Showing papers in "Journal of Physical Chemistry A in 2001"

ReaxFF: A Reactive Force Field for Hydrocarbons

Abstract: To make practical the molecular dynamics simulation of large scale reactive chemical systems (1000s of atoms), we developed ReaxFF, a force field for reactive systems. ReaxFF uses a general relationship between bond distance and bond order on one hand and between bond order and bond energy on the other hand that leads to proper dissociation of bonds to separated atoms. Other valence terms present in the force field (angle and torsion) are defined in terms of the same bond orders so that all these terms go to zero smoothly as bonds break. In addition, ReaxFF has Coulomb and Morse (van der Waals) potentials to describe nonbond interactions between all atoms (no exclusions). These nonbond interactions are shielded at short range so that the Coulomb and van der Waals interactions become constant as Rij → 0. We report here the ReaxFF for hydrocarbons. The parameters were derived from quantum chemical calculations on bond dissociation and reactions of small molecules plus heat of formation and geometry data for...

Structure and Dynamics of the TIP3P, SPC, and SPC/E Water Models at 298 K

Abstract: Molecular dynamics simulations of five water models, the TIP3P (original and modified), SPC (original and refined), and SPC/E (original), were performed using the CHARMM molecular mechanics program. All simulations were carried out in the microcanonical NVE ensemble, using 901 water molecules in a cubic simulation cell furnished with periodic boundary conditions at 298 K. The SHAKE algorithm was used to keep water molecules rigid. Nanosecond trajectories were calculated with all water models for high statistical accuracy. The characteristic self-diffusion coefficients D and radial distribution functions, gOO, gOH, and gHH for all five water models were determined and compared to experimental data. The effects of velocity rescaling on the self-diffusion coefficient D were examined. All these empirical water models used in this study are similar by having three interaction sites, but the small differences in their pair potentials composed of Lennard-Jones (LJ) and Coulombic terms give significant difference...

Addition of Polarization and Diffuse Functions to the LANL2DZ Basis Set for P-Block Elements

Abstract: Diffuse and polarization functions have been optimized for the LANL2DZ basis set for elements in groups 14−17. The optimized exponents are in most cases similar to those optimized with different effective core potentials, valence basis sets, or computational models. The average of the LANL2DZ results for different models is taken to be the best generalized set of exponents. The extended basis set gives good results (average deviation from experiment 0.11 eV) for atomic electron affinities with the B3LYP model, but is consistently low with the MP2 model. The extended basis set gives similar performance to the all-electron 6-31+G(d) basis set in calculations of vibrational frequencies and bond energies in selected main-group compounds, and is intermediate in speed between the 6-31+G(d) basis set and the unmodified LANL2DZ basis set.

The Effective Fragment Potential Method: A QM-Based MM Approach to Modeling Environmental Effects in Chemistry

Abstract: The effective fragment potential (EFP) method is described and its capabilities illustrated using several applications. The original method, EFP1, was primarily developed to describe aqueous solvation, by representing Coulombic, induction and repulsive interactions via one-electron terms in the ab initio Hamiltonian. It is demonstrated, using water clusters, the Menshutkin reaction and the glycine neutral/zwitterion equilibrium, that agreement with both fully ab initio calculations and experiment are excellent. More recently, the model has been extended so that it can treat any solvent, as well as more difficult links across covalent bonds.

How Well Can Hybrid Density Functional Methods Predict Transition State Geometries and Barrier Heights

Abstract: We compare hybrid Hartree−Fock density-functional theory to ab initio approaches for locating saddle point geometries and calculating barrier heights on a Born−Oppenhiemer potential energy surface. We located reactant, product, and saddle point stationary points for 22 reactions by the MP2 and QCISD ab initio methods and the B3LYP, BH&HLYP, mPW1PW91, and MPW1K hybrid Hartree−Fock DFT methods. We examined all of these methods with two basis sets, 6-31+G(d,p) and MG3. By comparison to calculations on five systems where the saddle point has been optimized at a high level of theory, we determined that the best saddle point geometries were obtained using the MPW1K and QCISD levels of theory. Of the methods tested, mPW1PW91 and B3LYP are the least effective for determining saddle point geometries and have mean unsigned error in barrier heights of 3.4−4.2 kcal/mol, depending on the basis set. In contrast, the MPW1K level of theory predicts the most accurate saddle point geometries and has a mean unsigned error o...

