ChemPhysChem

Jacobus van't Hoff proposed in 1874 that molecules have three-dimensional structures. The three-dimensional shape of molecules is responsible for many of their chemical and physical properties, such as the optical rotation of light. The cover picture illustrates a novel reaction microscope that was developed at LaserLaB Amsterdam to study and control molecular photodynamics. It employs advanced single-particle imaging detectors that measure the full three-dimensional velocity distribution of correlated electrons and (fragment) ions emitted from an excited molecule. Shaped femtosecond laser fields are used to control and manipulate the dynamics in the excited molecule in order to steer the outcome of the photochemical reaction. Recent experimental results are reviewed by M. H. M. Janssen and coworkers on p. 1459, illustrating the wealth of detailed information that can be obtained about the interplay between shaped laser fields, femtosecond dynamics, ionization processes and multichannel pathways in three-dimensional (chiral) molecules.

Chirality is a many-layered thing. At surfaces molecular handedness is joined by adsorption “footedness”, leading to multi-faceted chiral behavior in two dimensions, as reviewed by R. Raval et al. on p. 1474.

A roller-coaster ride: The rate-determining step concept is revised, since the difficulties on its definition are evidence of its inconsistency. In view of this, a physically correct kinetic concept is proposed: the rate-determining state.

On the ROA to somewhere: The first combined study of measured and computed Raman optical activity (ROA) of a transition metal complex under non-resonant scattering conditions is reported. ROA measurements of the two enantiomers of the dichloro[ethylenebis(4,5,6,7-tetrahydro-1-indenyl)] zirconium(IV) complex yield virtually mirror-image ROA spectra. The experimental spectra are directly comparable with predicted ROA spectra.

Clutching the ball: A comparative electrochemical study of the N-tritylpyrrolidino derivatives of Sc₃N@I_h-C₈₀ (picture, left) and Lu₃N@I_h-C₈₀ (picture, right) shows a significant influence of the encapsulated clusters on the physical and electrochemical properties of endohedral fullerene derivatives.

Making and breaking foams: A facile method for foam control is reported using a designed biosurfactant. The new biosurfactant is based on a four-helix bundle design which is interfacially active across a broad pH range. At pH 8.5 stable foam forms, yet decreasing the pH to 7.5 causes rapid foam dissipation. This unique behaviour can be linked to subtle molecular and specific-ion effects which lead to switchable changes in the state of the foam thin film (see picture).

Electrical double layer: The detailed outer Helmholtz plane (HP) structure composed of Cs⁺ above c(2×2)-Br on Ag(100) electrode using in situ surface X-ray diffraction in electrochemical environments is described. Hydrated Cs⁺ cations are accommodated in the hollow sites of the c(2×2)-Br layer. The coverage of Cs⁺ and the distance of the Cs⁺ layer from the surface depend on the electrode potential. A single layer of water is intercalated between the inner and outer HP ions (see picture).

CO₂adsorption complexes in K-FER zeolite are investigated by a combination of variable-temperature IR spectroscopy and periodic DFT calculations augmented for description of dispersion interactions. Calculated adsorption enthalpies on single and dual K⁺ sites are in excellent agreement with experimental values. The picture shows the CO₂ adsorption complex in the M channel viewed along the direction of the M channel (O red, Si gray, Al black, K⁺ violet).

Travelling wave ion mobility mass spectrometry was used to determine the intrinsic mobility of a series of gaseous supra-cation and supra-anion aggregates of the type [(DAI)_n+1(X)_n]⁺ (DAI is the 1,3- dialkylimidazolium cation and X is the anion) compared to the corresponding anions [(DAI)_n(X)_n+1]⁻ for n=0 to 9 (see graphic). There is a linear dependence of drift time on mass of the ionic liquid aggregates, with little effect of the sign of the charge.

Solvent matters: Solvent effects on PtN spin–spin coupling constants of cisplatin and three derivatives are investigated using first-principles theory. Good agreement with experiment is obtained when explicit solvent molecules are considered and when the computations are performed with a hybrid functional. The possibility of NMR detection of a Pt⋅⋅⋅H(water) coupling constant from an inverse hydration structural pattern is discussed.

