An experimental and computational view of the photoionization of diol-water clusters.

In the interstellar medium, diols and other prebiotic molecules adsorb onto icy mantles surrounding dust grains. Water in the ice may affect the reactivity and photoionization of these diols. Ethylene glycol (EG), 1,2-propylene glycol, and 1,3-propylene glycol clusters with water clusters were used as a proxy to study these interactions. The diol-water clusters were generated in a continuous supersonic molecular beam, photoionized by synchrotron-based vacuum ultraviolet light from the Advanced Light Source, and subsequently detected by reflectron time-of-flight mass spectrometry. The appearance energies for the detected clusters were determined from the mass spectra, collected at increasing photon energy. Clusters of both diol fragments and unfragmented diols with water were detected. The lowest energy geometry optimized conformers for the observed EG-water clusters and EG fragment-water clusters have been visualized using density functional theory (DFT), providing insight into hydrogen bonding networks and how these affect fragmentation and appearance energy. As the number of water molecules clustered around EG fragments (m/z 31 and 32) increased, the appearance energy for the cluster decreased, indicating a stabilization by water. This trend was supported by DFT calculations. Fragment clusters from 1,2-propylene glycol exhibited a similar trend, but with a smaller energy decrease, and no trend was observed from 1,3-propylene glycol. We discuss and suggest that the reactivity and photoionization of diols in the presence of water depend on the size of the diol, the location of the hydroxyl group, and the number of waters clustered around the diol.

Rotational and Vibrational Spectra of the Pyridyl Radicals: A Coupled-Cluster Study.

Pyridyl is a prototypical nitrogen-containing aromatic radical that may be a key intermediate in the formation of nitrogen-containing aromatic molecules under astrophysical conditions. On meteorites, a variety of complex molecules with nitrogen-containing rings have been detected with nonterrestrial isotopic abundances, and larger nitrogen-containing polycyclic aromatic hydrocarbons (PANHs) have been proposed to be responsible for certain unidentified infrared emission bands in the interstellar medium. In this work, the three isomers of pyridyl (2-, 3-, and 4-pyridyl) have been investigated with coupled cluster methods. For each species, structures were optimized at the CCSD(T)/cc-pwCVTZ level of theory and force fields were calculated at the CCSD(T)/ANO0 level of theory. Second-order vibrational perturbation theory (VPT2) was used to derive anharmonic vibrational frequencies and vibrationally corrected rotational constants, and resonances among vibrational states below 3500 cm-1 were treated variationally with the VPT2+K method. The results yield a complete set of spectroscopic parameters needed to simulate the pure rotational spectrum of each isomer, including electron-spin, spin-spin, and nuclear hyperfine interactions, and the calculated hyperfine parameters agree well with the limited available data from electron paramagnetic resonance spectroscopy. For the handful of experimentally measured vibrational frequencies determined from photoelectron spectroscopy and matrix isolation spectroscopy, the typical agreement is comparable to experimental uncertainty. The predicted parameters for rotational spectroscopy reported here can guide new experimental investigations into the yet-unobserved rotational spectra of these radicals.

Coupled Cluster Characterization of 1-, 2-, and 3-Pyrrolyl: Parameters for Vibrational and Rotational Spectroscopy.

Pyrrolyl (C4H4N) is a nitrogen-containing aromatic radical that is a derivative of pyrrole (C4H5N) 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.

Quantum electronic control on chemical activation of methane by collision with spin-orbit state selected vanadium cation.

