Characterization of the potential energy surfaces of two small but challenging noncovalent dimers: (P2)2 and (PCCP)2

This work characterizes eight stationary points of the P₂ dimer and six stationary points of the PCCP dimer, including a newly identified minimum on both potential energy surfaces. Full geometry optimizations and corresponding harmonic vibrational frequencies were computed with the second‐order Møller–Plesset (MP2) electronic structure method and six different basis sets: aug‐cc‐pVXZ, aug‐cc‐pV(X+d)Z, and aug‐cc‐pCVXZ where X = T, Q. A new L‐shaped structure with C₂ symmetry is the only minimum for the P₂ dimer at the MP2 level of theory with these basis sets. The previously reported parallel‐slipped structure with C_2h symmetry and a newly identified cross configuration with D₂ symmetry are the only minima for the PCCP dimer. Single point energies were also computed using the canonical MP2 and CCSD(T) methods as well as the explicitly correlated MP2‐F12 and CCSD(T)‐F12 methods and the aug‐cc‐pVXZ (X = D, T, Q, 5) basis sets. The energetics obtained with the explicitly correlated methods were very similar to the canonical results for the larger basis sets. Extrapolations were performed to estimate the complete basis set (CBS) limit MP2 and CCSD(T) binding energies. MP2 and MP2‐F12 significantly overbind the P₂ and PCCP dimers relative to the CCSD(T) and CCSD(T)‐F12 binding energies by as much as 1.5 kcal mol^?1 for the former and 5.0 kcal mol^?1 for the latter at the CBS limit. The dominant attractive component of the interaction energy for each dimer configuration was dispersion according to several symmetry‐adapted perturbation theory analyses. © 2014 Wiley Periodicals, Inc.     

A Theoretical Study of the Kinetics of the Hydrogen Atom Abstraction Reactions from Cyclopropane by H,O (3P), and Cl (2P3/2) Atoms and OH Radicals

The rate constants of the H‐abstraction reactions from cyclopropane by H, O (³P), Cl (²P_3/2), and OH radicals have been calculated over the temperature range of 250?2500 K using two different levels of theory. Calculations of optimized geometrical parameters and vibrational frequencies are performed using the MP2 method combined with the cc‐pVTZ basis set and the 6–311++G(d,p) basis set. Single‐point energy calculations have been carried out with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (perturbatively) electron excitations CCSD(T) using either the cc‐pVTZ, aug‐cc‐pVTZ, and aug‐cc‐pVQZ basis sets or the 6–311++G(3df,3pd) basis set. The CCSD(T) calculated potential energies have been extrapolated to the complete basis limit (CBS) limit. The Full Configuration Interaction (FCI) energies have been also estimated using the continued‐fraction approximation as proposed by Goodson (J. Chem. Phys., 2002, 116, 6948–6956). Canonical transition‐state theory combined with an Eckart tunneling correction has been used to predict the rate constants as a function of temperature using two kinetic models (direct abstraction or complex mechanism) at two levels of theory (CCSD(T)‐cf/CBS//MP2/cc‐pVTZ and CCSD(T)‐cf/6–311++G(3df,3pd)//MP2/6–311++G(d,p)). The calculated kinetic parameters are in reasonable agreement with their literature counterparts for all reactions. In the light of these trends, the use of the Pople‐style basis sets for studying the reactivity of other systems such as larger cycloalkanes or halogenated cycloalkanes is recommended because the 6–311++G(3df,3pd) basis set is less time consuming than the aug‐cc‐pVQZ basis set. Based on our calculations performed at the CCSD(T)‐cf/CBS//MP2/cc‐pVTZ level of theory, the standard enthalpy of formation at 298 K for the cyclopropyl radical has been reassessed and its value is (290.5 ± 1.6) kJ mol^?1.     

