HSAB principle applied to the time evolution of chemical reactions

Abstract

Time evolution of various reactivity parameters such as electronegativity, hardness, and polarizability associated with a collision process between a proton and an X- atom/ion (X = He, Li⁺, Be²⁺, B³⁺, C⁴⁺) in its ground (¹S) and excited(¹P,¹D,¹F) electronic states as well as various complexions of a two-state ensemble is studied using time-dependent and excited-state density functional theory. This collision process may be considered to be a model mimicking the actual chemical reaction between an X-atom/ion and a proton to give rise to an XH⁺ molecule. A favorable dynamical process is characterized by maximum hardness and minimum polarizability values according to the dynamical variants of the principles of maximum hardness and minimum polarizability. An electronic excitation or an increase in the excited-state contribution in a two-state ensemble makes the system softer and more polarizable, and the proton, being a hard acid, gradually prefers less to interact with X as has been discerned through the drop in maximum hardness value and the increase in the minimum polarizability value when the actual chemical process occurs. Among the noble gas elements, Xe is the most reactive. During the reaction: H₂ + H⁺ → H₃⁺ hardness maximizes and polarizability minimizes and H₂ is more reactive in its excited state. Regioselectivity of proton attack in the O-site of CO is clearly delineated wherein HOC⁺ may eventually rearrange itself to go to the thermodynamically more stable HCO⁺.