Bond In Ethane Research Articles

Origin of the Different Reactivity of the Triatomic Anions HMoN- and ZrNH- toward Alkane: Compositions of the Active Orbitals.

The reactivity of the triatomic anions HMoN- and ZrNH- toward alkanes was investigated by means of mass spectrometry in conjunction with density functional theory calculations. HMoN- can activate C-H bond of ethane with the liberation of ethene and hydrogen molecules, and the generation of hydrogen molecules is the major reaction channel; however, no C-H bond activation of ethane was observed over ZrNH- ion, and the density functional theory calculations suggest this pathway is hampered by intrinsic energy barrier. In sharp contrast, another triatomic anion HNbN- can bring about methane activation under thermal conditions, as reported previously. A strong dependence of the chemical reactivity of alkane activation on compositions of active orbitals in the above-mentioned systems is discussed. This combined experimental/computational study may provide new insights into the importance of compositions of active orbitals and their essential role in the reactions of related systems with alkanes.

Zero Steric Potential and bond order

The variation of Zero Steric Potential (ZSP) through a C–C bond shows two maximums, which their values depend on the bond order (BO). A good relationship (R2=1) is observed between the mean values of maximum ZSPs and the bond orders of C–C bonds in ethane, ethylene and acetylene, as reference molecules (Ln BO=1.956ZSP‾max-0.898). The obtained equation is used to predict the C–C bond orders of more than twenty aromatic and aliphatic hydrocarbons. The results show that the obtained bond orders from ZSP‾max are more reliable than those which are evaluated using NBO and Laplacian methods.

Identifying Different Types of Catalysts for CO2 Reduction by Ethane through Dry Reforming and Oxidative Dehydrogenation

AbstractThe recent shale gas boom combined with the requirement to reduce atmospheric CO2 have created an opportunity for using both raw materials (shale gas and CO2) in a single process. Shale gas is primarily made up of methane, but ethane comprises about 10 % and reserves are underutilized. Two routes have been investigated by combining ethane decomposition with CO2 reduction to produce products of higher value. The first reaction is ethane dry reforming which produces synthesis gas (CO+H2). The second route is oxidative dehydrogenation which produces ethylene using CO2 as a soft oxidant. The results of this study indicate that the Pt/CeO2 catalyst shows promise for the production of synthesis gas, while Mo2C‐based materials preserve the CC bond of ethane to produce ethylene. These findings are supported by density functional theory (DFT) calculations and X‐ray absorption near‐edge spectroscopy (XANES) characterization of the catalysts under in situ reaction conditions.

Identifying Different Types of Catalysts for CO2 Reduction by Ethane through Dry Reforming and Oxidative Dehydrogenation.

The recent shale gas boom combined with the requirement to reduce atmospheric CO2 have created an opportunity for using both raw materials (shale gas and CO2 ) in a single process. Shale gas is primarily made up of methane, but ethane comprises about 10 % and reserves are underutilized. Two routes have been investigated by combining ethane decomposition with CO2 reduction to produce products of higher value. The first reaction is ethane dry reforming which produces synthesis gas (CO+H2 ). The second route is oxidative dehydrogenation which produces ethylene using CO2 as a soft oxidant. The results of this study indicate that the Pt/CeO2 catalyst shows promise for the production of synthesis gas, while Mo2 C-based materials preserve the CC bond of ethane to produce ethylene. These findings are supported by density functional theory (DFT) calculations and X-ray absorption near-edge spectroscopy (XANES) characterization of the catalysts under in situ reaction conditions.

Selective C-H and C-C Bond Activation: Electronic Regimes as a Tool for Designing d(10) MLn Catalysts.

We wish to understand how a transition-metal catalyst can be rationally designed so as to selectively activate one particular bond in a substrate, herein, C-H and C-C bonds in ethane. To this end, we quantum chemically analyzed the activity and selectivity of a large series of model catalysts towards ethane and, for comparison, methane, by using the activation strain model and quantitative molecular orbital theory. The model catalysts comprise d(10) MLn complexes with coordination numbers n=0, 1, and 2; metal centers M=Co(-), Rh(-), Ir(-), Ni, Pd, Pt, Cu(+), Ag(+), and Au(+); and ligands L=NH3, PH3, and CO. Our analyses reveal that rather subtle electronic differences between bonds can be exploited to induce a lower barrier for activating one or the other, depending, among other factors, on the catalysts electronic regime (i.e., s-regime versus d-regime catalysts). Interestingly, the concepts and design principles emerging from this work can also be applied to the more challenging problem of differentiating between activation of the C-H bonds in ethane versus those in methane.

