Detailed mechanism and kinetics of reactions of anti- and syn-CH3CHOO with HC(O)OH: infrared spectra of conformers of hydroperoxyethyl formate

The reaction of CH3CHOO with HC(O)OH has a large rate coefficient so that it might play a significant role in the formation of secondary organic aerosols (SOA) in the atmosphere. We investigated the detailed mechanism and kinetics of the reactions of Criegee intermediate anti- and syn-CH3CHOO with HC(O)OH with a step-scan Fourier-transform infrared spectrometer by recording time-resolved absorption spectra of transient species and end products produced upon irradiation at 308 nm of a flowing mixture of CH3CHI2/O2/HC(O)OH at 298 K and 60 Torr. Thirteen bands of hydroperoxyethyl formate [HC(O)OCH(CH3)OOH, HPEF], the hydrogen-transferred adduct of CH3CHOO and HC(O)OH, were observed. Careful analysis deconvoluted these bands into absorption of three conformers of HPEF: a transient HPEF (P2*/P3*), a more stable open-form HPEF (mainly P2), and a stable intramolecularly hydrogen-bonded HPEF (mainly P1). At a later period, the end-product formic acetic anhydride [CH3C(O)OC(O)H, FAA], a dehydrated product of HPEF, was observed; this end-product is the same as that observed in CH2OO + CH3C(O)OH. Theoretical calculations on the reaction pathway scheme were performed to elucidate these reaction paths. Syn-CH3CHOO + HC(O)OH produced conformers P2*/P3* initially, followed by conversion to conformers P2, whereas anti-CH3CHOO + HC(O)OH produced conformers P2 and P1 directly. We derived a rate coefficient for the reaction CH3CHOO + HC(O)OH to be k = (2.1 ± 0.7) × 10-10 cm3 molecule-1 s-1 at 298 K and 40-80 Torr; the rate coefficient appeared to show insignificant conformation-specificity. We also found that FAA was produced mainly from the dehydration of the open-form HPEF (P2) with a rate coefficient k = (1420 ± 70) s-1; the intramolecularly hydrogen-bonded HPEF (P1) is stable.

Structures and Anharmonic Analyses of the O-H Stretching Vibrations of Jet-Cooled Benzoic Acid (BA), (BA)(H2O)n, and (BA)2(H2O)n (n = 1, 2) Clusters, and Their Ring-Deuterated Isotopologues Measured with IR-VUV Spectroscopy─Unraveling the Complex Anharmonic Couplings in the Cyclic Structures

The IR spectra of benzoic acid (BA), (BA)(H2O)n and (BA)2(H2O)n (n = 1, 2) clusters, and their ring-deuterated isotopologues in the 2800-3750 cm-1 region were measured with IR-vacuum ultraviolet spectroscopy under the jet-cooled condition. For (BA)(H2O) and (BA)(H2O)2, only a single isomer was observed for each species, whereas for (BA)2(H2O) and (BA)2(H2O)2, more than one isomers were present. The observed IR spectra were very complex and showed similar structures between (BA)m(H2O)n and their ring-deuterated isotopologues (BA-d5)m(H2O)n for specific values of m and n. The anharmonic analysis based on the vibrational second-order perturbation theory indicated that the complexity of the IR spectra in these clusters was due to the appearance of many bands of (i) the overtone and combination modes involving the O-H bend of H2O and the in-plane C-O-H bends and the C═O stretch of BA, and (ii) the combination modes involving the hydrogen-bonded O-H stretch and low-frequency intermolecular vibrations, with considerable intensities.

Detection of a C4 Criegee Intermediate: Fourier-Transform Microwave Spectroscopy of Methacrolein Oxide

Pure rotational transitions of methacrolein oxide (MACRO) were observed by Fourier-transform microwave spectroscopy. Among the four low-lying conformers existing within an energy window of 3 kcal/mol, only the lowest-energy conformer, the anti-trans conformer, was detected in a discharged jet of a 1,3-diiode-2-methylprop-1-ene and O2 mixture diluted in Ar. Nineteen pure rotational transitions, in the frequency range from 10 to 25 GHz, most of them showing A/E splitting due to the methyl-top internal rotation, were observed and analyzed by the XIAM program, yielding the internal rotation barrier of 559 cm-1, which very well agrees with a theoretically calculated value, 558 cm-1, at the CCSD(T)/cc-pVTZ level of theory.

Infrared Characterization of the Products of the Reaction between the Criegee Intermediate CH3CHOO and HCl

The rapid reactions between Criegee intermediates and hydrogen halides play important roles in atmospheric chemistry, particularly in the polluted urban atmosphere. Employing a step-scan Fourier transform spectrometer, we recorded infrared absorption spectra of transient species and end products of the reaction CH3CHOO + HCl in a flowing mixture of CH3CHI2/HCl/O2/N2 irradiated at 308 nm. Bands at 1453.6, 1383.7, 1357.9, 1323.8, 1271.8, 1146.2, 1098.2, 1017.5, 931.5, and 847.0 cm-1 were observed and assigned to the anti-conformer of chloroethyl hydroperoxide (anti-CEHP or anti-CH3CHClOOH). In addition, absorption bands of H2O and acetyl chloride [CH3C(O)Cl, at 1819.1 cm-1] were observed; some of them were produced from the secondary reactions of CH3CHClO + O2 → CH3C(O)Cl + HO2 and OH + HCl → H2O + Cl, according to temporal profiles of H2O and CH3C(O)Cl. These secondary reactions are conceivable because the nascent formation of CH3CHClO + OH via decomposition of internally excited CEHP was predicted by theory, and both HCl and O2 are major species in the system. The nascent formation of CH3CHClO + OH appears to be more important than that of CH3C(O)Cl + H2O, consistent with theoretical predictions. By adding methanol to deplete some anti-CH3CHOO, we observed only anti-CEHP with a reduced proportion; this observation indicates that the conversion from syn-CEHP, expected to be produced from syn-CH3CHOO + HCl, to anti-CEHP is facile. We also estimated the overall rate coefficient of the reaction syn-/anti-CH3CHOO + HCl to be kHCl = (2.7 ± 1.0) × 10-10 cm3 molecule-1 s-1 at ∼70 Torr and 298 K; this rate coefficient is about six times the only literature value kHClsyn = (4.77 ± 0.95) × 10-11 cm3 molecule-1 s-1 reported for syn-CH3CHOO + HCl by Liu et al., indicating that anti-CH3CHOO reacts with HCl much more rapidly than syn-CH3CHOO.

