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.

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

We have studied the palladium-mediated activation of C( sp n )-X bonds (n = 1-3 and X = H, CH 3 , Cl) in archetypal model substrates H 3 C-CH 2 -X, H 2 C=CH-X and HC≡C-X by catalysts PdL n  with L n  = no ligand, Cl - , and (PH 3 ) 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( sp 3 )-X, to C( sp 2 )-X, to C( sp )-X. Activation strain and matching energy decomposition analyses reveal that the decreased oxidative addition barrier going from  sp 3  to  sp 2  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( sp n )-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( sp 3 )-X to C( sp )-X.

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.

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.

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.

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.

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.

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.