Assessment of the performance of commonly used DFT functionals vs. MP2 in the study of IL-Water, IL-Ethanol and IL-(H2O)3 clusters

We present a comparative study of the accuracy of different DFT approaches vs. MP2 for evaluating ionic liquids (ILs) + cosolvent. Namely, we are interested in [XBmim] + cosolvent (X being Cl−, BF4−, PF6−, and CH3SO3− anions and cosolvent being water or ethanol) and [XBmim] + (H2O)3 clusters. In this study the B3LYP, B3LYP-D3, M06, M06-2X and M06-HF functionals with Pople and Dunning basis sets are considered. We find that the influence of the basis sets is a factor to take into consideration. As already seen for weakly bonded systems when the basis set quality is low the uncorrected counterpoise (unCP) or averaging counterpoise (averCP) energies must be used due to cancellation errors. Besides, the inclusion of extra diffuse functions and polarization is also required specially in the case of ILs interacting with water clusters. The B3LYP functional does not reproduce either the structure or the interaction energies for ILs + H2O and ILs + EtOH aggregates, the energetic discrepancies being more significant than the structural ones. Among the dispersive corrected functionals, M06-2X results resemble to a great extent the reference data when the unCP interaction energies are considered for both water and ethanol. In turn, M06 and B3LYP-D3 functionals are the best option for ILs containing polar and non-polar anions, respectively, whether the averCP interactions energies are taking into consideration. From the structural point of view, B3LYP and M06 functionals describe more open structures whereas B3LYP-D3, M06-2X and M06-HF structures resemble quite well MP2 results. When the number of water molecules increases the H bonding motif gains importance and the effect depends on the underlying functional. Only M06-2X and M06-HF behaviour is similar to that observed for one water molecule. This is important because to describe ILs-cosolvent solutions is not only necessary to take into account the ILs-cosolvent interactions but also the cosolvent-cosolvent ones in the ensemble of the system.Junta de Andalucía FQM282Ministerio de Ciencia e Innovación CTQ2011-2593

Empirical corrections and pair interaction energies in the fragment molecular orbital method

The energy and analytic gradient are developed for FMO combined with the Hartree-Fock method augmented with three empirical corrections (HF-3c). The auxiliary basis set approach to FMO is extended to perform pair interaction energy decomposition analysis. The FMO accuracy is evaluated for several typical systems including 3 proteins. Pair interaction energies computed with different approaches in FMO are compared for a water cluster and protein-ligand complexes.Comment: Revised version accepted in Chemical Physics Letter

A comparative study of semiempirical, ab initio, and DFT methods in evaluating metal-ligand bond strength, proton affinity, and interactions between first and second shell ligands in Zn-biomimetic complexes

International audienceAlthough theoretical methods are now available which give very accurate results, often comparable to the experimental ones, modeling chemical or biological interesting systems often requires less demanding and less accurate theoretical methods, mainly due to computer limitations. Therefore, it is crucial to know the precision of such less reliable methods for relevant models and data. This has been done in this work for small zinc-active site models including O- (H2O and OH-) and N-donor (NH3 and imidazole) ligands. Calculations using a number of quantum mechanical methods were carried out to determine their precision for geometries, coordination number relative stability, metal–ligand bond strengths, proton affinities, and interaction energies between first and second shell ligands. We have found that obtaining chemical accuracy can be as straightforward as HF geometry optimization with a double-f plus polarization basis followed by a B3LYP energy calculation with a triple-f quality basis set including diffuse and polarization functions. The use of levels as low as PM3 geometry optimization followed by a B3LYP single-point energy calculation with a double-tzeta quality basis including polarization functions already yields useful trends in bond length, proton affinities or bond dissociation energies, provided that appropriate caution is taken with the optimized structures. The reliability of these levels of calculation has been successfully demonstrated for real biomimetic cases

New Conceptual Understanding of Lewis Acidity, Coordinate Covalent Bonding, and Catalysis