Delocalization of Electrons in Molecules

Abstract: We report a general method for the investigation and quantification of delocalization in molecules. The method is based on the anisotropy of the current-induced density (ACID). Compared to the current density, which has been frequently used to investigate delocalization, the ACID approach has several advantages: it is a scalar field which is invariant with respect to the relative orientation of the magnetic field and the molecule, it is not a simple function of the overall electron density, it has the same symmetry as the wave function, and it can be plotted as an isosurface. Several selected examples demonstrate the predictive power and the general applicability of this method.

High Dimensional Model Representations

Abstract: In the chemical sciences, many laboratory experiments, environmental and industrial processes, as well as modeling exercises, are characterized by large numbers of input variables. A general objective in such cases is an exploration of the high-dimensional input variable space as thoroughly as possible for its impact on observable system behavior, often with either optimization in mind or simply for achieving a better understanding of the phenomena involved. An important concern when undertaking these explorations is the number of experiments or modeling excursions necessary to effectively learn the system input → output behavior, which is typically a nonlinear relationship. Although simple logic suggests that the number of runs could grow exponentially with the number of input variables, broadscale evidence indicates that the required effort often scales far more comfortably. This paper considers an emerging family of high dimensional model representation concepts and techniques capable of dealing with s...

Structure and Stability of Water Clusters (H2O)n, n ) 8-20: An Ab Initio Investigation

Abstract: Extensive ab initio calculations have been performed using the 6-31G(d,p) and 6-311++G(2d,2p) basis sets for several possible structures of water clusters (H2O)n, n = 8−20. It is found that the most stable geometries arise from a fusion of tetrameric or pentameric rings. As a result, (H2O)n, n = 8, 12, 16, and 20, are found to be cuboids, while (H2O)10 and (H2O)15 are fused pentameric structures. For the other water clusters (n = 9, 11, 13, 14, and 17−19) under investigation, the most stable geometries can be thought of as arising from either the cuboid or the fused pentamers or a combination thereof. The stability of some of the clusters, namely, n = 8−16, has also been studied using density functional theory. An attempt has been made to estimate the basis set superposition error and zero-point energy correction for such clusters at the Hartree−Fock (HF) level using the 6-311++G(2d,2p) basis set. To ensure that a minimum on the potential-energy surface has been located, frequency calculations have been c...

Perfluoroalkanes: Conformational Analysis and Liquid-State Properties from ab Initio and Monte Carlo Calculations

Abstract: Classical OPLS-AA force-field parameters are developed for perfluoroalkanes primarily by fitting to conformational profiles from gas-phase ab initio calculations (LMP2/cc-pVTZ(-f)//HF/6-31G*) and to experimental data for pure liquids. The ab initio C−C−C−C profile of n-C4F10 (perfluorobutane) is similar to those from prior high-level calculations and indicates the presence of gauche (g) and ortho (o) minima and of anti (a) minima slightly offset from 180°. Ab initio torsional profiles for n-C5F12 (perfluoropentane) and (CF3)2CFCF2CF3 (perfluoro-2-methylbutane) also show three sets of energy minima. Special OPLS-AA torsional parameters for these three molecules closely match ab initio and experimental geometries, conformational energies (ΔEmin), and conformational energy barriers. These specialized force fields were merged to provide a generalized force field for linear, branched, and cyclic perfluoroalkanes. The resultant parameters yield key ΔEmin values within 0.6 kcal/mol of the ab initio results for t...

The Cluster−Continuum Model for the Calculation of the Solvation Free Energy of Ionic Species

Abstract: A hybrid approach using a combination of explicit solvent molecules and the isodensity polarizable continuum model (IPCM) method is proposed for the calculation of the solvation thermodynamic prope...

