From ab Initio Potential Energy Surfaces to State-Resolved Reactivities: X + H₂O ↔ HX + OH [X = F, Cl, and O(³P)] Reactions

Abstract

We survey the recent advances in theoretical understanding of quantum state resolved dynamics, using the title reactions as examples. It is shown that the progress was made possible by major developments in two areas. First, an accurate analytical representation of many high-level ab initio points over a large configuration space can now be made with high fidelity and the necessary permutation symmetry. The resulting full-dimensional global potential energy surfaces enable dynamical calculations using either quasi-classical trajectory or more importantly quantum mechanical methods. The second advance is the development of accurate and efficient quantum dynamical methods, which are necessary for providing a reliable treatment of quantum effects in reaction dynamics such as tunneling, resonances, and zero-point energy. The powerful combination of the two advances has allowed us to achieve a quantitatively accurate characterization of the reaction dynamics, which unveiled rich dynamical features such as steric steering, strong mode specificity, and bond selectivity. The dependence of reactivity on reactant modes can be rationalized by the recently proposed sudden vector projection model, which attributes the mode specificity and bond selectivity to the coupling of reactant modes with the reaction coordinate at the relevant transition state. The deeper insights provided by these theoretical studies have advanced our understanding of reaction dynamics to a new level.

Communication: State-to-state dynamics of the Cl + H2O → HCl + OH reaction: Energy flow into reaction coordinate and transition-state control of product energy disposal

“…Much work has been done in this direction, particularly for gas-phase bimolecular reactions. [1][2][3][4][5] Sometime ago, Polanyi proposed two empirical rules concerning the mode specificity in atom-diatom reactions. 6 For a reaction with an early barrier, translational energy is more effective than the same amount of vibrational energy in surmounting the barrier and vice versa for a reaction with a late barrier.…”

“…We have recently studied the X + H 2 O → HX + OH (X = F, Cl, O( 3 P)) reactions systematically on accurate full-dimensional potential energy surfaces (PESs) and these studies have shown that the mode specificity and related bond selectivity can be largely understood in terms of the SVP model. 5 In this Communication, we address the question on how reactant excitations influence product state distributions. Such state-to-state mode specificity has seldom been addressed quantum mechanically for reactions involving polyatomic reactants, because of the difficulties associated with quantum state-to-state calculations.…”

“…This reaction and its reverse are of great importance in atmospheric chemistry 21 and have also served as a prototype for understanding mode specificity in reaction dynamics. 1,5 Experimental studies found that the excitation of the local OH bond to v OH = 4 in HOD enhances the O-H cleavage by eightfold, 22 and the released energy is mostly deposited into the internal excitation of the HCl product with internally cold OD. 23 These results suggest that the non-reactive OD moiety serves essentially as a spectator.…”

“…5 As a result, we focus on the HCl or H 2 product with the OH product in its vibrational ground state. In Fig.…”

Communication: An accurate full 15 dimensional permutationally invariant potential energy surface for the OH + CH4 → H2O + CH3 reaction

“…Despite the tremendous advances in electronic structure theory, the analytical representation of multi-dimensional global PESs based on large number of ab initio points has only recently become a reality for reactive systems beyond three atoms. [1][2][3] Indeed, several analytical functional forms with the necessary permutation invariance have been proposed to represent the complex topography of the PES. The two main forms are the permutation invariant polynomials (PIPs) 2 and neural network (NN) methods.…”