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A study shows that laser flash photolysis of 4-azidopyridine-1-oxide at 266 or 308 nm produces triplet 4-nitrenopyridine-1-oxide as the dominant reactive intermediate species with [k.sub.ISC] of approximately 2X[10.sub.7] [s.sup.-1]. Observations revealed no evidence of products occurring from the singlet nitrene, suggesting a slow rate of cyclization to the benzazirine and didehydroazepine species.

An array of glass and matrix isolation technique was used to study the photochemistry of azidopyridine 1-oxides. The results showed that the photochemistry of 4-azidopyridine 1-oxide is dominated by triplet nitrene chemistry at room temperature.

The formation mechanisms of siliconnitride and silicon nitrogenhydrogen films, both produced by chemical vapor deposition (CVD) techniquesand widely used in electronic device fabrication, are poorly understood.Identification of gas-phase intermediates formed from starting materials,typically silane, ammonia, and/or nitrogen, is a critical step inassessing the interplay between gas and surface processes in filmformation. Two potential intermediates in this process, HSiNSi andH3SiNSi, have now been detected in a molecular beam bymeans of rotational spectroscopy. Both molecules were produced inelectrical discharges of CVD-like gas mixtures and are the most readilyobserved silicon–nitrogen-containing molecules in the 6–20GHz frequency range, though neither has been the subject of priorexperimental or theoretical studies. HSiNSi and H3SiNSiare likely formed from reactions involving the silanitrile radical(SiN, isoelectronic to CN), implying that similar gas-phase reactionsmay be involved in film growth. [ABSTRACT FROM AUTHOR]

Broadband microwave spectra of N -ethyl maleimide (NEM) and N -ethyl succinimide (NES) have been recorded using chirped pulse Fourier transform microwave spectroscopy in the K a -band (26.5-40 GHz). The spectra for both molecules were fit to a Watson A-reduced Hamiltonian in the I r representation to obtain best fit experimental rotational constants (NEM: A 0 = 2143.1988(29), B 0 = 1868.7333(22), C 0 = C(6)-C(5)-N-C(4)), the ethyl group experiences significant steric strain when it is in the plane of the ring, associated with the interaction of the ethyl group with the two carbonyl oxygens. This leads to calculated barriers of θ A 0 = 2061.47756(14), B 0 = 1791.73517(12), C 0 = 1050.31263(11)), centrifugal distortion constants, and nuclear quadrupole coupling constants. While the heavy atoms of the five-membered ring of both molecules are planar, the ethyl chain has its terminal methyl group perpendicular to the ring. Along the relaxed potential energy curve for the ethyl dihedral angle (θ 1 = C(6)-C(5)-N-C(4)), the ethyl group experiences significant steric strain when it is in the plane of the ring, associated with the interaction of the ethyl group with the two carbonyl oxygens. This leads to calculated barriers of θ 1 = 1469 cm -1 and θ 1 = 1680 cm -1 in N -ethyl maleimide and N -ethyl succinimide, respectively.

Pyrrolyl (C 4 H 4 N) is a nitrogen-containing aromatic radical that is a derivative of pyrrole (C 4 H 5 N) and is an important intermediate in the combustion of biomass. It is also relevant for chemistry in Titan's atmosphere and may be present in the interstellar medium. The lowest-energy isomer, 1-pyrrolyl, has been involved in many experimental and theoretical studies of the N-H photodissociation of pyrrole, yet it has only been directly spectroscopically detected via electron paramagnetic resonance and through the photoelectron spectrum of the pyrrolide anion, yielding three vibrational frequencies. No direct measurements of 2- or 3-pyrrolyl have been made, and little information is known from theoretical calculations beyond their relative energies. Here, we present an ab initio quantum chemical characterization of the three pyrrolyl isomers at the CCSD(T) level of theory in their ground electronic states, with an emphasis on spectroscopic parameters relevant for vibrational and rotational spectroscopy. Equilibrium geometries were optimized at the CCSD(T)/cc-pwCVTZ level of theory, and the quadratic, cubic, and partial quartic force constants were evaluated at CCSD(T)/ANO0 for analysis using second-order vibrational perturbation theory to obtain harmonic and anharmonic vibrational frequencies. In addition, zero-point-corrected rotational constants, electronic spin-rotation tensors, and nuclear hyperfine tensors are calculated for rotational spectroscopy. Our computed structures and energies agree well with earlier density functional theory calculations, and spectroscopic parameters for 1-pyrrolyl are compared with the limited existing experimental data. Finally, we discuss strategies for detecting these radicals using rotational and vibrational spectroscopy on the basis of the calculated spectroscopic constants.

