Publications
1974
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(1974) The Journal of chemical physics. p. 3884-3888 Abstract
The asymmetry parameter 17 of the electric field gradient tensor having reflection symmetry is discussed for sp " hybridized atoms. It is shown that in this case η is a function of a single variable, which depends on the degree of hybridization and the occupation numbers of the hybrids. The model provides a physical picture for the various possible values of η as well as closed expressions for the occupation numbers. Two specific cases, sp 3 and sp2, are discussed in detail.
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(1974) The Journal of chemical physics. 61, 3, p. 1093-1100 Abstract
A quantitative analysis of the effect of spin lattice relaxation rates on pure quadrupole resonance pulse experiments for spins with I = 1 is given. A model is presented in which the three dimensional relaxation problem is reduced to a two dimensional one. In this model any possible population distribution of the energy levels corresponds to a point in a plane, the "v plane." The response of the system to spin-lattice relaxation effects and to rf pulses are described as translations of points in this plane. The experimental signal intensities are directly related to the coordinates of those points. The following cases are treated quantitatively: (i) CW saturation, (ii) CW saturation followed by a 90° pulse, (iii) two-pulse sequences, and (iv) continuous steady state pulse sequences. For each case explicit equations for the signal intensity as function of the experimental parameters are given. The analysis is made for both single crystals and powders.
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(1974) The Journal of chemical physics. 61, 6, p. 2265-2274 Abstract
Anomalous EPR line intensities observed in the photoexcited triplet state of porphyrins at 100°K are discussed. A kinetic model consisting of the singlet ground state and the first excited triplet state is developed. The EPR signal intensities are expressed in terms of the kinetic rate constants and are given as linear combinations of two exponents. With these expressions the transient effects on the line intensities can be explained. Experimental results for two porphyrin molecules H2TPP (tetraphenyl porphyrin free base) and H2(PF)TPP (pentafluorotetraphenyl porphyrin free base) are presented and compared with the theory. From the analysis we evaluated the spin lattice relaxation rate W, the depopulation rate constants kp and the ratio between the population rate constants Ap: For H 2TPP, kx = (6±2)×102 sec -1, ky = (3±1)×102 sec -1, kz = (1.5±0.5)×102 sec -1, Ax:Ay:Az≃1:0.6:0.2, and W = 3.3×103 sec-1. For H2(PF)TPP, k x = (5±2)×102 sec-1, ky = (1.4±0.4)×102 sec-1, kz = (1.0±0.3)×102 sec-1, Ax:A y:Az≃1:0.2:0.3, and W = 1.7×104 sec-1.
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(1974) The Journal of chemical physics. 61, 3, p. 1101-1106 Abstract
The description of the PNQR relaxation in the v plane is extended to the case of two nuclei of spin I = 1 interchanging by a dynamic process. The theory is applied to hydrazine, in which the two inequivalent nitrogen atoms interchange sites. Spin lattice relaxation measurements on all six 14N NQR lines were done at 228°K. Three types of experiments were performed: (i) steady state continuous pulse experiments, (ii) two-pulse experiments on the same transitions, and (iii) cross relaxation experiments in which the first pulse was applied to a transition of one site and the second pulse to the corresponding transition of the other site. Detailed analysis of these experiments gave accurate results for the relative orientations of the EFG tensors of the two 14N sites. The angles between the corresponding directions are: φXX = 93°.7, φYY = 16°.7, and φZZ = 91°.0.