NEXAFS Sensitivity to Bond Lengths in Complex Molecular Materials: A Study of Crystalline Saccharides

Detailed
analysis of the C K near-edge X-ray absorption fine structure
(NEXAFS) spectra of a series of saccharides (fructose, xylose, glucose,
galactose, maltose monohydrate, α-lactose monohydrate, anhydrous
β-lactose, cellulose) indicates that the precise determination
of IPs and σ* shape resonance energies is sensitive enough to
distinguish different crystalline saccharides through the variations
in their average C–OH bond lengths. Experimental data as well
as FEFF8 calculations confirm that bond length variations in the organic
solid state of 10–2 Å can be experimentally
detected, opening up the possibility to use NEXAFS for obtaining incisive
structural information for molecular materials, including noncrystalline
systems without long-range order such as dissolved species in solutions,
colloids, melts, and similar amorphous phases. The observed bond length
sensitivity is as good as that originally reported for gas-phase and
adsorbed molecular species. NEXAFS-derived molecular structure data
for the condensed phase may therefore be used to guide molecular modeling
as well as to validate computationally derived structure models for
such systems. Some results indicate further analytical value in that
the σ* shape resonance analysis may distinguish hemiketals from
hemiacetals (i.e., derived from ketoses and aldoses) as well as α
from β forms of otherwise identical saccharides.