Just add water dimers

Alkenes constitute a large fraction of the natural and human-made volatile organic compounds (VOCs) that are emitted into the troposphere. Their oxidation products degrade air quality and contribute to climate warming. Alkene oxidation is thought to involve Criegee intermediates (CIs), highly reactive molecules that form when ozone reacts with alkenes. However, the impact of CIs may be limited if they react rapidly with water. Modelers have found it difficult to quantify the effect of CIs on atmospheric composition, because laboratory data on CI reactions with water have been contradictory. On page 751 of this issue, Chao et al. (1) show that the simplest CI, formaldehyde oxide (CH_2OO), reacts rapidly with the water dimer, (H_2O)_2. Similar results are reported by Lewis et al. (2)

On the vibronic level structure in the NO_3 radical. Part III. Observation of intensity borrowing via ground state mixing

The A^2E" ← X^2A'_2 absorption spectrum exhibits vibronically allowed transitions from the ground state of NO_3 to upper state levels having a"_1 and e' vibronic symmetries. This paper explores the coupling mechanisms that lend intensities to these features. While transitions to e' vibronic levels borrow intensity from the very strong B^2E' ← X^2A'_2 electronic transition, those to a"_1 levels involve only negligible upper-state borrowing effects. Rather, it is the vibronic mixing of the ground vibronic level of NO_3 with vibrational levels in the B^2E' electronic state that permit the a"_1 levels to be seen in the spectrum. These ideas are supported by vibronic coupling calculations. The fact that the intensities of features corresponding to the two different vibronic symmetries are comparable is thus accidental

Cavity ringdown spectrum of the forbidden (A)over-tilde(2)E('')←(X)over-tilde(2)A(2)(') transition of NO_3: Evidence for static Jahn-Teller distortion in the (A)over-tilde state

The Jahn-Teller effect in the first two excited states of the nitrate radical NO3 has yet to be experimentally elucidated. In this paper, direct evidence of strong Jahn-Teller interactions in the A state is presented from the first complete absorption spectrum of the A(2)E(')←X(2)A(2)(') transition of NO3 in the gas phase in the region 5900-9000 cm(-1), at moderate resolution (0.15 cm(-1)). The observed spectrum is consistent with Herzberg-Teller selection rules, and reveals strong linear and quadratic Jahn-Teller interactions in the A state. Several of the vibronic bands have been tentatively assigned, including nu(2),nu(3), an irregular progression in nu(4), and combination bands involving nu(1). Our assignments are consistent with the previous works of Weaver [A. Weaver, D. W. Arnold, S. E. Bradforth, and D. M. Neumark, J. Chem. Phys. 94, 1740 (1991)] and Hirota [E. Hirota, T. Ishiwata, K. Kawaguchi, M. Fujitake, N. Ohashi, and I. Tanaka, J. Phys. Chem. 107, 2829 (1997)] The band origin is not observed, in accord with the selection rules, but is determined to be T-0=7064 cm(-1) from the observation of the 4(1)(0) hot band at 6695.7 cm(-1). Rotational contour analysis of this band indicates that the upper state is an asymmetric rotor, establishing that NO3 undergoes static Jahn-Teller distortion in the ground vibrational level of the A state

The Role of Torsion/Torsion Coupling in the Vibrational Spectrum of Cis−Cis HOONO

A three-dimensional model of the vibrational dynamics of HOONO is investigated. This model focuses on the couplings between the OH stretch vibration and the two torsions. The model is based on electronic energies, calculated at the CCSD(T)/cc-pVTZ level of theory and basis and dipole moment functions calculated at the CCSD/aug-cc-pVDZ level. The resulting points were fit to explicit functional forms, and the energies, wave functions, and intensities were evaluated using an approach in which the OH stretching motion was adiabatically separated from the torsional modes. It is found that the HOON torsion is strongly coupled to both the OONO torsion and OH stretch. Despite this, many of the conclusions that were drawn from earlier two-dimensional treatments, which did not include the OONO torsion, hold up on a semiquantitative level. In addition, we use this model to investigate the assignment of recently reported matrix isolated spectra of HOONO and DOONO. Finally, by comparing the results of this three-dimensional calculation to two-dimensional calculations and to the results of second-order perturbation theory, we investigate the question of how one determines the size of the reduced-dimensional system that is needed to describe the vibrational spectrum of molecules, like HOONO, that contain several large amplitude motions

Photodissociation of Cl_2O at 248 and 308 nm

Molecular beam studies of Cl_2O photolysis at 248 and 308 nm have been repeated and the analysis refined. At 248 nm, three distinct dissociation pathways that led to Cl+ClO products were resolved. At 308 nm, the angular distribution was slightly more isotropic than previously reported, leaving open the possibility that Cl_2O excited at 308 nm lives longer than a rotational period

