Dynamics of Proton Transfer in Mesoscopic Clusters

Proton transfer rates and mechanisms are studied in mesoscopic, liquid-state, molecular clusters. The proton transfer occurs in a proton-ion complex solvated by polar molecules comprising the cluster environment. The rates and mechanisms of the reaction are studied using both adiabatic and non-adiabatic molecular dynamics. For large molecular clusters, the proton-ion complex resides primarily on the surface of the cluster or one layer of solvent molecules inside the surface. The proton transfer occurs as the complex undergoes orientational fluctuations on the cluster surface or penetrates one solvent layer into the cluster leading to solvent configurations that favor the transfer. For smaller clusters the complex resides mostly on the surface of the cluster and proton transfer is observed only when the complex penetrates the cluster and solvent configurations that favor the proton transfer are achieved. Quantitative information on the cluster reaction rate constants is also presented.Comment: To appear in JCP (March). Postscript figures available on request ([email protected]

Prediction of the Size Distributions of Methanol-Ethanol Clusters Detected in VUV Laser/Time-of-flight Mass Spectrometry

The size distributions and geometries of vapor clusters equilibrated with methanol−ethanol (Me−Et) liquid mixtures were recently studied by vacuum ultraviolet (VUV) laser time-of-flight (TOF) mass spectrometry and density functional theory (DFT) calculations (Liu, Y.; Consta, S.; Ogeer, F.; Shi, Y. J.; Lipson, R. H. Can. J. Chem. 2007, 85, 843−852). On the basis of the mass spectra recorded, it was concluded that the formation of neutral tetramers is particularly prominent. Here we develop grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) frameworks to compute cluster size distributions in vapor mixtures that allow a direct comparison with experimental mass spectra. Using the all-atom optimized potential for liquid simulations (OPLS-AA) force field, we systematically examined the neutral cluster size distributions as functions of pressure and temperature. These neutral cluster distributions were then used to derive ionized cluster distributions to compare directly with the experiments. The simulations suggest that supersaturation at 12 to 16 times the equilibrium vapor pressure at 298 K or supercooling at temperature 240 to 260 K at the equilibrium vapor pressure can lead to the relatively abundant tetramer population observed in the experiments. Our simulations capture the most distinct features observed in the experimental TOF mass spectra: Et3H+ at m/z = 139 in the vapor corresponding to 10:90% Me−Et liquid mixture and Me3H+ at m/z = 97 in the vapors corresponding to 50:50% and 90:10% Me−Et liquid mixtures. The hybrid GCMC scheme developed in this work extends the capability of studying the size distributions of neat clusters to mixed species and provides a useful tool for studying environmentally important systems such as atmospheric aerosols

Effect of Droplet Size and Counterions on the Spatial Distribution of Ions

Charged droplets have become a new environment for accelerating chemical reactions by orders of magnitude relative to their bulk analogues. Nevertheless the reaction mechanisms still remain unknown. Here we investigate the ion spatial distributions and surface charge in aqueous droplets with diameters in the range of 5 nm to 16 nm with and without counterions using molecular dynamics. The charge carriers are Na, and Cl ions ions. We demonstrate the convergence of ion spatial distributions. Scaling of the ion distributions reveals underlying universal behavior. The convergence allows one to extrapolate the simulation results from nanoscopic dimensions to larger ones, which are still inaccessible to atomistic modeling.The surface excess charge and electric field are also computed. We find that the surface excess charge layer in the presence of Na and Cl ions is approximately 1.5 nm-1.7 nm thick and that approximately 55%-33 % (from smaller to larger droplets) of the total number of ions reside in this layer. For the first time droplet sizes that are accessible to experimental scrutiny are modeled atomistically. </div

Atomistic Modeling of Jet Formation in Charged Droplets

The first atomistic simulations that reveal the mechanism of Rayleigh fission are presented. It is demonstrated that simple ion or macroion ejection takes place through droplet deformation from a spherical into a distinct “tear” shape that contains a conical protrusion. We assert that the latter state is a free-energy minimum along an order parameter that measures the degree of droplet asphericity. The charged droplet’s long-time evolution proceeds by alternating between the two minima above and below the critical value that are reached through solvent evaporation and ion ejection, respectively. For the first time, this mechanism allows one to explain the nature of the progeny droplets and the percentage of charge lost during fission. The cone half angle is estimated and found to be in good agreement with the value predicted from the solution of the electrostatic equation for the dielectric liquid. It is found that the conical deformation is independent of the effect of electrohydrodynamic forces reported in experiments. Contrary to the experimental observations of two diametrically opposite jets for droplets suspended in the electric field, we find that a single jet is formed at the Rayleigh limit. The study provides insight into the mechanism of capture of a macroion in jets appearing in electrospray ionization mass spectrometry (ESI-MS) experiments and may explain the tolerance of the ESI-MS spectrum to salt contamination of the sample

