Linewidth Variations in ESR Spectra of o‐Dinitrobenzene Radicals

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69790/2/JCPSA6-46-10-4152-1.pd

ESR of Alkali Metal Dinitrobenzene Salts in Dimethoxyethane DME

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69774/2/JCPSA6-46-1-400-1.pd

ESR studies of the triplet state of [n.n] paracyclophanes

ESR experiments were performed on the triplet state of randomly oriented paracyclophanes in a variety of rigid glasses at temperatures ranging from 103 to 15°K. Spectra were recorded for [2.2]; [3.3]; (4,7,12,15)‐tetramethyl [2.2]‐paracyclophane and stagger‐ring paracyclophane. For all the samples except stagger ring, a four‐ringed paracyclophane, only the Hmin feature was observed from which D*, the root‐mean‐square zero‐field splitting, was calculated. For stagger ring the triplet spectrum has two features in the Δms = 1Δms=1 region in addition to the Hmin feature. From these, the zero‐field splittings, D and E, were calculated. The triplet spectra for the paracyclophanes show that there is strong transannular interaction with electron delocalization over all benzene rings. There is evidence for strongly coupled intramolecular exciton effects. The effect of increasing the inter‐ring separation from [2.2] paracyclophane to [3.3] paracyclophane is to decrease the transannular interaction. The effect of methyl substitution is to increase transannular effects relative to the parent compound. Transannular interactions in stagger ring are greater than in [2.2] paracyclophane despite the increased electron delocalization possible through the introduction of more than two rings. The large value of E for stagger ring represents a significant deviation from axial symmetry for the zero‐field‐splitting tensor and indicates that the methylene bridges, the methyl substituents, or the ring distortion, may make important contributions to the electronic distribution of the triplet state.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70725/2/JCPSA6-58-2-420-1.pd

ESR Studies of Nitrobenzene, o‐Dinitrobenzene, and m‐Dinitrobenzene Alkali‐Metal Salts

The hyperfine splittings for the lithium, sodium, potassium, and cesium salts of nitrobenzene and the sodium, potassium, and cesium salts of o‐dinitrobenzene over a range of temperature in DME are reported, as well as the splittings for the nitrobenzene and o‐dinitrobenzene radicals produced by electrolytic reduction in DME. The sodium salt of nitrobenzene was also studied in a mixture of DME and DMSO. Results for nitrobenzene and o‐dinitrobenzene are correlated with those previously reported for m‐dinitrobenzene salts. For all three anions it was found that the perturbation of the anion on ion‐pair formation increases along the series Cs+ to Li+. Line‐broadening effects associated with cation motion from one nitro group to the other in the o‐ and m‐dinitrobenzene salts correlate well with the extent of the perturbation of the anion by the metal cation. On the basis of (1) similar magnitudes and similar variation with temperature of the alkali‐metal hyperfine splittings for all three anions, (2) the absence of line‐broadening depending on alkali‐metal quantum number, and (3) the observations in the mixed solvent system, a model for the ion pairs in DME is proposed in which the cation interacts with a single nitro group at a given instant of time, the cation is tightly bound to the nitro group with a fairly strongly bound cage of DME molecules solvating the nitro group cation complex, and temperature variation of the alkali‐metal splitting is attributed to motion of the cation in the vicinity of the nitro group. Simple molecular‐orbital calculations were performed in which the electrostatic interaction of the cation with the nitro group was simulated by increasing both the oxygen and nitrogen Coulomb integral parameters and good agreement between calculated and experimental changes of anion hyperfine splittings on going from the nitrobenzene free ion in DME to the sodium salt was obtained.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70373/2/JCPSA6-47-9-3475-1.pd

ESR Studies of Phosphorescent Corannulene; Evidence for Pseudorotation

Electron spin resonance (ESR) spectra have been recorded for the lowest triplet state of corannulene. The experiments were performed by ultraviolet irradiation of the molecule in rigid glass solutions at 77 and 15°K. In the high‐field (Δm  =  1)(Δm=1) region the line shape and line positions of the absorptions associated with the XX and YY magnetic axes were strongly temperature dependent. From an analysis of the spectra the following conclusions were drawn: Corannulene in its first triplet state has a configuration of lowest energy which is not fivefold symmetric; at 15°K corannulene gives in the high‐field region the spectrum of a molecule “frozen” in conformations of less than threefold symmetry; tunneling between equivalent conformations at 77°K is believed to occur at a rate of about 109 Hz.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69781/2/JCPSA6-52-11-5656-1.pd

