In five-coordinate transition metal complexes, isomerization between square-pyramidal and trigonal-bipyramidal geometries is generally assumed to be rapid relative to reactions such as reductive elimination. Herein we report a system in which two square-pyramidal isomers of the same compound, [(NCN)Pt(Me)(ap)(Ph)(ba)]OTf and [(NCN)Pt(Ph)(ap)(Me)(ba)]OTf (ap = apical, ba = basal), undergo reductive elimination to form different products. For [(NCN)Pt-(Me)(ap)(Ph)(ba)]OTf, methyl migration to the pincer aryl group occurs to form the sigma-agostic complex [(N(C-Me)N)Pt(Ph)]OTf. In contrast, reductive elimination from [(NCN)Pt(Ph)(ap)(Me)(ba)]OTf affords toluene and (NCN)Pt(OTf). The observation of distinct products from these isomers demonstrates that isomerization between five-coordinate structures is slower than reductive elimination. DFT studies indicate that the barriers to these C-C couplings are dominated by the need for aryl groups to adopt a "face-on" configuration during reductive elimination, the steric requirements of which are kinetically prohibitive for some pathways in this system. Although the calculated barrier to isomerization is low enough for isomerization to be rapid at room temperature as expected, the activation barriers leading to experimentally observed carbon-carbon reductive elimination products are lower still, in agreement with experiment.

We present a fundamental description of the electron transfer (ET) step from substituted oligo(p-phenylene) (OPP) radical anions to CO2, with the larger goal of assessing the viability of underexplored, organic photoredox routes for utilization of anthropogenic CO2. This work varies the electrophilicity of para-substituents to OPP and probes the dependence of rate coefficients and interfragment interactions on the substituent Hammett parameter, σp, using constrained density functional theory (CDFT) and energy decomposition analysis (EDA). Large electronic coupling elements across substituents indicate an adiabatic electron transfer process for reactants at contact. As one might intuitively expect, free energy changes dominate trends in ET rate coefficients in most cases, and rates increase with substituent electron-donating ability. However, we observe an unexpected dip in rate coefficients for the most electron- donating groups, due to the combined impact of flattening free energies and a steep increase in reorganization energies. Our analysis shows that flattening OPP LUMO levels lower the marginal increase in free energy with decreasing σp. Reorganization energies do not exhibit a direct dependence on σp. They are higher for substituents containing lone pairs of electrons since substituent orientation varies with OPP charge. EDA reveals that interfragment orbital relaxation, or charge transfer interaction, plays a critical role in stabilizing the vertically excited charge transfer state. Subsequent relaxation to the final state geometry lowers charge transfer stabilization. A concurrent increase in long-range electrostatic interactions is observed, which are more favorable for electron-withdrawing substituents. Our study therefore suggests that while a wide range of ET rates are observed, there is an upper limit to rate enhancements achievable by tuning substituent electrophilicity.

Low-lying UV spectroscopy of trans-1,3-butadiene has been extensively studied by experimentalists and theorists. Though a host of techniques has been applied to understand its lowest electronic states, there are still important open questions. Among these are the positions of the two lowest valence excited states and the factors responsible for the spectral shape of the lowest allowed transitions. We present results from EOM-CC calculations in extended basis sets that are used to parametrize a three-electronic-state Koppel, Domcke, and Cederbaum (KDC) model. We test the sensitivity of the KDC model to a variety of parameters and address several outstanding questions regarding the spectrum. We find that the overall shape of the spectrum is determined primarily by the Franck-Condon envelope of the 11Bu state and that the princple impact of the doubly excited 21Ag state is to broaden the 11Bu peaks. There is only modest sensitivity to the relative position of these two states. We find that the lowest Rydberg state, the 11Bg state, has an unexpected impact on the third peak in the spectrum, and its effect is considerably more energy-dependent than that of the 21Ag state.

Endiandric acids and related polyketide natural products arise from polyene precursors and occur naturally as fused and bridged tetracycles. In some cases, the intramolecular Diels-Alder reactions that produce fused and bridged tetracycles result from a diene tether that may act as either a 4π or 2π component in the cycloaddition. To examine the preference for fused or bridged products, we applied density functional theory (using the M06-2X and B3LYP functionals) to reactants with various substituents for both fused and bridged transition states. Fused products were generally preferred except when disfavored by extreme steric hindrance (e.g., a tert-butyl group). These computational results are consistent with experimental data and suggest the existence of as-yet undiscovered natural products.

We investigate several representative density functional theory approaches for the calculation of relative activation energies and free energies of a set of model pericyclic reactions, some of which have been studied experimentally. In particular, we use a standard hybrid functional (B3LYP), the same hybrid functional augmented with a basis set superposition error and dispersion correction, a meta-hybrid functional developed to treat transition states and weak interactions (M06-2X), and the recently implemented random phase approximation (RPA) based on Kohn-Sham orbitals from conventional density functional theory by Furche and co-workers. We apply these methods to calculate relative activation energies and estimated free energies for the amide acetal Claisen rearrangement. We focus on relative activation energies to assess the effects of steric and weak interactions in the various methods and compare with experiment where possible. We also discuss the advantages of using this set of reactions as a test bed for the comparison of treatments of weak interactions. We conclude that all methods yield similar trends in relative reactivity, but the RPA yields results in best agreement with the experimental values.

We present a simple quasi-diabatization scheme applicable to spectroscopic studies that can be applied using any wavefunction for which one-electron properties and transition properties can be calculated. The method is based on rotation of a pair (or set) of adiabatic states to minimize the difference between the given transition property at a reference geometry of high symmetry (where the quasi-diabatic states and adiabatic states coincide) and points of lower symmetry where quasi-diabatic quantities are desired. Compared to other quasi-diabatization techniques, the method requires no special coding, facilitates direct comparison between quasi-diabatic quantities calculated using different types of wavefunctions, and is free of any selection of configurations in the definition of the quasi-diabatic states. On the other hand, the method appears to be sensitive to multi-state issues, unlike recent methods we have developed that use a configurational definition of quasi-diabatic states. Results are presented and compared with two other recently developed quasi-diabatization techniques.

ADVERTISEMENT RETURN TO ISSUESpecial Issue Prefac...Special Issue PrefaceNEXTTribute to John R. Miller and Marshall D. NewtonRobert J. Cave and Piotr PiotrowiakView Author Information Harvey Mudd College Rutgers University, NewarkCite this: J. Phys. Chem. B 2015, 119, 24, 7117–7119Publication Date (Web):June 18, 2015Publication History Published online18 June 2015Published inissue 18 June 2015https://doi.org/10.1021/jp5113069Copyright © 2015 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views260Altmetric-Citations1LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (4 MB) Get e-AlertsSUBJECTS:Amorphous materials,Charge transfer,Electron tunneling,Solvents,Tunneling Get e-Alerts