Power law relationships between bond length, bond strength and electron density distributions

The strength of a bond, defined as p=s/r, where s is the Pauling bond strength and r is the row number of an M cation bonded to an oxide anion, is related to a build-up of electron density along the MO bonds in a relatively large number of oxide and hydroxyacid molecules, three oxide minerals and three molecular crystals. As p increases, the value of the electron density is observed to increase at the bond critical points with the lengths of the bonds shortening and the electronegativities of the M cations bonded to the oxide anion increasing. The assertion that the covalency of a bond is intrinsically connected to its bond strength is supported by the electron density distribution and its bond critical point properties. A connection also exists between the properties of the electron density distributions and the connectivity of the bond strength network formed by the bonded atoms of a structure. Received: 20 August 1997 / Revised, accepted: 3 November 1997     

A molecular orbital study of bonding in sulfate molecules: Implications for sulfate crystal structures

Molecular orbital calculations have been completed on sulfate monomers and a dimer in a determination of minimum-energy geometries and electron density distributions. SO bond lengths calculated for the monomer and dimer correlate linearly with the fractional s-characters of the bonds, as observed for sulfate groups in crystals. With increasing coordination number of S, the bonded radii of S and O, as determined from electron density maps, increase at the same rate. This is at variance with the assumption that the radius of the oxide ion is nearly constant and that bond length variations arise primarily from changes in cation radii. The dimer shows a relatively large change in energy as its SOS angle is deformed from its minimum-energy value (125.6°) to 180°, in conformity with the small variation among observed angles. This is in contrast to the wide variation of bridging angles observed for silicate and phosphate dimers in crystals and molecules, and may imply that significant differences should be expected in the behavior of sulfates with respect to polymorphism and glass formation. The reaction energy of SO₃ + H₂O → H₂SO₄, calculated with second-order Møller-Plesset perturbation theory, agrees with the experimental value. Other properties of H₂SO₄ are also calculated and compared with experimental observations and previous calculations.     

Molecular mimicry of the bond length-bond strength variations in oxide crystals

Minimum energy theoretical bond lengths R _t obtained with robust split-basis molecular orbital calculations for 27 hydroxyacid molecules containing first- and second-row cations X ⁿ⁺ reproduce XO bond lengths in crystals. Plots of ln(R _t ) vs. ln(s), where s is the Pauling bond strength, define two different but essentially parallel trends (for first- and second-row cations, respectively) as observed for crystals. A new bond strength parameter p=s/r is defined where r=1 for first- and r=2 for second-row main-group cations. When a ln(R _t ) vs. ln(p) plot is prepared with these theoretical bond lengths, a single trend is obtained. A regression analysis of this data set shows that more than 99 percent of the variation of ln(R _t can be explained in terms of a linear dependence on ln(p), yielding R=1.39 p ^?0.22 as an estimator of the bond lengths. A comparison of 153 mean XO bond lengths compiled by Shannon (1976) for main-group closed-shell X-cations from all 6 rows of the periodic table with those estimated with this formula for r=1, 2, ..., 6, respectively, shows that these bond lengths are estimated within 0.05 Å on average with nearly 85 percent estimated within 0.10 Å of the observed value. More than 97 percent of the variation of these observed bond lengths can be ranked in terms of a linear dependence on the estimated bond lengths. The success of these calculations is further evidence that the forces that govern bond length variations in oxide crystals behave as if they are short-ranged.     

Copper,zinc, and organotin as long-term factors governing the distribution of organisms in the fal estuary in Southwest England

Evidence of long-term metal pollution in the Fal Estuary is provided by analyses of Cu in oyster (Ostrea edulis) communities extending over more than 120 yr and by analyses of Cu and Zn in sediments dating back to 1921. Indications are that levels of Cu pollution from mining have declined very slowly but that Zn pollution has not. In Restronguet Creek, the most heavily polluted area, metal levels in waters and sediments are elevated by orders of magnitude. The invertebrate fauna of the creek is limited, but far richer than might be predicted from laboratory toxicity data. The cockleCerastoderma edule exemplifies organisms that are excluded from the creek owing to the toxicity of dissolved Cu, whereasOstrea edulis, by virtue of its innate tolerance to Cu and Zn, grown in the Iower reaches. On the other hand, the population of the ragwornNereis diversicolor, which penetrates into the most heavily polluted areas, is demonstrably far more tolerant of Cu and Zn than population from clean areas and appears to be a tolerant strain. To a considerable degree, therefore, the distribution of species is dependent not only on the innate ability of some species to tolerate Cu and Zn, but on the capacities of other populations to develop metal resistance and thereby maintain their original range of distribution. Present-day problems in the Fal Estuary appear to stem not only from mining wastes but also from the increased usage of tributyltins in antifouling paints. In the dogwhelkNucella lapillus, the incidence of imposex (the appearance of a penis and other male characters in females) is virtually 100% at sites along the English Channel coast of southwest England. The degree of imposex is related to the body concentration of hexane-extractable organotin, which includes both tributyltin and dibutyltin fractions. The highest degree of imposex, where the female penis approaches that of the male in size, occurs in the Fal Estuary; also, in some males the penis becomes deformed. Populations of dogwhelks have declined dramatically in this estuary, and are also characterised by the presence of relatively fewer females and by the absence of juveniles and egg capsules. Reproductive failure in females is now known to be caused by blockage of the pallial oviduct during the development of imposex.     

