Search

An o-nitro-O-aryl oxime was observed to exhibit a short OO contact, which exhibited characteristics consistent with a chalcogen bond. The O-N bond length of the oxime was appreciably longer than the expected value, and NBO calculations indicated the presence of a n(O) → σ(O-N) orbital delocalisation. Topological analysis of the experimental electron density of two analogues shows the presence of a bond path between the two oxygen atoms, with ρ(r) and ∇2ρ(r) values consistent with an electrostatic interaction. Finally, electrostatic potential calculations indicate the presence of a σ-hole, the "smoking gun" indicating a Ch-bond. These results are unusual as oxygen is not typically considered to be a Ch-bond donor, especially in unactivated systems such as oximes.

Orphans in post-conflict settings have unique needs that have not been well-characterized. In post-conflict Liberia, maternal orphans are more likely to be without care than paternal orphans. This study examined the experiences of maternal orphans in Liberia, as they attempted to care for themselves and seek care from others, and the barriers they faced. In-depth interviews were conducted with 75 post-conflict Liberian orphans. We performed a secondary narrative analysis of interview transcripts from all maternal or double orphans (n = 17). We identified similar elements across narratives: traumatic loss, disconnection from family and community, and the desire for a savior. Female high-risk orphans were more likely to have formal substitute caregiving arrangements in which they were living with someone who was a relative or had been selected by a relative. Male orphans more commonly lacked arranged substitute care, but this allowed them to form relationships with substitute caregivers of their choosing. Sex also played a role in the provision of caregiving; substitute care was provided by women. Findings highlighted the syndemic relationship between poverty, violence, transactional sex, trauma, and substance use that traps high-risk Liberian orphans. Interventions are needed to improve access to mental health care, sober communities, housing, and education support. The need to integrate these services into indigenous institutions and address barriers related to stigma is explored.

The variable oxygen probe (VOP) is a crystallographic technique that has been used to explore the relative donor abilities of various filled orbitals ranging from vicinal lone pairs to polarized heteroatom-carbon bonds, remote π functionalities, and strained carbon-carbon (CC) bonds. In this study, the donor-acceptor interactions which underlie the VOP have been explored in the gas phase using density functional theory on the model systems 1-13 with natural bond orbital analysis of the various donor-acceptor interactions involving both neutral and charged σ* antibonding orbitals as the acceptor probes. Updated values for the VOP slopes of 1-13 were shown to relate qualitatively with the sum of all significant donor-acceptor interactions present in these derivatives. Application of the VOP to calculated structures of 1-13 with various -OR substituents revealed a similar relationship between the C-OR bond distance to pKa (ROH). However, the VOP slopes in the gas phase were significantly smaller in magnitude than those obtained from crystal structural data, likely due to the valence form (C+-OR) being disfavored in the former, highlighting the advantage of the VOP as an experimental technique to discriminate donor ability more effectively than calculated structures.

Collision-induced dissociation mass spectrometry of the ammonium ions 4a and 4b results in the formation of the seleniranium ion 5, the structure and purity of which were verified using gas-phase infrared spectroscopy coupled to mass spectrometry and gas-phase ion-mobility measurements. Ion-molecule reactions between the ion 5 (m/z = 261) and cyclopentene, cyclohexene, cycloheptene, and cyclooctene resulted in the formation of the seleniranium ions 7 (m/z = 225), 6 (m/z = 239), 8 (m/z = 253), and 9 (m/z = 267), respectively. Further reaction of seleniranium 6 with cyclopentene resulted in further π-ligand exchange giving seleniranium ion 7, confirming that direct π-ligand exchange between seleniranium ion 5 and cycloalkenes occurs in the gas phase. Pseudo-first-order kinetics established relative reaction efficiencies for π-ligand exchange for cyclopentene, cyclohexene, cycloheptene. and cyclooctene as 0.20, 0.07, 0.43, and 4.32. respectively. DFT calculations at the M06/6-31+G(d) level of theory provide the following insights into the mechanism of the π-ligand exchange reactions; the cycloalkene forms a complex with the seleniranium ion 5 with binding energies of 57 and 62 kJ/mol for cyclopentene and cyclohexene, respectively, with transition states for π-ligand exchange having barriers of 17.8 and 19.3 kJ/mol for cyclopentene and cyclohexene, respectively.

