N-heterocyclic carbene self-assembled monolayers on copper and gold : dramatic effect of wingtip groups on binding, orientation and assembly

Funding: EPSRC PhD studentship (EP/M506631/1).Self‐assembled monolayers of N‐heterocyclic carbenes (NHCs) on copper are reported. The monolayer structure is highly dependent on the N,N‐substituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surface while methyl‐ or ethyl‐substituted NHCs lie flat. Temperature‐programmed desorption studies show that the NHC binds to Cu(111) with a desorption energy of Edes=152±10 kJ mol−1. NHCs that bind upright desorb cleanly, while flat‐lying NHCs decompose leaving adsorbed organic residues. Scanning tunneling microscopy of methylated NHCs reveals arrays of covalently linked dimers which transform into adsorbed (NHC)2Cu species by extraction of a copper atom from the surface after annealing.Publisher PDFPeer reviewe

Tribochemistry of Aldehydes Sheared between (0001) Surfaces of α-Alumina from First-Principles Molecular Dynamics

First-principles molecular dynamics (FPMD) simulations are used to explore the tribological behavior of systems consisting of two Al₂O₃ (0001) surfaces separated by acetaldehyde molecules. The simulations were performed with normal pressures, <i>P</i>, that ranged from 0 to 20 GPa. The simulations show that sliding occurs with little or no significant changes in the structure of the aldehydes when <i>P</i> is low. Meanwhile, tribochemical reactions between aldehydes to yield oligomers, and between the oligomers and the surfaces, occur at higher <i>P</i>. The occurrence of these reactions leads to slip mechanisms that are dominated by the dissociation of chemical bonds. The different slip mechanisms affect the friction forces required to maintain motion of the surfaces. Slip mechanisms that do not involve bond rupture require low forces, with a friction coefficient of 0.034 to 0.044. The friction forces are much larger for the slip processes involving the rupture of bonds. Interestingly, the results indicate that the friction forces associated with slip mechanisms involving bond rupture are lower when the longer oligomers are involved in the slip process. Overall, this work sheds light on the atomic-level chemical processes that occur when lubricated surfaces move past one another, and may aid in the rational use of tribochemical reactions in functional lubrication

Behavior of Two-Dimensional Hydrogen-Bonded Networks under Shear Conditions: A First-Principles Molecular Dynamics Study

Static quantum chemical calculations and first-principles molecular dynamics simulations are used to examine the behavior of two-dimensional hydrogen-bonded systems under sliding conditions, with the goal of assessing whether such systems may be useful as lubricants. The results demonstrate that these systems can be effective lubricants if the hydrogen bonds (HBs) in the system are of moderate strength, evenly distributed in the system, and restricted to reside within the layers. One system that meets these conditions was found to exhibit friction forces and friction coefficients that are comparable to layered systems consisting of sheets of atoms connected via covalent or ionic bonds, such as graphite and MoS₂. The results also show that the flexibility associated with the HBs allows this system to reversibly undergo large structural deformations. This ability allowed this system to undergo a slip mechanism in which the layers buckled, which was found to reduce the slip barrier. The ability to reversibly accommodate structural changes may represent an advantage of systems comprising sheets consisting of covalently or ionically bonded components, which can be damaged irreversibly as a result of large structural deformations

GAMESS-UJ, Carter group GAMESS-UJ documentation

We have developed two methods for calculating U and J from unrestricted Hartree-Fock calculations. Our initial effort is described in Phys. Rev. B 76, 155123 (2007), followed by a rotationally-invariant formalism described in J. Chem. Phys. 129, 014103 (2008). “GAMESS-UJ “ refers to the second method implemented in the GAMESS packag

DFT Computational Study of the Methanolytic Cleavage of DNA and RNA Phosphodiester Models Promoted by the Dinuclear Zn^(II) Complex of 1,3-Bis(1,5,9-triazacyclododec-1-yl)propane

