Facile reaction of the model urease complex [Ni2(OAc)3(urea)(tmen)2][OTf] (A) with acetohydroxamic acid (AHA) gives the monobridged hydroxamate complex (I) [Ni2(OAc)2(AA)(urea)(tmen)2][OTf] with a Ni−Ni distance of 3.434(1) Å compared to that of 3.5 Å in urease (OAc, CH3COO-; tmen, N,N,N‘,N‘-tetramethylethylenediamine; OTf, CF3SO3; AHA, acetohydroxamic acid; AA, acetohydroxamate anion). I is a close model of one proposed mode of urease inhibition by hydroxamic acids, recently observed in the acetohydroxamate-inhibited C319A variant of Klebsiella aerogenes urease. Reaction of [Ni2(OH2)(OAc)4(tmen)2] (B) with AHA gives the dibridged hydroxamate complex (II) [Ni2(OAc)(AA)2(tmen)2][OAc] with a Ni−Ni distance of 3.005(1) Å. Infrared spectroscopic studies provide evidence for the bridging acetate groups undergoing carboxylate shifts thereby assisting replacement of acetate by hydroxamate. Both I and II show ferromagnetic exchange coupling.

The nuclear electric quadrupole moment of 41Ca, which contains a single neutron outside closed proton and neutron shells (N=Z=20) has been determined. For this purpose the hyperfine structure (hfs) of the metastable 4s4p 3P 1 state was measured by a highly sensitive technique of combined radiofrequency laser saturation spectroscopy. Because of the small size of the quadrupole interaction constant B corrections for second order effects had to be included in the analysis. Based on corrected hfs constants for the 4s4p 3p~ state of the isotopes 41'43'45Ca and on a recent ab initio calculation of (r -3) integrals [1] one obtains in a consistent way: Q(41Ca)= -80(8) mb Q(43Ca)= -49(5) mb [1] Q(45Ca) = + 46(14) mb.These values include corrections for the electronic core polarization and relativistic effects. Moreover, the hfs anomaly in the 3p~ state was derived from the corrected Afactors and the nuclear magnetic moments of 41'43Ca. The value is 41A43= -1.16(39). 10 -4.

We have measured the neutron lifetime using a superconducting electron spectrometer and a pulsed beam of cold neutrons. Spatially defined neutron bunches are completely contained within the spectrometer's active volume while the /3-decay rate is measured. The flux is determined from the radioactivity of nearly totally absorbing thick samples of cobalt and gold exposed to the neutron beam. We obtain r" =876 ± 21 sec.PACS numbers: 13.30.Ce, 14.20.Dh, 23.40.Bw The measured neutron lifetime provides information about fundamental parameters, in particular the vector and axial-vector weak-coupling constants, Gv and GA> Knowing these quantities is crucial for an understanding of the weak interaction and there are important consequences for cosmology and astrophysics. x At present Gv is best determined from nuclear /?-decay experiments and the experimentally determined neutron lifetime is used to infer GA/GVIn previous experiments electrons or protons were detected from neutrons decaying in continuous thermal or cold beams. Published lifetimes 2 vary between 877 and 937 sec, with errors between 8 and 18 sec. The discrepancies are much larger than the stated errors. Until recently, precise values for GA/GV came only from the experimental lifetime, but the inconsistencies introduce a large systematic uncertainty. Fortunately, a recent measurement of the neutron p asymmetry determines GA/GV better than the lifetime measurements, mitigating the difficult choice between inconsistent lifetimes. 3 Nevertheless, it is important to resolve the lifetime discrepancies. The combination of very precise neutron-decay experiments will eventually allow us to determine both GA and Gv without resorting to more complex nuclear systems.We measured the neutron lifetime with a new method using a pulsed neutron beam. This pulsed beam passes through an "in-beam" electron spectrometer with large active volume. We detect decay electrons during short time intervals of length At ~ 0.5 msec, when neutron bunches are fully contained within the spectrometer and all neutron decays with electron energies above threshold have the same detection probability. The neutron lifetime r n is obtained from the decay law AN n /At = -N n /T n . The integrated number of neutrons N n comes from measurements of the y activity, n r =N n /r r , of thick samples of high-purity 5 Co and 197 Au exposed to the beam. The neutron-decay rate ANjAt is obtained from the measured p rate n p =Np/At. Thus the experimental method is summarized by the simple relation Tn^tyiriy/np) which relates the neutron lifetime to the well-known nuclear lifetimes x y .Compared to continuous-beam lifetime measurements our method does not require precise knowledge of the effective length of the spectrometer or the neutron velocity distribution. In many previous experiments sensitivity to the neutron velocity spectrum is avoided by the measurement of the neutron-capture flux directly with thin detectors containing 3 He or 10 B. However, this procedure relies on the exactness of the l/v dependenc...

Seismic field tests conducted near Tulsa, Oklahoma, and the Handy area in north‐central Texas used linear arrays of vibrators to concentrate sound waves into a beam, which is directed vertically when all vibrators operate in‐phase or at an angle from the vertical when relative time delays are introduced to each vibrator. The sound wave directivity was verified in the Tulsa area by recordings from subsurface seismometers, and at the same time reflection enhancement by wave beaming was exhibited from surface seismometers. Visual inspection and statistical analysis of reflection continuity indicated that it makes no difference in the Handy area whether beam‐forming techniques are used in the field or are applied later in processing. This result was anticipated, since previous seismic work in the Handy area indicated that the random noise level was low enough to minimize the theoretical advantage of field summing.