Thionitrous acid (HSNO), a potential key intermediate in biological signaling pathways, has been proposed to link NO and H2S biochemistries, but its existence and stability in vivo remain controversial. We establish that HSNO is spontaneously formed in high concentration when NO and H2S gases are mixed at room temperature in the presence of metallic surfaces. Our measurements reveal that HSNO is formed by the reaction H2S + N2O3 → HSNO + HNO2, where N2O3 is a product of NO disproportionation. These studies also suggest that further reaction of HSNO with H2S may form HNO and HSSH. The length of the S-N bond has been derived to high precision and is found to be unusually long: 1.84 Å, the longest S-N bond reported to date for an R-SNO compound. The present structural and, particularly, reactivity investigations of this elusive molecule provide a firm foundation to better understand its potential physiological chemistry and propensity to undergo S-N bond cleavage in vivo.

The rotational spectrum of nitrogen-protonated nitrous oxide (HNNO(+)), an isomer whose existence was first inferred from kinetic studies more than 30 years ago, has now been detected by Fourier transform microwave spectroscopy, guided by new high-level coupled-cluster calculations of its molecular structure. From high-resolution measurements of the hyperfine splitting in its fundamental rotational transition, the rotational constant (B + C)/2 and the quadrupole tensor element χaa(N) for both nitrogen atoms have been precisely determined. The derived constants agree well with quantum-chemical calculations here and others in the literature. The χaa(N) values for the two isomers of protonated nitrous oxide are qualitatively consistent with the valence bond description of H-N═N(+)═O for the electronic structure of the nitrogen-protonated form and N≡N(+)-O-H for the oxygen-protonated form. HNNO(+) is found to be 2-4 times less abundant than NNOH(+) under a range of experimental conditions, as might be expected because this metastable isomer is known to be only ∼6 kcal mol(-1) less stable than ground-state NNOH(+) from kinetic measurements by Ferguson and co-workers.

The fundamental molecular ion H3(+) has impacted astronomy, chemistry, and physics, particularly since the discovery of its rovibrational spectrum. Consisting of three identical fermions, its properties are profoundly influenced by the requirements of exchange symmetry, most notably the nonexistence of its ground rotational state. Spectroscopy of H3(+) is often used to infer the relative abundances of its two nuclear spin modifications, ortho- and para-H3(+), which are important in areas as diverse as electron dissociative recombination and deuterium fractionation in cold interstellar clouds. In this paper, we explore in detail the impact of exchange symmetry on the states of H3(+), with a particular focus on the state degeneracies necessary for converting spectral transition intensities to relative abundances. We address points of confusion in the literature surrounding these issues and discuss the implications for proton-transfer reactions of H3(+) at low temperatures.

Sulfoxylic acid (HOSOH), a chemical intermediate roughly midway along the path between highly reduced (H2S) and highly oxidized sulfur (H2SO4), has been detected using Fourier transform microwave spectroscopy and double resonance techniques, guided by new high-level CCSD(T) quantum-chemical calculations of its molecular structure. Rotational spectra of the two most stable isomers of HOSOH, the putative ground state with C2 symmetry and the low-lying C(s) rotamer, have been measured to high precision up to 71 GHz, allowing accurate spectroscopic parameters to be derived for both isomers. HOSOH may play a role in atmospheric and interstellar chemistry, and the present work provides the essential data to enable remote sensing and/or radioastronomical searches for these species. Spectroscopic characterization of HOSOH suggests that other transient intermediates in the oxidation of SO2 to H2SO4 may be amenable to laboratory detection as well.