A New Glimpse into the CO2‐Philicity of Carbonyl Compounds

We report a theoretical study on non‐conventional structures of 1:1 complexes between carbon dioxide and carbonyl compounds. These structures have never been reported before but are relevant for understanding the solubility of carbonyl compounds in supercritical CO₂. The work is based on the results of ab initio calculations at the MP2 and CCSD(T) levels using aug‐cc‐pVDZ and aug‐cc‐pVTZ basis sets. Investigated systems include aldehydes, ketones and esters, together with some fluorinated derivatives. The results are interpreted in terms of natural bond orbital analyses. Harmonic vibrational frequency calculations have also been done in order to compare them with available experimental data. We show for the first time that complexes where CO₂ behaves globally as a Lewis base are stable in the case of ketones and esters, but not in the case of aldehydes, and their stability is similar to that of traditional complexes in which CO₂ behaves as a Lewis acid. This finding considerably modifies the concept of CO₂‐philicity and may have important ramifications in the development of green reactions in supercritical CO₂.     

Influence of hydrogen bonds between sidechains and cooperativity on self‐assembly of the cyclopeptides

The self‐assembly of four cyclic D,L‐octapeptides, [‐(D‐Ala‐Gln)₄‐], [‐(D‐Val‐Gln)₄‐], [‐(D‐Leu‐Gln)₄‐], and [‐(D‐Phe‐Gln)₄‐], was investigated on the theory level in detail. Based on these cyclic peptides, which contain L‐Gln residues and possess C₄ symmetry, a series of oligomers were constructed according to different stacking modes as well as interaction patterns. We employed the semiempirical molecular orbital method AM1 to optimize the structures of all the oligomers, some of which were further studied using density functional method B3PW91/6‐31G to calculate the interaction energies. The studies indicate that when these cyclopeptides aggregate to form oligomers, or even nanotubes, four more hydrogen bonds could form between the sidechains of L‐Gln residues in addition to eight hydrogen bonds formed between the backbones of adjacent two cyclic peptides, a result that would clearly affect the self‐assembling process of cyclic peptides. The main effects can be summarized as follows. First, the dimers of these cyclic peptides with C₄ symmetry are more stable than those with D₄ symmetry due to their additional H‐bonds between Gln sidechains. Second, for the self‐assembly of the cyclopeptides, there is a competition between parallel and antiparallel stacking modes in lower oligomers such as dimers. However, with an increasing degree of oligomerization, energetically there is an increased possibility for the cyclic peptides to take the parallel stacking mode in assembly. Finally, the synergetic effect of weak interactions is the fundamental driving force for cyclic peptides to form stable nanotubes. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Ab initio study of the structure,cooperativity, and vibrational properties in the mixed hydrogen‐bonded trimers of hydrogen isocyanide and water

The five trimers of H₂O···HNC···H₂O, H₂O···H₂O···HNC, HNC···H₂O···H₂O, H₂O···HNC···HNC, and HNC···HNC···H₂O have been studied with quantum chemical calculations. Their structures, harmonic vibrational frequencies and interaction energies have been calculated at the B3LYP and MP2 levels with the aug‐cc‐pVDZ and aug‐cc‐pVTZ basis sets. The cooperative effect on these properties has also been studied quantitatively. For HNC:(H₂O)₂ systems, the cyclic H₂O···H₂O···HNC trimer is most stable with an interaction energy of ?16.01 kcal/mol and a large cooperative energy of ?3.25 kcal/mol at the MP2/aug‐cc‐pVTZ level. For H₂O:(HNC)₂ systems, the interaction energy and cooperative energy in the H₂O···HNC···HNC trimer are larger than those in the HNC···HNC···H₂O trimer. The NH stretch frequency has a blue shift for the terminal HNC molecule in the HNC···H₂O···H₂O and HNC···HNC···H₂O trimers and a red shift in other cases. A many‐body analysis has also been performed to understand the interaction energies in these hydrogen‐bonded clusters. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

