Experiment

Experiments were performed on a Cu surface held at 4.6 K. Two physisorbed states of mDIB were observed: one termed ‘Row’28 and the other ‘Diagonal’, the latter being the principal subject of the present report. An STM image of the Diagonal adsorption state on Cu(110) is shown in Fig. 1a, with a theoretical simulation directly below.

Figure 1: Bond-selective electron-induced reaction of physisorbed mDIB on Cu(110). STM images (EXPT) and simulations (TH) show the selective dissociation of the C-I bond. (a) An intact physisorbed mDIB. The white cross indicates the midpoint of mDIB in the STM image. The two C-I bond directions, ‘AL’ and ‘AC’, are given by blue lines and designated as C-I(AL) and C-I(AC). (b) A reactive outcome as a result of the breaking of C-I(AL) in the prior mDIB reagent. (c) A reactive outcome for the single observed case of mDIB breaking C-I(AC). In the pictured final states (PIC), the green dashed circle indicates the Cu atom to which the IPh was bound, and the blue dashed line the direction of the broken C-I bond. All experimental STM images shown here are 25 × 17 Å2 in size, recorded at a sample bias of +0.1 V and a tunnelling current of 0.1 nA. Full size image

From the simulation, the benzene ring is located at a short-bridge site with one C-I directed along the Cu row and the other at 126° from the Cu row (measured clockwise from [1 0]), leading to the two lobes that were observed, corresponding to the two C-I bonds with I atoms located atop Cu atoms, as evident in the simulated STM image. The bond lengths of both C-I bonds in the physisorbed state were computed as ∼2.1 Å, and the heat of adsorption as 1.0 eV. This Diagonal adsorption state was also predicted by a computational study performed by Panosetti and Hofer29. The midpoint between the two lobes in Fig. 1a, EXPT, is indicated by a white cross used as the origin of the spatial distribution of the products following electron-induced reaction; see Fig. 1a–c.

Electron-induced reaction of physisorbed mDIB was initiated by tunnelling electrons from the STM tip placed over the centre of the intact physisorbed reagent at a sample bias ≥+1.0 V. The experimental findings are illustrated in Fig. 1, with, at left, sample STM images for the initial and final states of the major reaction path breaking the C-I bond AL the Cu rows, and, at the right, the minor path breaking the C-I bond directed AC the Cu rows. For 139 reactive cases, the breaking of only one of the two C-I bonds was observed, namely, the AL bond of the physisorbed mDIB, in preference to the AC bond. The bond selectivity amounted to a 99.3% probability for C-I(AL), as against 0.7% for the alternate C-I(AC).

A representative case of C-I(AL) bond breaking is shown in Fig. 1b. The reaction products are a chemisorbed I atom and a chemisorbed iodophenyl (IPh). A comparison between experiment (Fig. 1b, EXPT) and theory (Fig. 1b, TH and PIC) allowed us to identify the binding sites of both products with respect to the prior mDIB. The I atom was found at the closest four-fold hollow site adjacent to the initial I atom position in the reagent. The IPh product was bound to the atop Cu atom underneath the reagent mDIB (green dashed circle, Fig. 1b, PIC). The I atom and IPh were found to recoil in opposite directions AL the Cu row (Supplementary Fig. 1a and Supplementary Note 1), as expected for the dissociation of C-I(AL) aligned along the row. The most probable recoil distance of the I atom measured from the white cross of Fig. 1 was 4.7 Å. Figure 1c and Supplementary Fig. 1b show the minor path consisting of 1 out of 139 cases. In this case, the products were found to recoil in the opposite direction AC the rows (Supplementary Note 1), from which we conclude that the bond dissociated was C-I(AC).

The observed strong preference for the breaking of C-I(AL) rather than C-I(AC) shows that the electron-induced reaction of the Diagonal physisorbed mDIB is markedly bond-selective. As the reaction of Row physisorbed mDIB did not exhibit any detectable bond selectivity between its symmetrical C-I bonds28, the bond-selective reaction of Diagonal is attributed to its asymmetric adsorption geometry. This reactive bond selectivity will be examined below by the MD theory.

The number of electrons involved in triggering a reactive event was determined experimentally for the major path by measuring the average reaction rate as a function of the tunnelling current in the range of 0.6–18 nA, at a constant sample bias of +1.3 V. As shown in Fig. 2a, the reaction rate scaled linearly with the tunnelling current, evidencing a one-electron process. This linear relationship also excludes an electric field effect as a major cause of reaction30.

