A GEOMETRIC REPRESENTATION OF NUCLEAR MODULATION EFFECTS - THE EFFECTS OF HIGH ELECTRON-SPIN MULTIPLICITY ON THE ELECTRON-SPIN ECHO ENVELOPE MODULATION SPECTRA OF MN-2-RAS P21( COMPLEXES OF N)
Rg. Larsen et al., A GEOMETRIC REPRESENTATION OF NUCLEAR MODULATION EFFECTS - THE EFFECTS OF HIGH ELECTRON-SPIN MULTIPLICITY ON THE ELECTRON-SPIN ECHO ENVELOPE MODULATION SPECTRA OF MN-2-RAS P21( COMPLEXES OF N), The Journal of chemical physics, 98(9), 1993, pp. 6704-6721
A theoretical treatment is presented for the analysis of ESEEM spectra
of I = 1/2 nuclei coupled to an electron spin of high multiplicity, w
ith specific attention to the case of S = 5/2. This treatment is shown
to account for the observed spectral behavior of N-15 and P = 31 nucl
ei coupled to Mn2+ in a GDP complex with the protein N-ras p21. The tr
eatment involves the decomposition of the multilevel electron spin sys
tem into a set of noninteracting two level systems, an approximation t
hat is valid when the dispersed EPR spectral width is large compared t
o the microwave excitation bandwidth. The consequent spectral selectiv
ity of the microwave excitation is accounted for, in ESEEM simulations
, by attaching a weight to the ESEEM subspectra associated with each E
PR transition, and calculating the total ESEEM spectrum as a weighted
superposition of the subspectra. The simplest means of estimating the
appropriate weight factors-identifying them with the cw EPR intensity
of each transition, as deduced by simulation of the EPR spectra-leads
to ESEEM simulations that account for the key features of the observed
spectra, in particular, features that are peculiar to high multiplici
ty spin systems. In the studied Mn2+ system, no clear indication of or
ientation selective effects were found. A simple geometric representat
ion is presented which enables the facile understanding of ESEEM spect
ra of nuclear spin I = 1/2 coupled to an electron spin of high spin mu
ltiplicity in orientationally disordered solids. Analytical expression
s are derived for the ESEEM frequencies, frequency dispersions and amp
litudes. It is shown that in these systems external field variation ca
n lead to an array of spectral line-narrowing and amplitude resonance
phenomena analogous to those observed in S = 1/2 systems.