Chanchal Chaudhuri

Two photoionization thresholds of N3 produced by ClN3 photodissociation at 248 nm: Further evidence for cyclic N3

We present results of near-threshold photoionization of N3 photofragments produced by laser photodissociation of ClN3 at 248 nm. The time of flight of recoiling N3 is used to resolve two photochemical channels producing N3, which exhibit different translational energy release. The two forms of N3 resolved in this way exhibit different photoionization thresholds, consistent with their assignment to linear (X 2pi(g)) and cyclic N3. This result agrees with the existing theoretical calculations of excited and ionic states of N3 and strengthens previous experimental results which suggested that the ClN3 photolysis produces a cyclic form of N3.

Development of a stable source of atomic oxygen with a pulsed high-voltage discharge and its application to crossed-beam reactions

To investigate the reactions of oxygen atoms with ethene and silane in a crossed-beam condition, we developed a stable, highly intense, and short-pulsed source of atomic oxygen with a transient high-voltage discharge. Mixtures of O(2) and He served as discharge media. Utilizing a crossed molecular-beam apparatus and direct vacuum-ultraviolet ionization, we measured the temporal profiles of oxygen atoms and the time-of-flight spectra of reaction products. With O(2) 3% seeded in He as a discharge medium, oxygen atoms might have a full width as small as 13.5 micros at half maximum at a location 193 mm downstream from the discharge region. Most population of oxygen atoms is in the ground state (3)P but some in the first excited state (1)D, depending on the concentration of precursor O(2). This discharge device analogously generates carbon, nitrogen, and fluorine atoms from precursors CO, N(2), and F(2), respectively.

Observation of Photochemical C−N Bond Cleavage in CH3N3: A New Photochemical Route to Cyclic N3

We report VUV-photoionization based photofragmentation-translational spectroscopy data, providing a comprehensive study of the collision free photochemistry of methyl azide (CH3N3) at 193 nm. We report the first observation of the production of methyl and the N3 radical and derive the translational energy release distribution of this reaction. The most probable translation energy is only 8%, and the maximum translational energy is only 60% of the available energy, taking CH3 + linear N3 as the zero of energy. However, the maximum translational energy release is quantitatively consistent with production of the higher energy isomer cyclic N3. Threshold photoionization of the N3 fragment using tunable synchrotron radiation shows results consistent with theoretical predictions of the cyclic N3 ionization potential. The secondary dissociation of N3 --> N(2D) + N2 is also observed and its translational energy release is derived. This distribution peaks at approximately 6 and extends to 11 kcal/mol as would be expected from the size of the exit channel barrier for spin-allowed dissociation of cyclic N3 (7 kcal/mol) and, furthermore, inconsistent with the barrier height of the spin-allowed dissociation of linear N3 (3 kcal/mol). A large fraction (approximately 45%) of the N3 does not dissociate on the microsecond time scale of the experiment suggesting methyl azide may be the most attractive photochemical precursor of cyclic N3 yet found.

The simplest all-nitrogen ring: Photolytically filling the cyclic-N3 well

We report evidence that cyclic-N(3) is exclusively produced in the 157-nm photolysis of ClN(3). Photoproduct translational energy measurements reveal a single-peaked distribution for an N(3)-formation channel with maximum and minimum translational energies matching the theoretically predicted minimum and maximum binding energies of cyclic-N(3), respectively. The absence of linear-N(3) greatly simplifies the data analysis. The zero-Kelvin heat of formation of cyclic-N(3) is derived experimentally (142+/-3.5 kcal/mol) and is in excellent agreement with the best existing determinations from other studies.

Collision-free photochemistry of methylazide: Observation of unimolecular decomposition of singlet methylnitrene

Methylazide photolysis at 248 nm has been investigated by ionizing photofragments with synchrotron radiation in a photofragmentation translational spectroscopy study. CH3N and N2 were the only observed primary products. The translational energy release suggests a simple bond rupture mechanism forming singlet methylnitrene, 1CH3N, and N2. Thus, these experiments reveal the unimolecular decomposition of this highly unstable species. We explain our observations through a mechanism which is initiated by the isomerization of 1CH3N to a highly internally excited methanimine H2C=NH isomer, which decomposes by 1,1-H2 elimination forming HNC+H2 as well as sequential H-atom loss (N-H followed by C-H bond cleavage), to form HCN. No evidence for dynamics on the triplet manifold of surfaces is found.

Photodissociation dynamics of vinyl fluoride (CH2CHF) at 157 and 193nm: Distributions of kinetic energy and branching ratios

Using photofragment translational spectroscopy and tunable vacuum-ultraviolet ionization, we measured the time-of-flight spectra of fragments upon photodissociation of vinyl fluoride (CH2CHF) at 157 and 193 nm. Four primary dissociation pathways--elimination of atomic F, atomic H, molecular HF, and molecular H2--are identified at 157 nm. Dissociation to C2H3 + F is first observed in the present work. Decomposition of internally hot C2H3 and C2H2F occurs spontaneously. The barrier heights of CH2CH --> CHCH + H and cis-CHCHF --> CHCH + F are evaluated to be 40+/-2 and 44+/-2 kcal mol(-1), respectively. The photoionization yield spectra indicate that the C2H3 and C2H2F radicals have ionization energies of 8.4+/-0.1 and 8.8+/-0.1 eV, respectively. Universal detection of photoproducts allowed us to determine the total branching ratios, distributions of kinetic energy, average kinetic energies, and fractions of translational energy release for all dissociation pathways of vinyl fluoride. In contrast, on optical excitation at 193 nm the C2H2 + HF channel dominates whereas the C2H3 + F channel is inactive. This reaction C2H3F --> C2H2 + HF occurs on the ground surface of potential energy after excitation at both wavelengths of 193 and 157 nm, indicating that internal conversion from the photoexcited state to the electronic ground state of vinyl fluoride is efficient. We computed the electronic energies of products and the ionization energies of fluorovinyl radicals.

