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A theoretical study of the halogen-bondedcomplexes (A–X···C)formed between halogenated derivatives (A–X; A = F, Cl, Br,CN, CCH, CF3, CH3, H; and X = Cl, Br) and anitrogen heterocyclic carbene, 1,3-dimethylimidazole-2-ylidene (MeIC)has been performed using MP2/aug′-cc-pVDZ level of theory.Two types of A–X:MeIC complexes, called here type-I and -II,were found and characterized. The first group is described by longC–X distances and small binding energies (8–54 kJ·mol–1). In general, these complexes show the traditionalbehavior of systems containing halogen-bonding interactions. The secondtype is characterized by short C–X distances and large bindingenergies (148–200 kJ·mol–1), and onthe basis of the topological analysis of the electron density, theycorrespond to ion-pair halogen-bonded complexes. These complexes canbe seen as the interaction between two charged fragments: A–and +[X–CIMe] with a high electrostatic contributionin the binding energy. The charge transfer between lone pair A(LP)to the σ* orbital of C–X bond is also identified as asignificant stabilizing interaction in type-II complexes. [ABSTRACT FROM AUTHOR]

Radical additions to substituted alkenes arc among the most important reactions in radical chemistry. Nonetheless, there is still some controversy in the literature about the factors that affect the rate and regioselectivity in these addition reactions, in this paper, the orientation of(nucleophilic) radical additions to electron-rich. -neutral, and -poor monosubstituted substrates (ii reactions in total) is investigated through the use of chemical concepts and reactivity descriptors. The regioselectivity of the addition of nueleophilic radicals on electron-rich and -neutral alkenes is thermodynamically controlled. An excellent correlation of 94% is found between the differences in activation barriers and in product stabilities (unsubstituted versus substituted site attack). Polar effects at the initial stage of the reaction play a significant role when electron-poor substrates are considered. lowering the extent of regioselectivity toward the unsubstituted sites, as predicted from the stability differences. This is nicely confirmed through an analysis for each of the ii reactions using the spin-polarized dual descriptor, matching electrophilic and nucleophilic regions. [ABSTRACT FROM AUTHOR]

In our continuous effort to retrieve the Woodward-Hoffmann rules from conceptual density functional theory (DFT), we have examined the last type of pericyclic reactions, i.e., chelotropic reactions. Both the initial hardness response and the dual descriptor have been investigated to predict the allowed and forbidden character for the addition of SO2 to butadiene {An system) and 1,3,5-hexatriene {An + 2 system). It is shown that with both electronic descriptors, the conrota- tory/disrotatory mode for the linear and nonlinear mechanisms are retrieved based on a density-only approach, free from consideration of orbital and (or) wave function symmetry. The dual descriptor moreover reveals that stabilizing interactions are presented only for the linear path, which can be considered as an overall favourable mechanism for a chelotropic reaction. [ABSTRACT FROM AUTHOR]

The chemical reactivity of the first- and second-generation Grubbs catalysts has always been a significant issue in olefin metathesis. In the present work, we study the [2+2] cycloreversion/cycloaddition and the alkylidene rotation involved into the interconversion of the ruthenacyclobutane intermediate, through the reaction force and reaction force constant analysis. It has been found that the structural contribution controls the barrier energy in the interconversion of ruthenacyclobutane via [2+2] cycloreversion/cycloaddition, which is slightly lower in the second generation of Grubbs catalysts while its electronic contribution is slightly higher, which unveils a major rigidity and donor/acceptor properties of the NHC. This finding explains a greater structural contribution in the rate constant. Moreover, on the basis of the reaction force constant, the process can be classified as "two-stage"-concerted reactions, noting a more asynchronous process when the first generation is used as a catalyst. Finally, a similar analysis into the alkylidene rotation was performed. It was determined that [2+2] cycloreversion and alkylidene rotations take place in a sequential manner, the energy barrier is again controlled by structural reorganization, and the pathway is less asynchronous. [ABSTRACT FROM AUTHOR]

