Assessment of density functional approximations for N2 and CO2 physisorption on benzene and graphene.

Experimental isotherms of N2 and CO2 on carbon-based porous materials and models of the physisorption of gases on surfaces are used to obtain the pore size distribution (PSD). An accurate modelization of the physisorption of N2 and CO2 on the surface of carbon-based porous materials is important to obtain accurate N2 and CO2 storage capacities and reliable PSDs. Physisorption depends on the dispersion interactions. High precision ab initio methods, such as CCSD(T), consider accurately the dispersion interactions, but they are computationally expensive. Double hybrid, hybrid and DFT-based methods are much less expensive. In the case of graphene, there are experimental data of the adsorption of N2 and CO2 on graphite that can be used to build the Steele interaction potential of these gases on graphene. The goal is to find out hybrid and/or DFT methods that are as accurate as the CCSD(T) on benzene and as accurate as the experimental results on graphene. Calculations of the interaction energy curves of N2 and CO2 on benzene and graphene have been carried out using the CCSD(T) method and several double hybrid, hybrid, and DFT methods that consider the dispersion interactions. The energy curves on benzene have been compared to the CCSD(T) and the energy curves on graphene have been compared with the Steele energy curves. The comparisons indicate that double hybrids with dispersion corrections and ωB97 based DFT methods are accurate enough for benzene. For graphene, only the PBE-XDM functional has a good agreement with the Steele energy curves.

Structural Trends in Monoboronyl Compounds: Analysis of the Interaction of Second-Row Elements with BO.

A theoretical study of the monoboronyl compounds of second-row elements, [XBO] (X = Na, Si, P, S, Cl), has been carried out. It is observed that the preference for the XBO arrangement is higher when moving to the right of the period. In the case of sodium monoboronyl three minima were characterized, all lying rather close in energy: linear NaBO, linear NaOB, and an L-shaped structure. Linear NaBO and the L-shaped structure are nearly isoenergetic, whereas linear NaOB is located 2.11 kcal/mol above linear NaBO. The barrier for the conversion of the L-shaped structure into linear NaBO is about 5.1 kcal/mol, suggesting that both species could be potential targets for experimental detection. For silicon monoboronyl, two minima, linear SiBO and linear SiOB, are found, the latter lying about 13 kcal/mol above SiBO. The barrier for the isomerization of SiOB into SiBO is estimated to be 11.4 kcal/mol. For phosphorus, sulfur, and chlorine monoboronyls the linear XBO isomer is clearly the most stable one, and the barriers for the conversion into XOB species are relatively high, suggesting that quite likely the linear XBO isomer should be the main experimental target. All studied monoboronyls are relatively stable, with dissociation energies increasing from left to right of the second-row (69.8 kcal/mol for NaBO and 118.98 kcal/mol for ClBO). An analysis of the bonding for second-row monoboronyls has been carried out, emphasizing the different characteristics of the X-B and X-O bonds along the second row.

Molecular Structure and Bonding in Plutonium Carbides: A Theoretical Study of PuC3.

The most relevant species of plutonium tricarbide were characterized using theoretical methods. The global minimum is predicted to be a fan structure where the plutonium atom is bonded to a quasi-linear C3 unit. A rhombic isomer, shown to be a bicyclic species with transannular C-C bonding, lies about 39 kJ/mol above the fan isomer. A linear PuCCC isomer and a three-membered ring CPuC2 isomer were found to be higher in energy (150 and 195 kJ/mol, respectively, above the predicted global minimum). The possible processes for the formation of these species are discussed, and the IR spectra were predicted to help in possible experimental detection. The nature of the Pu-C interaction has been analyzed in terms of a topological analysis of the electronic density, showing that Pu-C bonding is essentially ionic with a certain degree of covalent character.

Enhanced association for C70 over C60 with a metal complex with corannulene derivate ligands.

The geometry imposed by the coordination sphere around the metal, together with the choice of the "arms" can be advantageously used to build corannulene-based molecular tweezers, which show great affinities for C60 and C70, as revealed by NMR titration experiments, mass spectroscopy, DFT calculations and the single crystal X-ray structural analysis of the compound C60 ⊂1.

Molecular structure of uranium carbides: isomers of UC3.

In this article, the most relevant isomers of uranium tricarbide are studied through quantum chemical methods. It is found that the most stable isomer has a fan geometry in which the uranium atom is bonded to a quasilinear C3 unit. Both, a rhombic and a ring CU(C2) structures are found about 104-125 kJ/mol higher in energy. Other possible isomers including linear geometries are located even higher. For each structure, we provide predictions for those molecular properties (vibrational frequencies, IR intensities, dipole moments) that could eventually help in their experimental detection. We also discuss the possible routes for the formation of the different UC3 isomers as well as the bonding situation by means of a topological analysis of the electron density.

