P. A. Narayana

Electron spin echo envelope modulation of trapped radicals in disordered glassy systems: Application to the molecular structure around excess electrons in γ‐irradiated 2‐methyltetrahydrofuran glass

Three specifically deuterated 2‐methyltetrahydrofurans (MTHF) have been synthesized to delineate the proton (deuteron) arrangement and hence the molecular arrangement around a trapped excess electron in γ‐irradiated glassy MTHF at 77 K. The electron–deuteron anisotropic hyperfine interaction was detected by its modulation of the two‐pulse electron spin echo envelope. Comparison of experimental and simulated modulation patterns shows that the excess electron is located about 3.7 A below the center of the plane of the ring carbons in MTHF on the side opposite the methyl group and that three or four MTHF molecules constitute the first solvation shell. A second moment analysis of the EPR linewidths of the excess electron in the variously deuterated MTHF’s independently supports the same structural conclusions. The shortest electron–proton distance is then ∼3.1 A, which compares moderately well with previous, less accurate ENDOR results. This structure seems somewhat more expanded than that indicated by theore...

ESR line shape studies of trapped electrons in γ‐irradiated 17O enriched 10M NaOH alkaline ice glass: Model for the geometrical structure of the trapped electron

The ESR line shape and width of a trapped electron (et−) has been measured in 34% 17O enriched 10M NaOH alkaline ice glass at 77 K. The second moment of et− due to 17O coupling is 134 G2. Analysis of the second moment contributions from 17O coupling together with the constraint from electron spin echo studies that the et−–O distance is greater than 2 A show that the et−–17O hyperfine coupling is mainly isotropic. Simulation of the et− ESR line under these constraints on 17O coupling and previous ones determined for H or D coupling for various numbers n of equivalent water molecules in the first solvation shell of et− give a best fit to the experimental line shape for n=6. For n?4 definite structure would be expected in the et− line shape which is not seen experimentally. The n=6 result together with previous data allows a rather complete geometrical description of the trapped, hydrated electron to be defined.

Electron spin echo envelope modulation of trapped radicals in disordered glassy systems: Application to the molecular structure around excess electrons in γ‐irradiated 10M sodium hydroxide alkaline ice glass

Electron spin echo signals associated with trapped electrons in aqueous matrices show a modulation due to hyperfine interaction with the surrounding magnetic nuclei. The three pulse electron spin echo modulation gives information about the next nearest neighbor deuterons surrounding the electron. Analysis of the phase change in the three pulse modulation gives an effective deuteron interaction distance of 3.6 A. The two pulse spin echo modulation depends on both the nearest and next nearest neighbor deuterons. Simulations of this modulation support a structural model in which the number of equivalent nearest neighbor deuterons to the electron is 6 at a distance of 2.1 A and with an isotropic coupling constant of 0.9 MHz (5.8 MHz for protons). The orientation of the water molecules in the first solvation shell is not uniquely determined and two possible models are proposed. In model I there are three first solvation shell water molecules with their molecular dipoles oriented toward the electron so that the...

Electron spin echo modulation studies of silver atom solvation and desolvation in ice matrices

Electron spin echo modulation patterns have been obtained at 4.2 K and analyzed for desolvated silver atoms produced at 4.2 K by radiolysis of deuterated ice matrices containing AgF and for solvated silver atoms produced by brief warming to 77 K. The analysis shows that desolvated Ag0 is surrounded by eight equivalent deuterons at 3.1±0.1 A which suggests a tetrahedral model of four waters with their dipoles pointed away from Ag0 (i.e., a solvated Ag+ geometry). Solvated Ag0 appears to be formed by rotation of one or two waters around one of their OD bonds to put one or two deuterons at ∼1.8 A from Ag0. These close deuterons do not contribute to the echo modulation pattern because the nuclear splitting is larger than the microwave magnetic field, and the remaining six or seven deuterons at 3.1 A produce the modulation pattern.

