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