Article pubs.acs.org/JPCC
Retention of Bond Direction in Surface Reaction: A Comparative Study of Variously Aligned p‑Dihalobenzenes on Cu(110) Lydie Leung,† Tingbin Lim,† Zhanyu Ning,† John C. Polanyi,*,† Wei Ji,‡ and Chen-Guang Wang†,‡ †
Lash Miller Chemical Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada ‡ Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-Nano Devices, Renmin University of China, Beijing 100872, China S Supporting Information *
ABSTRACT: Previous studies indicated that the reagent bond direction of a bond being broken in surface reaction dominated the subsequent product recoil direction. Here we test this in an STM study of the electron-induced bond breaking for three clearly different alignments of each of two dihalobenzene reactions on Cu(110). A strong correlation was observed between the physisorbed adsorbate bond direction and the subsequent recoil direction of the chemisorbed halogen-atom product. The correlation was also evident in the theoretical modeling for the case of variously aligned diiodobenzene. The theory employed the impulsive two-state (I2S) approach to compute the reaction dynamics following electron attachment. This showed that the correlation between the prior bond direction and the subsequent product angular distribution was due to the directionality of the antibonding repulsion responsible for extending the molecule’s carbon−halogen bond, en route to reaction. Retention of bond direction in reaction dominated the effect of differing roughness of the surface along markedly different crystal axes.
■
INTRODUCTION The study of reaction dynamics, the motion of atoms and molecules in chemical reaction, was initiated half a century ago with the investigation of angular scattering in crossed molecular beams of reagents.1 Since then the field has moved to the study of reactive scattering at defined crystalline surfaces.2 The systems presently under study are today more complex, involving both an adsorbate and an underlying slab. The dynamical problem is nonetheless tractable, owing to the newly found ability to observe adsorbed molecules at surfaces individually by scanning tunnelling microscopy (STM)3 together with the power of molecular dynamics (MD) theory enabling calculations of the motions of the hundreds of atoms in the adsorbate plus substrate. The present work addresses the basic question of whether a simple constant of motion can be used to illuminate the complex process of surface reaction. Earlier work had suggested that the direction of the bond being broken, with respect to the underlying surface, would provide a guide to the directionality of the ensuing reaction. Evidence was found for this retention of direction in reaction for individual cases of both thermal4 and electron-induced2,5,6 surface reactions. Here we provide a more stringent test by examining, for two different surface reactions, the effect on the observed product distributions of a systematic variation between three different reagent alignments that are attainable for each of these reactions. The finding in every case is that the observed initial bond directions provide a useful guide to the product angular distributions. This finding is © 2015 American Chemical Society
shown to conform to the expectation from MD computations, employing an impulsive model of electron-induced reaction taking place across a multilayer slab corresponding in composition to the substrate.7−12 The concept of the retention of bond direction in going from a directed bond in a physisorbed reagent to formation of a chemisorbed reaction product is examined here. This correlation extended beyond retention of alignment to include retention of orientation, the direction of the terminal atom or group. These correlations were described in the earlier work as constituting “linear” or “vectorial” reaction dynamics.2,4−6 Retention of reagent conformation has also been reported for the case of the electron-induced dissociation of CH3SSCH3 on Au(111), in which the chemisorbed product CH3S retained its alignment.13 Studies of the electron-induced reaction of pdiiodobenzene (pDIB),7,14 p-dichlorobenzene (pDCB),8 and odiiodobenzene (oDIB)9 on Cu(110) have given several examples of the preferential formation of atomic product along the extension of the direction of the prior bond. This directionality applies markedly to the I atom and to a lesser extent to the polyatomic fragment which can be deflect by rotation. The degree to which retention of directionality constitutes a guide to the reaction dynamics can be expected to depend on Received: September 21, 2015 Revised: October 22, 2015 Published: October 23, 2015 26038
DOI: 10.1021/acs.jpcc.5b09211 J. Phys. Chem. C 2015, 119, 26038−26045
Article
The Journal of Physical Chemistry C
anionic state of duration t* simulated by a pseudopotential approach.22,25,26 The molecular dynamics (MD) in the anionic state were calculated until t* whereupon the system was returned to the ground state with atomic momenta and positions taken from the anionic excited state and continued until the final state was reached. The dynamics were calculated for motion across the Cu slab of 160 atoms in which the top three of the five layers were permitted to move. The MD calculations were run while conserving the number of atoms, the volume of the system, and the total energy. A time step of 0.5 fs was used throughout the dynamical calculations.
the roughness of the surface across which recoil and chemisorption take place. For an anisotropic crystal face such as Cu(110), this roughness will vary with the surface direction in which the products are scattered. In the present study we are able to determine the effect of changing surface direction in the applicability of the vectorial model. We find, for three markedly different alignments of each of the reagent molecules, pDIB and pDCB, that the concept of direction retention provides a valuable guide to the dynamics, despite change in surface roughness with recoil direction. This work is the first to allow a systematic test of the concept, owing to the availability of single reagents in multiple in-plane surface alignments, ranging from across the copper rows, to diagonally, and along the rows. The test was applied in each case, pDIB and pDCB, to the analysis of the initial electron-induced reaction, namely, the breaking of the first carbon−halogen bond. In each case the products were found to recoil preferentially along the direction of the bond being broken, indicating that the correlation is a strong one.
■
RESULTS AND DISCUSSION In this section we describe the three alignments observed for each physisorbed reagent molecule, p-diiodobenzene (pDIB) and p-dichlorobenzene (pDCB), and report the dynamics obtained by STM for the first bond breaking in each case. The three different molecular alignments observed for physisorbed pDIB and pDCB, on Cu(110) at 4.6 K, are evident in the STM images of Figure 1. The most common alignment had its C−X axis approximately along the [001]
■
METHODS Experimental Section. The experiments were performed in a UHV low-temperature STM (Omicron) with a base pressure
