Quasiclassical Direct Dynamics Simulations Research Articles

Dynamic hydrogen bonds promote C–H functionalization driven by Cl− anion

Most of studies for hydrogen bonds focus on the static model especially between two polar atoms. In contrast, introducing the third polar atom may emerge the competitive hydrogen bonds, which would represent a distinct perspective to perturb the catalytic chemical transformation. Herein, we report quantum mechanics calculations and quasi-classical direct dynamics simulations that demonstrate a triangle form of proton accepters enabled by Cl− anion can afford diverse hydrogen bonds, which control the reactivity and selectivity of Rh catalyzed phenol functionalization. A redox mechanism for carbene insertions with notable ligand effect was discovered and supported by both calculations and the experimental kinetic isotopic effect. The quaternary ammonium additive can stabilize key oxonium ylide intermediates with O–H⋯Cl hydrogen bonds that inhibit the common O–H insertion and promote the carbene C–H insertion product. The nonclassical dynamic hydrogen bonds coupling with Rh complex dissociation may result in an intermediate shuttle of oxonium ylides, which unify the site-selective of the direct phenol functionalization.

Timing and Structures of σ-Bond Metathesis C–H Activation Reactions from Quasiclassical Direct Dynamics Simulations

Metal-mediated σ-bond metathesis and σ-complex-assisted metathesis (σ-CAM) reactions represent a major class of alkane C–H activation reactions. Here, we present quasiclassical direct dynamics traj...

Experiments and Direct Dynamics Simulations That Probe η2-Arene/Aryl Hydride Equilibria of Tungsten Benzene Complexes.

Key steps in the functionalization of an unactivated arene often involve its dihaptocoordination by a transition metal followed by insertion into the C-H bond. However, rarely are the η2-arene and aryl hydride species in measurable equilibrium. In this study, the benzene/phenyl hydride equilibrium is explored for the {WTp(NO)(PBu3)} (Bu = n-butyl; Tp = trispyrazoylborate) system as a function of temperature, solvent, ancillary ligand, and arene substituent. Both face-flip and ring-walk isomerizations are identified through spin-saturation exchange measurements, which both appear to operate through scission of a C-H bond. The effect of either an electron-donating or electron-withdrawing substituent is to increase the stability of both arene and aryl hydride isomers. Crystal structures, electrochemical measurements, and extensive NMR data further support these findings. Static density functional theory calculations of the benzene-to-phenyl hydride landscape suggest a single linear sequence for this transformation involving a sigma complex and oxidative cleavage transition state. Static DFT calculations also identified an η2-coordinated benzene complex in which the arene is held more loosely than in the ground state, primarily through dispersion forces. Although a single reaction pathway was identified by static calculations, quasiclassical direct dynamics simulations identified a network of several reaction pathways connecting the η2-benzene and phenyl hydride isomers, due to the relatively flat energy landscape.

Time-Dependent Perspective for the Intramolecular Couplings of the N-H Stretches of Protonated Tryptophan.

Quasi-classical direct dynamics simulations, performed with the B3LYP-D3/cc-pVDZ electronic structure theory, are reported for vibrational relaxation of the three NH stretches of the −NH3+ group of protonated tryptophan (TrpH+), excited to the n = 1 local mode states. The intramolecular vibrational energy relaxation (IVR) rates determined for these states, from the simulations, are in good agreement with the experiment. In accordance with the experiment, IVR for the free NH stretch is slowest, with faster IVR for the remaining two NH stretches which have intermolecular couplings with an O atom and a benzenoid ring. For the free NH and the NH coupled to the benzenoid ring, there are beats (i.e., recurrences) in their relaxations versus time. For the free NH stretch, 50% of the population remained in n = 1 when the trajectories were terminated at 0.4 ps. IVR for the free NH stretch is substantially slower than for the CH stretch in benzene. The agreement found in this study between quasi-classical direct dynamics simulations and experiments indicates the possible applicability of this simulation method to larger biological molecules. Because IVR can drive or inhibit reactions, calculations of IVR time scales are of interest, for example, in unimolecular reactions, mode-specific chemistry, and many photochemical processes.

