Ab initio, density functional theory, and continuum solvation model prediction of the product ratio in the S(N)2 reaction of NO2(-) with CH3CH2Cl and CH3CH2Br in DMSO solution

The reaction pathways for the interaction of the nitrite ion with ethyl chloride and ethyl bromide in DMSO solution were investigated at the ab initio level of theory, and the solvent effect was included through the polarizable continuum model. The performance of BLYP, GLYP, XLYP, OLYP, PBE0, B3PW91, B3LYP, and X3LYP density functionals has been tested. For the ethyl bromide case, our best ab initio calculations at the CCSD(T)/aug-cc-pVTZ level predicts product ratio of 73% and 27% for nitroethane and ethyl nitrite, respectively, which can be compared with the experimental values of 67% and 33%. This translates to an error in the relative DeltaG* of only 0.17 kcal mol(-1). No functional is accurate (deviation <0.5 kcal mol(-1)) for predicting relative DeltaG*. The hybrid X3LYP functional presents the best performance with deviation 0.82 kcal mol(-1). The present problem should be included in the test set used for the evaluation of new functionals.     

Absolute solvation free energy of Li+ and Na+ ions in dimethyl sulfoxide solution: a theoretical ab initio and cluster-continuum model study

The solvation of the lithium and sodium ions in dimethyl sulfoxide solution was theoretically investigated using ab initio calculations coupled with the hybrid cluster-continuum model, a quasichemical theory of solvation. We have investigated clusters of ions with up to five dimethyl sulfoxide (DMSO) molecules, and the bulk solvent was described by a dielectric continuum model. Our results show that the lithium and sodium ions have four and five DMSO molecules into the first coordination shell, and the calculated solvation free energies are -135.5 and -108.6 kcal mol(-1), respectively. These data suggest a solvation free energy value of -273.2 kcal mol(-1) for the proton in dimethyl sulfoxide solution, a value that is more negative than the present uncertain experimental value. This and previous studies on the solvation of ions in water solution indicate that the tetraphenylarsonium tetraphenylborate assumption is flawed and the absolute value of the free energy of transfer of ions from water to DMSO solution is higher than the present experimental values.     

Chemical reactions inside structured nano-environment: S(N)2 vs. E2 reactions for the F(-) + CH(3)CH(2)Cl system

A new receptor for S(N)2 transition states, named NPTROL, is proposed. This molecule has a cavity and four hydroxyl groups that are able to interact with ionic S(N)2 and E2 transition states. Its catalytic effect and selectivity was investigated through high level ab initio calculations using the fluoride ion plus ethyl chloride in DMSO solution as a model system. Calculations at the ONIOM[CCSD(T)/6-311+G(2df,2p)?:?MP2/BASIS1] level of theory and solvent effects, included through a continuum solvation model, indicate that NPTROL is able to catalyze the S(N)2 pathway and has an inverse effect on the E2 pathway. Inside the NPTROL cavity, the ΔG(?) for the S(N)2 transition state is 5.00 kcal mol(-1) lower than that for E2, and as a consequence this reaction becomes highly selective toward the S(N)2 product.     

Ab initio benchmark study for the oxidative addition of CH4 to Pd: importance of basis-set flexibility and polarization

To obtain a state-of-the-art benchmark potential energy surface (PES) for the archetypal oxidative addition of the methane C-H bond to the palladium atom, we have explored this PES using a hierarchical series of ab initio methods (Hartree-Fock, second-order M?ller-Plesset perturbation theory, fourth-order M?ller-Plesset perturbation theory with single, double and quadruple excitations, coupled cluster theory with single and double excitations (CCSD), and with triple excitations treated perturbatively [CCSD(T)]) and hybrid density functional theory using the B3LYP functional, in combination with a hierarchical series of ten Gaussian-type basis sets, up to g polarization. Relativistic effects are taken into account either through a relativistic effective core potential for palladium or through a full four-component all-electron approach. Counterpoise corrected relative energies of stationary points are converged to within 0.1-0.2 kcal/mol as a function of the basis-set size. Our best estimate of kinetic and thermodynamic parameters is -8.1 (-8.3) kcal/mol for the formation of the reactant complex, 5.8 (3.1) kcal/mol for the activation energy relative to the separate reactants, and 0.8 (-1.2) kcal/mol for the reaction energy (zero-point vibrational energy-corrected values in parentheses). This agrees well with available experimental data. Our work highlights the importance of sufficient higher angular momentum polarization functions, f and g, for correctly describing metal-d-electron correlation and, thus, for obtaining reliable relative energies. We show that standard basis sets, such as LANL2DZ+1f for palladium, are not sufficiently polarized for this purpose and lead to erroneous CCSD(T) results. B3LYP is associated with smaller basis set superposition errors and shows faster convergence with basis-set size but yields relative energies (in particular, a reaction barrier) that are ca. 3.5 kcal/mol higher than the corresponding CCSD(T) values.     