A theoretical analysis of substituted aromatic compounds

The substituents ? CH₃, ? F, ? NO₂, ? OCH₃, and ? CH₂?CH₂ were placed at the ortho, meta, and para positions on the aromatic molecules aniline, benzaldehdye, nitrobenzene, and phenol. MMFF94, AM1, B3LYP, M06, M06‐2X, ωB97X, ωB97X‐d, and RI‐MP2 using cc‐pVDZ and cc‐pVTZ and CCSD(T) with cc‐pVDZ basis sets were used to calculate the geometries and energies of all regiomers of the molecules. Relative energies of the ortho and meta regiomers relative to the para regiomers were calculated and compared to the CCSD(T) values. A good basis set correlation between cc‐pVDZ and cc‐pVTZ was observed in RI‐MP2. Overall, RI‐MP2 gave the best correlation with the CCSD(T) results. All of the hybrid functionals showed similar accuracy and could effectively describe the intramolecular hydrogen‐bonding interactions of these compounds. The methoxy group at the para position in methoxyaniline, methoxyphenol, methoxynitrobenzene, and methoxybenzaldehyde was rotated around the phenyl‐O bond. HF, along with the cc‐pVDZ basis with the other methods, generated inaccurate energy profiles for p‐methoxyphenol. For the density functional theory methods, it was necessary to use improved grids to get smooth curves. © 2013 Wiley Periodicals, Inc.     

Density functional theory study of formaldehyde oligomers

Hydrogen‐bonded formaldehyde oligomers (dimer to pentamer) are studied using density functional theory (DFT), the B3LYP method, and the 6‐311+G* basis set. Many‐body interaction energies are obtained to study the contribution of many‐body terms to binding energy. The basis set superposition error (BSSE)‐corrected total energies are ?229.08170, ?343.61410, ?458.16660, and ?572.70901 hartrees for dimer, trimer, tetramer, and pentamer, respectively, with corresponding binding energies ?2.55, ?4.86, ?6.99, and ?9.49 kcal/mol. Two‐body energies have been found to contribute significantly to the total binding energy in dimer to pentamer, whereas higher‐order interaction energies are negligible. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Highly accurate relativistic universal Gaussian basis set for Dirac–Fock–Breit calculations

Ion mobilities of H₂O⁺ drifting in helium are calculated and compared with experiment. These calculations employ global potential energy surfaces of the H₂O⁺–He complex, which in the present case were calculated ab initio at the unrestricted MP2 level of theory using a basis set of aug‐cc‐pVTZ quality, and treating the ion as a rigid body. Details are presented of the general characteristics of both the ground and first‐excited electronic states of the complex. Although only the ground‐state surface was used for the mobility calculations, the ab initio determination of the ground state necessitated the inclusion of the first‐excited state owing to the presence of a crossing between the two. This crossing is also described. Mobilities calculated from the global surfaces are in good agreement with experiment. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Ab initio calculations of the Ar–ethane intermolecular potential energy surface using bond function basis sets

The intermolecular potential energy surface (PES) of argon with ethane has been studied by ab initio calculations at the levels of second‐order Møller–Plesset perturbation (MP2) theory and coupled‐cluster theory with single, double, and noniterative triple configurations (CCSD(T)) using a series of augmented correlation‐consistent basis sets. Two sets of bond functions, bf1 (3s3p2d) and bf2 (6s6p4d2f), have been added to the basis sets to show a dramatic and systematic improvement in the convergence of the entire PES. The PES of Ar–ethane is characterized by a global minimum at a near T‐shaped configuration with a well depth of 0.611 kcal mol^?1, a second minimum at a collinear configuration with a well depth of 0.456 kcal mol^?1, and a saddle point connecting the two minima. It is shown that an augmented correlation‐consistent basis set with a set of bond functions, either bf1 or bf2, can effectively produce results equivalent to the next larger augmented correlation‐consistent basis set, that is, aug‐cc‐pVDZ‐bf1 ≈ aug‐cc‐pVTZ, aug‐cc‐pVTZ‐bf1 ≈ aug‐cc‐pVQZ. Very importantly, the use of bond functions improves the PES globally, resulting accurate potential anisotropy. Finally, MP2 method is inadequate for accurate calculations, because it gives a potentially overestimated well depth and, more seriously, a poor potential anisotropy. © 2012 Wiley Periodicals, Inc.     