Ab Initio Calculations on Conventional and Unconventional Hydrogen BondsStudy of the Hydrogen Bond Strength

Abstract: Different measures of H-bond strength based on X−H proton donating bond properties and on parameters of H···Y distance (Y−proton acceptor within X−H···Y H bridges) are investigated. Correlations between such measures and H-bond energy are studied. The parameters of H-bonds are taken from geometry of simple complexes optimized within HF/6-311++G** and MP2/6-311++G** levels of theory. The Bader theory of atoms in molecules is also applied for an estimation of electronic densities at bond critical points and Laplacians of these densities, these topological parameters are also used to define H-bond strength measures. Apart from the conventional statistical analysis, the factor analysis is applied to study the properties of H bridges. The results show that the set of geometrical, energetic, and topological variables describing the H bridge may be replaced by one new variable, one factor. It is also shown that the geometrical and topological parameters of the proton donating bond better correlate with the H-bon...

Patterns of Ring Currents in Conjugated Molecules: A Few-Electron Model Based on Orbital Contributions

Abstract: In the CTOCD-DZ (continuous transformation of origin of current density-diamagnetic zero) formulation of coupled Hartree−Fock theory for magnetic response of closed-shell systems, induced current density at each point is calculated with the gauge origin at that point. In addition to its economy and accuracy for total current maps, CTOCD-DZ is shown to yield a unique and physically motivated definition of, and symmetry criteria for, orbital contributions to current density. This leads to a few-electron interpretation of ring currents. Only the four HOMO electrons of an aromatic (4n+2)-electron monocycle contribute significantly to the ring current, and in general only a small subset of the high-lying π electrons dominate the more complex patterns of current in polycyclic π systems. Benzene, naphthalene, hexacene, pyracylene, coronene, and corannulene are treated as examples.

Conical Intersections: The New Conventional Wisdom

Abstract: During the past decade the perception of conical intersections has changed. It is now appreciated that what was once viewed largely as a theoretical curiosity is an essential aspect of electronically nonadiabatic processes. Concomitantly, our understanding of this singular consequence of the Born−Oppenheimer separation of nuclear and electronic motion has grown enormously. In this work the theory of conical intersections is reviewed.

Calculation of Optical Rotation Using Density Functional Theory

Abstract: We report calculations of the frequency-dependent electric dipole−magnetic dipole polarizability tensor, βαβ(ν), using ab initio density functional theory (DFT). Gauge invariant (including) atomic orbitals (GIAOs) are used to guarantee origin-independent values of β = (1/3)Tr [βαβ]. Calculations of β at the sodium D line frequency, β(D), for 30 rigid chiral molecules are used to predict their specific rotations, [α]D. Calculations have been carried out using the B3LYP functional and the 6-31G*, DZP, 6-311++G(2d,2p), aug-cc-pVDZ, and aug-cc-pVTZ basis sets. Comparison to experimental [α]D values for 28 of the 30 molecules yields average absolute deviations of calculated and experimental [α]D values in the range 20−25° for the three large basis sets, all of which include diffuse functions. The accuracies of [α]D values calculated using the 6-31G* and DZP basis sets, which do not include diffuse functions, are significantly lower: average deviations from experiment are 33° and 43°, respectively. Hartree−Foc...

Spectroscopic Study of Nitric Acid and Water Adsorption on Oxide Particles: Enhanced Nitric Acid Uptake Kinetics in the Presence of Adsorbed Water

Abstract: In this study, the heterogeneous reactivity of nitric acid on oxide particles of some of the most abundant crustal elements is investigated at 296 K. The oxide particles are used as models for mineral dust aerosol found in the atmosphere. Transmission FT-IR spectroscopy is used to probe changes in the spectrum of the oxide particle surface following adsorption of HNO3 on SiO2, α-Al2O3, TiO2, γ-Fe2O3, CaO, and MgO. It is found that HNO3 molecularly and reversibly adsorbs on SiO2. For the other oxides investigated, HNO3 dissociatively and irreversibly adsorbs to form surface nitrate. There is also a small amount of molecularly adsorbed nitric acid (<10% of the adsorbed nitrate) on the oxide particle surface in the presence of gas-phase nitric acid. Because adsorbed water may play a role in the heterogeneous uptake of nitric acid in the atmosphere, transmission FT-IR spectroscopy is used to investigate H2O adsorption on SiO2, α-Al2O3, TiO2, γ-Fe2O3, CaO, and MgO particles as well. Uptake of water on the oxid...