Ab initio calculation of the inversion barrier in the germyl radical

Equilibrium geometry, vibrational frequencies and inversion barrier of the GeH₃ radical have been calculated using ab initio molecular orbital methods at both the SCF and correlated levels. At the SCF level, the effect of several small and medium size basis sets of Ge on these properties are studied systematically. Electron correlation corrections as incorporatedvia fourth-order unrestricted Møller-Plesset perturbation theory in conjunction with large Gaussian basis sets were found to reduce the GeH₃ inversion barrier appreciably relative to the UHF values. The final barrier height of 4.5 kcal/mol can be compared with a recent estimate of 4.4 kcal/mol inferred from the REMPI study.     

Conformational and structural studies of 1-chloropropane and 1-bromopropane from temperature-dependant FT-IR spectra of rare gas solutions and ab initio calculations

Variable temperature (−55 to −150°C) studies of the infrared spectra (3500–400 cm⁻¹) of 1-chloropropane (CH₃CH₂CH₂Cl) and 1-bromopropane (CH₃CH₂CH₂Br) dissolved in liquid krypton and xenon, respectively, have been recorded. Utilizing two conformer pairs in krypton solution for chloride and three conformer pairs in xenon solution for bromide, enthalpy differences of 52±3 cm⁻¹ (0.62±0.06 kJ/mol) and 72±7 cm⁻¹ (0.86±0.08 kJ/mol) were obtained for the chloride and bromide, respectively, with the gauche form being the more stable conformer for both molecules. From these data, it is estimated that 28 and 26% of trans form are present at ambient temperature for the chloride and bromide, respectively. The conformation stabilities, harmonic force constants, fundamental frequencies, infrared intensities and Raman activities have been obtained from RHF/6-31G(d) and/or MP2/6-31G(d) ab initio calculations for both halopropanes and these quantities have been compared to the experimental values when appropriate. The optimized geometries have also been obtained with several different ab initio basis sets with full electron correlation by the perturbation method up to MP2/6-311+G(2d,2p). The r₀ structural parameters of both halopropanes have been obtained by combining the ab initio data with the previously reported microwave rotational constants for both conformers. The quantities are compared to the corresponding results for some similar molecules.     

Theoretical study of the neutral hydrolysis of methyl formate via a concerted and stepwise water-assisted mechanism using free-energy curves and molecular dynamics simulation

A procedure previously described by us is used for the theoretical study of chemical reactions in solution by means of molecular dynamics simulation, with solute–solvent interaction potentials LJ (12-6-1) derived from ab initio quantum calculations. We apply the procedure to the case of the neutral hydrolysis of methyl formate, HCOOCH₃ + 3H₂O → HCOOH + CH₃OH + 2H₂O in aqueous solution, via concerted and stepwise water-assisted mechanisms. We use the solvent as reaction coordinate, and the free-energy curves for the calculation of the activation energies. The theoretical calculation for the thermodynamics of this hydrolysis reaction in aqueous solution, assisted by three water molecules, is in agreement with the available experimental information. In particular our study gives values of ΔG ^≠ = 28.88 and 28.17 kcal/mol for the concerted and stepwise mechanisms, close to the experimental activation barrier of 28.8 kcal/mol, and a significant improvement over the values of 48.05 and 45.66 kcal/mol found in another similar study using the PCM model.     

