571 research outputs found
Optically tailored access to metastable electronic states
On irradiating a molecular system with a laser beam of ultraviolet or visible frequency, photon absorption occurs when an electronic state is at a suitable energy level relative to an initial state. Despite meeting this criterion, interesting metastable states can remain inaccessible because of symmetry constraints. In this Letter a mechanism, based on the input of an off-resonant beam, is shown to enable the population of such states. This is achievable because the laser-modified process involves different selection rules compared to conventional photon absorption. The effects of applying the stimulus beam to either a one- or two-photon process are examined
Bound and free waves in non-collinear second harmonic generation
We analyze the relationship between the bound and the free waves in the
noncollinear SHG scheme, along with the vectorial conservation law for the
different components arising when there are two pump beams impinging on the
sample with two different incidence angles. The generated power is
systematically investigated, by varying the polarization state of both
fundamental beams, while absorption is included via the Herman and Hayden
correction terms. The theoretical simulations, obtained for samples which are
some coherence length thick show that the resulting polarization mapping is an
useful tool to put in evidence the interference between bound and free waves,
as well as the effect of absorption on the interference patternComment: 10 pages, 7 figure. to be published on Optics Expres
Phenyl vs. Ferrocenyl Cyclometallation Selectivity: Diastereoselective Synthesis of an Enantiopure Iridacycle
Ferrocenyl (Fc) and phenyl (Ph) containing imines FcCH=NCH(R)Ph and FcCH(R)N=CHPh (R = H and Me) were cycloiridated using [Cp*IrCl2]2 with NaOAc in CH2Cl2. All resulted in the formation of neutral chloride ligated half-sandwich iridacycles as a result of ortho-phenyl and not alpha-ferrocenyl C–H activation. The complexes derived from FcCH=NCH(R)Ph (R = H, Me) were obtained as a mixture of E and Z imine isomers, and with R = Me the product obtained from the (S)-imine was isolated by recrystallisation as a single diastereoisomer. The configuration was determined by an X-ray crystal structure analysis as SC,RIr,E
Ruthenacycles and Iridacycles as Catalysts for Asymmetric Transfer Hydrogenation and Racemisation
Ruthenacycles, which are easily prepared in a single step by reaction between enantiopure aromatic amines and [Ru(arene)Cl2]2 in the presence of NaOH and KPF6, are very good asymmetric transfer hydrogenation catalysts. A range of aromatic ketones were reduced using isopropanol in good yields with ee’s up to 98%. Iridacycles, which are prepared in similar fashion from [IrCp*Cl2]2 are excellent catalysts for the racemisation of secondary alcohols and chlorohydrins at room temperature. This allowed the development of a new dynamic kinetic resolution of chlorohydrins to the enantiopure epoxides in up to 90% yield and 98% enantiomeric excess (ee) using a mutant of the enzyme Haloalcohol dehalogenase C and an iridacycle as racemisation catalyst.
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