Theoretical investigation of an acid catalyst for viable release of H2 from BN nanotubes: A local pair natural orbital coupled cluster approach

Catalytic removal of H₂ from boron-nitride (BN)-based nanomaterials at ambient conditions is of paramount importance in order to develop lightweight hydrogen storage media. Here, the DLPNO-CCSD(T) technique is used to calculate accurate relative energies and activation barriers of Brønsted acid-initiated removal of H₂ from hydrogenated BN nanotubes (HBNNTs) with six different acids. Three crucial steps are identified in the mechanism: first H₂ release, catalyst regeneration via proton transfer, and second H₂ release to ensure feasibility of the dehydrogenation proposal. Our computational studies reveal that sulfonic acids with appropriate electrophilicity can facilitate dehydrogenation of HBNNT at a low free energetic cost (∆G^‡ = 17 kcal/mol). Importantly, these findings illustrate the possibility of H₂ release from BN nanomaterials at ambient conditions and provides hope for a sustainable chemical hydrogen storage strategy.     

DLPNO‐CCSD(T) scaled methods for the accurate treatment of large supramolecular complexes

In this work, we present scaled variants of the DLPNO‐CCSD(T) method, dubbed as (LS)DLPNO‐CCSD(T) and (NS)DLPNO‐CCSD(T), to obtain accurate interaction energies in supramolecular complexes governed by noncovalent interactions. The novel scaled schemes are based on the linear combination of the DLPNO‐CCSD(T) correlation energies calculated with the standard (LoosePNO and NormalPNO) and modified (Loose2PNO and Normal2PNO) DLPNO‐CCSD(T) accuracy levels. The scaled DLPNO‐CCSD(T) variants provide nearly TightPNO accuracy, which is essential for the quantification of weak noncovalent interactions, with a noticeable saving in computational cost. Importantly, the accuracy of the proposed schemes is preserved irrespective of the nature and strength of the supramolecular interaction. The (LS)DLPNO‐CCSD(T) and (NS)DLPNO‐CCSD(T) protocols have been used to study in depth the role of the CH–π versus π–π interactions in the supramolecular complex formed by the electron‐donor truxene‐tetrathiafulvalene (truxTTF) and the electron‐acceptor hemifullerene (C₃₀H₁₂). (NS)DLPNO‐CCSD(T)/CBS calculations clearly reveal the higher stability of staggered (dominated by CH–π interactions) versus bowl‐in‐bowl (dominated by π–π interactions) arrangements in the truxTTF•C₃₀H₁₂ heterodimer. Hemifullerene and similar carbon‐based buckybowls are therefore expected to self‐assemble with donor compounds in a richer way other than the typical concave–convex π–π arrangement found in fullerene‐based aggregates. © 2017 Wiley Periodicals, Inc.     

Exergonic and Spontaneous Production of Radicals through Beryllium Bonds

High‐level ab initio calculations show that the formation of radicals, by the homolytic bond fission of Y?R (Y=F, OH, NH₂; R=CH₃, NH₂, OH, F, SiH₃, PH₂, SH, Cl, NO) bonds is dramatically favored by the association of the molecule with BeX₂ (X=H and Cl) derivatives. This finding is a consequence of two concomitant effects, the significant activation of the Y?R bond after the formation of the beryllium bond, and the huge stabilization of the F^. (OH^., NH₂^.) radical upon BeX₂ attachment. In those cases where R is an electronegative group, the formation of the radicals is not only exergonic, but spontaneous.     

Computationally-assisted approach to the vibrational spectra of molecular crystals: study of hydrogen-bonding and pseudo-polymorphism.

A new computationally-assisted methodology (PiMM), which accounts for the effects of intermolecular interactions in the crystal, is applied to the complete assignment of the Raman and infrared vibrational spectra of room temperature forms of crystalline caffeine, theobromine, and theophylline. The vibrational shifts due to crystal packing interactions are evaluated from ab initio calculations for a set of suitable molecular pairs, using the B3LYP/6-31G* approach.The proposed methodology provides an answer to the current demand for a reliable assignment of the vibrational spectra of these methyl-xanthines, and clarifies several misleading assignments. The most relevant intermolecular interactions in each system and their effect on the vibrational spectra are considered and discussed. Based on these results, significant insights are obtained for the structure of caffeine in the anhydrous form (stable at room temperature), for which no X-ray structure has been reported. A possible structure based on C((8))--H...N((9)) and C((1,3))--H...O intermolecular interactions is suggested.     