Figure 2: Evidence of electron-induced process. (a) Rate versus current plot (in log scale) for the electron-induced reaction of physisorbed mDIB on Cu(110). The plot is shown for C-I(AL) reaction (major path) with a linear fit (red line) of the data. The slope of the linear fit was determined to be 1.1±0.2. The good quality of the fit was evidenced by the coefficient of determination (R2) of 0.927. Reaction rates were measured at +1.3 V. The error bar is the standard error from the exponential fitting of the data at each current (see Methods for details). (b) Projected-density-of-states (pDOS) calculation for the physisorbed mDIB. The pDOS shows the LUMO of mDIB to be 0.8 eV above the Fermi level. The pDOS of the mDIB molecule is given by the black line; the dashed and dotted lines give the pDOS of the I atoms and the C atoms next to the I atoms. The inset in panel b visualizes the LUMO of mDIB (isocontour=0.0005 e Å−3). The nodal planes between the C and I atoms show the LUMO to be of σ*(C-I) antibonding character. Full size image

As in previous studies of the electron-induced reaction of aryl iodides by ∼1 eV electrons on Cu(110)26,27,28,31,32, we ascribe the reaction to adsorbate electronic excitation. The computed projected density of states is shown in Fig. 2b. It exhibits a nodal plane between carbon and iodine atoms for the lowest unoccupied molecular orbital (LUMO), indicative of a σ* antibonding character with respect to the C-I bond. This C-I antibonding orbital was computed to be 0.8 eV above the Fermi level. The yield for electrons of ∼1.3 eV was measured as ∼10−9 reactive events per electron for the major path. The high single-electron energy of at least 1.0 eV required to give reaction argues against vibrational excitation as the source of induced reaction.

Theory

The observed bond selectivity can be understood in terms of the different recoil directions of the AL and AC fragments following an electron-induced repulsive impulse in the LUMO of one or the other C-I bond, leading to different reaction paths across the anisotropic Cu(110) surface. As in previous work25,26,27,28,31,32 we used the ‘Impulsive Two-State’ (I2S) model to simulate the MD of the electron-induced reaction. In this two-electronic state model, the MD was first followed for the 192-atom system on an approximate anionic repulsive potential-energy surface (PES) obtained by the transfer of an electron to the valence shell of a halogen atom for a period of femtoseconds (a time t*). Thereafter, the atoms were returned with their accumulated momenta to the ab initio DFT ground PES for MD over a period of picoseconds needed to reach the reacted final states. In the present case, the added charge comprises one electron at the I atom of C-I(AL) to favour breaking of that bond, or alternatively one electron at the I atom of C-I(AC) to favour the alternate bond breaking.

Figure 3a shows the dynamics for an electron added at C-I(AL), using the minimum t* of 20 fs required for reaction. Figure 3a shows that the impulse, with momenta carried over to the ground PES, stretched C-I(AL) causing it to break at ∼150 fs concurrently with the formation of the C-Cu (i.e., IPh-Cu) and I-Cu bonds. The distance versus time plot of Fig. 3a′ in the panel below shows bond extension of C-I(AL) from its initial separation of 2.1 to a 3.0 Å, accompanied by the formation of C-Cu and I-Cu bonds at their equilibrium separations (all equilibrium separations being indicated by horizontal dotted lines). Figure 3b shows that for the same t* the placing of one electron in the I atom of the alternate C-I(AC) bond did not break that C-I bond. This failure to break C-I(AC), as evident from the distance versus time plot of Fig. 3b′, is connected with the larger separation between the C atom of C-I(AC) from its nearest Cu atom neighbour (C-Cu is 2.3 Å in the AL case, 3.0 Å for AC). We show below that this difference in C-Cu separation is significant for the extent of C-Cu binding energy.