Spectroscopic Studies of Intracluster Chemistry

Infrared predissociation spectroscopy of ion-solvent clusters has allowed us to examine the effects of sequential hydration on the reactivity of cations such as NO^+, NO_2^+, and protonated formaldehyde, H_2COH^+, stable gas phase ions which are known to undergo rapid reactions in aqueous solution. Our experiments demonstrate that these ions undergo hydration reactions at critical cluster sizes. The smaller clusters have spectra characteristic of H_2O ligans bound to stable ion cores, but as the cluster size increases, there is a sudden onset for intracluster rearrangements, e.g. NO_2^+ + H_2O yields H_3O^+ + HNO_3 which occurs upon hydration with four water molecules. With this approach, we can probe microscopic aspects of solvent effects on chemical reactions including the hydration of carbonyls and acid-catalyzed hydrolysis of amides

BREATHING EASIER THROUGH SPECTROSCOPY: STUDYING FREE RADICAL REACTIONS IN AIR POLLUTION CHEMISTRY

Air pollution arises from the oxidation of volatile organic compounds emitted into the atmosphere from both anthropogenic and biogenic sources. Free radicals dominate the gas phase chemistry leading to the formation of tropospheric ozone, oxygenated organic molecules and organic aerosols, but this chemistry is complex. In this presentation, advances in our understanding of the spectroscopy and chemistry of atmospheric free radicals will be described that have come from exploiting the sensitivity and specificity of methods such as Cavity Ringdown Spectroscopy, Multiplexed Photoionization Mass Spectrometry and Cavity-Enhanced Frequency Comb Spectroscopy

New insights into the Jahn–Teller effect in NO_3 via the dark à 2E" state

The recent cavity ringdown (CRD) measurement of the forbidden Ã^2 E"←X~^2 A2' transition of the nitrate radical NO_3 reveals a rich, well-resolved spectrum in the near-infrared. The spectroscopic detail provides a new window onto the Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) effects in NO_3. This paper reviews the current experimental evidence for vibronic coupling in the à state and discusses the theoretical issues in the context of new preliminary EOMIP/CCSD and CCSD(T) calculations. The theoretical results to date indicate that the à 2E" state of NO_3 undergoes a relatively strong JT distortion which may require inclusion of higher order vibronic couplings. The intensity of this transition may involve multiple intensity borrowing mechanisms via PJT coupling among the X~, à and B~ states

Generation of charged droplets by field ionization of liquid helium

Positively charged helium droplets were produced by ionization of liquid helium in an electrostatic spraying experiment, in which fluid emerging from a thin glass capillary was ionized by applying a high voltage to a needle inside the capillary. At 2.2 K, fine droplets (<10 mu m in diameter) were produced in pulsed sprays or showers with total currents as high as 0.4 mu A at relatively low voltages (2-4 kV). Ionization was accompanied by a visible glow at the needle and glass tips. Droplet formation was suppressed at 3.5 K. In contrast, liquid nitrogen formed a well-defined Taylor cone with droplets having diameters comparable to the jet (approximate to 100 mu m) at much lower currents (3 nA) and higher voltages (9 kV), in agreement with previous results. The mechanism for charging in these liquids was proposed to be field ionization, identical to the processes leading to conduction in cryogenic insulating liquids observed by Gomer. The high currents resulting from field ionization in helium, together with the intrinsically low surface tension of helium I, led to charge densities that greatly exceeded the Rayleigh limit, thus preventing formation of a Taylor cone and resulting in Coulomb explosion of the liquid

Primary and secondary dissociation pathways in the ultraviolet photolysis of Cl_2O

The photodissociation of dichlorine monoxide (Cl_2O) at 308, 248, and 193 nm was studied by photofragment translational energy spectroscopy. The primary channel upon excitation at 308 and 248 nm was Cl–O bond fission with production of ClO+Cl. A fraction of the ClO photoproducts also underwent spontaneous secondary dissociation at 248 nm. The center-of-mass translational energy distribution for the ClO+Cl channel at 248 nm appeared to be bimodal with a high energy component that was similar in shape to the 308 nm distribution and a second, low energy component with a maximum close to the threshold for the 2Cl+O(3P) channel. Observation of a bimodal distribution suggests that two pathways with different dissociation dynamics lead to ClO+Cl products. The high product internal energy of the second component raises the possibility that ClO is formed in a previously unobserved spin-excited state a 4∑−. Following excitation at 193 nm, a concerted dissociation pathway leading to Cl_2+O was observed in addition to primary Cl–O bond breakage. In both processes, most of the diatomic photofragments were formed with sufficient internal energy that they spontaneously dissociated. The time-of-flight distributions of the Cl_2+O products suggest that these fragments are formed in two different channels Cl_2(3II)+O(3P) and Cl_2(X1∑)+O(1D)