Photoelectron imaging spectroscopy of nitroethane anions

We present low-energy velocity map photoelectron imaging results for bare and Ar solvated nitroethane anions. We report an improved value for the adiabatic electron affinity of nitroethane of (191 ± 6) meV which is used to obtain a C-NO(2) bond dissociation energy of (0.589 ± 0.019) eV in nitroethane anion. We assign a weak feature at (27 ± 5) meV electron binding energy to the dipole-bound anion state of nitroethane. Photoelectron angular distributions exhibit increasing anisotropy with increasing kinetic energies. The main contributions to the photoelectron spectrum of nitroethane anion can be assigned to the vibrational modes of the nitro group. Transitions involving torsional motion around the CN bond axis lead to strong spectral congestion. Interpretation of the photoelectron spectrum is assisted by ab initio calculations and Franck-Condon simulations.     

Evaporation of Lennard-Jones fluids

Evaporation and condensation at a liquid/vapor interface are ubiquitous interphase mass and energy transfer phenomena that are still not well understood. We have carried out large scale molecular dynamics simulations of Lennard-Jones (LJ) fluids composed of monomers, dimers, or trimers to investigate these processes with molecular detail. For LJ monomers in contact with a vacuum, the evaporation rate is found to be very high with significant evaporative cooling and an accompanying density gradient in the liquid domain near the liquid/vapor interface. Increasing the chain length to just dimers significantly reduces the evaporation rate. We confirm that mechanical equilibrium plays a key role in determining the evaporation rate and the density and temperature profiles across the liquid/vapor interface. The velocity distributions of evaporated molecules and the evaporation and condensation coefficients are measured and compared to the predictions of an existing model based on kinetic theory of gases. Our results indicate that for both monatomic and polyatomic molecules, the evaporation and condensation coefficients are equal when systems are not far from equilibrium and smaller than one, and decrease with increasing temperature. For the same reduced temperature T/T(c), where T(c) is the critical temperature, these two coefficients are higher for LJ dimers and trimers than for monomers, in contrast to the traditional viewpoint that they are close to unity for monatomic molecules and decrease for polyatomic molecules. Furthermore, data for the two coefficients collapse onto a master curve when plotted against a translational length ratio between the liquid and vapor phase.     

Synthesis, characterization, and photophysical study of fluorescent N-substituted benzo[ghi]perylene "swallow tail" monoimides

A set of N-substituted benzoperylene monoimide (BPI) fluorophores was synthesized and characterized structurally and photophysically. Condensation of benzo[ghi]perylene-1,2-dicarboxylic anhydride in the presence of "swallow tail" alkyl amines produced fluorophores that are soluble in a range of organic solvents, highly absorbing in the near-UV (ε(334) = 79,000 M(-1) cm(-1)), and fluorescent in the visible range. Photophysical behavior of the compounds was studied with steady-state and time-correlated single photon counting. The synthesized BPIs exhibit positive solvachromatic emission (λ(em) (hexane) = 469 nm; λ(em) (ethanol) = 550 nm) as a function of solvent polarity with little change in their excited-state lifetime (9.6-6.5 ns) and fluorescence quantum yield (0.27-0.44) over the polarity range studied. Solvachromatic shifts were analyzed using the Lippert-Mataga approach. In nonpolar hydrocarbon solvents evidence of dual emission from closely spaced (562 cm(-1)) S(1) and S(2) excited states is observed. Preliminary peak assignments for the anomalous S(2) emission are made.     

Further studies of intramolecular Michael reactions of nitrosoalkenes for construction of functionalized bridged ring systems

A wide variety of stabilized carbanions have been found to participate as nucleophiles in intramolecular Michael-type conjugate additions to in situ generated nitrosoalkenes to form bridged carbocyclic systems. The vinylnitroso platforms for these cyclizations have been prepared via two key steps involving ring-closing metathesis of vinyl chlorides and regioselective conversion of vinyl chlorides to α-chloroketones with sodium hypochlorite in glacial acetic acid/acetone. An alternative approach to preparation of some cyclization substrates has involved use of more reactive enol ethers as precursors to the requisite α-chloroketones. A sulfonamide anion has also been found to be an effective nucleophile in this type of reaction, leading to formation of a 6-azabicyclo[3.2.1]octane.     

Comparative molecular dynamics study of vapor-exposed basal, prismatic, and pyramidal surfaces of ice

We present the results of molecular dynamics simulations in which ice I(h) slabs with free basal, prismatic, 28° pyramidal, and 14° pyramidal facets are exposed to vapor. All simulations were carried out at 250 K using a six-site intermolecular potential. Characteristics common to all facets include spontaneous development of a quasi-liquid layer (QLL) within ～10 ns and QLL stratification into outer (ε(1)) and inner (ε(2)) sublayers having on average two and three hydrogen bonds, respectively. Vapor pressure, based on the rate of escape of molecules from the QLL to the vapor phase, is found to be greatest for the 14° pyramidal and basal facets (～230 Pa), while significantly lower values are obtained for the prismatic and 28° pyramidal facets (～200 Pa). The geometric thickness of the QLL also varies between facets, with the 14° pyramidal having the greatest thickness. The free prismatic and pyramidal facets exhibit significant anisotropic diffusivity, in-plane motion being faster in the trans-prismatic direction than in the basal-to-basal direction. The in-plane diffusion length is greatest for the 28° pyramidal facet and smallest for the prismatic facet. This diversity of facet-specific properties provides a rich set of possibilities for mechanisms of ice crystal growth and ablation.     

