Self-selection in olefin cross-metathesis: the effect of remote functionality

Olefin cross-metathesis (CM) is potentially an attractive method for generating dynamic combinatorial libraries (DCLs). In order for the CM reaction to be useful for DCL production, the course of the reaction and product distribution must be relatively insensitive to functionality remote from the reacting centers. We report on the CM of a series of allyl- and homoallylamides that are strongly dependent on remote functionality. This includes an unusual example of a cis-selective CM. [Reaction: see text]     

Control of oxo-molybdenum reduction and ionization potentials by dithiolate donors

The compounds (L-N3)MoO(qdt) and (L-N3)MoO(tdt) [(L-N3) = hydrotris(3,5-dimethyl-1-pyrazolyl)borate; tdt = toluene-3,4-dithiolate; qdt = quinoxaline-2,3-dithiolate] have been studied by cyclic voltammetry and photoelectron, magnetic circular dichroism, and electronic absorption spectroscopies, and the experimental data have been interpreted in the context of ab initio molecular orbital calculations on a variety of dithiolate dianion ligands. The PES data reveal very substantial differences between (L-N3)MoO(qdt) and (L-N3)MoO(tdt) in that the first ionization (originating from the Mo dxy orbital) for (L-N3)MoO(qdt) is about 0.8 eV to deeper binding energy than that of (L-N3)MoO(tdt). This stabilizing effect is also reflected in the solution reduction potentials, where (L-N3)MoO(qdt) is approximately 220 mV easier to reduce than (L-N3)MoO(tdt). A direct correlation between the relative donating ability of a given dithiolate ligand and the reduction potential of the (L-N3)MoO(dithiolate) complex has been observed, and a linear relationship exists between the calculated Mulliken charge on the S atoms of the dithiolate dianion and the Mo reduction potential. The study confirms previously communicated work (Helton, M. E.; Kirk, M. L. Inorg. Chem. 1999, 38, 4384-4385) that suggests that anisotropic covalency contributions involving only the out-of-plane S orbitals of the coordinated dithiolate control the Mo reduction potential by modulating the effective nuclear charge of the metal, and this has direct relevance to understanding the mechanism of ferricyanide inhibition in sulfite oxidase. Furthermore, these results indicate that partially oxidized pyranopterins may play a role in facilitating electron and/or atom transfer in certain pyranopterin tungsten enzymes which catalyze formal oxygen atom transfer reactions at considerably lower potentials.     

Exploring the origin of tip‐enhanced Raman scattering; preparation of efficient TERS probes with high yield

Tip‐enhanced Raman scattering (TERS) spectroscopy is a promising technique for nanoscale chemical analysis. However, there are several challenges preventing widespread application of this technology, including reproducible fabrication of efficient TERS probes. These problems reflect a lack of clear understanding of the origins of, and the parameters influencing TERS. It is believed that the coating characteristics at the apex of the tip have a major effect on the near‐field optical enhancement and thus the TERS activity of a metalized probe. Here we show that the aspect ratio of the tip can play a significant role in the efficiency of TERS probes. We argue that the electrostatic field arising from the lightning‐rod effect has a substantial role in the observed TERS effect. This argument is supported by ‘edge‐enhanced Raman scattering’ which is shown for a noble metal film. Furthermore, it is reported that an associated tip‐surface‐enhanced Raman scattering effect can be achieved by using a TERS‐inactive metalized probe on a surface‐enhanced Raman spectroscopy‐inactive roughened surface. This observation can be explained by an interparticle enhancement of the electromagnetic field. Copyright © 2011 John Wiley & Sons, Ltd.     

Infrared spectra of 1-phosphapropyne,CH3CP,and its perdeuteride,CD3CP

The infrared spectra of 1-phosphapropyne, CH₃CP, and its perdeuteride, CD₃CP, have been measured in the gaseous and solid states. The Q_K branches of perpendicular bands have been analyzed in terms of the usual quadratic expression in K. Fermi resonances were identified for the ν₁, ν₂ + ν₃, 2ν₃, 2ν₆⁰; and ν₅, 2ν₃ + ν₈ band systems of CH₃CP and the ν₁, 2ν₃, 2ν₆⁰; ν₆, ν₇ + ν₈; and ν₇, 3ν₈¹ band systems of CD₃CP. The xy Coriolis interaction was also identified between the ν₃ and ν₆ bands of the two species. All the fundamentals were assigned and the normal coordinate treatment was carried out along with the Coriolis constants, ζ^z.     

High resolution infrared spectrum and molecular structure of dichloroacetylene

The infrared spectrum of dichloroacetylene, prepared by the pyrolysis of dichloromaleic anhydride at 1000°C, has been recorded at both low and high resolution. In the low resolution spectrum a number of combination bands not previously observed have been assigned, the infrared active fundamentals have been reassigned, and a center wavenumber value has been determined for the Raman activev ₁ fundamental. The high resolution spectra of a number of fundamental bands, summation bands, and one difference band for the isotopomers,³⁵CICC³⁵CI and³⁵CICC³⁷CI, have been assigned, while a more limited number of bands has been assigned for the species³⁷CICC³⁷CI and³⁵CI¹³CC³⁵CI. The resultant rotational and vibration-rotation constants have been used to obtainr _o,r _e,r _s, and partial r_s structural parameters. The most reliable bond lengths are obtained from the partialr _s treatment and are 164.105(53) pm for the C-CI bond and 119.203(79) pm for the CC bond.