Gerhard L. Closs

Electron Paramagnetic Resonance Spectra of the Ground‐State Triplet Diphenylmethylene and Fluorenylidene Molecules in Single Crystals

The paramagnetic resonance spectra of the triplet ground states of diphenylmethylene and fluorenylidene molecules, oriented in single crystals, have been investigated. The fine structures and 13C hyperfine structures of these spectra have been studied and the parameters for best fit by a spin Hamiltonian have been determined. The implications of these results with respect to electron spin distribution and molecular structures have been examined.

Observation of a CIDNP pumped nuclear overhauser effect: a caveat for the interpretation of CIDNP spectra

Abstract The CIDNP spectrum generated in the pyrolysis of the dispiro 1 is explained in the framework of S—T mixing followed by a nuclear Overhauser effect in the product I.

Application of Fourier transform‐NMR spectroscopy to submicrosecond time‐resolved detection in laser flash photolysis experiments

The physical principles that are the foundation of the time‐resolved experiment are summarized by means of a general expression for the experimental observable—the free induction decay: the reaction dynamics are treated within the framework of the radical pair theory of chemically induced dynamic nuclear polarization (CIDNP) in a strong magnetic field. The basic instrumentation used consists of a pulsed nitrogen laser and a commercial, high‐resolution Fourier transform‐NMR spectrometer operating at a resonant frequency of 60 MHz. Modifications to the NMR spectrometer are described that provide for kinetic experiments with a time resolution of 1.2×10−7s and a spectral resolution on the order of hertz. Selected results from the flash photolysis of two ketones that react by Norrish type I cleavage are reported and discussed to exemplify the information obtainable with this new technique. The advantages and the limitations of NMR detection in flash photolysis experiments are considered.

Cyclization and disproportionation kinetics of triplet generated, medium chain length, localized biradicals measured by time-resolved CIDNP

Cinetique de formation de cyclanones et des produits de dismutation a partir de diradicaux oxo-1 polymethylenes

On the overhauser mechanism of chemically induced nuclear polarization as suggested by Adrian

Abstract The recently proposed Overhauser mechanism of nuclear polarization for certain types of reaction is compared with the conventional radical pair mechanism. It is concluded that both mechanisms will work in these reactions if the lifetimes of the free radicals are of the same order of magnitude or longer than the nuclear T 1 .

Polarization transfer in CIDNP experiments via scalar paramagnetic exchange mechanisms

Abstract A theory is outlined explaining how chemically induced nuclear polarization can be transfered by a diamagnetic-paramagnetic exchange reaction from a group of nuclei to another group not originally polarized. The mechanism depends critically on the scalar nuclear spin-spin coupling in the diamagnetic state. Experiments on the photochemical charge transfer between N,N-diethylaniline and pyrene are given as an example of this polarization transfer.

Endor frequencies of randomly oriented triplets by electron spin echo spectroscopy

Abstract ENDOR frequencies of randomly oriented triplets have been measured from the fast Fourier transform of electron spin echo envelope modulation. Important experimental conditions are low temperatures, high microwave power and long phase memory time in a proper form of solvent. The technique is exemplified by using diphenylmethylenes in rigid glasses, polycrystalline forms and single crystals of benzophenones.

Triplet spin dynamics of bacteriochlorophyll-a from transient EPR lineshape analysis

Abstract A study of the transient photoexcited triplet of bacteriochlorophyll-a in a glass matrix at 60 K utilizing light modulation EPR spectroscopy is reported. In view of widely deviating results for the triplet sublevel depopulation rates, this problem was engaged via EPR lineshape analysis rather than direct kinetic measurements. This method also yields the relative population and spin—lattice relaxation rates.

Low-Field Effects and CIDNP of Biradical Reactions

The field dependence of the CIDNP effect based on the radical pair theory is discussed in terms of simple theory. Much use is made of perturbation arguments. Several experimental examples are taken from the literature and compared with theory. Next, the theory of CIDNP derived from biradical reactions is developed based on a simple model for the dynamic processes. The field dependence of the signals is discussed with the aid of experiments involving Norrish Type I cleavage of cycloalkanones. Some deficiencies of the model are pointed out. Two other reactions are also included in the discussion.

The mechanism of the Stevens rearrangement

The influence of solvent viscosity and reaction temperature upon the stereoselectivity and intramolecularity of the Stevens [1,2] rearrangement of the ylide (1) has been examined; the results are compatible with two possible mechanisms: (i) a radical pair mechanism with an average geminate recombination rate which is exceptionally fast, or (ii) dual pathways involving concurrent radical pair and concerted processes.