By coupling a newly developed quantum-electronic-state-selected supersonically cooled vanadium cation (V+) beam source with a double quadrupole-double octopole (DQDO) ion-molecule reaction apparatus, we have investigated detailed absolute integral cross sections (σ's) for the reactions, V+[a5DJ (J = 0, 2), a5FJ (J = 1, 2), and a3FJ (J = 2, 3)] + CH4, covering the center-of-mass collision energy range of Ecm = 0.1-10.0 eV. Three product channels, VH+ + CH3, VCH2+ + H2, and VCH3+ + H, are unambiguously identified based on Ecm-threshold measurements. No J-dependences for the σ curves (σ versus Ecm plots) of individual electronic states are discernible, which may indicate that the spin-orbit coupling is weak and has little effect on chemical reactivity. For all three product channels, the maximum σ values for the triplet a3FJ state [σ(a3FJ)] are found to be more than ten times larger than those for the quintet σ(a5DJ) and σ(a5FJ) states, showing that a reaction mechanism favoring the conservation of total electron spin. Without performing a detailed theoretical study, we have tentatively interpreted that a weak quintet-to-triplet spin crossing is operative for the activation reaction. The σ(a5D0, a5F1, and a3F2) measurements for the VH+, VCH2+, and VCH3+ product ion channels along with accounting of the kinetic energy distribution due to the thermal broadening effect for CH4 have allowed the determination of the 0 K bond dissociation energies: D0(V+-H) = 2.02 (0.05) eV, D0(V+-CH2) = 3.40 (0.07) eV, and D0(V+-CH3) = 2.07 (0.09) eV. Detailed branching ratios of product ion channels for the titled reaction have also been reported. Excellent simulations of the σ curves obtained previously for V+ generated by surface ionization at 1800-2200 K can be achieved by the linear combination of the σ(a5DJ, a5FJ, and a3FJ) curves weighted by the corresponding Boltzmann populations of the electronic states. In addition to serving as a strong validation of the thermal equilibrium assumption for the populations of the V+ electronic states in the hot filament ionization source, the agreement between these results also confirmed that the V+(a5DJ, a5FJ, and a3FJ) states prepared in this experiment are in single spin-orbit states with 100% purity.

Chemical Activation of Water Molecule by Collision with Spin-Orbit-State-Selected Vanadium Cation: Quantum-Electronic-State Control of Chemical Reactivity.

We have obtained absolute integral cross sections (σ's) for the reactions of spin-orbit-state-selected vanadium cations, V+[a5DJ(J = 0, 2), a5FJ(J = 1, 2), and a3FJ(J = 2, 3)], with a water molecule (H2O) in the center-of-mass collision energy range Ecm = 0.1-10.0 eV. On the basis of these state-selected σ curves (σ versus Ecm plots) observed, three reaction product channels, VO+ + H2, VH+ + OH, and VOH+ + H, from the V+ + H2O reaction are unambiguously identified. Contrary to the previous guided ion beam study of the V+(a5DJ) + D2O reaction, we have observed the formation of the VO+ + H2 channel from the V+(a5DJ) + H2O ground reactant state at low Ecm's (<3.0 eV). No spin-orbit J-state dependences for the σ curves of individual electronic states are discernible, indicating that spin-orbit interactions are weak with little effect on chemical reactivity of the titled reaction. For the three product channels identified, the triplet σ(a3FJ) values are overwhelmingly higher than the quintet σ(a5DJ) and σ(a5FJ) values, showing that the reaction is governed by a "weak quintet-triplet spin crossing" mechanism, favoring the conservation of total electron spins. The σ curves for exothermic product channels are found to exhibit a rapid decreasing profile as Ecm is increased, an observation consistent with the prediction of the charge-dipole and induced-dipole orbiting model. This experiment shows that the V+ + H2O reaction can be controlled effectively to produce predominantly the VO+ + H2 channel via the V+(a3FJ) + H2O reaction at low Ecm's (≤0.1 eV) and that the ion-molecule reaction dynamics can be altered readily by selecting the electronic state of V+ cation. On the basis of the measured Ecm thresholds for the σ(a5DJ, a5FJ, and a3FJ: VH+) and σ(a5DJ, a5FJ, and a3FJ: VOH+) curves, we have deduced upper bound values of 2.6 ± 0.2 and 4.3 ± 0.3 eV for the 0 K bond dissociation energies, D0(V+-H) and D0(V+-OH), respectively. After correcting for the kinetic energy distribution resulting from the Doppler broadening effect of the H2O molecule, we obtain D0(V+-H) = 2.2 ± 0.2 eV and D0(V+-OH) = 4.0 ± 0.3 eV, which are in agreement with D0 determinations obtained by σ curve simulations.