Spin-Orbit Coupling and Intersystem Crossing in Conjugated Polymers: A Configuration Interaction Description

Abstract: Configuration−interaction calculations are performed to describe the singlet and triplet excited states of oligothiophene and oligo(phenylene ethynylene) conjugated chains. Intersystem crossing from the singlet to the triplet manifold is made possible by spin−orbit coupling, which leads to a mixing of the singlet (Sn) and triplet (Tn) wave functions. The electronic spin−orbit S1−Ti matrix elements, obtained from first-order perturbation theory, are used to compute the rates of intersystem crossing from the lowest singlet excited state, S1, into low-lying triplet states, Ti. On the basis of these results, a general mechanism is proposed to describe the intersystem crossing process in conjugated oligomers and polymers. The roles of chain length, heavy-atom derivatization, and ring twists are evaluated.

Impact of Ultrasonic Frequency on Aqueous Sonoluminescence and Sonochemistry

Abstract: A comprehensive investigation of ultrasonic frequency and its role in sonochemical activity and sonoluminescence (SL) has been performed. SL spectra and intensity were examined at four frequencies (205, 358, 618, and 1071 kHz) and in the presence of varying argon and oxygen saturation ratios. A series of high-energy reactions induced by the extreme temperatures and pressures obtained within a microbubble during acoustic cavitation contribute to the broad continuum characteristic of SL spectra. Chemical reactivity was also measured at all four frequencies. 1,4-Dioxane decomposition and hydrogen peroxide formation were chosen as representative sonochemical processes. A 358 kHz value was the optimal frequency for maximum SL intensity and chemical reaction rates. The impact of a hydroxyl radical scavenger, bicarbonate ion, on SL intensity and H2O2 formation was also examined. Results from this investigation indicate that nonlinear bubble implosions play a more significant role at lower frequencies whereas hig...

Interaction of Ozone and Water Vapor with Spark Discharge Soot Aerosol Particles Coated with Benzo[a]pyrene: O3 and H2O Adsorption, Benzo[a]pyrene Degradation, and Atmospheric Implications

Abstract: The interaction of ozone and water vapor with spark discharge soot particles coated with the five-ring polycyclic aromatic hydrocarbon benzo[a]pyrene (BaP) has been investigated in aerosol flow tube experiments at ambient temperature and pressure (296 K, 1 atm). The investigated range of ozone volume mixing ratio (VMR) and relative humidity (RH) was 0−1 ppm and 0−25%, respectively. The observed gas-phase ozone losses and pseudo-first-order BaP decay rate coefficients exhibited Langmuir-type dependencies on gas-phase ozone concentration and were reduced in the presence of water vapor, which indicates rapid, reversible and competitive adsorption of O3 and H2O on the particles followed by a slower surface reaction between adsorbed O3 and BaP. At low ozone VMR and RH, the half-life of surface BaP molecules was found to be shorter than previously reported (∼ 5 min at 30 ppb O3 under dry conditions). At higher RH and for multilayer BaP surface coverage, however, a strong increase of BaP half-life was observed a...

Laser Spectroscopy of Jet-Cooled Biomolecules and Their Water-Containing Clusters: Water Bridges and Molecular Conformation

Abstract: The techniques of laser-induced fluorescence (LIF), resonant two-photon ionization spectroscopy (R2PI), UV−UV hole-burning spectroscopy, fluorescence-dip infrared spectroscopy (FDIRS), and resonant ion-dip infrared spectroscopy (RIDIRS) have been used to study the ultraviolet and infrared spectra of individual conformations of small, flexible biomolecules cooled in a supersonic expansion. The water-containing clusters of these molecules and of other rigid molecules that possess multiple H-bonding sites are considered. The water molecules in many of the solute−(water)n clusters form hydrogen-bonded bridges between donor and acceptor sites on the solute molecule. The infrared spectroscopy of these bridges has been explored in some detail. Water bridges are also formed when one of the H-bonding sites is on a flexible side chain. These bridges have a profound influence on the conformational preferences of the flexible biomolecules.