Reaction mechanism and tautomeric equilibrium of 2-mercaptopyrimidine in the gas phase and in aqueous solution: a combined Monte Carlo and quantum mechanics study

A combined Monte Carlo and quantum mechanical study was carried out to analyze the tautomeric equilibrium of 2-mercaptopyrimidine in the gas phase and in aqueous solution. Second- and fourth-order M?ller-Plesset perturbation theory calculations indicate that in the gas phase thiol (Pym-SH) is more stable than the thione (Pym-NH) by ca. 8 kcal/mol. In aqueous solution, thermodynamic perturbation theory implemented on a Monte Carlo NpT simulation indicates that both the differential enthalpy and Gibbs free energy favor the thione form. The calculated differential enthalpy is DeltaH(SH)(-->)(NH)(solv) = -1.7 kcal/mol and the differential Gibbs free energy is DeltaG(SH)(-->)(NH)(solv) = -1.9 kcal/mol. Analysis is made of the contribution of the solute-solvent hydrogen bonds and it is noted that the SH group in the thiol and NH group in the thione tautomers act exclusively as a hydrogen bond donor in aqueous solution. The proton transfer reaction between the tautomeric forms was also investigated in the gas phase and in aqueous solution. Two distinct mechanisms were considered: a direct intramolecular transfer and a water-assisted mechanism. In the gas phase, the intramolecular transfer leads to a large energy barrier of 34.4 kcal/mol, passing through a three-center transition state. The proton transfer with the assistance of one water molecule decreases the energy barrier to 17.2 kcal/mol. In solution, these calculated activation barriers are, respectively, 32.0 and 14.8 kcal/mol. The solvent effect is found to be sizable but it is considerably more important as a participant in the water-assisted mechanism than the solvent field of the solute-solvent interaction. Finally, the calculated total Gibbs free energy is used to estimate the equilibrium constant.     

The role of correlation in calculations on 1,2-difluoroethylenes. The cis-trans energy difference

Correlation-corrected ab initio calculations predict cis-1,2-difluoroethylene to be more stable than trans. With second-order Rayleigh-Schrödinger Møller-Plesset theory, the cis form is lower by 0.9-1.3 kcal/mol, depending on the basis set, in agreement with the experimental energy difference ΔE = 1.1 kcal/mol. The positive ΔE is primarily due to greater intrapair correlation energy in the cis form.     

Thermochemistry of aqueous hydroxyl radical from advances in photoacoustic calorimetry and ab initio continuum solvation theory

Photoacoustic signals from dilute ( approximately 30 mM) solutions of H2O2 were measured over the temperature range from 10 to 45 degrees C to obtain the reaction enthalpy and volume change for H2O2(aq) --> 2 OH(aq) from which we ultimately determined DeltafG degrees , DeltafH degrees and partial molal volume, v degrees , of OH (aq). We find DeltarH = 46.8 +/- 1.4 kcal/mol, which is 4 kcal/mol smaller than the gas-phase bond energy, and DeltaVr = 6.5 +/- 0.4 mL/mol. The v degrees for OH in water is 14.4 +/- 0.4 mL/ml: smaller than the v degrees of water. Using ab intio continuum theory, the hydration free energy is calculated to be -3.9 +/- 0.3 kcal/mol (for standard states in number density concentration units) by a novel approach devised to capture in the definition of the solute cavity the strength and specific interactions of the solute with a water solvent molecule. The shape of the cavity is defined by "rolling" a three-dimensional electron density isocontour of water on the ab initio water-OH minimum interaction surface. The value of the contour is selected to reproduce the volume of OH in water. We obtain for OH(aq): DeltafH degrees = -0.2 +/- 1.4 and DeltafG degrees = 5.8 +/- 0.4 kcal/mol that are in agreement with literature values. The results provide confidence in the pulsed PAC technique for measuring aqueous thermochemistry of radicals and open the way to obtaining thermochemistry for most radicals that can be formed by reaction of OH with aqueous substrates while advancing the field of continuum solvation theory toward ab initio-defined solute cavities.     

