Methyl Substitution Destabilizes Alkyl Radicals.

We have quantum chemically investigated how methyl substituents affect the stability of alkyl radicals Mem H3-m C⋅ and the corresponding Mem H3-m C-X bonds (X = H, CH3 , OH; m = 0 - 3) using density functional theory at M06-2X/TZ2P. The state-of-the-art in physical organic chemistry is that alkyl radicals are stabilized upon an increase in their degree of substitution from methyl

C-X Bond Activation by Palladium: Steric Shielding versus Steric Attraction.

The C-X bond activation (X = H, C) of a series of substituted C(n°)-H and C(n°)-C(m°) bonds with C(n°) and C(m°) = H3 C- (methyl, 0°), CH3 H2 C- (primary, 1°), (CH3 )2 HC- (secondary, 2°), (CH3 )3 C- (tertiary, 3°) by palladium were investigated using relativistic dispersion-corrected density functional theory at ZORA-BLYP-D3(BJ)/TZ2P. The effect of the stepwise introduction of substituents was pinpointed at the C-X bond on the bond activation process. The C(n°)-X bonds become substantially weaker going from C(0°)-X, to C(1°)-X, to C(2°)-X, to C(3°)-X because of the increasing steric repulsion between the C(n°)- and X-group. Interestingly, this often does not lead to a lower barrier for the C(n°)-X bond activation. The C-H activation barrier, for example, decreases from C(0°)-X, to C(1°)-X, to C(2°)-X and then increases again for the very crowded C(3°)-X bond. For the more congested C-C bond, in contrast, the activation barrier always increases as the degree of substitution is increased. Our activation strain and matching energy decomposition analyses reveal that these differences in C-H and C-C bond activation can be traced back to the opposing interplay between steric repulsion across the C-X bond versus that between the catalyst and substrate.

C(spn )-X (n=1-3) Bond Activation by Palladium.

We have studied the palladium-mediated activation of C(spn )-X bonds (n = 1-3 and X = H, CH3 , Cl) in archetypal model substrates H3 C-CH2 -X, H2 C=CH-X and HC≡C-X by catalysts PdLn with Ln = no ligand, Cl- , and (PH3 )2 , using relativistic density functional theory at ZORA-BLYP/TZ2P. The oxidative addition barrier decreases along this series, even though the strength of the bonds increases going from C(sp3 )-X, to C(sp2 )-X, to C(sp)-X. Activation strain and matching energy decomposition analyses reveal that the decreased oxidative addition barrier going from sp3 , to sp2 , to sp, originates from a reduction in the destabilizing steric (Pauli) repulsion between catalyst and substrate. This is the direct consequence of the decreasing coordination number of the carbon atom in C(spn )-X, which goes from four, to three, to two along this series. The associated net stabilization of the catalyst-substrate interaction dominates the trend in strain energy which indeed becomes more destabilizing along this same series as the bond becomes stronger from C(sp3 )-X to C(sp)-X.

Land-based climate change mitigation measures can affect agricultural markets and food security.

Earlier studies have noted potential adverse impacts of land-related emissions mitigation strategies on food security, particularly due to food price increases-but without distinguishing these strategies' individual effects under different conditions. Using six global agroeconomic models, we show the extent to which three factors-non-CO2 emissions reduction, bioenergy production and afforestation-may change food security and agricultural market conditions under 2 °C climate-stabilization scenarios. Results show that afforestation (often simulated in the models by imposing carbon prices on land carbon stocks) could have a large impact on food security relative to non-CO2 emissions policies (generally implemented as emissions taxes). Respectively, these measures put an additional 41.9 million and 26.7 million people at risk of hunger in 2050 compared with the current trend scenario baseline. This highlights the need for better coordination in emissions reduction and agricultural market management policies as well as better representation of land use and associated greenhouse gas emissions in modelling.

Not Carbon s-p Hybridization, but Coordination Number Determines C-H and C-C Bond Length.

A fundamental and ubiquitous phenomenon in chemistry is the contraction of both C-H and C-C bonds as the carbon atoms involved vary, in s-p hybridization, along sp3 to sp2 to sp. Our quantum chemical bonding analyses based on Kohn-Sham molecular orbital theory show that the generally accepted rationale behind this trend is incorrect. Inspection of the molecular orbitals and their corresponding orbital overlaps reveals that the above-mentioned shortening in C-H and C-C bonds is not determined by an increasing amount of s-character at the carbon atom in these bonds. Instead, we establish that this structural trend is caused by a diminishing steric (Pauli) repulsion between substituents around the pertinent carbon atom, as the coordination number decreases along sp3 to sp2 to sp.