An improved line shape model for the matrix ENDOR response in disordered systems

An improved line shape model for the matrix ENDOR response near the free‐nucleus precession frequency in disordered systems is developed and applied to a spin system with S=1/2 and I=1/2 using a density matrix formalism. The model includes the radiofrequency and microwave field intensities as well as different spin relaxation paths explicitly. Application of this model to trapped electrons in 2‐methyltetrahydrofuran glass gives geometrical and magnetic data which are in excellent agreement with existing data from electron spin echo experiments. The effects of the various parameters in the theory of the matrix ENDOR line shape are discussed.

Geometrical structure of solvated electrons in γ‐irradiated 3‐methylpentane glass at 77 K from an analysis of the modulation of the electron spin echo decay envelope

Deuterium modulation of the electron spin echo decay envelope from trapped electrons in γ‐irradiated 3‐methylpentane‐d14 glass at 77 K has been analyzed to give a solvation structure characterized by 18–21 equivalent nearest neighbor deuterons to the electron at a distance of 3.0–3.2 A and with an isotropic coupling constant of essentially zero. From the structure of 3‐methylpentane it is suggested that there are six to seven first solvation shell molecules around the electron.

High power electron spin echo modulation studies of silver atom solvation and desolvation in ice matrices: Geometrical models for silver cation and silver atom aqueous solvation shells

High power electron spin echo modulation patterns have been obtained at 4.2 K and analyzed for presolvated silver atoms produced at 4.2 K in a nonequilibrium environment by radiolysis of deuterated ice matrices containing AgF, and for solvated silver atoms produced by brief annealing at 77 K. The analysis shows that presolvated Ag0 is surrounded by eight equivalent deuterons at 3.1±0.1 A with zero isotropic hyperfine coupling which suggests a tetrahedral model of four waters with their dipoles pointed away from Ag0 (i.e., a solvated Ag+ geometry). Solvated Ag0 has one deuteron at 1.7±0.05 A with a 1.8 MHz isotropic hyperfine coupling and seven deuterons at 3.1±0.1 A with zero isotropic hyperfine coupling. The solvation process appears to occur by rotation of one water around one of its OD bonds with no change in the Ag–O distance.

Effects of quadrupole interaction on electron spin echo modulation patterns in disordered systems

Abstract Effects of quadrupole interactions on two- and three-pulse electron spin echo modulation patterns are studied by calculating the modulation patterns for model systems. It is found that a small quadrupole coupling gives a characteristic pattern for two-pulse modulation patterns but has a considerably smaller effect on three-pulse modulation patterns.

General matrix ENDOR line shape model applied to the methyl radical in lithium acetate dihydrate using single crystal data

A new general matrix ENDOR line shape model is tested against ENDOR data on the methyl radical in γ‐irradiated lithium acetate dihydrate. All parameters in the line shape model are determined or narrowly limited by experiment. These include the dipolar tensors of all protons within 5 A of the methyl radical, the microwave and radiofrequency magnetic fields, and the electronic and nuclear spin–lattice and spin–spin relaxation times. The theoretical simulations agree satisfactorily with the experimental lineshape, microwave magnetic field dependence, radiofrequency magnetic field dependence and angular variation of single crystal ENDOR line intensities. The effective nuclear spin–lattice relaxation time is ∼10 ms at 77 K and seems to be dominated by an angularly independent nuclear relaxation mechanism. The angularly dependent electron–nuclear dipolar interaction is found to be of much less importance for the ENDOR response. It is noted that the angular variation of ENDOR intensities is an important paramet...

Analysis of three-pulse electron spin-echo modulation: Phase reversal

Abstract Three-pulse electron spin-echo modulation patterns are analyzed with particular attention paid to the phase reversal of the modulation. For zero isotropic hyperfine coupling a simple expression is derived which relates the time of phase reversal to the dipolar distance to the interacting magnetic nuclei. However, even a small isotrophc hyperfine coupling dramatically affects the time of phase reversal. The absence of phase reversal in the three-pulse echo modulation from trapped electrons in methyltetrahydrofuran glass is shown to be consistent with the results obtained from two-pulse echo modulation studies.