Temperature dependent ESR spectra

The bridging silicon in polyphenyl silanes separates practically independent 7r-systems (18). However, temperature-dependent ESR spectra attributed to electron exchange between the aryl substituents have been... 

In addition, we will present the mechanistic aspects concerning the electron-entry on four-coordinated phosphorus compounds. There are indications promoting the concept that a TBP is involved with the odd electron in an equatorial position. This presumption is based on the temperature-dependent ESR spectra of the radicals derived from 5. X-irradiation of a single crystal of 5 at liquid nitrogen temperature (77 K) yields the spectrum of radical 13 exclusively with ap//= 1120 G, api = 930 G, while,splitting is not resolved. From these values one calculates apls° = 993 G, indicating the phosphorus 3s spin density is 0.27, while the anisotropic contribution places 0.61 of the spin density in its 3p orbital, giving a total spin density of 0.88 on phosphorus. 

Intramolecular transfer of excitation is of considerable importance in photochemistry. Leermakers, Byers, Lamola, and Hammond (332) have demonstrated by optical emission the occurrence of intramolecular electronic energy transfer in 4-(l-naphthylalkyl)benzophenone. de-Groot and van der Waals (333) have examined the temperature-dependent ESR spectra of the phosphorescent states of benzene, toluene, triptycene, and tribenzotriptycene. The latter two molecules consist of three benzene or naphthalene systems joined together, and their ESR spectra reveal the intramolecular excitation transfer between the benzene or naphthalene subsystems. At 20°K, the excitation is mainly localized in one of the subsystems, but at such... 

ESR spectra of NO2 adsorbed on X- and Y-type zeolites were observed in the temperature range 77-346 K. Based upon spectral simulation using a Brownian diffusion model, motional dynamics of NO2 adsorbed on zeolite surface were analyzed quantitatively. In the case of X-type zeolite, it was found that the ESR spectra below 100 K is near the rigid limit. Above 230 K, the average rotational correlation time decreased from 1.7 x 10 (230 K) to 7.5 x lO sec (325 K) with increasing temperature and its degree of anisotropy was very close to one (N = 1.25). On the other hand, the temperature-dependent ESR spectra of NO2 adsorbed on Y-type zeolite were observed to be somewhat different from that for X-type zeolite. 

More than one decade ago we have reported an ESR study on the molecular motion of NO2 adsorbed on porous Vycor glass [8,9] and it was found that the NO2 adsorbed on the Vycor glass and Cu-metal supported on Vycor glass gave strongly temperature-dependence ESR spectra. The present study aimed to further apply our dynamic ESR technique to elucidate motional dynamics of NO2 adsorbed on various zeolite with different structures. The results can provide fundamental and useful information which are required to develop new active catalyst and to clarify the reaction mechanism. 

Figure 1 shows the temperature-dependent ESR spectra of NO2 adsorbed on the X-type zeolite. Below 100 K, the ESR spectra of an anisotropic triplet-triplet were observed as shown in Fig. 1(a) The observed ESR line shape was similar to that obtained for N02A ycor system at 4.8 K [9]. The principal values of g and A tensors were evaluated by the ESR spectral simulation method = 2 0055, gyy = 1.9915, g = 2.0015, and =142.6, Ayy =... 

Fig. 11 Temperature-dependent esr spectra of the monoanion of biperylene [17] in MTHF.

NO2 is a stable paramagnetic gaseous molecule at normal temperatures. The ESR parameters of NO2 trapped in a solid matrix have been well established from single-crystal ESR measurements and have been related to the electronic structure by molecular orbital studies [39]. Thus, the NO2 molecule has potential as a spin probe for the study of molecular dynamics at the gas-solid interface by ESR. More than two decade ago temperature-dependent ESR spectra of NO2 adsorbed on porous Vycor quartz glass were observed [40] Vycor is the registered trademark of Coming, Inc. and more information is available at their website. The ESR spectral line-shapes were simulated using the slow-motional ESR theory for various rotational diffusion models developed by Freed and his collaborators [41]. The results show that the NO2 adsorbed on Vycor displays predominantly an axial symmetrical rotation about the axis parallel to the O—O inter-nuclear axis below 77 K, but above this temperature the motion becomes close to an isotropic rotation probably due to a translational diffusion mechanism. 

Strongly temperature dependent ESR spectra were observed for the [(CH3)3NCH2] radical cation stabilized in Al-offretite and SAPO-42, see... 

