A Theoretical Study of the Reaction of ClONO₂ with HCl on Ice

The direct ClONO2 + HCl → Cl2 + HNO3 reaction on ice, implicated in polar stratospheric ozone depletion, is studied quantum chemically on a model ice lattice comprising nine water molecules. The reaction path is calculated at the HF/(HW*,3-21G) level, using the Hay−Wadt effective core potential for Cl. At these geometries, energies are recalculated at the MP2/(SBK+*,6-31+G*) level, with the Stevens−Bash−Krauss effective core potential for Cl. HCl is found to be ionized in the reactant complex. The calculated reaction internal energy barrier, including zero-point energy correction, is 6.4 kcal/mol. The reaction mechanism involves proton transfer in the ice lattice, accompanied by nucleophilic attack of Cl- on the Clδ+ in ClONO2; the lattice is an active participant in the reaction. Implications for heterogeneous atmospheric chemistry are discussed

Temperature Dependence of Cluster Ion Decomposition in a Quadrupole Ion Trap^†

Rate coefficients for the thermal decomposition of H+(H2O)3,4, H+(CH3OH)3, H+(C2H5OH)2, H+(CH3CN)2, H+((CH3)2CO)2, NO3-(HNO3)1,2, and Cl-H2O are measured as a function of temperature in a quadrupole ion trap over the pressure range 0.2−2 mTorr of He. The kinetics are in the low pressure limit and the decomposition activation energies are significantly less than the bond energies. The difference between the bond energy and the activation energy is reproduced by theory. The vibrational frequencies of the cluster ions necessary for the theoretical treatment of the dissociation rate constants are calculated ab initio at the HF/6-31G* level. This work demonstrates that cluster ion bond energies may be determined accurately from activation energies for dissociation at the low-pressure limit. The measurements also yield fundamental information about the intermolecular energy transfer between the helium buffer gas and the cluster ions

A Theoretical Study of ClONO₂ + Cl^- → Cl₂ + NO₃^- on Ice

The title reaction, of interest in connection with the central heterogeneous reaction HCl + ClONO2 → Cl2 + HNO3 for polar stratospheric ozone depletion, is studied quantum chemically at the Hartree−Fock level via a Cl-·ClONO2 complex anion embedded in a model ice lattice Cl-·ClONO2·(H2O)8·W29 comprising both quantum chemical and classical polarizable water molecules (W). The calculated reaction mechanism involves the nucleophilic attack of Cl- on the electrophilic Clδ+ in ClONO2, assisted by changes in hydrogen bonding involving both the desolvation of Cl- and the increased solvation of the nitrate group. The calculated reaction barrier at 0 K, including the zero-point energy correction, is 5.7 kcal/mol. This result is compared with previous results for the HCl + ClONO2 → Cl2 + HNO3 reaction (Bianco, R.; Hynes, J. T. J. Phys. Chem. A 1999, 103, 3797), and implications for stratospheric heterogeneous chemistry are discussed

Infrared Signatures of HNO₃ and NO₃⁻ at a Model Aqueous Surface. A Theoretical Study

The infrared signatures of nitric acid HNO3 and its conjugate anion NO3− at the surface of an aqueous layer are derived from electronic structure calculations at the HF/SBK+∗ level of theory on the HNO3·(H2O)3 → NO3−·H3O+·(H2O)2 model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters, mimicking various degrees of solvation [Bianco, R.; Wang, S.; Hynes, J. T. J. Phys. Chem. A 2007, 111, 11033]. The molecular level character of the various bands is discussed, and the solvation patterns are described in terms of hydrogen bonding and resulting polarization of the species’ intramolecular bonds. Connection is made with assorted experimental results, including surface-sensitive Sum Frequency Generation spectroscopy of aqueous nitric acid solutions, infrared spectroscopy of amorphous thin films of nitric acid monohydrate (NAM) and dihydrate (NAD), and infrared and Raman spectroscopic results for bulk aqueous solutions of nitric acid and nitrate salts

Theoretical Study of the First Acid Dissociation of H₂SO₄ at a Model Aqueous Surface^†

Electronic structure calculations on the H2SO4·(H2O)4,6 model system embedded at the surface of an aqueous layer have been performed to examine the feasibility of the first acid dissociation of H2SO4 to an ·H3O+ contact ion pair over a wide temperature range, with a special focus on the 190−250 K range relevant for atmospheric sulfate aerosols. The results indicate that the acid dissociation can be either thermodynamically favored or disfavored depending on the degree of solvation of the acid and the produced ions, as well as on the temperature

