87,009 research outputs found
Charge-displacement analysis via natural orbitals for chemical valence: Charge transfer effects in coordination chemistry
No full textWe recently devised a simple scheme for analyzing on quantitative grounds the Dewar-Chatt-Duncanson donation and back-donation in symmetric coordination complexes. Our approach is based on a symmetry decomposition of the so called Charge-Displacement (CD) function quantifying the charge flow, upon formation of a metal (M)-substrate (S) bond, along the M-S interaction axis and provides clear-cut measures of donation and back-donation charges in correlation with experimental observables [G. Bistoni et al.; Angew. Chem.; Int. Ed. 52, 11599 (2013)]. The symmetry constraints exclude of course from the analysis most systems of interest in coordination chemistry. In this paper, we show how to entirely overcome this limitation by taking advantage of the properties of the natural orbitals for chemical valence [M. Mitoraj and A. Michalak, J. Mol. Model. 13, 347 (2007)]. A general scheme for disentangling donation and back-donation in the CD function of both symmetric and non-symmetric systems is presented and illustrated through applications to M-ethyne (M = Au, Ni and W) coordination bonds, including an explicative study on substrate activation in a model reaction mechanism
Inactivated Candida albicans and immunostimulation. P. Marconi, L. Scaringi, L. Tissi, A. Cassone, F. Bistoni. In: Agriculture. Adjuvants, Interferon and Non-Specific Immunity. Report EUR 8675 EN. (F.M. Cancellotti and D. Galassi Eds.). pp. 143-152, 1984.
No full textLondon dispersion effects in the coordination and activation of alkanes in σ-complexes: A local energy decomposition study
No full textLocal energy decomposition (LED) analysis decomposes the interaction energy between two fragments calculated at the domain-based local pair natural orbital CCSD(T) (DLPNO-CCSD(T)) level of theory into a number of chemically meaningful contributions. Herein, this scheme is applied to the interaction between the transition metal (TM) and the alkane in σ-complexes. It is demonstrated that the often-neglected London dispersion (LD) energy is a fundamental component of the TM-alkane interaction for a wide range of experimentally characterized σ-complexes. LD effects determine the structure and the thermodynamic stability of σ-complexes and influence the selectivity of CH activation reactions. The magnitude of the LD energy can be modulated by increasing the size of the alkane and of the ancillary ligands on the TM. These results provide further evidence on the fundamental role that London dispersion plays in organometallic chemistry
Local energy decomposition analysis of hydrogen-bonded dimers within a domain-based pair natural orbital coupled cluster study
No full textThe local energy decomposition (LED) analysis allows for a decomposition of the accurate domain-based local pair natural orbital CCSD(T) [DLPNO-CCSD(T)] energy into physically meaningful contributions including geometric and electronic preparation, electrostatic interaction, interfragment exchange, dynamic charge polarization, and London dispersion terms. Herein, this technique is employed in the study of hydrogen-bonding interactions in a series of conformers of water and hydrogen fluoride dimers. Initially, DLPNO-CCSD(T) dissociation energies for the most stable conformers are computed and compared with available experimental data. Afterwards, the decay of the LED terms with the intermolecular distance (r) is discussed and results are compared with the ones obtained from the popular symmetry adapted perturbation theory (SAPT). It is found that, as expected, electrostatic contributions slowly decay for increasing r and dominate the interaction energies in the long range. London dispersion contributions decay as expected, as r−6. They significantly affect the depths of the potential wells. The interfragment exchange provides a further stabilizing contribution that decays exponentially with the intermolecular distance. This information is used to rationalize the trend of stability of various conformers of the water and hydrogen fluoride dimers
Understanding the Role of Dispersion in Frustrated Lewis Pairs and Classical Lewis Adducts: A Domain-Based Local Pair Natural Orbital Coupled Cluster Study
