From photoelectron detachment spectra of BrHBr−, BrDBr− and IHI−, IDI− to vibrational bonding of BrMuBr and IMuI

Abstract

Photoelectron detachment XLX(-)(00(0)0) + hν → XLX(vib) + e(-) + KER (X = Br or I, L = H or D) at sufficiently low temperatures photoionizes linear dihalogen anions XLX(-) in the vibrational ground state (v1v2 (l)v3 = 00(0)0) and prepares the neutral radicals XLX(vib) in vibrational states (vib). At the same time, part of the photon energy (hν) is converted into kinetic energy release (KER) of the electron [R. B. Metz, S. E. Bradforth, and D. M. Neumark, Adv. Chem. Phys. 81, 1 (1992)]. The process may be described approximately in terms of a Franck-Condon type transfer of the vibrational wavefunction representing XLX(-)(00(0)0) from the domain close to the minimum of its potential energy surface (PES) to the domain close to the linear transition state of the PES of the neutral XLX. As a consequence, prominent peaks of the photoelectron detachment spectra (pds) correlate with the vibrational energies EXLX,vib of states XLX(vib) which are centered at linear transition state. The corresponding vibrational quantum numbers may be labeled vib = (v1v2 (l)v3) = (00(0)v3). Accordingly, the related most prominent peaks in the pds are labeled v3. We construct a model PES which mimics the "true" PES in the domain of transition state such that it supports vibrational states with energies EXLX,pds,00(0)v3 close to the peaks of the pds labeled v3 = 0, 2, and 4. Subsequently, the same model PES is also used to calculate approximate values of the energies EXMuX,00(0)0 of the isotopomers XMuX(00(0)0). For the heavy isotopomers XHX and XDX, it turns out that all energies EXLX,00(0)v3 are above the threshold for dissociation, which means that all heavy XLX(00(0)v3) with wavefunctions centered at the transition state are unstable resonances with finite lifetimes. Turning the table, bound states of the heavy XLX are van der Waals (vdW) bonded. In contrast, the energies EXMuX,00(0)0 of the light isotopomers XMuX(00(0)0) are below the threshold for dissociation, with wavefunctions centered at the transition state. This means that XMuX(00(0)0) are vibrationally bonded. This implies a fundamental change of the nature of chemical bonding, from vdW bonding of the heavy XHX, XDX to vibrational bonding of XMuX. For BrMuBr, the present results derived from experimental pds of BrHBr(-) and BrDBr(-) confirm the recent discovery of vibrational bonding based on quantum chemical ab initio calculations [D. G. Fleming, J. Manz, K. Sato, and T. Takayanagi, Angew. Chem., Int. Ed. 53, 13706 (2014)]. The extension from BrLBr to ILI means the discovery of a new example of vibrational bonding. These empirical results for the vibrational bonding of IMuI, derived from the photoelectron spectra of IHI(-) and IDI(-), are supported by ab initio simulations of the spectra and of the wavefunction representing vibrational bonding of IMuI.

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

“…Very recently, we have developed the ground-state potential energy surface for the I + HI reaction including the spin-orbit interaction and used it to simulate the photodetachment spectra of the IHI À and IDI À anions as well as the vibrational bonding phenomenon in IMuI [43]. We found that the simulated spectra were generally in good agreement with the experimentally observed spectra although the agreement becomes somewhat worse at a higher energy region.…”

“…These simulated spectra were calculated from the wave packet dynamics calculations on the ground-state potential energy surface. For the calculated spectra, both high-and low-resolution (0.6 and 13 meV) spectra are shown, which are obtained from the calculated autocorrelation functions by multiplying by an appropriate Gaussian damping function exp(Àt 2 /a 2 ) [43,50]. The experimental energy resolutions are reported to be 1-2 and >8 meV for high-and low-resolution spectra, respectively [17,19,23].…”

“…In the current work, we extend the recent study of Ref. [43] and discuss the photodetachment spectra in a wide range of energy and report detailed computational results on vibrational-bonding in IMuI.…”

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

“…For the IHI -anion, electronic structure calculations were performed at the CCSD(T) level of theory with the same basis sets used in the neutral system. Quantum dynamics calculations were performed using both time-dependent and time-independent numerical techniques [43]. We used cylindrical coordinates [20] where the two I atoms are taken to lie on the z-axis with separation of R and with their midpoint being fixed at z = 0.…”