This
paper explores the strengths and limitations of valence-to-core X-ray
emission spectroscopy (V2C XES) as a probe of coordination environments.
A library was assembled from spectra obtained for 12 diverse Cr complexes
and used to calibrate density functional theory (DFT) calculations
of V2C XES band energies. A functional dependence study was undertaken
to benchmark predictive accuracy. All 7 functionals tested reproduce
experimental V2C XES energies with an accuracy of 0.5 eV. Experimentally
calibrated, DFT calculated V2C XES spectra of 90 Cr compounds were
used to produce a quantitative spectrochemical series showing the
V2C XES band energy ranges for ligands comprising 18 distinct classes.
Substantial overlaps are detected in these ranges, which complicates
the use of V2C XES to identify ligands in the coordination spheres
of unknown Cr compounds. The ligand-dependent origins of V2C intensity
are explored for a homologous series of [CrIII(NH3)5X]2+ (X = F, Cl, Br, and I) to rationalize
the variable intensity contributions of these ligand classes.
Conceptually, a detection threshold represents the lowest concentration at which an individual or a group of individuals can reliably perceive a given stimulus, with a commonly used operational definition of 50% performance above chance. Estimated detection thresholds (DTs), however, are often reported in the literature with little attention given to the matrix in which the stimuli were evaluated. Here, we highlight the influence of matrix effects on DTs for two odor-active compounds commonly found in Vitis Labrusca wines. Differences in orthonasal DTs for methyl anthranilate (MA) and 2-aminoacetophenone (2AAP) in water, a model wine system, and wine were demonstrated using a within-subject design and forced choice (i.e., criterion free) psychophysical methods. Six sample triads, each containing two blanks and one spiked sample, were presented to participants with the instructions to choose the “different” sample, and this was repeated in different matrices (water, model wine, and wine). The estimated DTs for both compounds were significantly lower in water versus the model wine system and wine. This finding recapitulates the strong need to carefully consider the nature of the delivery matrix when determining and comparing threshold estimates across studies. Additionally, data from prior reports have suggested DTs for MA and 2AAP may differ by two orders of magnitude in spite of their structural similarity. We failed to confirm this difference here: although 2AAP thresholds were somewhat lower than MA thresholds, differences were much smaller than what had been suggested previously. This, again, emphasizes the need to make comparisons within the same individuals, using appropriate methods with sufficient numbers of participants.
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