Infrared Spectra of (Z)- and (E)-•CH2C(CH3)CHI Radicals Produced upon Photodissociation of (Z)- and (E)-(CH2I)(CH3)C═CHI in Solid para-Hydrogen

Ozonolysis of isoprene is important in atmospheric chemistry because of the abundant emission of isoprene. This process produces the Criegee intermediates CH₂OO, methyl vinyl ketone oxide (MVKO, C₂H₃C(CH₃)OO), and methacrolein oxide (MACRO, CH₂C(CH₃)CHOO). Gaseous MACRO was recently produced and identified in laboratories after photolysis of a mixture of 1,3-diiodo-2-methyl-prop-1-ene [(CH₂I)(CH₃)C═CHI] and O₂, but the conformation-dependent formation mechanism remains unexplored. We report conformation-distinct IR spectra of (E)- and (Z)-(CH₂I)(CH₃)C═CHI isolated in solid p-H₂. Upon irradiation near 300 nm of (E)- and (Z)-(CH₂I)(CH₃)C═CHI in solid p-H₂ at 3.3 K, 3-iodo-2-methyl-prop-1-en-3-yl [•CH₂C(CH₃)CHI] radicals were characterized, with intense infrared absorption lines at 2991.3, 1458.7, 1434.7, 1317.4, 1190.4, 786.3, 677.9, and 467.2 cm^-1 and additional 11 weaker ones assigned to (E)-•CH₂C(CH₃)CHI and intense lines at 3108.5, 3076.2, 3028.5, 2970.0, 1174.2, 796.0, 683.6, and 609.5 cm^-1 and additional 7 weaker ones to (Z)-•CH₂C(CH₃)CHI. The assignments were derived according to the behaviors of secondary photolysis at 495 and 460 nm and a comparison of the vibrational wavenumbers and IR intensities of the observed lines with those calculated with the B2PLYP-D3/aug-cc-pVTZ-pp method. These observations confirm that only the allylic C-I bond, not the vinylic one, was photodissociated at 290 nm, and in solid p-H₂, the excess energy upon photolysis induced no conformational change. When O₂ was present in the matrix, several intense lines at 1147.5, 1025.7, 914.4, and 728.7 cm^-1, and 4 weaker ones were tentatively assigned to the adduct CH₂C(CH₃)CHIOO; the assignments were supported by ¹⁸O₂ isotopic experiments. Unlike in the gaseous phase, the remaining C-I bond of this adduct could not break to form MACRO because of the efficient quenching in a low-temperature matrix.

Structures of (Pyrazine)2 and (Pyrazine)(Benzene) Dimers Investigated with Infrared-Vacuum Ultraviolet Spectroscopy and Quantum-Chemical Calculations: Competition among π-π, CH···π, and CH···N Interactions

The structures of a pyrazine dimer (pyrazine)₂ and (pyrazine)(benzene) hetero-dimer cooled in a supersonic beam were investigated by the measurement of the infrared spectra in the C-H stretching region with infrared-vacuum ultraviolet (IR-VUV) spectroscopy and quantum-chemical calculations. The stabilization energy calculation at the CCSD(T)/aug-cc-pVTZ level of theory predicted three isomers for (pyrazine)₂ and three for (pyrazine)(benzene) with energy within 6 kJ/mol. Among them, the cross-displaced π-π stacked structure is the most stable in both dimers. In the observed IR spectra, both dimers exhibited two intense bands near 3065 cm^-1, with intervals of 8 cm^-1 in (pyrazine)₂ and 11 cm^-1 in (pyrazine)(benzene), while only one band appeared in the monomer. For (pyrazine)(benzene), we also measured the IR spectrum of (pyrazine)(benzene-d₆), where the interval of the two bands was unchanged. The analysis of the observed IR spectra with anharmonic calculations suggested the coexistence of three isomers of (pyrazine)₂ and (pyrazine)(benzene) in a supersonic jet. For (pyrazine)₂, the two isomers which were previously assigned to the H-bonded planar and the π-π stacked structures respectively were reassigned to the cross-displaced π-π stacked and T-shaped structures, respectively. In addition, the quantum chemical calculation and IR-VUV spectral measurement suggested the coexistence of the H-bonded planar isomer in the jet. For (pyrazine)(benzene), the IR spectrum of the (pyrazine) site showed a similar spectral pattern to that of (pyrazine)₂, especially the split at ∼3065 cm^-1. However, the anharmonic analysis suggested that they are assigned to the different vibrational motions of (pyrazine). The anharmonic vibrational analysis is essential to associate the observed IR spectra with the correct structures of the dimer.

Infrared spectra of isoquinolinium (iso-C9H7NH+) and isoquinolinyl radicals (iso-C9H7NH and 1-, 3-, 4-, 5-, 6-, 7- and 8-iso-HC9H7N) isolated in solid para-hydrogen

Protonated polycyclic aromatic nitrogen heterocycles (H⁺PANH) are prospective candidates that may contribute to interstellar unidentified infrared (UIR) emission bands because protonation enhances the relative intensities of the bands near 6.2, 7.7 and 8.6 μm, and the presence of the N atom induces a blue shift of the ring-stretching modes so that the spectra of H⁺PANH match better with the 6.2 μm feature in class-A UIR spectra. We report the infrared (IR) spectra of protonated isoquinoline (the 2-isoquinolinium cation, iso-C₉H₇NH⁺), its neutral counterpart (the 2-isoquinolinyl radical, iso-C₉H₇NH), and another mono-hydrogenated product (the 6-isoquinolinyl radical, 6-iso-HC₉H₇N), produced on the electron-bombardment of a mixture of isoquinoline (iso-C₉H₇N) with excess para-hydrogen (p-H₂) during matrix deposition at 3.2 K. To generate additional isomers of hydrogenated isoquinoline, we irradiated iso-C₉H₇N/Cl₂/p-H₂ matrices at 365 nm to generate Cl atoms, followed by IR irradiation to generate H atoms via Cl + H₂ (v = 1) → HCl + H; the H atoms thus generated reacted with iso-C₉H₇N. In addition to iso-C₉H₇NH and 6-iso-HC₉H₇N observed in the electron-bombardment experiments, we identified six additional hydrogenated isoquinoline species, 1-, 3-, 4-, 5-, 7- and 8-iso-HC₉H₇N, via their IR spectra; hydrogenation on the N atom and all available carbon atoms except for the two sharing carbon atoms on the fused ring was observed. Spectral groupings were achieved according to their behaviors after maintenance of the matrix in darkness and on secondary photolysis at various wavelengths. The assignments were supported via comparison of the experimental results with the vibrational wavenumbers and IR intensities of possible isomers predicted using the B3LYP/6-311++G(d,p) method. The implications in the identification of the UIR band are discussed.

Spectral Evidence of Bevel-Gear-Type Rotation of Benzene around Br in Solid p-H2: Infrared Spectrum of the C6H6Br Radical

Whether the structure of C₆H₆X (X = halogen), an intermediate in the halogenation of benzene, is an open or a bridged form has been debated. We produced Br to react with C₆H₆ upon photolysis in situ of a Br₂/C₆H₆/p-H₂ matrix at 3.2 K. In contrast to the C₆H₆Cl σ-complex reported previously, the observed infrared spectrum indicates that C₆H₆Br is an open-form π-complex. Furthermore, lines of the two CH out-of-plane bending modes associated mainly with even- and odd-numbered carbons, predicted near 672 and 719 cm^-1, merged into a broad line at 697.3 cm^-1, indicating that these modes become nearly equivalent as Br migrates from one carbon atom to another. Quantum-chemical calculations support that the benzene ring performs a bevel-gear-type rotation with respect to Br. Observation of only trans-ortho- and trans-para-C₆H₆Br₂ suggests that this gear-type motion allows the additional Br atom to attack C₆H₆Br only from the opposite side of the Br atom in C₆H₆Br.

