Biradical state

In some systems, triplet BET can occur, as deduced from time-resolved optical spectroscopy, magnetic field effects, CIDNP, or optoacoustic calorimetry. Triplet BET is governed by energetic factors, which determine rates, and by the relative topologies of the potential surfaces of parent molecule, radical ions, and of accessible triplet or biradical states. Divergent topologies for different states may cause rearrangements. 

B. Valence isomerization. The conjugated polyenes in their excited biradical state can lead to intramolecular cyclization in a number of ways giving rise to a number of products. In such cyclization, valence bonds are reorganized without migration of atoms or groups but by migration of c or it electrons only. For example, on excitation of pentadiene, the following products are observed ... 

One curious case of the effect of light on electron-transfer equilibrium involves the reduction of ,p-di(t-butyl)stilbcnc with potassium in DME. The reaction leads directly to a diamagnetic dianion a solution of this dianion remains ESR silent unless subjected to ultraviolet irradiation by a Hg/Xe lamp. The anion radical of a,(3-di(t-butyl)stilbene then formed from the dianion by the loss of an electron. The electron reverted within 5-10 min after ultraviolet irradiation was turned off, transforming the anion radical into the dianion (Gerson et al. 1996). This case deserves to be clarified. Maybe the light effect consists simply in singlet-triplet transformation of the dianion, with the formation of some more or less stable biradical state of the dianion, which possesses two unpaired electrons and can even be a paramagnetic one. 

The formation of paramagnetic sites in the reactions of DOSGs with saturated hydrocarbon molecules indicate that, in one of the steps, the system is transformed into the biradical state. The 0-0 bond of the cycle is the most probable precursor for formation reactive in such processes. It can be assumed that, in both cases, the process occurs through the biradical state. However, in the first case, the low-molecular radical escapes from the reaction zone through the gas phase, and paramagnetic sites are formed. In the second case, the spatial separation of free radical sites is impossible, and they combine or disproportionate to form non-paramagnetic reaction products. 

The establishment of such an equilibrium should have two effects after a short flash the apparent lifetime of the radical-pair should vary with n (because of the 5ns lifetime, at most, of Chi and of the occurrence of other traps in the antenna), and the amount of radical-pair should decrease while excitation resides in the antenna. To check that hypothesis we have varied n, measuring the amount of biradical state and its lifetime in several PS-II preparations with a different antenna size (n=5 to 200) (Hansson et al., 1987). 

In the high-resolution ESR (326 GHz) study of the biradical state Qa - Qb - in the Rb. Spheroids, RC determines the exchange integral in the biradical (Jo = 109 s 1) (Calvo et al., 2001). Because the rate constant of electron transfer from Qa to Qb is essentially less (kET 104 s 1) (Feher et al., 1992 Xu et al, 2000) than expected for an nonadiabatic activationless ET and the kET values considerably deviate from the dependence of the supperexchange attenuation parameter (yET) on the distance between donor and acceptor centers in RCs (Fig. XXX), we can conclude that the ET is adiabatic and requires thermal activation. 

Keywords Internal Conversion, Intramolecular Charge Transfer, Biradical State, Nucleobases, tro1 ... 

To understand these highly interesting photophysical properties of nucleobases, we have carried out CIS and coupled cluster (CC) calculations of the potential energy profiles of cytosine and its derivatives at optimized CIS geometries [10]. The results indicate that the Sj S0 internal conversion occurs through a barrierless state switch from the initially excited 1 tttt state to a biradical state, which intersects... 

Figure 15-4. The energy of the Sj state of cytosine (left) and uracil (right) as a function of the C4C5C6N] dihedral angle. EOM-CCSD(T) values are shifted uniformly upward by the amount indicated in the figure. The insert shows the side view of the structure of the minimum of the biradical state of cytosine. (Reprinted with permission from Refs. [10] and [11].)...

It is interesting that the biradical geometries of the purine bases, resulting from the twisting of the N3-C2 bond, are very similar to the geometries of the nucleobases at the S1(tttt )/S0 conical intersections located by higher levels of theory [17-20], There is also a close geometrical similarity between the biradical state of pyrimidine... 

Figure 15-9. CIS and CC2/cc-pVDZ potential-energy profiles of the lowest energy tttt state of TMC as a function of CC-d V.C dihedral angle (left panel). CC2/cc-pVDZ potential-energy profiles of the 1 tttt" and biradical states of cytosine (right panel). The number (330 cm-1) refers to the energy barrier (CC2) for the state switch from the tttt minimum to the biradical state. The dihedral angle at the ground-state minimum is 0°. (Reprinted with permission from Ref. [23].)...

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