Bond Hydrogenolysis Research Articles

Efficient C–O Bond Hydrogenolysis Over Polyoxometalate Catalysts in Mild Hydrodeoxygenation of Lignin-Derived Phenols to Cycloalkanes

Efficient C–O Bond Hydrogenolysis Over Polyoxometalate Catalysts in Mild Hydrodeoxygenation of Lignin-Derived Phenols to Cycloalkanes

Water-assisted preparation of Ni/La(OH)3 catalyst for efficient catalytic C-O bond hydrogenolysis of tetrahydrofuran-dimethanol

Due to easy dehydration of La(OH)3 to LaOOH by thermal treatment, it is challenging to prepare highly efficient Ni/La(OH)3 catalyst for hydrogenolysis reactions. Herein, Ni/La(OH)3 (Ni/La-2) catalyst is prepared by thermal reduction of NiO/La(OH)3 in hydrogen flow with vaporized water, while La(OH)3 is partially dehydrated to LaOOH without water vapor affording Ni/(La(OH)3 + LaOOH) (Ni/La-1). Ni/La-2 displays 93 % and 3 % yields of 1,2,6-hexanetriol (HTO) and 1,6-hexanediol (HDO) respectively towards C-O hydrogenolysis of tetrahydrofuran-dimethanol (THFDM) in batch reactor and high stability (200 h) in fixed-bed reactor, which is the best among the reported catalysts to the best of our knowledge. Ni/La-2 possesses pure La(OH)3 as support, enhanced dispersion of Ni species and abundant Niδ+-O-La interfaces in comparison to Ni/La-1, which results in the exceptional catalytic performances in C-O hydrogenolysis of THFDM. A fine control of reaction temperature and time is required to optimize the yields HTO and HDO in hydrogenolysis of THFDM over Ni/La-2. The water-assisted preparation method provides a new perspective for the preparation of La(OH)3 supported metal catalysts.

One-pot synthesis of Ir subnanosized clusters supported on MWW nanosheets as effective catalysts for C-C bond hydrogenolysis

The rational design synthesis of metal catalysts with high dispersion and ordered structure is a challenging subject in materials chemistry and heterogeneous catalysis. In this study, we report one-pot synthesis of Ir subnanosized particles supported on MWW nanosheets and their application in hydrogenolysis of decalin. Here, it has been found that C-C bond cleavage occurs via a synergistic carbocation and carbene mechanism on Ir-containing catalysts, and via carbocation in the case of solid acids. The combination of optimal acidic and metallic site characteristics in the subnanosized Ir supported on MWW nanosheets Pill_Ir-ZN_150 sample contributes to the high activity of the final catalysts. We expect that these Ir catalytic systems will open up more opportunities for hydrogenolysis of naphthenics to high-value products.

Inverse ceria-nickel catalyst for enhanced C-O bond hydrogenolysis of biomass and polyether.

Regulating interfacial electronic structure of oxide-metal composite catalyst for the selective transformation of biomass or plastic waste into high-value chemicals through specific C-O bond scission is still challenging due to the presence of multiple reducible bonds and low catalytic activity. Herein, we find that the inverse catalyst of 4CeOx/Ni can efficiently transform various lignocellulose derivatives and polyether into the corresponding value-added hydroxyl-containing chemicals with activity enhancement (up to 36.5-fold increase in rate) compared to the conventional metal/oxide supported catalyst. In situ experiments and theoretical calculations reveal the electron-rich interfacial Ce and Ni species are responsible for the selective adsorption of C-O bond and efficient generation of Hδ- species, respectively, which synergistic facilitate cleavage of C-O bond and subsequent hydrogenation. This work advances the fundamental understanding of interfacial electronic interaction over inverse catalyst and provides a promising catalyst design strategy for efficient transformation of C-O bond.