Figure 3: Computed trajectories for the bond-selective reaction of mDIB using the system’s 192 atoms. The trajectories, calculated using the I2S model with t*=20 fs, give the time evolution of the system to its final state. (a) Trajectory for one electron placed at the I atom of C-I(AL); the green circle indicates the nearest Cu atom to the C atom of C-I(AL). (b) Trajectory for one electron placed at the I atom of C-I(AC). The yellow highlights indicate that the system is in the anionic state. In (a′) and (b′) below, the time evolution of the C-I, I-Cu and C-Cu separations are shown as black, blue and green lines, respectively, for each trajectory, AL and AC. The C-Cu bond separation is measured from the Cu atom nearest to the carbon of the C-I bond: C-I(AL) in (a′) and C-I(AC) in (b′). The horizontal dotted lines give the equilibrium separations of the atomic pairs, as indicated. I2S, Impulsive Two-State. Full size image

In Fig. 4, we superimpose the MD taken from a full calculation of the motion of the 192 atoms onto a two-dimensional cut through the potential-energy hypersurface that gives the dependence of the potential energy on the two C-I separations. In drawing this PES we have kept the angle between the two C-I bonds fixed at its initial 120°. This constant angle is a good approximation to the PES governing the initial reaction dynamics, since the motion calculated for the full hypersurface shows the angle between the two C-I bonds decreasing by only 10° in the first ∼200 fs, during which C-I(AL) extends by 1.0 Å. On this restricted PES, the energy barrier to dissociate C-I(AL) is shown to be 0.7 eV for that coordinate, while the barrier to dissociate C-I(AC) is markedly higher for the alternate bond extension (the crest of the barrier along C-I(AC) is at 1.5 eV, lying beyond the range of Fig. 4).

Figure 4: Trajectories using the I2S model superimposed on a restricted ground PES. The restricted cut through the ground potential-energy hypersurface was obtained by varying only the two C-I bond distances at a fixed bond angle of 120°. The trajectory from the I2S model was based on a full molecular dynamics calculation using all 192 atoms in the system. (a) The trajectory for one electron added to the I atom of the AL bond, C-I(AL). (b) The trajectory for one electron added to the I atom of the AC bond, C-I(AC). The energy values shown are relative to the initial state (IS). The yellow arrow in the trajectories encompass the 20 fs duration of the repulsion on the anionic PES; the red line indicates the subsequent ∼3 ps motion on the ground PES. I2S, Impulsive Two-State. Full size image

We attribute the lower barrier along C-I(AL) to the greater C-Cu binding, and hence greater stabilization along this coordinate than along C-I(AC) (see Fig. 3a′ which show the C-Cu bond forming AL, but not AC). The extent of stabilization due to C-Cu binding can be estimated from the Bader version of the bond-length versus bond-order (n) relationship for atomic pairs33, which gives n=0.5 for C-Cu at the transition state (C-I(AL)=3.0 Å) in Fig. 4a as compared with n=0.3 for extension of the other coordinate, C-I(AC), to 3.0 Å. For a computed ∼3.0 eV bond energy of C-Cu, this leads to ∼0.6 eV greater stabilization due to C-Cu binding in the AL coordinate than the AC coordinate. Accordingly, the lower barrier for the C-I(AL) (0.7 eV) than C-I(AC) (>1.2 eV) in Fig. 4 can be ascribed to stronger C-Cu binding and hence greater stabilization AL rather than AC. This finding shows that the binding of the products to the surface, late in the course of reaction, can influence reaction barriers with resultant bond selectivity.

The trajectories shown in Fig. 4 are taken from the motion on the full potential, with the two C-I bond separations plotted here on the ‘fixed bond-angle’ PES whose energy contours are shown. Bond selectivity is caused by the impulsive motion on the anionic PES, which impels the system on the ground PES to stretch C-I(AL) giving surface reaction over a 0.7 eV barrier along r 1 , but does not stretch C-I(AC) sufficiently to surmount the higher barrier along r 2 . The low barrier for C-I(AL), as indicated above, is due to the proximity of an underlying Cu atom along the row direction, stabilizing the system through C-Cu bond formation.

It should be noted that the bond selectivity being reported is specific to the impulsive dynamics (see Fig. 4) being a consequence of the directed recoil of the atoms liberated in electron-induced reaction. The C-I bond breaking could also in principle be obtained thermally, although this was not studied here. We have, however, calculated, ab initio, the different energy barriers to reaction for the individual breaking of the two C-I bonds by C-I extension along the alternative minimum-energy paths of the ground PES. The computed barrier heights relevant to thermal reaction were found to be 0.3 eV for the previously favoured AL C-I and 0.2 eV for the previously disfavoured AC C-I. Thermal reaction is therefore predicted to exhibit a modest bond selectivity but in the opposite sense to that observed and modelled here for electron-induced reaction.