Semiautomated Transition State Localization for Organometallic Complexes with Semiempirical Quantum Chemical Methods.

We present an efficient computational protocol for robust transition state localization that can be routinely applied to complex (organometallic) reactions. The capabilities of the combination of extended tight-binding semiempirical methods (GFNn-xTB) with a state-of-the-art transition state localization algorithm (mGSM) is demonstrated on a modified version of the MOBH35 benchmark set, consisting of 29 organometallic reactions and transition states. Furthermore, for three examples we demonstrate how error-prone the conventional (manual) approach based on chemical intuition can be and how errors are avoided by a semiautomated generation of reaction profiles. The performance of the GFNn-xTB methods is carefully assessed and compared with that of the widely used PM6-D3H4 and PM7 semiempirical methods. The GFNn-xTB methods show much higher success rates of 89.7% (GFN1-xTB) and 86.2% (GFN2-xTB) compared with 72.4% for PM6-D3H4 and 69.0% for PM7. The barrier heights and reaction energies are computed with much better accuracy at reduced computational cost for the GFNn-xTB methods compared with the PMx methods, allowing a semiquantitative assessment of possible reaction pathways already at a semiempirical level. The mean error of GFN2-xTB for the barrier heights (8.2 kcal mol-1) is close to what low-cost density functional approximations provide and substantially smaller than the corresponding error of the competitor methods.

Comprehensive Thermochemical Benchmark Set of Realistic Closed-Shell Metal Organic Reactions.

We introduce the new MOR41 benchmark set consisting of 41 closed-shell organometallic reactions resembling many important chemical transformations commonly used in transition metal chemistry and catalysis. It includes significantly larger molecules than presented in other transition metal test sets and covers a broad range of bonding motifs. Recent progress in linear-scaling coupled cluster theory allowed for the calculation of accurate DLPNO-CCSD(T)/CBS(def2-TZVPP/def2-QZVPP) reference energies for 3d,4d,5d-transition metal compounds with up to 120 atoms. Furthermore, 41 density functionals, including seven GGAs, three meta-GGAs, 14 hybrid functionals, and 17 double-hybrid functionals combined with two different London dispersion corrections, are benchmarked with respect to their performance for the newly compiled MOR41 reaction energies. A few wave function-based post-HF methods as, e.g., MP2 or RPA with similar computational demands are also tested and in total, 90 methods were considered. The double-hybrid functional PWPB95-D3(BJ) outperformed all other assessed methods with an MAD of 1.9 kcal/mol, followed by the hybrids ωB97X-V (2.2 kcal/mol) and mPW1B95-D3(BJ) (2.4 kcal/mol). The popular PBE0-D3(BJ) hybrid also performs well (2.8 kcal/mol). Within the meta-GGA class, the recently published SCAN-D3(BJ) functional as well as TPSS-D3(BJ) perform best (MAD of 3.2 and 3.3 kcal/mol, respectively). Many popular methods like BP86-D3(BJ) (4.9 kcal/mol) or B3LYP-D3(BJ) (4.9 kcal/mol) provide significantly worse reaction energies and are not recommended for organometallic thermochemistry considering the availability of better methods with the same computational cost. The results regarding the performance of different functional approximations are consistent with conclusions from previous main-group thermochemistry benchmark studies.

Fully Automated Quantum-Chemistry-Based Computation of Spin-Spin-Coupled Nuclear Magnetic Resonance Spectra.

We present a composite procedure for the quantum-chemical computation of spin-spin-coupled 1 H NMR spectra for general, flexible molecules in solution that is based on four main steps, namely conformer/rotamer ensemble (CRE) generation by the fast tight-binding method GFN-xTB and a newly developed search algorithm, computation of the relative free energies and NMR parameters, and solving the spin Hamiltonian. In this way the NMR-specific nuclear permutation problem is solved, and the correct spin symmetries are obtained. Energies, shielding constants, and spin-spin couplings are computed at state-of-the-art DFT levels with continuum solvation. A few (in)organic and transition-metal complexes are presented, and very good, unprecedented agreement between the theoretical and experimental spectra was achieved. The approach is routinely applicable to systems with up to 100-150 atoms and may open new avenues for the detailed (conformational) structure elucidation of, for example, natural products or drug molecules.

trans-cis C-Pd-C rearrangement in hemichelates.

Kinetically unstable heteroleptic trans-bispalladacycles were isolated by using hemichelation. Three structures of trans isomers and five of cis isomers were characterized by X-ray diffraction analysis. The ready trans-to-cis isomerization of such hemichelates that was monitored by variable temperature NMR experiments is facilitated dynamically because the Pd(ii) center can preserve its square planar coordination in a rather low lying transition state, which was localized by methods of the density functional theory. This process is not achievable in the isomerization of conventional homoleptic trans-bispalladacycles since it involves the preliminary partial chelate decoordination and an unfavorable high-lying planar trigonal coordinated - or Y-shaped-Pd(ii) transition state according to DFT investigations.

Developing adaptive QM/MM computer simulations for electrochemistry.

We report the development of adaptive QM/MM computer simulations for electrochemistry, providing public access to all sources via the free and open source software development model. We present a modular workflow-based MD simulation code as a platform for algorithms for partitioning space into different regions, which can be treated at different levels of theory on a per-timestep basis. Currently implemented algorithms focus on targeting molecules and their solvation layers relevant to electrochemistry. Instead of using built-in forcefields and quantum mechanical methods, the code features a universal interface, which allows for extension to a range of external forcefield programs and programs for quantum mechanical calculations, thus enabling the user to readily implement interfaces to those programs. The purpose of this article is to describe our codes and illustrate its usage. © 2016 Wiley Periodicals, Inc.