Theoretical Investigation of the Photoexcited NO2+H2O reaction at the Air–Water Interface and Its Atmospheric Implications

The atmospheric role of photochemical processes involving NO₂ beyond its dissociation limit (398 nm) is controversial. Recent experiments have confirmed that excited NO₂^* beyond 420 nm reacts with water according to NO₂^*+H₂O→HONO+OH. However, the estimated kinetic constant for this process in the gas phase is quite small (k≈10⁻¹⁵–3.4×10⁻¹⁴ cm³ molecule⁻¹ s⁻¹) suggesting minor atmospheric implications of the formed radicals. In this work, ab initio molecular dynamics simulations of NO₂ adsorbed at the air–water interface reveal that the OH production rate increases by about 2 orders of magnitude with respect to gas phase, attaining ozone reference values for NO₂ concentrations corresponding to slightly polluted rural areas. This finding substantiates the argument that chemistry on clouds can be an additional source of OH radicals in the troposphere and suggests directions for future laboratory experimental studies.     

Simulation of amino acid diffusion across water/hydrophobic interfaces

The mechanism of amino acid transfer across water/hydrophobic interfaces has important biological relevance but is poorly understood. Combined QM/MM simulations show that zwitterionic Gly enters the hydrophobic phase till ca. 1 nm before neutralisation occurs. Interfacial effects significantly affect the relative tautomers stability.     

A Local Model for the Energy Transfer in a Saturated Static Mixture

In the present work the transient energy transfer in a nonsaturated porous medium is studied, using a mixture theory viewpoint. The porous matrix is assumed homogeneous, rigid and isotropic, while the fluid is a Newtonian incompressible one and both are assumed static. Since the homogeneous matrix is not saturated, gradients of concentration are present. The porous medium and the fluid (a liquid) will be regarded as continuous constituents of a mixture that will have also a third constituent, an inert gas, assumed with zero mass density and thermal conductivity. The problem is described by a set of two partial differential equations which represent the energy balances for the fluid and the solid constituents. Isovalues for these two constituents are plotted, considering representative time instants and selected values for the energy equations coefficients and for the saturation.     

Isoprene Reactivity on Water Surfaces from ab initio QM/MM Molecular Dynamics Simulations

Isoprene is the most abundant volatile organic compound in the atmosphere after methane. While gas-phase processes have been widely studied, the chemistry of isoprene in aqueous environments is less well known. Nevertheless, some experiments have reported unexpected reactivity at the air-water interface. In this work, we have carried out combined quantum-classical molecular dynamics simulations of isoprene at the air-water interface, as well as ab initio and density functional theory calculations on isoprene-water complexes. We report the first calculation of the thermodynamics of adsorption of isoprene at the water surface, examine how hydration influences its electronic properties and reactivity indices, and estimate the OH-initiated oxidation rate. Our study indicates that isoprene interacts with the water surface mainly through H−π bonding. This primary interaction mode produces strong fluctuations of the π and π^* bond stabilities, and therefore of isoprene's chemical potential, nucleophilicity and ionization potential, anticipating significant dynamical effects on its reactivity at the air-water interface. Using data from the literature and free energies reported in our work, we have estimated the rate of the OH-initiated oxidation process at the air-water interface (5.0×10¹² molecule cm⁻³ s⁻¹) to be about 7 orders of magnitude larger than the corresponding rate in the gas phase (8.2×10⁵ molecule cm⁻³ s⁻¹). Atmospheric implications of this result are discussed.     

On the nature of the unusually long OO bond in HO(3) and HO(4) radicals

The HO(3) and HO(4) polyoxide radicals have attracted some attention due to their potential role in ozone chemistry. Experimentally, the geometrical structure of HO(3) is known whereas that of HO(4) is not. Moreover, the existence of the latter radical has been questioned. Theoretical calculations on the two species have been reported before, showing important structural differences depending on the computational level. Both radicals present an unusually long OO bond (around 1.7-1.8 A) that can be associated with an intricate interaction between HO, or HO(2), with O(2). The nature of such interaction is investigated in detail using large scale ab initio methods (CASSCF, CASPT2, MRCI, QCISD, CCSD(T)) and density functional techniques (B3LYP) in connection with extended basis sets. Stabilization enthalpies at 298 K with respect to HO (or HO(2)) and O(2) have been computed amounting to -3.21 kcal mol(-1) for HO(3) (trans conformation) and 11.33 kcal mol(-1) for HO(4) (cis conformation). The corresponding formation enthalpies are 6.12 and 11.83 kcal mol(-1). The trans conformation of HO(4) is less stable than the cis one by 6.17 kcal mol(-1). Transition states for HO(4) dissociation and for cis/trans conversion are also described.     

Computation of electronic molecular polarizabilities by a variational method at the CISD level

The variational method proposed earlier has been generalized, using a trial function of the form: ψ = (Λ₀ + Σ_sΛ_sm_s)ψ₀ in which m_s = r^2p+1C^m_l, s standing for a triplet (p, l, m) and implemented into the program Hondo-8.4. The second-order density matrices are used to take into account the mono and bi-excited states (DM1 and DM2 matrices, GUGA Algorithm). This allows us to compute the dipole (α), dipole-quadrupole (A) and quadrupole (C) polarizability tensors at the CISD level. The results obtained for a series of test molecules: CO, HF, NH₃, and methane with various gaussian basis sets are compared with experimental results (when available for A and C) and those obtained with other theoretical methods. The correlation is found to lower the values of the dipole polarizability which was generally too high when computed by the variational method at the RHF level and the values obtained here are in good agreement with the experimental ones. © 1996 John Wiley & Sons, Inc.     

Long-range electrostatic interactions in hybrid quantum and molecular mechanical dynamics using a lattice summation approach

A method based on a lattice summation technique for treating long-range electrostatic interactions in hybrid quantum mechanics/molecular mechanics simulations is presented in this article. The quantum subsystem is studied at the semiempirical level, whereas the solvent is described by a two-body potential of molecular mechanics. Molecular dynamics simulations of a (quantum) chloride ion in (classical) water have been performed to test this technique. It is observed that the application of the lattice summations to solvent-solvent interactions as well as on solute-solvent ones has a significant effect on solvation energy and diffusion coefficient. Moreover, two schemes for the computation of the long-range contribution to the electrostatic interaction energy are investigated. The first one replaces the exact charge distribution of the quantum solute by a Mulliken charge distribution. The long-range electrostatic interactions are then calculated for this charge distribution that interacts with the solvent molecule charges. The second one is more accurate and involves a modified Fock operator containing long-range electron-charge interactions. It is shown here that both schemes lead to similar results, the method using Mulliken charges for the evaluation of long-range interactions being, however, much more computationally efficient.