Identification of CH3OH2+ and H3O+-centered cluster isomers from fragment-dependent vibrational predissociation spectra of H+(CH3OH)4H2O

Cluster isomers of H+(CH3OH)(4)H2O have been identified by vibrational predissociation spectroscopy in combination with mass-selected detection of photofragments. Ab initio calculations indicate that the cluster ion can exist in either CH3OH2+(CH3OH)(3)H2O or H3O+(CH3OH)(4) isomeric forms. They can dissociate via a methanol loss or water loss channel, depending on the structure of the isomers. While water loss is the dominant channel of the dissociation, the methanol loss channel is readily accessible by the H3O+-centered cluster isomer. We demonstrate in this study that mass-selected detection of photofragments produced by vibrational excitation is an effective way of identifying cluster isomers in the gas phase

Comparative Studies of H+(C6H6)(H2O)1,2 and H+(C5H5N)(H2O)1,2 by DFT Calculations and IR Spectroscopy

Protonated benzene–water and pyridine–water complexes have been investigated by density functional theory (DFT) calculations and infrared (IR) spectroscopy. The calculations performed at the B3LYP/6–31+G* level predict that there exist several stable isomers for H+(C6H6)(H2O)1,2 with two distinct ion cores, C6H7+ and H3O+. In contrast, only the C5H5NH+-centred form can be found for H+(C5H5N)(H2O)1,2, arising from the higher proton affinity of pyridine compared to that of benzene and water. Vibrational predissociation spectroscopic measurements of H+(C6H6)(H2O)2 and H+(C5H5N)(H2O)2 support the predictions.

Hydration-induced conformational changes in protonated 2,4-pentanedione in the gas phase

Starting with H+[CH3C(O)CH2C(O)CH3] (denoted H+PD), the protonated diketone-water clusters H+PD(H2O) n (n = 1–3) have been characterized by density functional theory calculations in combination with vibrational predissociation spectroscopy to explore the conformational changes of a protonated bifunctional ion solvated by water in the gas phase. Theoretical calculations for H+PD revealed that the ion contains an intramolecular hydrogen bond (IHB), with two oxygen atoms bridged by the extra proton in an O—H+ … O form. Attachment of one water molecule to it readily ruptures this IHB, replacing the H+ by the H3O+ moiety. Further replacement of the IHB by two water molecules occurs at n = 2 and the −C(O)CH2C(O)- chain is fully opened (or unfolded) after transfer of the extra proton to the water trimer at n = 3. To verify the computational findings, infrared spectroscopic measurements were performed using a vibrational predissociation ion trap spectrometer to identify cluster isomers from the signatures of hydrogen bonded and non-hydrogen bonded OH stretching spectra of H+PD(H2O)2,3 produced in a corona discharge supersonic expansion. Besides open form isomers, evidence for the formation of water-bridged structures has been found for H+PD(H2O)3 at an estimated temperature of 200 K. A detailed illustration of the unfolding steps as well as the energy profiles for the evolution of a two-water bridge isomer from the protonated H+PD monomer are analysed pictorially (including both stable intermediates and transition states) in the present investigation.

Exploring the dynamics of reaction N+SiH4 with crossed molecular-beam experiments and quantum-chemical calculations

We investigated the reaction N((4)S,(2)D,(2)P)+SiH(4) in crossed molecular beams at a collision energy of 4.7 kcal mol(-1) with a time-of-flight mass spectrometer and selective photoionization. Ion signals were observed at m/z=42-45, associated with two product channels, HSiNH/SiNH(2)+H+H and HSiN/HNSi+H(2)+H. The species producing the signal at m/z=43 is assigned to product HSiN/HNSi and that at m/z=44 to product HSiNH/SiNH(2). The signal observed at m/z=42 is attributed to daughter ions of those two products and that at m/z=45 to (29)Si and (30)Si isotopic variants. We report time-of-flight spectra as a function of laboratory angle and simulations for the two products, from which both kinetic-energy and angular distributions of products in the center-of-mass (c.m.) frame were derived. The dependence of release of kinetic energy on the c.m. scattering angle is weak. The average translational energy released is 7.7 kcal mol(-1) for product channel HSiNH/SiNH(2)+H+H and 30.3 kcal mol(-1) for product channel HSiN/HNSi+H(2)+H. Through consecutive triple fragmentation, the angular distribution is slightly anisotropic for product HSiNH/SiNH(2) but isotropic for product HSiN/HNSi. Assuming equal efficiencies of detection, we estimate the branching ratios of products HSiNH/SiNH(2) and HSiN/HNSi to be roughly 15:85. To facilitate an understanding of the reaction mechanisms, we calculated the potential-energy surface for reaction N((2)D)+SiH(4) with quantum-chemical methods. Reactions N((2)D)+SiH(4)-->SiNH(2)+H+H and N((2)D)+SiH(4)-->HNSi+H(2)+H account satisfactorily for the present experimental results. Isomeric products HSiNH and HSiN are minor in this work.