No abstract

No abstract

The general principles discussed in Chapter 3 also apply to reactions of organometallic complexes. Because these systems do not have a wide range of structurally similar complexes with different metal atoms for comparative studies across the Periodic Table, comparisons are usually made down a particular group. However, there is a wide range of ligands available for studies of entering and leaving group effects. This area has been the subject of several recent reviews. A major difference from the systems discussed in Chapter 3 is that many of these complexes are soluble in organic solvents, including hydrocarbons. This can minimize the complicating factor of solvent coordination, but these solvents often have quite low dielectric constants so that various types of preassociation are more probable. The metal carbonyl family of compounds is typical of the range of structures and reactivities of organometallic complexes. The rate of CO exchange was examined in early studies, and this work is the subject of a recent review. The order of reaction rates is as follows: Where the rate law has been determined, the reaction is first-order in [M(CO)R] and zero-order in [CO]. This implies a D mechanism, since a solvent intermediate is unlikely for the "noncoordinating" solvents. This mechanism also is probable for other ligand substitutions. The main mechanistic exception to the above generalizations is V(CO)6, which has an Ia mechanism for PR3 substitution reactions. This compound is unique in that it is the only 17-electron metal carbonyl and also is by far the most labile. Some kinetic results for substitution on V(CO)6 in hexane are given in Table 5.1. The substitution rates have rather low ΔH* values, and the negative ΔS* values are typical of an associative process. The rates for various entering groups correlate with the basicity rather than the size, as measured by the cone angle. It has been suggested that formation of a 19-electron associative intermediate from a 17-electron reactant is much more favorable than a 20-electron intermediate from an 18-electron reactant.

No abstract

A new method for the selection of wavelengths from near infrared spectra using partial least squares(PLS) analysis is presented. The method aims to find wavelengths that produce significant improvements in PLS prediction accuracy over using all wavelengths. The method is based on data splitting and evaluation of the appropriate prediction errors. Analysis of interactance spectra of kiwifruit using three evaluation criteria are compared with the results obtained from full spectrum analysis and with the recently proposed feature selection method. Using the recommended criterion, the method was found to produce models with lower standard errors than the optimum model obtained using the feature selection method 87% of the time. Properties of initiating the search method from starting points selected by three procedures are compared and recommendations are given for selecting the initial wavelengths. The new search method also has a low probability of obtaining significant correlations through chance.

No abstract

Different measurement modes (reflectance, interactance and transmission) and spectral windows (500-1100 nm) were compared for their ability to predict non-destructively the harvest soluble solids content (SSC) and titratable acidity (TA) of Satsuma Mandarin (Citrus reticulata cv. "Miyagawa"). A direct transmission measurement mode (source and sensor opposite sides of fruit) coupled with a single spectral window from 700 to 930 nm delivered the most accurate SSC predictions (R 2 = 0.93, RMSEP = 0.32%). Accurate TA prediction was not possible with any measurement mode and the predictive ability that was achieved (best results: R 2~0 .65, RMSEP~0.15%) was only afforded indirectly through a correlation with the skin chlorophyll changes that occur as the fruit mature.

No abstract