Using Kohn−Sham Orbitals in Symmetry-Adapted Perturbation Theory to Investigate Intermolecular Interactions

Abstract: This is the first reported use of a hybrid method involving density functional theory (DFT) and symmetry-adapted perturbation theory (SAPT) to calculate intermolecular interactions. This work was stimulated by the reported failures of supermolecular DFT calculations to adequately predict intermolecular (and interatomic) interactions, particularly of the van der Waals type. The goals are to develop a hybrid scheme that will calculate intermolecular interaction energies accurately and in a computationally efficient fashion, while including the benefits of the energy decomposition provided by SAPT. The computational savings result from replacing the costly perturbation theory treatment with DFT, which should include the intramolecular correlation effects on the intermolecular interaction energies. The accuracy of this new hybrid approach (labeled SAPT(DFT)) is evaluated by comparisons with higher level calculations. The test cases include He2, Ar2, Ar−H2, (H2O)2, (HF)2, CO2−CH3CN, and CO2-dimethylnitramine. ...

Femtosecond Time-Resolved Fluorescence Study of Photoisomerization of trans-Azobenzene

Abstract: The electronic relaxation and isomerization mechanism of trans-azobenzene after the S2(ππ*) ← S0 photoexcitation were investigated in solution by steady-state and femtosecond time-resolved fluorescence spectroscopy. In the steady-state fluorescence spectrum, two bands were observed with their peaks at ∼390 nm (∼25 750 cm-1) and ∼665 nm (∼15 000 cm-1). These fluorescence bands showed good mirror images of the S2(ππ*) ← S0 and S1(nπ*) ← S0 absorption bands, so that they were assigned to the fluorescence from the S2(ππ*) and S1(nπ*) states having “planar” structures. The lifetimes of the S2 and S1 states were determined as ∼110 fs (S2) and ∼500 fs (S1) by time-resolved measurements. The quantum yield of the S2 → S1 electronic relaxation was evaluated by comparing the intensity of the S2 and S1 fluorescence, and it was found to be almost unity. This implies that almost all molecules photoexcited to the S2(ππ*) state are relaxed to the “planar” S1(nπ*) state. The present fluorescence data clarified that the is...

The effects of low molecular weight dicarboxylic acids on cloud formation

Abstract: The ubiquitous presence of organic compounds in tropospheric particles requires that their role in aerosol/cloud interactions be accounted for in climate models. In this paper, we present studies that investigate the hygroscopic behavior of organic compounds and their efficiency as ice nuclei. Specifically, results for soluble and partially soluble dicarboxylic acids that have been observed in atmospheric aerosol are discussed. At room temperature, we use a humidified tandem differential mobility analyzer (HTDMA) and a condensation particle counter interfaced with a cloud condensation nuclei counter to characterize the water uptake behavior of these acids. The HTDMA data agree quite well with modeled hygroscopic behavior. However, we find that some of the compounds retain water to very low humidities, never exhibiting efflorescence. The studies are extended to lower temperatures using a continuous flow ice thermal diffusion chamber to investigate the role of these species in ice nucleation at cirrus condi...

