Mechanochemistry and Thermochemistry are Different: Stress-Induced Strengthening of Chemical Bonds

Most chemical reactions require activation which is conventionally supplied by heat. In stark contrast, mechanical activation by applied external forces opens intriguing novel possibilities. Here, the first direct comparison of mechanical versus thermal activation of bond breaking is provided. Studying both thiolate-copper interfaces and junctions provides evidence for vastly different reaction pathways and product classes. This is understood in terms of directional mechanical manipulation of coordination numbers and system fluctuations in the process of mechanical activation.

Figure 1

TD of the thiolated Cu surface obtained by ab initio MD. C–S distance $d_{\mathrm{C}–\mathrm{S}}$ (solid line) and average S–Cu distance ${\overline{d}}_{\mathrm{S}–\mathrm{Cu}}$ (dotted line; obtained at each time step as the average of the five shortest S–Cu distances $d_{\mathrm{S}–\mathrm{Cu}}$) are shown shortly before fragmentation. I, II, and III label the three key stages of the bond rupture process as discussed in the text. The snapshot shows the configuration at $t\approx 7.45\mathrm{ps}$ where ${\mathrm{CH}}_3\mathrm{\text{−}}{\mathrm{CH}}_2$ is about to detach while S remains on the surface (white dotted line and white arrow symbolize the broken C–S bond and the repulsive $\mathrm{H}/\mathrm{S}$ interaction as described in the text, respectively, while thicker bonds highlight the increased fivefold S–Cu coordination, compared to $n_{\mathrm{S}–\mathrm{Cu}}\approx 3$ at equilibrium).

Figure 2

Mechanically induced decomposition of the thiolated Cu surface obtained by mechanically pulling out one of the four thiolates. Vertical fragmentation energy for C–S bond breaking $E_{\mathrm{C}–\mathrm{S}}^{\mathrm{frag}}$ (solid line, left axis) and applied pulling force -$F$ (dashed line, right axis) as a function of extension $D$; use of the minus sign follows convention. The insets show representative configurations at extensions $D$ indicated by the arrows. The important bonds are highlighted by thick bonds. The reduction of the S coordination number $n_{\mathrm{S}–\mathrm{Cu}}$ from 3 to 2 to 1 is highlighted by the shaded areas.

Figure 3

Mechanically induced decomposition of the ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{S}/{\mathrm{Cu}}_9$ cluster obtained by mechanical pulling of the single thiolate. For definitions and labeling see Fig. 2.