Surprising Reactivities in the Gas Phase

Starting in the 1960s, a technique called ion cyclotron resonance (ICR) mass spectrometry, developed hy John Brauman (1937- ) at Stanford University, facilitated measurement of the products formed from gas-phase reactions between molecules and ions. Jesse L. Beauehamp (1942- ), at Caltech, was an early investigator of this new gas-phase chemistry. In contrast to the solution-phase chemistry, in the gas phase toluene is a stronger acid than methanol  

CH30 (gas) + C6H5CH3 (gas) CH3OH (gas) + CelljCHj (gas) Strong Base Strong Acid Weak Acid Weak Base 

Is there something strange or more complex about the new gas-phase chemistry The answer is no. To the contrary, gas-phase chemistry is much simpler to understand than the solution chemistry that has been the foundation of decades (indeed centuries) of chemical laboratory practice. The classical textbook explanation above considers the two acids and two bases only and mentions nothing of the solvent that surrounds them. In the gas phase there really are no solvent molecules and the acidity of any HA derives from the sum of the energies of just three processes  

The reaction coordinate for a typical (solution-phase) 5 2 reaction is depicted in chapter 4 CH3CI and OH react in a single step forming a transient activated complex that instantaneously transforms to the CH3OH and Cl products. In 1977, John Brauman discovered that reactions of this type behave very differently in the gas phase, without a 

Although these examples of gas-phase chemistry might appear to be esoteric, the ability to separate intrinsic reactivities of molecules and ions from the influences of solvents is fundamental to understanding chemistry. It may also be relevant for chemistry in interstellar space. 

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