Ab initio MO calculations on the stable conformations and their binding energies of the ion-molecule complexes: Ion = H+, Li+, Na+, K+ Be2+, and molecule = CO and N2

The most stable conformation of ion-molecule complexes involving a CO molecule were surveyed by the use of Hartree-Fock (HF) MO and third-order Moller-Plesset perturbation (MP3) methods with a 6–31G* basis set ion = H⁺, Li⁺, Na⁺, K⁺, Bc²⁺, Mg²⁺, and Ca²⁺. The MP3 level of theory reveals the ion-CO conformation in which the ion bonds to a carbon atom of CO to be the most stable; these MP3 results are contrary to the HF ones. Binding energies of ion-molecule complexes involving CO and N₂ were computed; MP3 energies are in good agreement with the experimental ones. The computed binding energies of cation-N₂ are about one-third of cation-NH₃ due to the absence of dipole moment and the smaller polarizability of N₂. The decrease in binding energy in cation-CO and -N₂ complexes, with increasing cation size, is mainly caused by the decrease of the electrostatic and polarization stabilizations.     

Theoretical study of the relative stabilities of H+(X)2 and H+(X)3 conformers and their clustering energies: X  CO and N2

Third-order Møller–Plesset perturbation theory (MP 3) with a 6-31G** basis set was applied to study the relative stabilities of H⁺(X)₂ conformations (X ? CO and N₂) and their clustering energies. The effect of both basis set extensions and electron correlation is not negligible on the relative stabilities of the H⁺(CO)₂ clusters. The most stable conformation of H⁺(CO)₂ is found to be a C_∞v structure in which a carbon atom of CO bonds to the proton of H⁺(CO), whereas that of H⁺(N₂)₂ is a symmetry D_∞h structure. The second lowest energy conformations of H⁺(CO)₂ and H⁺(N₂)₂ lie within 2 kcal/mol above the energies of the most stable structures. Clustering energies computed using MP 3 method with the 6-31G** basis set are in good agreement with the experimental findings of Hiraoka, Saluja, and Kebarle. The low-lying singlet conformations of H⁺(X)₃ (X ? CO and N₂) have been studied by the use of the Hartree–Fock MO method with the 6-31G** basis set and second-order Møller–Plesset perturbation theory with a 4-31G basis set. The most stable structure is a T-shaped structure in which a carbon atom of CO (or a nitrogen atom of N₂) attacks the proton of the most stable conformation of H⁺(X)₂ clusters.     

Many‐body energy decomposition analysis of cooperativity in hydrogen fluoride clusters

This article studies the cooperativity present in hydrogen fluoride clusters, (FH)_n, by means of a many‐body decomposition of the binding energy. With the aim of quantifying how the results depend on the calculation level, the partition was performed from dimer to hexamer at the RHF, MP2, and density functional (B3LYP) levels, and for the heptamer and octamer at the RHF and B3LYP levels, using a 6‐31++G(d, p) basis set in all cases. We obtain that, for a proper representation of the cooperative effects in hydrogen fluoride, at least the inclusion of the three‐body terms is fundamental. The contributions are found to be underestimated at the RHF level and overestimated at the B3LYP level, with respect to the MP2 results. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Revealing the physical nature and the strength of charge‐inverted hydrogen bonds by SAPT(DFT), MP2, SCS‐MP2, MP2C,and CCSD(T) methods

The physical nature of charge‐inverted hydrogen bonds in H₃XH YH₃ (X = Si, Ge; Y = Al, Ga) dimer systems is studied by means of the SAPT(DFT)‐based decomposition of interaction energies and supermolecular interaction energies based on MP2, SCS‐MP2, MP2C, and CCSD(T) methods utilizing dimer‐centered aug‐cc‐pCVnZ (n = D, T, Q) basis sets as well as an extrapolation to the complete basis set limit. It is revealed that charge‐inverted hydrogen bonds are inductive in nature, although dispersion is also important. Computed interaction energies form the following relation: . It is confirmed that the aug‐cc‐pCVDZ basis set performs poorly and that very accurate values of interaction and dispersion energies require basis sets of at least quadrupole‐ζ quality. Considerably large binding energies suggest potential usefulness of charge‐inverted hydrogen bonds as an important structural motif in molecular binding. Terminology applying to σ‐ and π‐hole interactions as well as to triel and tetrel bonds is discussed. According to this new terminology the charge‐inverted hydrogen bond would become the first described case of a hydride‐triel bond. © 2017 Wiley Periodicals, Inc.     