A comparison of procrystal and ab initio model representations of the electron-density distributions of minerals

 The procrystal calculation of the electron density is a very rapid procedure that offers a quick way to analyze various bonding properties of a crystal. This study explores the extent to which the positions, number, and properties of bond-critical points determined from the procrystal representations of the electron density for minerals are similar to those of first-principles ab initio model distributions. The purpose of the study is to determine the limits imposed upon interpretation of the procrystal electron density. Procrystal calculations of the electron density for more than 300 MO bonds in crystals were compared with those previously calculated using CRYSTAL98 and TOPOND software. For every bond-critical point found in the ab initio calculations, an equivalent one was also found in the procrystal model, with similar magnitudes of electron density, and at similar positions along the bonds. The curvatures of the electron densities obtained from the ab initio and the procrystal distributions are highly correlated. It is concluded that the procrystal distributions are capable of providing good estimates of the bonded radii of the atoms and the properties of the electron-density distributions at the bond-critical points. Because the procrystal model is so fast to compute, it is especially useful in addressing the question as to whether a pair of atoms is bonded or not. If the Bader criteria for bonding are accepted, then the successful generation of the bond-critical points by the procrystal model demonstrates that bonding is an atomic feature. The main difference between the critical-point properties of the procrystal and the ab initio model is that the curvature in the electron density perpendicular to the bond path of the ab initio model is sharper than for the procrystal model. This is interpreted as indicating that the electrons that migrate into a bond originate from its sides, and not from the regions closer to the nuclei. This observation also suggests that ab initio optimization routines could see an improvement in speed if the parameters relating to the angular components of atomic wave functions were to vary before the radial components. Received: 6 August 2001 / Accepted: 21 November 2001     

Electron density distribution and bond critical point properties for forsterite, Mg2 SiO4, determined with synchrotron single crystal X-ray diffraction data

A generalized X-ray scattering factor model experimental electron density distribution has been generated for the orthosilicate forsterite, using an essentially extinction and absorption free set of single crystal diffraction data recorded with intense, high energy synchrotron X-ray radiation (E=100.6 keV). A refinement of the model converged with an R(F)=0.0061. An evaluation of the bond critical point, bcp, properties of the distribution at the (3, –1) stationary points for the SiO and MgO bonded interactions, yielded values that agree typically within ~5%, on average, with theoretical values generated with quantum chemical computational strategies, using relatively robust basis sets. On the basis of this result, the modeling of the experimental distribution is considered to be adequate. As the bcp properties increase in magnitude, the MgO and SiO bonds decrease in length as calculated for a number of rock forming silicates. As asserted by Coppens (X-ray charge densities and chemical bonding. Oxford University Press, Oxford, 1997), large negative ²(r_c) values, characteristic of shared interactions involving first row atoms, may not be characteristic of closed shell covalent bonded interactions involving second row Si, P and S atoms bonded to O. This study adds new evidence to the overall relatively good agreement between theoretical bcp properties generated with computational quantum strategies, on the one hand, and experimental properties generated with single crystal high energy synchrotron diffraction data on the other. The similarity of results not only provides a basis for using computational strategies for studying and modeling structures, defects and the reactivity of representative structures, but it also provides a basis for improving our understanding of the crystal chemistry of earth materials and the character of the SiO bonded interaction.     

Conflict in Common: Heritage-making in Cape York

The outstanding natural and cultural values of Cape York have been acknowledged for decades, but those decades have been characterised by deep conflict. Non-government organisation intervention in local politics has seen a forceful push for nominating some or all of the Cape York Peninsula as a World Heritage Site. We illuminate the authorised heritage discourse at work in heritage-making, and highlight contested issues of ownership, governance, authenticity, and value. These themes contribute to the possibility of marginalising the voices of local people who wish to contribute to heritage-making in Cape York. Politics infuses all aspects of heritage-making in Cape York, and the specific experiences on Cape York reflect larger political processes occurring in World Heritage discourse. The paper draws on interviews undertaken in May and June 2012.     

The organic geochemistry of chrysotile asbestos from the Eastern Townships,Quebec

Extractions of selected specimens of chrysotile from the Eastern Townships of Quebec yielded small quantities of organic material. Infrared and gas Chromatographic studies indicate the presence of predominantly n-alkanes. These were probably derived by distillation from Precambrian sediments although a magmatic origin is not completely ruled out. Sample collection, handling, methods of analysis and problems associated with such studies of chrysotile are discussed.     

Environmental perverse incentives in coastal monitoring

It can be argued that the intensity of monitoring of coastal marine environments lags behind the equivalent terrestrial environments. This results in a paucity of long-term time series of key environmental parameters such as turbidity. This lack of management information of the sources and sinks, and causes and impacts of stressors to the coastal marine environment, along with a lack of co-ordination of information collection is compromising the ability of environmental impact assessments of major coastal developments to discriminate between local and remote anthropogenic impacts, and natural or background processes. In particular, the quasi outsourcing of the collection of coastal information can lead to a perverse incentive whereby in many cases nobody is actively or consistently monitoring the coastal marine environment effectively. This is particularly the case with regards to the collection of long-term and whole-of-system scale data. This lack of effective monitoring can act to incentivise poor environmental performance.