A comparison of different methods in determining energies and geometries of open-shell systems

Three methods of obtaining eigenvectors for open-shell systems, namely the Roothaan restricted open-shell method, the “half-electron” method, and the use of ground state orbitals, are compared with each other both on a formal basis, and by contrasting theab initio energies predicted by these schemes for a series of radicals and triplets. The r.m.s. improvement in the energy of eight radicals by use of the Roothaan method rather than the half-electron approximation amounts to 4.8 kcal mole^?1, whereas the r.m.s. improvement for nineteen triplet states is 8.2 kcal mole^?1. Optimum geometries predicted by these two methods do not differ appreciably. The use of closed-shell eigenvectors rather than those of the half-electron method leads to very large errors for triplet states whose electron density distributions differ appreciably from those of the ground state.     

Photolytic covalent binding of indoleacrylic acid to DNA

E-beta-Indol-3-ylacrylic acid (IA), radiolabelled at the 2 position with 14C, and calf thymus DNA have been irradiated with UV light (lambda greater than 280 nm) in phosphate buffer (pH 7.0). Re-isolation of the DNA indicates covalent binding of IA at levels of up to 295 nmol IA/mg native DNA (0.097 IA/base). Binding is observed for both native and heat denatured DNA, but is more efficient with the latter. Quantum efficiencies of 2.60 X 10(-5) and 2.30 X 10(-4) mol IA bound to native DNA/mol photon absorbed have been measured at 308 and 266 nm, respectively. Studies with the four polyribonucleotides indicate a strong preference for binding to poly[U]. Photolysis of either untreated or enzymatically degraded labeled native DNA with 254 nm light leads to the reformation of IA, and a 2 + 2 photocycloadduct of IA and thymidine has been isolated and characterized and matched by HPLC to a DNA derived adduct. Equilibrium dialysis studies provide no evidence for preassociation of IA to DNA.     

MNDO study of catenated sulfur: The molecules and ions S3 to S8

Semiempirical molecular orbital calculations by the MNDO method are reported for sulfur rings and chains S₃ through S₈, for the corresponding dications, S through S, and for S. The MNDO method seems quite successful in predicting the geometries of neutral catenated sulfur molecules, even the unusual bond-length alternation and extent of coplanarity in cyclo-S₇. In contrast to hydrocarbon rings, for which its prediction of strain is erratic, MNDO is consistent in its calculated strain energies in small cyclosulfur rings; unfortunately all the strain energies are overestimated by 70%. As a consequence of this error, the method must be considered unreliable in its predictions of structures for the dications S, since many of these ions could potentially exist as strained bicyclic systems. In addition, MNDO appears to have difficulty handling long, partial SS σ bonds, as are found to occur in S. It may be for this reason that MNDO predicts, apparently incorrectly, that the open-chain isomers of S are more stable than are any of the cyclic forms, at least for S to S. With respect to neutral Sⁿ molecules, however, the MNDO predictions appear more reliable than ab initio molecular orbital (MO ) calculations using small basis sets without polarization functions and without configuration interaction (CI ). However, MNDO apparently underestimates by about a factor of two the strength of the three-electron π bonds present in the terminal links of sulfur diradical chains.     

Spectrophotometric determination of copper, nickel, cobalt, iron and manganese as their pyridine thiocyanates Simultaneous determination in mixtures

Spectrophotometric determinations of copper, nickel, cobalt, iron, and manganese, based on the chloroform extraction of the metal pyridine thiocyanates, have been investigated. Optimum conditions require the pH of the aqueous solution to be in the range about 5–8; tartaric acid is used to prevent precipitation of hydrous oxides. An excess of pyridine must be used because chloroform readily extracts pyridine from the aqueous solution. Results are improved by making the extraction from a solution of high ionic strength (2 or above), which is provided by magnesium nitrate. Perchlorate decreases the absorbance, but the effect is essentially constant over a perchlorate concentration range of 0.8 to 2M. Although the metal pyridine thiocyanates are extracted by benzene, substituted benzenes, and halogenated hydrocarbons, chloroform is superior to other solvents in extraction efficiency and in colour stability of the extracted species. An example is given of the simultaneous determination of copper, nickel, cobalt, and iron in the same solution. Anions that also form metal pyridine compounds must be absent.     

Detection of benzo[a]pyrene sulfate and glucuronide conjugates in cell culture medium by directly coupled microbore high-performance liquid chromatography-fast atom bombardment mass spectrometry.

An improved method is described for detecting glucuronide and sulfate conjugates of benzo[a]pyrene in medium from cell cultures treated with benzo[a]pyrene. This method is based on a microbore high-performance liquid chromatograph directly coupled to a high-resolution continuous-flow fast atom bombardment mass spectrometer. Sulfate and glucuronide conjugates, as well as some structural isomers of glucuronide conjugates, were fully separated by the reversed-phase microbore high-performance liquid chromatography conditions used in this study. Since the method does not rely on the use of radiolabeled materials, it may be used to detect conjugates of a wide variety of hydrocarbons. The high sensitivity and selectivity of the method were demonstrated by detecting conjugates in the media of cell cultures derived from mice, hamsters and humans.     

On relative stability of linear stationary feedback control systems with time delay

A method is presented whereby the absolute and relative stabilityof linear control systems containing transport lag can be determined.As a result feedback systems with variable time delay and loopgain, may be investigated in straightforward manner.     

What Factors Affect Attitudes Toward Women in Science Held by High School Biology Students?

To achieve the goal of science for all Americans, students of both genders must believe that careers in science are equally appropriate for women. Yet male and female students in high school science classes do not have the same views of women in science. This study investigated the influence of 17 factors on high school students' attitudes toward women in science. Data were collected from 844 students enrolled in biology classes in an urban school district in Georgia. Multiple regression determined that the 17 factors significantly influence students' attitudes toward women in science, accounting for 28% of common variance. The four most significant factors - student gender, science ability, level of education the student plans to complete, and career interest-accounted for 24.6% of total variance. Female students who have high science ability, plan to complete high levels of education, and who have career interests in science showed more favorable attitudes toward women in science. Males with low science ability, low levels of education they plan to complete, and no interest in science as a career had the least favorable attitudes toward women in science. Male students with less positive attitudes toward science careers for women need to be included in programs aimed at encouraging all students to consider science careers.     

New chemical reactivity in aromatic dithiocarbamate and monothiocarbamate ligands: syntheses of Mo2L4·2THF (L = Pyrrole or indole monothiocarbamate or pyrrole dithiocarbamate) and Mo2L4 (L = indoline monothiocarbamate)

The reactions between Mo₂(C₂H₃O₂)₄ and several new dithio- and monothiocarbamates have been studied. The first example of a dimolybdenum compound of a dithiocarbamate with the “molybdenum acetate” structure is reported (Mo₂L₄·2THF; L = pyrrole dithiocarbamate). In addition, the synthesis of Mo₂L′₄·2THF (L′ = pyrrole monothiocarbamate, indole monothiocarbamate, indole dithiocarbamate) and Mo₂L″₄ (L″ = indoline monothiocarbamate) are reported. A discussion of the unique stabilization of the dithiocarbamate bridge by “aromatic” dithio- and monothiocarbamates is presented.