The Importance of Motions that Accompany Those Occurring Along the Reaction Coordinate

Abstract

The reaction coordinate is a well known quantity used to define the motions critical to chemical reactions, but many other motions always accompany it. These other motions are typically ignored but this is not always possible. Sometimes it is not even clear as to which motions comprise the reaction coordinate: spectral measurements that one may assume are dominated by the reaction coordinate could instead be dominated by the accompanying modes. Examples of different scenarios are considered. The assignment of the visible absorption spectrum of chlorophyll-a was debated for 50 years, with profound consequences for the understanding of how light energy is transported and harvested in natural and artificial solar-energy devices. We recently introduced a new, comprehensive, assignment, the centrepiece of which was determination of the reaction coordinate for an unrecognized photochemical process. The notion that spectroscopy and reactivity are so closely connected comes directly from Hush's adiabatic theory of electron-transfer reactions. Its basic ideas are reviewed, similarities to traditional chemical theories drawn, key analytical results described, and the importance of the accompanying modes stressed. Also highlighted are recent advances that allow this theory to be applied to general transformations including isomerization processes, hybridization, aromaticity, hydrogen bonding, and understanding why the properties of first-row molecules such as NH 3 (bond angle 1088) are so different to those of PH 3 -BiH 3 (bond angles 90-938). Historically, the question of what is the reaction coordinate and what is just an accompanying motion has not commonly been at the forefront of attention. In our new approach in which all chemical processes are described using the same core theory, this question becomes thrust forward as always being the most important qualitative feature to determine. Chemists mostly consider reactions using a single nuclear coordinate, the reaction coordinate.[1] A simple example of this is apparent in the London-Eyring-Polanyi-Sato [2] (LEPS) potentialenergy surface for the chlorine atom-exchange reaction [3] Clthat is shown in Fig. 1a as a function of the bond lengths R ab and R bc . The reaction coordinate indicates the lowest-energy path that connects reactants to products and is shown in blue in the figure. When a reaction occurs, molecular geometries change holistically, and this coordinate in general is therefore complex, embodying what happens to all atoms including atoms in any solvent or surrounding medium. All other motions are thought to smoothly and perhaps even instantaneously adjust to motion along the reaction coordinate. Fig. 1b displays the energy profile [3] for the chlorine atom-exchange reaction as a function of the reaction coordinate, showing how the energy increases from that at the floor of the reactant valley as the approaching atom gets closer to the molecule, going through a maximum at the reaction's transition state (TS). However, Fig. 1c shows how the distance R ac bet...

Relating transition-state spectroscopy to standard chemical spectroscopic processes

“…In its simplest analytical form as applied here, adiabatic electron-transfer theory assumes parabolic surfaces and includes only one type of nuclear motion, motion along the reaction coordinate. These assumptions lead to three limitations in its applicability to transition-state spectroscopy: i) often motions other than the reaction coordinate are critical to observed phenomena [4,[72][73][74], including continuous motions like solvent friction [75]; ii) conical intersections intrinsically involve at least two nuclear coordinates; and iii) many reactions of interest involve bimolecular processes that cannot easily be discussed using harmonic potentials. However, the basic model does provide an underlying basis for critical aspects of these more complicated phenomena.…”

Diabatic models with transferrable parameters for generalized chemical reactions

“…The critical feature is the universality of the model parameters for the qualitative description of system properties. Simple models can also be extended to universal quantitative ones provided that full quantum solutions [26,50], sufficient interfering processes [43,[51][52], and motions orthogonal to the reaction coordinate are also included [27,[53][54]. The assignment [55] of the Q-band spectrum of chlorophyll-a, arguably the world's most important chromophore whose properties display a strong pseudo-Jahn-Teller effect, following 50 years of intense debate, provides another example of the power of this approach.…”

Putting David Craig’s Legacy to Work in Nanotechnology and Biotechnology

“…Reproduced from ref. [178], with permission of CSIRO Publishing. in 1950 demonstrated this effect for benzene. [13] Modern nanotechnological applications to which electron correlation is critical will be discussed also in later sections.…”

“…[107,108] This allowed the bacterial reaction centre cation to be understood completely, [99,102] facilitated assignment of the spectrum of Chl-a, [47] and led to the understanding of a range of phenomena where properties were determined by more than just the reaction coordinate, as has been recently reviewed. [178] It unified critical aspects of the works of Craig and of Hush, playing on the non-uniqueness of diabatic descriptions for chemical and spectroscopic phenomena. [179] Unification of basic theories for chemistry and spectroscopy touches the very foundations of chemical understanding.…”