Infrared Characterization of the Products and Rate Coefficient of the Reaction between Criegee Intermediate CH2OO and HNO3

The reactions of Criegee intermediates with HNO₃ are important in the polluted urban atmosphere because of their large rate coefficients and the significant concentration of HNO₃. Employing a step-scan Fourier-transform spectrometer, we recorded infrared spectra of transient species and end products in the reaction CH₂OO + HNO₃ upon irradiation of a flowing mixture of CH₂I₂/HNO₃/N₂/O₂ at 308 nm. Eight bands at 1686, 1426, 1348, 1294, 1052, 965, 891, and 825 cm^-1 were assigned to the absorption of the adduct nitrooxymethyl hydroperoxide (NMHP, NO₃CH₂OOH). Additional products from two dissociation channels were observed. Four bands at 1709, 1325, 1276, and 886 cm^-1 were assigned to H₂C(O)ONO₂ (with coproduct OH), produced from the fission of the O-O bond of internally hot NMHP (NMHP*). Simultaneous detection of H₂CO (1746 cm^-1), NO₂ (1617 cm^-1), and HO₂ (1392 and 1098 cm^-1) indicated a direct cleavage of the N-OC and C-OO bonds of NMHP*. The relative yields of these three channels in pressure range 10-150 Torr were estimated. At 10 Torr, the absorption of internally excited HNO₃ near 885 and 1320 cm^-1 was also detected at an early stage of the reaction. We investigated also the rate coefficient of the reaction CH₂OO + HNO₃ by probing the temporal profiles of the formation of NMHP and NO₂ under total pressures of 40 and 70 Torr at 298 K. The rate coefficient k_HNO3 = (2.4 ± 0.4) × 10^-10 cm³ molecule^-1 s^-1 is less than half the only literature value, (5.4 ± 1.0) × 10^-10 cm³ molecule^-1 s^-1, reported by Foreman et al. (Angew. Chem. Int. Ed. 2016, 55, 10419-10422).

Infrared and Laser-Induced Fluorescence Spectra of Sumanene Isolated in Solid para-Hydrogen

The para-hydrogen (p-H₂) matrix-isolation technique has been scarcely used to record electronic absorption and emission spectra. It is expected that its small matrix shifts due to diminished molecular interactions and the softness of the lattice might be advantageous to help identify the carriers of the diffuse interstellar bands. In this article, we present infrared, fluorescence excitation, and dispersed fluorescence spectra of sumanene (C₂₁H₁₂), a bowl-shaped polycyclic aromatic hydrocarbon and a fragment of C₆₀, isolated in solid p-H₂. The recorded vibrational wavenumbers from infrared and dispersed fluorescence agree with the scaled harmonic vibrational wavenumbers calculated with the B3PW91/6-311++G(2d,2p) and B3LYP/6-311++G(2d,2p) methods. The recorded fluorescence excitation spectra are consistent with the spectra of jet-cooled gas-phase C₂₁H₁₂ reported previously by Kunishige et al. We found a rather small matrix shift of 55 cm^-1 for the S₁-S₀ electronic transition origin located at 27 888 cm^-1. Vibrational wavenumbers associated with the S₁ state of C₂₁H₁₂ inferred from the experimental spectrum can be assigned mostly to fundamental normal modes; they are in satisfactory agreement with scaled harmonic vibrational wavenumbers calculated at the TD-B3PW91/6-311++G(2d,2p) level of theory. Significantly more vibrational modes of the S₁ state were identified as compared with those in the reported gas-phase work. The potential of p-H₂ matrix-isolation spectroscopy to provide electronic excitation spectra suitable for comparison to astronomical observations is discussed by comparing the spectra of C₂₁H₁₂ isolated in solid p-H₂ and in solid Ne, a matrix host commonly employed in astrochemistry.

Mechanism and kinetics of the reaction of the Criegee intermediate CH2OO with acetic acid studied using a step-scan Fourier-transform IR spectrometer

Acetic acid, CH₃C(O)OH, plays an important role in the acidity of the troposphere. The reactions of Criegee intermediates with CH₃C(O)OH have been proposed to be a potential source of secondary organic aerosol in the atmosphere. We investigated the detailed mechanism and kinetics of the reaction of the Criegee intermediate CH₂OO with CH₃C(O)OH. The time-resolved infrared absorption spectra of transient species produced upon irradiation at 308 nm of a flowing mixture of CH₂I₂/O₂/CH₃C(O)OH at 298 K were recorded using a step-scan Fourier-transform infrared spectrometer. The decrease in the intensity of the bands of CH₂OO was accompanied by the appearance of bands near 886, 971, 1021, 1078, 1160, 1225, 1377, 1402, 1434, and 1777 cm^-1, assigned to the absorption of hydroperoxymethyl acetate [CH₃C(O)OCH₂OOH, HPMA], the hydrogen-transferred adduct of CH₂OO and CH₃C(O)OH. Two types of conformers of HPMA, an open form and an intramolecularly hydrogen-bonded form, were identified. At a later reaction period, bands of the open-form HPMA became diminished, and new bands appeared at 930, 1045, 1200, 1378, 1792, and 1810 cm^-1, assigned to formic acetic anhydride [CH₃C(O)OC(O)H, FAA], a dehydrated product of HPMA. The intramolecularly hydrogen-bonded HPMA is more stable. From the temporal profiles of HPMA and FAA, we derived a rate coefficient k = (1.3 ± 0.3) × 10^-10 cm³ molecule^-1 s^-1 for the reaction CH₂OO + CH₃C(O)OH to form HPMA and a rate coefficient k = 980 ± 40 s^-1 for the dehydration of the open-form HPMA to form FAA. Theoretical calculations were performed to elucidate the CH₂OO + CH₃C(O)OH reaction pathway and to understand the distinct reactivity of these two forms of HPMA.

Hydrogen-Atom-Assisted Uphill Isomerization of N-Methylformamide in Darkness

N-Methylformamide, HC(O)NH(CH₃), is the smallest amide detected in the interstellar medium that can exist as cis and trans isomers. We performed reactions of H atoms with trans-NMF in solid para-hydrogen at 3.3 K and found that the cis-NMF isomer, which has higher energy, increased continuously in darkness, demonstrating a previously overlooked and seemingly unlikely isomerization of prebiotic molecules through H-atom tunneling reactions in the absence of light. Infrared spectra of radical intermediates trans-•C(O)NH(CH₃) and trans-HC(O)NH(•CH₂) were identified. Further H addition and H abstraction enhanced the formation of CH₃NCO, HNCO, and CH₂NH in the H-rich experiments. These results indicate that, unlike the dual cycle of H-abstraction and H-addition channels chemically linking formamide and HNCO, the H addition to CH₃NCO produced only cis-radicals that led to cis-NMF. Furthermore, H-atom-induced fragmentation by breaking the C-C bond provides links between NMF and HCNO/CH₂NH. These endothermic isomerization/decomposition reactions become possible through the coupling with H + H → H₂.

