The average local ionization energy as a tool for identifying reactive sites on defect-containing model graphene systems.

In a continuing effort to further explore the use of the average local ionization energy [Formula: see text] as a computational tool, we have investigated how well [Formula: see text] computed on molecular surfaces serves as a predictive tool for identifying the sites of the more reactive electrons in several nonplanar defect-containing model graphene systems, each containing one or more pentagons. They include corannulene (C20H10), two inverse Stone-Thrower-Wales defect-containing structures C26H12 and C42H16, and a nanotube cap model C22H6, whose end is formed by three fused pentagons. Coronene (C24H12) has been included as a reference planar defect-free graphene model. We have optimized the structures of these systems as well as several monohydrogenated derivatives at the B3PW91/6-31G* level, and have computed their I(r) on molecular surfaces corresponding to the 0.001 au, 0.003 au and 0.005 au contours of the electronic density. We find that (1) the convex sides of the interior carbons of the nonplanar models are more reactive than the concave sides, and (2) the magnitudes of the lowest I(r) surface minima (the I S, min) correlate well with the interaction energies for hydrogenation at these sites. These I S, min values decrease in magnitude as the nonplanarity of the site increases, consistent with earlier studies. A practical benefit of the use of I(r) is that a single calculation suffices to characterize the numerous sites on a large molecular system, such as graphene and defect-containing graphene models.

Σ-holes, π-holes and electrostatically-driven interactions.

A positive π-hole is a region of positive electrostatic potential that is perpendicular to a portion of a molecular framework. It is the counterpart of a σ-hole, which is along the extension of a covalent bond to an atom. Both σ-holes and π-holes become more positive (a) in going from the lighter to the heavier atoms in a given Group of the periodic table, and (b) as the remainder of the molecule is more electron-withdrawing. Positive σ- and π-holes can interact in a highly directional manner with negative sites, e.g., the lone pairs of Lewis bases. In this work, the complexes of 13 π-hole-containing molecules with the nitrogen lone pairs of HCN and NH(3) have been characterized computationally using the MP2, M06-2X and B3PW91 procedures. While the electrostatic interaction is a major driving force in π-hole bonding, a gradation is found from weakly noncovalent to considerably stronger with possible indications of some degree of coordinate covalency.

Intra- and intermolecular electrostatic interactions and their significance for the structure, acidity, and tautomerization behavior of trinitromethane.

We have addressed several interesting issues related to trinitromethane: the propellerlike arrangement of its nitro groups, its lower-than-predicted (although still high) acidity, its aci tautomerization, and the absence of expected very short C-HO intermolecular bridges in the crystal. A combination of crystallographic and computed data was used in our analysis. The structural features mentioned and the anomalous acidity can be attributed to intra- and/or intermolecular electrostatic interactions. Decomposition via C-NO(2) bond scission may occur before aci tautomerization can take place.

Evaluation of three short-listing methodologies for selection into postgraduate training in general practice.

OBJECTIVE: This study aimed to evaluate the effectiveness and efficiency of three short-listing methodologies for use in selecting trainees into postgraduate training in general practice in the UK. METHODS: This was an exploratory study designed to compare three short-listing methodologies. Two methodologies - a clinical problem-solving test (CPST) and structured application form questions (AFQs) - were already in use for selection purposes. The third, a new situational judgement test (SJT), was evaluated alongside the live selection process. An evaluation was conducted on a sample of 463 applicants for training posts in UK general practice. Applicants completed all three assessments and attended a selection centre that used work-related simulations at final stage selection. Applicant scores on each short-listing methodology were compared with scores at the selection centre. RESULTS: Results indicate the structured AFQs, CPST and SJT were all valid short-listing methodologies. The SJT was the most effective independent predictor. Both the structured AFQs and the SJT add incremental validity over the use of the CPST alone. Results show that optimum validity and efficiency is achieved using a combination of the CPST and SJT. CONCLUSIONS: A combination of the CPST and SJT represents the most effective and efficient battery of instruments as, unlike AFQs, these tests are machine-marked. Importantly, this is the first study to evaluate a machine-marked SJT to assess non-clinical domains for postgraduate selection. Future research should explore links with work-based assessment once trainees are in post to address long-term predictive validity.