A structural, spectroscopic and computational study of a series of oximes was undertaken to investigate how geometric and structural changes relevant to the reaction coordinate for the Beckmann reaction (normal Beckmann) and Beckmann fragmentation (abnormal Beckmann) manifest in the ground state. X-ray structures of a range of oximes derived from dihydrolevoglucosan (Cyrene™; which undergoes the abnormal Beckmann reaction), in which the oxygen substituent was systematically varied were determined. As the electron demand of the OR group increased, the major structural changes included lengthening of the N-OR bond distance, and a decrease in the magnitude of the C2[double bond, length as m-dash]N-O bond angle, consistent with the changes seen for cyclohexanone oximes, which undergo the normal Beckmann reaction. However, unique to the Cyrene oximes, an increase in the length of the fissile C1-C2 bond was observed, which correlated with a decrease in the 13C-13C 1-bond coupling constants as the electron demand of the OR substituent increased. Computational studies of Cyrene and cyclohexanone oximes using Natural Bond Orbital analysis support an electronic structure involving n(O) → σ*C1-C2 and σC1-C2 → σ*N-O localized orbital interactions.

Computational, solution phase, and crystal structure analysis of 2- and 4-organoselenylmethyl-substituted pyridinium ions (10a-c and 11a-c) provides strong evidence for C-Se hyperconjugation (σ(C-Se)-π*) between the C-Se σ-bond and the π-deficient aromatic ring and a through-space interaction (n(Se)-π*) between the selenium p-type lone pair and the π-deficient aromatic ring. There is also a weak anomeric-type interaction (n(Se)-σ*(CC)) involving the selenium p-type lone pair electrons and the polarized CH(2)-C(Ar) σ-bond. NBO analysis of calculated cations with varying electron demand (B3LYP/6-311++G**) show that C-Se hyperconjugation (σ(C-Se)-π*) is the predominant mode of stabilization in the weakly electron-demanding pyridinium ions (10d, 11d, 14, and 15); however, the through-space (n(Se)-π*) interaction becomes more important as the electron demand of the β-Se-substituted carbocation increases. The anomeric interaction (n(Se)-σ*(CC)) is relatively weak in all ions.

Selenium substituents which are disposed β to an electron deficient centre, such as a carbocation p-orbital, or the π* orbital of an electron deficient p-system, interact in a stabilising way by a combination of C-Se hyperconjugation (σ(Se-C)-π* interaction), and a through-space homoconjugative n(Se)-π* interaction. The relative importance of these two modes of interaction is dependant on the electron demand of the cation, with hyperconjugation predominating for low electron demand systems, and the n(Se)-π* interaction predominating for high electron demand cations.

The O-(3,5-dinitrobenzoyl) derivative of benzohydroxamic acid 1 crystallizes in its iminol tautomeric form, hydrogen bonded to a molecule of dimethyl sulfoxide (DMSO) when crystallized from methanol in the presence of DMSO. In contrast, in the absence of DMSO, 1 crystallizes in its amide tautomeric form. Computations suggest that in the absence of DMSO, the amide tautomer of 1 is significantly more stable than the iminol form. Conversely, in the presence of DMSO, the two forms have comparable stabilities. Solution infrared spectra of 1 run in dichloromethane are consistent with the amide tautomeric form predominating; however, titration of aliquots of DMSO results in the formation of a second species with spectral characteristics consistent with the iminol tautomer.

The gas-phase reactivity of the vanadium hydroxides [VO(2)(OH)(2)](-) and [V(2)O(5)(OH)](-) toward methanol was examined using a combination of ion-molecule reactions (IMRs) and collision-induced dissociation (CID) in a quadrupole ion trap mass spectrometer. Isotope-labeling experiments with CD(3)OH, (13)CH(3)OH, and CH(3)(18)OH were used to confirm the stoichiometry of ions and the observed sequence of reactions. The experimental data were interpreted with the aid of density functional theory calculations, carried out at the B3LYP/SDD6-311++G** level of theory. While [VO(2)(OH)(2)](-) is unreactive, [V(2)O(5)(OH)](-) undergoes a metathesis reaction to yield [V(2)O(5)(OCH(3))](-). The DFT calculations reveal that the metathesis reaction of methanol with [VO(2)(OH)(2)](-) suffers from a barrier of +0.52 eV (relative to separated reactants) but that the reaction of [V(2)O(5)(OH)](-) with methanol readily proceeds via addition/elimination reactions with both transition states being below the energy of the separated reactants. CID of [V(2)O(5)(OCH(3))](-) (m/z 213) yields three ions arising from activation of the methoxo ligand: [V(2), O(6), C, H](-) (m/z 211); [V(2), O(5), H](-) (m/z 183); and [V(2), O(4), H](-) (m/z 167). Additional experiments and DFT calculations suggest that these ions arise from losses of H(2), formaldehyde and the sequential losses of H(2) and CO(2), respectively. The use of an (18)O-labeled methoxo ligand in [V(2)O(5)((18)OCH(3))](-) (m/z 215) showed the competing losses of H(2)C(16)O and H(2)C(18)O and [H(2) and C(16)O(18)O] and [H(2) and C(16)O(2)], highlighting that (16)O/(18)O exchange between the methoxo ligand and the vanadium oxide occurs prior to the subsequent fragmentation of the ligand. DFT calculations reveal that a key step involves hydrogen atom transfer from the methoxo ligand to the oxo ligand of the same vanadium center, producing the intermediate [V(2)O(4)(OH)(OCH(2))](-) containing a ketyl radical ligand and a hydroxo ligand. This intermediate can either undergo CH(2)O loss, or the ketyl radical can couple with an oxo ligand of the adjacent vanadium center, producing [V(2)O(3)(μ(2)-O(2)CH(2))](-), which is a key intermediate in the (16)O/(18)O scrambling and in the H(2) loss channel.