A chemical link between methylamine and methylene imine and implications for interstellar glycine formation

Methylamine CH₃NH₂ is considered to be an important precursor of interstellar amino acid because hydrogen abstraction might lead to the aminomethyl radical •CH₂NH₂ that can react with •HOCO to form glycine, but direct evidence of the formation and spectral identification of •CH₂NH₂ remains unreported. We performed the reaction H + CH₃NH₂ in solid p-H₂ at 3.2 K and observed IR spectra of •CH₂NH₂ and CH₂NH upon irradiation and when the matrix was maintained in darkness. Previously unidentified IR spectrum of •CH₂NH₂ clearly indicates that •CH₂NH₂ can be formed from the reaction H + CH₃NH₂ in dark interstellar clouds. The observed dual-cycle mechanism containing two consecutive H-abstraction and two H-addition steps chemically connects CH₃NH₂ and CH₂NH in interstellar media and explains their quasi-equilibrium. Experiments on CD₃NH₂ produced CD₂HNH₂, in addition to •CD₂NH₂ and CD₂NH, confirming the occurrence of H addition to •CD₂NH₂.

Infrared Spectra of 1-Quinolinium (C9H7NH+) Cation and Quinolinyl Radicals (C9H7NH and 3-, 4-, 7-, and 8-HC9H7N) Isolated in Solid para-Hydrogen

Large protonated polycyclic aromatic hydrocarbons (H⁺PAH) and the corresponding nitrogen heterocycles (H⁺PANH) have been proposed as possible carriers of unidentified infrared (UIR) emission bands from galactic objects. The nitrogen atom in H⁺PANH is expected to induce a blue shift of the band associated with the CC-stretching mode of H⁺PAH near 6.3 μm so that their emission bands might agree better with the UIR band near 6.2 μm. We report the IR spectrum of protonated quinoline (1-quinolinium cation, C₉H₇NH⁺) and its neutral species (1-quinolinyl radical, C₉H₇NH) measured upon electron bombardment during the deposition of a mixture of quinoline (C₉H₇N) and para-hydrogen (p-H₂) at 3.2 K, indicating that the protonation and hydrogenation occur mainly at the N atom site. Additional experiments on the irradiation of C₉H₇N/Cl₂/p-H₂ matrices at 365 nm to generate Cl atoms, followed by irradiation with IR light to generate H atoms via Cl + H₂ (v = 1), were performed to induce the reaction H + C₉H₇N. This method proved to be efficient for hydrogenation reactions in solid p-H₂; we identified, in addition to C₉H₇NH observed in electron-bombardment experiments, four radicals with hydrogenation at the C-atom site─3-, 4-, 7-, and 8-HC₉H₇N. Spectral assignments were achieved according to the behavior upon secondary photolysis and a comparison of experimental results with vibrational wavenumbers and IR intensities predicted with the B3LYP/6-311++G(d,p) method. The observed lines at 1641.4, 1598.4, and 1562.0 cm^-1 associated with the CC-stretching mode of C₉H₇NH⁺ are blue-shifted from those at 1618.7, 1580.8, 1556.7, and 1510.0 cm^-1 of the corresponding protonated naphthalene (C₁₀H₉⁺).

Dynamics of Reaction CH3CHI + O2 Investigated via Infrared Emission of Products CO, CO2, and OH

The reaction CH₃CHI + O₂ has been commonly employed in laboratories to produce a methyl-substituted Criegee intermediate CH₃CHOO, but the detailed dynamics of this reaction remain unexplored. We carried out this reaction by irradiating a flowing mixture of CH₃CHI₂ (∼70 mTorr) and O₂ (∼4 and 8 Torr) at 308 or 248 nm and observed infrared emission of the products with a step-scan Fourier-transform spectrometer. Upon irradiation at 248 nm with O₂ ∼4 Torr, a Boltzmann distribution of CO (v ≤ 4, J ≤ 25) with average vibrational energy (12 ± 2) kJ mol^-1 and of OH (v = 1, J ≤ 5.5) were observed and assigned to be produced from the decomposition of CH₃C(O)OH* to form CO + CH₃OH and OH + CH₃CO, respectively. The observed broadband emission of CO₂ was simulated with two vibrational distributions of average energies (42 ± 3) and (114 ± 6) kJ mol^-1 and assigned to be produced from the decomposition of CH₃C(O)OH* and (methyl dioxirane)*, respectively. The results upon irradiation of the sample at 308 nm are similar, likely indicating a small fraction of energy partition into these products and rapid thermalization of CH₃CHI*. Compared with reaction CH₂I + O₂, the title reaction yielded products with much less internal excitation, consistent with the expectation that these observed products receive much less fraction of available energy upon fragmentation when an additional methyl moiety was present in the parent. The large-v component of CO observed in experiments of CH₂I + O₂ at 248 nm, produced from secondary reaction HCO + O₂, was absent in this work because the corresponding secondary reaction CH₃CO + O₂ in decomposition of CH₃CHOO* produces α-lactone + OH or H₂CO + CO + OH.

Structures of Pyridine-Water Clusters Studied with Infrared-Vacuum Ultraviolet Spectroscopy

The infrared (IR) spectra of the O-H stretching vibrations of pyridine-water clusters (Pyd)_m(H₂O)_n, with m, n = 1-4, have been investigated with infrared-vacuum ultraviolet (VUV) spectroscopy under a jet-cooled condition. The time-of-flight mass spectrum of (Pyd)_m(H₂O)_n⁺ by VUV ionization at ∼9 eV showed an unusual intensity pattern with very weak ion signals for m = 1 and 2 and stronger signals for m ≥ 3. This unusual mass pattern was explained by a drastic structural change of (Pyd)_m(H₂O)_n upon the VUV ionization, which was followed by the elimination of water molecules. Among the recorded IR spectra, only one spectrum monitored, (Pyd)₂⁺ cation, showed a well-resolved structure. The spectrum was analyzed by comparing with the simulated ones of possible stable isomers of (Pyd)₂(H₂O)_n, which were obtained with quantum-chemical calculations. Most of the calculated (Pyd)₂(H₂O)_n clusters had the characteristic structure in which H₂O or (H₂O)₂ forms a hydrogen-bonded bridge between two pyridines to form the π-stacked (Pyd)₂, and an additional H₂O molecule(s) extends the H-bonded network. The π-stacked (Pyd)₂(H₂O)_n moiety is very stable and is thought to exist as a local structure in a pyridine/water mixed solution. The Fermi resonance between the O-H stretch fundamentals and the overtones of the O-H bending vibrations in (Pyd)_m(H₂O)_n was found to be less pronounced in the case of (Pyd)_m(NH₃)_n studied previously.

