Absolute total electron impact ionization cross-sections for many-atom organic and halocarbon species.

The experimental determination of absolute total electron impact ionization cross-sections for polyatomic molecules has traditionally been a difficult task and restricted to a small range of species. This article reviews the performance of three models to estimate the maximum ionization cross-sections of some 65 polyatomic organic and halocarbon species. Cross-sections for all of the species studied have been measured experimentally using the same instrument, providing a complete data set for comparison with the model predictions. The three models studied are the empirical correlation between maximum ionization cross-section and molecular polarizability, the well-known binary encounter Bethe (BEB) model, and the functional group additivity model. The excellent agreement with experiment found for all three models, provided that calculated electronic structure parameters of suitably high quality are used for the first two, allows the prediction of total electron-impact ionization cross-sections to at least 7% precision for similar molecules that have not been experimentally characterized.

Adapting APSIM to model the physiology and genetics of complex adaptive traits in field crops.

Progress in molecular plant breeding is limited by the ability to predict plant phenotype based on its genotype, especially for complex adaptive traits. Suitably constructed crop growth and development models have the potential to bridge this predictability gap. A generic cereal crop growth and development model is outlined here. It is designed to exhibit reliable predictive skill at the crop level while also introducing sufficient physiological rigour for complex phenotypic responses to become emergent properties of the model dynamics. The approach quantifies capture and use of radiation, water, and nitrogen within a framework that predicts the realized growth of major organs based on their potential and whether the supply of carbohydrate and nitrogen can satisfy that potential. The model builds on existing approaches within the APSIM software platform. Experiments on diverse genotypes of sorghum that underpin the development and testing of the adapted crop model are detailed. Genotypes differing in height were found to differ in biomass partitioning among organs and a tall hybrid had significantly increased radiation use efficiency: a novel finding in sorghum. Introducing these genetic effects associated with plant height into the model generated emergent simulated phenotypic differences in green leaf area retention during grain filling via effects associated with nitrogen dynamics. The relevance to plant breeding of this capability in complex trait dissection and simulation is discussed.

On the electron affinity of nitromethane (CH3NO2).

A high-level systematic computational study is presented on an accurate value for the adiabatic valence electron affinity of nitromethane, CH(3)NO(2), to resolve literature disagreements in theoretical and experimental reported values. Density functional methods with triple-zeta quality basis sets gave good fortuitous agreement to early experimental determinations, while single-reference wave function based methods employing up to CCSD(T) gave poor or fortuitous agreement depending on the experimental reference value. DFT methods in general cannot accurately describe electron attachment from the result of unphysical self-interaction. It is found that multireference methods with aug-cc-pVTZ or similar basis sets are required to converge to an experimental value. Our highest level of theory 3S-MCQDPT2 and 7S-MCQDPT2 calculations with an aug-cc-pVTZ quality basis description yield values of 0.188 and 0.176 eV (0.170 eV with polynomial extrapolation), in excellent agreement with the most recent experimental value of 0.172 +/- 0.006 eV. CCSD(T)//aug-cc-pVTZ provides a fortuitously reasonable description. The isolated dipole-bound anion binding energy is tentatively calculated to be 7-8 meV.

Crossed-beam studies of electron transfer to oriented trichloronitromethane, CCl(3)NO(2), molecules.

Fast potassium atoms donate an electron to CCl(3)NO(2) molecules to form K(+) ions and the negative ions O(-), Cl(-), NO(2) (-), CCl(3) (-), CCl(2)NO(2) (-), CCl(3)NO(-), and CCl(3)NO(2) (-). Threshold energies are measured for these ions and electron affinities for CCl(2)NO(2) (-), CCl(3)NO(-), and CCl(3)NO(2) (-) are estimated to be 2.35, 2.35, and 1.89 eV (+/-0.6 eV), respectively. The threshold energies show that the C-N and N-O bonds are substantially weaker than in nitromethane. The CCl(3)NO(2) molecules are oriented before the collision and at energies near 2.5 eV the electron appears to transfer to the pi( *) (NO) orbital forming the parent negative ion, CCl(3)NO(2) (-), which is stabilized by interacting with the K(+) donor. As the collision energy increases the parent negative ion fragments and the orientation dependence of the fragment ions helps understand the fragmentation pathway.

Fiber-optic infrared reflection absorption spectroscopy for trace analysis on surfaces of varying roughness. Part II: Acetaminophen on stainless steel.

Investigations of the effects of surface roughness on the utility of grazing-angle Fourier transform infrared reflection absorption spectroscopy (IRRAS) as a method for quantifying trace contamination of metal surfaces have been extended to acetaminophen, a model active pharmaceutical agent, on 316 stainless steel. The effects are more complicated than for the surfactant sodium dodecyl sulfate (SDS) on stainless steel; they include a strong surface-finish dependence of sensitivity and nonlinear behavior at surface loadings above approximately 1-2 microg cm(-2). Using data from samples in the loading range 0-0.5 microg cm(-2), unbiased partial least squared calibrations can be readily achieved for individual surface finishes with detection limits of L(D) approximately 0.15 microg cm(-2). However, as found for SDS on stainless steel, models built using data from samples of mixed surface roughness are more problematic.

Steric effects in electron transfer from potassium to pi-bonded oriented molecules CH3CN, CH3NC, and CCl3CN.

