The magnetic and electronic structure of vanadyl pyrophosphate from density functional theory

We have studied the magnetic structure of the high symmetry vanadyl pyrophosphate ((VO)_(2)P_(2)O)7, VOPO), focusing on the spin exchange couplings, using density functional theory (B3LYP) with the full three-dimensional periodicity. VOPO involves four distinct spin couplings: two larger couplings exist along the chain direction (a-axis), which we predict to be antiferromagnetic, J_(OPO) = −156.8 K and J_O = −68.6 K, and two weaker couplings appear along the c (between two layers) and b directions (between two chains in the same layer), which we calculate to be ferromagnetic, J_layer = 19.2 K and J_chain = 2.8 K. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that J_(OPO) originates from a super-exchange interaction through the bridging –O–P–O– unit. In contrast, J_O results from a direct overlap of 3d_(x^2 − y^2) orbitals on two vanadium atoms in the same V_(2)O_8 motif, making it very sensitive to structural fluctuations. Based on the variations in V–O bond length as a function of strain along a, we found that the V–O bonds of V–(OPO)_(2)–V are covalent and rigid, whereas the bonds of V–(O)_(2)–V are fragile and dative. These distinctions suggest that compression along the a-axis would have a dramatic impact on J_O, changing the magnetic structure and spin gap of VOPO. This result also suggests that assuming J_O to be a constant over the range of 2–300 K whilst fitting couplings to the experimental magnetic susceptibility is an invalid method. Regarding its role as a catalyst, the bonding pattern suggests that O_2 can penetrate beyond the top layers of the VOPO surface, converting multiple V atoms from the +4 to +5 oxidation state, which seems crucial to explain the deep oxidation of n-butane to maleic anhydride

Approximate density matrix functionals applied to hetero-atomic bond dissociation

A two-orbital two-electron diatomic model resembling LiH is used to investigate the differences between the exact L\"owdin-Shull and approximate Hartree-Fock-Bogoliubov and Baerends-Buijse density matrix functionals in the medium- to long-distance dissociation region. In case of homolytic dissociation (one electron on each atom), the approximate functionals fail to generate the correct energy due to a compromise between the Hartree-Fock component (which favors partial charge transfer) and the strong correlation component (which hampers charge transfer). The exact functional is able to generate the physically correct answer by enforcing the equi-charge distribution of the bonding and antibonding orbitals. Besides, the approximate functionals also have issues in correctly describing heterolytic dissociation (two electrons on one atom) due to the strong correlation component hampering charge transfer. In this work, we propose a new scheme in which the homolytic dissociation problem for approximate functionals is avoided by adding a Lagrange multiplier that enforces equi-charge distribution of the bonding and antibonding orbitals. The symmety based nature of the findings implies that they are most likely transferable to other cases in which one uses an approximate one-particle method in conjunction with a symmetrical particle-hole correction factor

The para-substituent effect and pH-dependence of the organometallic Baeyer–Villiger oxidation of rhenium–carbon bonds

We studied the Baeyer–Villiger (BV) type oxidation of phenylrhenium trioxide (PTO) by H2O2 in the aqueous phase using Quantum Mechanics (density functional theory with the M06 functional) focusing on how the solution pH and the para-substituent affect the Gibbs free energy surfaces. For both PTO and MTO (methylrhenium trioxide) cases, we find that for pH > 1 the BV pathway having OH− as the leaving group is lower in energy than the one involving simultaneous protonation of hydroxide. We also find that during this organometallic BV oxidation, the migrating phenyl is a nucleophile so that substituting functional groups in the para-position of phenyl with increased electron-donating character lowers the migration barrier, just as in organic BV reactions. However, this substituent effect also pushes electron density to Re, impeding HOO− coordination and slowing down the reaction. This is in direct contrast to the organic analog, in which para-substitution has an insignificant influence on 1,2-addition of peracids. Due to the competition of the two opposing effects and the dependence of the resting state on pH and concentration, the reaction rate of the organometallic BV oxidation is surprisingly unaffected by para-substitution