Anisotropic Electron Spin Resonance Spectrum of PF2 in Low‐Temperature Matrices

The anisotropic electron spin resonance spectrum of the PF2 radical has been observed. P2F2 was thermally decomposed by passing the gas over a hot wire. The radicals produced were trapped in an argon matrix on a flat sapphire rod held at 20°K. The same radicals were formed by photolysis of PF2H in an argon matrix. In contrast to N2F4, PF2 radicals were not detected unless the P2F4 was heated above 200°C. The observed 12‐line ESR spectrum is interpreted as being due to randomly oriented nonrotating radicals with axially symmetric gg and hyperfine tensors. Based on computer‐simulated spectra, the following assignment has been made: g‖  =  2.0011,g⊥  =  1.9922;C‖  =  (P)  =  307G,C⊥(P)  =  − 83.0G;C‖(F)  =  127G,C⊥(F)  =  33.5Gg‖=2.0011,g⊥=1.9922;C‖=(P)=307G,C⊥(P)=−83.0G;C‖(F)=127G,C⊥(F)=33.5G. The same radical was also trapped in different matrices. The nature of the trapping sites and the effect of environment on the radicals is discussed. Comparison with data available for NF2 indicates that the unpaired electron in PF2 is localized on the central atom to a greater extent than it is in NF2. Extended Hückel calculations also give this result.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71316/2/JCPSA6-52-3-1592-1.pd

Anisotropic Electron Spin Resonance Spectra of PCl2 and NCl2 in Low Temperature Matrices

Well‐resolved ESR spectra of PCl2 were observed from the thermolysis (1000°C) of PCl3, in argon, krypton, xenon, and nitrogen matrices. Satisfactory computer simulated spectra were obtained using two slightly different interpretations. One assumed a rhombic g tensor and an axially symmetric phosphorus hyperfine tensor; gxx = 2.0024, gyy = 20011, gzz = 1.9962; C∥(P) = 293 G; C⊥(P) = ±30.5 G; C∥(Cl) = 17.5 G; C⊥(Cl)<0.3 Ggxx=2.0024,gyy=20011,gzz=1.9962;C∥(P)=293G;C⊥(P)=±30.5G;C∥(Cl)=17.5G;C⊥(Cl)<0.3G. The other assumed an axial g tensor and a rhombic phosphorus hyperfine tensor; g∥ = 2.0024, g⊥ = 1.9986; Cxx(P) = 293 G; Cyy(P) = ±39.0 G; Czz(P) = ±22.5 G; C∥(Cl) = 17.5 G; C⊥(Cl)<0.3 Gg∥=2.0024,g⊥=1.9986;Cxx(P)=293G;Cyy(P)=±39.0G;Czz(P)=±22.5G;C∥(Cl)=17.5G;C⊥(Cl)<0.3G. In situ photolysis of PCl3 in argon produced a spectrum tentatively assigned to the asymmetric PClCl radical. The ESR spectrum due to NCl2 radicals was observed from a direct room temperature sprayon of NCl3 mixed with nitrogen or argon. Experiments using 15NCl3 were also performed. The following assignment was obtained: gxx = 2.006, gyy = 2.025, gzz = 2.023; Cxx(14N) = 40 G; Cyy(14N) = 14 G; Czz(14N) = 6 G; C∥(Cl) = 24 G; C⊥(Cl) = 17 Ggxx=2.006,gyy=2.025,gzz=2.023;Cxx(14N)=40G;Cyy(14N)=14G;Czz(14N)=6G;C∥(Cl)=24G;C⊥(Cl)=17G. A discussion of the spin density distribution in the nitrogen and phosphorus dihalide radicals (NF2, NCl2, PF2, and PCl2) and of the electronic structure of these species using the available experimental data, atomic electronegativities, and the results of extended Hückel molecular orbital calculations is presented. Some information on the decomposition reactions of NCl3 was obtained. A test of the reference values of the atomic nuclear moment—valence electron spin coupling parameters is possible by using the total spin densities determined for these radicals. The reference values calculated by Hurd and Cooden are shown to be preferable.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71279/2/JCPSA6-57-6-2431-1.pd