Triplet-ESR spectroscopy

The extreme sensitivity of the ZFS-components to small changes in the electron distribution provides the basis for the application of the triplet-esr spectroscopy as an analytical tool. A prerequisite for such applications is the assignment of the ZFS-components to the molecular axes. In the past this goal has been achieved by incorporating the molecules of interest in a suitable host single crystal [80] or by a magneto-photoselection technique [81]. 

Many early studies in ESR spectroscopy were concerned with anions, cations and triplet states derived from conjugated molecules. The unpaired electron(s)... 

UV-vis spectra of matrix-isolated intermediates are not so informative as matrix IR spectra. As a rule, an assignment of the UV spectrum to any intermediate follows after the identification of the latter by IR or esr spectroscopy. However, UV-vis spectra may sometimes be especially useful. It is well known, for example, that the energy of electronic transitions in singlet ground-state carbenes differs from that of the triplet species. In this way UV spectroscopy allows one to identify the ground state of the intermediate stabilized in the matrix in particular cases. This will be exemplified below. 

From the point of view of ESR spectroscopy, the distinction between molecules with one unpaired electron and those with more than one lies in the fact that electrons interact with one another these interactions lead to additional terms in the spin Hamiltonian and additional features in the ESR spectrum. The most important electron electron interaction is coulombic repulsion with two unpaired electrons, repulsion leads to the singlet-triplet splitting. As we will see, this effect can be modeled by adding a term, JS St, to the spin Hamiltonian,... 

Since most of the carbenes 1 have triplet ground states, ESR spectroscopy allows to see the unpaired electrons and determine the local symmetry at the carbene center and the amount of spin delocalization.13-18 Most of the ESR spectra of carbenes reported in the literature have been recorded in organic glasses or powder samples at temperatures between 4 and 77 K. Many carbenes are slightly colored and exhibit characteristic absorptions extending to the visible region of the spectrum. UV/vis spectroscopy not only provides information on the excited states of carbenes, which in many cases are the reactive species during precursor photolyses, but also links low temperature spectroscopy to LFP in solution at room temperature. 

Attempts to detect a thermally populated triplet state ( A ) of 8 by ESR spectroscopy were unsuccessful. This was attributed to the high reactivity of the diradical, which presumably easily abstracts hydrogen atoms in hydrocarbon matrices (to form p-quinodimethane), even at very low temperatures. In this context, the triplet state of /7-phenylenebis(phenylmethylene) has been observed. Apparently, substitution of the carbenic hydrogens of 8 by phenyl groups confers sufficient stability (thermodynamic and perhaps kinetic) to the biradical, which allows its observation. According to variable-temperature ESR spectroscopy, the triplet state of /7-phenylenebis(phenylmethylene) is thermally populated and the singlet state lies 0.5-1 kcal/mol lower in energy [76-79]. 

A molecular structure, similar to that of hexasilylated benzene derivative 229, was obtained from tetracyclic hexasilylbenzene 230 by Kira, Sakurai and coworkers, where the six silicon centres are incorporated in three five-membered ring systems (Scheme 80) °. In dimetalated compound 231, two lithium centres, coordinated by a quinuclidine ligand each, are capping the phenyl ring plane from both sides in the solid state. Moreover, it could be found that compound 231 has a thermally accessible triplet state, investigated by temperature-dependent ESR spectroscopy °°. 

A crucial methodological step forward was the discovery " that one could observe weU-defined electron spin resonance (ESR) spectra of frozen solutions of triplet species in random orientation. By the early 1960s, spectra of the triplet states of a number of carbenes had been recorded. Thus, when Dowd showed that photolysis of frozen matrices of the diazene (11) or the ketone (12) (Scheme 5.1) gave TMM (4), the spectroscopic tools for the characterization of this key non-Kekule compound lay to hand. Trimethylenemethane was the first non-Kekule molecule to be identified by ESR spectroscopy. 

A non-Kekule molecule conceptually formed by fusion of two TMM units and also predicted" " to have a triplet ground state is 2,4-dimethylenecyclo-butane diyl (20), which ultimately was prepared by two independent syntheses. Matrix ESR spectroscopy and gas-phase photodetachment photoelectron spectroscopy (PES) (see Section 4.1.4) eventually agreed that the ground state is triplet. 

In solution, the triplet biradical 14b dimerizes, and the dimeric products are formed with strong chemically induced nuclear polarization. The absolute rate of the dimerization at 146 K, as monitored in viscous solution by ESR spectroscopy, is just about that predicted by the spin-corrected encounter frequency under those conditions. The cycloaddition of the triplet with a typical alkene, acrylonitrile, also can be followed in this way. 

So far, there have been no experimental tests of the subtle distinction between the parity and disjoint mnemonics on the parent PMP, which remains an unknown substance. However, a number of PMP derivatives have been synthesized, starting with the dicarbene 58, a disjoint, equal-parity structure that was studied by Itoh as early as and shown by ESR spectroscopy to consist of both triplet and... 

ESR spectroscopy (a) J. E. Wertz and J. R. Bolton, in Electron Spin Resonance Elementary Theory and Practical Applications McGraw-Hill, New York, 1972. (b) J. A. Berson, in The Chemistry of the Quinonoid Compounds, Vol. II, S. Patai and Z. Rappoport, Eds., John Wiley Sons, Inc., New York, 1988. (c) W. Gordy, in Theory and Applications of Electron Spin Resonance, Vol. 15, A. Weissberger series Ed., W. West, Ed., John Wiley Sons, Inc., New York, 1980, p. 589. (d) E. Wasseiman, W. A. Yager, and L. C. Snyder, Electron spin resonance (E.S.R.) of the triplet states of randomly oriented molecules, J. Chem. Phys. 1964, 41, 1763. 

The photolysis of the 4-alkylidene-l-pyrazoline (94) gives rise to two isomeric methylene cyclopropanes (95 and 96).76 The available evidence points to the intermediacy of a trimethylenemethyl species (97) in the triplet state which can cyclize in three ways. The same species is postulated in the photolysis of a series of 4-alkylidene-l-pyrazoline-3-carboxylates.77 This appears to be a general route to derivatives of trimethylenemethyl trimethylenemethyl itself has been generated from 4-methylene-1-pyrazoline and the triplet nature of the intermediate identified by electron spin resonance (ESR) spectroscopy.78... 

---