Ultrafast N-H vibrational dynamics of cyclic doubly hydrogen-bonded homo- and heterodimers

Hydrogen-bonded interfaces are essential structural elements in biology. Furthermore, they can mediate electron transport by coupling the electron to proton transfer within the interface. The specific hydrogen-bonding configuration and strength have a large impact on the proton transfer, which exchanges the hydrogen-bonded donor and acceptor species (i.e., NH...O --> N...HO). Modulations of the hydrogen-bonding environment, such as the hydrogen-bond stretch and twist modes, affect the proton-transfer dynamics. Here, we present transient grating and echo peak shift measurements of the NH stretch vibrations of four doubly hydrogen-bonded cyclic dimers in their electronic ground state. The equilibrium vibrational dynamics exhibit strong coherent modulations that we attribute to coupling of the high-frequency NH vibration to the low-frequency interdimer stretch and twist modes and not to interference between multiple Fermi resonances that dominate the substructure of the linear spectra.     

A ground-state-directed optimization scheme for the Kohn-Sham energy

Kohn-Sham density-functional calculations are used in many branches of science to obtain information about the electronic structure of molecular systems and materials. Unfortunately, the traditional method for optimizing the Kohn-Sham energy suffers from fundamental problems that may lead to divergence or, even worse, convergence to an energy saddle point rather than to the ground-state minimum-in particular, for the larger and more complicated electronic systems that are often studied by Kohn-Sham theory nowadays. We here present a novel method for Kohn-Sham energy minimization that does not suffer from the flaws of the conventional approach, combining reliability and efficiency with linear complexity. In particular, the proposed method converges by design to a minimum, avoiding the sometimes spurious solutions of the traditional method and bypassing the need to examine the structure of the provided solution.     

Ultrasound assisted synthesis of 5,9-dimethylpentadecane and 5,9-dimethylhexadecane--the sex pheromones of Leucoptera coffeella

Racemic 5,9-dimethylpentadecane and 5,9-dimethylhexadecane, the major and minor constituents, respectively, of the sex pheromone of Leucoptera coffeella, have been synthesized from citronellol in 56-58% overall yield through six steps. Ultrasound irradiation efficiently supported tosylation of alcohols (two steps) as well as the subsequent cross coupling reactions with the pertinent Grignard reagents (also two steps).     

The electronic spectra of conjugated free radicals

Ensemble averaging schemes are applied within the PPP model to calculate excitation spectra of free conjugated hydrocarbon radicals and further, more general forms of the pairing and particle-hole properties for conjugated hydrocarbons are shown to be valid.     

Synthesis and molecular modelling of double-functionalised nucleosides with aromatic moieties in the 5'-(S)-position and minor groove interactions in DNA zipper structures

A series of six double-functionalised nucleosides, in which aromatic moieties were inserted into the 5'-(S)-C-position, were synthesised and incorporated into DNA duplexes. The aromatic moieties were thymine-1-yl, phenyl, 1,2,3-triazol-1-yl, 1,2,3-triazol-4-yl, 4-(uracil-5-yl)-1,2,3-triazol-1-yl and 4-phenyl-1,2,3-triazol-1-yl. The DNA duplexes were studied with UV melting curves, CD spectroscopy and molecular modelling. The results showed that the aromatic moieties in some cases interact in the minor groove forming DNA zipper structures. The strongest specific interaction was found between two thymines or between a thymine and a phenyl group in a crossed (-3)-zipper motif (i.e., with two base pairs interspacing the modifications). Modelling revealed that the interaction is aromatic stacking across the minor groove. Also, the extended uracil-triazole moiety demonstrated zipper contacts in the minor groove as well as binding to the floor of the groove.     

Control of orbital angular momentum of light with optical fibers

We present a fiber-based method for generating vortex beams with a tunable value of orbital angular momentum from -1? to +1? per photon. We propose a new (to our knowledge) method to determine the modal content of the fiber and demonstrate high purity of the desired vortex state (97% after 20 m, even after bends and twists). This method has immediate utility for the multitude of applications in science and technology that exploit vortex light states.     

Long range 13C?13C coupling constants. A review

A large number of ¹³C? ¹³C coupling constants from a diversity of compounds are now available. Attempts have been made to classify this information in order to illustrate how these data are obtained, how signs are determined and how both two- and three-bond couplings can lead to structural information. Coupling constants in aromatic compounds are now well documented, substituent effect trends have been established and correlations with bond orders are noticed. Comparison of ¹³C? ¹³C couplings with ¹³C? ¹H couplings is mentioned. Theoretical calculations are dealt with, and these can now be properly evaluated as both magnitudes and signs of the experimental couplings are known. Finally, the occurrence of long range couplings in biosynthetic material is briefly surveyed.     

Brueckner coupled cluster response functions

A derivation of the linear response function for the Brueckner coupled cluster method is presented that enables the calculation of second-order molecular properties such as frequency-dependent polarizabilities. By using the Brueckner orbital variant of coupled cluster theory, the spurious pole structure inherent in the standard coupled cluster approach with orbital relaxation is avoided. © 1994 John Wiley & Sons, Inc.