Interfacial Nanostructure and Hydrogen Bond Networks of Choline Chloride and Glycerol Mixtures Probed with X-ray and Vibrational Spectroscopies.

The molecular distribution at the liquid-vapor interface and evolution of the hydrogen bond interactions in mixtures of glycerol and choline chloride are investigated using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Nanoscale depth profiles of supersaturated deep eutectic solvent (DES) mixtures up to ∼2 nm measured by ambient-pressure XPS show the enhancement of choline cation (Ch+) concentration by a factor of 2 at the liquid-vapor interface compared to the bulk. In addition, Raman spectral analysis of a wide range of DES mixtures reveals the conversion of gauche-conformer Ch+ into the anti-conformer in relatively lower ChCl concentrations. Finally, the depletion of Ch+ from the interface (probing depth = 0.4 nm) is demonstrated by aerosol-based velocity map imaging XPS measurements of glyceline and water mixtures. The nanostructure of liquid-vapor interfaces and structural rearrangement by hydration can provide critical insight into the molecular origin of the deep eutectic behavior and gas-capturing application of DESs.

Fraction of Free-Base Nicotine in Simulated Vaping Aerosol Particles Determined by X-ray Spectroscopies.

A new generation of electronic cigarettes is exacerbating the youth vaping epidemic by incorporating additives that increase the acidity of generated aerosols, which facilitate uptake of high nicotine levels. We need to better understand the chemical speciation of vaping aerosols to assess the impact of acidification. Here we used X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to probe the acid-base equilibria of nicotine in hydrated vaping aerosols. We show that, unlike the behavior observed in bulk water, nicotine in the core of aqueous particles was partially protonated when the pH of the nebulized solution was 10.4, with a fraction of free-base nicotine (αFB) of 0.34. Nicotine was further protonated by acidification with equimolar addition of benzoic acid (αFB = 0.17 at pH 6.2). By contrast, the degree of nicotine protonation at the particle surface was significantly lower, with 0.72 < αFB < 0.80 in the same pH range. The presence of propylene glycol and glycerol completely eliminated protonation of nicotine at the surface (αFB = 1) while not affecting significantly its acid-base equilibrium in the particle core. These results provide a better understanding of the role of acidifying additives in vaping aerosols, supporting public health policy interventions.

Probing growth of metal-organic frameworks with X-ray scattering and vibrational spectroscopy.

Nucleation and crystallization arising from liquid to solid phase are involved in a multitude of processes in fields ranging from materials science to biology. Controlling the thermodynamics and kinetics of growth is advantageous to help tune the formation of complex morphologies. Here, we harness wide-angle X-ray scattering and vibrational spectroscopy to elucidate the mechanism for crystallization and growth of the metal-organic framework Co-MOF-74 within microscopic volumes enclosed in a capillary and an attenuated total reflection microchip reactor. The experiments reveal molecular and structural details of the growth processes, while the results of plane wave density functional calculations allow identification of lattice and linker modes in the formed crystals. Synthesis of the metal-organic framework with microscopic volumes leads to monodisperse and micron-sized crystals, in contrast to those typically observed under bulk reaction conditions. Reduction in the volume of reagents within the microchip reactor was found to accelerate the reaction rate. The coupling of spectroscopy with scattering to probe reactions in microscopic volumes promises to be a useful tool in the synthetic chemist's kit to understand chemical bonding and has potential in designing complex materials.

A Direct Probe of the Hydrogen Bond Network in Aqueous Glycerol Aerosols.

The properties of aerosols are of paramount importance in atmospheric chemistry and human health. The hydrogen bond network of glycerol-water aerosols generated from an aqueous solution with different mixing ratios is probed directly with X-ray photoelectron spectroscopy. The carbon and oxygen X-ray spectra reveal contributions from gas and condensed phase components of the aerosol. It is shown that water suppresses glycerol evaporation up to a critical mixing ratio. A dielectric analysis using terahertz spectroscopy coupled with infrared spectroscopy of the bulk solutions provides a picture of the microscopic heterogeneity prevalent in the hydrogen bond network when combined with the photoelectron spectroscopy analysis. The hydrogen bond network is composed of three intertwined regions. At low concentrations, glycerol molecules are surrounded by water forming a solvated water network. Adding more glycerol leads to a confined water network, maximizing at 22 mol %, beyond which the aerosol resembles bulk glycerol. This microscopic view of hydrogen bonding networks holds promise in probing evaporation, diffusion dynamics, and reactivity in aqueous aerosols.