Electron transfer from K atoms to oriented CH3CN, CH3NC, and CCl3CN is studied in crossed beams at energies near the threshold for forming an ion pair. For the methyl compounds, the dominant ions are K+ and CN-; the steric asymmetry is very small and energy-independent, characteristic of sideways attack with the electron apparently entering the pi*CN antibonding orbital. Migration of the electron to the sigma*CC orbital to break the C-C bond is greatly facilitated by interaction with the atomic donor. CH2CN- is formed in collisions preferring CH3-end attack, and the steric asymmetry becomes very large near threshold. CCl3CN mostly forms Cl- in collisions slightly favoring the CCl3 end with a small energy dependence with the electron apparently entering the sigma* LUMO. CN- is formed in much smaller yield with a slight preference for the CN end. The parent negative ion CCl3CN- is observed, and a lower limit for its electron affinity is estimated to be 0.3 eV. Fragment ions CCl2CN- and CClCN- are also observed with upper limits for the quantity bond dissociation energy - electron affinity (BDE - EA) estimated to be 0.6 and 1.0 eV, respectively.

Grazing-angle fiber-optic fourier transform infrared reflection-absorption spectroscopy for the in situ detection and quantification of two active pharmaceutical ingredients on glass.

Fourier transform infrared reflection-absorption spectroscopy has been used with a fiber-optic grazing-angle reflectance probe as a rapid, in situ method for trace surface analysis of acetaminophen and aspirin at loadings of approximately 0-2 microg cm(-2) on glass. Partial least-squares multivariate regression permits the loadings to be quantified, simultaneously, with root-mean-squared errors of prediction of RMSEP approximately 0.1 microg cm(-2) for both compounds. The detection limits are estimated to be LD approximately 0.2 microg cm(-2).

Fiber-optic infrared reflection absorption spectroscopy for trace analysis on surfaces of varying roughness: Sodium dodecyl sulfate on stainless steel.

The effect of surface roughness on the analytical performance of a fiber-optic grazing-angle infrared reflection absorption spectroscopy (IRRAS) system has been investigated. The instrument was used to develop calibrations to quantify trace surface loadings for sodium dodecyl sulfate (SDS) on 316 stainless steel with three different surface finishes. Partial least squares (PLS) calibrations were developed for both individual finishes and combinations of finishes. For SDS on individual surface finishes, the root mean square (rms) error of cross-validation (RMSECV) varied between 0.08 and 0.12 microg cm(-2) with values of R2 between 0.89 and 0.96. A combined model for SDS on all surfaces gave an RMSECV of 0.18 microg cm(-2) with an R2 of 0.85.

Steric asymmetry in electron transfer from potassium atoms to oriented nitromethane (CH3NO2) molecules.

Electrons are transferred in collisions between potassium atoms and CH(3)NO(2) molecules that have been oriented in space prior to collision. The electron transfer produces K(+) ions, parent negative ions CH(3)NO(2)(-), and the fragment ions e(-), NO(2)(-), and O(-) in amounts that depend on the energy. The positive and negative ions are detected in coincidence by separate time-of-flight mass spectrometers at various collision energies for both CH(3)-end attack and NO(2)-end attack. The steric asymmetry for electrons and CH(3)NO(2)(-) is essentially zero, but the steric asymmetry for NO(2)(-) shows that NO(2)(-) is formed mainly in CH(3)-end collisions. There is evidence that the electrons and NO(2)(-) have the same transient precursor, despite having different steric asymmetries. It appears likely that the precursor is formed by electron transfer mainly in collisions normal to the molecular axis leading to near zero steric asymmetry for the electron. This transient precursor can also eject an NO(2)(-) ion, which is more likely to be removed as KNO(2) salt when K(+) ions are near the NO(2) end of the molecule, with the result that CH(3)-end collisions seem to produce more NO(2)(-).

Electron transfer from sodium to oriented nitromethane, CH3NO2: probing the spatial extent of unoccupied orbitals.

Beams of sodium atoms with energies of a few eV are crossed with a beam of oriented CH3NO2 molecules to study the effect of collision energy and orientation on electron transfer. The electron transfer produces Na+ ions and free electrons, parent negative ions (CH)NO2-), and fragmentation ions NO2- and O- in proportions that depend on the collision energy. The steric asymmetry is very small or zero and suggests that production of all of the ions is favored by sideways attack with respect to the permanent dipole along the C-N axis. In these experiments, the electron appears to be transferred into the 2B1 state of the anion comprising mainly the pi*NO LUMO, producing a valence-bound state rather than a dipole-bound state.

Dramatic steric behavior in electron transfer from various donors to CF3Br.

Different alkali metal atoms are observed to donate electrons to CF(3)Br molecules that are oriented in space. For collision energies high enough to overcome the Coulomb attraction, a positive ion/negative ion pair is observed and mass-analyzed using coincident time-of-flight mass spectroscopy. The alkali metal cation and various negative ions are observed. The most abundant negative ion is the bromide ion, Br(-), formed preferentially by attack at the Br end of the molecule. The steric asymmetry to produce Br(-) is almost identical for all of the alkali metal donors. Fluoride ions are formed in smaller abundance and reflect completely different behavior among the donors. Sodium and potassium have high thresholds and prefer the CF(3) end of the molecule, whereas cesium prefers the Br end of the molecule. Sodium and potassium apparently interact with the transient CF(3)Br(-) molecular negative ion while it is in the process of decomposing.