Theoretical study on the mechanisms of the nucleophilic substitution reactions of hydrosulfide ion and halomethanes

Three species involved in the nucleophilic substitution reaction of hydrosulfide ion and halomethanes are investigated by ab initio calculations. Geometries for stationary structures along the reaction paths are fully performed with the second‐order Møller–Plesset perturbation approximation with the cc‐pVDZ basis set. The monomer geometries determined by the MP2 method match the experimental results very well. Single point energy calculations are carried out at the coupled cluster with perturbative triple excitations CCSD (T) theory with aug‐cc‐pVDZ basis set. Halomethanes have three conformers here, which lead to the three product channels, HSCH₃ + F^?1, HSCH₃ + Cl^?1, and HSCH₃ + Br^?1. The investigation encompasses the six complexes formed among three channels, respectively. By selecting the six complexes as the model, we investigate the binding energy, topological property of the electron charge density and their Laplacian in detail theoretically. Electrostatic density potential maps of halomethanes are generated for the determination of attractive interaction sites. It is proved that the similar misshaped electron clouds of the three halogen atoms result in the similar properties of the carbon‐halogen bonds, and reveals that the product ion‐dipole complexes interactions are predominantly electrostatic in nature. The calculated results predict the binding energy of the most stable complex in six complexes is ?47.06 kcal/mol at the MP2 level of theory. The second channel has the lowest energy barrier, which is ?3.63 kcal/mol at the CCSD (T) levels of theory, is expected to be the most important pathway. It occurs via C? Cl cleavage accompanied by C? S bond formation. The other two channels have higher energy barriers and are expected to have smaller rates. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Ab Initio and Quantum Chemical Topology studies on the isomerization of HONO to HNO2. Effect of the basis set in QCT

The article focus on the isomerization of nitrous acid HONO to hydrogen nitryl HNO₂. Density functional (B3LYP) and MP2 methods, and a wide variety of basis sets, have been chosen to investigate the mechanism of this reaction. The results clearly show that there are two possible paths: 1) Uncatalysed isomerisation, trans‐HONO → HNO₂, involving 1,2‐hydrogen shift and characterized by a large energetic barrier 49.7 ÷ 58.9 kcal/mol, 2) Catalysed double hydrogen transfer process, trans‐HONO + cis‐HONO → HNO₂ + cis‐HONO, which displays a significantly lower energetic barrier in a range of 11.6 ÷ 18.9 kcal/mol. Topological analysis of the Electron Localization Function (ELF) shows that the hydrogen transfer for both studied reactions takes place through the formation of a ‘dressed’ proton along the reaction path. 1 Use of a wide variety of basis sets demonstrates a clear basis set dependence on the ELF topology of HNO₂. Less saturated basis sets yield two lone pair basins, V₁(N), V₂(N), whereas more saturated ones (for example aug‐cc‐pVTZ and aug‐cc‐pVQZ) do not indicate a lone pair on the nitrogen atom. Topological analysis of the Electron Localizability Indication (ELI‐D) at the CASSCF (12,10) confirms these findings, showing the existence of the lone pair basins but with decreasing populations as the basis set becomes more saturated (0.35e for the cc‐pVDZ basis set to 0.06e for the aug‐cc‐pVTZ). This confirms that the choice of basis set not only can influence the value of the electron population at the particular atom, but can also lead to different ELF topology. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Estimation of isotropic nuclear magnetic shieldings in the CCSD(T) and MP2 complete basis set limit using affordable correlation calculations