A density functional theory study of the cleavage of a DNA model [<i>p</i>-nitrophenyl methyl phosphate (<b>2</b>)] and two RNA models [<i>p</i>-nitrophenyl 2-hydroxypropyl phosphate (<b>3</b>) and phenyl 2-hydroxypropyl phosphate (<b>4</b>)] promoted by the dinuclear Zn^(II) complex of 1,3-bis­(1,5,9-triazacyclododec-1-yl)­propane formulated with a bridging methoxide (<b>1a</b>) was undertaken to determine possible mechanisms for the transesterification processes that are consistent with experimental data. The initial substrate-bound state of <b>2</b>:<b>1a</b> or <b>3</b>:<b>1a</b> has the two phosphoryl oxygens bridging Zn^(II)₁ and Zn^(II)₂. For each of <b>2</b> and <b>3</b>, four possible mechanisms were investigated, three of which were consistent with the overall free energy for the catalytic cleavage step for each substrate. The computations revealed various roles for the metal ions in the three mechanisms. These encompass concerted or stepwise processes, where the two metal ions with associated alkoxy groups [Zn^(II)₁:(⁻OCH₃) and Zn^(II)₁:(⁻O-propyl)] play the role of a direct nucleophile (on <b>2</b> and <b>3</b>, respectively) or where Zn^(II)₁:(⁻OCH₃) can act as a general base to deprotonate an attacking solvent molecule in the case of <b>2</b> or the attacking 2-hydroxypropyl group in the case of <b>3</b>. The Zn^(II)₂ ion can serve as a spectator (after exerting a Lewis acid role in binding one of the phosphates’ oxygens) or play active additional roles in providing direct coordination of the departing aryloxy group or positioning a hydrogen-bonding solvent to assist the departure of the leaving group. An important finding revealed by the calculations is the flexibility of the ligand system that allows the Zn–Zn distance to expand from ∼3.6 Å in <b>1a</b> to over 5 Å in the transforming <b>2</b>:<b>1a</b> and <b>3</b>:<b>1a</b> complexes during the catalytic event

<i>N</i>-heterocyclic carbene self-assembled monolayers on copper and gold:dramatic effect of wingtip groups on binding, orientation and assembly

Self‐assembled monolayers of N‐heterocyclic carbenes (NHCs) on copper are reported. The monolayer structure is highly dependent on the N,N‐substituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surface while methyl‐ or ethyl‐substituted NHCs lie flat. Temperature‐programmed desorption studies show that the NHC binds to Cu(111) with a desorption energy of Edes=152±10 kJ mol−1. NHCs that bind upright desorb cleanly, while flat‐lying NHCs decompose leaving adsorbed organic residues. Scanning tunneling microscopy of methylated NHCs reveals arrays of covalently linked dimers which transform into adsorbed (NHC)2Cu species by extraction of a copper atom from the surface after annealing

Stepwise Intramolecular Photoisomerization of NHC-Chelate Dimesitylboron Compounds with C–C Bond Formation and C–H Bond Insertion

C,C-chelate dimesitylboron (BMes₂) compounds containing an <i>N</i>-heterocyclic carbene (NHC) donor have been obtained. Single-crystal X-ray diffraction analyses established that the boron atom in these compounds is bound by four carbon atoms in a distorted tetrahedral geometry. Compared to previously reported N,C-chelate dimesitylboron compounds, the new C,C-chelate boron compounds have a much larger HOMO–LUMO energy gap (>3.60 eV). They do, however, respond to UV irradiation (300 nm) in the same manner as N,C-chelate BMes₂ compounds do, undergoing photoisomerization and converting to an intensely colored (yellow or orange) isomer <b>A</b> quantitatively, with a high quantum efficiency (0.60–0.75). NMR and single-crystal X-ray diffraction analyses established that the structure of <b>A</b> is similar to the dark isomers obtained from N,C-chelate BMes₂ compounds. However, unlike the N,C-chelate dark isomers that have the tendency to thermally reverse back to the light colored isomers, the isomers <b>A</b> of the C,C-chelate BMes₂ are thermally stable and no reverse isomerization was observed even when heated to 80 °C (or 110 °C) for hours. The most unusual finding is that isomers <b>A</b> undergo further photoisomerization when irradiated at 350 nm, forming a new colorless species <b>B</b> nearly quantitatively. NMR and single-crystal X-ray diffraction analyses established the structure of isomer <b>B</b>, which may be considered as an intramolecular C–H insertion product via a borylene intermediate. Mechanistic aspects of this unusual two-step photoisomerization process have been examined by DFT computational studies