Computational Chemistry

DH-f298ð 1 ; 4 #BQ;CBS#QB3Þ

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

#½Šþ# 6 ðÞþ# 37 : 785377 4 ðÞþ# 0 : 499818 2 ðÞ# 74 : 987629 ðÞ 380 : 861093 h

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

¼ 5624 :9 kJ mol#^1 ##½Š# 378 :68679 h 380 :861093 h# 31 :92 kJ mol#^1

¼ 5624 :9 kJ mol#^1 # 2 : 17430 ' 2625 :5 kJ mol#^1 # 31 :92 kJ mol#^1

¼ 5624 : 9 # 5708 : 62 # 31 :92 kJ mol#^1 ¼# 115 :6 kJ mol#^11 ; 4 #BQ,CBS-QB3

The best experimental value for the heat of formation of 1,4-benzoquinone
appears to be#122.6)3.8 kJ mol#^1 (#29.3)0.9 kcal mol#^1 ), although an
overlapping value of#115.9)12.6 kJ mol#^1 has been reported [ 204 ].
It follows from this method that to calculate the heat of formation of 1,2-
benzoquinone we now need only the value for 1,4-benzoquinone and the 298 K
enthalpies of the two quinones, since they are isomers:
For the G3(MP2) value,DH-f298(1,2-BQ, G3(MP2))¼DH-f298(heat of formation
1,4-BQ)þ[enthalpy 1,2-BQ – enthalpy 1,4-BQ]

¼# 119 : 7 þ#½Š' 380 : 886048 ##ðÞ 380 : 898776 2625 :5 kJ mol#^1

¼# 119 : 7 þ 33 :42 kJ mol#^1 ¼# 86 :3kJ mol#^11 ; 2 #BQ;G3 MP2ðÞ

For the CBS-QB3 value,DH-f298(1,2-BQ, CBS-QB3)¼DH-f298(heat of formation
1,4-BQ)þ[enthalpy 1,2-BQ – enthalpy 1,4-BQ]

¼# 115 : 6 þ#½Š' 380 : 848379 ##ðÞ 380 : 861093 2625 :5 kJ mol#^1

¼# 115 : 6 þ 33 :38 kJ mol#^1 ¼# 81 :9 kJ mol#^11 ; 2 #BQ;CBS-QB3

The best experimental value for the heat of formation of 1,2-benzoquinone
appears to be#87.9)13.0 kJ mol#^1 [ 204 ].
Both G3(MP2) and CBS-4M give reasonably satisfactory heats of formation for
these quinones by the atomization method, but it must be conceded that, in general,
an isodesmic approach with currently practical high-accuracy methods at least,
gives more accurate heats, and especially free energies, of formation; CBS-QB3
was somewhat limited in the size of molecule it could handle [ 191 ]:
Enthalpies, errors in kJ mol#^1

Atomization, G3(MP2)/CBS-QB3 9.0/8.1
Isodesmic, G3(MP2)/CBS-QB3 3.7/4.5

Free energies, errors in kJ mol#^1

Atomization, G3(MP2)/CBS-QB3 12.1/13.1
Isodesmic, G3(MP2)/CBS-QB3 4.2/5.6

322 5 Ab initio Calculations