Direct Measurements of Covalently Bonded Sulfuric Anhydrides from Gas-Phase Reactions of SO with Acids under Ambient Conditions.

Sulfur trioxide (SO) is an important oxide of sulfur and a key intermediate in the formation of sulfuric acid (HSO, SA) in the Earth's atmosphere. This conversion to SA occurs rapidly due to the reaction of SO with a water dimer. However, gas-phase SO has been measured directly at concentrations that are comparable to that of SA under polluted mega-city conditions, indicating gaps in our current understanding of the sources and fates of SO.

A systematic study on the kinetics of H-shift reactions in pristine acyl peroxy radicals.

A series of acyl peroxy radical H-shifts were systematically studied using computational approaches. Acyl peroxy radicals were categorized into small- (ethanal-pentanal), medium- (hexanal and heptanal) and large-sized (octanal and nonanal) molecules. The H-shifts spanning from 1,4 to 1,9 were inspected for each studied system.

Molecular rearrangement of bicyclic peroxy radicals is a key route to aerosol from aromatics.

The oxidation of aromatics contributes significantly to the formation of atmospheric aerosol. Using toluene as an example, we demonstrate the existence of a molecular rearrangement channel in the oxidation mechanism. Based on both flow reactor experiments and quantum chemical calculations, we show that the bicyclic peroxy radicals (BPRs) formed in OH-initiated aromatic oxidation are much less stable than previously thought, and in the case of the toluene derived ipso-BPRs, lead to aerosol-forming low-volatility products with up to 9 oxygen atoms on sub-second timescales.

Solving the discrepancy between the direct and relative-rate determinations of unimolecular reaction kinetics of dimethyl-substituted Criegee intermediate (CH)COO using a new photolytic precursor.

We have performed direct kinetic measurements of the thermal unimolecular reaction of (CH)COO in the temperature range 243-340 K and pressure range 5-350 Torr using time-resolved UV-absorption spectroscopy. We have utilized a new photolytic precursor, 2-bromo-2-iodopropane ((CH)CIBr), which photolysis at 213 nm in the presence of O produces acetone oxide, (CH)COO. The results show that the thermal unimolecular reaction is even more important main loss process of (CH)COO in the atmosphere than direct kinetic studies have suggested hitherto.

An experimental and master-equation modeling study of the kinetics of the reaction between resonance-stabilized (CH)CCHCH radical and molecular oxygen.

The kinetics of the reaction between resonance-stabilized (CH)CCHCH radical (R) and O has been investigated using photoionization mass spectrometry, and master equation (ME) simulations were performed to support the experimental results. The kinetic measurements of the (CH)CCHCH + O reaction (1) were carried out at low helium bath-gas pressures (0.2-5.

Time-resolved, broadband UV-absorption spectrometry measurements of Criegee intermediate kinetics using a new photolytic precursor: unimolecular decomposition of CHOO and its reaction with formic acid.

We present a time-resolved broadband cavity-enhanced UV-absorption spectrometer apparatus that we have constructed and utilized for temperature- and pressure-dependent kinetic measurements of formaldehyde oxide (CHOO) reactions. We also introduce and utilize a new photolytic precursor, bromoiodomethane (CHIBr), which photolysis at 213 nm in presence of O produces CHOO. Importantly, this precursor appears to be free from secondary reactions that may regenerate CHOO in kinetic experiments.

Binding and Release between Polymeric Carrier and Protein Drug: pH-Mediated Interplay of Coulomb Forces, Hydrogen Bonding, van der Waals Interactions, and Entropy.

The accelerating search for new types of drugs and delivery strategies poses challenge to understanding the mechanism of delivery. To this end, a detailed atomistic picture of binding between the drug and carrier is quintessential. Although many studies focus on the electrostatics of drug-vector interactions, it has also been pointed out that entropic factors relating to water and counterions can play an important role.

Dynamical Interactions of 5-Fluorouracil Drug with Dendritic Peptide Vectors: The Impact of Dendrimer Generation, Charge, Counterions, and Structured Water.

Molecular dynamics simulations are utilized to investigate the interactions between the skin cancer drug 5-fluorouracil (5FU) and peptide-based dendritic carrier systems. We find that these drug-carrier interactions do not conform to the traditional picture of long-time retention of the drug within a hydrophobic core of the dendrimer carrier. Rather, 5FU, which is moderately soluble in its own right, experiences weak, transient chattering interactions all over the dendrimer, mediated through multiple short-lived hydrogen bonding and close contact events.

