Optimization procedure for pulse separation in cross-correlation PIV

In PIV, the optimal time separation (t) between successive laser pulses is influenced by a number of parameters. In the present paper, only two kinds of error affecting the choice of t are studied: (i) random error arising from noise during recording of the flow seeded with tracer particles and subsequent interrogation of the particle images, and (ii) acceleration error arising from approximation of the local Eulerian velocity based on small (but non-zero) particle displacements. These two kinds of error place conflicting requirements on t. A model to optimize t with respect to these errors is described, and the model is confirmed by the results of a Monte Carlo simulation. This model for optimal t is extended to various acceleration distributions. An estimate for the spatial resolution of the velocity field resulting from cross-correlation PIV is proposed.We wish to thank the University of Delaware and the Department of Mechanical Engineering for providing a graduate fellowship to support this work.     

Crystal structure and magnetic properties of bis(2,2′-bipyridine) azido copper(II) perchlorate

The title compound has been synthesized and its crystal structure has been solved at room temperature. It crystallizes in the triclinic system, space groupP, witha=7.853(1) Å,b=10.177(1) Å,c=13.651(4) Å,=74.57(1)°,=88.49(1)°, =80.675(4)°, andZ=2. The structure consists of isolated [Cu(bipy)₂N₃]⁺ cations (bipy=2,2-bipyridine) and perchlorate anions. The metallic cation environment has a distorted trigonal-bipyramidal geometry. The EPR spectrum is rhombic, which indicates that the distortion with respect to the perfect trigonal bipyramid is sufficiently pronounced to mixz ²- andx ²–y ²-type orbitals in the ground state. The temperature dependence of the molar magnetic susceptibility has been investigated in the 300-2.2 K temperature range.     

Copper and iron interactions with angiotensin-converting enzyme inhibitors. A study by fast-atom bombardment tandem mass spectrometry

Fast atom bombardment, combined with high-energy collision-induced tandem mass spectrometry, has been used to investigate gas-phase metal-ion interactions with captopril, enalaprilat and lisinopril, all angiotensin-converting enzyme inhibitors.Suggestions for the location of metal-binding sites are presented. For captopril, metal binding occurs most likely at both the sulphur and the nitrogen atom. For enalaprilat and lisinopril, binding preferably occurs at the amine nitrogen. Copyright 1999 John Wiley & Sons, Ltd.     

ESI‐MS/MS of expanded porphyrins: a look into their structure and aromaticity

Electrospray mass spectrometry/mass spectrometry was used to investigate the gas‐phase properties of protonated expanded porphyrins, in order to correlate those with their structure and conformation. We have selected five expanded meso‐pentafluorophenyl porphyrins, respectively, a pair of oxidized/reduced fused pentaphyrins (22 and 24 π electrons), a pair of oxidized/reduced regular hexaphyrins (26 and 28 π electrons) and a regular doubly N‐fused hexaphyrin (28 π electrons). The gas‐phase behavior of the protonated species of oxidized and reduced expanded porphyrins is different. The oxidized species (aromatic Hückel systems) fragment more extensively, mainly by the loss of two HF molecules. The reduced species (Möbius aromatic or Möbius‐like aromatic systems) fragment less than their oxidized counterparts because of their increased flexibility. The protonated regular doubly fused hexaphyrin (non‐aromatic Hückel system) shows the least fragmentation even at higher collision energies. In general, cyclization through losses of HF molecules decreases from the aromatic Hückel systems to Möbius aromatic or Möbius‐like aromatic systems to non‐aromatic Hückel systems and is related to an increase in conformational distortion. Copyright © 2016 John Wiley & Sons, Ltd.     

100 Picosecond Diffraction Catches Structural Transients of Laser‐Pulse Triggered Switching in a Spin‐Crossover Crystal

We study by 100 picosecond X‐ray diffraction the photo‐switching dynamics of single crystal of the orthorhombic polymorph of the spin‐crossover complex [(TPA)Fe(TCC)]PF₆, in which TPA=tris(2‐pyridyl methyl)amine, TCC^2?=3,4,5,6‐Cl₄‐Catecholate^2?. In the frame of the emerging field of dynamical structural science, this is made possible by using optical pump/X‐ray probe techniques, which allow following in real time structural reorganization at intra‐ and intermolecular levels associated with the change of spin state in the crystal. We use here the time structure of the synchrotron radiation generating 100 picosecond X‐ray pulses, coupled to 100 fs laser excitation. This study has revealed a rich variety of structural reorganizations, associated with the different steps of the dynamical process. Three consecutive regimes are evidenced in the time domain: 1) local molecular photo‐switching with structural reorganization at constant volume, 2) volume relaxation with inhomogeneous distribution of local temperatures, 3) homogenization of the crystal in the transient state 100 µs after laser excitation. These findings are fundamentally different from those of conventional diffraction studies of long‐lived photoinduced high spin states. The time‐resolution used here with picosecond X‐ray diffraction probes different physical quantities on their intrinsic time‐scale, shedding new light on the successive processes driving macroscopic switching in a functionalized material. These results pave the way for structural studies away from equilibrium and represent a first step toward femtosecond crystallography.     

Spin crossover of ferric complexes with catecholate derivatives. Single-crystal X-ray structure, magnetic and Mössbauer investigations

Complexes of general formula [(TPA)Fe(R-Cat)]X.nS were synthesised with different catecholate derivatives and anions (TPA = tris(2-pyridylmethyl)amine, R-Cat2- = 4,5-(NO2)2-Cat2- denoted DNC(2-); 3,4,5,6-Cl4-Cat2- denoted TCC2-; 3-OMe-Cat(2-); 4-Me-Cat(2-) and X = BPh4-; NO3-; PF6-; ClO4-; S = solvent molecule). Their magnetic behaviours in the solid state show a general feature along the series, viz., the occurrence of a thermally-induced spin crossover process. The transition curves are continuous with transition temperatures ranging from ca. 84 to 257 K. The crystal structures of [(TPA)Fe(DNC)]X (X = PF6-; BPh4-) and [(TPA)Fe(TCC)]X.nS (X = PF6-; NO3- and n= 1, S = H2O; ClO4- and n= 1, S = H2O; BPh4- and n= 1, S = C3H6O) were solved at 100 (or 123 K) and 293 K. For those two systems, the characteristics of the [FeN(4)O(2)] coordination core and those of the dioxolene ligands appear to be consistent with a prevailing Fe(III)-catecholate formulation. This feature is in contrast with the large quantum mixing between Fe(III)-catecholate and Fe(II)-semiquinonate forms recently observed with the more electron donating simple catecholate dianion. The thermal spin crossover process is accompanied by significant changes of the molecular structures as shown by the average variation of the metal-ligand bond distances which can be extrapolated for a complete spin conversion from ca. 0.123 to 0.156 A. The different space groups were retained in the low- and high-temperature phases.     

Dynamic triggering of a spin-transition by a pulsed magnetic field

We report the first study of the effect of a high pulsed magnetic field on a spin transition complex in the solid state. The high spin fraction was determined by optical reflectivity. Sizeable effects are observed for the well-known spin transition solid Fe(Phen)₂(NCS)₂. In the hysteresis loop temperature range, an increase in the HS fraction is obtained, with an irreversible (reversible) character in the ascending (descending) branch of the loop. The time dependence of the HS fraction provides information on the kinetics of the spin-crossover process at the spin transition. Received 23 February 1999 and Received in final form 8 June 1999