Primary and Secondary Glyoxal Formation from Aromatics: Experimental Evidence for the Bicycloalkyl−Radical Pathway from Benzene, Toluene, and p-Xylene

Abstract: A new approach is presented to study the ring-cleavage process of benzene, toluene, and p-xylene (BTX). DOAS (differential optical absorption spectroscopy) was used for the simultaneous measurement of the respective ring-retaining products as well as glyoxal (a ring-cleavage product) in a series of experiments at the EUPHORE outdoor simulation chamber, Valencia/Spain. The good time resolution of the DOAS measurements (1-2 min) allowed the primary formation of glyoxal to be separated from any further contributions through additional pathways via reactions of stable intermediate compounds (secondary glyoxal formation). The ring-retaining products and glyoxal were identified as primary products. The primary glyoxal yield was found to be essentially identical to the overall yield of glyoxal formed over the time scale of the experiments. The negligible contribution from secondary glyoxal formation pathways was quantitatively understood for the experimental conditions of this study and was found to be representative for the troposphere. The yield of glyoxal was determined to be 35% ( 10% for benzene and about 5% higher for toluene and p-xylene. For benzene, the yield of hexadienedial was estimated to be e 8%. It is concluded that ringcleavage pathways involving the bicycloalkyl radical are major pathways in the oxidation of monocyclic aromatic hydrocarbons, i.e., BTX. The branching ratio for the bicycloalkyl radical intermediate, proposed to form from the reaction of the aromatic-OH adduct with atmospheric oxygen, could be directly identified with the primary glyoxal yield for the benzene system and as a lower limit in the case of toluene and p-xylene. Implications for the chemical behavior of aromatic hydrocarbons in the atmosphere are discussed.

Photoinduced Electron and Proton Transfer in Phenol and Its Clusters with Water and Ammonia

Abstract: Ab initio (RHF, MP2, CASSCF, and CASPT2) calculations have been performed for the electronic ground and lowest excited singlet states of phenol, the complexes of phenol with water and ammonia, and the corresponding cations. In agreement with recent experiments it is found that proton transfer is a barrierless process in the phenol−(H2O)3 and phenol−NH3 cations, whereas no proton transfer occurs in the phenol−H2O cation. Novel aspects of the reaction dynamics in the excited-state manifold of the neutral clusters are revealed by the calculations. Predissociation of the S1(ππ*) state by a low-lying 1πσ* state leads to a concerted electron and proton-transfer reaction from the chromophore to the solvent. The excited-state reaction is endothermic in phenol−H2O and phenol−(H2O)3 clusters but exothermic (though activated) in the phenol−NH3 complex. These results substantiate recent reinterpretations of spectroscopic and kinetic data on hydrogen-transfer reactions in phenol−ammonia clusters. The close relationshi...

Absolute Hydration Free Energy of the Proton from First-Principles Electronic Structure Calculations

Abstract: The absolute hydration free energy of the proton, ΔGhyd298(H+), is one of the fundamental quantities for the thermodynamics of aqueous systems. Its exact value remains unknown despite extensive experimental and computational efforts. We report a first-principles determination of ΔGhyd298(H+) by using the latest developments in electronic structure theory including solvation effects. High level ab initio calculations have been performed with a supermolecule-continuum approach based on a recently developed self-consistent reaction field model known as surface and volume polarization for electrostatic interaction (SVPE) or fully polarizable continuum model (FPCM). In the supermolecule-continuum approach, part of the solvent surrounding the solute is treated quantum mechanically and the remaining bulk solvent is approximated by a dielectric continuum medium. With this approach, the calculated results can systematically be improved by increasing the number of quantum mechanically treated solvent molecules. ΔGh...

An Extremely Small Reorganization Energy of Electron Transfer in Porphyrin−Fullerene Dyad

Abstract: Both charge-transfer absorption and emission have been observed in porphyrin-linked fullerene where the C60 moiety is closely located on the porphyrin plane. Electron-transfer parameters including reorganization energies, free energy changes, and electronic coupling matrix elements were determined by analyzing the charge-transfer absorption and emission in benzene. The reorganization energy is estimated as 0.23 ± 0.11 eV, which is the smallest value among inter- and intramolecular donor−acceptor systems ever reported and is comparable to the smallest ones for the primary charge separation in the photosynthetic reaction center. The results clearly show that fullerenes combined with porphyrins are potential components for constructing artificial photosynthetic systems.

