2 research outputs found
Identifying Molecular Species on Surfaces by Scanning Tunneling Microscopy: Methyl Pyruvate on Pd(111)
The structures of low coverages of
methyl pyruvate on a Pd(111)
surface at 120 K were studied using scanning tunneling microscopy
in ultrahigh vacuum. The experimentally observed images were assigned
to adsorbate structures using a combination of density functional
theory calculations and by simulating the images using the Bardeen
method. Two forms of methyl pyruvate were identified. The first, previously
found using reflection–absorption infrared spectroscopy, was
a flat-lying, keto form of <i>cis</i>-methyl pyruvate. It
was characterized by elongated, two-lobed images with the long axes
of the images oriented at ∼0 and ∼30° to the close-packed
directions. The structure was simulated using clean, CO- and methyl-functionalized
gold tips, and the simulated images agreed well with those found experimentally.
The simulated structures were not strongly dependent on the tip structure
or tip bias. This approach was used to identify the nature of the
second species as the enol form of <i>cis</i>-methyl pyruvate
with the carbonyl groups located over atop and bridge sites. Again,
the orientation of the image with respect to the underlying Pd(111)
lattice as well as the calculated image shape agreed well with the
experimental images
Formation of Chiral Self-Assembled Structures of Amino Acids on Transition-Metal Surfaces: Alanine on Pd(111)
The structure and self-assembly of
alanine on Pd(111) is explored using X-ray photoelectron spectroscopy
(XPS), low-energy electron diffraction (LEED), reflection–absorption
infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM),
and supplemented by density functional theory (DFT) calculations to
explore the stability of the proposed surface structures formed by
adsorbing alanine on Pd(111) and to simulate the STM images. Both
zwitterionic and anionic species are detected using RAIRS and XPS,
while DFT calculations indicate that isolated anionic alanine is significantly
more stable than the zwitterion. This observation is rationalized
by observing dimeric species when alanine is dosed at ∼270
K and then cooled to trap metastable surface structures. The dimers
form due to an interaction between the carboxylate group of anionic
alanine with the NH<sub>3</sub><sup>+</sup> group of the zwitterion.
Adsorbing alanine at 290 K results in the formation of dimer rows
and tetramers resulting in only short-range order, consistent with
the lack of additional diffraction spots in LEED. The stability of
various structures is explored using DFT, and the simulated STM images
are compared with experiment. This enables the dimer rows to be assigned
to the assembly of anionic-zwitterionic dimers and the tetramer to
the assembly of two dimers in which three of the alanine molecules
undergo a concerted rotation by 30°