Small carbides of third-row main group elements: structure and bonding in C3X compounds (X = K-Br).

The molecular structures of third-row main group tricarbides C(3)X (X = K-Br) have been studied by quantum chemical methods. It is found that less electronegative elements (K, Ca, Ga, Ge) favor either fan or rhombic structures (resulting from side interactions with either linear or triangular C(3) units), whereas the more electronegative elements (As, Se, Br) favor linear or three-membered ring structures (resulting from σ-type interactions with either linear or triangular C(3) units). The predicted global minima are of fan type for C(3)K, rhombic for C(3)Ca, C(3)Ga, and C(3)Ge, linear for C(3)As and C(3)Se, and a three-membered ring for C(3)Br. In order to aid in their possible experimental identification the molecular geometries, vibrational frequencies, IR intensities, and dipole moments have been provided. The nature of the interactions has been characterized through an analysis of the electronic charge density. In addition, the relative stability of the different isomers has been also rationalized in terms of an energy decomposition analysis.

On the molecular structure of uranium dicarbide: T-shape versus linear isomers.

A theoretical study of the molecular structure of uranium dicarbide has been carried out employing DFT, coupled cluster, and multiconfigurational methods. A triangular species, corresponding to a (5)A(2) electronic state, has been found to be the most stable UC(2) species. A triplet linear CUC species, which has been observed in recent infrared spectroscopy experiments, lies much higher in energy. A topological analysis of the electronic density has also been carried out. The triangular species is shown to be in fact a T-shape structure with a U-C interaction which can be considered to be a closed-shell interaction.

Computational study of the reaction of P+ with acetylene: does spin-crossing play a significant role?

A computational study of the reaction of P(+)((3)P) with acetylene has been carried out. The only exothermic products correlating with the reactants are PCCH(+)((2)Π) + H((2)S). Two different pathways leading to these products that are apparently barrier-free have been found. Both pathways involve isomerization into open-chain intermediates followed by direct elimination of a hydrogen atom. The possibility of spin-crossing has been considered because the species on the singlet surface are considerably more stable than those on the triplet one. On the singlet surface, there are other possible channels for the reaction, namely, cyclic PC(2)H(+)((2)A') + H((2)S) and CCP(+)((1)Σ) + H(2) ((1)Σ(g)(+)). A computational kinetic study shows that, in agreement with the experimental evidence, the major products are PCCH(+)((2)Π) + H((2)S) at all temperatures. Only at very high temperatures is CCP(+)((1)Σ) + H(2) ((1)Σ(g)(+)) formed in non-negligible amounts. Therefore, only PCCH(+) should be formed in the interstellar medium.

Ab initio benchmark calculations on Ca(II) complexes and assessment of density functional theory methodologies.

A set of benchmark results for the geometries, binding energies, and protonation affinities of 24 complexes of small organic ligands with Ca(II) is provided. The chosen level of theory is CCSD(T)/CBS obtained by means of a composite procedure. The performance of four density functionals, namely, PW91, PBE, B3LYP, and TPSS and several Pople-type basis sets, namely, 6-31G(d), 6-31+G(d), 6-31+G(2d,p) and 6-311+G(d) have been assessed. Additionally, the nature of the metal ligand bonding has been analyzed by means of the Symmetry Adapted Perturbation Theory (SAPT). We have found that the B3LYP hybrid functional, in conjunction with either the polarized double-ζ 6-31+G(2d,p) or the triple-ζ 6-311+G(d) basis sets, yields the closest results compared to the benchmark data. The SAPT analysis stresses the importance of induction effects in the binding of these complexes and suggests that consideration of classical electrostatic contributions alone may not be reliable enough for the prediction of relative binding energies for Ca(II) complexes.

Structure and bonding in third-row main group dicarbides C2X (X=K-Br).

The molecular structures of third-row main group dicarbides C(2)X (X=K-Br) have been studied by theoretical methods. It is found that K, Ca, and Ga favor C(2v)-symmetric (T-shape) ground states, whereas As, Se, and Br have linear or quasilinear ground states. In the case of germanium. a very flat potential energy surface is found and an L-shape structure seems to be the ground state. Dissociation energies into X+C(2) are relatively high. The main features of these compounds, in particular, the preference for linear or cyclic structures, have been rationalized in terms of the most relevant interactions between the third-row atom and dicarbon with the help of an energy decomposition analysis.

Cyanide complexes of Ti(IV): a computational study.