Non-energetic, Low-Temperature Formation of Cα-Glycyl Radical, a Potential Interstellar Precursor of Natural Amino Acids

The reaction of H atoms with glycine was investigated at 3.1 K in para-H₂, a quantum-solid host. The reaction was followed by IR spectroscopy, with the spectral analysis aided by quantum chemical computations. Comparison of the experimental IR spectrum with computed anharmonic frequencies and intensities proved that, regardless of the reactant glycine conformation, C_α-glycyl radical is formed in an H-atom-abstraction process with great selectivity. The product of the second H-atom abstraction, iminoacetic acid, was also observed in a smaller amount. The C_α-glycyl radical is sensitive to UV light and decomposes to iminoacetic acid and H atom upon 280 nm radiation. Since the reactive radical center is located on the C_α-atom, it is suggested that natural α-amino acids can be formed from glycine via the C_α-glycyl radical by non-energetic mechanisms in the solid phase of the interstellar medium.

Vacuum Ultraviolet Photoionization Induced Proton Migration and Formation of a New C-N Bond in Pyridine Clusters Revealed by Infrared Spectroscopy and Mass Spectrometry

The structures and reactions of pyridine (Pyd) cluster cations in a supersonic molecular beam generated upon photoionization at 9.2-9.4 eV were investigated by infrared (IR) action spectroscopy. The mass spectrum showed prominent peaks of (Pyd)_m⁺ and H⁺(Pyd)_m, m = 1-5. In the pyridine/pyridine-d₅ mixture, the mass pattern indicated that H⁺ and D⁺ migrated during the formation and dissociation of the cluster cations. The IR photodissociation spectra of both (Pyd)₂⁺ and H⁺(Pyd)₂ revealed a N-H stretching band near 3400 cm^-1, indicating that their structures are 1-(2-pyridyl)pyridin-1-ium and pyridinium-pyridine, respectively. Observation of the former product implies that the reaction proceeds via an α-distonic cation intermediate, while the latter product is formed via proton migration. The IR spectra of (Pyd)_m⁺ and H⁺(Pyd)_m, m ≥ 3, suggested that these clusters consist of a covalently bound (Pyd)₂⁺ or H⁺(Pyd)₂ core, respectively, with additional pyridines attached to them via hydrogen bonds and/or weak dispersive interactions.

Infrared characterization of the products and the rate coefficient of the reaction between Criegee intermediate CH2OO and HCl

Reactions between Criegee intermediates and hydrogen halides might be significant, particularly in the polluted urban atmosphere, because of their large rate coefficients. Employing a Fourier-transform spectrometer in a step-scan mode or a continuous-scan mode, we recorded infrared spectra of transient species and end products in a flowing mixture of CH₂I₂/HCl/N₂/O₂ irradiated at 308 nm. Five bands near 823.2, 1061.1, 1248.4, 1309.2, and 1359.6 cm^-1 were observed and assigned to the gauche-conformer of chloromethyl hydroperoxide (CMHP, CH₂ClOOH). At a later time of the reaction, absorption bands of H₂O and formyl chloride (CHClO) at 1782.9 cm^-1 were observed; these species were likely produced from the secondary reactions of CH₂ClO + O₂→ CHClO + HO₂ and OH + HCl → H₂O + Cl according to temporal profiles of CMHP, H₂O, and CHClO; formation of CH₂ClO + OH via decomposition of internally excited CMHP was predicted by theory and both HCl and O₂ are major species in the system. We investigated also the rate coefficient of the reaction CH₂OO + HCl on probing CH₂OO with a continuous-wave infrared quantum-cascade laser absorption system under total pressure 5.2-8.2 torr at 298 K. The rate coefficient k_HCl = (4.8 ± 0.4) × 10^-11 cm³ molecule^-1 s^-1, is comparable to the only literature value k_HCl = (4.6 ± 1.0) × 10^-11 cm³ molecule^-1 s^-1 reported by Foreman et al.

Formation and Infrared Spectrum of the Open-Form 2-Bromoethyl Radical (2-C2H4Br•) from Ultraviolet Irradiation of a C2H4/Br2/p-H2 Matrix

The addition reaction of halogens to alkenes is important in organic synthesis, but the reaction intermediate has rarely been detected. Whether the structure of the intermediate bromoethyl (C₂H₄Br^•) radical is a bridged form or an open form is unclear. We took advantage of the diminished cage effect of solid p-H₂ and employed infrared (IR) absorption to record the IR spectrum of C₂H₄Br^• after photolysis of a C₂H₄/Br₂/p-H₂ matrix at 254 nm, followed by annealing. New spectral features at 676.9, 776.7, 1068.5, 1148.0, 3041.8, and 3126.8 cm^-1 are assigned to the open-form 2-bromoethyl radical, according to their photolytic behavior and comparison with scaled harmonic vibrational wavenumbers and IR intensities calculated with the B2PLYPD3/6-311++G(2df,2p) method.

A Direct Mapping Approach to Understand Carrier Relaxation Dynamics in Varied Regions of a Polycrystalline Perovskite Film

We developed a direct mapping approach to overlay the image of a polycrystalline perovskite film obtained from the transient absorption microscope (TAM) with that from the scanning electron microscope (SEM). By mapping these imaging data pixel by pixel, we are able to observe the relaxation dynamics of the photo-generated charge carriers on varied regions of the film. The carrier relaxation dynamics contain a dominated single-exponential decay component owing to the recombination of charge carriers. The lifetime distribution of charge recombination shows a bimodal feature, for which the rapid and slow distributions are assigned as free and trapped carriers, respectively. The charge recombination was slower in the grain boundary (GB) region than in the grain interior (GI) region. The small grains have longer lifetimes than the large grains for the crystal size smaller than 500 nm. Therefore, GB with retarded charge recombination might play a positive role in a perovskite solar cell.

IR-VUV spectroscopy of pyridine dimers, trimers and pyridine-ammonia complexes in a supersonic jet

The infrared spectra of the C-H stretching vibrations of (pyridine)m, m = 1-3, and the N-H stretching vibrations of (pyridine)m-(NH3)n, m = 1, 2; n = 1-4, complexes were investigated by infrared (IR)-vacuum ultraviolet (VUV) spectroscopy under jet-cooled conditions. The ionization potential (IP0) of the pyridine monomer was determined to be 74 546 cm-1 (9.242 eV), while its complexes showed only smooth curves of the ionization thresholds at ∼9 eV, indicating large structural changes in the ionic form. The pyridine monomer exhibits five main features with several satellite bands in the C-H stretching region at 3000-3200 cm-1. Anharmonic calculations including Fermi-resonance were carried out to analyze the candidates of the overtone and combination bands which can couple to the C-H stretching fundamentals. For (pyridine)2 and (pyridine)3, most C-H bands are blue-shifted by 3-5 cm-1 from those of the monomer. The structures revealed by random searching algorithms with density functional methods indicate that the π-stacked structure is most stable for (pyridine)2, while (pyridine)3 prefers the structures stabilized by dipole-dipole and C-Hπ interactions. For the (pyridine)m-(NH3)n complexes, the mass spectrum exhibited a wide range distribution of the complexes. The observed IR spectra in the N-H stretching vibrations of the complexes showed four main bands in the 3200-3450 cm-1 region. These features are very similar to those of (NH3)n complexes, and the bands are assigned to the anti-symmetric N-H stretching band (ν3), the symmetric N-H stretching (ν1) band, and the first overtone bands of the N-H bending vibrations (2ν4). The anharmonic calculations including the Fermi-resonance between ν1 and 2ν4 well reproduced the observed spectra.