Ab initio molecular dynamics study of carbon dioxide and bicarbonate hydration and the nucleophilic attack of hydroxide on CO2

We apply ab initio molecular dynamics (AIMD) to study the hydration structures of the carbon dioxide molecule and the bicarbonate and carbonate anions in liquid water. We also compute the free energy change associated with the nucleophilic attack of the hydroxide ion on carbon dioxide. CO2 behaves like a hydrophobic species and exhibits weak interactions with water molecules. The bicarbonate and carbonate ions are strongly hydrated and coordinate to an average of 6.9 and 8.7 water molecules, respectively. The energetics for the reaction in the gas phase are investigated using density functional theory and second-order M?ller-Plesset perturbation theory (MP2) in conjunction with high-quality basis sets. Using umbrella sampling techniques, we compute the standard state, aqueous phase free energy difference associated with the reaction CO2+OH--->HCO3- after correcting AIMD energies with MP2 results. Our predictions are in good agreement with experiments. The hydration structures along the reaction coordinate, which give rise to a predicted 9.7 kcal/mol standard state free energy barrier, are further analyzed.     

Kinetics of the Zn(II)/Zn(Hg) electrode reactions in DMSO solutions of halide ions

The electrode reaction Zn(II)/Zn(Hg) in complex chloride, bromide, and iodide solutions with DMSO as solvent and ammonium perchlorate as supporting electrolyte has been studied at the equilibrium potential by the faradaic impedance method and a square-wave method. Furthermore, double-layer data have been determined by electrocapillary measurements. The results indicate that the zinc chloride and bromide complexes do not contribute noticeably to the exchange current density, while in the iodide system both the solvated zinc ion and the first complex take part in the charge transfer. From the dissimilar results valid for water and DMSO solutions the conclusion is made that probably ligand bridging at the amalgam by the halide ions is operative in water solutions, whereas in DMSO the larger solvent molecules adsorbed can form a steric hindrance to ligand bridging by chloride or bromide ions.     

A new approach to theab initio energy of the homodesmic reaction for the resonance energy of benzene

Summary A scheme of the basis set superposition error (BSSE) correction is first proposed and introduced to determine theab initio energy of the homodesmic reaction for the resonance energy of benzene. Calculations with 6-31G*(5D) and 6-31G*(6D) basis sets at the complete fourth-order Møller-Plesset perturbation level furnish the energy value of 21.35 kcal/mol after the correction, which is in complete agreement with the experimental value of 21.3±0.2 kcal/mol. The energy values at the lower theoretical levels are generally underestimated but they are superior to the uncorrected values. The inclusion of triple excitations displays the dominant contribution of the correlation energy. Detailed analysis of the results reveals some of the similarities between the homodesmic reaction of benzene and the interaction of van der Waals molecule, which provides further justification of the BSSE correction scheme presented in this study.     

An ab initio correlated study of the potential energy surface for the HOBr.H2O complex

The potential energy surface (PES) for the HOBr.H(2)O complex has been investigated using second- and fourth-order M?ller-Plesset perturbation theory (MP2, MP4) and coupled cluster theory with single and doubles excitations (CCSD), and a perturbative approximation of triple excitations (CCSD-T), correlated ab initio levels of theory employing basis sets of triple zeta quality with polarization and diffuse functions up to the 6-311++G(3dp,3df ) standard Pople's basis set. Six stationary points being three minima, two first-order transition state (TS) structures and one second-order TS were located on the PES. The global minimum syn and the anti equilibrium structure are virtually degenerated [DeltaE(ele-nuc) approximately 0.3 kcal mol(-1), CCSD-T/6-311++G(3df,3pd) value], with the third minima being approximately 4 kcal mol(-1) away. IRC analysis was performed to confirm the correct connectivity of the two first-order TS structures. The CCSD-T/6-311++G(3df,3pd)//MP2/6-311G(d,p) barrier for the syn<-->anti interconversion is 0.3 kcal mol(-1), indicating that a mixture of the syn and anti forms of the HOBr.H(2)O complex is likely to exist.     