Mechanism of Oxidation of Ethane to Ethanol at Iron(IV)-Oxo Sites in Magnesium-Diluted Fe2(dobdc).

The catalytic properties of the metal-organic framework Fe2(dobdc), containing open Fe(II) sites, include hydroxylation of phenol by pure Fe2(dobdc) and hydroxylation of ethane by its magnesium-diluted analogue, Fe0.1Mg1.9(dobdc). In earlier work, the latter reaction was proposed to occur through a redox mechanism involving the generation of an iron(IV)-oxo species, which is an intermediate that is also observed or postulated (depending on the case) in some heme and nonheme enzymes and their model complexes. In the present work, we present a detailed mechanism by which the catalytic material, Fe0.1Mg1.9(dobdc), activates the strong C-H bonds of ethane. Kohn-Sham density functional and multireference wave function calculations have been performed to characterize the electronic structure of key species. We show that the catalytic nonheme-Fe hydroxylation of the strong C-H bond of ethane proceeds by a quintet single-state σ-attack pathway after the formation of highly reactive iron-oxo intermediate. The mechanistic pathway involves three key transition states, with the highest activation barrier for the transfer of oxygen from N2O to the Fe(II) center. The uncatalyzed reaction, where nitrous oxide directly oxidizes ethane to ethanol is found to have an activation barrier of 280 kJ/mol, in contrast to 82 kJ/mol for the slowest step in the iron(IV)-oxo catalytic mechanism. The energetics of the C-H bond activation steps of ethane and methane are also compared. Dehydrogenation and dissociation pathways that can compete with the formation of ethanol were shown to involve higher barriers than the hydroxylation pathway.

Mild oxidation of ethane to acetic acid by H2O2 catalyzed by supported μ-nitrido diiron phthalocyanines

Low temperature selective transformation of light alkanes of natural gas to useful products continues to be an important challenge in chemistry and industry. μ-Nitrido diiron phthalocyanines were recently identified as powerful oxidation catalysts capable of oxidizing methane, benzene and transforming poly- and perfluorinated aromatic compounds under oxidative conditions. Herein, we have supported two μ-nitrido diiron complexes onto silica, graphite or nafion and their catalytic performance has been evaluated in the heterogeneous oxidation of ethane in water. Acetic acid was obtained with selectivity up to 71%. Turnover number up to 58 and 50–65 % product yields can be achieved. The cleavage of C–C bond also occurred and formic acid was formed as side product. In contrast to methane oxidation, no strong influence of 75 mM acid was observed in ethane oxidation. The reactivities of C–H bonds in methane, ethane and propane were very similar when the reaction was performed in water. In contrast, in diluted acidic solution, the μ-nitrido diiron phthalocyanine – H2O2 system exhibits 5 times higher activity in the oxidation of the methane C–H bond compared to the ethane C–H bond.

Photophysics, photochemistry and thermal stability of diarylethene-containing benzothiazolium species

Abstract The photophysics, photochemistry and thermal stability of four 1-aryl-2-(N-methyl-2-benzothiazolium) ethene iodides (aryl: phenyl, 1-naphthyl, 9-phenanthryl and 9-anthryl) were studied. Although the absorption spectra are found to be hypsochromic-shifted, fluorescence spectra are bathochromic-shifted. The dipole moment in the relaxed excited state was found to be larger than that in the ground state. To investigate effects of N-methylation and the aryl-ring size, a detailed comparison was made between those in the present work (of charged compounds) and previous studies of their neutral analogues, with computed electron affinity and ionisation potentials serving to rationalise the experimentally observed bathochromic shifts in absorption and emission spectra. The kinetics of thermal isomerisation depend strongly on the nature of the aryl moiety and solvent; the larger the aryl ring, the slower the rate of isomerisation. The fastest isomerisation process was found to take place in MeOH. The anthryl derivative did not isomerize either by light- or heat-exposure, due to high energy barriers of rotation around the ethenic bond. Based on the significant blue-shift of the Z-isomer absorption maximum relative to that of the E-isomer, and the high percentage of Z-isomers in the photostationary state, these compounds may serve as potential promising candidates for optical data-storage applications.

