Iridium-catalyzed C–H borylation of THF was theoretically investigated as example of sp3 C–H functionalization. DFT computations show that β-regioselective borylation occurs more easily than does ...α-regioselective, as reported experimentally, through oxidative addition of C–H bond to iridium(III) species and reductive elimination of B–C bond. The reductive elimination is both a rate-determining step and a regioselectivity-determining step. The lower energy transition state (TS) of the reductive elimination of β-boryloxolane arises from the Ir···(β-oxolanyl) interaction at TS being stronger than the Ir···(α-oxolanyl) one. The Ir···(β-oxolanyl) interaction being stronger than the Ir···(α-oxolanyl) one is a result of the valence orbital energy of the α-oxolanyl group being higher than that of the β-oxolanyl group due to antibonding overlap of the valence orbital with O 2p orbital, where SOMO of oxolanyl radical is taken as valence orbital hereinafter. Reactivity of substrate decreases following the order primary (β) C–H of ethyl ether > primary C–H of n-pentane ∼ secondary (β) C–H of THF > secondary C–H of cyclopentane > secondary (α) C–H of THF ∼ secondary C–H of n-pentane > secondary (α) C–H of ethyl ether. The primary C–H bond is more reactive than the secondary one because of its smaller steric repulsion and lower energy valence orbital of the primary alkyl group. The β-C–H bond of THF is more reactive than the secondary C–H bond of cyclopentane because of valence orbital energy of the β-oxolanyl group being lower than that of the cyclopentyl group. Both steric and electronic factors are important for determining reactivity of substrate. Bidentate ligand consisting of pyridine and N-heterocyclic carbene is predicted to be better than 3,4,7,8-tetramethyl-1,10-phenanthroline used experimentally.
We report a method that ensures the selective alkylation of benzamides and aromatic ketones at the para-position via cooperative nickel/aluminum catalysis. Using a bulky catalyst/cocatalyst system ...allows reactions between benzamides and alkenes to afford the corresponding para-alkylated products. The origin of the high para-selectivity has also been investigated by density functional theory calculations.
Levoglucosan (1,6‐anhydro‐β‐D‐glucopyranose) formation by the thermal degradation of native cellulose was investigated by MP4(SDQ)//DFT(B3LYP) and DFT(M06‐2X)//DFT(B3LYP) level computations. The ...computational results of dimer models lead to the conclusion that the degradation occurs by a concerted mechanism similar to the degradation of methyl β‐D‐glucoside reported in our previous study. One‐chain models of glucose hexamer, in which the interchain hydrogen bonds of real cellulose crystals are absent, do not exhibit the correct reaction behavior of levoglucosan formation; for instance, the activation enthalpy (Ea=≈38 kcal mol−1) is considerably underestimated compared to the experimental value (48–60 kcal mol−1). This problem is solved with the use of two‐chain models that contain interchain hydrogen bonds. The theoretical study of this model clearly shows that the degradation of the internal glucosyl residue leads to the formation of a levoglucosan precursor at the chain end and levoglucosan is selectively formed from this levoglucosan end. The calculated Ea (56–62 kcal mol−1) agrees well with the experimental value. The computational results of three‐chain models indicate that this degradation occurs selectively on the crystalline surface. All these computational results provide a comprehensive understanding of several experimental facts, the mechanisms of which have not yet been elucidated.
Cellulose unchained: Quantum chemical computations disclose that the degradation of a cellulose chain on the crystalline surface forms a levoglucosan end (LG‐end) and then levoglucosan is successively produced from the LG‐end. These reaction features are consistent with experimental observations, and the hydrogen network plays crucial roles in the reaction.
Carbon monoxide (CO) produced in many large-scale industrial oxidation processes is difficult to separate from nitrogen (N₂), and afterward, CO is further oxidized to carbon dioxide. Here, we report ...a soft nanoporous crystalline material that selectively adsorbs CO with adaptable pores, and we present crystallographic evidence that CO molecules can coordinate with copper(II) ions. The unprecedented high selectivity was achieved by the synergetic effect of the local interaction between CO and accessible metal sites and a global transformation of the framework. This transformable crystalline material realized the separation of CO from mixtures with N₂, a gas that is the most competitive to CO. The dynamic and efficient molecular trapping and releasing system is reminiscent of sophisticated biological systems such as heme proteins.
This work clearly disclosed that the lower energy d-valence band-top of M13@Pt42 is the origin of the higher activity of the M13@Pt42, which arises from the smaller exchange repulsion of the Pt42 ...shell with the M13 core than with the Pt13 core.
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•Ni13@Pt42 and Co13@Pt42 are more active than Pt55 for oxygen reduction reaction.•Rate-determining step is OH formation in Ni13@Pt42 and Co13@Pt42.•Rate-determining step is H2O formation in Pt55.•Ni13 and Co13 cores accelerate H2O formation to enhance ORR catalytic activities.•Lower energy d-valence band-top leads to higher activity for ORR.
