25 research outputs found
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).Selfâassembled monolayers of Nâheterocyclic carbenes (NHCs) on copper are reported. The monolayer structure is highly dependent on the N,Nâsubstituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surface while methylâ or ethylâsubstituted NHCs lie flat. Temperatureâprogrammed desorption studies show that the NHC binds to Cu(111) with a desorption energy of Edes=152±10â
kJâmolâ1. NHCs that bind upright desorb cleanly, while flatâlying 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
Finite Temperature Structure and Dynamics of Zinc Dialkyldithiophosphate Wear Inhibitors:Â A Density Functional Theory and ab Initio Molecular Dynamics Study
Computational Investigation of the Oxygen Evolution Reaction Catalyzed by Nickel (Oxy)hydroxide Complexes
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 Al<sub>2</sub>O<sub>3</sub> (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 MoS<sub>2</sub>. 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). âGAMESS-UJ â 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<sup>(II)</sup> 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<sup>(II)</sup> complex of
1,3-bisÂ(1,5,9-triazacyclododec-1-yl)Âpropane 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<sup>(II)</sup><sub>1</sub> and Zn<sup>(II)</sup><sub>2</sub>. 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<sup>(II)</sup><sub>1</sub>:(<sup>â</sup>OCH<sub>3</sub>) and Zn<sup>(II)</sup><sub>1</sub>:(<sup>â</sup>O-propyl)] play the role of a direct
nucleophile (on <b>2</b> and <b>3</b>, respectively) or
where Zn<sup>(II)</sup><sub>1</sub>:(<sup>â</sup>OCH<sub>3</sub>) 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<sup>(II)</sup><sub>2</sub> ion
can serve as a spectator (after exerting a Lewis acid role in binding
one of the phosphatesâ 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 ZnâZn
distance to expand from âŒ3.6 Ă
in <b>1a</b> to over
5 Ă
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
Selfâassembled monolayers of Nâheterocyclic carbenes (NHCs) on copper are reported. The monolayer structure is highly dependent on the N,Nâsubstituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surface while methylâ or ethylâsubstituted NHCs lie flat. Temperatureâprogrammed desorption studies show that the NHC binds to Cu(111) with a desorption energy of Edes=152±10â
kJâmolâ1. NHCs that bind upright desorb cleanly, while flatâlying 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 CâC Bond Formation and CâH Bond Insertion
C,C-chelate dimesitylboron (BMes<sub>2</sub>) 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 HOMOâLUMO energy gap (>3.60 eV). They do,
however,
respond to UV irradiation (300 nm) in the same manner as N,C-chelate
BMes<sub>2</sub> compounds do, undergoing photoisomerization and converting
to an intensely colored (yellow or orange) isomer <b>A</b> quantitatively,
with a high quantum efficiency (0.60â0.75). 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
BMes<sub>2</sub> 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 BMes<sub>2</sub> 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 CâH insertion product
via a borylene intermediate. Mechanistic aspects of this unusual two-step
photoisomerization process have been examined by DFT computational
studies