Combining density based dynamical correlation with a reduced density matrix strong correlation description

A combined density and first-order reduced-density-matrix (1RDM) functional method is proposed for the calculation of potential energy curves (PECs) of molecular multibond dissociation. Its 1RDM functional part, a pair density functional, efficiently approximates the ab initio pair density of the complete active space self-consistent-field (CASSCF) method. The corresponding approximate on top pair density {\Pi} is employed to correct for double counting a correlation functional of density functional theory (DFT). The proposed ELS-DM{\Pi}DFT method with the extended L\"owdin-Shull (ELS) 1RDM functional with dispersion and multibond (DM) corrections augmented with the {\Pi}DFT functional closely reproduces PECs of multibond dissociation in the paradigmatic N_2 , H_2O, and H_2CO molecules calculated with the recently proposed CAS{\Pi}DFT (CASSCF augmented with a {\Pi} based scaled DFT correlation correction) method. Furthermore, with the additional M-correction, ELS-DM{\Pi}DFT+M reproduces well the benchmark PEC of the N_2 molecule by Lie and Clementi

Functionalization of Rhenium Aryl Bonds by O-Atom Transfer

Aryltrioxorhenium (ArReO_3) has been demonstrated to show rapid oxy-functionalization upon reaction with O-atom donors, YO, to selectively generate the corresponding phenols in near quantitative yields. (18)^O-Labeling experiments show that the oxygen in the products is exclusively from YO. DFT studies reveal a 10.7 kcal/mol barrier (Ar = Ph) for oxy-functionalization with H_2O_2 via a Baeyer-Villiger type mechanism involving nudeophilic attack of the aryl group on an electrophilic oxygen of YO coordinated to rhenium

Prediction of Nontrivial Band Topology and Superconductivity in Mg2_2Pb

The interplay of BCS superconductivity and nontrivial band topology is expected to give rise to opportunities for creating topological superconductors, achieved through pairing spin-filtered boundary modes via superconducting proximity effects. The thus-engineered topological superconductivity can, for example, facilitate the search for Majorana fermion quasiparticles in condensed matter systems. Here we report a first-principles study of Mg$_2$Pb and predict that it should be a superconducting topological material. The band topology of Mg$_2$Pb is identical to that of the archetypal quantum spin Hall insulator HgTe, while isostructural and isoelectronic Mg$_2$Sn is topologically trivial; a trivial to topological transition is predicted for Mg$_2$Sn$_{1-x}$Pb$_x$ for x~0.77. We propose that Mg$_2$Pb-Mg$_2$Sn quantum wells should generate robust spin-filtered edge currents in analogy to HgTe/CdTe quantum wells. In addition, our calculations predict that Mg$_2$Pb should become superconducting upon electron doping. Therefore, Mg$_2$Pb is expected to provide a practical material platform for studying emergent phenomena arising from the interplay of superconductivity and band topology.Comment: 5 figure

A homolytic oxy-functionalization mechanism: intermolecular hydrocarbyl migration from M–R to vanadate oxo

A new mechanism for generating C–O bonds from metal-hydrocarbyls involving homolytic, intermolecular migration of the hydrocarbyl group to a vanadium oxo is reported. Responsible for the C–O bond in phenol formed by the reaction of OVCl_3 with HgPh_2, it may provide air-regenerable metal oxos a role in aerobic alkane oxidations

Diet, Physical Activity, and Stress Among Wheelchair Users With Multiple Sclerosis: Examining Individual and Co-Occurring Behavioral Risk Factors