Hydrogenolysis of guaiacol and lignin to phenols over Ni/Nb2O5[sbnd]HZSM-5 catalyst

Selective hydrogenolysis of CAr-O bonds in lignin to produce aromatic compounds typically necessitates severe conditions. We developed a Ni/Nb2O5HZSM-5 catalyst that facilitates direct cleavage of guaiacol's aryl ether bonds at reduced temperatures (200 °C) and pressure (0.1 MPa H2), achieving a conversion of 89.5 % with the selectivity of phenol at 81.7 %, while retaining its activity after five cycles. The Ni/Nb2O5HZSM-5 exhibits a higher yield of phenol (49.1 mmolphenol·gNi−1·h−1), currently achieving the highest phenol yield among Ni-based catalysts. The addition of Nb2O5 enhances the dispersion of Ni and augments the effective surface area. In addition, the strong interaction of Nb with the HZSM-5 changed the electronic state of Nb and enhanced the resistance of the catalyst to high temperature and mechanical stress. Employing this catalyst for lignin depolymerization in an aqueous medium led to a 17.0 wt% yield of alkyl phenolic compounds. This approach represents an advancement in biomass resource conversion, circumventing the dependency on high-pressure and precious-metal catalysts, and signaling a new trajectory for sustainable biomass utilization in scientific research.

Demystifying group-4 polyolefin hydrogenolysis catalysis: Gaseous propane hydrogenolysis mechanism over the same catalysts

A kinetic/mechanistic investigation of gaseous propane hydrogenolysis over the single-site heterogeneous polyolefin depolymerization catalysts AlS/ZrNp2 and AlS/HfNp2 (AlS = sulfated alumina, Np = neopentyl), is use to probe intrinsic catalyst properties without the complexities introduced by time- and viscosity-dependent polymer medium effects. In a polymer-free automated plug-flow catalytic reactor, propane hydrogenolysis turnover frequencies approach 3,000 h-1 at 150 °C. Both catalysts exhibit approximately linear relationships between rate and [H2] at substoichiometric [H2] with rate law orders of 0.66 ± 0.09 and 0.48 ± 0.07 for Hf and Zr, respectively; at higher [H2], the rates approach zero-order in [H2]. Reaction orders in [C3H8] and [catalyst] are essentially zero-order under all conditions, with the former implying rapid, irreversible alkane binding/activation. This rate law, activation parameter, and DFT energy span analysis support a scenario in which [H2] is pivotal in one of two plausible and competing rate-determining transition states-bimolecular metal-alkyl bond hydrogenolysis vs. unimolecular β-alkyl elimination. The Zr and Hf catalyst activation parameters, ΔH‡ = 16.8 ± 0.2 kcal mol-1 and 18.2 ± 0.6 kcal mol-1, respectively, track the relative turnover frequencies, while ΔS‡ = -19.1 ± 0.8 and -16.7 ± 1.4 cal mol-1 K-1, respectively, imply highly organized transition states. These catalysts maintain activity up to 200 °C, while time-on-stream data indicate multiday activities with an extrapolated turnover number ~92,000 at 150 °C for the Zr catalyst. This methodology is attractive for depolymerization catalyst discovery and process optimization.

Unveiling the mechanism of triphos-Ru catalysed C–O bond disconnections in polymers

Ruthenium complexes with facially coordinating tripodal phosphine ligands are privileged catalysts for a broad range of (de-)hydrogenation-based transformations. Among these, C–O bond hydrogenolysis holds potential for the depolymerisation of both the biopolymer lignin and epoxy resins applied in wind turbine blades, aircrafts and more. However, this methodology is poorly understood in mechanistic terms. Here, we present a detailed investigation on the triphos-Ru catalysed C–O bond scission on a molecular level. A combination of experimental, spectroscopical and theoretical studies elucidates the reactivity of the ruthenium trimethylenemethane precatalyst, revealing the key roles of ruthenium phenolates in both catalyst activation as well as the catalytic cycle itself. Furthermore, a Ru(0)/Ru(II) oxidative addition into the C–O bond is disclosed, with a triphos-Ru(0) dihydrogen complex as entry point. With the molecular nature of the operating triphos-Ru species and the thermodynamics and kinetics of the catalysis unravelled, improvements of established methods as well as design of related transformations may become possible.