Ab initio aqueous thermochemistry: application to the oxidation of hydroxylamine in nitric acid solution

Ab initio molecular orbital calculations were performed and thermochemical parameters estimated for 46 species involved in the oxidation of hydroxylamine in aqueous nitric acid solution. Solution-phase properties were estimated using the several levels of theory in Gaussian03 and using COSMOtherm. The use of computational chemistry calculations for the estimation of physical properties and constants in solution is addressed. The connection between the pseudochemical potential of Ben-Naim and the traditional standard state-based thermochemistry is shown, and the connection of these ideas to computational chemistry results is established. This theoretical framework provides a basis for the practical use of the solution-phase computational chemistry estimates for real systems, without the implicit assumptions that often hide the nuances of solution-phase thermochemistry. The effect of nonidealities and a method to account for them is also discussed. A method is presented for estimating the solvation enthalpy and entropy for dilute aqueous solutions based on the solvation free energy from the ab initio calculations. The accuracy of the estimated thermochemical parameters was determined through comparison with (i) enthalpies of formation in the gas phase and in solution, (ii) Henry's law data for aqueous solutions, and (iii) various reaction equilibria in aqueous solution. Typical mean absolute deviations (MAD) for the solvation free energy in room-temperature water appear to be ~1.5 kcal/mol for most methods investigated. The MAD for computed enthalpies of formation in solution was 1.5-3 kcal/mol, depending on the methodology employed and the type of species (ion, radical, closed-shell) being computed. This work provides a relatively simple and unambiguous approach that can be used to estimate the thermochemical parameters needed to build detailed ab initio kinetic models of systems in aqueous solution. Technical challenges that limit the accuracy of the estimates are highlighted.     

Ab initio study of the S(N)2 and E2 mechanisms in the reaction between the cyanide ion and ethyl chloride in dimethyl sulfoxide solution

[reaction: see text] Reliable theoretical calculations predict a free energy barrier for nitrile formation from the reaction between the cyanide ion and ethyl chloride in DMSO solvent of 24.1 kcal/mol, close to the experimental value of 22.6 kcal/mol. We have also predicted that the isonitrile formation is less favorable by 4.7 kcal/mol, while the elimination mechanism is less favorable by more than 10 kcal/mol. These results indicate that isonitrile formation and bimolecular elimination are not significant side reactions for primary alkyl chloride reactions.     

Polarization Effects for Hydrogen-Bonded Complexes of Substituted Phenols with Water and Chloride Ion

Variations in hydrogen-bond strengths are investigated for complexes of nine para-substituted phenols (XPhOH) with a water molecule and chloride ion. Results from ab initio HF/6-311+G(d, p) and MP2/6-311+G(d, p)//HF/6-311+G(d, p) calculations are compared with those from the OPLS-AA and OPLS/CM1A force fields. In the OPLS-AA model, the partial charges on the hydroxyl group of phenol are not affected by the choice of para substituent, while the use of CM1A charges in the OPLS/CM1A approach does provide charge redistribution. The ab initio calculations reveal a 2.0-kcal/mol range in hydrogen-bond strengths for the XPhOH?OH(2) complexes in the order X = NO(2) > CN > CF(3) > Cl > F > H >OH >CH(3) > NH(2). The pattern is not well-reproduced with OPLS-AA, which also compresses the variation to 0.7 kcal/mol. However, the OPLS/CM1A results are in good accord with the ab initio findings for both the ordering and range, 2.3 kcal/mol. The hydrogen bonding is, of course, weaker with XPhOH as acceptor, the order for X is largely inverted, and the range is reduced to ca. 1.0 kcal/mol. The substituent effects are found to be much greater for the chloride ion complexes with a range of 11 kcal/mol. For quantitative treatment of such strong ion-molecule interactions the need for fully polarizable force fields is demonstrated.     

