Toward asymmetric aziridination with an iron complex supported by a D2-symmetric tetra-NHC.

A neutral D2-symmetric macrocyclic tetra-N-heterocyclic carbene ligand was synthesized. The macrocycle was ligated to iron(II) via transmetalation from an isolated silver complex that has two conformers. The iron complex catalyzed the first stereospecific aziridination between aryl azides and aliphatic alkenes, albeit with low ee's.

Correction: Interfacial acidity on the strontium titanate surface: a scaling paradigm and the role of the hydrogen bond.

Correction for 'Interfacial acidity on the strontium titanate surface: a scaling paradigm and the role of the hydrogen bond' by Robert C. Chapleski, Jr. et al., Phys. Chem. Chem. Phys., 2021, 23, 23478-23485, DOI: 10.1039/D1CP03587H.

Interfacial acidity on the strontium titanate surface: a scaling paradigm and the role of the hydrogen bond.

A fundamental understanding of acidity at an interface, as mediated by structure and molecule-surface interactions, is essential to elucidate the mechanisms of a range of chemical transformations. While the strength of an acid in homogeneous gas and solution phases is conceptually well understood, acid-base chemistry at heterogeneous interfaces is notoriously more complicated. Using density functional theory and nonlinear vibrational spectroscopy, we present a method to determine the interfacial Brønsted-Lowry acidity of aliphatic alcohols adsorbed on the (100) surface of the model perovskite, strontium titanate. While shorter and less branched alkanols are known to be less acidic in the gas phase and more acidic in solution, here we show that shorter alcohols are less acidic whereas less substituted alkanols are more acidic at the gas-oxide interface. Hydrogen bonding plays a critical role in defining acidity, whereas structure-acidity relationships are dominated by van der Waals interactions between the alcohol and the surface.

Elucidation of the Reaction Mechanism of C2 + N1 Aziridination from Tetracarbene Iron Catalysts.

A combined computational and experimental study was undertaken to elucidate the mechanism of catalytic C2 + N1 aziridination supported by tetracarbene iron complexes. Three specific aspects of the catalytic cycle were addressed. First, how do organic azides react with different iron catalysts and why are alkyl azides ineffective for some catalysts? Computation of the catalytic pathway using density functional theory (DFT) revealed that an alkyl azide needs to overcome a higher activation barrier than an aryl azide to form an iron imide, and the activation barrier with the first-generation catalyst is higher than the activation barrier with the second-generation variant. Second, does the aziridination from the imide complex proceed through an open-chain radical intermediate that can change stereochemistry or, instead, via an azametallacyclobutane intermediate that retains stereochemistry? DFT calculations show that the formation of aziridine proceeds via the open-chain radical intermediate, which qualitatively explains the formation of both aziridine diastereomers as seen in experiments. Third, how can the formation of the side product, a metallotetrazene, be prevented, which would improve the yield of aziridine at lower alkene loading? DFT and experimental results demonstrate that sterically bulky organic azides prohibit formation of the metallotetrazene and, thus, allow lower alkene loading for effective catalysis. These multiple insights of different aspects of the catalytic cycle are critical for developing improved catalysts for C2 + N1 aziridination.

Catalytic aziridination with alcoholic substrates via a chromium tetracarbene catalyst.

The first examples of aziridination catalysis with a chromium complex are communicated. This tetracarbene chromium complex provides novel catalytic aziridination reactions with protic substrates such as alcohols or amines on the alkene or organic azide and is the most effective catalyst at low alkene loading for aliphatic alkenes to date.

Chemistry at molecular junctions: Rotation and dissociation of O2 on the Ag(110) surface induced by a scanning tunneling microscope.

