TIME-RESOLVED EPR-SPECTRA OF SHORT BIRADICALS (1,3 TO 1,8) IN SOLUTION - THE INTERCONNECTING ROLES OF THE EXTERNAL MAGNETIC-FIELD, DIPOLAR COUPLING, EXCHANGE INTERACTION, AND MOLECULAR-MOTION IN THE SPIN POLARIZATION AND RELAXATION MECHANISMS
Mde. Forbes et Sr. Ruberu, TIME-RESOLVED EPR-SPECTRA OF SHORT BIRADICALS (1,3 TO 1,8) IN SOLUTION - THE INTERCONNECTING ROLES OF THE EXTERNAL MAGNETIC-FIELD, DIPOLAR COUPLING, EXCHANGE INTERACTION, AND MOLECULAR-MOTION IN THE SPIN POLARIZATION AND RELAXATION MECHANISMS, Journal of physical chemistry, 97(50), 1993, pp. 13223-13233
Time-resolved electron paramagnetic resonance spectra of biradicals in
solution with chain lengths of 3-8 carbon atoms are reported. Spin po
larization arises from the spin-correlated radical pair (SCRP) mechani
sm and in some cases also from the triplet mechanism (TM). The observa
tion of spectra at two magnetic fields, X-band (3375 G) and Q-band (12
500 G), provides qualitative estimates of the relative importance of
each mechanism for a given biradical. The influence of several magneti
c properties, such as the spin-spin coupling J, the dipolar coupling D
, and the electron-nuclear hyperfine interactions a(H), on the time de
pendence and spectral shape is discussed. The sip and magnitude of D i
n the precursor triplet state play a strong role in the degree of TM p
olarization observed in the biradical EPR spectrum. In the biradical,
the ratio of J to the Zeeman energy and that of a(H) to J determine th
e sign and the magnitude of SCRP polarization. Molecular orbital calcu
lations support the observation that the value of J is lowered by delo
calization of electron density. The biradical also has a dipolar inter
action, and it is shown how this term and the rotational orientation t
ime tau(c) can contribute to spin relaxation. Observation of 1,4-bis(b
enzyl) and photoenol type biradicals was hindered by intermolecular re
actions. The effects of conformational flexibility and tumbling on the
polarization and relaxation mechanisms are discussed. The Closs birad
ical 7 is a special case, with a triplet ground state and an encounter
rate k(en) defined by a vibrational normal mode rather than a series
of conformational jumps.