7 research outputs found
Pseudocarbynes: Charge-Stabilized Carbon Chains
Carbyne is the long-sought
linear allotrope of carbon. Despite
many reports of solid carbyne, the evidence is unconvincing. A recent
report of supposed carbyne shows gold clusters in transmission electron
microscopy (TEM) images. In order to determine the effects of such
clusters, we performed ab initio calculations of uncapped and capped
linear carbon chains and their complexes with gold clusters. The results
indicate that gold dramatically alters the electron densities of the
CC bonds. The resulting charge-stabilization of the carbon
chains leads to pseudocarbynes. These findings are corroborated in
calculations of the structures of crystals containing isolated carbon
chains and those intercalated with gold clusters. Calculated Raman
spectra of these pseudocarbynes with gold clusters are in better agreement
with experiment than calculated spectra of isolated carbon chains.
The current work opens the way toward the design and development of
a new class of metal-intercalated carbon compounds
Probing the Nature of Charge Transfer at Nano–Bio Interfaces: Peptides on Metal Oxide Nanoparticles
Characterizing the nano–bio
interface has been a long-standing
endeavor in the quest for novel biosensors, biophotovoltaics, and
biocompatible electronic devices. In this context, the present computational
work on the interaction of two peptides, A6K (Ac-AAAAAAK-NH<sub>2</sub>) and A7 (Ac-AAAAAAA-NH<sub>2</sub>) with semiconducting
TiO<sub>2</sub> nanoparticles is an effort to understand the peptide–metal
oxide nanointerface. These investigations were spurred by recent experimental
observations that nanostructured semiconducting metal oxides templated
with A6K peptides not only stabilize large proteins like photosystem-I
(PS-I) but also exhibit enhanced charge-transfer characteristics.
Our results indicate that α-helical structures of A6K are not
only energetically more stabilized on TiO<sub>2</sub> nanoparticles,
but the resulting hybrids also exhibit enhanced electron transfer
characteristics. This enhancement can be attributed to substantial
changes in the electronic characteristics at the peptide-TiO<sub>2</sub> interface. Apart from understanding the mechanism of electron transfer
(ET) in peptide-stabilized PS-I on metal oxide nanoparticles, the
current work also has implications in the development of novel solar
cells and photocatalysts
Electrochemical Capture and Release of Carbon Dioxide Using a Disulfide–Thiocarbonate Redox Cycle
We describe a new electrochemical
cycle that enables capture and
release of carbon dioxide. The capture agent is benzylthiolate (RS<sup>–</sup>), generated electrochemically by reduction of benzyldisulfide
(RSSR). Reaction of RS<sup>–</sup> with CO<sub>2</sub> produces
a terminal, sulfur-bound monothiocarbonate, RSCO<sub>2</sub><sup>–</sup>, which acts as the CO<sub>2</sub> carrier species, much the same
as a carbamate serves as the CO<sub>2</sub> carrier for amine-based
capture strategies. Oxidation of the thiocarbonate releases CO<sub>2</sub> and regenerates RSSR. The newly reported <i>S</i>-benzylthiocarbonate (IUPAC name benzylsulfanylformate) is characterized
by <sup>1</sup>H and <sup>13</sup>C NMR, FTIR, and electrochemical
analysis. The capture–release cycle is studied in the ionic
liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide
(BMP TFSI) and dimethylformamide. Quantum chemical calculations give
a binding energy of CO<sub>2</sub> to benzyl thiolate of −66.3
kJ mol<sup>–1</sup>, consistent with the experimental observation
of formation of a stable CO<sub>2</sub> adduct. The data described
here represent the first report of electrochemical behavior of a sulfur-bound
terminal thiocarbonate
A Nickel Phosphine Complex as a Fast and Efficient Hydrogen Production Catalyst
Here
we report the electrocatalytic reduction of protons to hydrogen
by a novel S<sub>2</sub>P<sub>2</sub> coordinated nickel complex,
[Ni(bdt)(dppf)] (bdt = 1,2-benzenedithiolate, dppf = 1,1′-bis(diphenylphosphino)ferrocene).