Expansion of the sigma-hole concept.

The term "sigma-hole" originally referred to the electron-deficient outer lobe of a half-filled p (or nearly p) orbital involved in forming a covalent bond. If the electron deficiency is sufficient, there can result a region of positive electrostatic potential which can interact attractively (noncovalently) with negative sites on other molecules (sigma-hole bonding). The interaction is highly directional, along the extension of the covalent bond giving rise to the sigma-hole. Sigma-hole bonding has been observed, experimentally and computationally, for many covalently-bonded atoms of Groups V-VII. The positive character of the sigma-hole increases in going from the lighter to the heavier (more polarizable) atoms within a Group, and as the remainder of the molecule becomes more electron-withdrawing. In this paper, we show computationally that significantly positive sigma-holes, and subsequent noncovalent interactions, can also occur for atoms of Group IV. This observation, together with analogous ones for the molecules (H3C)2SO, (H3C)2SO2 and Cl3PO, demonstrates a need to expand the interpretation of the origins of sigma-holes: (1) While the bonding orbital does require considerable p character, in view of the well-established highly directional nature of sigma-hole bonding, a sizeable s contribution is not precluded. (2) It is possible for the bonding orbital to be doubly-occupied and forming a coordinate covalent bond.

Unusual odd-electron fragments from even-electron protonated prodiginine precursors using positive-ion electrospray tandem mass spectrometry.

Reports of anticancer and immunosuppressive properties have spurred recent interest in the bacterially produced prodiginines. We use electrospray tandem mass spectrometry (ES-MS/MS) to investigate prodigiosin, undecylprodiginine, and streptorubin B (butyl-meta-cycloheptylprodiginine) and to explore their fragmentation pathways to explain the unusual methyl radical loss and consecutive fragment ions that dominate low-energy collision-induced dissociation (CID) mass spectra. The competition between the formation of even-electron ions and radical ions is examined in detail. Theoretical calculations are used to optimize the structures and calculate the energies of both reactants and products using the Gaussian 03 program. Results indicate that protonation occurs on the nitrogen atom that initially held no hydrogen, thus allowing formation of a pseudo-seven-membered ring that constitutes the most stable ground state [M + H](+) structure. From this precursor, experimental data show that methyl radical loss has the lowest apparent threshold but, alternatively, even-electron fragment ions can be formed by loss of a methanol molecule. Computational modeling indicates that methyl radical loss is the more endothermic process in this competition, but the lower apparent threshold associated with methyl radical loss points to a lower kinetic barrier. Additionally, this characteristic and unusual loss of methyl radical (in combination with weaker methanol loss) from each prodiginine is useful for performing constant neutral loss scans to quickly and efficiently identify all prodiginines in a complex biological mixture without any clean-up or purification. The feasibility of this approach has been proven through the identification of a new, low-abundance prodigiosin analog arising from Hahella chejuensis.

Why are dimethyl sulfoxide and dimethyl sulfone such good solvents?

We have carried out B3PW91 and MP2-FC computational studies of dimethyl sulfoxide, (CH(3))(2)SO, and dimethyl sulfone, (CH(3))(2)SO(2). The objective was to establish quantitatively the basis for their high polarities and boiling points, and their strong solvent powers for a variety of solutes. Natural bond order analyses show that the sulfur-oxygen linkages are not double bonds, as widely believed, but rather are coordinate covalent single S(+)-->O(-) bonds. The calculated electrostatic potentials on the molecular surfaces reveal several strongly positive and negative sites (the former including sigma-holes on the sulfurs) through which a variety of simultaneous intermolecular electrostatic interactions can occur. A series of examples is given. In terms of these features the striking properties of dimethyl sulfoxide and dimethyl sulfone, their large dipole moments and dielectric constants, their high boiling points and why they are such good solvents, can readily be understood.