Ring–Opening Mechanism of O‐heterocycles into α,ω‐Diols over Ni−La(OH)3: C−O Bond Hydrogenolysis of THFA to 1,5‐Pentanediol as a Case Study

AbstractThe elucidation of the ring–opening reaction mechanism is a critical step towards improving the catalytic performance for the conversion of biomass into value–added chemicals. Herein, we focused on the stepwise C−O bond hydrogenolysis mechanism of oxygen–containing heterocycles (O‐heterocycles) into α,ω‐diols, in particular THFA to 1,5‐PeD, over selective Ni−La(OH)3 in hydrogen–donor isopropanol. A mechanistic study was carried out on a structurally well–defined Ni−La(OH)3, where the mechanism was elucidated using a combination of kinetic and 13C NMR isotope labeling experiments. It was suggested that both Ni nanoparticles and La(OH)3 support play a critical role in the reaction mechanism, where basic hydroxide species of the support initially deprotonate the CH2OH and −OH modified furan, tetrahydrofuran and tetrahydropyran rings, adsorbing them chemically on the catalyst surface. This step is followed by a direct hydride attack on the second carbon atom of these rings, which is proposed to be the key step for ring cleavage, as indicated by the increased deuteration at this position. In addition to the direct hydrogenolysis using gaseous H2, it is assumed that a catalytic transfer hydrogenolysis reaction (CTH) reaction of THFA is proposed to proceed via the dehydrogenation of isopropanol over isolated Ni species, forming hydrogen species that can be adsorbed on the Ni surface or desorb as H2 molecules.

Catalytic Hydrogenolysis by Atomically Dispersed Iron Sites Embedded in Chemically and Redox Non-innocent N-Doped Carbon.

Atomically dispersed first-row transition metals embedded in nitrogen-doped carbon materials (M-N-C) show promising performance in catalytic hydrogenation but are less well-studied for reactions with more complex mechanisms, such as hydrogenolysis. Their ability to catalyze selective C-O bond cleavage of oxygenated hydrocarbons such as aryl alcohols and ethers is enhanced with the participation of ligands directly bound to the metal ion as well as longer-range contributions from the support. In this article, we describe how Fe-N-C catalysts with well-defined local structures for the Fe sites catalyze C-O bond hydrogenolysis. The reaction is facilitated by the N-C support. According to spectroscopic analyses, the as-synthesized catalysts contain mostly pentacoordinated FeIII sites, with four in-plane nitrogen donor ligands and one axial hydroxyl ligand. In the presence of 20 bar of H2 at 170-230 °C, the hydroxyl ligand is lost when N4FeIIIOH is reduced to N4FeII, assisted by the H2 chemisorbed on the support. When an alcohol binds to the tetracoordinated FeII sites, homolytic cleavage of the O-H bond is accompanied by reoxidation to FeIII and H atom transfer to the support. The role of the N-C support in catalytic hydrogenolysis is analogous to the behavior of chemically and redox-non-innocent ligands in molecular catalysts based on first-row transition metal ions and enhances the ability of M-N-Cs to achieve the types of multistep activations of strong bonds needed to upgrade renewable and recycled feedstocks.

Cleavage of the Disulfide Bonds via Hydrogenolysis for Recovering Iso‐octyl Thioglycolate from Distillation Residue in Iso‐octyl Thioglycolate Production

AbstractAbout 10–15 wt% of distillation residue, which is mainly composed of the oxidation products of iso‐octyl thioglycolate, harmful to the environment and difficult to dispose, is generated during refining through distillation. Herein we report an effective approach to recover iso‐octyl thioglycolate from the distillation residue by hydrogenolysis of the disulphide bonds. The effects of reaction temperature, reaction time, and feeding speed were examined. It is found that the rate of hydrogen formation is a key factor affecting the yield of iso‐octyl thioglycolate. The orthogonal test shows that the influence order is: reaction time > mass ratio of distillation residue/HCl > the molar ratio of HCl/Zn > reaction temperature. About 63.64 % iso‐octyl thioglycolate could be harvested after hydrogenolysis at 30 °C for 3.0 h. The mechanism study shows that iso‐octyl thioglycolate was regenerated by cleavage the disulfide bonds in iso‐octyl thioglycolate disulfides. Therefore, an alternative way for resource utilization of iso‐octyl thioglycolate distillation residue was developed.

Atomically Dispersed Pt and NiO Clusters Synergistically Enhanced C–O Bond Hydrogenolysis

Atomically Dispersed Pt and NiO Clusters Synergistically Enhanced C–O Bond Hydrogenolysis

Hydrodeoxygenation of anisole over SBA-15-supported Ni, Pd, and Pt mono- and bimetallic catalysts: effect of the metal's nature on catalytic activity and selectivity.

