Physical Chemistry Third Edition

13.6 Experimental Molecular Study of Chemical Reaction Mechanisms 613

05

Emission

intensity

v = 3→ 2

transitions

10

t/ms

15 20

P 3 → 2 (3)

P 3 → 2 (4)

P 3 → 2 (5)

P 3 → 2 (6)

P 3 → 2 (7)

05

v = 2→ 1

10

t/ms

15 20

P 2 → 1 (3)

P 2 → 1 (4)

P 2 → 1 (5)

P 2 → 1 (6)

P 2 → 1 (7)

P 2 → 1 (8)

P 2 → 1 (9)

05

v = 1→ 0

10

t/ms

15 20

P 1 → 0 (3)

P 1 → 0 (4)

P 1 → 0 (5)

P 1 → 0 (6)

P 1 → 0 (7)

P 1 → 0 (8)

P 1 → 0 (9)

P 1 → 0 (10)

Figure 13.23 Intensity of Laser Pulses as a Function of Time for Excited HF

Molecules.In each plot, the vertical axis represents the intensity of a laser pulse emit-

ted by HF formed in a flash-initiated reaction. The horizontal axis represents time mea-

sured in microseconds. Each curve is labeled with both the initial and the final values of

the vibrational quantum numberv. The number in parentheses is the final value ofJ, the

rotational quantum number (the initial value ofJis smaller than this value by unity). From

R. D. Levine and R. B. Bernstein,Molecular Reaction Dynamics and Chemical Reactivity,

Oxford University Press, New York, 1987, p. 213.

The second reaction in the mechanism of Eq. (13.6-3) has been thoroughly studied:^41

F+H 2 −→H+HF (13.6-4)

The molar energy change of the reaction from ground state to ground state is∆E 0 

−122 kJ mol−^1. The molar activation energy∆E 0 is equal to 6.7 kJ mol−^1. The small-

ness of the activation energy correlates with an activated complex that is similar in struc-

ture to the reactants, in agreement withHammond’s postulate, which states that “the

more exoergic the reaction, the more the transition state will resemble the reagents.”

Levine and Bernstein reviewed the experimental and theoretical study of this reac-

tion up to the time of publication of their book. The first investigation was a chemical

(^41) R. D. Levine and R. B. Bernstein,Molecular Reaction Dynamics and Chemical Reactivity, Oxford
University Press, New York, 1987, pp. 306ff, 396ff.