Molecular holograms: The three-dimensional structure of molecules leads to three-dimensional patterns of ejected electrons and ions after excitation by photons of sufficient energy. The application of advanced photoelectron and photoion coincidence imaging techniques (see figure) to make pictures of those three-dimensional photon-induced patterns is discussed. These experiments foster our insight into molecular photodynamics and chemical structure.

Order in chaos: Racemic proline adsorbed on Cu(110) is a random solid solution of enantiomers. However examination with STM supported by ab initio calculations shows that the while the distribution of enantiomers and conformers is indeed random, the footprint chirality obeys a strict heterochiral arrangement like a racemic compound (see picture).

Let′s tango: Recognition and binding of specific sites on DNA by proteins is central for many cellular functions. In the search for its target site, the DNA-associated protein is facing both thermodynamic and kinetic difficulties. Herein, the history of the DNA search problem, the theoretical background and the various experimental methods used to study the kinetics of protein searching for target sites on DNA are discussed (see picture).

The introduction and control of defined chiral centers on the scaffold of DOTA (see structure) imparts rigidity to the system and increases its structural homogeneity. Different locations of these chiral elements lead to different degrees of conformational prevalence.

Single-molecule spectroscopy on the conjugated polymer MEH-PPV at 1.2 K is used to investigate the nature of the well-known red subpopulation of emitters by emission, excitation and time-resolved techniques. The results point to longer chromophoric units as a consequence of packing effects. Moreover, fluorescence excitation spectra (see spectrum) allow the first direct measurement of energy transfer rates via the line widths of donor chromophores.

Flexible backbone: Conformations of the pseudo-peptide backbone of valinomycin in dioxane and methanol, some of which were not previously seen by NMR spectroscopy, were identified by means of experimental and calculated Raman optical activity (ROA) spectra. The asymmetrical and symmetrical bracelet forms are favored in dioxane, while structures based on the flexible propeller conformer dominate in methanol (see picture).

Stretched thin: Upon stretching low-density polyethylene films, unoriented dopant molecules become oriented in an all-or-nothing scenario (see picture). The oriented fraction resides on the surfaces of PE crystals.

Enantioselective sensor: We introduce a simple approach in which a cyclodextrin is assembled onto the silver surface of a SERS platform for chiral recognition of enantiomers (see picture). Trapping properties, chiral discrimination and quantification of the R,R- and S,S-enantiomers of hydrobenzoin by surface-enhanced Raman scattering (SERS) spectroscopy are demonstrated.

On the spot: Correlated atomic force, optical absorption, and fluorescence microscopy of individual gold nanoparticles (NPs) are applied to measure the luminescence quantum yield of NPs with diameters of 5 nm to 80 nm (see picture).

Neighborhood similarity measure: Vibrational circular dichroism and Raman optical activity are powerful spectroscopic techniques to establish absolute configuration (AC). Their reliability and objectivity is further enhanced by plotting how good a new assignment of AC is compared to earlier successful AC assignments (see picture).

A well-trodden path: The thus far most systematically studied enzyme in directed evolution of stereoselectivity, the lipase PAL (see picture), is investigated once more with emphasis on substrate acceptance of bulky chiral esters, as iterative saturation mutagenesis in the form of CASTing is more efficient than traditional techniques.

Twisted crystals: Ferdinand Bernauer proposed in “Gedrillte” Kristalle (1929), that a great number of simple, crystalline substances grow from solution or from the melt as polycrystalline spherulites with helically twisting radii that give rise to concentric optical bands between crossed polarizers (see picture). In fact, rhythmic precipitation and helical twisting often coexist, complicating optical analyses and presenting Bernauer with difficulties in the characterization and classification of the objects of his interest.

Racemic malic acid adsorbed on Cu(110) forms enantiomorphous domains by opposite relative arrangement of heterochiral pairs. A pronounced chiral restructuring of the metal surface is directly identified, including chirality transfer to surface areas that are not decorated with molecules.

Let′s do the twist: Compact expressions to calculate the transition energies, absorption line strengths, and rotational line strengths of circular dichroism for the excitonic states in a helical arrangement of N identical chromophores are presented (see picture). An analytical expression can be given when only next-neighbor interactions are considered.

Three new photostable rylene derivatives were synthesised and characterized with regard to fluorescence labels in biological applications. Single-molecule techniques show excellent photostability of these dyes. Additionally it is shown that one of them (4-PEG-TDI) can be used for membrane labelling (see picture).