Paddle Ball Dynamics during Conversion of a Rh–Methyl Hydride Complex to a Rh–Methane σ-Complex through Reductive Coupling

We used quasiclassical direct dynamics simulations to examine reaction pathways for protonation-induced reductive coupling of a pincer (PONOP)Rh–methyl hydride complex. These dynamics simulations revealed that the majority of trajectories (>90%) emerging from a vibrationally averaged velocity distribution of the Rh–methyl hydride reductive coupling transition state led to the Rh–methane σ-complex. Only a few trajectories (<10%) dynamically deviated and did not sample this σ-complex structure and resulted in direct methane reductive elimination/dissociation. This indicates that the Rh–methane σ-complex is directly dynamically connected to the Rh–methyl hydride reductive coupling transition state and is not the result of methane rebound due to a solvent cage. Unexpectedly, trajectories that resulted in a Rh–methane σ-complex occurred with paddle ball motion where methane began to leave the Rh coordination sphere but then returned to the σ-complex, and so the distinction between reductive coupling and elimin...

Dynamically Bifurcating Hydride Transfer Mechanism and Origin of Inverse Isotope Effect for Heterodinuclear AgRu-Catalyzed Alkyne Semihydrogenation

The mechanism and heterodinuclear cooperative effects for AgRu-catalyzed alkyne semihydrogenation were analyzed with density-functional theory (DFT) and experiment. This combined effort revealed the following: (1) AgRu-catalyzed diphenylacetylene hydrogenation initially gives a kinetic mixture of cis-stilbene and trans-stilbene by an ionic Ag–H hydride transfer transition state and post-transition state bifurcation, which was identified by quasiclassical direct dynamics simulations. (2) The hydrogenation reaction exhibits an unexpected inverse kinetic isotope effect (KIE < 1) resulting from an inverse equilibrium isotope effect (EIE) for heterodinuclear H2/D2 activation. (3) The Ag–Ru heterodinuclear cooperative effective is critical for both H2 activation and vinylsilver protonolysis reaction steps. (4) Rate studies and computational analysis show that electron-donating groups accelerate catalysis.

Dynamical Mechanism May Avoid High-Oxidation State Ir(V)-H Intermediate and Coordination Complex in Alkane and Arene C-H Activation by Cationic Ir(III) Phosphine.

Organometallic reaction mechanisms are assumed to be appropriately described by minimum energy pathways mapped out by density functional theory calculations. For the two-step oxidative addition/reductive elimination mechanism for C-H activation of methane and benzene by cationic Cp*(PMe3)IrIII(CH3), we report quasiclassical direct dynamics simulations that demonstrate the IrV-H intermediate is bypassed in a significant amount of productive trajectories initiated from vibrationally averaged velocity distributions of oxidative addition transition states. This organometallic dynamical mechanism is akin to the σ-bond metathesis pathway but occurs on the oxidative addition/reductive elimination energy surface and blurs the line between two- and one-step mechanisms. Quasiclassical trajectories also reveal that the momentum of crossing the reductive elimination structure always induces complete alkane and arene dissociation from the Ir metal center, skipping weak C-H σ and π coordination complexes. This suggests that these weak coordination complexes after reductive elimination are not necessarily on the reaction pathway and likely result from a solvent cage.

Dynamics of the Degenerate Rearrangement of Bicyclo[3.1.0]hex-2-ene

Quasiclassical direct dynamics simulations are applied to a 4-fold degenerate rearrangement which yields a nonstatistical product distribution. The simulated product ratio agrees with experiment and is found to be entirely dynamically determined. Trajectory lifetimes are on the order of a low-frequency vibrational period. The interaction of reaction momentum with the geometric features of the potential surface produces selectivity despite a common energy barrier. A geometric model is described for qualitatively estimating much of the dynamically determined product ratio independently of trajectory calculations. The characteristics of this reaction are expected also to apply to others involving modestly stabilized diradical intermediates.

Ab initio direct dynamics study of cyclopropyl radical ring-opening.

Quasiclassical direct dynamics simulations, at the CASSCF(3,3)/6-31G(d) level of theory, are used to study the stereochemistry of the electrocyclic ring-opening reaction of the cyclopropyl radical. The trajectories are initiated at the reaction's transition state (TS), with their initial conditions sampled from the TS's 174 degrees C Boltzmann distribution. Intrinsic reaction coordinate calculations predict the overall reaction to have disrotatory stereochemistry. Though this is the preferred initial reaction stereochemistry in the trajectories, 43% of the trajectories follow the conrotatory path. Four unique trajectory types are observed during 200 fs dynamics of the product allyl radical. Intramolecular vibrational energy redistribution and internal rotation are incomplete on this time scale, and a statistical distribution of the allyl isomers is not observed.