Bending the curve of terrestrial biodiversity needs an integrated strategy.

Increased efforts are required to prevent further losses to terrestrial biodiversity and the ecosystem services that it  provides1,2. Ambitious targets have been proposed, such as reversing the declining trends in biodiversity3; however, just feeding the growing human population will make this a challenge4. Here we use an ensemble of land-use and biodiversity models to assess whether-and how-humanity can reverse the declines in terrestrial biodiversity caused by habitat conversion, which is a major threat to biodiversity5. We show that immediate efforts, consistent with the broader sustainability agenda but of unprecedented ambition and coordination, could enable the provision of food for the growing human population while reversing the global terrestrial biodiversity trends caused by habitat conversion. If we decide to increase the extent of land under conservation management, restore degraded land and generalize landscape-level conservation planning, biodiversity trends from habitat conversion could become positive by the mid-twenty-first century on average across models (confidence interval, 2042-2061), but this was not the case for all models. Food prices could increase and, on average across models, almost half (confidence interval, 34-50%) of the future biodiversity losses could not be avoided. However, additionally tackling the drivers of land-use change could avoid conflict with affordable food provision and reduces the environmental effects of the food-provision system. Through further sustainable intensification and trade, reduced food waste and more plant-based human diets, more than two thirds of future biodiversity losses are avoided and the biodiversity trends from habitat conversion are reversed by 2050 for almost all of the models. Although limiting further loss will remain challenging in several biodiversity-rich regions, and other threats-such as climate change-must be addressed to truly reverse the declines in biodiversity, our results show that ambitious conservation efforts and food system transformation are central to an effective post-2020 biodiversity strategy.

Projecting terrestrial biodiversity intactness with GLOBIO 4.

Scenario-based biodiversity modelling is a powerful approach to evaluate how possible future socio-economic developments may affect biodiversity. Here, we evaluated the changes in terrestrial biodiversity intactness, expressed by the mean species abundance (MSA) metric, resulting from three of the shared socio-economic pathways (SSPs) combined with different levels of climate change (according to representative concentration pathways [RCPs]): a future oriented towards sustainability (SSP1xRCP2.6), a future determined by a politically divided world (SSP3xRCP6.0) and a future with continued global dependency on fossil fuels (SSP5xRCP8.5). To this end, we first updated the GLOBIO model, which now runs at a spatial resolution of 10 arc-seconds (~300 m), contains new modules for downscaling land use and for quantifying impacts of hunting in the tropics, and updated modules to quantify impacts of climate change, land use, habitat fragmentation and nitrogen pollution. We then used the updated model to project terrestrial biodiversity intactness from 2015 to 2050 as a function of land use and climate changes corresponding with the selected scenarios. We estimated a global area-weighted mean MSA of 0.56 for 2015. Biodiversity intactness declined in all three scenarios, yet the decline was smaller in the sustainability scenario (-0.02) than the regional rivalry and fossil-fuelled development scenarios (-0.06 and -0.05 respectively). We further found considerable variation in projected biodiversity change among different world regions, with large future losses particularly for sub-Saharan Africa. In some scenario-region combinations, we projected future biodiversity recovery due to reduced demands for agricultural land, yet this recovery was counteracted by increased impacts of other pressures (notably climate change and road disturbance). Effective measures to halt or reverse the decline of terrestrial biodiversity should not only reduce land demand (e.g. by increasing agricultural productivity and dietary changes) but also focus on reducing or mitigating the impacts of other pressures.

Afforestation for climate change mitigation: Potentials, risks and trade-offs.

Afforestation is considered a cost-effective and readily available climate change mitigation option. In recent studies afforestation is presented as a major solution to limit climate change. However, estimates of afforestation potential vary widely. Moreover, the risks in global mitigation policy and the negative trade-offs with food security are often not considered. Here we present a new approach to assess the economic potential of afforestation with the IMAGE 3.0 integrated assessment model framework. In addition, we discuss the role of afforestation in mitigation pathways and the effects of afforestation on the food system under increasingly ambitious climate targets. We show that afforestation has a mitigation potential of 4.9 GtCO2 /year at 200 US$/tCO2 in 2050 leading to large-scale application in an SSP2 scenario aiming for 2°C (410 GtCO2 cumulative up to 2100). Afforestation reduces the overall costs of mitigation policy. However, it may lead to lower mitigation ambition and lock-in situations in other sectors. Moreover, it bears risks to implementation and permanence as the negative emissions are increasingly located in regions with high investment risks and weak governance, for example in Sub-Saharan Africa. Afforestation also requires large amounts of land (up to 1,100 Mha) leading to large reductions in agricultural land. The increased competition for land could lead to higher food prices and an increased population at risk of hunger. Our results confirm that afforestation has substantial potential for mitigation. At the same time, we highlight that major risks and trade-offs are involved. Pathways aiming to limit climate change to 2°C or even 1.5°C need to minimize these risks and trade-offs in order to achieve mitigation sustainably.