Theoretical Study of the Dissociation of Nitric Acid at a Model Aqueous Surface

The issue of acid dissociation of nitric acid at an aqueous surface is relevant in various portions of the atmosphere in connection with ozone depletion. This proton-transfer reaction is studied here via electronic structure calculations at the HF/SBK+(d) level of theory on the HNO3·(H2O)3 model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters with an increasing degree of solvation of the nitrate group. Free energy estimates for all the cases examined favor undissociated, molecular nitric acid over the 0−300 K temperature range, including that relevant for the upper troposphere, where it is connected to the issue of the mechanism of nitric acid uptake by water ice aerosols. The presence of molecular HNO3 at 300 K at the surface is further supported by vibrational band assignments in good agreement with a very recent surface-sensitive vibrational spectroscopy study of diluted HNO3/H2O solutions

Theoretical Study of O–O Single Bond Formation in the Oxidation of Water by the Ruthenium Blue Dimer

The first key step in the oxidation of water to O2 by the oxidized species [(bpy)2(O)RuVORuV(O)(bpy)2]4+ of the Ru blue dimer is studied using density functional theory (DFT) and an explicit solvent treatment. In the model reaction system [L2(O)RuVORuV(O)L2]4+·(H2O)4·W76, the surrounding water solvent molecules W are described classically while the inner core reaction system is described quantum mechanically using smaller model ligands (L). The reaction path found for the O–O single bond formation involves a proton relay chain: direct participation of two water molecules in two proton transfers to yield the product [L2(HOO)RuIVORuIV(OH)L2]4+·(H2O)3·W76. The calculated ∼3 kcal/mol reaction free energy and ∼15 kcal/mol activation free energy barrier at 298 K are consistent with experiment. Structural changes and charge flow along the intrinsic reaction coordinate, the solvent’s role in the reaction barrier, and their significance for water oxidation catalysis are examined in detail

Infrared Signatures of HNO₃ and NO₃⁻ at a Model Aqueous Surface. A Theoretical Study

The infrared signatures of nitric acid HNO3 and its conjugate anion NO3− at the surface of an aqueous layer are derived from electronic structure calculations at the HF/SBK+∗ level of theory on the HNO3·(H2O)3 → NO3−·H3O+·(H2O)2 model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters, mimicking various degrees of solvation [Bianco, R.; Wang, S.; Hynes, J. T. J. Phys. Chem. A 2007, 111, 11033]. The molecular level character of the various bands is discussed, and the solvation patterns are described in terms of hydrogen bonding and resulting polarization of the species’ intramolecular bonds. Connection is made with assorted experimental results, including surface-sensitive Sum Frequency Generation spectroscopy of aqueous nitric acid solutions, infrared spectroscopy of amorphous thin films of nitric acid monohydrate (NAM) and dihydrate (NAD), and infrared and Raman spectroscopic results for bulk aqueous solutions of nitric acid and nitrate salts

Infrared Signatures of HNO₃ and NO₃⁻ at a Model Aqueous Surface. A Theoretical Study

The infrared signatures of nitric acid HNO3 and its conjugate anion NO3− at the surface of an aqueous layer are derived from electronic structure calculations at the HF/SBK+∗ level of theory on the HNO3·(H2O)3 → NO3−·H3O+·(H2O)2 model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters, mimicking various degrees of solvation [Bianco, R.; Wang, S.; Hynes, J. T. J. Phys. Chem. A 2007, 111, 11033]. The molecular level character of the various bands is discussed, and the solvation patterns are described in terms of hydrogen bonding and resulting polarization of the species’ intramolecular bonds. Connection is made with assorted experimental results, including surface-sensitive Sum Frequency Generation spectroscopy of aqueous nitric acid solutions, infrared spectroscopy of amorphous thin films of nitric acid monohydrate (NAM) and dihydrate (NAD), and infrared and Raman spectroscopic results for bulk aqueous solutions of nitric acid and nitrate salts

Infrared Signatures of HNO₃ and NO₃⁻ at a Model Aqueous Surface. A Theoretical Study

The infrared signatures of nitric acid HNO3 and its conjugate anion NO3− at the surface of an aqueous layer are derived from electronic structure calculations at the HF/SBK+∗ level of theory on the HNO3·(H2O)3 → NO3−·H3O+·(H2O)2 model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters, mimicking various degrees of solvation [Bianco, R.; Wang, S.; Hynes, J. T. J. Phys. Chem. A 2007, 111, 11033]. The molecular level character of the various bands is discussed, and the solvation patterns are described in terms of hydrogen bonding and resulting polarization of the species’ intramolecular bonds. Connection is made with assorted experimental results, including surface-sensitive Sum Frequency Generation spectroscopy of aqueous nitric acid solutions, infrared spectroscopy of amorphous thin films of nitric acid monohydrate (NAM) and dihydrate (NAD), and infrared and Raman spectroscopic results for bulk aqueous solutions of nitric acid and nitrate salts