Oxidation of ethane to ethanol by N2O in a metal-organic framework with coordinatively unsaturated iron(II) sites.

Enzymatic haem and non-haem high-valent iron-oxo species are known to activate strong C-H bonds, yet duplicating this reactivity in a synthetic system remains a formidable challenge. Although instability of the terminal iron-oxo moiety is perhaps the foremost obstacle, steric and electronic factors also limit the activity of previously reported mononuclear iron(IV)-oxo compounds. In particular, although nature's non-haem iron(IV)-oxo compounds possess high-spin S = 2 ground states, this electronic configuration has proved difficult to achieve in a molecular species. These challenges may be mitigated within metal-organic frameworks that feature site-isolated iron centres in a constrained, weak-field ligand environment. Here, we show that the metal-organic framework Fe2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) and its magnesium-diluted analogue, Fe0.1Mg1.9(dobdc), are able to activate the C-H bonds of ethane and convert it into ethanol and acetaldehyde using nitrous oxide as the terminal oxidant. Electronic structure calculations indicate that the active oxidant is likely to be a high-spin S = 2 iron(IV)-oxo species.

Toward Better Understanding of the Catalytic Action of Acidic Zeolites: Investigation in Methane and Ethane Activation and Transformation

Studies of cracking reactions of alkanes with three or more carbon atoms have been central to the development of our understanding of the catalytic action of acidic zeolites, which are important catalysts in petrochemical and chemical industries. However, the mechanisms of the simplest cracking reactions, that is, the cracking of the C–H and C–C bonds in methane and ethane, have only been studied theoretically, not experimentally. Here we show that ethane is converted over a H-MFI zeolite at 510–550 °C with formation of such primary products as ethene, hydrogen, methane, and propane. To explain these results, we suggest and consider two catalytic cycles of the reaction. The first cycle involves protolytic cracking of the C–H bond with formation of hydrogen and ethoxide group, the latter decomposing into ethene and the zeolite acid site. We propose that the second cycle is initiated by the protolytic cracking of the C–C bond that results in formation of methane and a methoxide group as an intermediate. We theorize that this reacts with ethane molecules regenerating the zeolite acid site and producing (i) methane and ethene via hydrogen transfer and (ii) propane via a C–C bond formation reaction. Both suggested catalytic cycles are fully supported by the kinetic results of this study and are in a good agreement with recent theoretical work. We also demonstrate that the cracking of the C–H bond in methane (that could proceed via methoxide intermediate) does not occur over H-MFI zeolites up to 700 °C most likely because of a high activation energy. The proposed involvement of methoxide groups, as active intermediates, in ethane transformation provides a basis and excellent opportunity for theoretical studies of such interesting reactions as hydrogen transfer and C–C bond formation with participation of these surface species.

Activation of C–H and C–C bonds of ethane by gas-phase Pt atom: Potential energy surface and reaction mechanism

The reaction mechanism of the gas-phase Pt atom with C2H6 has been systematically studied on the singlet and triplet potential energy surfaces at CCSD(T)//BPW91/6-311++G(d, p), Lanl2dz level. For the formation of the main products, PtC2H4+H2 and (H)2Pt+C2H4, the minimal energy reaction pathway involves the rate-determining step of the release of a H2 or ethene molecule by a reductive elimination mechanism at the exit. For the formation of the side products, PtCH2+CH4, the optimal pathway proceeds through the σ-complex cis-HPtC2H5 from initial C–H bond cleavage, which assists the C–C σ-bond metathesis. This reactivity mode is complementary for the classical reactivity picture through the C–C cleavage intermediate M(CH3)2. Besides, these results are in qualitative agreement with the experimental results, in which the C–H insertion product is experimentally observed and the C–C insertion product is not formed in observable quantity.

The rotational barrier in ethane: a molecular orbital study.

The energy change on each Occupied Molecular Orbital as a function of rotation about the C-C bond in ethane was studied using the B3LYP, mPWB95 functional and MP2 methods with different basis sets. Also, the effect of the ZPE on rotational barrier was analyzed. We have found that σ and π energies contribution stabilize a staggered conformation. The σs molecular orbital stabilizes the staggered conformation while the stabilizes the eclipsed conformation and destabilize the staggered conformation. The πz and molecular orbitals stabilize both the eclipsed and staggered conformations, which are destabilized by the πv and molecular orbitals. The results show that the method of calculation has the effect of changing the behavior of the energy change in each Occupied Molecular Orbital energy as a function of the angle of rotation about the C–C bond in ethane. Finally, we found that if the molecular orbital energy contribution is deleted from the rotational energy, an inversion in conformational preference occurs.

