Marzilli, et aZ.suggested by Heilmeier and Harrison." This would enhance the shielding of the s electrons by the d electrons and thereby decrease the s-electron density on the iron nucleus. Therefore, this can account for the larger chemical shifts observed. The degree of delocalization of electrons in d orbitals required to produce the observed increase can be roughly estimated as 0.02 electron. This estimate is based on the observation that a complete removal of a d electron, as in going from Fez' to Fe3+, results in a change of about 1 mmlsec in the isomer shift. orbitals toward the V,, are negative and therefore a slight increase in the electronic density in these orbitals could only lead to a small diminution in the quadrupole splittings of the fi forms. However, we observe an enhancement of A. It seems that the two axial nitrogens are not situated exactly on the octahedral positions but form a very slightly distorted octahedron. The slightly asymmetrical situation of the nitrogens presumably results in a stronger interaction with one of the 3d,,,,, orbitals and thereby lifts the degeneracy of the orbitals. This would lead to a nonzero value for the asymmetry parameter of the field gradient, 7 . The decrease in quadrupole splitting due to flow of about 0.02 electron in the 3d,,,,, can be more than compensated by the increase in A arising from the asymmetry parameter 7 , i.e., '/3(Zfif)2 in the expression. An orbital occupation of approximately (3d,,)'.6 (3d,,)''4 or vice versa for the fi forms can account for the experimentally observed increase in quadrupole splitting. port from NSF under grant GB-3642 1.Cl,, 16413-89-1; phthalic anhydride, 8544-9; urea, 57-13-6.Regarding the differences in A , the contributions of 3d,,,y, a Isomer shifts are reported with respect to iron. * A. Hudson and H.The pmr spectra of cobaloximes LCo(DH),X, with X = NO,, SeCN, C1, Br, NCS, N,, SCN, i-PrXan, CN, NO,, and CH,, L = 4-terr-butylpyridine (t-Bu(py)) and tri-n-butylphosphine (Bu,P), and DH = monoanion of dimethylglyoxime, are reported. Except for the linear triatomics, which form a bent bond to cobalt (N,, SCN, SeCN), the chemical shifts of the cis dioxime methyl resonance and the trans o! H resonance of (t-Bu(py))Co(DH),X are linearly related. The slope of the line is approximately in agreement with the screening formula of an axially anisotropic group with the cobalt at the center. Analysis of the dependence of the chemical shifts on X in (t-Bu(py))Co(DH),X and of the variation of coupling constants in Bu,PCo-(DH),X is consistent with rehybridization of the Co-X bond. Specifically, it is suggested that the degree of 4s character in the Co-X bond increases along the series. According to Ramsey's equation, the paramagnetic contribution perpendicular to the axial direction should decrease. This interpretation differs from a previous study of cobaloximes.' that previous work utilized incorrectly formulated compounds. More specific synthetic procedures are reported, as is the fractional separation of the N-and S-bonded thiocy...

An antifibrin antibody (T2G1s) Fab' fragment labeled with technetium-99m was tested for its ability to produce images of fresh thrombi in dogs. In gamma camera images, all thrombi were evident by 2-4 hours after injection. Mean thrombus-to-blood and thrombus-to-muscle ratios averaged 4.0 and 69 at four hours after injection and increased to 24 and 270, respectively, by 24 hours after injection. When compared with I-125 fibrinogen injected into the same dogs, Tc-99m-antifibrin Fab' had lower absolute uptake in thrombus but higher thrombus-to-blood ratios due to a faster rate of disappearance from the blood. The primary route of excretion was through the kidneys. Tc-99m-antifibrin Fab' was highly stable in vivo, with an average of 82% of the circulating radioactivity able to bind to fibrin at 4 hours after injection. When compared with an In-111-labeled Fab fragment of antifibrin antibody 59D8, thrombus-to-blood and thrombus-to-muscle ratios were slightly higher for the Tc-99m-labeled antibody, and the blood disappearance rate was slightly faster. The absolute uptake in thrombus, however, was not significantly different, and the thrombus was visualized at about the same time after injection. These studies suggest that Tc-99m T2G1s Fab' is a potential agent for detecting thrombi in a clinical setting.

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