The catalysis is fast and efficient with a turnover frequency of 1240
s<sup>–1</sup> and an overpotential of only 265 mV for half
activity at low acid concentrations. Furthermore, catalysis is possible
using a weak acid, and the complex is stable for at least 4 h in acidic
solution. Calculations of the system carried out at the density functional
level of theory (DFT) are consistent with a mechanism for catalysis
in which both protonations take place at the nickel center
Catalytic Hydrogen Evolution by Fe(II) Carbonyls Featuring a Dithiolate and a Chelating Phosphine
Two pentacoordinate mononuclear iron
carbonyls of the form (bdt)Fe(CO)P<sub>2</sub> [bdt = benzene-1,2-dithiolate;
P<sub>2</sub> = 1,1′-diphenylphosphinoferrocene
(<b>1</b>) or methyl-2-{bis(diphenylphosphinomethyl)amino}acetate
(<b>2</b>)] were prepared as functional, biomimetic models for
the distal iron (Fe<sub>d</sub>) of the active site of [FeFe]-hydrogenase.
X-ray crystal structures of the complexes reveal that, despite similar
ν(CO) stretching band frequencies, the two complexes have different
coordination geometries. In X-ray crystal structures, the iron center
of <b>1</b> is in a distorted trigonal bipyramidal arrangement,
and that of <b>2</b> is in a distorted square pyramidal geometry.
Electrochemical investigation shows that both complexes catalyze electrochemical
proton reduction from acetic acid at mild overpotential, 0.17 and
0.38 V for <b>1</b> and <b>2</b>, respectively. Although
coordinatively unsaturated, the complexes display only weak, reversible
binding affinity toward CO (1 bar). However, ligand centered protonation
by the strong acid, HBF<sub>4</sub>·OEt<sub>2</sub>, triggers
quantitative CO uptake by <b>1</b> to form a dicarbonyl analogue <b>[1(H)-CO]<sup>+</sup></b> that can be reversibly converted back
to <b>1</b> by deprotonation using NEt<sub>3</sub>. Both crystallographically
determined distances within the bdt ligand and density functional
theory calculations suggest that the iron centers in both <b>1</b> and <b>2</b> are partially reduced at the expense of partial
oxidation of the bdt ligand. Ligand protonation interrupts this extensive
electronic delocalization between the Fe and bdt making <b>1(H)<sup>+</sup></b> susceptible to external CO binding
A Nickel Phosphine Complex as a Fast and Efficient Hydrogen Production Catalyst
Here
we report the electrocatalytic reduction of protons to hydrogen
by a novel S<sub>2</sub>P<sub>2</sub> coordinated nickel complex,
[Ni(bdt)(dppf)] (bdt = 1,2-benzenedithiolate, dppf = 1,1′-bis(diphenylphosphino)ferrocene).
The catalysis is fast and efficient with a turnover frequency of 1240
s<sup>–1</sup> and an overpotential of only 265 mV for half
activity at low acid concentrations. Furthermore, catalysis is possible
using a weak acid, and the complex is stable for at least 4 h in acidic
solution. Calculations of the system carried out at the density functional
level of theory (DFT) are consistent with a mechanism for catalysis
in which both protonations take place at the nickel center
CO<sub>2</sub> Preactivation in Photoinduced Reduction via Surface Functionalization of TiO<sub>2</sub> Nanoparticles
Salicylate and salicylic acid derivatives act as electron
donors
via charge-transfer complexes when adsorbed on semiconducting surfaces.
When photoexcited, charge is injected into the conduction band directly
from their highest occupied molecular orbital (HOMO) without needing
mediation by the lowest unoccupied molecular orbital (LUMO). In this
study, we successfully induce the chemical participation of carbon
dioxide in a charge transfer state using 3-aminosalicylic acid (3ASA).
We determine the geometry of CO<sub>2</sub> using a combination of
ultraviolet–visible spectroscopy (UV–vis), surface enhanced
Raman scattering (SERS), <sup>13</sup>C NMR, and electron paramagnetic
resonance (EPR). We find CO<sub>2</sub> binds on Ti sites in a carbonate
form and discern via EPR a surface Ti-centered radical in the vicinity
of CO<sub>2</sub>, suggesting successful charge transfer from the
sensitizer to the neighboring site of CO<sub>2</sub>. This study opens
the possibility of analyzing the structural and electronic properties
of the anchoring sites for CO<sub>2</sub> on semiconducting surfaces
and proposes a set of tools and experiments to do so