S. J. Knak Jensen

Femtosecond photolysis of aqueous HOCl

This paper reports an experimental study of the photolysis of aqueous HOCl using femtosecond pulses at 266 nm. The formation of photoproducts is monitored by transient absorption spectroscopy from 230 to 400 nm. The HOCl molecules dissociate with unity quantum yield to form OH+Cl faster than 1 ps, and as a result of the potential along the HO–Cl reaction coordinate, all excess energy is given to the fragments as translational energy. After dissociation, and solvent cage escape, the majority of the Cl and OH fragments recombine after diffusion on a time scale of 50 ps. The diffusion dynamics is studied using a simple model for diffusive recombination and a more extensive molecular dynamics simulation. A minor fraction of the Cl atoms (∼10%) reacts with HOCl in a diffusion limited reaction to form Cl2+OH.

STRUCTURAL CHANGES OF HYDROGEN BONDED HEXAGONAL TRIMERS UPON IONIZATION

Abstract Hexagonal trimers, (QH)3, formed from the monomers QH=(NH3, H2O, and HF) are investigated by ab initio quantum chemical geometry optimizations. All trimers have three hydrogen bonds, which decrease in length as the electronegativity of Q increases. The structures of the cations, ( QH ) 3 + , are also determined. The hexagonal structure of the trimers is absent in the cations, which have two hydrogen bonds of shorter length than found in the trimer. The length of the hydrogen bonds in the cations decrease with increasing electronegativity of Q. The structures of the cations may be described as open chain complexes of the form Q–[HQH]+–QH.

Is there an O–H⋯C hydrogen bond in the cation of cis o-cresol?

Ab initio and DFT geometry optimization, followed by the application of Bader’s theory atoms in molecules (AIM) have been applied to probe the proposed O‐H· · ·C3‐ hydrogen bond in cis o-cresol. No evidence was found to support the presence of such novel type hydrogen bond. It is concluded that the observed ir frequency shift (25 cm 21 ) should be explained by some

Structural changes upon ionization of simple hydrogen bonded hexagonal dimers

Abstract The geometry of hexagonal dimers, (QH)2, derived from small molecules, containing two electronegative atoms, QH=H3COH, H3CF, H2NOH, N2H4, NH2F, H2O2 and HOF are investigated by ab initio quantum chemical geometry optimizations. All the considered (neutral) dimers, have some symmetry and two hydrogen bonds of equal length. The geometry of the corresponding cations, (QH)2+, are determined in order to investigate the structural changes upon ionizatioin. The structural changes can be divided into two groups. One, where the structural changes include migration of H/H+ and one, without such migration. In the former group the cations can be considered as adducts of QH2+ and Q. In the latter group the cations can be described either as adducts of QH+ and QH or as [QH, QH]+ cations. In each group there may be ring openings or ring reductions. Generally, the cations have fewer and shorter hydrogen bonds than the corresponding neutral dimers, but there are exceptions. All the hydrogen bonds investigated here are found to be unsymmetrical, in the sense that the H atom in the hydrogen bond have different distances to the neighbouring electronegative atoms.

On the hydrogen atom transfer in the reaction of O−, with H20. Ab initio calculations of the potential energy along two reaction paths

Abstract The geometry corresponding to the minimum potential energy of an O − , H2O cluster is calculated at the second order Moller-Plesset approximation level as a function of the O-O − distance (path 1) and H-O − distance (path 2). Along path 1 the oxygen atoms are inequivalent for O-O − distances larger than 2 A and equivalent for smaller distances. The finding of clusters with equivalent oxygen atoms is in accordance with the observation of equal probability of proton and hydrogen atom transfer in collisions in the gas phase between O − , and H2O at thermal energies. Along path 2 the oxygen atoms of the cluster remain inequivalent for all H-O − distances. The comparison of energies of selected configurations along the two paths offers a qualitative explanation for the findings that the reaction O − + H2O → OH → OH- in aqueous solution proceeds predominantly by H+ ion transfer and that the contribution from H atom transfer increases with increasing temperature.

