state-specific quartic distortion constants.

In this work, we present the spectral analysis of 1H- and 2H-1,2,3-triazole vibrationally excited states alongside provisional and practical computational predictions of the excited-state quartic centrifugal distortion constants. The low-energy fundamental vibrational states of 1H-1,2,3-triazole and five of its deuteriated isotopologues ([1-H]-, [4-H]-, [5-H]-, [4,5-H]-, and [1,4,5-H]-1H-1,2,3-triazole), as well as those of 2H-1,2,3-triazole and five of its deuteriated isotopologues ([2-H]-, [4-H]-, [2,4-H]-, [4,5-H]-, and [2,4,5-H]-2H-1,2,3-triazole), are studied using millimeter-wave spectroscopy in the 130-375 GHz frequency region. The normal and [2-H]-isotopologues of 2H-1,2,3-triazole are also analyzed using high-resolution infrared spectroscopy, determining the precise energies of three of their low-energy fundamental states.

Rotamers of Methanediol: Composite Predictions of Structures, Frequencies, and Rovibrational Constants.

Geminal diols are known to be important intermediates in atmospheric ozonolysis and the aerosol cycle. Recently, the simplest member of this class, methanediol, was interrogated in the gas phase with infrared spectroscopy. To aid in future spectroscopic investigations of methanediol, including in the interstellar medium, we report fundamental frequencies and rovibrational constants for the two rotamers of this molecule using composite methods along with vibrational perturbation theory.

Millimeter/Submillimeter-wave Spectroscopy and the Semi-experimental Equilibrium () Structure of 1-1,2,4-Triazole (-CHN).

The millimeter/submillimeter spectrum of 1-1,2,4-triazole is reported from 70 to 700 GHz, providing spectral frequencies directly comparable to radio telescopes and enabling an astronomical search. Using four deuteriated samples of 1,2,4-triazole, we measured, assigned, and least-squares fit transitions for 26 isotopologues to sextic A- and S-reduced Hamiltonians. An accurate and precise semi-experimental () structure from 50 independent moments of inertia has been obtained.

Helium droplet infrared spectroscopy of the butyl radicals.

Butyl radicals (n-, s-, i-, and tert-butyl) are formed from the pyrolysis of stable precursors (1-pentyl nitrite, 2-methyl-1-butyl nitrite, isopentyl nitrite, and azo-tert-butane, respectively). The radicals are doped into a beam of liquid helium droplets and probed with infrared action spectroscopy from 2700 to 3125 cm, allowing for a low temperature measurement of the CH stretching region. The presence of anharmonic resonance polyads in the 2800-3000 cm region complicates its interpretation.

Infrared spectroscopy of the protonated HCl dimer and trimer.

The protonated HCl dimer and trimer complexes were prepared by pulsed discharges in supersonic expansions of helium or argon doped with HCl and hydrogen. The ions were mass selected in a reflectron time-of-flight spectrometer and investigated with photodissociation spectroscopy in the IR and near-IR regions. Anharmonic vibrational frequencies were computed with VPT2 at the MP2/cc-pVTZ level of theory.

Thermal Decomposition of CHO: A Curious Case of Pressure-Dependent Tunneling Effects.

The thermal unimolecular decomposition of a methoxy radical (CHO), a key intermediate in the combustion of methane, methanol, and other hydrocarbons, was studied using high-level coupled-cluster calculations, followed by ,-resolved master equation analyses. The experimental results available for a wide range of temperature and pressure are in striking agreement with the calculations. In line with a previous theoretical study that used a one-dimensional master equation, the tunneling correction is found to exhibit a marked pressure dependence, being the largest at low pressure.

How to VPT2: Accurate and Intuitive Simulations of CH Stretching Infrared Spectra Using VPT2+K with Large Effective Hamiltonian Resonance Treatments.

This article primarily discusses the utility of vibrational perturbation theory for the prediction of X-H stretching vibrations with particular focus on the specific variant, second-order vibrational perturbation theory with resonances (VPT2+K). It is written as a tutorial, reprinting most important formulas and providing numerous simple examples. It discusses the philosophy and practical considerations behind vibrational simulations with VPT2+K, including but not limited to computational method selection, cost-saving approximations, approaches to evaluating intensity, resonance identification, and effective Hamiltonian structure.

Infrared photodissociation spectroscopy and anharmonic vibrational study of the HO molecular ion.

Molecular cations of HO and DO are produced in a supersonic expansion. They are mass-selected, and infrared photodissociation spectra of these species are measured with the aid of argon-tagging. Although previous theoretical studies have modeled these systems as proton-bound dimers of molecular oxygen, infrared spectra have free OH stretching bands, suggesting other isomeric structures.

Sulfurous and sulfonic acids: Predicting the infrared spectrum and setting the surface straight.

Sulfurous acid (HSO) is an infamously elusive molecule. Although some theoretical papers have supposed possible roles for it in more complicated systems, it has yet to be experimentally observed. To aid experiment in detecting this molecule, we have examined the HO + SO potential energy surface at the CCSDT(Q)/CBS//CCSD(T)-F12b/cc-pVTZ-F12b level of theory to resolve standing discrepancies in previous reports and predict the gas-phase vibrational spectrum for HSO.

tert-Butyl peroxy radical: ground and first excited state energetics and fundamental frequencies.

Alkylperoxy radicals (RO2˙) are key intermediates in combustion and atmospheric oxidation processes. As such, reliable detection and monitoring of these radicals can provide a wealth of information about the underlying chemistry. The tert-butyl peroxy radical is the archetypal tertiary peroxy radical, yet its vibrational spectroscopy is largely unexplored.