A linear correlation between isotropic nuclear magnetic shielding constants for seven model molecules (CH₂O, H₂O, HF, F₂, HCN, SiH₄ and H₂S) calculated with 37 methods (34 density functionals, RHF, MP2 and CCSD(T)), with affordable pcS‐2 basis set and corresponding complete basis set results, estimated from calculations with the family of polarization‐consistent pcS‐n basis sets is reported. This dependence was also supported by inspection of profiles of deviation between CBS estimated nuclear shieldings and shieldings obtained with the significantly smaller basis sets pcS‐2 and aug‐cc‐pVTZ‐J for the selected set of 37 calculation methods. It was possible to formulate a practical approach of estimating the values of isotropic nuclear magnetic shielding constants at the CCSD(T)/CBS and MP2/CBS levels from affordable CCSD(T)/pcS‐2, MP2/pcS‐2 and DFT/CBS calculations with pcS‐n basis sets. The proposed method leads to a fairly accurate estimation of nuclear magnetic shieldings and considerable saving of computational efforts. Copyright © 2013 John Wiley & Sons, Ltd.     

Adsorption of carbon monoxide on Pd low‐index surfaces and (110) reconstructed surface

Adsorption and diffusion of carbon monoxide on Pd low‐index surfaces and missing‐row Pd (110) reconstructed surface have been investigated by the extended London–Eyring–Polyani–Sato (LEPS) method constructed by means of a five‐parameter Morse potential. All critical characteristics, such as adsorption site, adsorption geometry, binding energy, CO vibrational frequency have been obtained and compared with the experimental and theoretical data. On these surfaces, the stable adsorption sites of CO are changed with increasing CO coverage. On the missing‐row Pd (110) reconstructed surface, there are five stable adsorption sites: H₁, H₂ (H₁ and H₂ are threefold hollow sites on (111) subsurface), B (bridge site on the second layer), SB (short‐bridge site), and T (top site). Copyright © 2010 John Wiley & Sons, Ltd.     

Molecular parameters of tetraatomic carbonyls X2CO and XYCO (X,Y = H,F, Cl) in the ground and lowest excited electronic states,part 1: A test of ab initio methods

Geometrical parameters of tetraatomic carbonyl molecules X₂CO and XYCO (X, Y = H, F, Cl) in the ground (S₀) and lowest excited singlet (S₁) and triplet (T₁) electronic states as well as values of barriers to inversion in S₁ and T₁ states and S₁ ← S₀ and T₁ ← S₀ adiabatic transition energies were systematically investigated by means of various quantum‐chemical techniques. The following methods were tested: HF, MP2, CIS, CISD, CCSD, EOM‐CCSD, CCSD(T), CR‐EOM‐CCSD(T), CASSCF, MR‐MP2, CASPT2, CASPT3, NEVPT2, MR‐CISD, and MR‐AQCC within cc‐pVTZ and cc‐pVQZ basis sets. The accuracy of quantum‐chemical methods was estimated in comparison with experimental data and rather accurate structures of excited electronic states were obtained. MP2 and CASPT2 methods appeared to be the most efficient and CCSD(T), CR‐EOM‐CCSD(T), and MR‐AQCC the most accurate. It was found that at equilibrium all the molecules under study are nonplanar in S₁ and T₁ electronic states with CO out‐of‐plane angle ranging from 34° (H₂CO, S₁) to 52° (F₂CO, T₁), and height of barrier to inversion varying from 300 (H₂CO, S₁) to 11,000 (F₂CO, T₁) cm^?1. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Selective Production of Carbon Monoxide via Methane Oxychlorination over Vanadyl Pyrophosphate

A catalytic process is demonstrated for the selective conversion of methane into carbon monoxide via oxychlorination chemistry. The process involves addition of HCl to a CH₄–O₂ feed to facilitate C?H bond activation under mild conditions, leading to the formation of chloromethanes, CH₃Cl and CH₂Cl₂. The latter are oxidized in situ over the same catalyst, yielding CO and recycling HCl. A material exhibiting chlorine evolution by HCl oxidation, high activity to oxidize chloromethanes into CO, and no ability to oxidize CO, is therefore essential to accomplish this target. Following these design criteria, vanadyl pyrophosphate (VPO) was identified as an outstanding catalyst, exhibiting a CO yield up to approximately 35 % at 96 % selectivity and stable behavior. These findings constitute a basis for the development of a process enabling the on‐site valorization of stranded natural‐gas reserves using CO as a highly versatile platform molecule.