Fundamental Change in the Nature of Chemical Bonding by Isotopic Substitution

Abstract

Isotope effects are important in the making and breaking of chemical bonds in chemical reactivity. Here we report on a new discovery, that isotopic substitution can fundamentally alter the nature of chemical bonding. This is established by systematic, rigorous quantum chemistry calculations of the isotopomers BrLBr, where L is an isotope of hydrogen. All the heavier isotopomers of BrHBr, BrDBr, BrTBr, and Br(4)HBr, the latter indicating the muonic He atom, the heaviest isotope of H, can only be stabilized as van der Waals bound states. In contrast, the lightest isotopomer, BrMuBr, with Mu the muonium atom, alone exhibits vibrational bonding, in accord with its possible observation in a recent experiment on the Mu+Br2 reaction. Accordingly, BrMuBr is stabilized at the saddle point of the potential energy surface due to a net decrease in vibrational zero point energy that overcompensates the increase in potential energy.

First-principles simulations of transition state spectra of the I + HI and I + DI reactions and vibrational bonding in IMuI

“…BrH + Br reaction [39] and used it successfully for simulation of some important properties of BrHBr À and BrDBr À [40]. In addition, we also found that the system has the ''vibrational-bonding'' bound state when the hydrogen atom is replaced by Mu atom (=muonium with a mass of 0.114 u) [40]. This result is qualitatively in accord with recent experimental observations [41,42].…”

“…1 and 2, the ground state potential energy surface has a relatively deep van der Waals well with a depth of 0.076 eV, corresponding to the linear IÁ Á ÁHI van der Waals complex (C 1v ). We did not find a stable bent IÁ Á ÁHI van der Waals complex on our potential energy surface in contrast with the Br + HBr case [40,49]. Similarly, both the second and third states have linear van der Waals minima but the well depth on the third state is found to comparable to that on the ground state.…”

“…5 and 6, it is seen that the IMuI system has a bound vibrational-bonding state while the other four isotopomers show only van der Waals bonding character, as in Ref. [40] for BrLBr. Also shown in Fig.…”

“…Recently, we have developed an accurate ab initio-level global potential energy surface for the Br + HBr ? BrH + Br reaction [39] and used it successfully for simulation of some important properties of BrHBr À and BrDBr À [40]. In addition, we also found that the system has the ''vibrational-bonding'' bound state when the hydrogen atom is replaced by Mu atom (=muonium with a mass of 0.114 u) [40].…”

“…The details of our quantum calculations are also described in Refs. [40,43]. Electronically nonadiabatic effects have been completely ignored in this work.…”