Excitation Energies from Time-Dependent Density Functional Theory for Linear Polyene Oligomers: Butadiene to Decapentaene

Abstract: Time-dependent density functional theory (TDDFT) is applied to calculate vertical excitation energies of trans-1,3-butadiene, trans−trans-1,3,5-hexatriene, all-trans-1,3,5,7-octatetraene, and all-trans-1,3,5,7,9-decapentaene. Attachment and detachment densities for transitions in butadiene and decapentaene from the ground state to the 2 1Ag and 1 1Bu excited states are also calculated and analyzed. Based on comparisons with experimental results and high level ab initio calculations in the literature, significant improvement over configuration−interaction singles is observed for the 2 1Ag state of the polyenes, which has been known to have significant double excitation character. For the 1 1Bu state, TDDFT underestimates the excitation energy by 0.4−0.7 eV. In this case we have observed a significant difference between the results for TDDFT and TDDFT within the Tamm−Dancoff approximation, both in excitation energies and, at least for butadiene, in the character of the excited state.

The Reaction Probability of OH on Organic Surfaces of Tropospheric Interest

Abstract: Using a flow-tube reactor coupled to a chemical ionization mass spectrometer, we investigated the heterogeneous loss of OH on Halocarbon wax, two types of organized organic monolayers, and several solid organic surfaces (paraffin wax, stearic acid-palmitic acid mixture, pyrene, and soot) that are representative of surfaces found in the troposphere. The heterogeneous reaction is very efficient: the reaction probability is greater than 0.1 for all the organic surfaces investigated, except for Halocarbon wax. These results indicate that OH-organic heterogeneous reactions will significantly modify the hygroscopic properties and cloud condensation nuclei (CCN) ability of organic surfaces in the troposphere, and thus may play an important role in the Earth’s radiative balance by affecting the properties of clouds. We also determined the diffusion coefficient of OH in helium to be 665 ( 35 Torr cm 2 s -1 . This value is close to that of its polar analogue, H 2O, suggesting that the diffusion coefficient of OH can be calculated accurately with H2O transport properties.

Bond Dissociation Energies and Radical Stabilization Energies Associated with Substituted Methyl Radicals

Abstract: Bond dissociation energies (BDEs) and radical stabilization energies (RSEs) associated with a series of 22 monosubstituted methyl radicals (•CH2X) have been determined at a variety of levels including, CBS-RAD, G3(MP2)-RAD, RMP2, UB3-LYP and RB3-LYP. In addition, W1′ values were obtained for a subset of 13 of the radicals. The W1′ BDEs and RSEs are generally close to experimental values and lead to the suggestion that a small number of the experimental estimates warrant reexamination. Of the other methods, CBS-RAD and G3(MP2)-RAD produce good BDEs. A cancellation of errors leads to reasonable RSEs being produced from all the methods examined. CBS-RAD, W1′ and G3(MP2)-RAD perform best, while UB3-LYP performs worst. The substituents (X) examined include lone-pair-donors (X = NH2, OH, OCH3, F, PH2, SH, Cl, Br and OCOCH3), π-acceptors (X = BH2, CH=CH2, C≡CH, C6H5, CHO, COOH, COOCH3, CN and NO2) and hyperconjugating groups (CH3, CH2CH3, CF3 and CF2CF3). All substituents other than CF3 and CF2CF3 result in radical stabilization, with the vinyl (CH=CH2), ethynyl (C≡CH) and phenyl (C6H5) groups predicted to give the largest stabilizations of the π-acceptor substituents examined and the NH2 group calculated to provide the greatest stabilization of the lone-pair-donor groups. The substituents investigated in this work stabilize methyl radical centers in three general ways that delocalize the odd electron: π-acceptor groups (unsaturated substituents) delocalize the unpaired electron into the π-system of the substituent, lone-pair-donor groups (heteroatomic substituents) bring about stabilization through a three-electron interaction between a lone pair on the substituent and the unpaired electron at the radical center, while alkyl groups stabilize radicals via a hyperconjugative mechanism. Polyfluoroalkyl substituents are predicted to slightly destabilize a methyl radical center by inductively withdrawing electron density from the electron-deficient radical center.