The P analogues of methyldiazonium ion (CH 3 NN) + (1), methyldiphosphonium ion (CH 3 PP) + (2), methylphosphoazonium ion (CH 3 NP) + (3), and methylazophosphonium ion (CH 3 PN) + (4) were studied with ab initio techniques. Potential energy surface analysis shows that the C 3 v-symmetric ions all are minima. 4 corresponds to a shallow minimun along the automerization path of its isomer 3

Atomic properties are central to discussions of bonding and reactivity, and topological electron density analysis provides a powerful framework for their rigorous determination. Whereas the topological method can be applied routinely to comparatively small molecules, its application to large molecules is somewhat impeded by the rather considerable amounts of computer time required. Here, we describe two new methods for the determination of integrated atomic properties that greatly reduce the integration times while maintaining the generality, the rigor, and the accuracy of the topological partitioning method. The principle of the methods consists in reducing the total molecular electron density function to atom-specific electron density functions that accurately describe the electron density distribution in the basin of the atom whose properties are being determined. It is shown that this task can be accomplished either by a reduction of the space of the primitives in which the wave function is expanded or via electron density functions defined by atom-specific subsets of selected localized molecular orbitais. With the current integration algorithm the latter method is more efficient. The two methods can be combined. The theoretical principles and the computational implementation of the methods are discussed. Their performances have been tested for a series of polyines C 2 n H 2 and polynitriles C n N n H n +2 ( n =1–5 and 10) and the topological characteristics and the integrated properties are found to be in excellent agreement with results obtained by the conventional technique. Importantly, it is shown that with the subset selection, integration time requirements approach an upper limit as the size of the molecule increases. The methods perform equally well for all kinds of basis sets, allowing for the analysis of large molecules described by spliced basis sets. The rigorous electron density analysis of very large molecular systems becomes feasible.

Eight ester and ether derivatives of propargyl alcohol with varying electron demand were structurally characterised using low temperature X-ray crystallography, these were combined with seven derivatives obtained from the Cambridge Structural Database. Variable oxygen probe analysis of these derivatives provided evidence that the ethynyl substituent is a relatively weak π-electron donor, and is a slightly less effective donor than the C–C bond of an ethyl substituent.

Application of the variable oxygen probe to derivatives of (4-methoxycarbonyl)cubylmethanol 11 demonstrated a strong response of C-OR bond distance to the electron demand of the OR substituent, consistent with an enhanced σ-donor ability of the strained C-C bonds of cubane. The extent of cubane donor ability was found to be superior to an unstrained donor 13, comparing data extracted from the Cambridge Structural Database (T. W. Cole, Ph.D. Dissertation, University of Chicago, 1966), but weaker than the previously studied cyclopropane donors. Structural evidence is also found for σCC-π*CO interactions in these structures.

The X-ray structures of a range of oxime derivatives (1 and 4), of cyclohexanone and 4-tert-butylcyclohexanone, where the electron demand of the oxygenated substituent on the oxime nitrogen (OR) is systematically varied were determined. It was established that as the OR group becomes more electron demanding, then the N–OR bond distance increases, consistent with the early stages of bond breakage. Concomitant with this structural effect was a noticeable closing up of the N1–C1–C2 bond angle, consistent with the early stages of migration of the antiperiplanar carbon onto the nitrogen substituent. These structural effects are consistent with the manifestation of the early stages of the Beckmann rearrangement in the ground state structures of these oxime derivatives. The carbon–carbon bond distances of the participating carbons in this rearrangement, however, did not vary in a systematic way with the electron demand of the OR substituent, suggesting that the structural effects are too small to be detected using X-ray crystallography. However, the 13C–13C 1-bond coupling constants, which are sensitive to the effects of hyperconjugation, were shown to vary in a systematic way with the electron demand of the OR substituent. Structural effects in the oxime 5 derivatives of 2,2-dimethylcyclohexanone, a substrate that is prone to Beckmann fragmentation rather than Beckmann rearrangement, were similar but smaller in magnitude.