Infrared Spectra of Monohydrogenated Aniline, ortho- and para-HC6H5NH2, Generated in Solid para-Hydrogen

The isomers of monohydrogenated aniline (HC₆H₅NH₂) are regarded as important intermediates in reduction reactions of aniline, but their spectral identification has been limited to electron paramagnetic resonance in an adamantane matrix. We report here infrared (IR) spectra of two least-energy isomers of HC₆H₅NH₂, produced on electron bombardment during the deposition of a matrix of aniline and para-hydrogen at 3.2 K. The intensities of IR lines of HC₆H₅NH₂ increased during maintenance of the electron-bombarded matrix in darkness for a prolonged period because of the neutralization of protonated aniline, H⁺C₆H₅NH₂, by trapped electrons and further reactions between aniline and the unreacted hydrogen atoms that were produced during electron bombardment. The observed lines were grouped according to their behaviors on secondary photolysis with light at 520, 465, and 375 nm. On comparison of experimental spectra with quantum chemically predicted spectra for four possible isomers of HC₆H₅NH₂, lines in one group were assigned to the most stable ortho-HC₆H₅NH₂ and those in the other group were assigned to the secondmost stable para-HC₆H₅NH₂. Their photolytic behaviors at varied wavelengths are consistent with predicted ultraviolet absorption bands. The mechanisms of formation of these isomers are discussed according to semiquantitative analysis.

UV/Vis+ photochemistry database: Structure, content and applications

The "science-softCon UV/Vis⁺ Photochemistry Database" (www.photochemistry.org) is a large and comprehensive collection of EUV-VUV-UV-Vis-NIR spectral data and other photochemical information assembled from published peer-reviewed papers. The database contains photochemical data including absorption, fluorescence, photoelectron, and circular and linear dichroism spectra, as well as quantum yields and photolysis related data that are critically needed in many scientific disciplines. This manuscript gives an outline regarding the structure and content of the "science-softCon UV/Vis⁺ Photochemistry Database". The accurate and reliable molecular level information provided in this database is fundamental in nature and helps in proceeding further to understand photon, electron and ion induced chemistry of molecules of interest not only in spectroscopy, astrochemistry, astrophysics, Earth and planetary sciences, environmental chemistry, plasma physics, combustion chemistry but also in applied fields such as medical diagnostics, pharmaceutical sciences, biochemistry, agriculture, and catalysis. In order to illustrate this, we illustrate the use of the UV/Vis⁺ Photochemistry Database in four different fields of scientific endeavor.

Dynamics of the reaction CH2I + O2 probed via infrared emission of CO, CO2, OH and H2CO

The reaction CH2I + O2 has been widely employed recently for the production of the simplest Criegee intermediate CH2OO in laboratories, but the detailed dynamics of this reaction have been little explored. Infrared emission of several products of this reaction, initiated on irradiation of CH2I2 and O2 (∼8 Torr) in a flowing mixture at 308 or 248 nm, was recorded with a step-scan Fourier-transform spectrometer; possible routes of formation were identified according to the observed vibrational distribution of products and published theoretical potential-energy schemes. Upon irradiation at 308 nm, Boltzmann distributions of CO (v ≤ 5, J ≤ 19) with an average vibrational energy of 32 ± 3 kJ mol-1 and OH (v ≤ 3, J ≤ 5.5) with an average vibrational energy of 29 ± 4 kJ mol-1 were observed and assigned to the decomposition of HCOOH* to form CO + H2O and OH + HCO, respectively. The broadband emission of CO2 was simulated with two vibrational distributions of average energies (91 ± 4) and (147 ± 8) kJ mol-1 and assigned to be produced from the decomposition of HCOOH* and methylene bis(oxy), respectively. Upon irradiation of samples at 248 nm, the emission of OH and CO2 showed similar distributions with slightly greater energies, but the distribution of CO (v ≤ 11, J ≤ 19) became bimodal with average vibrational energies of (23 ± 4) and (107 ± 29) kJ mol-1, and branching (56 ± 5) : (44 ± 5). The additional large-v component is assigned to be produced from a secondary reaction HCO + O2 to form CO + HO2; HCO is a coproduct of OH. The branching between CO and OH is (50 ± 5) : (50 ± 5) at 308 nm and (64 ± 5) : (36 ± 4) at 248 nm, consistent with the mechanism according to which an additional channel to produce CO opens at 248 nm. Highly internally excited H2CO was also observed. With O2 at 16 Torr, the extrapolated nascent internal distributions are similar to those with O2 at 8 Torr except for a slight quenching effect.

Infrared Spectra of (Z)- and (E)-•C2H3C(CH3)I Radicals Produced upon Photodissociation of (Z)- and (E)-(CH2I)HC═C(CH3)I in Solid para-Hydrogen

Ozonolysis of isoprene to produce Criegee intermediates such as methyl vinyl ketone oxide (MVKO), C₂H₃C(CH₃)OO, is an important process in atmospheric chemistry. MVKO was recently produced and identified in laboratories after photolysis of a gaseous mixture of 1,3-diiodo-but-2-ene, (CH₂I)HC═C(CH₃)I, and O₂, but the mechanism of its formation remains unexplored. We synthesized pure (Z)- and (E)-1,3-diiodo-but-2-ene and measured their distinct IR spectra. Upon irradiation at 280 nm of (Z)- and (E)-1,3-diiodo-but-2-ene in solid p-H₂ at 3.3 K, the fission of the terminal C-I bond yields (Z)- and (E)-3-iodo-but-2-en-1-yl [^•C₂H₃C(CH₃)I] radicals, respectively. These radicals were characterized with infrared absorption lines at 2962.4, 1423.8, 1265.3, 1120.9/1127.0, 921.4/922.3, and 792.5/791.7 cm^-1, and 16 additional weaker lines for (Z)-^•C₂H₃C(CH₃)I and 1405.2, 1208.2, 1106.0/1103.9, 934.2/933.4, and 785.1/784.9 cm^-1 and five additional weaker ones for (E)-^•C₂H₃C(CH₃)I. The assignments were derived according to behavior on secondary photolysis and comparison of the vibrational wavenumbers and the IR intensities of observed lines with those calculated with the B2PLYP-D3/aug-cc-pVTZ-pp method. These observations confirmed that only the terminal I atom, not the central one, was photodissociated at 280 nm and, in solid p-H₂, the excess energy after photodissociation induced no change in conformation. These new spectra of ^•C₂H₃C(CH₃)I radicals can provide valuable information for the understanding of the mechanism of formation of Criegee intermediate MVKO from the source reaction of photolysis of (CH₂I)HC═C(CH₃)I in O₂ in the laboratory.

