Ams = 1 Transition

A single triplet has three resonant fields, two due to Amv= l transitions and one due to Ams 2 transitions. For amorphous or polycrystalline samples, two triplet powder patterns are formed due to contributions from all possible orientations of triplets with respect to the applied field. The full-field triplet powder pattern due to Ahia-= 1 transitions is centered about If and has the following critical points ... 

T. J. Kemp, University of Warwick Noting the very low quantum yield for intramolecular electron transfer in low temperatures displayed by your porphyrin-quinone model compound, would it not be possible to shock-freeze a solution undergoing irradiation at a higher temperature (and giving a workable concentration of paramagnetic species) in order to determine a low-temperature spectrum with the particular aim of observing a possible Am = 2 transition ... 

Hesse and Heusinger130 studied the ESR signal due to Am = 2 transition of radical pairs in a number of /-irradiated polymers including 1,2-polybutadiene (both atactic and isotactic) and 3,4-polyisoprene. It was found that the distance between the radicals in the pair is 0.53 0.04 pm 1.0 0.5% of the radicals in 1,2-polybutadiene and 3,4-isoprene are arranged in pairs. 

Subnormal V02+ complexes (93) of SBs derived from 2-hydroxynaphthaldehyde and o-aminophenol and some substituted derivatives have also been obtained.765 The ESR spectra of the complexes (93 R = R = R" = H R = Cl, R = R" = H) in polycrystalline solids at room temperature exhibit two parallel lines, two perpendicular lines and a broad line around 1600 G due to the AM% = 2 transition. At 77 K, both high- and low-field spectra exhibit hyperfine splittings. Several complexes of type (93) were also studied by Carlisle et al. (Table 41, 27-34).766,767 v(V=0) values of complexes (93) lie in the range 955-1000 cm-1 and the compounds exhibit only one d-d band in the region 620-700 nm. As in the case of (91 R = R = H), on treatment of compound (93 R = R = R" = H) with phen the dimer is broken and a mixed-ligand complex is obtained.765... 

Of comparable importance was the subsequent discovery that Ams = + 2 transitions could be detected at low fields for triplet molecules in random orientations in rigid media (van der Waals and de Groot, 1959, 1960). These are best observed with the apparatus modified so that the static and oscillating magnetic fields are parallel, but if weak spectra can be detected with the normal arrangement of static and oscillating fields perpendicular to each other then the combined results are almost as informative as those from single crystal studies. 

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 2. Time-resolved CIDEP spectrum of the lowest triplet of GAV at 77 K. a, Am = 2 transitions at half field, b, Ani = 1 transition at the canonical orientations. The strong emissive signal at g = 2 is due to phenoxy radicals.

FIGURE 7.11 Variation of energy levels for / = 1 with angle 6 and illustration of the constant energy separation for the Am = 2 transition. 

FIGURE 3. Fine structure of the triplet state of naphthalene as a function of the external field Hp. Heavy lines H0 parallel to y-axis light lines H0 parallel to z-axis dotted arrows the aM = 2 transition. 

D. However, D and E can be obtained directly from the AMS=2 transition by using spectrometers with the micro-wave field parallel to the applied field (41,42). 

As was pointed out in the introduction, optical detection of the Zeeman transitions of the triplet state preceded the optical detection in zf. Since these former experiments resemble those in ionic crystals, researchers in this field called this technique MODR (microwave-optical double resonance). The assignment of the zf transitions as well as the relative order of the zf levels could be concluded also from the MODR techniques as in the PMDR technique. The first reported MODR experiment was made by Sharnoff (15), in which the Am = 2 transition of the C10D8 tr Plet state in a biphenyl host, using amplitude modulation of the microwave power. A few months later Kwiram reported the optical detection of the Am = + 1 for phenanthrene in biphenyl (16). The experiments were... 

Triplet signals were first noted by Kosower and Waits in concentrated solutions of stable pyridinyl radicals (4 ) and confirmed for (CHs) and 4 in MTHF glasses at 77 K K Three pairs of shoulders around the g = 2 signal at 3295G for the monomeric pyridinyl and a AM = 2 transition at 1645G suggested the presence of triplet pair. The zero-field parameters were D == 0.0098 cm and E = 0.0011 cm , for which a radical-radical separation of 6.5 A was estimated from the relationship for spin-spin dipolar interaction (Eq. 12). 

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. 

