(641f) Competing Forces in Chiral Catalytic Surface Chemistry: Enantiospecificity Versus Enantiomer Disproportionation

Equilibrium
adsorption of gas phase mixtures of D-
and L-alanine (Ala), serine (Ser),
phenylalanine (Phe) and aspartic acid (Asp) onto the naturally chiral Cu{3,1,17}^R&S
and achiral Cu(111) and Cu(100) surfaces has been studied by both experiment
and DFT-based modeling.  Isotopically labelled *L-amino acids (HO₂¹³CCH(NH₂)-R) and
unlabeled D-amino acids allow mass
spectrometric enantiodifferentiation of the adsorbed species during temperature
programmed decomposition, following equilibrium adsorption.  Measurements of
the relative equilibrium coverages of D-
and *L-amino acids on the Cu{3,1,17}^R&S
surfaces, q_D/R /q_*L/R = q_*L/S
/q_D/S, at gas phase partial pressure ratios of P_*L/P_D
= 0.5, 1, and 2 indicate that the D-Ala
and *L-Ala conglomerate phases are more
energetically stable than a D*L-Ala racemate phase, but that their adsorption
energies are not measurably enantiospecific, DDE_DL » 0.  In contrast,
the adsorption of D-Asp and *L-Asp on the Cu{3,1,17}^R&S surfaces
exhibits measurable enantiospecificity, DDE_DL » 3 kJ/mole.  The
surprising observation is that even on achiral surfaces such as Cu{111},
exposure to non-racemic mixtures of D-Asp
and *L-Asp in the gas phase, P_*L/P_D
= 0.5 or 2, can result in dramatic enhancement of enantiomeric excess on the surface,
q_D /q_*L = 0.06 and 16, respectively.

In general,
we show that the enantiospecific adsorption of chiral molecules on chiral
surfaces is dictated by two competing forces the enantiospecificity of
adsorption energetics and the propensity of enantiomers to disproportionate
into homochiral (conglomerate) or heterochiral (racemate) clusters.  These
phenomena have been studied by measuring the surface enantiomeric excess, ee_s, of mixtures of chiral amino acids adsorbed on Cu
surfaces and in equilibrium with gas phase mixtures of varying enantiomeric
excess, ee_g.  Ala adsorption
on Cu{3,1,17}^R&S surfaces
is non-enantiospecific, ee_s =
ee_g, because Ala enantiomers
do not interact with either the surface or one another enantiospecifically.  Asp
adsorbs enantiospecifically on the Cu{3,1,17}^R&S surfaces; ee_s ¹ ee_g, even during
exposure to a  racemic mixture in the gas phase, ee_g = 0.  Exposure of the achiral Cu{111} surface to
non-racemic Asp, ee_g ¹ 0, results in local amplification of enantiomeric excess, |ee_s|>
|ee_g|, as a result of
homochiral disproportionation.  Finally, in spite of the fact that the Cu{653}^R&S surface are chiral, the adsorption of Asp mixtures
is dominated by homochiral disproportionation of adsorbed enantiomers rather
than enantiospecific adsorbate-surface interactions, |ee_s|>
|ee_g|.  All of these types of
behavior are captured by a Langmuir-like adsorption isotherm that also
describes competition between enantiospecific adsorption and both homochiral
(conglomerate) and heterochiral (racemate) clustering of adsorbed molecules
(Figure).

Figure. The surface enantiomeric excess, ee_s, versus gas phase enantiomeric excess, ee_g, for adsorption of enantiomer mixture under
conditions in which the dominant surface interactions vary from disproportionation
into conglomerate clusters, x = 0, to disproportionation
into racemate clusters, x = 1.