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via Decoded Science : Some chemical reactions seem to occur within a blink of an eye, while others take place over long time frames.

The rate (speed) at which a reaction proceeds is fundamental to all chemistry, but particularly in the areas of kinetics and chemical dynamics.

As sub-disciplines of physical chemistry, kinetics and chemical dynamics are concerned with the measurement of reaction rates, and most notably, for controlling the end products of a reaction.

The Simplest Characterization of a Reaction: Change

In simple terms, a chemical reaction may occur when we observe a change in a substance. In everyday observations, we may see this when rust forms on an iron surface, or skin becomes burnt from overexposure to the sun. We do not notice all chemical changes immediately; chemical change also occurs over longer spans of time.

Write the reaction in a chemical shorthand (known as a chemical equation) to see exactly what is reacting:

A + B -> C

This chemical equation tells us that putting A and B together gives C. However, this equation gives no clues as to how A and B managed to come into contact, how long they were in contact, or any other information surrounding the reaction.

Chemical Reactivity: Kinetics, Transition States, and Dynamics

The next step in understanding chemical reactions is the concept of molecular collisions. Molecules react and products result when there are enough molecules of the right types, and they meet sufficient localized conditions. For example, scientists have found that applying heat, increasing pressures in the vessel, or reducing vessel volumes can accelerate reactivity.

When reactions are viewed from left to right in the diagram, the reactant molecules must (metaphorically speaking) climb a barrier to eventually become the product molecules. The top of the barrier is [AB]* known as transition from reactants to products. Femtochemistry utilizes transition states to execute reactions with pulsed laser light. Copyright image by John A. Jaksich, all rights reserved.

From the turn 0f the 20th century to present times, physical chemists experimented with different reactions in the hope of simplifying how chemical reactions take place.
Many chemists, by the end of the 20th century, questioned whether reaction transition states truly existed–or whether reactions took place in a concerted manner. (Concerted reactions have no true Transition State: the breaking and forming of chemical bonds in a reaction do not possess a Transition State.)

While scientists recognized that they could accelerate reactions through an increase of temperature, physical chemists turned to the molecules, themselves, for a deeper understanding. Researchers wanted to manipulate individual molecules in real time.

Physical chemist, Ahmed Zewail and co-workers tweaked individual bonds in the molecules–and Zewail received the 1999 Nobel prize in Chemistry for his work. By doing so, science realized a fundamental understanding of how reactions take place.

Utilizing pulses of energetic light (laser light), the researchers focused the pulses to initiate and drive reactions to a Transition State and observe them to completion.

Thus it may be simply envisioned as follows:

A + B —> [AB]* —> C.

However, as always, the devil is in the details – Zewail and co-workers initiated the sequence of reactions in the following manner for the break-up of cyclobutane to two molecules of ethylene gas (One molecule produces two):

Reaction dynamics are illustrated by this break-up of cyclobutane to two molecules of ethylene gas. The transition state, butylide di-radical, is shown to exist from Zewail’s work. This transition state existed for approximately 500 -700 femtoseconds (One femtosecond equals one quadrillionth of a second.) during experimentation. The alternate mechanism consisted of a concerted bond-breaking-forming mechanism without the Transition State. The concerted mechanism was ruled out since could it not be readily observed in experimentation. Copyright image by John A. Jaksich, all rights reserved.

Do All Reactions Proceed Via a Transition State?

Do all reactions proceed via a Transition State species that is measurable? Since Zewail’s breakthrough in the late 20th century, physical chemists are utilizing shorter time sequences and enhanced molecular species for probing. To do so, they are attempting attosecond (One attosecond is equal to one quintillionth of a second) laser pulses– the newer technique is 1000 times more sensitive than the Femtosecond laser. The molecular species are energized to the excited state (known as a Rydberg state) prior to losing or gaining an electron in the bond formation process.

Although it is not completely clear that all reactions proceed via a definable Transition State, a newer generation of chemists continue the work carried out by Professor Zewail.


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