Infrared Spectrum of Fulvenallene and Fulvenallenyl in Helium Droplets.

Fulvenallene is the global minimum on the CH potential energy surface. Rearrangement of fulvenallene to other CH species and dissociation to produce fulvenallenyl radical (CH) is carried out in a continuous-wave SiC pyrolysis furnace at 1500 K. Prompt pick-up and solvation by helium droplets allows for the acquisition of vibrational spectra of these species in the CH stretching region.

Ethyl + O in Helium Nanodroplets: Infrared Spectroscopy of the Ethylperoxy Radical.

Helium-solvated ethylperoxy radicals (CHCHOO) are formed via the in situ reaction between A' ethyl radical and Σ dioxygen. The reactants are captured sequentially through the droplet pick-up technique. Helium droplets are doped with ethyl radical via pyrolysis of di- tert-amyl peroxide or n-propylnitrite in an effusive, low-pressure source.

Rotamers of Isoprene: Infrared Spectroscopy in Helium Droplets and Ab Initio Thermochemistry.

Isoprene (CH) is an abundant, reactive tropospheric hydrocarbon, derived from biogenic emissions. A detailed understanding of the spectroscopy of isoprene is therefore desirable. Isoprene monomer is isolated in helium droplets and its infrared spectrum is measured in the CH stretching region.

Sequential Capture of O(P) and HCN by Helium Nanodroplets: Infrared Spectroscopy and ab Initio Computations of the Σ O-HCN Complex.

Catalytic thermal cracking of O is employed to dope helium droplets with O(P) atoms. Mass spectrometry of the doped droplet beam reveals an O dissociation efficiency larger than 60%; approximately 26% of the droplet ensemble is doped with single oxygen atoms. Sequential capture of O(P) and HCN leads to the production of a hydrogen-bound O-HCN complex in a Σ electronic state, as determined via comparisons of experimental and theoretical rovibrational Stark spectroscopy.

Helium Nanodroplet Isolation of the Cyclobutyl, 1-Methylallyl, and Allylcarbinyl Radicals: Infrared Spectroscopy and Ab Initio Computations.

Gas-phase cyclobutyl radical (CH) is produced via pyrolysis of cyclobutylmethyl nitrite (CH(CH)ONO). Other CH radicals, such as 1-methylallyl and allylcarbinyl, are similarly produced from nitrite precursors. Nascent radicals are promptly solvated in liquid He droplets, allowing for the acquisition of infrared spectra in the CH stretching region.

Infrared laser spectroscopy of the n-propyl and i-propyl radicals: Stretch-bend Fermi coupling in the alkyl CH stretch region.

The n-propyl and i-propyl radicals were generated in the gas phase via pyrolysis of n-butyl nitrite [CH(CH)ONO] and i-butyl nitrite [(CH)CHCHONO], respectively. Nascent radicals were promptly solvated by a beam of He nanodroplets, and the infrared spectra of the radicals were recorded in the CH stretching region. Several previously unreported bands are observed between 2800 and 3150 cm.

Experimental evaluation of protein identification by an LC/MALDI/on-target digestion approach.

Tryptic digestion of proteins continues to be a workhorse of proteomics. Traditional tryptic digestion requires several hours to generate an adequate protein digest. A number of enhanced accelerated digestion protocols have been developed in recent years.

Macrolactin is the polyketide biosynthesis product of the pks2 cluster of Bacillus amyloliquefaciens FZB42.

In the genome of Bacillus amyloliquefaciens FZB42, three operons pks1, pks2, and pks3 were identified which encode the biosynthesis of polyketides. pks1 and pks3 have been attributed to the production of bacillaene and difficidin/oxydifficidin, respectively, while the pks2 product remained hitherto unknown. Mass spectrometric analysis of the culture filtrates of the wild-type B.

Structural and functional characterization of three polyketide synthase gene clusters in Bacillus amyloliquefaciens FZB 42.

Although bacterial polyketides are of considerable biomedical interest, the molecular biology of polyketide biosynthesis in Bacillus spp., one of the richest bacterial sources of bioactive natural products, remains largely unexplored. Here we assign for the first time complete polyketide synthase (PKS) gene clusters to Bacillus antibiotics.

Whole cell matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and in situ structure analysis of streptocidins, a family of tyrocidine-like cyclic peptides.

Streptocidins, a family of tyrocidine-like cyclic decapeptides, are an ideal demonstration object for the detection and in situ structure analysis of natural compounds directly in microbial cells using whole cell matrix-assisted laser desorption/ionization time-of-flight-mass spectrometry (MALDI-TOFMS), an emerging technique that can be used for rapid sensitive metabolic profiling of microorganisms. Five main members of the streptocidin family (A-E) were detected in Brevibacillus cells picked from agar plates and identified by in situ structure analysis with post-source decay MALDI-TOFMS. This efficient modern method allows the precise detection of metabolites within minutes without the need to isolate and purify the target compounds.

Initiation of surfactin biosynthesis and the role of the SrfD-thioesterase protein.

In this paper, the initiation reactions in surfactin biosynthesis by Bacillus subtilis OKB 105 were investigated. Evidence for a specific role of the SrfD protein, the external thioesterase enzyme in surfactin biosynthesis, was obtained for the first time. The action of SrfD was investigated both with the native, but only partially purified, enzyme and the highly purified, His-tagged protein overexpressed in Escherichia coli.