3.2. Effect of Hydrogen Bonding on EPR spectral parameters
Experimental results suggest that EPR spectra are more sensitive to the
HFCC of the nitrogen atom and of the hydrogen atoms of the amino- and
methyl-groups in acidic solution. In addition, EPR spectra are more
sensitive to HFCC of the nitrogen atom, the -hydrogen atom, and the
hydrogen atoms of methyl group in alkali solution. Figs. 4 and 5 show
the average of the g -tensor and the HFCC of the atoms considering
200 snapshots along the trajectory and optimization structure in the gas
phase radicals, respectively. The solvent induces an increase ingxx (435 ppm) and in gyy(199 ppm) for the radicals but does not have a similarly significant
effect on their gzz (-84 ppm). Meanwhile,
solvation decreases the HFCCs [N (-9.84 MHz),
Havg.(amino) (-28.50 MHz) and
Havg.(methyl) (-12.27 MHz)].
For radicals, on the other hand, solvation decreasesgxx (-163 ppm) and gyy(-116 ppm) and have no significant effects on neithergzz (+13 ppm) nor on the HFCC of N (-2.55 MHz),
of Havg.(amino) (+0.76 MHz), and of Hα(+0.67 MHz)]. According to previous studies [18-25] and consistent
with Eqns. (2) and (3), the values of gxx ,gyy , and of atomic HFCC are strongly dependent on
the geometric parameters and on spin density.
<Fig. 4 >
<Fig. 5 >
As can be seen from Eq. (2), the main contribution of g -tensor
components is spin-orbit coupling. Considering the directions of theg -tensor components (Fig. 1) and the shape of the spin density
iso-surface, we observe significant contributions fromgxx and gyy but a
negligible contribution from gzz , yielding the
following order: gxx >
gyy > gzz (negligible).
The greater value of the spin density iso-surface of radical in
comparison with that for the and radicals (Fig. 1) we can argue that the
values of the g -tensor components of the radical are greater. It
may be concluded from the results displayed in Fig. 5 and listed in
Table 2 that the transfer from the gas to the solution phase is
accompanied by a decrease in the HFCC of atoms for radical since it
exhibits a lower spin density than the gas phase. In contrast, the
radical exhibits a spin density that remains quasi-constant on passing
from the gas to the solution phase and this is accompanied by the
constancy of the HFCC of the atoms of this radical. In the radicals, the
oxygens of carboxyl groups have more spin-orbit coupling due to the
highest spin-density on the oxygen atoms. This leads to an increase ing values in radical compared to their values for and , in
agreement with previous work such as in Ref. [30].
Variations of the spin density distribution on the oxygen atoms (which
have the most spin-orbital coupling) results in substantial variation of
the gxx and gyycomponents. Fig. 6 shows variation of g -tensor components versus
the magnetic moment of the oxygen atoms of the carboxylic groups in the
, and radicals. From the figure, the gxx(gyy ) components have a linear dependence with
Pearson correlation coefficients of 0.86 (0.76), 0.85 (0.43) and 0.80
(0.73) on the magnetic moment of the oxygen atom for the , and radicals,
respectively. In contrast, it is found that thegzz component is not linearly correlated
with the magnetic moment of the oxygen atoms of the carboxyl group as
the data exhibit a marked scatter. The little change of thegzz component is probably due to some
spin-orbital coupling of the oxygen atoms of the water molecules (since
these oxygen atoms form hydrogen bonds with the radicals toward
direction of the magnetic moment).
Therefore, in the case of radical, the transfer from the gas to the
solution phases results in an increase in the spin density of the oxygen
atoms of the carboxyl groups (Table 2) and, consequently, a concomitant
increase in the gxx andgyy components. On the contrary, the radical this
inter-phase transfer results in the reduction of thegxx and the gyy components
due to a decrease in the spin density on the oxygen atoms (Table 2).
<Fig. 6 >
The HFCCs is known to depend on the spin density at the nuclei of the
atom and is a direct measure of unpaired electron delocalization through
the Fermi contact term, dominant for s -electrons (the only
orbitals with a maximum rather than a node at the nucleus) - (see
discussions in Refs. [49-50]). Fig. 7 shows the correlation of the
HFCC of the methyl group and its magnetic moment for , and radicals with
a linear Pearson correlation coefficient of 0.97.
<Fig. 7 >
<Table 4 >
Table 4 collects the MD-averaged calculated isotropic HFCC andg -tensors for radicals along with their experimental counterparts
in acidic and alkaline solutions [27-28]. In the case of the
alkaline pH (11-13.5), the calculated HFCC for the N atom and for the
methyl group are in good agreement with experiment. A similarly good
agreement also exists between the calculated and experimental HFCC of
amino group and the isotropic giso .
At acidic pH (2.5-5), the calculated HFCC of the N atom and of the
methyl group for the have better agreement with the experiment compared
to the results obtained for the radical. On the other hand, the
calculation giso and the HFCC for the
Hα atom of is in better agreement with the experimental
results. Therefore, both radicals can exist in acid solution.