smiles:CC1=NC2=C(N1C)C(=O)N(C(=O)N2)C

Same atoms, joined differently. Every one of these is a separate compound with its own boiling point, smell and reactions. These are the ones PubChem has on file, which for a larger formula is fewer than the number that could exist on paper.

• Caffeine
• Enprofylline
• Isophthalyl dihydrazide
• Isocaffeine
• 2,4-Dicyano-3-methylglutaramide
• 3,9-Dihydro-1,3,8-trimethyl-1H-purine-2,6-dione
• 1-Propylxanthine
• Terephthalohydrazide
• 1-Ethyl-3-methyl-3,7-dihydro-purine-2,6-dione
• 2,4,6-Trimethyl-2,3,5,6-tetraazabicyclo[5.2.0]nona-1(7),3-diene-8,9-dione
• 1-(4-Pyridoyl)-4-methylsemicarbazide
• Isophthalamidoxime
• N1,N4-Dihydroxyterephthalimidamide
• 3-Amino-5-carbamimidamidobenzoic acid
• N-(6-acetamidopyrimidin-4-yl)acetamide
• 4-(furan-2-ylmethylamino)-2,5-dihydro-1H-1,3,5-triazin-6-one
• 3-Ethyl-1-methyl-3,7-dihydro-1H-purine-2,6-dione
• 2-(N'-hydroxycarbamimidoyl)-N-(pyridin-2-yl)acetamide
• 1-hydroxy-2-[(E)-(4-hydroxyphenyl)methylideneamino]guanidine
• 1-hydroxy-2-[(E)-(2-hydroxyphenyl)methylideneamino]guanidine
• 4,7-Dimethyl-1,4,6,7-tetrahydroimidazo[4,5-E][1,4]diazepine-5,8-dione
• 5-ethyl-7-methyl-1H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione
• 1,4-Dimethyl-4,5,7,8-tetrahydroimidazo-[4,5-E]-1,4-diazepin-5,8(6H)-dione
• 1-hydroxy-3-[(E)-(6-methyl-2-pyridinyl)methylideneamino]urea
• N-(4-(Hydrazinecarbonyl)pyridin-2-yl)acetamide
• 1-Methyl-3-(pyridine-3-carbonylamino)urea
• Cyclo(-Gly-L-His)
• 3-Isopropyl-3,7-dihydro-purine-2,6-dione
• 7-Propylxanthine
• 9-Propylxanthine
• 4-Amino-6-methyl-7,8-dihydropyrimido[5,4-f][1,4]oxazepin-5-one
• 1,2-Hydrazinedicarboxamide, N-phenyl-
• 3-Amino-1-ethylpyrazolo[4,5-c]pyridine-4,6-diol
• [4-(Carbamoylamino)phenyl]urea
• 2-(Cyanoimino)-N-formyl-1-pyrrolidineacetamide
• 1,2-Propanedione, 1-(4-imino-1,4-dihydro-1-pyridyl)-, dioxime
• 6-(3-Ethyl-1,2,4-oxadiazol-5-yl)-2,3,4,5-tetrahydropyridazin-3-one
• 4,5-dimethyl-4H,7H-(1,2,4)triazolo(1,5-a)pyrimidine-6-carboxylic acid
• 5-(methoxymethyl)-2-methyl[1,2,4]triazolo[1,5-a]pyrimidin-7(4H)-one
• 2-Amino-6,7-dimethyl-3,7-dihydropyrimido[4,5-b][1,4]oxazin-4-one
• 1-(2-hydroxyethyl)-6-methyl-2H-pyrazolo[3,4-d]pyrimidin-4-one
• 3-ethyl-7-methyl-1H-pyrazolo[1,5-a][1,3,5]triazine-2,4-dione
• Phthalohydrazide
• 6-Amino-1,3-dimethylpyrrolo[3,4-d]pyrimidine-2,4-dione
• 1,8,7-Trimethylxanthin
• 3,5,7-trimethyl-2H-pyrazolo[3,4-d]pyrimidine-4,6-dione
• 3-(Propan-2-yl)-2H-pyrazolo[4,3-d]pyrimidine-5,7(4H,6H)-dione
• 9-(2-Hydroxyethyl)-2-methyl-9H-purin-6-ol