Triplet spin density

FIGURE 10.7 Ketosulfone derivatives plot of the triplet spin density of the p-phenyl... 

The triplet spin density distribution along the polyene chain of dodecapentaenal has been calculated using the semiempirical MINDO method (Kok and Groenen, 1996). The spin density at carbon 2 has been measured with ESEEM on deuterated molecules and agrees weU with these calculations and validates them (Kok and Groenen, 1996). 

Figure 36 The singlet HOMO, LUMO, and triplet spin density surfaces of (ppz)2lr(bpy)+ are shown.

The CNDO and CNDO/S methods apply the ZDO approximation to all integrals, regardless of whether the orbitals are loeated on the same atom or not. In the INDO method, whieh was designed to improve the treatment of spin densities at nuelear eenters and to handle singlet-triplet energy differenees for open-shell speeies, exehange integrals... 

The consistent total energy makes it possible to compute singlet-triplet gaps using RHF for the singlet and the half-electron calculation for the triplet. Koopman s theorem is not followed for half-electron calculations. Also, no spin densities can be obtained. The Mulliken population analysis is usually fairly reasonable. 

The EPR spectrum of the ethyl radical presented in Fig. 12.2b is readily interpreted, and the results are relevant to the distribution of unpaired electron density in the molecule. The 12-line spectrum is a triplet of quartets resulting from unequal coupling of the electron spin to the a and P protons. The two coupling constants are = 22.38 G and Op — 26.87 G and imply extensive delocalization of spin density through the a bonds Note that EPR spectra, unlike NMR and IR spectra, are displayed as the derivative of absorption rather than as absorption. 

First, try to draw resonance contributors for both ground state and triplet anthrone. Then display a spin density surface for the triplet state of anthrone. (Note that the spin density surface shows the location of both unpaired electrons, one of which may be in a 7t orbital and one of which may be in a o orbital.) Where are the two unpaired electrons Are they localized or delocalized Given that spin delocalization generally leads to stabilization, would you expect the triplet state of anthrone to be stable ... 

Spin density surface for triplet anthrone locates the two unpaired electrons. 

CNDO/2 for predicting molecular geometries, it gave a vastly superior treatment of singlet-triplet splittings and spin densities. 

A very similar application of the modified Bloch equations was based in the work of Adams and Connelly.4 ESR spectra (Figure 5.8) of [Mo P(0 Me)3 2(MeC = CMc)Cp] show the expected triplet (two equivalent 31P nuclei) at 280 K, but only a doublet at 160 K. At intermediate temperatures, the lines broaden. The interpretation is that the alkyne undergoes a pendulum oscillation, which in the extrema diverts spin density from one or the other phosphite. Interestingly, the diamagnetic cation undergoes a similar motion on the NMR time scale, but then the alkyne undergoes a complete rotation. Thus, analysis of the effect leads to a measure of the rate of the oscillation. The... 

The detection of zero-field splitting for dianions of [18] or [21] is very important it reveals not only the existence of a triplet state, but it also provides information on the mode of spin density distribution. Even more... 

The third intra-pair reaction to be discussed involves bond formation between radical anion and cation without intervening transfer both singlet and triplet radical ion pairs can couple. For example, the bifunctional radical cation 24 generates two chloranil adducts, most likely via zwitterions (e.g., 74 and 75 ), initiated by forming a C O bond. The CIDNP results indicate that 74 and 75 are formed from a singlet radical ion pair. Adduct 75 is a minor product, as the major spin density of 24 + is located in the allyl function which, therefore, is expected to be the principal site of coupling. 

In spite of those highly favorable stmctural factors, 16a is very ephemeral. Its lifetime in degassed benzene is 0.5 ps, which is shorter even than that of parent triplet DPC. Product analysis smdies have shown that 16a forms a trimer of dianthrylcarbene (116) as the main product (50-60%) (Scheme 9.37). The trimer is the one formed as a result of a threefold coupling at position 10 of the carbene. This observation suggests that delocalization of the unpaired electrons in 16a leads to their leaking out from the carbene center to position 10, where sufficient spin density builds up for the trimerization to take place. At the same time, the lack of formation of olefin-type dimers through coupling of two units of 16a at their carbene centers indicates that the carbene center itself is indeed well shielded and stabilized. 

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