Abstract ESR spectroscopic applications to polymer science are presented. ESR parameters used for the molecular and material characterization of polymer materials are reviewed. It is emphasized that ESR studies of the polymer science are particularly effective in three areas. (1) Intermediate species such as free radicals produced in chemical reactions of polymer materials can be directly detected. (2) The temperature dependent ESR spectra of free radicals trapped in the polymer matrices are very effective for the evaluation of molecular mobility (molecular motion) of polymer chains. (3) The mobility of electron, the structure of solitons, and the doping behavior in conduction polymers can be observed in detail in order to clarify the mechanism of conduction. 

Fig. 7.4 Temperature dependent ESR spectra of PMMA propagating radical, (a) In anisole at -120°C. (b) In benzonitrile at temperatures shown in the figure. The figure is adapted from [4] with permission from the Society of Polymer Science, Japan...

Since destruction of polymer materials is very important for practical purposes, a large number of investigations on fracture phenomena in polymers have been carried out from both the experimental and theoretical points of view. Several reports provide indirect evidence for main chain scissions, for example decreases in molecular weight or initiations of the graft or block copolymerization after mastication. Direct evidence for chemical bond scission can be obtained from ESR measurements on fractured polymer materials [21]. The high reactivity and high mobility of free radicals produced by mechanical fracture (mechano-radicals) can also be followed. The ESR application to mechanical destruction of polymer materials is presented below. Temperature-dependent ESR spectra of polymer radicals produced... 

In Section 7.6.1, principal directions of the g-tensor of PTFE peroxy radicals were determined and rotation of the radicals around the PTFE chain axis, rather than around the 0-0 bond, was demonstrated. In this section, we describe a detailed study of the ESR spectra and the molecular motion of peroxy radicals when they are trapped in various regions with many kinds of heterogeneities in polymer matrices. Temperature dependent ESR spectra of peroxy radicals in the powdered polyethylene having a low crystallinity are shown in Fig. 7.32 [33]. 

Structure and mobility of the peroxy radicals in the amorphous region had heterogeneity. The B-radicals begin to move at lower temperatures while the A-radicals (rigid radical) convert to the B-radical (mobile radical). The peroxy radicals have actually a distribution of mobility, which is described here with two representative components. Clearer temperature dependent ESR spectra of the peroxy radicals have been observed in the urea-PE complex (UPEC) as shown in Fig. 7.35 [34]. The spectra could be simulated with two components due to A- and B-radicals as described above. This suggests that the radicals had a distribution of mobility, even in the UPEC crystals. For the peroxy radicals produced from the alkyl radicals in the UPEC, the principal values of the g-tensor at 77 K were gi = 2.0022, g2 = 2.0081, and g3 = 2.0366. 

Abstract Measurements of temperature dependent ESR spectra of spin labels trapped in the polymer matrices is very effective for the evaluation of molecular mobility (molecular motion) of polymer chains. We can characterize the molecular motion of the particular sites in different region by the ESR method. It is possible to detect the mobility of polymer material at the segment or atomic level. ESR studies help to relate the features of the nanometer scale to macroscopic properties of the polymer materials. We present examples of spin labeling studies in the polymer science to help readers new to the field understand how and for what areas the method is effective. In the first part, we give a simple review of the spin labeling method and present applications in the polymer physics related to relaxation phenomena in various systems. In the second part applications to biopolymer system are introduced to help the clarification of various mechanisms of bio-membranes. 

Fig. 8.2 Temperature dependent ESR spectra of spin-labeled PS at chain end Mn = 21 kDa) and at inside site (right,...

Fig. 8.6 Temperature dependent ESR spectra of spin-labeled PE of different ctystaUinity. Separation between arrows shows extreme separation width.

Fig. 11. Temperature-dependent ESR spectra of the tris(trimethylsilylmethyl)amine radical cation (cf. Fig. 10), proving its cogwheel dynamics in H2CCI2 solution above 260 K (see text) [1],...

Temperature dependent ESR spectra of the most persistent radicals (4 R = Me, Et) reveal permutation processes via a a radical in an M4 process (Scheme 1). ... 

The advances in the molecular design of new polymeric materials with targeted properties require advanced molecular characterization of the polymers. ESR techniques are among the methods under continuous development in the quest for more comprehensive physical and chemical information that could correlate microscopic properties with materials performance. ESR spectroscopy has been used in various areas of polymer science, with different goals, such as to study mechanisms of chemical reactions in polymerization and radiation effects, to identify intermediate species, to observe decay and conversion of different species, or to investigate relaxation phenomena of polymer chains by observing temperature-dependent ESR spectra of radical species trapped in solid and liquid polymers. 

---