Hydrogen abstraction in astrochemistry: formation of ˙CH2CONH2 in the reaction of H atom with acetamide (CH3CONH2) and photolysis of ˙CH2CONH2 to form ketene (CH2CO) in solid para-hydrogen

The amide bond of acetamide is unaffected by hydrogen exposure, but the hydrogen abstraction on its methyl site activates this molecule to react with other species to extend its size as a first step to form interstellar complex organic molecules.

Infrared spectrum of hydrogenated corannulene rim-HC20H10 isolated in solid para-hydrogen

Hydrogenated polycyclic aromatic hydrocarbons have been proposed to be carriers of the interstellar unidentified infrared (UIR) emission bands and the catalysts for formation of H₂; spectral characterizations of these species are hence important. We report the infrared (IR) spectrum of mono-hydrogenated corannulene (HC₂₀H₁₀) in solid para-hydrogen (p-H₂). In experiments of electron bombardment of a mixture of corannulene and p-H₂ during deposition of a matrix at 3.2 K, two groups of spectral lines increased with time during maintenance of the matrix in darkness after deposition. Lines in one group were assigned to the most stable isomer of hydrogenated corannulene, rim-HC₂₀H₁₀, according to the expected chemistry and a comparison with scaled harmonic vibrational wavenumbers and IR intensities predicted with the B3PW91/6-311++G(2d,2p) method. The lines in the other group do not agree with predicted spectra of other HC₂₀H₁₀ isomers and remain unassigned. Alternative hydrogenation was achieved with H atoms produced photochemically in the infrared-induced reaction Cl + H₂ (v = 1) → H + HCl in a Cl₂/C₂₀H₁₀/p-H₂ matrix. With this method, only lines attributable to rim-HC₂₀H₁₀ were observed, indicating that hydrogenation via a quantum-mechanical tunneling mechanism produces preferably the least-energy rim-HC₂₀H₁₀ regardless of similar barrier heights and widths for the formation of rim-HC₂₀H₁₀ and hub-HC₂₀H₁₀. The mechanisms of formation in both experiments are discussed. The bands near 3.3 and 3.4 µm of rim-HC₂₀H₁₀ agree with the UIR emission bands in position and relative intensity, but other bands do not match satisfactorily with the UIR bands.

Effects of solvent molecules on hemi-bonded (CH3SH)2+: infrared absorption of [(CH3SH)2-X]+ with X = H2O, (CH3)2CO, or NH3 and (CH3SH)n+ (n = 3-6)

Three-electron two-center (3e-2c) hemi-bonds play important roles in the oxidation and electron transport of proteins and are implicated to be involved in some neurodegenerative diseases. Our previous investigations on infrared (IR) spectra of (CH3SH)2+ using vacuum-ultraviolet photoionization, infrared dissociation, and time-of-flight detection have shown that (CH3SH)2+ is (3e-2c)-bonded. To investigate the influence of the solvent molecules on the (3e-2c)-bonded (CH3SH)2+ in a supersonic jet, we added H2O or (CH3)2CO or NH3 or (CH3SH)n (n = 1-4) to (CH3SH)2+ and investigated their IR action spectra. The (3e-2c)-bonded (CH3SH)2+ ion core was maintained when a molecule of H2O or (CH3)2CO or CH3SH binds, indicating that the ion core is more stable than the hydrogen bond, whereas the (3e-2c)-bond became broken by a NH3 molecule because the proton transfer led to a more stable hydrogen-bonded structure. The spectral features of the SH-stretching modes of (CH3SH)n+ (n = 3-6) indicate that the (3e-2c)-bonded (CH3SH)2+ ion core is maintained and the first two additional CH3SH are H-bonded to the free SH groups of the ion core. For larger clusters with n = 5 and 6, the additional solvent molecules likely bind to the first solvation shell. These results show also that the (3e-2c)-bonded S∴S structure is more stable than the S∴O and S∴N structures in [(CH3SH)2-X]+ with X = H2O or (CH3)2CO or CH3SH or NH3.

Formation and infrared identification of protonated fluoranthene isomers 3-, 9-, and 10-C16H11+ in solid para-H2

Polycyclic aromatic hydrocarbons (PAH) and their derivatives are prospective carriers of unidentified infrared (UIR) emission features observed in interstellar media. Fluoranthene (C16H10) is a simple planar PAH with five- and six-membered rings; it can be considered as a fragment of C60, which, along with its cationic counterpart, has been identified in interstellar media. Protonated fluoranthene, C16H11+, was generated upon electron bombardment during deposition at 3.2 K of p-H2 containing fluoranthene in a small proportion. The intensities of infrared features of C16H11+ decreased after maintaining the matrix in darkness because of its neutralization with trapped electrons. According to the correlations in intensities upon neutralization and secondary photolysis, observed lines were classified into three groups which are assigned to isomers 3-C16H11+, 9-C16H11+, and 10-C16H11+. Experimental vibrational wavenumbers and relative IR intensities of the features agree with corresponding calculated values predicted for these three isomers of C16H11+ with the B3PW91/6-311++G(2d,2p) method. 3-C16H11+ and 9-C16H11+ are predicted to have the lowest energy (within 5 kJ mol-1), whereas 10- and 1-C16H11+ are lying above the global minimum 3-C16H11+ by ∼20 kJ mol-1. However, definitive identification of 1-C16H11+ could not be made as only the most intense line is tentatively assigned. Although the observed spectra of these isomers match unsatisfactorily with the UIR bands, they will facilitate the potential terrestrial and extraterrestrial identification of these species.

Detailed mechanism and kinetics of the reaction of Criegee intermediate CH2OO with HCOOH investigated via infrared identification of conformers of hydroperoxymethyl formate and formic acid anhydride

Infrared spectra of hydroperoxymethyl formate indicate that the open-form conformer decomposes to formic-acid anhydride, but the hydrogen-bonded one does not.

Rate coefficient of the reaction CH2OO + NO2 probed with a quantum-cascade laser near 11 μm

Employing a cw quantum-cascade laser coupled with Herriott mirrors to probe CH₂OO, we report a rate coefficient k = (1.0 ± 0.2) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ for the reaction CH₂OO + NO₂ at 298 K, which is much smaller than literature values.

Tunable quad-band transmission response, based on single-layer metamaterials

We investigated the electromagnetically induced transparency (EIT)-like effects in planar metamaterials (MMs) at microwave (GHz) frequencies. The specific MMs that were used in this study consist of cut-wire resonator/ring resonator, which achieved the dual EIT-like effects in a single-layer through the bright- and quasi-dark-mode coupling and the lattice mode coupling. In addition, by varying the distance between the two resonators, the quad-band EIT spectral response in the microwave region was obtained, and the group refractive index at the EIT-like resonance of proposed design reached up to 4,000. This study provides the design approach to the multispectral EIT-like effects and might suggest potential applications in a variety of fields, for example, low-loss slow-light device, multiple switching sensor, and other sensing devices.