Oxidative Addition of the CαCβ Bond in β‐O‐4 Linkage of Lignin to Transition Metals Using a Relativistic Pseudopotential‐Based ccCA‐ONIOM Method

A multi-level multi-layer QM/QM method, the relativistic pseudopotential correlation-consistent composite approach within an ONIOM framework (rp-ccCA-ONIOM), was applied to study the oxidative addition of the C(α)-C(β) bond in an archetypal arylglycerol β-aryl ether (β-O-4 linkage) substructure of lignin to Ni, Cu, Pd and Pt transition metal atoms. The chemically active high-level layer is treated using the relativistic pseudopotential correlation-consistent composite approach (rp-ccCA), an efficient methodology designed to reproduce an accuracy that would be obtained using the more computationally demanding CCSD(T)/aug-cc-pCV∞Z-PP, albeit at a significantly reduced computational cost, while the low-level layer is computed using B3LYP/cc-pVTZ. The thermodynamic and kinetic feasibilities of the model reactions are reported in terms of enthalpies of reactions at 298 K (ΔH°(298)) and activation energies (ΔH-act). The results obtained from the rp-ccCA:B3LYP hybrid method are compared to the corresponding values using CCSD(T) and several density functionals including B3LYP, M06, M06 L, B2PLYP, mPWPLYP and B2GP-PLYP. The energetics of the oxidative addition of CC bond in ethane to Ni, Cu, Pd and Pt atoms are also reported to demonstrate that the rp-ccCA method effectively reproduces the accuracy of the CCSD(T)/aug-cc-pCV∞Z method. Our results show that in the catalytic activation of the C(α)-C(β) bond of β-O-4, the use of platinum metal catalysts will lead to the most thermodynamically favored reaction with the lowest activation barrier.

Room Temperature Dehydrogenation of Ethane to Ethylene

The transient titanium alkylidyne, (PNP)Ti≡C(t)Bu (PNP = N[2-P(i)Pr(2)-4-methylphenyl](2)(-)), activates a C-H bond of ethane at room temperature, and a β-hydrogen of the resulting ethyl ligand is subsequently transferred to the adjacent alkylidene ligand to form an ethylene adduct of titanium. Treatment of the ethylene complex with two-electron oxidants such as organic azides results in extrusion of ethene concomitant with formation of a mononuclear titanium imido complex.

Time-dependent density functional theory gradients in the Amsterdam density functional package: geometry optimizations of spin-flip excitations

An implementation of time-dependent density functional theory (TDDFT) energy gradients into the Amsterdam density functional theory program package (ADF) is described. The special challenges presented by Slater-type orbitals in quantum chemical calculation are outlined with particular emphasis on details that are important for TDDFT gradients. Equations for the gradients of spin-flip TDDFT excitation energies are derived. Example calculations utilizing the new implementation are presented. The results of standard calculations agree well with previous results. It is shown that starting from a triplet reference, spin-flip TDDFT can successfully optimize the geometry of the four lowest singlet states of CH2 and three other isovalent species. Spin-flip TDDFT is used to calculate the potential energy curve of the breaking of the C–C bond of ethane. The curve obtained is superior to that from a restricted density functional theory calculation, while at the same time the problems with spin contamination exhibited by unrestricted density functional theory calculations are avoided.

Photochemistry and DNA-affinity of some stilbene and distyrylbenzene analogues containing pyridinium and imidazolium iodides

The relaxation properties of the excited states of some trans-1,2-diarylethene analogues (where one aryl group is a methylpyridinium or dimethylimidazolinium group and the other one is a π-excessive furyl or pyrrolyl group) and two all-trans-distyrylbenzene analogues (where the central ring is a methylpyridinium group and the side rings are furyl or methylpyrrolyl groups) have been investigated in buffered (pH 7) aqueous solutions. The compounds of the diarylethene series generally undergo efficient trans → cis photoisomerization while the yield of the radiative deactivation is very small at room temperature. The corresponding distyrylbenzenes display small yields of radiative/reactive pathways and mainly deactivate by internal conversion. The solvent effect on the spectral behaviour indicates the occurring of intramolecular charge transfer (“push–pull” compounds) which can induce interesting non-linear optical properties. Some experiments on the interactions with DNA, which might affect the cell metabolism, showed a modest binding affinity for the compounds with one ethenic bond and two aromatic rings. The complexation constant increases substantially in compounds with two ethenic bonds (three aromatic rings) and in the halogen-substituted compounds. The formation of ligand–DNA complexes affects only slightly the competition of the radiative/reactive relaxation from the lowest excited singlet state.

