Benchmarking the polyatomic reaction dynamics of X+methane

Abstract

With recent developments of sophisticated experimental techniques and advanced theoretical methods/computations, the field of chemical dynamics has reached the point that theoryexperiment comparisons can be made at a quantitative level in very fine details for a prototypical A+BC system. As the system becomes larger, more degrees of freedom are involved and the complexity increases exponentially. At the same time, the multifaceted nature of polyatomic systems also opens up the possibilities for observing many new chemistry and novel phenomena-a land of opportunities. For the past 15 years or so my laboratory has delved into the reaction dynamics of methane+X (X: F, Cl, O(3 P), and OH). This effort shifts the paradigm in the field of reaction dynamics by making the title reaction a benchmark polyatomic system. In this account, I shall disclose my thinking behind some of the key concepts and methods we introduced and how the unexpectedly discovered phenomena led to other uncharted territories. Those findings not only enrich our understanding of the specific reactions we studied at the most fundamental level and inspire the theoretical developments, but also shape our thinking and lay the foundation for future explorations of different aspects of the multifaceted nature of polyatomic reactivity.

Multifaceted Stereoselectivity in Polyatomic Reactions

“…However, this differential stereoselectivity for different |jK⟩-state is completely washed out for the unpolarized reactants, leading to an apparent loss-of-memory phenomenon alluded to earlier. 5,6,38,39 As for the Cl + CH 4 (v 3 = 1, |101⟩) reaction (Figure 8, top), 48 its unpolarized vibrational branching is essentially identical to the other two Cl + CHD 3 reactions. Its polarized branching appears comparable to the Cl + CHD 3 (v 1 = 1, |10⟩) case (middle panel, R(0)), which can be more readily seen in Figure 10a that directly compares the ratios of [HCl(v = 1)/HCl(v = 0)] under different collision geometries of the three reactions.…”

“…The detailed dynamics of the | jK ⟩-selected reaction does depend on the collisional geometry and in a very sensitive, distinct manner with respective to the rotational identity of reactants. However, this differential stereoselectivity for different | jK ⟩-state is completely washed out for the unpolarized reactants, leading to an apparent loss-of-memory phenomenon alluded to earlier. ,,, …”

“…As a chemical reaction occurs, old bonds are broken and new bonds formed. Because of the directional properties of a chemical bond and the intermolecular interaction, this bond cleavage and formation must proceed with certain preferred geometry in accord with the transition-state structure, leading to the concepts of steric requirement and steric hindrance of chemical reactivity. − Yet, a clear understanding of the origin of this stereoselectivity has been hampered by several interwoven factors. Not only the landscapes of the potential energy surface (PES) both in the entrance valley and near the transition-state region exert profound influence on reactivity, but also the kinematic factorsthe interaction time (governed by the masses and collisional energy, E c ) and the rotational velocity of reactantscome into play in a subtle way. − To unravel the physical origin from the entangled potential and kinematic factors becomes a daunting challenge even for a simple atom + diatom (or A + BC) reaction; for example, very different mechanistic interpretations could be put forward to account for the same observation. ,− Nevertheless, several decades of studies, mostly theoretical investigations on A + BC systems, culminated in the formulation of a conceptual framework to guide our thinking .…”

Pair-Correlated Imaging of Cl + CH3D(v4, v1-I, v1-II = 1, |jK⟩) → CH2D(vi) + HCl(v)

“…Driving a chemical reaction to a specific product continues to be a central theme of chemistry. − If a chemical reaction is envisioned as the transformation from reagents to products along a minimum energy path, an intuitive way of driving the reaction to the desired product is to vibrationally excite the reagent so that its motion has a large projection along that energy path. ,,, With the advent of powerful, tunable infrared (IR) lasers and the developments of sophisticated experimental techniques as well as theoretical capability, recent years have witnessed tremendous advances in the area of vibrational control of chemical reactivity. − The concept of mode- and bond-selectivity in reactions with polyatomic reagents has also been firmly established. ,,, …”

“…Those vibrations can be as simple as a bond stretch or can be as complex as the collective motion of many atoms of the molecule, such as the bend, rock, and so on. The premise of the mode- and bond-selective concept has been, until very recently, − rooted on the amplitude of the vibrational motion, whose effects on reactivity can often, though not always, be physically understood on the ground of a classical oscillator. − However, there are other types of molecular vibrations of a very different nature, which cannot be pictured in such a way. One notable example is the so-called Fermi-coupled vibration, which occurs typically between one-quantum excitation of a stretching mode and the overtone of a bending mode.…”