The focus of this dissertation is to correct misconceptions about Lewis acidity, uncover the physical nature of the coordinate covalent bond, and discusses how Lewis acid catalysts influence the rate enhancement of the Diels-Alder reaction. Large-scale quantum computations have been employed to explore many of Lewis\u27 original ideas concerning valency and acid/base behavior. An efficient and practical level of theory able to model Lewis acid adducts accurately was determined by systematic comparison of computed coordinate covalent bond lengths and binding enthalpies of ammonia borane and methyl substituted ammonia trimethylboranes with high-resolution gas-phase experimental work. Of all the levels of theory explored, M06-2X/6-311++G(3df,2p) provided molecular accuracy consistent with more resource intensive QCISD(T)/6 311++G(3df,2p) computations. Coordinate covalent bond strength has traditionally been used to judge the strength of Lewis acidity; however, inconsistencies between predictions from theory and computation, and observations from experiment have arisen, which has resulted in consternation within the scientific community. Consequently, the electronic origin of Lewis acidity was investigated. It has been determined that the coordinate covalent bond dissociation energy is an inadequate index of intrinsic Lewis acid strength, because the strength of the bond is governed not only by the strength of the acid, but also by unique orbital interactions dependent upon the substituents of the acid and base. Boron Lewis acidity is found to depend upon both substituent electronegativity and atomic size. Originally deduced from Pauling\u27s electronegativities, boron\u27s substituents determine acidity by influencing the population of its valence by withdrawing electron density. However, size effects manifest differently than previously considered, where greater σ-bond orbital overlap, rather than π-bond orbital overlap, between boron and larger substituents increase the electron density available to boron, thereby decreasing Lewis acidity. The computed electronegativity and size effects of substituents establish unique periodic trends that provide a novel and clearer understanding of boron Lewis acidity, consistent with first principle predictions. Lastly, it is discovered that the energetics associated with the transition structure converge much slower than what was observed for coordinate covalent bonded ground states. Consequently, it is harder to model activation barriers, as compared to binding energies, to within experimental accuracy, because larger basis sets must be employed. Hyperconjugation within dienophile ground states, initiated by geminal Lewis acid interactions, is found to govern the strength of the coordinate covalent bond between the Lewis acid and the dienophile. A novel interpretation is presented where the strength of the coordinate covalent bond within the Lewis acid activated dienophile is governed by donor-acceptor orbital interactions between the π-density present on the carbonyl group to the σ* orbitals on the Lewis acid, rather than the main donor-acceptor motif between the oxygen lone pair and the empty 2p orbital on the Lewis acid. Moreover, the same hyperconjugation within the dienophile controls the rate enhancement of the Lewis acid catalyzed Diels-Alder reaction, by modulating the energy of the dienophile\u27s lowest unoccupied molecular orbital. A new understanding of Lewis acidity and coordinate covalent bonding has been achieved to better describe and predict the structure and electronic mechanism of organic reactions

The lithium-thiophene interaction: a critical study using highly correlated electronic structure approaches of quantum chemistry

International audienceThe fundamental multicentric interaction of a lithium atom with a single thiophene ring is addressed. A systematic study of the interaction energy (IE) and geometry for the Li-T charge-transfer complex is done at the MP2 and CCSD(T) levels using increasingly large basis sets up to aug-cc-pVQZ (AVQZ). Basis set superposition errors (BSSE) are evaluated and shown to have a major impact on the value of the IE. The Fixed-Node Diffusion Monte Carlo (FN-DMC) method is used as an alternative basis-set-free approach to obtain what is likely to be the most accurate estimate of the IE obtained so far. While counterpoise-corrected MP2/AVQZ and CCSD(T)/AVTZ interaction energies are found to be −3.8 and −7.5 kcal/mol, the FN-DMC method yields +1.3 ± 1.7 kcal/mol. The slow convergence of the ab initio IE (and some key structural parameters) with respect to basis set quality and the discrepancy with the FN-DMC result is discussed. A visualization of the electron pairing using the electron pair localization function (EPLF) for the Li-doped versus undoped thiophene is also presented

Understanding Interaction Capacity of CO2 with Organic Compounds at Molecular Level: A Theoretical Approach

In this chapter, interactions of CO2 with a number of organic compounds at molecular level are discussed in detail. The naked and substituted hydrocarbons along with compounds functionalized by hydroxyl, carbonyl, thiocarbonyl, carboxyl, sulfonyl, and amide groups have attracted much attention as CO2-philic agents. In general, interaction capacity between the functionalized organic compounds with CO2 is stronger than the hydrocarbon and its derivatives. An addition of more CO2 molecules into the interaction system formed by the functionalized organic compounds and CO2 leads to an increase in the stability of the complexes. The obtained results indicate that π…π linkages between CO2 and aromatic rings can significantly contribute to the interactions between CO2 and MOF/ZIF materials. Formic acid (HCOOH) is likely to be the most soluble compound as compared to the remaining host molecules (CH3OH, CH3NH2, HCHO, HCOOCH3, and CH3COCH3) when dissolved in CO2. The carbonyl (>C═O, >C═S) and sulfonyl (>S═O, >S═S) compounds have presented a higher stability, as compared to other functionalized groups, when they interact with CO2. Therefore, they can be valuable candidates in the design of CO2-philic materials and in the search of materials to adsorb CO2

Quantifying the hydration structure of sodium and potassium ions: taking additional steps on Jacob's Ladder

The ability to reproduce the experimental structure of water around the sodium and potassium ions is a key test of the quality of interaction potentials due to the central importance of these ions in a wide range of important phenomena. Here, we simulate the Na+ and K+ ions in bulk water using three density functional theory functionals: (1) the generalized gradient approximation (GGA) based dispersion corrected revised Perdew, Burke, and Ernzerhof functional (revPBE-D3) (2) the recently developed strongly constrained and appropriately normed (SCAN) functional (3) the random phase approximation (RPA) functional for potassium. We compare with experimental X-ray diffraction (XRD) and X-ray absorption fine structure (EXAFS) measurements to demonstrate that SCAN accurately reproduces key structural details of the hydration structure around the sodium and potassium cations, whereas revPBE-D3 fails to do so. However, we show that SCAN provides a worse description of pure water in comparison with revPBE-D3. RPA also shows an improvement for K+, but slow convergence prevents rigorous comparison. Finally, we analyse cluster energetics to show SCAN and RPA have smaller fluctuations of the mean error of ion-water cluster binding energies compared with revPBE-D3