Blue shifts vs red shifts in sigma-hole bonding.

Sigma-hole bonding is a noncovalent interaction between a region of positive electrostatic potential on the outer surface of a Group V, VI, or VII covalently-bonded atom (a sigma-hole) and a region of negative potential on another molecule, e.g., a lone pair of a Lewis base. We have investigated computationally the occurrence of increased vibration frequencies (blue shifts) and bond shortening vs decreased frequencies (red shifts) and bond lengthening for the covalent bonds to the atoms having the sigma-holes (the sigma-hole donors). Both are possible, depending upon the properties of the donor and the acceptor. Our results are consistent with models that were developed earlier by Hermansson and by Qian and Krimm in relation to blue vs red shifting in hydrogen bond formation. These models invoke the derivatives of the permanent and the induced dipole moments of the donor molecule.

Sigma-hole bonding: molecules containing group VI atoms.

It has been observed both experimentally and computationally that some divalently-bonded Group VI atoms interact in a noncovalent but highly directional manner with nucleophiles. We show that this can readily be explained in terms of regions of positive electrostatic potential on the outer surfaces of such atoms, these regions being located along the extensions of their existing covalent bonds. These positive regions can interact attractively with the lone pairs of nucleophiles. The existence of such a positive region is attributed to the presence of a "sigma-hole." This term designates the electron-deficient outer lobe of a half-filled p bonding orbital on the Group VI atom. The positive regions become stronger as the electronegativity of the atom decreases and its polarizability increases, and as the groups to which it is covalently bonded become more electron-withdrawing. We demonstrate computationally that the sigma-hole concept and the outer regions of positive electrostatic potential account for the existence, directionalities and strengths of the observed noncovalent interactions.

An overview of halogen bonding.

Halogen bonding (XB) is a type of noncovalent interaction between a halogen atom X in one molecule and a negative site in another. X can be chlorine, bromine or iodine. The strength of the interaction increases in the order Cl

Analysis of the reaction force for a gas phase S(N)2 process: CH3Cl + H2O --> CH3OH + HCl.

The "reaction force" F(R(c)) is the negative derivative of a system's potential energy V(R(c)) along the intrinsic reaction coordinate of a process. If V(R(c)) goes through a maximum, as is commonly the case, then F(R(c)) has a characteristic profile: a negative minimum followed by zero at the transition state and then a positive maximum. These features reflect four phases of the reaction: an initial one of reactant preparation, followed by two of transition to products, and then relaxation of the latter. In this study, we have analyzed, in these terms, a gas-phase S(N)2 substitution, selected to be CH3Cl + H2O --> CH3OH + HCl. We examine, at the B3LYP/6-31G level, the geometries, energetics, and molecular surface electrostatic potentials, local ionization energies, and internal charge separation.

An unusual feature of end-substituted model carbon (6,0) nanotubes.

We have examined the effects of substituents on the computed electrostatic potentials V(S)(r) and average local ionization energies I(S)(r) on the surfaces of model carbon nanotubes of the types (5,5), (6,1) and (6,0). For the (5,5) and the (6,1), the effects upon both V(S)(r) and I(S)(r) of substituting a hydroxyl group at one end are primarily localized to that part of the system. For the (6,0) tube, however, a remarkable change is observed over its entire length, with V(S)(r) showing a marked gradation from strongly positive at the substituted end to strongly negative at the other; I(S)(r) correspondingly goes from higher to lower values. Replacing OH by another resonance- donor, NH2, produces similar results in the (6,0) system, while the resonance withdrawing NO2 does the opposite, but in equally striking fashion. We explain these observations by noting that the arrangement of the C-C bonds in the (6,0) tube facilitates charge delocalization over the full length and entire surface of the tube. Substituting NH2 and NO2 at opposite ends of the (6,0) tube greatly strengthens the gradations in both V(S)(r) and I(S)(r). The first hyperpolarizability of this system was found to be nine times that of para-nitroaniline, suggesting possible nonlinear optical applications. [figure: see text]. HF/STO-5G electrostatic potential on outer surface of open (6,0) C72H10NH2NO2. The nitro group is at the right end of the tube, the amino group at the left. In eV: purple is less than 14, blue is between 14 and 15, green is between 15 and 16.5, yellow is between 16.5 and 17.5, and red is more than 17.5.

