Forbidden Ams = 2 transition

Weak absorptions corresponding to the forbidden Am,= 2 transitions have also been observed in certain cases (van der Waals and de Groot, 1959) the lines are fairly narrow, probably because the energy differences between the two states which do not involve the m, = 0 level are not so strongly anisotropic. 

A typical first-order derivative spectrum of randomly oriented triplet diradicals is given schematically in Figure 4, which shows how readily the D and E values can be read out. The six high-field signals belong to the Ams = +1 transitions, whereas the forbidden Ams = 2 transition appears at half-field. The latter is characteristic for triplet diradicals and serves as a criterion for their unambiguous assignment. Illustrative examples are the triplet cyclobutane-1,3-diyls (5) [13], the cyclopentane-1,3-diyls (6) [14 16], and the 4,5-diazacyclopentane-1,3-diyls (7) [17], whose zfs parameters are summarized in Table 1. 

Forbidden Ams = 2 transitions appear. Lines oecur at approximately half field of the allowed Ams = 1 ones. 

The spatial localization of H atoms in H2 and HD crystals found from analysis of the hyperfine structure of the EPR spectrum, is caused by the interaction of the uncoupled electron with the matrix protons [Miyazaki 1991 Miyazaki et al. 1991]. The mean distance between an H atom and protons of the nearest molecules was inferred from the ratio of line intensities for the allowed (without change in the nuclear spin projections. Am = 0) and forbidden (Am = 1) transitions. It equals 3.6-4.0 A and 2.3 A for the H2 and HD crystals respectively. It follows from comparison of these distances with the parameters of the hep lattice of H2 that the H atoms in the H2 crystal replace the molecules in the lattice nodes, while in the HD crystal they occupy the octahedral positions. 

This phenomenon had been known for some time in the field of transition-metal complexes, but had not been exploited. A simple and possibly oversimplified view of why these forbidden transitions give rise to narrow fines is that the anisotropy found for Ams = +1 transitions stems from a marked dependence of the zero-field splitting upon orientation. Since this is primarily confined to a separation between the ms = 0 and ms = + 1 levels, the low-field Ams = + 2 transitions are unaffected, since the mB = 0 level is not involved (Fig. 156). 

Figure 16 shows plots of the energy levels for both axial (E = 0) and rhombic (D > 3E >0) systems with D hv. In the axial system, it is obvious that a transition cannot be induced between =0 and m = 1 because of the large energy separation (D). A transition also cannot be induced between m = 1 and m = —1, since this is forbidden by selection rules (i.e., this is a Am = 2 transition while only Arris = 1 transitions are allowed). Therefore, transitions are almost never observed in purely axial integer-spin systems. 

Figure 3. Computer-simulated spectra of the M= +1/2 -1/2 fine-structure group at the X band as calculated from expressions calculated up to third order in perturbation for various values of D. The abscissa is in gauss with the zero of reference chosen at unperturbed resonance field Bo- Full hues refer to A/n = 0 transitions, dashed hnes to the hyperfine "forbidden" Am = 1 transitions. AH intensities are drawn on the same scale. These spectra are to be compared with Figure lb. A/g[iB = 93 G. (a) >/gp.B = 75 G, (b) D/g iB = 100 G, and (c) D/g[lB = 150 G. Adapted with permission from de Wijn and Van Balderin (1967).

Often ftrlly reduced samples of dinuclear iron containing proteins contain a ferromagnetically coupled dinuclear Fe " center that has as its ground state 5tot= 4 vide infra). In many cases formally forbidden AMs= 4 transitions between the Mg = 2 spin states produce resonances in the EPR spectrum with geff 16 (Fig. 5). 

The first order EPR spectrum of Co(acacen) consists of two sets of eight allowed Ams = 1, Amo, = 0 transitions (I = 7/2, two magnetically nonequivalent sites). This simple pattern, however, was only observed for orientations of B0 near the principal axis gj. If B0 lies near the plane spanned by gy and gz, forbidden Ams = 1, Amo, = 1, 2 transitions occur (Fig. 2 a). 

A six- or four-line ESR spectrum that can be fitted to a triplet spin Hamiltonian is strong evidence that the species in the sample embodies two unpaired electron spins. Support for the presence of a triplet spin system often can be found in the weak Ams = 2 line, which appears at one-half the field strength of the center of gravity of the Ams = 1 six-line pattern. This nominally forbidden Amj = 2 resonance results when the ESR spectrometer field and frequency produce a micro-wave quantum of energy just sufficient to jump the gap between the uppermost and lowermost triplet substates, that is, a transition over two quantum levels. 

Apparently the zero field splitting within the ground state is very weak since XmT does not exhibit any deviation from the Curie law below 10 K. This is confirmed by the EPR spectrum at 4.2 K where no fine structure is detected. This spectrum exhibits the AMS = 1 allowed transition at g = 1.991 and the AMS = 2, 3 and 4 forbidden transitions of decreasing intensity at half, third and quarter-field respectively. The theoretical expression for the magnetic susceptibility has been established from the spin state structure of Fig. 41 and the Zeeman perturbation expressed as ... 

That a dynamic Jahn-Teller effect indeed may be of importance in SmBe has also been suggested in a paper by Uemura et al. (1986a,b). They clearly observe unusual AM - 2 and AM = 3 transitions in the ESR spectra of Eu + in SmBe, although the mixing matrix elements in cubic symmetry should be negligible. Reasons for the appearance of the forbidden transitions could be a dynamical JT-effect and/or fluctuations between... 

NO— NO dimer species with a triplet state (referred to as the (NO)2 bi-radical or radical pair in the following). The ESR spectrum of (NO)2 bi-radical shows the forbidden transition. Arris = 2, at ca. 170 mT g 4), when the corresponding allowed transitions, Ams = 1, are observed for the same sample at ca. 340 mT ig 2). These verify the presence of the triplet electronic state. Thus the ESR study suggested that the zeolite can stabilize the (NO)2 dimer as the triplet rather than the usual singlet state, indicating a great affinity of the NO molecule for the zeolites. The (N0)2 bi-radical may play an important role as an intermediate species in the decomposition of NO [29], ESR studies have accordingly continued to be of interest in recent decades. 

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