You're asking for the 18 isomers of C12H26, not C12H18. C12H18 would be an alkene or an alkane with a ring structure, but C12H26 is the formula for dodecane, a straight-chain alkane.

Here's the breakdown:

C12H26 (Dodecane) Isomers:

1. n-Dodecane: The straight chain isomer.

2. 2-Methyl-undecane

3. 3-Methyl-undecane

4. 4-Methyl-undecane

5. 5-Methyl-undecane

6. 2,2-Dimethyl-decane

7. 3,3-Dimethyl-decane

8. 4,4-Dimethyl-decane

9. 2,3-Dimethyl-decane

10. 2,4-Dimethyl-decane

11. 2,5-Dimethyl-decane

12. 2,6-Dimethyl-decane

13. 3,4-Dimethyl-decane

14. 3,5-Dimethyl-decane

15. 3,6-Dimethyl-decane

16. 2,2,3-Trimethyl-nonane

17. 2,2,4-Trimethyl-nonane

18. 2,2,5-Trimethyl-nonane

Note: There are many more potential isomers, but these are the most common and well-known.

Important: The number of isomers increases rapidly with the number of carbon atoms in a hydrocarbon. It's crucial to understand the principles of structural isomerism and use systematic methods to identify and name isomers.