A new selection system to recruit general practice registrars: preliminary findings from a validation study.

OBJECTIVE: To design and validate a new competency based selection system to recruit general practice registrars, comprising a competency based application form, referees' reports, and an assessment centre. DESIGN: Longitudinal predictive validity study and a matched case comparison. SETTING: South Yorkshire and East Midlands region, United Kingdom, comprising three deaneries. PARTICIPANTS: 46 of 167 doctors were followed up in training after three months in practice, and 20 general practice trainers were selected by using traditional recruitment methods. MAIN OUTCOME MEASURES: Trainer ratings of trainee performance in practice on targeted competencies. RESULTS: Performance ratings of targeted competencies at the assessment centre predicted trainer ratings of performance in the job. Furthermore, those trainees recruited through the new competency based process performed significantly better in the job than those recruited through traditional recruitment processes. CONCLUSION: A new competency based selection process using assessment centres improves the validity of selection of general practice registrars compared with traditional selection techniques.

Comparative analysis of surface electrostatic potentials of carbon, boron/nitrogen and carbon/boron/nitrogen model nanotubes.

We have extended an earlier study, in which we characterized in detail the electrostatic potentials on the inner and outer surfaces of a group of carbon and B(x)N(x) model nanotubes, to include several additional ones with smaller diameters plus a new category, C(2x)B(x)N(x). The statistical features of the surface potentials are presented and analyzed for a total of 19 tubes as well as fullerene and a small model graphene. The potentials on the surfaces of the carbon systems are relatively weak and rather bland; they are much stronger and more variable for the B(x)N(x) and C(2x)B(x)N(x). A qualitative correlation with free energies of solvation indicates that the latter two categories should have considerably greater water solubilities. The inner surfaces are generally more positive than the corresponding outer ones, while both positive and negative potentials are strengthened by increasing curvature. The outsides of B(x)N(x) tubes have characteristic patterns of alternating positive and negative regions, while the insides are strongly positive. In the closed C(2x)B(x)N(x) systems, half of the C-C bonds are double-bond-like and have negative potentials above them; the adjacent rows of boron and nitrogens show the usual B(x)N(x) pattern. When the C(2x)B(x)N(x) tubes are open, with hydrogens at the ends, the surface potentials are dominated by the B+-H- and N(-)-H+ linkages.

Electrostatic potentials and covalent radii.

We begin with a brief overview of the electrostatic potential V(r) as a fundamental determinant of the properties of systems of electrons and nuclei. The minimum of V(r) along the internuclear axis between two bonded atoms is a natural and physically meaningful boundary point, at which the electrostatic forces of the two nuclei upon an element of charge exactly cancel. We propose that the distances from nuclei to V(r) bond minima provide the basis for a well-defined set of covalent radii. Density functional calculations at the B3PW91/6-311+G** level were carried out for 59 molecules to locate the V(r) minima in 95 bonds and use these as the basis for determining single- and multiple-bond covalent radii for eight first- and second-row atoms plus hydrogen. It was found to be unrealistic to assign a single covalent radius to each atom; different values are needed for bonds to first- and second-row atoms, as well as to hydrogen. Using these results, we are able to predict the bond lengths of 33 single and multiple bonds with average errors of less than 0.04 A relative to experimental data.