The scanning tunneling microscope (STM) is a fascinating tool used to perform chemical processes at the single-molecule level, including bond formation, bond breaking, and even chemical reactions. Hahn and Ho [J. Chem. Phys. 123, 214702 (2005)] performed controlled rotations and dissociations of single O2 molecules chemisorbed on the Ag(110) surface at precise bias voltages using STM. These threshold voltages were dependent on the direction of the bias voltage and the initial orientation of the chemisorbed molecule. They also observed an interesting voltage-direction-dependent and orientation-dependent pathway selectivity suggestive of mode-selective chemistry at molecular junctions, such that in one case the molecule underwent direct dissociation, whereas in the other case it underwent rotation-mediated dissociation. We present a detailed, first-principles-based theoretical study to investigate the mechanism of the tunneling-induced O2 dynamics, including the origin of the observed threshold voltages, the pathway dependence, and the rate of O2 dissociation. Results show a direct correspondence between the observed threshold voltage for a process and the activation energy for that process. The pathway selectivity arises from a competition between the voltage-modified barrier heights for rotation and dissociation, and the coupling strength of the tunneling electrons to the rotational and vibrational modes of the adsorbed molecule. Finally, we explore the "dipole" and "resonance" mechanisms of inelastic electron tunneling to elucidate the energy transfer between the tunneling electrons and chemisorbed O2.

Dynamical steering and electronic excitation in NO scattering from a gold surface.

Nonadiabatic coupling of nuclear motion to electronic excitations at metal surfaces is believed to influence a host of important chemical processes and has generated a great deal of experimental and theoretical interest. We applied a recently developed theoretical framework to examine the nature and importance of nonadiabatic behavior in a system that has been extensively studied experimentally: the scattering of vibrationally excited nitric oxide molecules from a Au(111) surface. We conclude that the nonadiabatic transition rate depends strongly on both the N-O internuclear separation and the molecular orientation and, furthermore, that molecule-surface forces can steer the molecule into strong-coupling configurations. This mechanism elucidates key features of the experiments and provides several testable predictions regarding the dependence of vibrational energy transfer on the initial vibrational energy, molecular orientation, and incident angle.

Nonadiabatic dynamics at metal surfaces: independent-electron surface hopping.

Recent experiments have shown convincing evidence for nonadiabatic energy transfer from adsorbate degrees of freedom to surface electrons during the interaction of molecules with metal surfaces. In this paper, we propose an independent-electron surface hopping algorithm for the simulation of nonadiabatic gas-surface dynamics. The transfer of energy to electron-hole pair excitations of the metal is successfully captured by hops between electronic adiabats. The algorithm is able to account for the creation of multiple electron-hole pairs in the metal due to nonadiabatic transitions. Detailed simulations of the vibrational relaxation of nitric oxide on a gold surface, employing a multistate potential energy surface fit to density functional theory calculations, confirm that our algorithm can capture the underlying physics of the inelastic scattering process.

Model Hamiltonian for the interaction of NO with the Au(111) surface.

We have constructed a model Hamiltonian to describe the interaction of a nitric oxide (NO) molecule with a Au(111) surface. The diagonal elements of the 2x2 Hamiltonian matrix represent the diabatic potential energy surfaces corresponding to the neutral and negative-ion states of the molecule. A position-dependent off-diagonal element controls the extent of mixing of the two diabatic states. The parameters of the Hamiltonian matrix were determined from ground-state density functional theory calculations, both in the absence and presence of a small applied electric field to perturb the extent of charge transfer to the molecule. The resulting model Hamiltonian satisfactorily reproduces the ab initio results, and scattering simulations of the incident translational energy dependence of trapping probability and final rotational energy of NO agree quite well with experiment. The explicit incorporation of neutral and ionic configurations should serve as a realistic and practical platform for elucidating the importance of charge transfer and nonadiabatic effects at metal surfaces, as well as provide a useful testing ground for the development of theories of nonadiabatic dynamics.

Vibrational relaxation of NO on Au(111) via electron-hole pair generation.

Recent experiments have demonstrated the breakdown of the Born-Oppenheimer approximation when NO undergoes inelastic scattering from a Au(111) surface. In this paper, we provide a simple theoretical model for understanding this phenomenon. Our model predicts multiquanta vibrational relaxation through the creation of high-energy electron-hole pair excitations in the metal. Using experimentally determined parameters, our model gives qualitatively accurate predictions for the final vibrational state populations of the scattered molecule and predicts efficient conversion of vibrational energy into electronic energy.