1. Some Resonance Structures for H, C, N, O Isomers For quantum calculated local-minimum-energy atomic arrangements

2. H, C, N, O isomers C in middle H C N O : : : H C N O : : : H C N O : : : + _ H C N O : : : + _ charge separation

3. H C N O : : : : H C N O : : : + _ : H C N O : : + _ + _ H C N O : : : : + _ H C N O : : : bad charge sestet bad charge H, C, N, O isomers N in middle

4. H C N O : : : : + _ H C N O : : : : _ + H C N O : : : : _ + H C N O : : : : + _ sestet bad charge sestet H, C, N, O isomers O in middle

5. H C N O : : : H C N O : : + : _ H C N O : : : : sestet bad charge H, C, N, O isomers cyclic H C N O : : : + _ sestet H C N O : : : + _ : bad charge Can’t even be calculated (opens up)

6. HNCO isomers H C N O : : : H C N O : : : H C N O : : : H C N O : : : : H C N O : : : + _ H C N O : : : : + _ H C N O : : : : _ + H C N O : : + : _

7. 150 100 50 0 200 kcal/mole H C N O H C N O H C N O H C N O H C N O H C N O H C N O H C N O E calc

8. References: Most of these structures and energies were calculated at a very high level of theory as reported in Journal of Chemical Physics, 120, 11586-11599 (2004). The much less stable structures with O in the middle were calculated using the highest level of theory available in the Spartan 04 package of quantum programs. This program could not find an energy minimum for the cyclic structure with H on oxygen.