ChemInform Abstract: The Oxidative Dehydrogenation of Ethane Over Chlorine-Promoted Lithium- Magnesium Oxide Catalysts

Abstract The addition of chlorine to a Li + MgO catalyst improves considerably the ethylene yield that may be attained during the oxidative dehydrogenation (OXD) of ethane. While operating at 650°C, Li + MgO catalysts, produced via a sol-gel method or by adding HCl to an unsintered Li + MgO catalyst, promoted a C2H6 conversion of 75–79% at a C2H4 selectivity of 70% after 50 hr on stream. Even after 250 hr the ethylene yield was 45%. Chlorine was slowly lost from the catalysts as a result of reaction with H 2 O, but the evidence indicates that the improved activity and selectivity is largely a result of heterogeneous rather than homogeneous chlorine-promoted reactions. Over the modified catalysts the rate of C 2 H 6 conversion was increased relative to the rate of C 2 H 4 conversion. The presence of chlorine in the solid significantly decreased the amount of C0 2 , a poison for the OXD reaction, that was taken up by the catalyst. Thus, C0 2 formed during the OXD of C2H6 may be less effective in poisoning the active centers on a chlorine-modified catalyst. The presence of chlorine also may alter the reactive forms of oxygen on the surface (e.g., O − ions) so that they are capable of activating the weaker CH bond in ethane, but are less effective in activating the stronger CH bond in ethylene. With respect to the oxidative coupling of CH 4 , these results demonstrate that at 650°C the primary step, which includes the activation of CH 4 and reactions involving CH 3 · radicals, is responsible mainly for the CO x products, not the subsequent oxidation of CH 4 and C 2 H 6 . This would be the case even at relatively high partial pressures of C 2 H 4 and C 2 H 6 .

Theoretical study of model elementary reactions of dissociative addition of light hydrocarbons to the Ti-doped aluminide cluster Al12Ti

The potential energy surfaces (PES), energies E, and activation barriers h of elementary reactions of dissociative addition of CH4 and C2H6 molecules to the Al12Ti cluster with a marquee structure in the singlet and triplet states were calculated within the B3LYP approximation of the density functional theory using the 6-31G* basis set. The first stage of the reaction Al12Ti + CH4 leads to the adsorption complex CH4 · Al12Ti with the R(TiC) distance of ∼2.4 A. The methane molecule is coordinated as a tridentate ligand the singlet state and as a bidentate ligand in the triplet state, although both coordination modes are close in energy. In the transition state, the CH4 molecule is coordinated through its active C-H bond to an inclined Ti-Al edge of the cluster, and the C-H bond is significantly elongated and weakened. The activation barrier height h referenced to the CH4 complex is ∼9 and ∼19 kcal/mol for the singlet and triplet, respectively, and that referenced to the primary products Al12Ti(CH3)(H) is ∼21 kcal/mol. The barrier to migration of the CH3 group around the metal cluster is estimated at ∼10 kcal/mol. At the initial stage of the reaction Al12Ti + C2H6, two types of C2H6 · Al12Ti adsorption complexes are formed. In one of them, the ethane molecule is coordinated through a methyl group (as the methane molecule); and in the other type, the coordination is through the C-C bond. This reaction can proceed through two paths by means of insertion into C-H or C-C bonds to give Al12Ti(C2H5)(H) or Al12Ti(CH3)2, respectively. The second path is impeded by a high barrier (∼30 kcal/mol) and is possible, if at all, only at high temperatures. Conversely, the insertion into a C-H bond in ethane is somewhat more favorable than in methane. Analogously, the PES of addition of the second methane molecule to Al12Ti(CH3)(H) was calculated. The second molecule is adsorbed and dissociates by the same mechanism as the first CH4 molecule, but with somewhat lower barriers and energy effect of formation of Al12Ti(CH3)2(H)2. The addition of propane and longer hydrocarbons is briefly considered. The results are compared with the results of previous analogous calculations of the PES of related reactions of dissociative adsorption of dihydrogen on the Al12Ti cluster, which are more exothermic, have lower barriers, and can occur under milder conditions.

