Ola Engkvist

Structure and vibrational dynamics of the benzene dimer

Point-wise evaluated coupled-cluster single double triple [CCSD(T)] stabilization energies are used to parameterize the nonempirical model (NEMO) empirical intermolecular potential of the benzene dimer in the ground electronic state. The potential is used for theoretical interpretation of the dimer structure and the dynamics of its intermolecular motions. Only one energy minimum, corresponding to the T-shaped structure, is found. A parallel displaced structure is the first-order transition structure separating the molecular symmetrically equivalent T-shaped structures. Due to a relatively high transition barrier (∼170 cm−1), the interconversion tunneling is unimportant in the energy region spanned by the available rotational spectra and is thus neglected (accordingly, the molecular symmetry group which is used for interpretation of the available experimental spectra is related to the T-shaped structure with two feasible internal rotations and nonequivalent monomers). The dimer undergoes a nearly free inte...

Benzene trimer and benzene tetramer: Structures and properties determined by the nonempirical model (NEMO) potential calibrated from the CCSD(T) benzene dimer energies

The energetics and structure of the benzene trimer and tetramer are investigated with the nonempirical model (NEMO) potential calibrated to high precision by comparison with CCSD(T) benzene dimer energies. From the obtained potential energy surface, possible configurations could be determined and the experimental observed structures could be identified. This potential also reproduces the binding energies and allows for the determination of all intermolecular modes. It could be shown that this potential is therefore well suited and important to predict the structure and thermodynamic data for larger clusters, which cannot be accurately computed by ab initio quantum chemical methods.

Adsorption of water on the NaCl(001) surface. III. Monte Carlo simulations at ambient temperatures

Adsorption of water on NaCl(001) is studied at room temperature using recently constructed intermolecular potentials. Coverages of 0.5 and 3.0 water molecules per NaCl were studied in molecular simulations. At low coverage water molecules cluster on the surface to form islands, while at higher coverage a layered structure appears. These results are in agreement with recent Fourier transform infrared spectroscopy measurements.

Adsorption of water on NaCl(001). I. Intermolecular potentials and low temperature structures

Water adsorption on the NaCl(001) surface has been extensively studied both theoretically and experimentally during recent years. Here we investigate it using intermolecular potentials derived from intermolecular perturbation theory (IMPT). The water–water interactions are described by the recently developed ASP-W4 potential. For the water–NaCl surface, repulsion parameters were developed using IMPT, and C6 dispersion coefficients were calculated using coupled Hartree–Fock perturbation theory. The binding energy between a single water molecule and the NaCl surface is found to be 40 kJ mol−1. A stable tetramer can form on the surface, similar to the water tetramer in the gas phase. At a coverage of one water molecule per NaCl unit, there are several different water monolayer structures with approximately the same energy. Some have all the water oxygens located close to Na+ ions, but others have some water molecules located above the Cl− ions. The latter are farther from the surface, and are hydrogen bonded...

Developments in computational studies of crystallization and morphology applied to urea

A new method of probing surface–surface interactions and calculating attachment energies for morphology predictions, based on the interactions between an infinite surface and a thin finite slice (a nano-crystallite), has been implemented in the ORIENT program package. This, together with existing capabilities for studying 2D periodic surface adlayers, or isolated molecular clusters on a surface, enables a wide range of complementary calculations to be performed to study crystallization phenomena of organic molecules with accurate anisotropic atom–atom intermolecular potentials, including distributed-multipole electrostatic models. Properties pertinent to the morphology and agglomeration of urea crystals are reported, including surface relaxation, attachment energies and surface energies, solvent and solute binding energies, and the inter-surface interaction energy. We correctly predict the two major forms {110} and {001} of vapour-grown urea crystals, including an observed aspect ratio. The polar cap facets of the crystals probably arise from the unusually large relaxation of a polar {111} surface which provides a further kinetic barrier to growth. A comparison of the binding energies of water and urea molecules to the different surfaces shows that the growth of the {110} surfaces will be particularly impeded by the presence of water. This rationalizes the increased morphological dominance of this face in crystals grown from solution. The interfacial energy between the dominant (110) and (001) crystal faces has also been calculated, and was found to be only about 20% smaller than the interaction between (110) surfaces.

Methylated uracil dimers: potential energy and free energy surfaces

Theoretical analysis of the formation of 1-methyluracil, 3-methyluracil and 1,3-dimethyluracil dimers was performed. Stabilization energies of these dimers were evaluated with the Cornell et al. force field (J. Am. Chem. Soc., 1995, 117, 5179). In total 16, 13 and 15 energy minima were studied for the three dimers. Thermodynamic data were obtained with the rigid rotor–harmonic oscillator–ideal gas approximation. Furthermore, populations of various structures were determined by molecular dynamic simulations in the NVE microcanonical ensemble and numerical evaluation of the configuration integrals in the NVT canonical ensemble. The potential energy surfaces (PESs) and the free energy surfaces (FESs) of these dimers differ. The largest difference was found for the 1-methyluracil dimer where the global and first local minima on the PES and FES do not coincide.

A MONTE CARLO SIMULATION STUDY OF THE TEMPERATURE DEPENDENCE FOR THE CONFORMATION DISTRIBUTION OF 1,2-DIMETHOXYETHANE IN WATER

Monte Carlo simulations of 1,2-dimethoxyethane in water were performed at two temperatures; 298 and 398 K. Analysis of the simulations shows that the anti–anti–anti conformer of 1,2-dimethoxyethane is the only of the most populated conformers that increases its probability with increasing temperature. This behavior suggests that clouding of polyethyleneoxide-water solutions is induced by conformational changes. In both simulations the amount of the anti–anti-gauche conformer is surprisingly large, even though convergence problems of the simulation at 298 K occurred. A high amount of the anti–anti-gauche conformer of 1,2-dimethoxyethane in water solution have not been reported before.