Prevalence of the alkyl/phenyl-folded conformation in benzylic compounds C6H5CH2-X-R (X=O,CH2, CO,S, SO,SO2): significance of the CH/pi interaction as evidenced by high-level ab initio MO calculations

Ab initio MO calculations were carried out to examine the conformational energies of various benzylic compounds C(6)H(5)CH(2)XR (X=O, CH(2), CO, S, SO, SO(2); R=CH(3), C(2)H(5), iC(3)H(7), tC(4)H(9)) at the MP2/6-311G(d,p)//MP2/6-31G(d) level. Rotamers with R/Ph in gauche relationship are generally more stable than the R/Ph anti rotamers. In these stable geometries, the interatomic distance in the interaction of alpha- or beta-CH in the alkyl group and the ipso-carbon atom of the phenyl ring is short. The computational results are consistent with experimental data from supersonic molecular jet spectroscopy on 3-n-propyltoluene and NMR and crystallographic data on structurally related ketones, sulfoxides, and sulfones. In view of this, the alkyl/phenyl-congested conformation of these compounds has been suggested to be a general phenomenon, rather than an exception. The attractive CH/pi interaction has been suggested to be a dominant factor in determining the conformation of simple aralkyl compounds.     

Towards an Understanding of Halide Interactions with the Carbonyl-Containing Molecule CH3CHO

The anion photoelectron spectra of Cl⁻⋅⋅⋅CD₃CDO, Cl⁻⋅⋅⋅(CD₃CDO)₂, Br⁻⋅⋅⋅CH₃CHO, and I⁻⋅⋅⋅CH₃CHO are presented with electron stabilisation energies of 0.55, 0.93, 0.48, and 0.40 eV, respectively. Optimised geometries of the singly solvated species featured the halide appended to the CH₃CHO molecule in-line with the electropositive portion of the C=O bond and having binding energies between 45 and 52 kJ mol⁻¹. The doubly solvated Cl⁻⋅⋅⋅(CH₃CHO)₂ species features asymmetric solvation upon the addition of a second CH₃CHO molecule. Theoretical detachment energies were found to be in excellent agreement with experiment, with comparisons drawn between other halide complexes with simple carbonyl molecules.     

Contributions of Various Noncovalent Bonds to the Interaction between an Amide and S‐Containing Molecules

N‐Methylacetamide, a model of the peptide unit in proteins, is allowed to interact with CH₃SH, CH₃SCH₃, and CH₃SSCH₃ as models of S‐containing amino acid residues. All of the minima are located on the ab initio potential energy surface of each heterodimer. Analysis of the forces holding each complex together identifies a variety of different attractive forces, including SH???O, NH???S, CH???O, CH???S, SH???π, and CH???π H‐bonds. Other contributing noncovalent bonds involve charge transfer into σ* and π* antibonds. Whereas some of the H‐bonds are strong enough that they represent the sole attractive force in several dimers, albeit not usually in the global minimum, charge‐transfer‐type noncovalent bonds play only a supporting role. The majority of dimers are bound by a collection of several of these attractive interactions. The SH???O and NH???S H‐bonds are of comparable strength, followed by CH???O and CH???S.     

Prediction of hydrocarbon enthalpies of formation by various thermochemical schemes

The correlation consistent composite approach (ccCA) has been used to compute the enthalpies of formation (ΔH_f′s) for 60 closed‐shell, neutral hydrocarbon molecules selected from an established set (Cioslowski et al., J. Chem. Phys. 2000 , 113, 9377). This set of thermodynamic values includes ΔH_f's for hydrocarbons that span a range of molecular sizes, degrees of aromaticity, and geometrical configurations, and, as such, provides a rigorous assessment of ccCA's applicability to a variety of hydrocarbons. The ΔH_f's were calculated via atomization energies, isodesmic reactions, and hypohomodesmotic reactions. In addition, for 12 of the aromatic molecules in the set that are larger than benzene, the energies of ring‐conserved isodesmic reactions were used to calculate the ΔH_f′s. Using an atomization energy approach to determine the ΔH_f′s, the lowest mean absolute deviation (MAD) from experiment achieved by ccCA for the 60 hydrocarbons was 1.10 kcal mol^?1. The use of the mixed Gaussian/inverse exponential complete basis set extrapolation scheme (ccCA‐P) in conjunction with hypohomodesmotic reaction energies resulted in a MAD of 0.87 kcal mol^?1. This value is compared with MADs of 1.17, 1.18, and 1.28 kcal mol^?1 obtained via the Gaussian‐4 (G4), Gaussian‐3 (G3), and Gaussian‐3(MP2) [G3(MP2)] methods, respectively (using the hypohomodesmotic reactions). © 2012 Wiley Periodicals, Inc.     

