Concise Physical Chemistry

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COMPUTATIONAL KINETICS 159

For example, if a bromide ion collided with an alkyl alcohol, in all likelihood it

would collide with the “wrong” part of the molecule.

Br−+CH 3 (CH 2 )nCH 2 OH→No Reaction

Only rarely would it collide with the OH group, resulting in reaction.

Br−

↙

CH 3 (CH 2 )nCH 2 OH→OH−+CH 3 (CH 2 )nCH 2 Br

The probability factorPwould be correspondingly low.

10.8 COMPUTATIONAL KINETICS

A critical factor determining the rate of a chemical reaction is the activation enthalpy,

which is the differenceaHbetween the reactant enthalpy and the relatively unstable

activated complex. We can write

aH=Hac−Hr

whereHacis the enthalpy of the activated complex andHris the enthalpy of the

reactant molecule. Hehre (2006) has computed several activation enthalpies, including

one for the isomerization of methyl isocyanide to acetonitrile:

CH 3 N C→CH 3 C N

The postulated activated intermediate is the three-membered ring

CH 3

N C

This energetic species does not lend itself to experimental study, but it can be treated

computationally. Results from some computational kinetics studies are roughly com-

parable to measured values, but this is a difficult field because computed activation

enthalpies are very sensitive to the estimated structure of the activated complex and

the rate constant is an exponential function ofaH.