Density functional theory (B3LYP) and coupled-cluster techniques [CCSD(T)] including solvent effects have been used to study the homoleptic and mixed cyanide/isocyanide complexes of Ti(IV), [Ti(CN)(n)](4-n) (n=1-6). The most stable isomer is found to be the isocyanide form except for n=6 where the cyanide isomer is preferred. Calculations accounting for solvent effects show that, irrespective of the solvent employed, the hexacyanocomplex should be formed. We have additionally analyzed the bonding situation in these complexes in order to shed some light on the reasons for the predicted cyano-/isocyano preference. We have found that the more advantageous sigma-bonding capabilities of the cyanide form become increasingly important for larger n eventually favoring the cyanoisomer for n=6. We finally compare the bonding situation in hexacyanotitanate(IV) with that of hexacyanoferrate(II).

Polyisocyanides of titanium.

Neutral Ti[CN](n) complexes have been investigated with quantum chemistry techniques. According to our theoretical predictions, these complexes are shown to prefer isocyanide arrangements. Therefore, these compounds are good candidates to be the first polyisocyanides to be characterized. The theoretical calculations predict Ti(NC)(4), a methane-like tetrahedral structure with four isocyanide ligands, as the most stable neutral complex. The fact that the isocyanide ligand is a better pi-donor than the cyanide one seems to be the key factor for the preference for isocyanides in neutral titanium complexes.

Gas-Phase Reaction of NH2(+) with Acetic Acid: Implications in Astrochemistry.

A theoretical study of the ion-molecule reaction, NH2(+) with acetic acid that could lead to precursors of glycine in the interstellar medium, has been carried out on the triplet and singlet potential energy surfaces. All stationary points and transition states on the (NO2C2H6)(+) triplet and singlet surfaces have been determined at the MP2(full) level with the cc-pVTZ basis set. Energetic data have been obtained at the CCSD(T) level employing the aug-cc-pVTZ basis set. The global minimum of the (NO2C2H6)(+) system is predicted to be protonated glycine in its singlet state, (1)A', and in general singlet states are more stable than the corresponding triplet ones. Formation of ionized glycine from this reaction is shown to be a feasible process under interstellar conditions, but the proton transfer channel and the formation of the compound CH2COOH(+) seem to be more favorable processes on the triplet and singlet potential surfaces, respectively.

Monoligand Zn(II) Complexes: Ab Initio Benchmark Calculations and Comparison with Density Functional Theory Methodologies.

A systematic theoretical study on several models of Zn(II) complexes has been carried out employing both ab initio correlated wave function and density functional methods. The performance of five different functionals namely PW91, PBE, B3LYP, MPWLYP1M, and TPSS in the prediction of metal-ligand bond distances, binding energies, and proton affinities has been assessed comparing the results to those obtained with the MP2 and CCSD(T) wave function methodologies. Several basis sets ranging from double-ζ up to quintuple-ζ quality have been used, including the recently developed all-electron correlation consistent basis sets for zinc. It is shown that all the tested functionals overestimate both the metal-ligand bond distances and the binding energies, being that the B3LYP and TPSS functionals are the ones that perform the best. An analysis of the metal-ligand interaction energy shows that induction and charge-transfer effects play a prominent role in the bonding of these systems, even for those complexes with the less polarizable ligands. This finding highlights the importance of a correct description of the polarization of the monomers' charge densities by any theoretical method which aims to be applied to the study of Zn(II) complexes.

Structure and bonding in first-row transition metal dicarbide cations MC2+.

A theoretical study of the first-row transition metal dicarbide cations MC2+ (M=Sc-Zn) has been carried out. Predictions for different molecular properties that could help in their eventual experimental detection have been made. Most MC2+ compounds prefer a C2v symmetric arrangement over the linear geometry. In particular, the C2v isomer is specially favored for early transition metals. Only for CuC2+ is the linear isomer predicted to be the global minimum, although by only 1 kcal/mol. In all cases the isomerization barrier between cyclic and linear species seems to be very small (below 2 kcal/mol). The topological analysis of the electronic density shows that most C2v isomers are T-shaped structures. In general, MC2+ compounds for early transition metals have larger dissociation energies than those formed by late transition metals. In most cases the dissociation energies for MC2+ compounds are much smaller than those obtained for their neutral analogues. An analysis of the bonding in MC2+ compounds in terms of the interactions between the valence orbitals of the fragments helps to interpret their main features.

Cyanides and isocyanides of first-row transition metals: molecular structure, bonding, and isomerization barriers.