Key determinants of global land-use projections.

Land use is at the core of various sustainable development goals. Long-term climate foresight studies have structured their recent analyses around five socio-economic pathways (SSPs), with consistent storylines of future macroeconomic and societal developments; however, model quantification of these scenarios shows substantial heterogeneity in land-use projections. Here we build on a recently developed sensitivity approach to identify how future land use depends on six distinct socio-economic drivers (population, wealth, consumption preferences, agricultural productivity, land-use regulation, and trade) and their interactions. Spread across models arises mostly from diverging sensitivities to long-term drivers and from various representations of land-use regulation and trade, calling for reconciliation efforts and more empirical research. Most influential determinants for future cropland and pasture extent are population and agricultural efficiency. Furthermore, land-use regulation and consumption changes can play a key role in reducing both land use and food-security risks, and need to be central elements in sustainable development strategies.

New concepts for designing d10 -M(L)n catalysts: d regime, s regime and intrinsic bite-angle flexibility.

Our aim is to understand the electronic and steric factors that determine the activity and selectivity of transition-metal catalysts for cross-coupling reactions. To this end, we have used the activation strain model to quantum-chemically analyze the activity of catalyst complexes d(10) -M(L)n toward methane C-H oxidative addition. We studied the effect of varying the metal center M along the nine d(10) metal centers of Groups 9, 10, and 11 (M=Co(-), Rh(-), Ir(-), Ni, Pd, Pt, Cu(+), Ag(+), Au(+)), and, for completeness, included variation from uncoordinated to mono- to bisligated systems (n=0, 1, 2), for the ligands L=NH(3), PH(3), and CO. Three concepts emerge from our activation strain analyses: 1) bite-angle flexibility, 2) d-regime catalysts, and 3) s-regime catalysts. These concepts reveal new ways of tuning a catalyst's activity. Interestingly, the flexibility of a catalyst complex, that is, its ability to adopt a bent L-M-L geometry, is shown to be decisive for its activity, not the bite angle as such. Furthermore, the effect of ligands on the catalyst's activity is totally different, sometimes even opposite, depending on the electronic regime (d or s) of the d(10) -M(L)n complex. Our findings therefore constitute new tools for a more rational design of catalysts.

Steric nature of the bite angle. A closer and a broader look.

The bite angle (ligand-metal-ligand angle) is known to greatly influence the activity of catalytically active transition-metal complexes towards bond activation. Here, we have computationally explored how and why the bite angle has such effects in a wide range of prototypical C-X bonds and palladium complexes, using relativistic density functional theory at ZORA-BLYP/TZ2P. Our model reactions cover the substrates H(3)C-X (with X = H, CH(3), Cl) and, among others, the model catalysts, Pd[PH(2)(CH(2))(n)PH(2)] (with n = 2-6) and Pd[PR(2)(CH(2))(n)PR(2)] (n = 2-4 and R = Me, Ph, t-Bu, Cl), Pd(PH(3))X(-) (X = Cl, Br, I), as well as palladium complexes of chelating and non-chelating N-heterocyclic carbenes. The purpose is to elaborate on an earlier finding that bite-angle effects have a predominantly (although not exclusively) steric nature: a smaller bite angle makes more room for coordinating a substrate by bending away the ligands. Indeed, the present results further consolidate this steric picture by revealing its occurrence in this broader range of model reactions and by identifying and quantifying the exact working mechanism through activation strain analyses.

Tandem mass spectrometry of silver-adducted ferrocenyl catalyst complexes.

Ferrocene is a popular template in material science due to its exceptional characteristics that offer the ability to optimize the selectivity and activity of catalysts by the addition of carefully selected substituents. In combinatorial catalyst development, the high susceptibility to electrophilic substitution reactions offers the opportunity for the rapid introduction of molecular diversity. Mass spectrometry (MS)-based continuous-flow systems can be applied to rapidly evaluate catalyst performance as well as to (provisionally) identify the introduced catalyst complexes. Herein, we describe the fragmentation characteristics of the [ferrocenyl bidentate + Ag](+) catalyst complexes in dedicated (high-resolution) MS(n) experiments. The investigation of the fragmentation patterns of a selected number of catalyst classes is accompanied with a density functional theory investigation of fragmentation intermediates in order to assess the viability of a selected fragmentation mechanism.

