Showing papers in "Journal of the American Chemical Society in 1961"

Hypochromism and other spectral properties of helical polynucleotides

Abstract: Exciton interaction in a double-stranded helical polymer, such as a homogeneous polynucleotide, leads to a splitting of each excited electronic energy level of the monomer into two exciton bands of energy levels Consideration of the selection rules derived from zeroth-order functions shows that for double-stranded polynucleotides pi yields pi * transitions are allowed to, at most, one exciton level of each band, both polarized perpendicularly to the helix axis; whereas, n yields pi * transitions are allowed only to one level of the upper band, that being polarized parallel to the helix axis Examination of the spectral properties of the four constituents of deoxribonucleic acid (DNA) reveals that although exciton bands probably exist in regions of spectral overlap, there should be isolated energy levels lying below the main exciton bands These levels may act as localized energy traps during excitation propagation through the helix In order to treat the problem of hypochromism, it is necessary to use first-order functions An equation is developed which takes into account the effect of exciton interaction and dispersion-force interaction on the oscillator strength of the helix Application of this equation to a helical, double-stranded polyadenylic acid having the configuration of DNA indicates that perturbationmore » resulting from dispersion-force interaction among the bases can account for the observed hypochromism (auth)« less

Electron Spin Resonance and Polarographic Investigation of Substituted Nitrobenzene Negative Ions

Abstract: : Electron spin resonance spectra were obtained for 14 para substixted nitrobenzene mononegative ions that were generated in acetonitrile solution by electrochemical reduction within a microwave cavity. Polarographic data were also obtained for these para substituents and for 8 meta substituents. An excellent linear correlation was found between the half-wave potentials for the meta substituents and sigma-n and sigma-o values. From gross deviation of p-ni.o and p-amino substituents from the meta correlation line, para resonance energies are estimated. The N14 coupling constant of the nitro group is found to decrease as the ease of reducing the parent molecule is increased by the para substitution and vice versa. Measurement of ring proton coupling constants leads to the conclusion that the total electron spin magnetization in the four central ring pi-orbitals is approximately constant. The para substituent merely effects a redistribution of the magnetization among these ring positions. Evidence based on hyperfine structure is presented for hindered rotation of the aldehyde group of para nitrobenzaldehyde anion, and a maximum rotational frequency of 2,800,000 c.p.s. is estimated. Comparison of the F19 hyperfine coupling constant of p-fluoronitrobenzene with the para proton coupling constant of nitrobenzene leads to the ratio F to H coupling constants = 2.12; from this ratio is estimated that Q-F = 47.5. (Author)