Identification and Self-Reaction Kinetics of Criegee Intermediates syn-CH3CHOO and CH2OO via High-Resolution Infrared Spectra with a Quantum-Cascade Laser

The Criegee intermediates, carbonyl oxides produced in ozonolysis of unsaturated hydrocarbons, play important roles in atmospheric chemistry. The two conformers of CH₃CHOO exhibit distinct reactivity toward several atmospheric species, but a distinct conformer-specific probe is challenging because ultraviolet and infrared absorption bands of syn- and anti-CH₃CHOO overlap at low-resolution. Employing a quantum-cascade laser and a Herriott cell, we recorded the O-O stretching bands of CH₂OO and syn-CH₃CHOO in region 880-932 cm^-1 at resolution 0.0015 cm^-1. In addition to completely resolved vibration-rotational lines of CH₂OO extending over 50 cm^-1, some spectral lines associated with hot bands were identified. Spectral lines solely due to syn-CH₃CHOO were also identified. Probing these lines, we determined the rate coefficient for the self-reaction of syn-CH₃CHOO to be k_self = (1.6 ± _0.6^0.5) × 10^-10 cm³ molecule^-1 s^-1, about twice that of CH₂OO.

Infrared spectra of 3-hydroxy-(1H)-pyridinium cation and 3-hydroxy-(1H)-pyridinyl radical isolated in solid para-hydrogen

As pyridine and its derivatives are regarded as building blocks of nitrogen-containing polycyclic aromatic hydrocarbons, spectral identifications of their protonated and hydrogenated species are important. The infrared (IR) absorption spectra of the 3-hydroxy-(1H)-pyridinium cation, 3-C₅H₄(OH)NH⁺, and the 3-hydroxy-(1H)-pyridinyl radical, 3-C₅H₄(OH)NH, produced on electron bombardment during deposition of a mixture of 3-hydroxypyridine, 3-C₅H₄(OH)N, and para-H₂ to form a matrix at 3.2 K were recorded. Intense IR absorption lines of trans-3-C₅H₄(OH)NH⁺ at 3594.4, 3380.0, 1610.6, 1562.2, 1319.4, 1193.8, 1167.5, and 780.4 cm^-1 and eleven weaker ones decreased in intensity after the matrix was maintained in darkness for 20 h, whereas lines of trans-3-C₅H₄(OH)NH at 3646.2, 3493.4, 3488.7, 1546.7, 1349.6, 1244.1, 1209.1, 1177.3, 979.8, and 685.2 cm^-1 and nine weaker ones increased. The intensities of lines of trans-3-C₅H₄(OH)NH decreased upon irradiation at 520 nm and diminished nearly completely upon irradiation at 450 nm, whereas those of trans-3-C₅H₄(OH)NH⁺ remained unchanged upon irradiation at 370, 450, and 520 nm. Observed vibrational wavenumbers and relative intensities of these species agree satisfactorily with the scaled harmonic vibrational wavenumbers and IR intensities predicted with the B3LYP/aug-cc-pVTZ method. The observed 3-C₅H₄(OH)NH⁺ cation and 3-C₅H₄(OH)NH radical are predicted to be the most stable species among all possible isomers by quantum-chemical calculations.

Spectral Characterization of Three-Electron Two-Center (3e-2c) Bonds of Gaseous CH3S∴S(H)CH3 and (CH3SH)2+ and Enhancement of the 3e-2c Bond upon Protonation

The three-electron two-center (3e-2c) bond plays an important role in structures and electron communication in biological systems involving cationic sulfur compounds. Although the nature of 3e-2c bonds and their theoretical formalism have attracted great interest, direct spectral identifications of 3e-2c-bound molecules are scarce. We observed the infrared spectra of the weakly 3e-2c-bound CH₃S∴S(H)CH₃ and the strongly 3e-2c-bound (CH₃SH)₂⁺ in a supersonic jet using infrared (IR) dissociation with vacuum-ultraviolet photoionization and time-of-flight detection. Protonation of CH₃S∴S(H)CH₃ to form [CH₃(H)S∴S(H)CH₃]⁺ significantly enhances the 3e-2c bond, characterized by a large red shift of the SH-stretching band with enhanced IR intensity, shortening of the calculated S-S distance from 3.00 to 2.86 Å, and a dissociation energy increased from ∼23 to 162 kJ mol^-1.

Vibrational autoionization of state-selective jet-cooled methanethiol (CH3SH) investigated with infrared + vacuum-ultraviolet photoionization

Vibrational autoionization of Rydberg states provides key information about nonadiabatic processes above an ionization threshold. We employed time-of-flight mass detection of CH₃SH⁺ to record vibrational-state selective photo-ionization efficiency (PIE) spectra of jet-cooled methanethiol (CH₃SH) on exciting CH₃SH to a specific vibrationally excited state with an infrared (IR) laser, followed by excitation with a tunable laser in the vacuum-ultraviolet (VUV) region for ionization. Autoionizing Rydberg states assigned to the ns, np, nd and nf series are identified. When IR light at 2601 (ν₃, SH stretching mode) and 2948 cm^-1 (ν₂, CH₃ symmetric stretching mode) was employed, the Rydberg series converged to the respective vibrationally excited (ν₃ and ν₂) states of CH₃SH⁺. When IR light at 3014 cm^-1 (overlapped ν₁/ν₉, CH₃ antisymmetric stretching and CH₂ antisymmetric stretching modes) was employed, Rydberg series converging to two vibrationally excited states (ν₁ and ν₉) of CH₃SH⁺ were observed. When IR light at 2867 cm^-1 (2ν₁₀, overtone of CH₃ deformation mode) and 2892 cm^-1 (2ν₄, overtone of CH₂ scissoring mode) was employed, both Δν = -1 and Δν = -2 ionization transitions were observed; there is evidence for direct ionization from the initial state into the CH₃SH⁺ (ν₄⁺ = 1) continuum. In all observed IR-VUV-PIE spectra, the ns and nd series show intensity greater than the other Rydberg series, which is consistent with the fact that the highest-occupied molecular orbital of CH₃SH is a p-like lone pair orbital on the S atom. The quantum yields for autoionization of various vibrational excited states are discussed. Values of ν₁ = 3035, ν₂ = 2884, ν₃ = 2514, and ν₉ = 2936 cm^-1 for CH₃SH⁺ derived from the converged limits agree satisfactorily with values observed for Ar-tagged CH₃SH⁺ at 3026, 2879, 2502, and 2933 cm^-1.

Spectroscopy of prospective interstellar ions and radicals isolated in para-hydrogen matrices

The p-H₂ matrix-isolation technique coupled with photolysis in situ or electron bombardment produces protonated or hydrogenated species important in astrochemistry.

High-resolution vibration–rotational spectra and rotational perturbation of the OO-stretching (ν6) band of CH2OO between 879.5 and 932.0 cm−1

We report the observation of a rotationally resolved ν₆ band associated with the OO-stretching mode of the simplest Criegee intermediate, CH₂OO, in the range of 879.5–932.0 cm⁻¹ (11.37–10.73 μm) at an optical resolution of 0.0015 cm⁻¹.