Table 1
PM3 Heats of formation (kCal/mol)
2
5
6
7
8
9
10
11
12
X=N
66.7
80.9
33.8
85.3
42.6
85.8
43.6
88.3
40.9 
X=O
36.2
43.3
1.2
44.7
7.4
35.9
16.2
48.7
6.6
X=S
74.6
85.1
44.3
86.3
47.9
57.2
49.6
89.7
49.2