Strong and symmetric hydrogen bonding in the hydrogen di-superoxide anion

Abstract The structure of the triplet state of the hydrogen di-superoxide anion, [O 2 –H–O 2 ] − , has been investigated by ab initio quantum chemical geometry optimizations. The anion has two short hydrogen bonds that are symmetry connected. At the highest level of theory considered (B3LYP/AUC-cc-pVTZ) the O–O bond length is 1.332 A, the hydrogen bond length is 1.220 A, the strength of the hydrogen bond is 155 kJ/mol and the standard enthalpy of formation of the anion is estimated to be −199±4 kJ/mol at 25°C.

Structural changes of triplet states of hydrogen bonded hexagonal dimers upon ionization and electron capture

Abstract The dimers of the S =1/2 radicals, HO 2 and N 2 H 3 , are investigated by ab initio quantum chemical geometry optimizations. Several isomers of the dimers are found. The main emphasis in this investigation is on dimers consisting of two monomers, hydrogen bonded head to tail in a hexagonal ring with two hydrogen bonds of equal length. The electronic wave function for these dimers is a spin triplet. The geometries of the cations and anions pertaining to the hexagonal dimers are likewise determined by the geometry optimizations in order to investigate the structural changes upon ionization and electron capture. The main structural change can be described as a migration of H/H + from one monomer to the other while the overall hexagonal form is conserved. In order to have a standard of comparison a similar investigation is performed for the spin triplet state of the hexagonal dimer of C 2 H 5 . The findings in this case are consistent with the expected low capacity for hydrogen bonding for the C 2 H 5 radical.

Molecular dynamics simulation of an overfilled Kr monolayer on graphite

A microscopic model for Kr physisorbed on graphite is studied by the molecular dynamics technique. The model contains a Lennard-Jones potential describing the interaction among the Kr atoms and an external potential with sixfold symmetry describing the interaction between the Kr atoms and the graphite surface. The number of Kr atoms and the area of the surface are chosen to correspond to a five per cent overfilling of the perfect Kr monolayer. The Kr atoms are confined to move in a plane implying that second-layer promotion is inhibited in this study. Most simulations are performed with rectangular lattices of dimensions up to 170.4 AA*147.6 AA with periodic boundary conditions. A few simulations have also been performed with a circular lattice with reflecting walls in order to study the effects of the boundary conditions on the domain structures. The structure factors indicate two phases for the model: a low-temperature incommensurate phase and a high-temperature fluid phase. The transition between the two phases proceeds via a first-order transition with coexistence of the two phases in a temperature interval. No commensurate phase is seen in the simulation, in contrast to the experiments.

Theoretical study of hydrates of the atomic oxygen radical anion

Abstract Clusters consisting of the atomic oxygen radical anion, O − , and n H 2 O molecules are investigated by quantum mechanical ab initio geometry optimizations. Stable structures are determined at the second-order Moller–Plesset level for n n =5 and 6. For each value of n >1 many different structures (isomers) are found. The energy separation between the isomers is a few kJ/mol only. Generally, the structures of the isomers fall into two categories. In one category, O − is attached to a number of hydrogens all belonging to different H 2 O molecules. These structures are all rather flat. The number of nearest neighbor hydrogens attached to O − does not exceed four. The other category of structures may be described as hydrated OH, OH − complexes. In most cases ( n >2), the local geometry around the OH − part of the complex is pyramidal like. The oxygen atom in OH − is attached to not more than four hydrogens—from different H 2 O molecules or from the OH part of the complex. The closeness in free energy of the two categories of structures offers an explanation for the observed reversible interconversion of OH and O − at low temperatures in radiolyzed ice.

Flip-flops in fluorinated o-cresol

Abstract Ab initio geometry optimizations are used to investigate the side chain fluorinated cis o -cresols, HOC 6 H 4 CH 3− n F n ( n =1, 2, and 3). The three ground states are found to contain hydrogen bonded hexagon, which may flip flop between two equivalent positions relative to the phenyl ring. In the cases n =2 and n =3 the activation energy for the flip-flop is about the same and less than 1 kJ/mol, whereas it is an order of magnitude higher for n =1. The difference in activation energies is associated with the dynamics of the flip-flop and the stoichiometry of the cresol.