The correlation consistent composite approach (ccCA): an alternative to the Gaussian-n methods

An alternative to the Gaussian-n (G1, G2, and G3) composite methods of computing molecular energies is proposed and is named the "correlation consistent composite approach" (ccCA, ccCA-CBS-1, ccCA-CBS-2). This approach uses the correlation consistent polarized valence (cc-pVXZ) basis sets. The G2-1 test set of 48 enthalpies of formation (DeltaHf), 38 adiabatic ionization potentials (IPs), 25 adiabatic electron affinities (EAs), and 8 adiabatic proton affinities (PAs) are computed using this approach, as well as the DeltaHf values of 30 more systems. Equilibrium molecular geometries and vibrational frequencies are obtained using B3LYP density functional theory. When applying the ccCA-CBS method with the cc-pVXZ series of basis sets augmented with diffuse functions, mean absolute deviations within the G2-1 test set compared to experiment are 1.33 kcal mol(-1) for DeltaHf,0.81 kcal mol(-1) for IPs, 1.02 kcal mol(-1) for EAs, and 1.51 kcal mol(-1) for PAs, without including the "high-level correction" (HLC) contained in the original Gn methods. Whereas the HLC originated in the Gaussian-1 method as an isogyric correction, it evolved into a fitted parameter that minimized the error of the composite methods, eliminating its physical meaning. Recomputing the G1 and G3 enthalpies of formation without the HLC reveals a systematic trend where most DeltaHf values are significantly higher than experimental values. By extrapolating electronic energies to the complete basis set (CBS) limit and adding G3-like corrections for the core-valence and infinite-order electron correlation effects, ccCA-CBS-2 often underestimates the experimental DeltaHf, especially for larger systems. This is desired as inclusion of relativistic and atomic spin-orbit effects subsequently improves theoretical DeltaHf values to give a 0.81 kcal mol(-1) mean absolute deviation with ccCA-CBS-2. The ccCA-CBS method is a viable "black box" method that can be used on systems with at least 10-15 heavy atoms.     

Potential energy surfaces of the microhydrated guanine...cytosine base pair and its methylated analogue.

A complete scan of the potential and free-energy surfaces of monohydrated and dihydrated guanine...cytosine and 9-methylguanine...1-methylcytosine base pairs was realized by the molecular dynamics/quenching technique using the force field of Cornell et al. implemented in the AMBER7 program. The most stable and populated structures localized were further fully reoptimized at the correlated ab initio level employing the resolution of identity M?ller-Plesset method with a large basis set. A systematic study of microhydration of these systems using a high-level correlated ab initio approach is presented for the first time. The different behavior of guanine...cytosine and adenine...thymine complexes is also discussed. These studies of nucleic acid base pairs are important for finding binding sites of water molecules around bases and for better understanding of the influence of the solvent on the stability of the structure of DNA.     

How important is the dispersion interaction for cyclobis(paraquat‐p‐phenylene)‐based molecular “shuttles”? A theoretical study

Inclusion complexes of cyclobis(paraquat‐p‐phenylene) and various aromatic molecules in their neutral and oxidized form were studied at the LMP2/6‐311+G**//BHandHLYP/6‐31G* level of theory, which represents the highest level theoretical study to date for these complexes. The results show that it is dispersion interaction that contributes most to the binding energy. One electron oxidation of a guest molecule leads to complete dissociation of inclusion complex generating strong repulsion potential between guest and host molecules. Electrostatic interactions also can play an important role, provided the guest molecule has a dipole moment; however, dispersion interactions always dominate in binding energy. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Accurate and fast treatment of large molecular systems: Assessment of CEPA and pCCSD within the local pair natural orbital approximation

The local pair natural orbital approach, which has been combined with two post‐Hartree–Fock methods, CEPA‐1 and pCCSD‐1a, recently, is assessed for its applicability to large real‐world problems without abundant computing resources. Test cases are selected based on being representative for computational chemistry problems and availability of reliable reference data. Both methods show a good performance and can be applied easily to systems of up to 100 atoms when very accurate energies are sought after. A considerable demand for basis sets of good quality has been identified and practical guidelines to satisfy this are mapped out. © 2012 Wiley Periodicals, Inc.     

Gas-phase enthalpies of formation of the fluorobenzenes family and their dewar isomers from ab initio calculations

The standard gas-phase enthalpies of formation, at T = 298.15 K, of the complete series of fluorobenzene and their corresponding dewar isomers have been determined by means of the CBS-QB3 and G3MP2B3 composite approaches. These values have been estimated by using appropriate supporting reactions, such as, reactions of atomization or of atom substitution. The results show that there is a linear dependence between the enthalpy of the most stable n-fluorobenzene and the corresponding n-fluorodewar benzene (n = 0, 1, …, 6). Further, the estimates are always more negative than the experimental results and so, suggested enthalpies of formation for 1,2,3-, 1,2,4- and 1,3,5-trifluorobenzenes and for 1,2,3,4- and 1,2,3,5-tetrafluorobenzenes are those retrieved from G3MP2B3 calculations added by 8 kJ/mol. The interaction of four different M⁺ ions with fluorobenzene and the three difluorobenzenes shows that the σ-interaction with 1,2-difluorobenzene is stronger than π-interaction on these fluorobenzenes.     

