Ultraviolet absorption studies of the alkali metal atom-oxygen molecule matrix reaction

Alkali metal atoms were codeposited with oxygen at high dilution in argon on a sapphire window cooled to 17 K. Ultraviolet spectra contained a medium intensity absorption near 250 nm which exhibited a long wavelength component between 269 and 275 nm for the heavier alkali metals. The 250 nm band arises from M⁺O₂^? and is assigned to a $\text{π → π}{}^1$ superoxide transition, and the absorption near 270 nm, which increased on sample warming, is attributed to M⁺O₄^?.     

Infrared spectra and structures for group 4 dihydroxide and tetrahydroxide molecules

Hafnium and zirconium atoms react with H(2)O(2) molecules and with H(2) + O(2) mixtures to form M(OH)(2) and M(OH)(4) molecules, which are trapped in solid argon and identified from isotopic shifts in the infrared spectra. Electronic structure calculations at the MP2 level converge to almost linear M(OH)(2) and tetrahedral M(OH)(4) molecules and predict vibrational frequencies for mixed isotopic molecules of lower symmetry that are in excellent agreement with experimental measurements, thus substantiating the identification of hafnium and zirconium dihydroxide and tetrahydroxide molecules. Titanium atoms react to give the same product molecules, but Ti(OH)(4) has an S(4) structure with bent Ti-O-H bonds, Ti(OH)(2) appears to be nearly linear, and the more stable tetravalent HM(O)OH isomer is more prominent for Ti. The Group 4 tetrahydroxides reported here are the first examples of pure metal tetrahydroxide molecules.     

Mining and visualizing large anticancer drug discovery databases

In order to find more effective anticancer drugs, the U.S. National Cancer Institute (NCI) screens a large number of compounds in vitro against 60 human cancer cell lines from different organs of origin. About 70,000 compounds have been tested in the program since 1990, and each tested compound can be characterized by a vector (i.e., "fingerprint") of 60 anticancer activity, or -[log(GI50)], values. GI50 is the concentration required to inhibit cell growth by 50% compared with untreated controls. Although cell growth inhibitory activity for a single cell line is not very informative, activity patterns across the 60 cell lines can provide incisive information on the mechanisms of action of screened compounds and also on molecular targets and modulators of activity within the cancer cells. Various statistical and artificial intelligence methods, including principal component analysis, hierarchical cluster analysis, stepwise linear regression, multidimensional scaling, neural network modeling, and genetic function approximation, among others, can be used to analyze this large activity database. Mining the database can provide useful information: (a) for the development of anticancer drugs; (b) for a better understanding of the molecular pharmacology of cancer; and (c) for improvement of the drug discovery process.     

Quantification of <Superscript>234</Superscript>Th recovery in small volume sea water samples by inductively coupled plasma-mass spectrometry

Summary Development of a small volume (2-4 liter) technique for measuring ²³⁴ Th in sea water has been instrumental in bringing to light small-scale structures in upper-ocean particle removal processes previously missed by standard ²³⁴ Th measurement techniques. Further development of this method to evaluate removal efficiencies of ²³⁴ Th via MnO₂ precipitation quantified using ICP-MS are presented in this work. Advantages to this approach are precise knowledge of ²³⁴ Th recovery, while maintaining high sample throughput afforded by ICP-MS analyzes. The improved technique includes the acidification of 4-liter sea water samples and the addition of ²³⁰ Th as a yield monitor prior to MnO₂ precipitation. Subsequent filtration and beta-counting of the high-energy daughter, ^234mPa, was followed by a final background count after 6 half-lives (144 d) of decay. Filtered precipitates were dissolved with H₂O₂, and an internal standard of ²²⁹ Th was added. Samples were purified using anion-exchange chromatography to remove high levels of manganese, and recoveries were determined by measured ratios of ²³⁰ Th/²²⁹ Th by ICP-MS. Application of this procedure for ²³⁴ Th derived export in the recent Southern Ocean Iron Experiment showed average recoveries of 91%. Corrections for rare low recoveries (25-80%) noticeably change ²³⁴ Th profiles, thus impacting subsequent elemental flux calculations.     

Plane partitions IV: A conjecture of Mills—Robbins—Rumsey

In this paper we consider a problem posed by W Mills D Robbins and H Rumsey for a certain plane partition generating functionZ _n (x, m) The special caseZ _n (1,m) is the generating function that arose in the weak Macdonald conjecture Mills—Robbins—Rumsey conjectured thatZ _n (2,m) also possesses a nice finite product representation Their conjecture is proved as Theorem 1 The method of proof resembles that of the evaluation ofZ _n (1,m) given previously Many results for the₃ F ₂ hypergeometric function are required including Whipple's theorem, the Pfaff-Saalschutz summation and contiguous relations In passing we note that our Lemma 2 provides a new and simpler representation ofZ _n (2,m) as a determinant $$Z_n (2,m) = \det \left( {\delta _{ij} + \sum\limits_{t = 0}^1 {\left( {\mathop {m + j + t}\limits_t } \right)} \left( {\mathop {m + t}\limits_{m + t} } \right)} \right)_{0 \leqq ij \leqq n - 1} $$ Conceivably this new representation may provide new interpretations of the combinatorial significance ofZ _n (2,m) In the final analysis, one would like a combinatorial explanation ofZ _n (2,m) that would provide an algorithmic proof of the Mills Robbins—Rumsey conjecture