Microsolvation of uracil and its conjugate bases: a DFT study of the role of solvation on acidity

The effect of microsolvation on the deprotonation energies of uracil was examined using DFT. The structures of uracil and its N(1) and N(3) conjugate bases were optimized with zero to six associated water molecules. Multiple configurations (upward of 93) of these hydrated clusters were located at PBE1PBE/6-311+G(d,p). Trends in these geometries are discussed, with the waters generally forming chains with small numbers of waters (one-three), rings (three-five waters), or cages (five-six waters). The difference in energy between the N1 and N3 conjugate bases is 13 kcal mol(-1) in the gas phase, and it decreases with each added water up to four. At this point the energy difference has been halved, but addition of a fifth or sixth water has little effect on the energy difference. This is understood in terms of the water structures and their ability to stabilize the negatively charged atoms in the conjugate bases.     

Interconversion study in 1,4-substituted six-membered cyclohexane-type rings. Structure and dynamics of trans-1,4-dibromo-1,4-dicyanocyclohexane

Cyclohexane is an extremely flexible molecule that oscillates, at room temperature, between two clearly distinct and extreme conformations that cannot be distinguished at room temperature; so much so that the NMR spectrum is a single line that includes all 12 protons be they axial or equatorial. This raises the interesting question as to what happens when there are equal substituents at the 1 and 4 carbon atoms of the ring. Therefore substitution in the 1,4-positions in the cyclohexane ring has been the subject of considerable interest because some form of interconversion between extreme conformations could lead to the existence of a rather unusual behavior. To study this problem, the interconversion in (di- or tetra-1,4)-substituted six-membered cyclohexane-type rings, trans-1,4-dibromo-1,4-dicyanocyclohexane, was found to be a particularly suitable candidate. Although X-ray diffraction studies on the crystalline solid found the molecule to be centrosymmetric, it still shows a significant dipole moment μ in solution, as determined with a procedure that leads to the vapor phase values of μ. Furthermore, the low magnetic field proton NMR spectrum at ambient temperature appears as a single line, a situation that changes with increasing field intensity and different solvents. Both these effects are attributed to dynamics, because small distortions can easily disrupt the exact cancellation of the individual dipoles (which are quite strong) associated with each end of the molecule. The molecule can exist in two forms, with both the bromines in an axial geometry or both in an equatorial position. Interconversion between these forms is observed, as in the parent cyclohexane. The single NMR line observed at low magnetic fields is due to fast exchange and requires that the two forms have roughly equal populations. Spectra obtained at low temperature confirm this, and variable-temperature studies allow measurement of the rates, leading to an enthalpy of activation of 62 kJ mol(-1). More details of the interconversion are provided by some new calculation methods. Even for a relatively small molecule like this, calculation of a full potential energy surface is prohibitive. However, methods are now available to follow the molecule along the reaction coordinate in quite an efficient way. The results of these calculations lead to an extremely detailed picture of chair-chair interconversion in a di- and tetrasubstituted six-membered ring of the cyclohexane family.     

Chemical tailoring of fullerene acceptors: synthesis, structures and electrochemical properties of perfluoroisopropylfullerenes

High-temperature syntheses of the new C(60)(i-C(3)F(7))(2,4,6) and C(70)(i-C(3)F(7))(2,4) isomers and their characterization by spectroscopic methods, X-ray crystallography, cyclic voltammetry and density functional theory provide compelling evidence that they are superior electron acceptors than trifluoromethylfullerenes.     

Solution structure of copper-seamed C-alkylpyrogallol[4]arene nanocapsules with varying chain lengths

The stability of copper-seamed C-alkylpyrogallol[4]arene hexamers with varying chain lengths in solution has been studied using small-angle neutron scattering (SANS). The progression in diameter of spherical capsules with increasing alkyl chain lengths of copper-seamed hexamers in solution suggests both robustness as well as a close correlation between the solid phase and solution phase structures.     

Catalytic deuteration of silanes mediated by N-heterocyclic carbene-Ir(III) complexes

The catalytic activity of a series of coordinatively unsaturated NHC-M(III) (M = Rh, Ir; NHC = N-heterocyclic carbene) complexes was tested in the deuteration of secondary and tertiary silanes. Among these, [IrCl(I(t)Bu')(2)] provides the highest conversions to the deuterated species. Mechanistic studies highlight the reversible nature of the ortho-metalation reaction.