A synchronous, concerted chemical process is rigorously divided by the reaction force F( R), the negative gradient of V( R), into 'reactant' and 'product' regions which are dominated by structural changes and an intervening 'transition' region which is electronically intensive. The reaction force constant κ( R), the second derivative of V( R), is negative throughout the transition region, not just at the nominal transition state, at which κ( R) has a minimum. This is consistent with experimental evidence that there is a transition region, not simply a specific point. We show graphically that significant nonsynchronicity in the process is associated with the development of a maximum of κ( R) in the transition region, which increases as the process becomes more nonsynchronous. (We speculate that for a nonconcerted process this maximum is actually positive.) Thus, κ( R) can serve as an indicator of the level of nonsynchronicity. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The reaction force F( ξ) is the negative gradient of the potential energy of a chemical process along the intrinsic reaction coordinate ξ. We extend the rigorous concept of F( ξ) to the 'activation strain model' of Bickelhaupt et al., to formulate the 'strain' force F( ξ) that retards a reaction and the 'interaction' force F( ξ) that drives it. These are investigated for a group of Diels-Alder cycloadditions. The results fully support the interpretation of the minimum of F( ξ) as defining the beginning of the transition from deformed reactants to eventual products. [ABSTRACT FROM AUTHOR]

Photoelectronspectroscopy measurements and density functionalcalculations are combined to determine the lowest electron bindingenergies of first-row transition-metal aqua ions, titanium throughcopper, with 3d1through 3d9electronic configurations,in their most common oxidation states. Vertical ionization energiesare found to oscillate considerably between 6.76 and 9.65 eV for thedications and between 7.05 and 10.28 eV for the respective trivalentcations. The metal cations are modeled as [M(H2O)n]q牛겵 (q= 2, 3, and 4; n= 6 and 18) surroundedby continuum solvent. The performance of 10 exchange–correlationfunctionals, two GGAs, three MGGAs, two HGGAs and three HMGGAs, combinedwith the MDF10(ECP)/6-31(d,p) basis set is assessed for 11 M–Obond distances, 10 vertical ionization energies, 6 adiabatic ionizationenergies, and the associated reorganization free energies. We findthat for divalent cations the HGGA and HMGGA functionals in combinationwith the 18 water model show the best agreement with experimentalvertical ionization energies and geometries; for trivalent ions, theMGGA functionals perform best. The corresponding reorganization freeenergies (λo) of the oxidized ions are significantlyunderestimated with all DFT functionals and cluster models. This indicatesthat the structural reorganization of the solvation shell upon ionizationis not adequately accounted for by the simple solvation models used,emphasizing the importance of extended sampling of thermally accessiblesolvation structures for an accurate computation of this quantity.The photoelectron spectroscopy measurements reported herein providea comprehensive set of transition-metal redox energetic quantitiesfor future electronic structure benchmarks. [ABSTRACT FROM AUTHOR]

We have analyzed the variation of the reaction force F( ξ) and the reaction force constant κ( ξ) along the intrinsic reaction coordinates ξ of the water-assisted proton transfer reactions of HX-N = Y (X,Y = O,S). The profile of the force constant of the vibration associated with the reactive mode, k( ξ), was also determined. We compare our results to the corresponding intramolecular proton transfers in the absence of a water molecule. The presence of water promotes the proton transfers, decreasing the energy barriers by about 12 - 15 kcal mol. This is due in part to much smaller bond angle changes being needed than when water is absent. The κ( ξ) profiles along the intrinsic reaction coordinates for the water-assisted processes show striking and intriguing differences in the transition regions. For the HS-N = S and HO-N = S systems, two κ( ξ) minima are obtained, whereas for HO-N = O only one minimum is found. The k( ξ) show similar behavior in the transition regions. We propose that this fine structure reflects the degree of synchronicity of the two proton migrations in each case. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]