Conspectus
Molecular chirality has been of scientific interest
since 1848
when Pasteur demonstrated its direct connection to the rotation of
light by solutions of chiral compounds. In the 1960s the connection
was made between the chirality of pharmaceutical compounds and their
physiological impact; one enantiomer can be therapeutic while the
other is toxic. That realization prompted enormous effort in the synthesis
of enantiomerically pure compounds for bioactive use (a $300B/yr market).
Until relatively recently, metals were ignored as potential substrates
for asymmetric surface chemistry because metals have highly symmetric, achiral bulk structures and the premise was that they could
not expose chiral surfaces. In 1996, we demonstrated that the high
Miller index surfaces of metals can be chiral, existing in two enantiomeric
forms M(hkl)
R&S
, and we hypothesized that they exhibit
enantiospecific interactions with chiral adsorbates. Most such intrinsically
chiral metal surfaces have ideal structural motifs based on low Miller
index terraces separated by kinked monatomic steps. This Account begins
with a short tutorial on the ideal and real structures of chiral metal
surfaces to provide a firm basis for understanding the origin of their
chirality. It then chronicles the evolution of our understanding of
their enantiospecific interactions with chiral adsorbates.
Detecting,
quantifying, and understanding enantiospecific surface
chemistry on intrinsically chiral metal surfaces has been far more
challenging than coming to the realization that such surfaces exist.
The first successes came from measurements and modeling of the enantiospecific
adsorption energetics of small chiral molecules such as propylene
oxide and trans-1,2-dimethylcyclopropane. These revealed
one of the core challenges to observing enantiospecificity, the fact
that the enantiospecificities of reaction energetics and barriers
tend to be small, i.e., a few kJ/mol. Measurements of the enantiospecific
adsorption energetics of R-3-methylcyclohexanone
on seven different Cu(hkl)
R&S
surfaces demonstrated their sensitivity
to surface structure, but again revealed variations of only a few
kJ/mol. One of the most important advances in our understanding of
chiral surface chemistry is that the limitations imposed by weakly
enantiospecific interactions can be circumvented by processes with
nonlinear kinetics or equilibria. As an example, the surface explosion
mechanism of d- and l-tartaric acid decomposition
on Cu(hkl)
R&S
surfaces leads to enantiospecific rates that differ by almost
2 orders of magnitude, in spite of the fact that the rate constants
are only weakly enantiospecific. More surprising is the observation
that equilibrium adsorption of nonracemic mixtures of d-
and l-aspartic acid can lead to autoamplification of enantiomeric
excess, even on achiral Cu(111) surfaces. Again, this arises from
a nonlinear adsorption isotherm. Most recently, we have developed
a high throughput method for identification of the most enantiospecific
surface orientation...