First-Principle Framework for Total Charging Energies in Electrocatalytic Materials and Charge-Responsive Molecular Binding at Gas-Surface Interfaces.

Heterogeneous charge-responsive molecular binding to electrocatalytic materials has been predicted in several recent works. This phenomenon offers the possibility of using voltage to manipulate the strength of the binding interaction with the target gas molecule and thereby circumvent thermochemistry constraints, which inhibit achieving both efficient binding and facile release of important targets such as CO2 and H2. Stability analysis of such charge-induced molecular adsorption has been beyond the reach of existing first-principle approaches.

Nanodusty plasma chemistry: a mechanistic and variational transition state theory study of the initial steps of silyl anion-silane and silylene anion-silane polymerization reactions.

The growth of nanodusty particles, which is critical in plasma chemistry, physics, and engineering. The aim of the present work is to understand the detailed reaction mechanisms of early steps in this growth. The polymerization of neutral silane with the silylene or silyl anion, which eliminates molecular hydrogen with the formation of their higher homologues, governs the silicon hydride clustering in nanodusty plasma chemistry.

Large entropic effects on the thermochemistry of silicon nanodusty plasma constituents.

Determination of the thermodynamic properties of reactor constituents is the first step in designing control strategies for plasma-mediated deposition processes and is also a key fundamental issue in physical chemistry. In this work, a recently proposed multistructural statistical thermodynamic method is used to show the importance of multiple structures and torsional anharmonicity in determining the thermodynamic properties of silicon hydride clusters, which are important both in plasmas and in thermally driven systems. It includes five different categories of silicon hydride clusters and radicals, including silanes, silyl radicals, and silenes.

Assessment of range-separated time-dependent density-functional theory for calculating C6 dispersion coefficients.

We assess a variant of linear-response range-separated time-dependent density-functional theory (TDDFT), combining a long-range Hartree-Fock (HF) exchange kernel with a short-range adiabatic exchange-correlation kernel in the local-density approximation (LDA) for calculating isotropic C6 dispersion coefficients of homodimers of a number of closed-shell atoms and small molecules. This range-separated TDDFT tends to give underestimated C6 coefficients of small molecules with a mean absolute percentage error of about 5%, a slight improvement over standard TDDFT in the adiabatic LDA which tends to overestimate them with a mean absolute percentage error of 8%, but close to time-dependent Hartree-Fock which has a mean absolute percentage error of about 6%. These results thus show that introduction of long-range HF exchange in TDDFT has a small but beneficial impact on the values of C6 coefficients.

Kinetics of the hydrogen atom abstraction reactions from 1-butanol by hydroxyl radical: theory matches experiment and more.

In the present work, we study the H atom abstraction reactions by hydroxyl radical at all five sites of 1-butanol. Multistructural variational transition state theory (MS-VTST) was employed to estimate the five thermal rate constants. MS-VTST utilizes a multifaceted dividing surface that accounts for the multiple conformational structures of the transition state, and we also include all the structures of the reactant molecule.

Thermochemistry of radicals formed by hydrogen abstraction from 1-butanol, 2-methyl-1-propanol, and butanal.

We calculate the standard state entropy, heat capacity, enthalpy, and Gibbs free energy for 13 radicals important for the combustion chemistry of biofuels. These thermochemical quantities are calculated from recently proposed methods for calculating partition functions of complex molecules by taking into account their multiple conformational structures and torsional anharmonicity. The radicals considered in this study are those obtained by hydrogen abstraction from 1-butanol, 2-methyl-1-propanol, and butanal.

Statistical thermodynamics of 1-butanol, 2-methyl-1-propanol, and butanal.

The purpose of the present investigation is to calculate partition functions and thermodynamic quantities, viz., entropy, enthalpy, heat capacity, and Gibbs free energies, for 1-butanol, 2-methyl-1-propanol, and butanal in the vapor phase. We employed the multi-structural (MS) anharmonicity method and electronic structure calculations including both explicitly correlated coupled cluster theory and density functional theory.

Kinetics of the Hydrogen Abstraction from Carbon-3 of 1-Butanol by Hydroperoxyl Radical: Multi-Structural Variational Transition-State Calculations of a Reaction with 262 Conformations of the Transition State.