Exploring the (Very Flat) Potential Energy Landscape of R−Br⋅⋅⋅π Interactions with Accurate CCSD(T) and SAPT Techniques

Halogen bonds involving an aromatic moiety as an acceptor, otherwise known as R?X???π interactions, have increasingly been recognized as being important in materials and in protein–ligand complexes. These types of interactions have been the subject of many recent investigations, but little is known about the ways in which the strengths of R?X???π interactions vary as a function of the relative geometries of the interacting pairs. Here we use the accurate CCSD(T) and SAPT2+3δMP2 methods to investigate the potential energy landscapes for systems of HBr, HCCBr, and NCBr complexed with benzene. It is found that only the separation between the complexed molecules have a strong effect on interaction strength while other geometric parameters, such as tilting and shifting R?Br???π donor relative to the benzene plane, affect these interactions only mildly. Importantly, it is found that the C_6v (T‐shaped) configuration is not the global minimum for any of the dimers investigated.     

Criteria for the elucidation of the pseudopericyclic character of the cyclization of (Z)-1,2,4,6-heptatetraene and its heterosubstituted analogues: magnetic properties and natural bond orbital analysis

The electrocyclization of heterosubstituted derivatives of (Z)-1,2,4,6-heptatetraene, (2Z)-2,4,5-hexatrien-1-imine and (2Z)-2,4,5-hexatrienal exhibit some features which suggest a pseudopericyclic mechanism. In order to examine this, a comprehensive study including the determination of magnetic properties to estimate aromaticity and an NBO analysis throughout the reaction path was conducted. The cyclization of 5oxo-2,4-pentadienal, a process of unequivocal pseudopericyclic nature, was studied for comparison. The results suggest that, although the lone electron pair on the heteroatom in the heptatetraene derivatives seemingly plays a crucial role in the reaction mechanism, it does not suffice to deprive the reaction from the essential features of a pericyclic disrotatory electrocyclization.     

Thermodynamic and ab initio analysis of the controversial enthalpy of formation of formaldehyde.

There are two values, -26.0 and -27.7 kcal mol(-1), that are routinely reported in literature evaluations for the standard enthalpy of formation, Delta(f) H(o)(298), of formaldehyde (CH(2)=O), where error limits are less than the difference in values. In this study, we summarize the reported literature for formaldehyde enthalpy values based on evaluated measurements and on computational studies. Using experimental reaction enthalpies for a series of reactions involving formaldehyde, in conjunction with known enthalpies of formation, its enthalpy is determined to be -26.05+/-0.42 kcal mol(-1), which we believe is the most accurate enthalpy currently available. For the same reaction series, the reaction enthalpies are evaluated using six computational methods: CBS-Q, CBS-Q//B3, CBS-APNO, G2, G3, and G3B3 yield Delta(f) H(o)(298)=-25.90+/-1.17 kcal mol(-1), which is in good agreement to our experimentally derived result. Furthermore, the computational chemistry methods G3, G3MP2B3, CCSD/6-311+G(2df,p)//B3LYP/6-31G(d), CCSD(T)/6-311+G(2df,p)//B3LYP/6-31G(d), and CBS-APNO in conjunction with isodesmic and homodesmic reactions are used to determine Delta(f) H(o)(298). Results from a series of five work reactions at the higher levels of calculation are -26.30+/-0.39 kcal mol(-1) with G3, -26.45+/-0.38 kcal mol(-1) with G3MP2B3, -26.09+/-0.37 kcal mol(-1) with CBS-APNO, -26.19+/-0.48 kcal mol(-1) with CCSD, and -26.16+/-0.58 kcal mol(-1) with CCSD(T). Results from heat of atomization calculations using seven accurate ab initio methods yields an enthalpy value of -26.82+/-0.99 kcal mol(-1). The results using isodesmic reactions are found to give enthalpies more accurate than both other computational approaches and are of similar accuracy to atomization enthalpy calculations derived from computationally intensive W1 and CBS-APNO methods. Overall, our most accurate calculations provide an enthalpy of formation in the range of -26.2 to -26.7 kcal mol(-1), which is within computational error of the suggested experimental value. The relative merits of each of the three computational methods are discussed and depend upon the accuracy of experimental enthalpies of formation required in the calculations and the importance of systematic computational errors in the work reaction. Our results also calculate Delta(f) H(o)(298) for the formyl anion (HCO(-)) as 1.28+/-0.43 kcal mol(-1).