BIOINORGANIC CHEMISTRY A Short Course Second Edition

78 INSTRUMENTAL METHODS

1. Moving an electron from a ground state to an excited state ( Eelectronic ),
   leading to observations in the ultraviolet – visible spectroscopic region


2. Reorienting an electron in a magnetic fi eld ( Eelectron spin ) as seen in elec-
   tron paramagnetic resonance (EPR) spectroscopy


3. Reorienting a nuclear moment in a magnetic fi eld ( Enuclear spin ) as observed
   in nuclear magnetic resonance (NMR) spectrometry


4. Detecting energy absorbed or emitted in deforming bonds between
   atoms in molecules ( Evibration ) as observed in infrared or resonance Raman
   spectroscopy

The entire spectrum of electromagnetic resonance, as shown in Figure 3.2
adapted from Cowan,^3 is available for various physical methods. High - energy,
short - wavelength transitions occur at the left end of Figure 3.2 , whereas low -
energy, long - wavelength transitions occur at the right.

3.2 X - RAY ABSORPTION SPECTROSCOPY (XAS) AND

EXTENDED X - RAY ABSORPTION FINE STRUCTURE (EXAFS)

3.2.1 Theoretical Aspects and Hardware,

X - ray diffraction from crystalline samples can result in a complete three -
dimensional crystal structure of a molecule, but requires a single crystal suit-

Figure 3.2 Illustrated energy transitions for several useful regions of the electromag-
netic spectrum. (Adapted with permission of John Wiley & Sons, Inc. from Figure 2.2
of reference 3. Copyright 1997, Wiley - VCH.)

γ-ray

core electronic

transitions

valence electronic

transitions

X-ray ultra

violet

visible infrared microwave radio

nuclear

transitions

λ (m) 10 -12 - 10-10 10 -10 - 10-8 10 -8 - 10-6 10 -6 - 10-4 10 -4 - 10-2 10 -2 - 10+2

ν (Hz) 1018 - 10^201016 - 10 (^181014) - 10^161012 - 10^141010 - 10^12106 - 10^10
Nohνa
(Jmol-1)
108 - 10^10106 - 10^8104 - 10^6102 - 10^4100 - 10^210 -4 - 10^0
aNo = Avogradro's number = 6.023 x 10 23