Monometallic Ni, Pd and Pt and bimetallic catalysts formed by combinations of the above metals supported on SBA-15 silica were synthesized, characterized and tested in the hydrodeoxygenation reaction of anisole. The objective of the work was to detect the effect of the nature of metals on the activity of the catalysts at different steps of anisole hydrodeoxygenation: hydrogenation of the aromatic ring of anisole and C-O bond cleavage in the intermediate cyclohexyl methyl ether. The support and the catalysts were characterized by N2 physisorption, X-ray diffraction, UV-vis diffuse reflectance spectroscopy, temperature-programmed reduction, scanning electron microscopy-energy dispersive X-ray spectroscopy, transmission electron microscopy and HAADF-STEM. The catalytic activity tests were carried out in a batch reactor at 280 °C and 7.3 MPa pressure. The activity results show that the NiPd/SBA-15 catalyst had the greatest ability for hydrogenation of the aromatic ring of anisole, while its NiPt/SBA-15 analog resulted in better activity for C-O bond hydrogenolysis. The bimetallic NiPt/SBA-15 catalyst showed the best catalytic performance in the HDO of anisole ascribed to the formation of a Ni-Pt alloy. On the other hand, the combination of Pd and Pt metals in the PdPt/SBA-15 catalyst resulted in the formation of bimetallic particles with Pd-rich and Pt-rich domains, showing high selectivity for the formation of the cyclohexyl methyl ether, which can be useful for the hydrogenation of aromatic rings in O-containing reactants with the formation of saturated O-containing products. According to the characterization results (HAADF-STEM), the different catalytic behavior of NiPd/SBA-15, NiPt/SBA-15, and PdPt/SBA-15 catalysts could be attributed to different characteristics of the bimetallic active phases in them.

Ru nanoparticles supported on various metal-modified (La, Zr, Cr, Ce and Ga) α-Al2O3 for the selective hydrogenolysis of lignin and its derived ethers

Controlling the C-O bond hydrogenolysis while suppressing the direct hydrogenation of aromatic rings in lignin-derived ethers is critical and challenging for the utilization of lignin. Herein, a series of metal-modified (La, Zr, Cr, Ce and Ga) α-Al2O3-supported Ru-based catalysts were synthesized for the conversion of diphenyl ether (DPE). The catalyst characterizations demonstrate that the metal modification cannot change the chemical valence of Ru but promotes the dispersion of Ru nanoparticles and adjusts the adsorption state of DPE. The results indicate that the metal modification inhibits the arenes hydrogenation at low temperatures but facilitates the C-O bond cleavage during heating (at low H2 pressures or an inert atmosphere). Ru-Cr/α-Al2O3 shows the highest reaction rate for DPE conversion at 1 MPa Ar and 200 °C. Moreover, Ru-Cr/α-Al2O3 not only efficiently converts different lignin model compounds by hydrogenolysis, but also achieves the controlled depolymerization of real lignin to generate guaiacols under mild conditions.

Selective hydrogenolysis of C-O bonds in lignin model compounds and Kraft lignin over highly efficient NixCoyAl catalysts

The selective hydrogenolysis of CO bonds is critical for the effective valorization of lignin. Herein, we synthesized a series of highly efficient NixCoyAl-T catalysts by a coprecipitation-calcination-reduction method and evaluated their catalytic hydrogenolysis performance towards the CO bonds of lignin-derived aryl ether, using isopropanol as the hydrogen donor source. The results revealed that the Ni0.5Co1.5Al-600 catalyst displayed superior activity and its catalytic performances were strongly influenced by the Ni/Co molar ratios and calcination temperatures. The enhanced dispersion of metal particles and the electron transfer from Ni to Co in Ni0.5Co1.5Al-600 catalyst facilitated the cleavage of CO bonds in lignin model compounds. Furthermore, the β-benzyl-H atom in 2-phenoxy-1-phenylethanol (PPE) also contributes to the cleavage of Caliphatic-O bonds. Under optimized reaction conditions (180 ℃, reaction time 2 h), PPE was completely converted over the Ni0.5Co1.5Al-600 catalyst, and the yield of ethylbenzene and cyclohexanol reached 95.91wt% and 85.73wt%, respectively. These findings demonstrate that the NixCoyAl-T catalytic system holds great promise for the production of valuable chemicals from lignin under mild conditions.

