Central-barrier dynamics in HCl dissociative chemisorption on Pt(111): Transition state bond elongation as a geometric indicator of vibrational efficacy.

We present the first six-dimensional quantum dynamics study of HCl dissociative chemisorption on Pt(111) based on an accurate neural network potential energy surface. The transition state H-Cl bond is stretched by Δr = 0.40 to 2.82 bohr, significantly shorter than those for HCl on Au(111) and Ag(111). This places HCl/Pt(111) in a distinct central barrier regime, leading to two atypical outcomes. First, unlike on Au(111) or Ag(111), where the top site is two orders of magnitude less reactive, the dissociation probability at the top site on Pt(111) is comparable to that at other sites. Second, translational energy is consistently more effective than vibrational excitation, with vibrational efficacy η < 1 for v = 1 and 2 across the entire total energy range. In contrast, rotational effects remain unaffected, showing the same helicopter preference and reduced rotational efficacy as on monometallic surfaces. By comparing eleven HCl + metal systems, we reveal a clear quantitative trend. When Δr is 0.40-0.50 bohr, η remains below unity. When Δr reaches 1.32 bohr, η is always above unity. This provides a geometric rationalization of Polanyi's early/late dichotomy.