Computational Chemistry

Reference [ 197 ] gives the 0 K G2 value by the atomization method as#195.7 kJ
mol#^1 and the experimental value as (from two sources)#190.7 or#189.8 kJ mol#^1.
To correct the 0 K heat of formation to that at 298.15 K we add the increase in
enthalpy of methanol on going from 0 to 298 K and subtract the corresponding
increases for the elements in their standard states. The value for methanol is the
difference of two quantities provided in the thermochemical summary at the end of
the G2 calculation as implemented in Gaussian 94 and subsequent versions:

DDH-ðCH 3 OHÞ¼G2 Enthalpy#G2ð0KÞ

¼# 115 : 53061 #ð# 115 : 53490 Þh

¼ 0 : 00429 ' 2625 :5 kJ mol#^1

¼ 11 :26 kJ mol#^1

(G2(0 K) is the G2 value for what we have calledDEtotal0K)
The experimental enthalpy increases for the elements are given in [ 197 ]; in
kJ mol#^1 :
DDH-(element)

C(graphite) 1.050

H 2 8.468

O 2 8.680

From these andDDHf-ðCH 3 OHÞ:

DHf298- ðCH 3 OHÞ¼DH-f0ðCH 3 OHÞþDDH-ðCH 3 OHÞ#ðDDH-ðCÞ

þ 2 DDH-ðH 2 Þþ

1

2

DDH-ðO 2 ÞÞð 5 : 192 Þ

¼# 195 : 7 þ 11 : 26 # 1 : 050 þ 28 ðÞþ: 468

1

2

ðÞ 8 : 680 kJ mol#^1



¼# 195 : 7 # 11 :07 kJ mol#^1 ¼# 206 :8 kJ mol#^1

The accepted 298 K experimental value [ 199 ] is# 205 )10 kJ mol#^1.
Note that ifDHf0-is not wanted,DH-f298can be calculated directly, since from
Eqs. (5.190) and (5.192) the 0 K ab initio energy of the compound is subtracted out
and it follows that

DH-f298ðCH 3 OHÞ¼DH-f0ðCÞþ 4 DH-f0ðHÞþDHf0-ðOÞ

# DEtotal0KðCÞþ 4 DEtotal0K ðHÞþDEtotal0KðOÞ

)*

þG2 Enthalpy

#ðDDH-ðCÞþ 2 DDH-ðH 2 Þþ

1

2

DDH-ðO 2 ÞÞð 5 : 193 Þ

5.5 Applications of the Ab initio Method 317