We estimated rate constants for the hydrogen abstraction from carbon-3 of 1-butanol by hydroperoxyl radical, a critically important reaction in the combustion of biofuel. We employed the recently developed multi-structural variational transition-state theory (MS-VTST), which utilizes a multifaceted dividing surface that allows us to include the contributions of multiple structures for reacting species and transition states. First, multiconfigurational Shepard interpolation-based on molecular-mechanics-guided interpolation of electronic-structure Hessian data obtained by the M08 HX/jun-cc-pVTZ electronic model chemistry-was used to obtain the portion of the potential energy surface needed for single-structure variational transition-state theory rate constants including multidimensional tunneling; then, the M08-HX/MG3S electronic model chemistry was used to calculate multi-structural torsional anharmonicity factors to complete the MS-VTST rate constant calculations.

Carbondioxide rare-gas systems: sensitivity of basis sets and double-hybrid density functionals.

This study emphasizes on the performance of six newly developed double-hybrid density functionals (DHDF) in explaining the potential energy curves of different carbondioxide rare-gas systems. The basis set sensitivity has also been explored with the use of three basis sets. Our results suggest that for lighter He/Ne-CO(2) complexes, proper choice of DHDF and basis set lead to results those matches exactly with earlier calculations and also with the experiment.

Nonlinear optical switching properties in the furylfulgide aberchrome 540-dihydrobenzofuran derivative pair of photochromic materials.

Time-dependent density functional theoretical investigation has been carried out to justify the switching action of nonlinear optical properties in the furylfulgide Aberchrome 540 (FFA) and dihydrobenzofuran derivative (DHBF) photochromic pair of molecules. The effect of solvents on this switching action has also been addressed. The calculations suggest that DHBF has a higher optical coefficient compared to that of FFA.

CH/pi interaction in benzene and substituted derivatives with halomethane: a combined density functional and dispersion-corrected density functional study.

The present work intends to establish the efficiency of dispersion-corrected density functionals in explaining the potential energy curves of benzene-methane, benzene-fluoroform, and 1,3,5-trifluoro benzene-methane complexes. The interaction energies of all of the complexes under investigation have been evaluated using both van der Waals-corrected and normal gradient-corrected Perdew-Burke-Ernzerhof and Becke-Lee-Yang-Parr density functionals. Our analyses suggest that the potential energy curves for both benzene-methane and benzene-fluoroform complexes are in excellent agreement with highly accurate coupled cluster (CCSD(T)) results as well as high-level counterpoise-corrected MP2 results.

Suitability of double hybrid density functionals and their dispersion-corrected counterparts in producing the potential energy curves for CO2-Rg (Rg: He, Ne, Ar and Kr) systems.

The present work aims to establish the suitability of double hybrid density functionals in explaining the potential energy curves of carbon dioxide-rare gas (CO(2)-Rg; Rg: He, Ne, Ar, and Kr) systems. The interaction energies of the most stable T-shaped configuration of all CO(2)-Rg systems have been evaluated using pure gradient-corrected functionals and double hybrid density functionals and their dispersion-corrected analogs with the use of Dunning's augmented correlation consistent polarized valence triple-zeta (aug-cc-pVTZ) basis function. The equilibrium separation distance, r, between CO(2) and Rg obtained from the potential energy curves for these CO(2)-Rg systems are then compared with the experimental as well as with some earlier theoretical non-density functional theory (non-DFT) results.

Searching of potential energy curves for the benzene dimer using dispersion-corrected density functional theory.

The present work aims to establish the utility of dispersion-corrected density functional theory for potential energy curves of the benzene dimer, a problem that has received significant attention for a long time. The interaction energies of parallel-stacked, T-shaped and parallel-displaced benzene dimer configurations have been evaluated using both dispersion- and normal gradient-corrected Perdew-Burke-Ernzerhof functionals along with Dunning's augmented correlation-consistent polarized valence triple-zeta (aug-cc-pVTZ) basis functions and compared with explicit correlation methods. The potential energy curves for the parallel-stacked and parallel-displaced benzene dimers are in excellent agreement with highly accurate coupled cluster (CCSD(T)) results, while for the T-shaped benzene dimer the dispersion-corrected results show a distinct deviation, being closer in that case to the MP2 level of results.

Magnetic interactions in alkyl substituted cyclohexane diradical systems: a broken symmetry approach.

Magnetic interactions in alkyl substituted cyclohexane diradical systems have been investigated within the framework of spin flip density functional theory. The investigations suggest a ferromagnetic interaction for both the alkyl substituted cyclohexane-1,3-diyls and cyclohexane-1,4-diyls. However, in the case of cyclohexane-1,3-diyls, the ferromagnetic interaction is much stronger than its 1,4 analogue.