The activation strain model of chemical reactivity.

Herein, we provide an account of the activation strain model of chemical reactivity and its recent applications. In this model, the potential energy surface DeltaE(zeta) along the reaction coordinate zeta is decomposed into the strain DeltaE(strain)(zeta) of the increasingly deformed reactants plus the interaction DeltaE(int)(zeta) between these deformed reactants, i.e., DeltaE(zeta) = DeltaE(strain)(zeta) + DeltaE(int)(zeta). The purpose of this fragment-based approach is to arrive at a qualitative understanding, based on accurate calculations, of the trends in activation barriers and transition-state geometries (e.g., early or late along the reaction coordinate) in terms of the reactants' properties. The usage of the activation strain model is illustrated by a number of concrete applications, by us and others, in the fields of catalysis and organic chemistry.

Catalyst selection based on intermediate stability measured by mass spectrometry.

The power of natural selection through survival of the fittest is nature's ultimate tool for the improvement and advancement of species. To apply this concept in catalyst development is attractive and may lead to more rapid discoveries of new catalysts for the synthesis of relevant targets, such as pharmaceuticals. Recent advances in ligand synthesis using combinatorial methods have allowed the generation of a great diversity of catalysts. However, selection methods are few in number. We introduce a new selection method that focuses on the stability of catalytic intermediates measured by mass spectrometry. The stability of the intermediate relates inversely to the reactivity of the catalyst, which forms the basis of a catalyst-screening protocol in which less-abundant species represent the most-active catalysts, 'the survival of the weakest'. We demonstrate this concept in the palladium-catalysed allylic alkylation reaction using diphosphine and IndolPhos ligands and support our results with high-level density functional theory calculations.

Comment on "The interplay between steric and electronic effects in SN2 reactions".

We respond to a paper by Fernández, Frenking, and Uggerud (FFU: Chem. Eur. J. 2009, 15, 2166) in which they conclude that not steric hindrance but reduced electrostatic attraction and reduced orbital interactions are responsible for the S(N)2 barrier, in particular in the case of more highly substituted substrates, for example, F(-) + C(CH(3))(3)F. We disagree with this conclusion, which we show is the result of neglecting geometry relaxation processes that are induced by increased Pauli repulsion in the sterically congested S(N)2 transition state.

Trends and anomalies in H-AH(n) and CH(3)-AH(n) bond strengths (AH(n) = CH3, NH2, OH, F).

Along the series of H-AH(n) bonds, with AH(n) = CH(3), NH(2), OH, and F, the bond dissociation energies show a steady increase as can be expected from the increasing difference in electronegativity along this series. However, in the same series for CH(3)-AH(n) the bond strength first decreases from CH(3)-CH(3) to CH(3)-NH(2) and only thereafter increases again along CH(3)-NH(2), CH(3)-OH and CH(3)-F. To understand the origin of the apparent anomaly occurring for the trend in C-A bond strengths, we have analyzed the bonding mechanism in H-AH(n), CH(3)-AH(n) and other model systems, using density functional theory at BLYP/TZ2P. We recover that increasing electronegativity difference across a bond causes an increasing stability. But we also find that the nature of the bond changes qualitatively for AH(n) = CH(3) due to the saturation of A with hydrogens. The need of the methyl group to adopt an umbrella shaped geometry plays a key role in the difference between bonds with CH(3)* and other second period AH(n)* radicals.

Reaction Coordinates and the Transition-Vector Approximation to the IRC.

The appearance of a reaction profile or potential energy surface (PES) associated with the reaction path (defined as the path of steepest descent from the saddle point) depends on the choice of reaction coordinate onto which the intrinsic reaction coordinate is projected. This provides one with the freedom, but also the problem, of choosing the optimal perspective (i.e., the optimal reaction coordinate) for revealing what is essential for understanding the reaction. Here, we address this issue by analyzing a number of different reaction coordinates for the same set of model reactions, namely, prototypical oxidative addition reactions of C-X bonds to palladium. We show how different choices affect the appearance of the PES, and we discuss which qualities make a particular reaction coordinate most suitable for comparing and analyzing the reactions. Furthermore, we show how the transition vector (i.e., the normal mode associated with a negative force constant that leads from the saddle point to the steepest descent paths) can serve as a useful and computationally much more efficient approximation (designated TV-IRC) for full IRC computations, in the decisive region around the transition state.