Two-dimensional-Raman-terahertz spectroscopy of water: theory

We discuss the hybrid 2D-Raman-THz spectroscopy of liquid water. This two-dimensional spectroscopy is designed to directly work in the low-frequency range of the intermolecular degrees of freedom. The information content of 2D-Raman-THz spectroscopy is similar to 2D-Raman or 2D-THz spectroscopy, but its experimental implementation should be easier. That is, 2D-Raman-THz spectroscopy is a 3rd-order nonlinear spectroscopy and as such completely avoids cascading of consecutive 3rd-order signals, which turned out to be a major difficulty in 5th-order 2D-Raman spectroscopy. On the other hand, it does not require any intense THz pump-pulse, the lack of which limits 2D-THz spectroscopy to the study of semiconductor quantum wells as the currently available pulse energies are too low for molecular systems. In close analogy to 2D-Raman spectroscopy, the 2D-Raman-THz response of liquid water is simulated from an all-atom molecular dynamics simulation, and the expected spectral features are discussed.     

Effect of pressure on the Ginzburg-Landau parameter kappa=lambda/xi in YB6

Measurements of the transition temperature Tc, the upper critical field Hc2, and the magnetic penetration depth lambda under hydrostatic pressure (up to approximately 9.2 kbar) in the YB6 superconductor were carried out. A pronounced and negative pressure effect (PE) on Tc and Hc2 with dTc/dp=-0.0547(4) K/kbar and micro0dHc2(0)/dp=-4.84(20) mT/kbar, and zero PE on lambda(0) were observed. The PE on the coherence length dxi(0)/dp=0.28(2) nm/kbar was calculated from the measured pressure dependence of Hc2(0). Together with the zero PE on the magnetic penetration depth lambda(0), our results imply that the Ginzburg-Landau parameter kappa(0)=xi(0)/lambda(0) depends on pressure and that pressure "softens" YB6, e.g., moves it to the type-I direction.     

Synthesis and characterization of bis[dicarboxylatotetraorganodistannoxane] units involving 5-[(E)-2-(aryl)-1-diazenyl]-2-hydroxybenzoic acids: An investigation of structures by X-ray diffraction, NMR, electrospray ionisation MS and assessment of in vitro cytotoxicity

Reactions of 5-[(E)-2-(aryl)-1-diazenyl]-2-hydroxybenzoic acids (LHH′, where the aryl group is an R-substituted phenyl ring such that for L¹HH′: R = H; L²HH′: R = 2′-CH₃; L³HH′: R = 3′-CH₃; L⁴HH′: R = 4′-CH₃; L⁵HH′: R = 4′-Cl; L⁶HH′: R = 4′-Br) with ⁿBu₂SnO in a 1:1 molar ratio yielded complexes of composition {[ⁿBu₂Sn(LH)]₂O}₂. The complexes have been characterized by ¹H, ¹³C, ¹¹⁹Sn NMR, ESI-MS, IR and ^119mSn Mössbauer spectroscopic techniques in combination with elemental analyses. The crystal structures of {[ⁿBu₂Sn(L¹H)]₂O}₂ (1), {[ⁿBu₂Sn(L⁴H)]₂O}₂ (4), {[ⁿBu₂Sn(L⁵H)]₂O}₂ (5) and {[ⁿBu₂Sn(L⁶H)]₂O}₂ (6) were determined. The compounds are centrosymmetric tetranuclear bis(dicarboxylatotetrabutyldistannoxane) complexes containing a planar Sn₄O₂ core in which two μ₃-oxo O-atoms connect an Sn₂O₂ ring to two exocyclic Sn-atoms. The four carboxylate ligands display two different modes of coordination where both modes involve bridging of two structurally distinct Sn-atoms. The solution structures were confirmed by ¹¹⁹Sn NMR spectroscopy by observing two tin resonances in compounds 1, and 4-6. The observed difference between the two tin resonances was about 3 ppm while the differences in ¹³C resonances were even smaller. Compounds {[ⁿBu₂Sn(L²H)]₂O}₂ (2) and {[ⁿBu₂Sn(L³H)]₂O}₂ (3) undergo a very complex exchange processes in deuteriochloroform solution. The in vitro cytotoxic activity of compounds 1 and 4 against WIDR, M19 MEL, A498, IGROV, H226, MCF7 and EVSA-T human tumour cell lines is reported.     

Acid-base properties of the nucleic-acid model 2'-deoxyguanylyl(5'-->3')-2'-deoxy-5'-guanylate, d(pGpG)3-, and of related guanine derivatives

The dinucleotide d(pGpG) is an often employed DNA model to study various kinds of interactions between DNA and metal ions, but its acid-base properties were not yet described in detail. In this study the six deprotonation reactions of H4[d(pGpG)]+ are quantified. The acidity constants for the release of the first proton from the terminal P(O)(OH)2 group (pKa = 0.65) and for one of the (N7)H+ sites (pKa = 2.4) are estimated. The acidity constants of the remaining four deprotonation reactions were measured by potentiometric pH titrations in aqueous solution (25 degrees C; I = 0.1 M, NaNO3): The pKa values for the deprotonations of the second (N7)H+, the P(O)2(OH)-, and the two (N1)H sites are 2.98, 6.56, 9.54 and 10.11, respectively. Based on these results we show how to estimate acidity constants for related systems that have not been studied, e.g. pGpG, which is involved in the initiation step of a rotavirus RNA polymerase. The relevance of our results for nucleic acids in general is briefly indicated.     

