Umbrella inversion of a carboxylic radical anion centre,

1
Kjell Tage Øhman Einar Sagstuen
Umbrella inversion of a carboxylic radical anion
centre, a study by EPR/ENDOR spectroscopy and DFT
Department of Physics University of Oslo,
Norway
Single crystals of l-o serine phosphate were
X-irradiated and studied at 10 K and 77 K using
EPR and ENDOR techniques. Two anion radicals,
Radical I 10 K (fig.2) and Radical I 77 K (fig.4)
were detected and characterized as two different
geometrical conformations of the O6-protonated
reduction product (fig.1). Radical I 10 K is only
observed after irradiation at 10 K, and upon
heating to 40 K Radical I 10 K transforms rapidly
and irreversibly into Radical I 77 K. This
transition increases the isotropic ß-hfc for the
H(4) and the hydroxylic H(9) protons, in contrast
to the their much reduced anisotropic hfcs after
the transition. An inversion of the carboxylic
acid centre accompanied with other minor
geometrical adjustments explain the major changes
of the observed isotropic and anisotropic hfcs.
DFT calculations (B3LYP/6-31G(d)//B3LYP/6-311G(2
df,p)) give hcf tensors in good agreement with
experimental data, see figs.5-6 and the tables.
Figure 1 Subsequent to electron capture,
protonation occurs from a neighbouring amino
group The PES illustrates how protonation
mini-mizes the total energy.
10 K
10 K 77 K
77 K
Figure 4 Positive spin density distribution for
the partial energy optimized structure Radical I
77 K.
Figure 2 Positive spin density distribution for
the partial energy optimized structure Radical I
10 K.
Figure 3 EPR spectra for Radical I 10 K (top)
and Radical I 77 K. The sentral triplet (red) is
due to the H4 and H9 proton couplings. The
decarboxylated cation gives the complex resonance
signals on the flanks (black).
Figure 5 Dihedral angle scans without
relaxations for the hydroxyl proton H9 in Radical
I 10 K (left) and Radical I 77 K (right) showing
the correlation between the isotropic hfc and the
ansotroipc hfc.
Figure 6 Experimental and DFT calculated high
frequency ENDOR resonances through three
orthogonal planes of rotation.
Radikal I 10 K
Radikal I 77 K
increasing iso. hfc / decreasing aniso. hfc