Cyanides and isocyanides of first-row transition metal M(CN) (M=Sc-Zn) are investigated with quantum chemistry techniques, providing predictions for their molecular properties. A careful analysis of the competition between cyanide and isocyanide isomers along the transition series has been carried out. In agreement with the experimental observations, late transition metals (Co-Zn) clearly prefer a cyanide arrangement. On the other hand, early transition metals (Sc-Fe), with the only exception of the Cr(CN) system, favor the isocyanide isomer. The theoretical calculations predict the following unknown isocyanides, ScNC(3Delta), TiNC(4Phi), VNC(5Delta), and MnNC(7Sigma+), and agree with the experimental observation of FeNC(6Delta) and the CrCN(6Sigma+) cyanide. First-row transition metal cyanides and isocyanides are predicted to have relatively large dissociation energies with values within the range 80-101 kcal mol(-1), except Zn(CN), which has a dissociation energy around 50-55 kcal mol(-1), and low isomerization barriers. A detailed analysis of the bonding has been carried out employing the topological analysis of the charge density and an energy decomposition analysis. The role of the covalent and electrostatic contributions to the metal-ligand bonding, as well as the importance of pi bonding, are discussed.

Structure and bonding in first-row transition-metal dicarbides: are they related to the stability of met-cars?

First-row transition-metal dicarbides MC(2) (M=Sc-Zn) have been investigated by using quantum-mechanical techniques. The competition between cyclic and linear isomers in these systems has been studied and the bonding scheme for these compounds is discussed through topological analysis of electron density. All of the systems have been found to prefer a C(2v)-symmetric arrangement, although for ZnC(2) the energy difference between this and the linear isomer is rather small. In most cases the C(2v)-symmetric structure corresponds to a T-shaped structure, with the exceptions of TiC(2), CoC(2), and NiC(2) which have been shown to be true rings. A detailed analysis of the variation of the energy of the system with geometry has been carried out. An analysis of the bonding, taking into account the main interactions between the valence orbitals of both fragments, the M atom and the C(2) molecule, has allowed the main features of these compounds to be interpreted. A clear correlation between the dissociation energies of the first-row transition-metal dicarbides and the bonding energies of the corresponding met-cars was observed.

Pseudorotation motion in tetrahydrofuran: an ab initio study.

The use of different models based on experimental information about the observed level splitings, rotational constants, and far-infrared transition frequencies leads to different predictions on the equilibrium geometry for tetrahydrofuran. High-level ab initio calculations [coupled cluster singles, doubles (triples)/complete basis set (second order Moller-Plesset triple, quadrupole, quintuple)+zero-point energy(anharmonic)] suggest that the equilibrium conformation of tetrahydrofuran is an envelope C(s) structure. The theoretical geometrical parameters might be helpful to plan further microwave spectroscopic studies in order to get a physical interpretation of the measurements.

A computational study of the reaction of ground-state nitrogen atoms with chloromethyl radicals.

A computational study of the N(4S) + CH2Cl reaction has been carried out. The first step of the reaction is the formation of an initial intermediate (NCH2Cl), which is relatively stable and does not involve any energy barrier. The two most exothermic products are those resulting from the release of a chlorine atom, H2C=N + Cl and trans-HC=NH + Cl. A kinetic study within the framework of the statistical theories suggests that the kinetically preferred product is also the most exothermic one. This is in contrast with the analogue reaction of nitrogen atoms with CH2F, where the preferred product from both thermodynamic and kinetic points of view is HFCN + H. Therefore, reactions of nitrogen atoms with chloromethyl radicals release chlorine atoms as major products. The rate coefficient for the title reaction is estimated to be about 3.09 x 10(-13) cm3 s(-1) molecule(-1) at 300 K, a value four times smaller than the rate coefficient for its fluorine analogue.

Computational study of the reaction of N(2D) atoms with CH2F radicals: an example of a barrier-free reaction involving very high internal energies.

The singlet potential-energy surface for the N(2D)+CH2F(2A') reaction has been studied employing both second-order Møller-Plesset and density-functional theories. The energies of the involved species have been refined using the Gaussian-2, complete basis set, and coupled-cluster singles and doubles (triples) methods. The reaction proceeds through the formation of an initial intermediate, which does not involve any activation barrier. Based on the energy profile for the singlet potential-energy surface, the preferred product should be the most exothermic one, namely, HCN+HF, followed by HNC+HF and FCN+H2. This result seems in contradiction with a computational study of the kinetics of the title reaction in terms of the statistical theories, which leads to the prediction that the production of HNC+HF should be the dominant channel. Consequently, a limited molecular-dynamics study has been carried out, concluding that in fact the system behaves in a nonstatistical way. According to the molecular-dynamics study, the most exothermic channel, HCN+HF, should be the dominant one. An analysis of the possible role of the singlet surface in the reaction of N(4S) with CH2F(2A') has also been carried out. The computational study shows that the microcanonical coefficients for the nonadiabatic channels are much smaller than the competing adiabatic ones. Therefore, the reaction of N(4S) with CH2F(2A') should proceed on the triplet surface without spin change.