new index atom name original index 1 C 1 2 C 2 3 C 3 4 C 4 5 C 8 6 C 5 7 C 6 8 C 7 9 Cl 12 10 N 9 11 N 10 12 O 11 13 H 13 14 H 15 15 H 14 16 H 16 17 H 17 18 H 18 19 H 19 20 H 20 21 H 21