No full textThe interaction of Lewis acids and bases in both classical Lewis adducts and frustrated Lewis pairs (FLPs) is investigated to elucidate the role that London dispersion plays in different situations. The analysis comprises 14 different adducts between tris(pentafluorophenyl)borane and a series of phosphines, carbenes, and amines with various substituents, differing in both steric and electronic properties. The domain-based local pair natural orbital coupled-cluster (DLPNO-CCSD(T)) method is used in conjunction with the recently introduced local energy decomposition (LED) analysis to obtain state-of-the-art dissociation energies and, at the same time, a clear-cut definition of the London dispersion component of the interaction, with the ultimate goal of aiding in the development of designing principles for acid/base pairs with well-defined bonding features and reactivity. In agreement with previous DFT investigations, it is found that the London dispersion dominates the interaction energy in FLPs, and is also remarkably strong in Lewis adducts. In these latter systems, its magnitude can be easily modulated by modifying the polarizability of the substituents on the basic center, which is consistent with the recently introduced concept of dispersion energy donors. By counteracting the destabilizing energy contribution associated with the deformation of the monomers, the London dispersion drives the stability of many Lewis adducts
Extrapolation to the Limit of a Complete Pair Natural Orbital Space in Local Coupled-Cluster Calculations
No full textThe domain-based local pair natural orbital (PNO) coupled-cluster DLPNO-CCSD(T) method allows one to perform single point energy calculations for systems with hundreds of atoms while retaining essentially the accuracy of its canonical counterpart, with errors that are typically smaller than 1 kcal/mol for relative energies. Crucial to the accuracy and efficiency of the method is a proper definition of the virtual space in which the coupled-cluster equations are solved, which is spanned by a highly compact set of pair natural orbitals (PNOs) that are specific for each electron pair. The dimension of the PNO space is controlled by the TCutPNO threshold: only PNOs with an occupation number greater than TCutPNO are included in the correlation space of a given electron pair, whilst the remaining PNOs are discarded. To keep the error of the method small, a conservative TCutPNO value is used in standard DLPNO-CCSD(T) calculations. This often leads to unnecessarily large PNO spaces, which limits the efficiency of the method. Herein, we introduce a new computational strategy to approach the complete PNO space limit (for a given basis set) that consists in extrapolating the results obtained with different TCutPNO values. The method is validated on the GMTKN55 set using canonical CCSD(T) data as the reference. Our results demonstrate that a simple two-point extrapolation scheme can be used to significantly increase the efficiency and accuracy of DLPNO-CCSD(T) calculations, thus extending the range of applicability of the technique
HFLD: A Nonempirical London Dispersion-Corrected Hartree-Fock Method for the Quantification and Analysis of Noncovalent Interaction Energies of Large Molecular Systems †
No full textA nonempirical quantum mechanical method for the efficient and accurate quantification and analysis of intermolecular interactions is presented and tested on existing benchmark sets. The leading idea here is to focus on the intermolecular part of the correlation energy that contains the all-important London dispersion (LD) interaction. To keep the cost of the method low, essentially at the level of a Hartree-Fock (HF) calculation, the intramolecular part of the correlation energy is neglected. We also neglect the nondispersive parts of the intermolecular correlation energy. This scheme that we denote as Hartree-Fock plus London dispersion (HFLD) can be readily realized on the basis of the recently reported multilevel implementation of the domain-based local pair natural orbital coupled-cluster (DLPNO-CC) theory in conjunction with the well-established local energy decomposition (LED) analysis. The accuracy and efficiency of the HFLD method are evaluated on rare gas dimers, on the S66 and L7 benchmark sets of noncovalent interactions, and on an additional set (LP14) consisting of bulky Lewis pairs held together by intermolecular interactions of various strengths, with interaction energies ranging from -8 to -107 kcal/mol. It is first shown that the LD energy calculated with this approach is essentially identical to that obtained from the full DLPNO-CCSD(T)/LED calculation, with a mean absolute error of 0.2 kcal/mol on the S66 benchmark set. Moreover, in terms of the overall interaction energies, the HFLD method shows an efficiency that is comparable to that of the HF method, while retaining an accuracy between that of the DLPNO-CCSD and DLPNO-CCSD(T) schemes. Since the underlying DLPNO-CCSD method is linear scaling with respect to the system size, the HFLD approach also does not lead to new bottlenecks for large systems. As an illustrative example of its efficiency, the HFLD scheme was applied to the interaction between the substrate and the residues in the active site of the cyclohexanone monooxygenase enzyme. The excellent cost/performance ratio indicates that the HFLD method opens new avenues for the accurate calculation and analysis of noncovalent interaction energies in large molecular systems
