Low-temperature data collection

Present-day diffraction facilities provide easy access to very low-temperature data collection and hence to an accurate determination of electron densities in crystals. Application of standard theorems of classical physics then provides an evaluation of the Coulombic interaction energies in crystal lattices [27]. These calculations are parameter-less and hence are as accurate as the electron density is. Moreover, for highly polar compounds, typically aminoacid zwitterions and the like, a fortunate coincidence cancels out all other attractive and repulsive contributions, and the Coulombic term almost coincides with the total interaction energy. 

In [59] the authors reported the structure of a tri-osmium complex containing a hydride and clearly stated that a low temperature X-ray diffraction experiment would not be useful to locate the hydride if an accurate absorption correction is not carried out. Curiously, a few years before they had contacted Prof A. Sironi and myself at the University of Milan proposing a low temperature data collection on that compound, with the purpose of locating the not so clearly visible hydride. As evident from [59], we were able to convince them on the real problems connected with the location of hydrogens close to heavy metals. 

If it can not be avoided, disorder may be reduced or its effects ameliorated by low-temperature data collection, which is almost always to be recommended. Modern crystallographic software provides many effective and powerful methods for modeling disorder, even in quite complex manifestations, and the judicious application of refinement techniques such as sensible geometrical constraints and restraints, and an appropriate treatment of atomic displacement parameters, can often salvage what would otherwise be a structural mess. 

When low-temperature studies are performed, the maximum resolution is imposed by data collection geometry and fall-off of the scattered intensities below the noise level, rather than by negligible high-resolution structure factor amplitudes. Use of Ag Ka radiation would for example allow measurement of diffracted intensities up to 0.35 A for amino-acid crystals below 30 K [40]. Similarly, model calculations show that noise-free structure factors computed from atomic core electrons would be still non-zero up to O.lA. 

The advantages of measuring at very low temperatures are well established [4], Because of the increased speed of data collection, it now becomes feasible to consider the use of liquid helium as a cryogen. A prototype open-flow helium cooling device using mainly off-the-shelf components has been developed [3],... 

Nitromalonamide (C3H,N304, Ri = R3 = NH2, R2 = N02) was chosen as a further example of a very short intramolecular keto-enol 0- H 0 hydrogen bond. It has one of the shortest known 0- H 0 distances at 2.38 A[5]. Both low-temperature X-ray (10 K) and neutron data sets (15 K) have been collected to examine whether our results from benzoylacetone are of a general nature. 

Accordingly, the bulk of data collected over the different catalytic systems showed that Pt catalyses the reduction of stored NO already at low temperature, and that the copresence of Pt and Ba on the same support is required for the occurrence of this route. 

In a study on another manganese enzyme, glutathione transferase, the Hoffmann group has proposed Q-band dispersion EPR at the unusually low temperature of 2 K as the optimal approach to collect data from Mn11 centers with D hv (Smoukov et al. 2002). This proposal would be practically limited by the fact that Q-band spectrometers running at 2 K can be counted on the fingers of one finger however, dispersion spectra are readily obtained in Q-band also at helium-flow temperatures (i.e., T>4.2K). 

However, the detailed description of the FT product distribution together with the reactant conversion is a very important task for the industrial practice, being an essential prerequisite for the industrialization of the process. In this work, a detailed kinetic model developed for the FTS over a cobalt-based catalyst is presented that represents an evolution of the model published previously by some of us.10 Such a model has been obtained on the basis of experimental data collected in a fixed bed microreactor under conditions relevant to industrial operations (temperature, 210-235°C pressure, 8-25 bar H2/CO feed molar ratio, 1.8-2.7 gas hourly space velocity, (GHSV) 2,000-7,000 cm3 (STP)/h/gcatalyst), and it is able to predict at the same time both the CO and H2 conversions and the hydrocarbon distribution up to a carbon number of 49. The model does not presently include the formation of alcohols and C02, whose selectivity is very low in the FTS on cobalt-based catalysts. 

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