14β-Arylpropiolylamino-17-cyclopropylmethyl-7,8-dihydronormorphinones and Related Opioids. Further Examples of Pseudoirreversible μ Opioid Receptor Antagonists

14beta-4'-Chlorocinnamoylaminodihydronormorphinone (2a), and analogues, are selective pseudoirreversible antagonists of the mu opioid receptor (MOR). The preparation of analogues with ethynic bonds, replacing the ethenic bond of 2a, is described. The new ligands, in mouse antinociceptive assays, had pseudoirreversible MOR antagonist activity, which, in the case of 8b was of longer duration than that of 2a. The related codeinone (9b) had only antagonist activity in vivo, in contrast to 2a's codeinone equivalent 3a, which had potent antinociceptive activity.

Dissecting the Hindered Rotation of Ethane

The existence of a rotational barrier of ca. 3 kcalmol 1 around the C C single bond in ethane has been known since the early studies of Ebert, Wagner, Eucken and Weigert and Pitzer done in the 1930s. Even the simplest ab initio methods are able to reproduce this fundamental result. However, the physical origin of this hindered rotation is still under controversy in the literature. There are two effects that have been regarded as responsible for the rotational barrier : a larger steric repulsion in the eclipsed conformation and an enhanced stabilization of the staggered conformation due to hyperconjugation. Clearly, the two effects (and possibly some other ones) coexist in ethane, so in order to quantify the magnitude of one of them one must be able to switch off the other in the model calculation. The steric repulsion still remains the most popular explanation of the hindered rotation of ethane. This effect is often understood as the increase in energy that accompanies the antisymmetrization of a wave function originally formed by strictly localized descriptions of two methyl groups brought up to the final ethane geometry where they overlap. This so-called Pauli repulsion is considered to be more important for the eclipsed conformation, where the overlap between the occupied sC H molecular orbitals is larger, thus giving rise to a hindered rotation. The non-orthogonality between the molecular orbitals (MOs) of the two fragments seems to be the main source of controversy of such models. It has been argued that a zerothorder unperturbed system formed by two non-orthogonal sets cannot be put in correspondence with a Hermitian Hamiltonian, raising doubts about any physical argument obtained from a perturbative approach using such a model. (We cannot accept this point of view, which would exclude all the perturbation theories of intermolecular interactions as being illegitimate.) The hyperconjugation effect is due to interactions/delocalizations between electrons of vicinal C H bonds mainly through the C C bond. In an MO picture, this corresponds to favorable two-electron two-orbital interactions between the occupied sC H orbital of one methyl group and the virtual antibonding s*C H orbital of the other. This also involves orbital s*C C. [21] The electron delocalization effect can easily be assessed in valence bond (VB) theory calculations by adding/ removing the appropriate resonance structures from the wave function. In an MO calculation it is not that simple due to the delocalized nature of the MOs. The hyperconjugation effect is estimated by constructing the wave function from sets of MOs localized in methyl moieties. The way such MOs are constructed, in particular whether they are variationally optimized, apparently led to opposite conclusions about the role of the hyperconjugation in the rotational barrier. Another factor to be taken into account is the geometry relaxation that accompanies the internal rotation, and in particular the change in the C C distance. It is somewhat striking that the C C bond is significantly shorter in the staggered conformation than in the eclipsed one although this bond is formally not changing during the rotation. This effect already appears at the simplest minimal basis SCF level of theory: at the STO6G level one gets C C bond lengths equal to 1.535 and 1.545 , respectively. At the same time the effect of geometry relaxation on the height of the barrier is minor. Nonetheless, previous studies concluded that different answers can be obtained for the relative importance of the steric repulsion and the hyperconjugation effect depending on whether the geometry relaxation has been considered or not. Previous energy decomposition analyses relied, in one way or another, on the definition of two methyl fragments. However, in the last years there have been a growing interest in other kinds of energy partitioning schemes, closely related to population analysis and bond-order techniques. Such schemes allow expressing the total energy of a system, exactly or up to a good accuracy, as a sum of atomic and diatomic contributions, as given by Equation (1):