Computational Chemistry

like AM1, discussed inChapter 6). Augmentation with experimental data is still
needed for accurate calculations when an ab initio calculation would involve an
extended, solid substance like graphite (see the discussion in connection with the
atomization method); other examples are phosphorus and sulfur. For estimation of
solid-state enthalpies see [ 202 ]; there has been work on the ab initio calculation of
enthalpies of sublimation [ 203 ].
Let us briefly compare the atomization calculation of the 298 K heats of for-
mation of 1,4- and 1,2-benzoquinone by the G3(MP2) and the CBS-QB3 methods.
1,4-Benzoquinone, using G3(MP2)

DH-f0(C,^3 P) 711.2 kJ mol#^1 experimental atomization E
DH-f0(H,^2 S) 216.035 kJ mol#^1 experimental atomization E
DH-f0(O,^3 P) 246.8 kJ mol#^1 experimental atomization E

DEtotal0K(C,^3 P) #37.789338 h G3(MP2) enthalpy at 0 K
DEtotal0K(H,^2 S) #0.501839 h G3(MP2) enthalpy at 0 K
DEtotal0K(O,^3 P) #74.989774 h G3(MP2) enthalpy at 0 K
H298K(1,4-BQ, G3(MP2)) #380.898776 h G3(MP2) enthalpy at 298.15 K

DDH-(C, graphite) 1.050 kJ mol#^1 experimental enthalpy increase, 0 K to 298 K
DDH-(H 2 ) 8.468 kJ mol#^1 experimental enthalpy increase, 0 K to 298 K
DDH-(O 2 ) 8.680 kJ mol#^1 experimental enthalpy increase, 0 K to 298 K

DHf298- ðÞ 1 ; 4 #BQ;G3 MP2ðÞ

¼½Š6 711ðÞþ: 2 4 216ðÞþ: 035 2 246ðÞ: 8 kJ mol#^1

#½Šþ# 6 ðÞþ# 37 : 78934 4 ðÞþ# 0 : 50184 2 ðÞ# 74 : 98977 ðÞ 380 : 89878 h

#ð 61 ðÞþ: 050 28 ðÞþ: 468 ðÞ 8 : 680 kJ mol#^1

¼ 5624 :9 kJ mol#^1 ##½Š# 378 :72294 h 380 :89878 h# 31 :92 kJ mol#^1

¼ 5624 :9 kJ mol#^1 # 2 : 17584 ' 2625 :5 kJ mol#^1 # 31 :92 kJ mol#^1

¼ 5624 : 9 # 5712 : 67 # 31 :92 kJ mol#^1 ¼# 119 :7 kJ mol#^11 ; 4 #BQ;G3 MP2ðÞ

1,4-Benzoquinone, using CBS-QB3

DH-f0(C,^3 P) 711.2 kJ mol#^1 experimental atomization E
DH-f0(H,^2 S) 216.035 kJ mol#^1 experimental atomization E
DH-f0(O,^3 P) 246.8 kJ mol#^1 experimental atomization E

DEtotal0K(C,^3 P) #37.785377 h CBS-QB3 enthalpy at 0 K
DEtotal0K(H,^2 S) #0.499818 h CBS-QB3 enthalpy at 0 K
DEtotal0K(O,^3 P) #74.987629 h CBS-QB3 enthalpy at 0 K
H298K(1,4-BQ, CBS-QB3) #380.861093 h CBS-QB3 enthalpy at 298.15 K

DDH-(C, graphite) 1.050 kJ mol#^1 experimental enthalpy increase, 0 K to 298 K
DDH-(H 2 ) 8.468 kJ mol#^1 experimental enthalpy increase, 0 K to 298 K
DDH-(O 2 ) 8.680 kJ mol#^1 experimental enthalpy increase, 0 K to 298 K

5.5 Applications of the Ab initio Method 321