Molecular dynamics simulation of liquid water: hybrid density functionals

The structure, dynamical, and electronic properties of liquid water utilizing different hybrid density functionals were tested within the plane wave framework of first-principles molecular dynamics simulations. The computational approach, which employs modified functionals with short-ranged Hartree-Fock exchange, was first tested in calculations of the structural and bonding properties of the water dimer and cyclic water trimer. Liquid water simulations were performed at the state point of 350 K at the experimental density. Simulations included three different hybrid functionals, a meta-functional, four gradient-corrected functionals, and the local density and Hartree-Fock approximations. It is found that hybrid functionals are superior in reproducing the experimental structure and dynamical properties as measured by the radial distribution function and self-diffusion constant when compared to the pure density functionals. The local density and Hartree-Fock approximations show strongly over- and understructured liquids, respectively. Hydrogen bond analysis shows that the hybrid functionals give slightly smaller average numbers of hydrogen bonds than pure density functionals but similar hydrogen bond populations. The average molecular dipole moments in the liquid from the three hybrid functionals are lower than those of the corresponding pure density functionals.     

Synthesis of fluoranthenes and indenocorannulenes: elucidation of chiral stereoisomers on the basis of static molecular bowls

Cycloaromatization of a peri diyne with an external alkyne provides a general route to indeno-fused polynuclear aromatics. Fluoranthenes 9 (or 13) are easily accessible in good to excellent yields (75-99%; 18 examples) from the reaction of symmetric (or asymmetric) diynes 4 and alkynes 5 (or norbornadiene) in the presence of Wilkinson's catalyst. This formal [(2+2)+2] cycloaddition can also be applied to generate various indenocorannulenes 28 from 2,3-diethynylcorannulene derivatives 27 and alkynes 5. The indenocorannulenes 28 exist in a static bowl form at room temperature with bowl-to-bowl inversion barriers higher than 24 kcal/mol. This barrier renders the rate of inversion slow enough at room temperature to establish a class of chiral, bowl-shaped stereoiomers containing no tetrahedrally ligated atoms. The crystal structure of 28g provides insight into the bowl-shaped geometry of these compounds. This new synthetic method occurs under neutral conditions and tolerates various functional groups (e.g., alkyl, aryl, alcohols, and esters).     

d8 rhodium and iridium complexes of corannulene

[(COE)2M]+ fragments (M = Rh, Ir; COE = cyclooctene) react with corannulene to give eta6-bound complexes [(COE)2Rh(eta6-C20H10)]PF6 and [(COE)2Ir(eta6-C20H10)]PF6. Both compounds were characterized by X-ray diffraction studies, and binding in the solid state is compared with that of their aromatic analogues [(COE)2M(eta6-arene)]PF6 (aryl = benzene and phenanthrene). Solution NMR studies show that the [(COE)2Rh]+ fragment walks over the curved aromatic surface of corannulene, whereas the Ir analogue is not fluxional. Experimental as well as computational studies suggest that inter-ring movement of the Rh complex follows a hub migration mechanism. Initial reactivity studies indicate that the [(COE)2M]+ fragments can undergo chemical transformations, such as transfer dehydrogenation and substitution reactions, while still bound to corannulene.     

The infrared-driven cis-trans isomerization of HONO. II: Vibrational relaxation and slow isomerization channel

In a recent paper [R. Schanz et al., J. Chem. Phys. 122, 044509 (2005)], we investigated the IR-driven cis-trans isomerization of HONO in a Kr matrix with the help of femtosecond IR spectroscopy. We found that isomerization occurs on a 20 ps time scale, however, with a cis-->trans quantum yield of only 10% that is significantly below the value reported in the literature (close to 100%). At the same time, we concluded that vibrational energy has not completely dissipated out of the molecule at the maximum delay time we reached in this study (500 ps). In order to verify whether additional, slower reaction channels exist, we extend the study here to delay times up to 100 ns. At a temperature of 32 K, we indeed find an additional isomerization channel on a 2 ns timescale, which increases the total cis-->trans quantum yield to approximately 30%. The trans-->cis quantum yield is approximately 7%. There is still a discrepancy between the quantum yields we observe and the literature values, however, we provide experimental evidence that this discrepancy is due to the different temperatures of our study. Vibrational cooling occurs on a 20 ns time scale, and cascades in a highly nonstatistical manner through one single normal mode (most likely the ONO bending mode nu(5)). Intermolecular energy dissipation into the rare gas matrix is more efficient than intramolecular vibrational energy redistribution and the matrix environment can certainly not be considered a weak perturbation.     

Proton transfer in imidazole-based molecular crystals

Heterocycles' aggregates show rather good proton conductivity. In particular, condensed structures formed by imidazole rings that are held together by polymeric chains have attracted some interest as possible candidate materials for fuel cell membranes. However, the details of the proton diffusion process could not be resolved by means of experimental measurements because of the fast rearrangement of the structure after each proton exchange. In this work, we report in detail the results of ab initio molecular dynamics calculations, which were briefly presented in a previous Letter [M. Iannuzzi and M. Parrinello, Phys. Rev. Lett. 93, 025901 (2004)]. The conformational changes associated with the diffusion of protons in model crystalline structures containing chains of imidazole rings are described in the framework of an atomistic approach. In particular, the bonding pattern characterizing the structure of imidazole-2-ethylene-oxide doped by an excess proton is also studied through the calculation of the 1H NMR chemical shifts. The unresolved resonances appearing in the experimental spectra could be associated with specific structural features, in connection with the fluctuating hydrogen bonding. The analysis of the distortions that induce or are induced by the mobility of the protons offers some new hints for the engineering of new proton conducting materials.