Computational Chemistry

an occupied orbital and in the other (EA) addition of an electron to a virtual (or a
half-occupied) orbital. The IE for an orbital is defined as the energy needed to
remove an electron from the orbital to infinite separation, while the EA of an orbital
is the energy released when the orbital accepts an electron from infinity [ 310 ].
These quantifies are commonly given in electron volts; 1 eV¼96.485 kJ mol–1¼
0.03675 hartrees, 1 h¼27.212 eV. A typical IE for an organic molecule is 8–9 eV
(e.g. benzene 9.24 eV), which is ca. 800 kJ mol#^1 or about twice the energy of a

C

C C

H H

H H

O

210.8

199.8

(206.0)

29.1

28.1

(30.6)

2.08

2.06

2.00

1.96

(2.05)

H

7.72

7.56

(7.37)

134.9

127.3

(130.9)

C

H 2 C CH 2

H H

0.14

0.32

(0.22)

• 4.2
  –1.8
  (–2.9)

H

H

H

H

H

7.57 131.0

139.6

0.21

1.03

24.3

0.31

–1.74

25.8

[7]paracyclophane

Only the HF / 6-31G*

chemical shifts are shown

H C H

H

H

–1.65

0.92

(–2.3)

0.46 6-311+ G(2d,p)

0.55 6-31G*

(ca. 0.9 experimental)

H H

Fig. 5.44 Calculated and experimental^1 H and^13 C NMR spectra: chemical shifts relative to TMS
H and C, respectively. The calculations were done on the B3LYP/6–31G geometry (B3LYP is a
density functional method;Chapter 7) at the HF/6–311+G(2d,p) and HF/6–31G levels using the
default NMR method (GIAO) implemented in Gaussian 94W [ 198 ]. The experimental values are
from ref. [ 235 ], except for the values for [7]paracyclophane [ 307 ]. The larger basis set may be
somewhat more accurate but takes longer. Compare withChapter 7, Fig. 7.9

362 5 Ab initio Calculations