As mentioned in Section IVA, mixed ligand complexes of formula [([12]aneN3)M2(L8)] (where M is Zn or Cu) have been synthesized and characterized spectroscopically (107). The data are consistent with an imidazolate bridged structure as illustrated in Fig. 33. EPR spectroscopy on the dicopper(I) complex shows evidence of coupling there is a weak AM = 2 transition at half field (ca. 1560 G compared with ca. 1540 G in CU2CU2SOD). Attempts to prepare heterodinuclear CuZn complexes as Cu2Zn2SOD models have, so far, been unsuccessful. However, some heterodinuclear complexes of related ligands have been... 

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. 

Figure 15. Transition from Cu-Cu dimer (bottom spectra) to Cu monomer (top spectra) as the ratio of ligand to metal increases. Effect of the ratio of molar concentrations of carnosine to Cu (from 100 1 to 1 1) on the ESR spectra (X-band) of frozen (77 K) aqueous solutions of 9.10 x 10" M copper(II) ion. All solutions contain 0.27 M sucrose and are adjusted to pH 7.2 + 0.1. The half-field AM = 2 transition of the copper(Il) dimer is shown at the bottom of the figure. Similar half-field transitions of lower intensity also were observed at concentration ratios of 5 1 and 10 1. Substitution of 0.82 M sodium perchlorate for the sucrose has no effect on the spectra. From [233], with permission.

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. 

The ESR spectrum of the fully reduced anion, PV2 Wio04o (Figure 7), has the 15-hne pattern ((g) = 1.95, (a) = 53 G) expected for the interaction of the unpaired electrons with two vanadium nuclei. Observation of the half-field (Am, = 2) transition in the spectrum of the solid potassium salt confirms the triplet state of the species responsible for the spectrum in Figure 7. It seems very likely that those isomers (1,2 and 1,6 with V-O-V groups) which are responsible for the 15-line spectrum of the mixed valence complex would have antiferromagnetically coupled spins in the fully reduced anion. The remote isomers with vanadium atoms separated by O-W-0 (1,4 and 1,5) or O-W-O-W-O (1,11) sequences are therefore probably responsible for the triplet state ESR spectrum. Further ESR and magnetic susceptibihty measurements are planned on individual isomers. 

Figure 7.7. Triplet spin energy levels vs magnetic field H, for (a) H x. (b) H y and (c) H z. Note the zero-field spitting the half-field resonance at C due to the Am = 2 transitions, and the full resonances which occur at different fields A and B for the Anis= -E 1 and —1 transitions [61].

Recently, ESR spectroscopy has been used to study srlver(II) porphyrins. Aggregation in metalloporphyrins, in tetrachloroethane (TCE) and 2-methyltetrahydrofuran (MTHF) has been studied at 77 K. In MTHF a well-resolved signal due to monomeric Ag mesoporphyrm(IX) dimethyl ester (MPD) was observed. In TCE, however, additional signals on the monomer spectrum and a g = 4 signal with a partially resolved structure were found when the gain was increased. The g = 4 signal was ascribed to the AM = 2 transition of the dimeric [Ag"MPD]2. In an equimolar molar mixture of Cu MPD and Ag MPD a heterodinuclear complex was formed. 

A simulation of such a superposition, using the fine-structure constants of the naphthalene molecule (D/hc = 0.100 cm , E/hc = -0.015 cm , and g = ge), and with a linewidth of AB = 1.2 mT for each component is shown in Fig. 7.13a. The discontinuities in the broad Am = 1 ESR spectrum correspond to the principal-axis orientations Bo u u = x,y,z). They occur because of the high density of resonance-field values in the neighbourhood of the principal-axis directions (compare Fig. 7.4). For B = 0, two of these discontinuities occur at the same place. The Am = 2 spectrum is narrow because the anisotropy of the Am = 2 transitions is to first approximation zero. The ESR signal (Fig. 7.13b) is - as usual in ESR due to the method of measurement - the first derivative of the spectrum (absorption vs. frequency or field strength. Fig. 7.13a). Even with a powder or a glass sample. 

MULTIP simulates ESR spectra of triplet states, biradicals, radical or ion pairs in rigid matrices for Ams = 1, and Ams = 2, transitions. Several programs are available according to the system symmetry and to the existence or not of hyperfine coupling. When the g, electron dipole and hyperfine coupling tensors have different principal axes, the spectrum is computed utilizing the second order treatment of Iwasaki [19]. 

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

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