Molecular Characterization of the Surface Excess Charge Layer in Droplets

&lt;div&gt;Charged droplets play a central role in native mass spectrometry, atmospheric aerosols and in serving as micro-reactors for accelerating chemical reactions. The surface excess charge layer (SECL) in droplets has often been associated with distinct chemistry. Using molecular simulations for droplets with Na+ and Cl- ions we have found that this layer is ≈ 1.5−1.7 nm thick and depending on the droplet size it includes 33%-55% of the total number of ions. Here, we examine the effect of droplet size and nature of ions in the structure of SECL by using molecular dynamics. We find that in the presence of simple ions the thickness of the surface excess charge layer is invariant not only with respect to droplet size but also with respect to the nature of the simple ions and it is not sensitive to fine details of different force fields used in our simulations.&lt;/div&gt;&lt;div&gt; In the presence of macroions the SECL may extend to 2.0. nm. For the same droplet size, iodide and model H3O+ ions show considerably higher concentration than the sodium and chloride ions. In nano-drops, the SECL does not have the highest concentration of ions. We identify the maximum ion concentration region that may overlap with SECL in nano-drops. We also find that the differences in the average water dipole orientation in the presence of cations and anions in this layer are reflected in the charge distributions. Within the surface charge layer, the number of hydrogen bonds reduces gradually relative to the droplet interior where the number of hydrogen bonds is on the average 2.9 for droplets of diameter &lt; 4 nm and 3.5 for larger droplets. The decrease in the number of hydrogen bonds from the interior to the surface is less pronounced in larger droplets. In droplets with diameter &lt; 4 nm and high concentration of ions the charge of the ions is not compensated only by the solvent polarization charge but by the total charge that also includes the other free charge. This finding shows exceptions to the commonly made assumption that the solvent compensates the charge of the ions in solvents with very high dielectric constant. The study provides molecular insight into the bi-layer droplet structure assumed in the equilibrium partitioning model (EPM) of C. Enke and assesses critical assumptions of the Iribarne-Thomson model for the ion-evaporation mechanism. We suggest the extension of the bi-layer droplet structure in EPM to include the maximum ion concentration region that may not coincide with SECL in nanodrops. We compute the ion concentrations in SECL, which are those that should enter the kinetic equation in the ion-evaporation mechanism, instead of the overall drop ion concentration that has been used thus far.&lt;br&gt;&lt;/div&gt;</jats:p

Molecular Simulation Methods of Evaporating Electrosprayed Droplets

Molecular simulations provide significant insights into the relationship between the release of a macromolecule from charged droplets and its charge state. These simulations also help determine the possible location of the charge in macroions detected by mass spectrometry. However, several significant challenges must be considered in the modeling process. These challenges include the effect of the droplet-size-dependent chemistry on the charge state of a macroion, limitations in force fields, efficient droplet evaporation at any temperature, and effective treatment of electrostatic interactions. Here, we present a robust methodology for molecular simulations that enables the study of the chemistry and interactions of macromolecules within a droplet, and the relationship between the dynamics of the process of interest and the solvent evaporation rate. The competition of these dynamic processes will determine the mass spectrum. The success of the approach depends on the accurate and effective treatment of electostatic interactions in combination with the efficient use of spherical boundary conditions. Multilevel summation method (MSM) has been developed [Hardy et al. ``Multilevel summation method for electrostatic force evaluation\u27\u27 J. Chem. Theory Comput. {\bf 11}, 766 (2015)] for the efficient treatment of electrostatics in non-periodic and semi-periodic systems, charged and neutral. We present comparison of MSM with particle-mesh Ewald (PME) method in order to show MSM\u27s ability to study conformational changes of macromolecules in droplets. We find that MSM produces the same conformations of macromolecules (within one standard deviation) as PME. We demonstrate the capability of spherical boundary condition and MSM to study physical and chemical processes in droplets by using the example of the Rayleigh jet formation and charge emission from it. We conclude that robust approaches for droplet simulations that can be used with a force field of any complexity are available and can be implemented within many of the available open-source molecular modeling softwares. In the near future, the presented approach may provide reliable reference mass spectra for experiments, where the deviations from the experimental data may reveal valuable information about the processes that take place within the instrument