OBJECTIVE: This study examined individual and co-occurring behavioral risk factors (diet, exercise, and stress) in wheelchair users with multiple sclerosis (MS) and potential association with MS symptoms (ie, fatigue, depression, anxiety, pain, sleep, and health-related quality of life [HRQOL]). DESIGN: Survey. SETTING: General Community. PARTICIPANTS: One hundred twenty-three wheelchair users with MS completed this study (N=123). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Participants were mailed instructions for accessing online questionnaires (demographic and clinical characteristics, Godin Leisure-Time Exercise Questionnaire, Perceived Stress Scale, Automated Self-Administered 24-Hour Dietary Assessment Tool, and MS symptoms). RESULTS: Standard cut-points were used to categorize behavioral risk factors and then identify the extent and distribution of these behaviors both individually and co-occurring. We then analyzed the associations between behavioral risk factors and MS symptoms using bivariate correlation analyses and Mann-Whitney U tests. The mean age of participants was 60.6±10.0 years, 76% identified as women, 82% had a progressive disease course, and the mean MS duration was 23.0±9.7 years. Seven participants were classified as having 0 negative health behaviors, 41 participants had 1 negative health behavior, 49 participants had 2 negative health behaviors, and 26 participants had 3 negative health behaviors. The number of negative health behaviors was significantly correlated with HRQOL (physical, r=.30; psychological, r=.47), sleep (r=.25), depressive symptoms (r=.36), and anxiety (r=.43). Mann-Whitney U tests indicated greater fatigue, depression, and anxiety as well as lower sleep quality and HRQOL among participants who reported 2 or 3 behavioral risk factors compared with 0 or 1 behavioral risk factor. CONCLUSIONS: Future research should examine the design and implementation of multiple health behavior change interventions targeting co-occurring behavioral risk factors among wheelchair users with MS

Synthesis and Characterization of the k^2-acac-O,O Complex Os_(IV)(acac)_2PhCl and Study of CH Activation with Benzene

We have synthesized and fully characterized the air-stable complex (κ^2-acac-O,O)2Os^(IV)(Ph)Cl (Cl-1-Ph; acac-O,O = acetylacetonate), which reacts with C_6D_6 to generate Cl-1-Ph-d_5 in high yield and catalyzes the H/D exchange reaction between benzene and toluene-d_8 upon heating to 140 °C. To our knowledge, this is the first example of stoichiometric and catalytic, homogeneous, intermolecular CH activation of arenes by a discrete Os complex. The reactions show extended induction periods. DFT studies of Cl-1-Ph and cis-(κ^2-acac-O,O)_2Os^(III)(C_6H_5)(C_6D_6) (cis-(C_6D_6)-2-Ph) found a mechanism involving CH activation by traces of Os(III) and Cl atom transfer between Cl-1-Ph and cis-(C_6D_6)-2-Ph. Experimental data showing that addition of reductants eliminates the induction periods suggest that CH activation occurs from an oxidation state lower than Os^(IV), consistent with the DFT predictions. Consistent with a Cl atom transfer mechanism, the triflate analogue of Cl-1-Ph, OTf-1-Ph, does not undergo a stoichiometric or catalytic reaction with C_6D_6

Perceptions of Physical Activity Guidelines Among Wheelchair Users With Multiple Sclerosis

BACKGROUND: Physical activity guidelines provide prescriptive resources for population-level promotion of behavior change to improve health outcomes. The National Multiple Sclerosis Society (NMSS) recently created and disseminated physical activity guidelines for persons with multiple sclerosis (MS) across the disease spectrum. OBJECTIVE: This study aimed to assess perceptions of the updated NMSS Physical Activity Guidelines among wheelchair users with MS. METHODS: One hundred thirty-four wheelchair users with MS participated in a cross-sectional study examining health behaviors. Outcomes were measured using an online questionnaire that included items related to demographic and clinical characteristics and a battery of questions regarding perceptions of the NMSS Physical Activity Guidelines based on Expanded Disability Status Scale groups 7.0-7.5 & 8.0-8.5. RESULTS: Among the 134 participants, 77 participants (58%) did not meet the general recommendations, 43 participants sometimes meet the general recommendations (32%), and 14 participants (10%) reported meeting the general recommendations. Participants reported positive perceptions across modalities (i.e. Breathing, Flexibility, Upper Extremity, Lower Extremity, and Core Exercises); however Upper and Lower Extremity Exercises were rated as the most challenging based on inability to complete independently. CONCLUSION: Wheelchair users with MS in this study generally rated the NMSS Guidelines for Physical Activity as appropriate