Computational Chemistry

Cl– C Cl

H

H H

Cl–

r = rC – Cl – rT = 0

TS

C

H

H H

rT

Cl Cl

0

100

50

r = rC–Cl – rT

–5 –4 –3 –2 –1 0 12345

Cl C

H

H

H

rC–Cl

TS

rT = 2.426 Å

r = 0

E = 101 kJ mol–1

E (kJ mol–1)

relative to E at rC–Cl = 25 Å, r = 22.574Å

r = rC – Cl – rT < 0 r = rC–Cl – rT > 0

Fig. 8.2 Profile for the SN2 reaction Cl"þCH 3 Cl in water. Calculations by the author using
B3LYP/6-31þG* with the continuum solvent method SM8 [ 22 ] as implemented in Spartan [ 31 ].
Note thatris the distance of the Cl"from the transition state bond length (2.426 A ̊), not the Cl"/C
distance; thusrmeasures the “deviation” from the transition state and becomes zero at the
transition state. This makes the graph symmetrical about the energy axis, as it should be presented
for this identity reaction. The zero of energy is taken asrC–Cl¼25 A ̊,r¼22.574 A ̊

Table 8.1 Some of the data used to construct Fig.8.2. Variation of energy with Cl-/C

distance for the SN2 reaction Cl"+ CH 3 Cl in water. Calculations by the author using

B3LYP/6-31+G* with the continuum solvent method SM8 [ 22 ] as implemented in

Spartan [ 31 ]. Therof the x-axis in Fig.8.2isrC–Cl"r(transition state)¼rC–Cl"

2.426. Hartrees were converted to kJ mol"^1 by multiplying by 2,626

rC–ClA ̊ rA ̊ SM8 E Relative E

Hartrees kJ mol"^1

25 22.574 "960.51789 0

5 2.574 "960.51663 3.3

4 1.574 "960.51495 7.7

3 0.574 "960.50604 31.1

2.5 0.074 "960.48412 88.6

2.426, transition state 0 "960.47955 101

528 8 Some “Special” Topics