Protonated carbonyl sulfide: prospects for the spectroscopic observation of the elusive HSCO+ isomer

Spurred by the apparent conflict between ab initio predictions and infrared spectroscopic evidence regarding the relative stability of isomers of protonated carbonyl sulfide, key stationary points on the isomerization surface of HOCS(+) have been examined via systematic extrapolations of ab initio energies. Electron correlation has been accounted for using second-order M?ller-Plesset perturbation theory and coupled cluster theory through triple excitations [CCSD, CCSD(T), and CCSDT] in conjunction with the correlation consistent hierarchy of basis sets, cc-pVXZ (X=D,T,Q,5,6). HSCO(+) is predicted to lie lower in energy than HOCS(+) by 4.86 kcal mol(-1), computed using the focal point extrapolation scheme of Allen and co-workers [J. Chem. Phys. 99, 4638 (1993)] with corrections for anharmonic zero-point vibrational energy, core correlation, non-Born-Oppenheimer, and scalar relativistic effects. A transition state has been located, constituting the barrier to isomerization of HSCO(+) to HOCS(+), lying 68.9 kcal mol(-1) higher in energy than HSCO(+). This is well above predicted exothermicity [DeltaH(r) (o)(0 K)=48.1 kcal mol(-1), cc-pVQZ CCSD(T)] for the reaction considered in the experiments (HSCO(+)+H(2)-->OCS+H(3) (+)). Though proton tunneling will lead to a lower effective barrier, this prediction is consistent with the lack of HSCO(+) in electrical discharges in H(2)OCS, since the relative populations of HOCS(+) and HSCO(+) will depend on the experimental details of the protonation route rather than the relative thermodynamic stability of the isomers. Anharmonic vibrational frequencies and vibrationally corrected rotational constants from cc-pVTZ CCSD(T) cubic and quartic force constants are provided, to aid in the spectroscopic observation of the energetically favorable but apparently elusive HSCO(+) isomer.     

New insight on the origin of the unusual acidity of Meldrum's acid from ab initio and combined QM/MM simulation study.

Ab initio molecular orbital and combined QM/MM Monte Carlo simulations have been carried out to investigate the origin of the unusually high acidity of Meldrum's acid. Traditionally, the high acidity of Meldrum's acid relative to that of methyl malonate has been attributed to an additive effect due to the presence of two E esters in the dilactone system. However, the present study reveals that there is significant nonadditive effect that also makes major contributions. This results from preferential stabilization of the enolate anion over that of Meldrum's acid due to anomeric stereoelectronic interactions. To investigate solvent effects on the acidity in aqueous solution, the relative acidities of Z and E conformers of methyl acetate have been determined in combined ab initio QM/MM simulations. There is significant solvent effect on the conformational equilibria for both the neutral ester and its enolate anion in water, leading to stabilization of the E stereoisomer. However, the computed solvent effect of 4.4 kcal/mol in favor of the E isomer of methyl acetate is largely offset by the favorable solvation of 3.4 kcal/mol for the E conformer of the enolate anion. This leads to an enhanced acidity of 3.4 kcal/mol for the (E)-methyl acetate in water over the Z conformer. In Meldrum's acid, it is the preferential stabilization of the enolate anion due to anomeric effects coupled with the intrinsically higher acidity of the E conformation of ester that is responsible for its high acidity.     

The microwave and infrared spectroscopy of benzaldehyde: conflict between theory and experimental deductions

Recently, it has been proposed that ab initio calculations cannot accurately treat molecules comprised of a benzene ring with a pi-conjugated substituent, for example, benzaldehyde. Theoretical predictions of the benzaldehyde barrier to internal rotation are typically a factor of 2 too high in comparison to the experimental values of 4.67 (infared) and 4.90 (microwave) kcal mol(-1). However, both experiments use Pitzer's 1946 model to compute the reduced moment of inertia and employ the experimentally observed torsional frequency to deduce benzaldehyde's rotational barrier. When Pitzer's model is applied to a system with a nonconjugated functional group, such as phenol, the model and theoretical values are in close agreement. Therefore, we conclude the model may not account for conjugation between the substituent and the pi-system of benzene. The experimental values of the benzaldehyde rotational barrier are therefore misleading. The true rotational barrier lies closer to the theoretically extrapolated limit of 7.7 kcal mol(-1), based on coupled cluster theory.