Harden M. McConnell

Harden M. McConnell (July 18, 1927 – October 8, 2014) was a scientist – one of the leading physical chemists of the last 50 years. His work provided the foundation for many areas within science today, and has been internationally recognized by the many awards he has received, including the National Medal of Science, the Wolf Prize, and election to the National Academy of Science."

Harden earned a B.S. degree in chemistry from George Washington University in 1947, and his Ph.D. in chemistry from the California Institute of Technology in 1951 with Norman Davidson. After serving for two years as a National Research Fellow in physics at the University of Chicago with Robert S. Mulliken and John Platt, he held a position as research chemist at Shell Development Company. He was recruited by Norman Davidson, John D. Roberts, and Linus Pauling at the California Institute of Technology in 1956 as Assistant Professor of Chemistry, he was promoted to Professor of Chemistry and Physics in 1963, and in 1964 he moved to Stanford University as a professor. In 1979 he was named Robert Eckles Swain Professor of Chemistry at Stanford University. From September 1989 to September 1992, he was Head of the Department of Chemistry at Stanford. On September 1, 2000, Harden was granted Emeritus status.

He did important research to the understanding of the relation between molecular electronic structure and electron and nuclear magnetic resonance spectra during the period of 1955 through 1965. After that, he developed the technique of spin-labels, whereby electron and nuclear magnetic resonance spectra can be used to study the structure and kinetics of proteins and membranes.

He recognized that the discovery of nuclear hyperfine interactions in aromatic free radicals represented a major breakthrough in the study of the electronic structure of unsaturated hydrocarbons. His theoretical and experimental studies of nuclear hyperfine interactions in such compounds showed conclusively that this interaction gave a measure of the unpaired electron spin densities on the carbon atoms (see McConnell equation for details). His theoretical and experimental investigations of the anisotropic nuclear hyperfine interactions laid a firm foundation for the analysis of the paramagnetic resonance spectra of organic free radicals in. molecular crystals. His work also provided the first experimental demonstration of a negative spin density at a proton. He also realized that certain nitric oxide free radicals had the potential of providing labels for studying molecular motions. His introduction of 'spin labels' has led to a deep understanding of such motions, and to extensive applications in many biological systems of great interest. These motions include the rates of translational diffusion of lipids in bilayer membranes as well as the rates of trans membrane phospholipid "flip-flop". In fact nitric oxide free radical "spin labels" provided some of the earliest evidence for the fluidity of biological membranes.

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