Identifying Molecular Species on Surfaces by Scanning Tunneling Microscopy: Methyl Pyruvate on Pd(111)

Abstract

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 cis-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 cis-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.

Enhanced hydrogenation activity and diastereomeric interactions of methyl pyruvate co-adsorbed with R-1-(1-naphthyl)ethylamine on Pd(111)

“…All the most stable complexes ( Figs 5 and 6 ) bind to the enol MP tautomer in a cis configuration 38 , with the C=C double bond located at Pd–Pd bridge sites. The methylene hydrogens are distorted from a planar geometry as found for ethylene on Pd(111) 54 .…”

“…Van der Waals corrections were included by using the method of Tkatchenko and Scheffler 51 . STM topography simulations were performed with the Tersoff-Hamman 52 approach as implemented in bSKAN 3.7, which has been found to reproduce the shapes of experimental STM images for MP on Pd(111) 38 as well as those obtained using the more rigorous Bardeen approach 71 .…”

“…NEA has also been found to be randomly distributed on surfaces thus implying that NEA should act as a one-to-one modifier to provide a chiral reaction site 30 31 . The chemistry of the simplest α -ketoester, MP has been explored on several transition-metal surfaces 33 34 35 36 37 38 where it exists as both the enol and keto tautomer (depicted in Supplementary Fig. 4 ) 36 38 and the energetics on Pd(111) have been discussed in detail 37 .…”

“…The chemistry of the simplest α -ketoester, MP has been explored on several transition-metal surfaces 33 34 35 36 37 38 where it exists as both the enol and keto tautomer (depicted in Supplementary Fig. 4 ) 36 38 and the energetics on Pd(111) have been discussed in detail 37 . However, MP tends to agglomerate on platinum, the most commonly used catalyst for the enantioselective hydrogenation of MP (ref.…”

Chirally-modified metal surfaces: energetics of interaction with chiral molecules

“…9 Molecular chirality has also attracted much interest in surface science research, 10,11 where adsorption of chiral molecules on welldefined single-crystal surfaces and their interaction with other adsorbates have been extensively studied using a variety of surface sensitive techniques. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] A recent review by Gellman, Tysoe and Zaera summarizes the latest report on chiral surface chemistry performed on model single crystalline surfaces. 28 As one of the major results of these studies, the authors put forward the idea that enantioselectivity arises from the energetic differences in the interaction of two opposite enantiomers with a chiral surface.…”