Triplet ESR spectrum

Parallel to the generation of the DR oligomers carbenoid AC oligomers (see Table 2) are formed upon UV-irradiation of the monomer crystals at low temperatures. The high field part of the AC triplet ESR spectra with the lines 1, 2, 3, and 4 is shown in Fig. 12. The unspecified lines are not relevant for our purposes they are attributed to the doubling of the unit cell in the low temperature phase of the TS-6 crystals In contrast to the DR triplets, the AC triplets are in their electronic ground state. This follows from the 1/T Curie law temperature dependence of the ESR intensities. The fine structure splitting and the pattern of the hyperfine structure of the DR and... 

Fig. 27. Triplet ESR spectra of PSI, MSII and MSIII in chloroform-toluene (2 1) at 100 K. Microwave power 5.0 mW, field modulation 20G(100KHz), excitation with square wave modulated (83 Hz) light of an Ar ion laser (514.5 nm, 0.5 W). Absorption and emission peaks have been labeled a and p respectively. Approximately 1 x 10"4 M solution was used for recording the spectra. (Taken from Ref. 205).

M. Hoshino, S. Konishi, M. Imamura, S. Watanabe, and Y. Hama, Optical Absorption and Triplet ESR Spectra of Oxovanadium(IV) Tetraphenylporphyrin Cation Radical, Chem. Phys. Lett., 102 (1983) 259. 

The triplet esr spectra are determined primarily by the dipole-dipole interaction of the two unpaired electrons. This interaction causes a splitting (the so called zero-field-splitting (ZFS)) of the threefold degenerate triplet state even in the absence of an external magnetic field H. The Hamiltonian for the triplet states is obtained by adding to Equ. (11) a term, H ps, which takes care for the dipole-dipole interaction ... 

The effect of the partial alignment on the intensity distribution of triplet esr-spectra is shown in Fig. 10b for naphthalene as guest. Spectra are shown for an mentation of the liquid crystal tic axis L either perpendicular (upper trace ... 

Photoirradiation of poly(vinyl alcohol) in vacuum causes formation of three kinds of radicals which give singlet, quartet and triplet ESR spectra [1621]. These spectra were assigned to alkoxy and methyl radicals which originated from the acetyl group remaining in the sample, and to carbon radicals gen-... 

Oxo-2,5-cyclohexadienylidene [83] was generated in solid argon at 9 K by irradiation of diazo compound [84] with visible light (A>495 nm) (Sander et al., 1988 Bucher and Sander, 1992 Bucher et al., 1992). The IR, UV, and esr spectra of [83] were in accord with a structure having a triplet state with one delocalized electron. In the IR spectrum of the carbene [83] the r (CO) mode was found at 1496 cm which indicates a bond order of the C—O bond considerably less than 2. The low-temperature reaction of carbene [83] with CO generated the keto-ketene [85]. Irradiation (A = 543 10 nm) of [83] led to its transformation into a very labile species, presumed to be [86], which rearranged back to [83] not only under UV or... 

ESR spectra for a number of nitrene intermediates produced by photolysis at 77°K have been reported.<50) Analysis of these spectra has resulted in their assignment to triplet ground state nitrenes. 

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... 

ESR Spectra of Biradicals, Triplet States, and other S>lf2 Systems... 

ESR Spectra of Biradicals, Triplet States, and other S>l/2 Systems At constant frequency, v0 = AE/h, the resonant fields are ... 

Fig. 4 Esr spectra of high-spin trianthrylene [21a] at 120 K in MTHF/K A, biradical triplet state [21a]2- (labelled O) B, triradical quartet state [21a]3- (labelled ) C, biradical triplet state [21a]4- (labelled ).

A related finding appears for the dianions of the doubly layered system [14] their esr spectra taken in the glass provide evidence for a triplet structure, and from the D-value the inter-plane distance in the charged species can be readily determined (Irmen et al., 1984 Alexander et al, 1989). 

Photolysis of vinyldiazomethane in an organic glass at 6 K leads to vinylcarbene in its triplet ground state,14,56,57 which — as is indicated by the ESR spectra — forms a pair of the s-cis and s-trans isomer. The delocalization of one unpaired electron in the rr-system is similar to that of the allyl radical, while the other unpaired electron is localized in a sp2-orbital at the carbenic C atom (see formula T-33 ).58... 

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. 

In electrochemical oxidation of l-hydroxy-3-imidazoline-3-oxides containing one to four H atoms at a-C, one observes in ESR-spectra not only triplet splitting of the nucleus 14N of the nitroxyl group (a v 15-16 G) but also splitting of the neighboring protons (a// 18-20 G), with multiplets corresponding to their number (from doublet to quintet) (101). Unlike spatially hindered hydroxylamines which show reversibility in electrochemical oxidation, hydroxylamines with H at a-C are oxidized irreversibly. Oxidation of hydroxylamines with nitroxyl radical proceeds easily and with quantitative yields (102). In the oxidation of asymmetric polylluorinated hydroxylamines with Mn02, isomeric polyfluorinated nitrones have been obtained (103). 

Figure 1 is an ESR spectrum acquired after 10 minutes of sonicating an Ar - saturated solution of 2-PCB and PBN at 20 kHz. Samples that were irradiated at different power intensities for the same time period (10 minutes) are compared. Peaks consist of a triplet of doublets with the parameters of och = 4.2, ocn =16.0, which are characteristic a values for phenyl radical trapped by PBN. These peaks appeared in all ESR spectra obtained from sonicated solutions of 2-PCB (n = 20) and 4-PCB (n = 19) with irradiation times longer than 10 minutes (n = number of experiments). 

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