Ethylene Intersystem Crossing Caught in the Act by Photofragment Sulfur Atoms.

Ethylene, C2H4, the simplest π-bonded molecule, is of enormous fundamental and commercial importance. Its lowest triplet state, in which the CH2 moieties occupy perpendicular planes, is well known from theory, but there has been no definitive experimental observation of this species. Here, velocity map imaging of the sulfur atoms in ethylene sulfide (c-C2H4S) photodissociation at 217 nm is used to reveal the internal state distribution of co-product ethylene. While both S (1D) and S (3P) translational energy distributions display three distinct regions that find their origins in singlet and triplet excited states of c-C2H4S, respectively, the S (3P) distribution is dominated by a fourth, low-recoil region. In this region, the distribution is fully isotropic at a recoil of 9 ± 1 kcal/mol, corresponding to the opening of the triplet ethylene channel. Multireference calculations suggest that this photodissociation pathway is mediated by a hot, transient biradical CH2CH2S that strongly favors CH2-hindered rotations in the predissociated complex. This photochemical ring-opening mechanism is invoked to account for the vibrational features observed in this low-recoil region, which are attributed to triplet ethylene relaxing to the torsional saddle point on the ground-state singlet surface. This study thereby gives for the first time the experimental confirmation of an adiabatic singlet-triplet splitting of 66 ± 1 kcal/mol and a torsional barrier height of 64 ± 1 kcal/mol in ethylene.

Photodissociation by Circularly Polarized Light Yields Photofragment Alignment in Ozone Arising Solely from Vibronic Interactions.

We present a direct determination of photofragment alignment produced by circularly polarized light in photolysis of a planar polyatomic molecule. This alignment arises via a new mechanism involving coherent excitation of two mutually perpendicular in-plane transition dipole moment components. The alignment is described by a new anisotropy parameter γ_{2}^{'} that was isolated by a unique laser polarization geometry. The determination of the parameter γ_{2}^{'} was realized in ozone photolysis at 266 nm where dc slice images of O(^{1}D_{2}) atomic fragments were acquired. A model developed for interpretation of the photolysis mechanism shows that it can exist only in case of failure of the Born-Oppenheimer approximation when electronic and vibrational (vibronic) interactions have to be taken into account. This finding suggests that determination of the alignment parameter γ_{2}^{'} can be used as a key for direct insight into vibronic interactions in photolysis of polyatomic molecules. The results obtained for ozone photolysis via the Hartley band showed significant γ_{2}^{'} alignment but little recoil speed dependence, consistent with the notion that, as opposed to the situation for derivative coupling, under our experimental conditions, the vibronic contributions to the nonadiabatic dynamics are not dependent on recoil speed.

Demonstration of multi-hit and multi-mass capability of 3D imaging in a conventional velocity map imaging experiment.

Coincidence and three-dimensional (3D) imaging offer unique capability in photodissociation and scattering experiments, and a variety of methods have been developed. The basic concept behind all these approaches is to register both the position (x, y) at which the particle hits the detector and the arrival time (t). A novel advance to the time and position sensitive detection was introduced recently by Li and co-workers [Rev. Sci. Instrum. 85(12), 123303 (2014)]. This method utilizes a high-speed digitizer and a computer algorithm along with the camera and detector usually employed in a conventional velocity map imaging apparatus. Due to the normal intensity variations of the ion spots, a correlation can be made between ion intensity recorded by the camera and peak intensity in the digitizer. This makes it possible to associate each ion spot's position with its respective arrival time, thereby constructing a 3D distribution. The technique was primarily introduced for ultrafast ion and electron imaging experiments at high repetition rate with single or few events per image frame. We have recently succeeded in adapting this approach at low repetition rate. Modifications were done to the initial setup to enhance the acquisition efficiency to obtain and correlate multiple hits per laser shot rather than single-hit events. The results are demonstrated in two experiments, dimethyl amine dissociative ionization at 205 nm and carbonyl sulfide photodissociation at 217 nm, with up to 27 events correlated in a single frame. Temporal and spatial slicing capabilities were achieved with good resolution, giving the photofragment velocity and angular distribution for multiple masses simultaneously.