ESR Studies on Ketyl Type Radical-Amine Complexes
J. Phys. Chem., Vol. 100, No. 24, 1996 10025
and zero for tertiary. We attributed the nubmer of H atoms of
amine to the factor controlling the magnitude of Ke. The
complex is produced through the hydrogen bond formation
between N atom of amine and H atom of OH group in ketyl.
Thus, the nonbonding electrons of N atom facing the H atom
of OH group are important. It is well-known that the structure
of amine depends on the degree of hydrogen substitution.
Primary amine has more pyramidal geometry than does tertiary
amine. This means that the electron density of nonbonding
orbital of N atom on the side of R-HB is the richest in primary
amine. Thus the bond is more tightly formed in R-HB-primary
amine case. This is in good consistence with the dependence
of Ke value.
Figure 9. Plots of aOH value vs free energy difference of complex
formation reaction.
Comparison of the Complexes of r-HB and HDPM. The
value of Ke is found to be smaller for HDPM complexes (<100
M-1) than that for R-HB complexes (167-1430 M-1). The
pKa of HDPM and R-HB are considered to be very similar since
the pKa of HDPM is 9.25 and that of acetophenone ketyl radical,
which has very similar chemical structure to that of R-HB, is
about 10.18 If the ability of proton donation controls the Ke
value, HDPM complex should show larger Ke value than R-HB
complexes. Thus, the difference in Ke value cannot be explained
by the difference in the ability of proton donation. The small
Ke value for HDPM may be interpreted by considering the
structural hindrance: R-HB has one bulky phenyl group while
HDPM has two. The extra phenyl group of HDPM will refuse
to form the complex, which results in relatively small Ke vlaue
in the HDPM complex.
The behavior of aOH value of R-HB against amines is different
from that of HDPM. aOH of HDPM in benzene is 2.5 G while
those in TEA and SBA complexes are 5.3 and 3.9 G,
respectively. The value itself increases when HDPM forms the
complex with TEA and SBA, which is the same result with
R-HB case. However, the aOH value of R-HB in the complexes
decreases from primary to tertiary amines while aOH for the
HDPM complex with SBA is smaller than that with TEA. The
behavior of aOH of the HDPM complex is fairly complicated
and no systematic explanation could be found. The extra phenyl
ring of HDPM may generate the slight difference of the averaged
position of the H atom in OH group depending on the structure
of countered amine. This would result in different dependence
of aOH value of the HDPM-amine complex.
of hfcc of â proton,7 aOH follows the equation aOH ) (B0 + B2
cos2 θ)FCR, where B2 is the coefficient reflecting the spin density
arising from hyperconjugation, B0 is due to other mechanism
and is generally negligible compared to B2, FC is the spin
R
population at the carbon atom, and θ is the dihedral angle
between the plane through C-O bond and the p orbital at carbon
atom and the plane through COH. According to this mecha-
nism, the aOH value depends on the averaged position of H
against molecular plane. On the other hand, the second
mechanism does not depend on the orientation between H and
the molecular plane. Since the aOH value of R-HB is close to
zero, it may be concluded that positive and negative contribu-
tions from C and O atoms cancel one another in bare R-HB.
This was confirmed by the temperature dependence of the aOH
value. The OH group may be torsionally oscillating out of the
molecular plane and the decrease of temperature restricts the
torsional oscillation to reduce the contribution of the first
mechanism. This mechanism was suggested for 9-hydroxy-
fluorenyl radical2 whose aOH showed similar negative temper-
ature dependence. Thus, the aOH value of negative sign
increases with decreasing temperature. The complex formation
may restrict the torsional motion of OH group similar to the
decrease of temperature in the bare R-HB system and positive
contribution decreases. This results in the increase of aOH value
in the complex formation.
Nature of the Bond of Complexes. As seen in Table 3, it
is remarkable that the formation constant shows various values
depending on aliphatic amines. It is very interesting that Ke is
relatively large, 919-1430 M-1 for primary amines and 410-
Acknowledgment. The present work is partly supported by
the Grant-in-Aid on Priority-Area-Research on Photoreaction
Dynamics from the Ministry of Education, Science, Sports, and
Culture of Japan (No. 06239103).
663 M-1 for secondary amines, and is small, 167-345 M-1
,
for tertiary amines. This suggests that stabilization of the
complex is the biggest in the complex with primary amines and
the smallest with tertiary amines. Another remarkable result is
that the value of aOH of R-HB is the largest in the complex
with primary amines while the smallest with tertiary amines.
This indicates that the hydrogen bond between amine and R-HB
depends on the degree of hydrogen substitution on the nitrogen
atom of the amine. The pKa value of amine might explain the
dependence of Ke and aOH. However, this is excluded because
pKa of cyclohexyl-, diethyl-, and triethylamines, which are
typical of primary, secondary, and tertiary amines, are 10.64,
10.93, and 10.72 in aqueous solution, respectively, and no
significant difference is found among the three. IP of amine is
another possible factor to control the Ke and aOH values. If
charge transfer interaction contributes to the complex formation,
tertiary amines should have larger Ke value than primary amines,
because IP of tertiary amines is generally smaller than that of
primary amines. However, the result is in the reverse of this
prediction and IP is excluded from the factor. The remarkable
difference among amines is the number of hydrogen (H) atoms
attached on the nitrogen: two for primary, one for secondary,
References and Notes
(1) Fisher, H. Z. Naturforsch. 1965, 20, 488.
(2) Wilson, R. J. Chem. Soc. (B) 1968, 84.
(3) Wilson, R. J. Chem. Soc. (B) 1968, 1581.
(4) Davidson, R. S.; Wilson, R. J. Chem. Soc. (B) 1970, 71.
(5) Livingston, R.; Zeldes, H. J. Chem. Phys. 1966, 44, 1245.
(6) Miyagawa, K.; Murai, H.; I’Haya, Y. J. Chem. Phys. Lett. 1985,
118, 140.
(7) Atherton, N. M. Principles of Electron Spin Resonance; Ellis
Horwood Ltd.: New York, 1993.
(8) Kawai, A.; Kobori, Y.; Obi, K. Chem. Phys. Lett. 1993, 215, 203.
(9) Sheppard, N. Hydrogen Bonding; Hadzi, D., Ed.; Pergamon: New
York, 1959. Mataga, N.; Kubota, T. Molecular Interactions and Electronic
Spectra; Marcel Dekker: New York, 1970. Nagakura, S. J. Am. Chem.
Soc. 1958, 80, 520.
(10) Kajii, Y.; Itabashi, H.; Shibuya, K.; Obi, K. J. Phys. Chem. 1992,
96, 7244.
(11) Cocivera, M.; Trozzolo, A. M. J. Am. Chem. Soc. 1970, 92, 1772.
Closs, G. L.; Paulson, D. R. J. Am. Chem. Soc. 1970, 92, 7229.
(12) Koyanagi, M.; Futami, H.; Mukai, M.; Yamauchi, S. Chem. Phys.
Lett. 1989, 154, 577.