In-situ deoxygenation of methyl palmitate to hydrocarbons on highly dispersed Ni–Re/TiO2 catalyst in aqueous phase using methanol as a hydrogen donor

Herein, TiO2 supported Ni–Re alloy catalysts (i.e., NiRe(x:y)/TiO2, x:y denotes the Ni/Re atomic ratio) were prepared and tested for in situ deoxygenation of methyl palmitate to hydrocarbons in aqueous phase using methanol as hydrogen donor. Raman spectra indicates that there is interaction between NiO and ReO4− species via forming Ni–O-Re(=O)3 species in the calcined catalysts, which contributes to the formation of Ni–Re alloy and promotes the Ni dispersion during the reduction with H2. Particularly, the Ni–Re alloy particles with average size of ∼1.4 nm highly disperse on spherical TiO2 particles (∼14 nm in diameter) in NiRe(3:4)/TiO2 (the nominal Ni content of 3 wt%). XPS reveals a charge transfer from metallic Ni to Re in the Ni–Re alloy. In in situ deoxygenation of methyl palmitate, compared with Ni/TiO2, NiRe(x:y)/TiO2 give remarkably higher deoxygenation activity and lower activity for the C–C bond hydrogenolysis, ascribed to the synergistic effect between Ni and Re species and the geometrical effect of Re, respectively, which are further verified by propanoic acid-TPD and TPSR. The yield of C6∼C16 liquid hydrocarbons can achieve as high as 98.0% on NiRe(3:4)/TiO2 at 330 °C. There is no obvious sintering of Ni–Re alloy and support TiO2 particles during the recycling of NiRe(3:4)/TiO2, while the carbon deposition and the leaching of Re species slightly reduce the catalyst activity.

Efficient deoxygenation of methyl esters to hydrocarbons on Al2O3 supported Ni-Sn intermetallic compounds

Al2O3 supported Ni-Sn alloy (Ni/Sn atomic ratio of 6) and Ni3Sn, Ni3Sn2 and Ni3Sn4 intermetallic compounds (IMCs) were prepared by co-precipitation method for the deoxygenation of fatty acid methyl esters to produce hydrocarbons. In comparison with metallic Ni, the catalytic evaluation and methyl propionate-TPD/TPSR results reveal that Ni-Sn IMCs remarkably inhibit the decarbonylation/decarboxylation pathway, C-C bond hydrogenolysis and methanation while promote the hydrodeoxygenation pathway, subsequently enhancing the yield of aiming hydrocarbons. Therein, 20%Ni3Sn2/Al2O3 has the best performance, and the hydrocarbons yield reach about 99% in the deoxygenation of methyl octanoate and methyl palmitate. We suggest that the geometric and electronic effects of Sn contribute the excellent performance of Ni-Sn IMCs, and there is synergetic effect between Ni and Sn sites for the activation of C-O/C=O bond, for which a suitable Ni-Sn atomic spacing is very important. Finally, the catalyst stability and deactivation were also investigated.

The Mechanism of Markovnikov-Selective Epoxide Hydrogenolysis Catalyzed by Ruthenium PNN and PNP Pincer Complexes.

The homogeneous catalysis of epoxide hydrogenolysis to give alcohols has recently received significant attention. Catalyst systems have been developed for the selective formation of either the Markovnikov (branched) or anti-Markovnikov (linear) alcohol product. Thus far, the reported catalysts exhibiting Markovnikov selectivity all feature the potential for Noyori/Shvo-type bifunctional catalysis, with either a RuH/NH or FeH/OH core structure. The proposed mechanisms of epoxide ring-opening have involved cooperative C-O bond hydrogenolysis involving the metal hydride and the acidic pendant group on the ligand, in analogy to the well-documented mechanism of polar double-bond hydrogenation exhibited by catalysts of this type. In this work, we present a combined computational/experimental study of the mechanism of epoxide hydrogenolysis catalyzed by Noyori-type PNP and PNN complexes of ruthenium. We find that, at least for these ruthenium systems, the previously proposed bifunctional pathway for epoxide ring-opening is energetically inaccessible; instead, the ring-opening proceeds through opposite-side nucleophilic attack of the ruthenium hydride on the epoxide carbon, without the involvement of the ligand N-H group. For both catalyst systems, the rate law and overall barrier predicted by density functional theory (DFT) are consistent with the results from kinetic studies.

