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Complex vibronic structure has been observed in the electric-dipole-forbidden charge transfer absorption bands of solid and matrix-isolated UF 6 in the temperature range 8-14 K. These bands have their maximum intensity near 260 and 375 nm. Associated with the 260 nm band are four electronic transitions with origins at 30331, 31032, 32120, and 32821 cm-" the first two being observed directly. Two more nophonon transitions are associated with the weak band at 375 nm, one at 24564 cm-I and another at 25265 cm-I . These levels are assigned via a weak j-j coupling scheme as excitations from the ligand tluG" orbital to empty uranium Sf orbitals. Uranium spin-orbit coupling in UF 6 charge transfer states strongly resembles that of UF 6 -. Several progressions in the symmetric stretch frequency VI = 580-595 cm-I are present which have as their origins various combinations of the above electronic levels with the bending modes V" vs, and V6 or their overtones 2v •• 2vs, and 2V6' Temperature effects, which account for the principal differences between the gas and solid spectra, are also discussed. A much more intense band at 214 nm is lacking in vibronic structure and is attributed to an allowed charge transfer transition.Although the Cary 14 spectrometer appeared to have more than adequate resolution for the UF 8 spectrum, a

The preferred conformation of cyclic 8-adenosine 3',5'-phosphate in aqueous solution has been determined from shifts of proton magnetic resonance signals caused by lanthanide ions. The conformation of the ribose and phosphate groups is consistent with the structure in the crystalline state. At pD 5.3, the purine base is in the syn conformation, with the best agreement (R = 0.048) of calculated and observed shifts at a glycosyl torsion angle of 86°. Reasonable error limits for the torsion angle determination are ±22°. Experiments were performed over a pD range of 2.2-5.3 with a nucleotide concentration range of 0.02-0.072 M, and a lanthanide nucleotide ratio ranging from 0 to 15, without significant changes in the association constant of the lanthanidenucleotide complexes. Keq at pD 5.3,13.0 ± 1.7, and at pD 2.2, Keq = 14.2 ± 2.9. Avery versatile hormonal messenger is cyclic 8-adenosine 3',5'-phosphate (3',5'-AMP).1 2 The crystal structure of 3',5'-AMP shows two molecules in the asymmetric unit which have very different orientations of the purine about the glycosyl bond.3 In one mole-(1) (a) Presented in part at the 29th Southwest Regional Meeting of the American Chemical Society, El Paso, Texas, Dec 1973. (b) Colorado State University, (c) Los Alamos Scientific Laboratory.(2) G.

Nmr of 017 in HC1 Solutions of Co2+ 121 1:1 complex as mentioned previously, in contrast to the 2-aminoethyl phosphate chelate system (the difference between successive stability constants, 0.39 in log K unit). Such a low stability of the 1:2 complex may be ascribed to the two dominating factors: the low basicity of pyridyl nitrogen and the steric effect of the pyridine ring, which may generate a strong repulsive force against the second ligand.

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