Molecular Quantum Dynamics

Part of the series Physical Chemistry in Action pp 59-80


Tunneling in Unimolecular and Bimolecular Reactions

  • Hua GuoAffiliated withDepartment of Chemistry and Chemical Biology, University of New Mexico Email author 
  • , Jianyi MaAffiliated withDepartment of Chemistry and Chemical Biology, University of New MexicoInstitute of Atomic and Molecular Physics, Sichuan University
  • , Jun LiAffiliated withDepartment of Chemistry and Chemical Biology, University of New Mexico

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Tunneling is an important quantum phenomenon in reaction dynamics. In this chapter, the effects of tunneling on photodissociation and reactive scattering are discussed using two prototypical examples. The first deals with a unimolecular decomposition reaction, namely the photodissociation of NH3 in its first (A) absorption band and the second is concerned with an important bimolecular reaction in combustion: HO + CO → H + CO2. In the former case, the lifetimes of low-lying vibrational resonances in the predissociative excited state are influenced by tunneling through a small barrier in the dissociation (N–H) coordinate, which is also responsible for a strong H/D isotope effect. The latter, on the other hand, is affected by tunneling through a tight barrier in the exit channel primarily along the H–O dissociation coordinate, which is manifested by the non-Arrhenius rate constant at low temperatures, kinetic isotope effects, and vibrational mode selectivity. In addition, the photodetachment of HOCO produces metastable HOCO species, the decomposition of which is dominated by deep tunneling to the H + CO2 products. Since both systems are influenced by multidimensional tunneling, an accurate characterization of the dynamics requires a quantum mechanical (QM) treatment, preferably with full dimensionality. In this chapter, we review the recent advances in understanding the effects of tunneling in these two reactive systems.