Oxygen reduction reaction (ORR) by Pt55 and core-shell M13@Pt42 (MCo or Ni) particles is explored theoretically. Rate-determining step (RDS) is the second OH formation in M13@Pt42 but the second H2O formation in Pt55. Activation energy of RDS decreases following the order Pt55 > Co13@Pt42 > Ni13@Pt42 and onset potential is larger in M13@Pt42 than in Pt55, showing M13@Pt42 is more active than Pt55. Lower energy d-valence band-top (φVB-top) of M13@Pt42 results in the higher activity of M13@Pt42 than that of Pt55, where φVB-top is the highest energy peak in d-valence band DOS. Thus, the φVB-top energy is important for ORR activity. The lower energy φVB-top of M13@Pt42 than that of Pt55 mainly arises from smaller exchange repulsion of Pt42 shell with M13 core than that with Pt13 core; smaller atomic size and d orbital size of Co and Ni than those of Pt are key factors.
Generation of H2 from methanol/water and hydrosilylation of CO2 catalyzed by tris(2-pyridylthio)methylzinc hydride κ3-TptmZnH 1 were investigated with DFT and MP2 methods. The hydrosilylation of CO2 ...occurs via the CO2 insertion into the Zn–H bond of 1 followed by the metathesis of a Zn–(η1-OCOH) bond with hydrosilane to yield silyl formate and regenerate 1. The CO2 insertion easily occurs, but the metathesis is difficult because of the formation of a very stable Zn–(η2-O2CH) species before the metathesis. The ΔG°⧧ value of the metathesis with triethoxysilane is much smaller than that with phenylsilane because electronegative methoxy groups stabilize the transition state bearing hypervalent Si center, which is consistent with the experimental result that triethoxysilane is used in the hydrosilylation of CO2. It is theoretically predicted here that hydrosilane with two electronegative OEt groups or one to three F groups can be applied to this reaction. In the generation of H2 from methanol/water by 1, the first step is the metathesis of 1 with the O–H bond of methanol/water to produce κ3-TptmZn(OMe)/k3-TptmZn(OH) and dihydrogen molecule. The next step is the metathesis of the Zn–OMe/Zn–OH bond with hydrosilane to yield silyl ether and regenerate 1. The first metathesis is rate-determining but the second one occurs with very small activation energy, indicating that various hydrosilanes can be applied to this reaction.
Oxides of base-metal elements play an important role as catalysts in alkene combustion, but correct knowledge of the reaction mechanism and determination factors for activity remains elusive. Herein, ...we report a systematic study of propene combustion catalyzed by ZnCr2O4(111) as one example using DFT + U calculations. The combustion occurs through three parallel reaction pathways starting from C–H σ-bond cleavage. In each pathway, acetate, carbonate, or formate is formed as a key intermediate, which is consistent with the experimental detection of several different kinds of intermediates in propene combustion. Effects of tetrahedral metal (MTd) and octahedral metal (MOh) atoms on the catalytic activity are discussed by comparing propene combustion by MgCr2O4(111), Cu-doped ZnCr2O4(111) (named Zn1 – x Cu x Cr2O4(111)), and Co-doped ZnCr2O4(111) (named ZnCr2 – x Co x O4(111)) with that by ZnCr2O4(111). Catalytic activity increases in the order MgCr2O4(111) < Zn1 – x Cu x Cr2O4(111) < ZnCr2O4(111) < ZnCr2 – x Co x O4(111). The higher activity of ZnCr2O4(111) than that of MgCr2O4(111) agrees with the experimental findings. ZnCr2 – x Co x O4(111) is computationally predicted here to be more active than ZnCr2O4(111) which has been experimentally used as a good catalyst. Based on computational findings, a discussion is presented on the relation between the activity and electronic structure of spinels.
The emission spectrum of the platinum(II) bipyridine complex Pt(CN)2(bpy) (bpy = 2,2′-bipyridine) interestingly exhibits blue shift and peak broadening in crystal as temperature increases from 15 ...to 292 K. Periodic QM(DFT)/MM calculations here showed that the emission spectrum was assigned as metal–metal to ligand charge transfer triplet (3MMLCT) excited state in which one-electron excitation occurred from dσ–dσ antibonding molecular orbital (MO) between several Pt atoms to the bpy π* orbital. Although the elongation of the Pt–Pt distance by the temperature rise was experimentally observed at the ground state, the QM/MM calculations indicated that the Pt–Pt distance at the 3MMLCT excited state did not change so much by the temperature rise. The broadening of the emission spectrum by the temperature rise was reproduced by taking account of thermal population of vibrationally excited states of Pt–Pt stretching at the 3MMLCT excited state. The blue shift of the emission peak by the temperature rise was explained by the thermal population of several possible structures at the 3MMLCT excited state; in the most plausible structure, two Pt(II) complexes participate in the 3MMLCT excited state (named 3 Pt 2 ), and in the second and the third ones, three and four Pt(II) complexes, respectively, participate in the excited state (named 3 Pt 3 and 3 Pt 4 ). Interestingly, 3 Pt 4 is more stable than 3 Pt 2 and 3 Pt 3 in the crystal structure at low temperature (LT = 10 K) but 3 Pt 3 is the most stable in the crystal structure at room temperature (RT = 293 K) because of the difference in the crystal structure. Considering thermal populations of these species, the emission peak was calculated at 1.95 eV in the LT crystal structure and 2.01 eV in the RT crystal structure, which agree well with the experimental values (1.87 and 1.94 eV at 15 and 292 K, respectively). These findings indicate that populations of the vibrationally excited state and several possible structures at the 3MMLCT exited state are crucially important for correctly understanding the emission spectrum in crystal.
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