Spin selectivity

Spectroscopists observed that molecules dissolved in rigid matrices gave both short-lived and long-lived emissions which were called fluorescence and phosphorescence, respectively. In 1944, Lewis and Kasha [25] proposed that molecular phosphorescence came from a triplet state and was long-lived because of the well known spin selection rule AS = 0, i.e. interactions with a light wave or with the surroundings do not readily change the spin of the electrons. 

Because of this spin selection rule, atoms which get into the lowest triplet state, 2 Si, do not easily revert to the ground 1 state the transition is forbidden by both the orbital and spin selection rules. The lowest triplet state is therefore metastable. In a typical discharge it has a lifetime of the order of 1 ms. 

At time [Case (2)] therefore, the hj rfine energy is approximately equal to the energy difference between the S and T states and can provide the driving force for T-S mixing. Now the h3q>erfine constants and Oj are a function of both nuclear and electronic spin states and thus one particular nuclear spin state for Hj and Hj will induce the T-S mixing more readily than the other. Thus nuclear spin selection occurs during the transition between S and T manifolds. However, this would yield no... 

The simple rules refer to nuclear spin selection during mixing of Tq and S states of the radical pair. If mixing of T+j and S states occurs instead, entirely different rules apply as discussed in Section IV (Kaptein, 1971b, 1972a Kaptein and den Hollander, 1972 Garst et al., 1971 J. I. Morris et al, 1972). 

Net F-polarization (A at 56-4 MHz) has been observed during the reaction of p-fluorobenzyl chloride with sodium naphthalene in tetra-hydrofuran solution to give p, p -difluorobibenzyl (Rakshys, 1971). The spin selection is believed to take place within a radical pair of the -------------------------------F... 

Regardless of the nature of the space parts, Q vanishes if V spin V spm- If Q vanishes, so does /. Thus we have the so-called spin-selection rule which denies the possibility of an electronic transition between states of different spin-multiplicity and we write AS = 0 for spin-allowed transitions. Expressed in different words, transitions between states of different spin are not allowed because light has no spin properties and cannot, therefore, change the spin. 

The first two terms in the expansion are strictly zero because of the spin selection rule, while the last two are non-zero, at least so far as the spin-selection rule is concerned. So a spin-forbidden transition like this, X VT , can be observed because the descriptions X and are only approximate that is why we enclose them in quotation marks. To emphasize the spin-orbit coupling coefficients for the first row transition elements are small, the mixing coefficients a and b are small, and hence the intensities of these spin-forbidden transitions are very weak. 

Behler J, Delley B, Lorenz S, Reuter K, SchefflerM. 2005. Dissociation of O2 at Al(lll) The role of spin selection rules. Phys Rev Lett 94 036104. 

A further technique exists for the determination of triplet energy levels. This technique, called electron impact spectroscopy, involves the use of inelastic scattering of low-energy electrons by collision with molecules. The inelastic collisions of the electrons with the molecules result in transfer of the electron energy to the molecule and the consequent excitation of the latter. Unlike electronic excitation by photons, excitation by electron impact is subject to no spin selection rule. Thus transitions that are spin and/or orbitally forbidden for photon excitation are totally allowed for electron impact excitation. 

Thus the change in the direction of the spin angular momentum of the electron effectively imparts some singlet character to a triplet state and, conversely, triplet character to a singlet state. This relaxes the spin selection rule since J S St dr is no longer strictly zero. The greater the nuclear charge,... 

Concerted fragmentation of the transition state in the peroxy radical recombination yields carbonyl compound molecules in the excited triplet state, alcohol in its singlet ground state, and oxygen in its triplet ground state, in fulfilment of the spin selection rules. 

A.J. Bard, University of Texas The mechanism you propose implies that there are spin selection rules operative which affect the relative rates of the electron transfer reactions. Is there any evidence that such spin selection rules are important in these kinds of reactions, especially in the presence of metallic centers ... 

J.R. Bolton We have not carried out any experiments as yet on metalloporphyrins linked to quinones. The spin selection rules should be operative in the radical pair. The singlet state of the radical pair should be able to return to the ground state with no spin inhibition however, the triplet state of the radical pair can return to the ground state only via spin interconversion or via the triplet state of the porphyrin. 

Ray SG, Daube SS, Leitus G, Vager Z, Naaman R (2006) Chirality-induced spin-selective properties of self-assembled monolayers of DNA on gold. Phys Rev Lett 96(3) 036101... 

In the lowest optically excited state of the molecule, we have one electron (t u) and one hole (/i ), each with spin 1/2 which couple through the Coulomb interaction and can either form a singlet 5 state (5 = 0), or a triplet T state (5 = 1). Since the electric dipole matrix element for optical transitions H em = (ep A)/(me) does not depend on spin, there is a strong spin selection rule (A5 = 0) for optical electric dipole transitions. This strong spin selection rule arises from the very weak spin-orbit interaction for carbon. Thus, to turn on electric dipole transitions, appropriate odd-parity vibrational modes must be admixed with the initial and (or) final electronic states, so that the weak absorption below 2.5 eV involves optical transitions between appropriate vibronic levels. These vibronic levels are energetically favored by virtue... 

From an historical point of view, the earliest indication of spin-selective reactivity of carbenes was exhibited by the stereochemistry of the cyclo-propanation reaction. The Skell Hypothesis (Skell and Woodworth, 1956) suggests that a spin-prohibition requires the addition of a triplet carbene to an olefin to occur in at least two steps. In turn, the obligatory formation of an... 

Spin selection rule An electronic transition takes place with no change in the total electron spin - that is, AS = 0 - hence singlet <- triplet transitions are forbidden or very weakly allowed. For example, the S0 —> Ti transition in anthracene has a molar absorption coefficient, emax, some 108 times less than that corresponding to the S0 —> Si transition. 

Spin selection rule The spin selection rule, AS = 0, specifies that there should be no change in the spin multiplicity. Weak spin-forbidden bands may occur when spin-orbit coupling is possible. Spin-forbidden transitions are more intense in complexes of heavy atoms as these lead to a larger spin-orbit coupling. 

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