Functional zirconium phosphate nanosheets enabled transfer hydrogenolysis of aromatic ether bonds over a low usage of Ru nanocatalysts

Catalytic hydrogenolysis of aromatic ether bonds is a highly promising strategy for upgrading lignin into small-molecule chemicals, which relies on developing innovative heterogeneous catalysts with high activity. Herein, we designed porous zirconium phosphate nanosheet-supported Ru nanocatalysts (Ru/ZrPsheet) as the heterogeneous catalyst by a process combining ball milling and molten-salt (KNO3). Very interestingly, the fabricated Ru/ZrPsheet showed good catalytic performance on the transfer hydrogenolysis of various types of aromatic ether bonds contained in lignin, i.e., 4-O-5, α-O-4, β-O-4, and aryl-O-CH3, over a low Ru usage (<0.5 mol%) without using any acidic/basic additive. Detailed investigations indicated that the properties of Ru and the support were indispensable. The excellent activity of Ru/ZrPsheet originated from the strong acidity and basicity of ZrPsheet and the higher electron density of metallic Ru0 as well as the nanosheet structure of ZrPsheet.

Highly efficient catalytic hydrogenolysis of lignin model compounds over hydrotalcite-derived Ni/Al2O3 catalysts

Catalytic hydrogenolysis of CO bonds in lignin provides a promising route to produce high value-added biofuels and chemicals. However, designing highly active and stable catalysts for this process is still a challenge. Herein, a series of Ni/Al2O3 catalysts were synthesized by calcination and reduction of layered double hydroxide precursors, and their activities were evaluated in the catalytic conversion of lignin-derived compounds. Diphenyl ether was completely converted with a 100% monomeric selectivity (cyclohexane and cyclohexanol) over the optimized Ni/Al2O3-500 under the optimal conditions of 200 ℃, 0.5 MPa H2 and 1.5 h, indicating the excellent catalytic activity in the cleavage of CO bonds of the catalyst. The excellent catalytic performances of Ni/Al2O3-500 were originated from its relatively large specific surface area, well-developed porosity, small crystallite size, high Ni0 concentration and strong interaction between the active metal and support. The reusability experiments show that Ni/Al2O3-500 possesses a relatively high stability for six reuses. Moreover, various lignin model compounds can be completely converted to corresponding monomeric products over Ni/Al2O3-500, indicating the applicability of this ingenious catalyst. The application of the hydrotalcite-derived Ni-based catalysts with high activity and stability provides a promising approach for the efficient hydrogenolysis of lignin derivatives.

Origin of the Ligand Ring-Size Effect on the Catalytic Activity of Cationic Calcium Hydride Dimers in the Hydrogenation of Unactivated 1-Alkenes.

Recently, it was shown that the double Ca-H-Ca-bridged calcium hydride cation dimer [LCaH2 CaL]2+ when stabilized by a larger macrocyclic N,N',N'',N''',N''''-pentadentate ligand showed evidently higher activity than when stabilized by a smaller N,N',N'',N'''-tetradentate ligand in the catalytic hydrogenation of unactivated 1-alkenes. In this DFT-mechanistic work, the origin of the observed ring-size effect is examined in detail using 1-hexene, CH2 =CH2 and H2 as substrates. It is shown that, at room temperature, both the N,N',N'',N''',N''''-stabilized dimer and the monomer are not coordinated by THF in solution, while the corresponding N,N',N'',N'''-stabilized structures are coordinated by one THF molecule mimicking the fifth N-coordination. Catalytic 1-alkene hydrogenation may occur via anti-Markovnikov addition over the terminal Ca-H bonds of transient monomers, followed by faster Ca-C bond hydrogenolysis. The higher catalytic activity of the larger N,N',N'',N''',N''''-stabilized dimer is due to not only easier formation of but also due to the higher reactivity of the catalytic monomeric species. In contrast, despite unfavorable THF-coordination in solution, the smaller N,N',N'',N'''-stabilized dimer shows a 3.2 kcal mol-1 lower barrier via a dinuclear cooperative Ca-H-Ca bridge for H2 isotope exchange than the large N,N',N'',N''',N''''-stabilized dimer, mainly due to less steric hindrance. The observed ring-size effect can be understood mainly by a subtle interplay of solvent, steric and cooperative effects that can be resolved in detail by state-of-the-art quantum chemistry calculations.