Effect of Electron Correlation on Intermolecular Interactions: A Pair Natural Orbitals Coupled Cluster Based Local Energy Decomposition Study
No full textThe development of post-Hartree-Fock (post-HF) energy decomposition schemes that are able to decompose the HF and correlation components of the interaction energy into chemically meaningful contributions is a very active field of research. One of the challenges is to provide a clear-cut quantification to the elusive London dispersion component of the intermolecular interaction. London dispersion is well-known to be a pure correlation effect, and as such it is not properly described by mean field theories. In this context, we have recently developed the local energy decomposition (LED) analysis, which provides a chemically meaningful decomposition of the interaction energy between two or more fragments computed at the domain-based local pair natural orbitals coupled cluster (DLPNO-CCSD(T)) level of theory. In this work, this scheme is used in conjunction with other interpretation tools to study a series of molecular adducts held together by intermolecular interactions of different natures. The HF and correlation components of the interaction energy are thus decomposed into a series of chemically meaningful contributions. Emphasis is placed on discussing the physical effects associated with the inclusion of electron correlation. It is found that four distinct physical effects can contribute to the magnitude of the correlation part of intermolecular binding energies (Eint C ): (i) London dispersion, (ii) the correlation correction to the reference induction energy, (iii) the correlation correction to the electron sharing process, and (iv) the correlation correction to the permanent electrostatics. As expected, the largest contribution to the correlation binding energy of neutral, apolar molecules is London dispersion, as in the argon dimer case. In contrast, the correction for the HF induction energy dominates Eint C in systems in which an apolar molecule interacts with charged or strongly polar species, as in Ar-Li + . This effect has its origin in the systematic underestimation of polarizabilities at the HF level of theory. For similar reasons, electron sharing largely contributes to the correlation binding energy of covalently bound molecules, as in the beryllium dimer case. Finally, the correction for HF permanent electrostatics significantly contributes to Eint C in molecules with strong dipoles, such as water and hydrogen fluoride dimers. This effect originates from the characteristic overestimation of dipole moments at the HF level of theory, leading in some cases to positive Eint C values. Our results are apparently in contrast to the widely accepted view that Eint C is typically dominated by London dispersion, at least, in the strongly interacting region. Clearly, post-HF energy decomposition schemes are very powerful tools to analyze, categorize, and understand the various contributions to the intermolecular interaction energy. Hopefully, this will eventually lead to insights that are helpful in designing systems with tailored properties. All analysis tools presented in this work will be available free of charge in the next release of the ORCA program package
Formation of Agostic Structures Driven by London Dispersion
No full textAgostic interactions between a C−H bond and a transition metal are commonly crucial in catalytic polymerization processes. Herein, a quantitative study of the nature of β-agostic interactions in a series of systems of importance in C−H bond activation reactions is reported. The analysis, characterized by the use of a coupled-cluster-based energy decomposition scheme, demonstrates that short-range London dispersion between the agostic C−H bond and the metal center plays a fundamental role in affecting the structural stability of these systems, contrary to a widely held view. These results are used to rationalize a series of previously published experimental findings
Protein microarrays: from serodiagnosis to whole proteome scale analysis of the immune response against pathogenic microorganisms.
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