Table 1. Level Splittings and the Separate Contributions from Asymmetry, d, and Tunneling, ℏΩ, All in cm^–1a.

	DFT(B3LYP) PES			Δ-ML PES
isotopomer	d	ℏΩ	Δ	d	ℏΩ	Δ	ΔΔ-MLcorr	Δexpt
parent	0	1.33	2.67	0	0.34	0.68	0.92	0.9735,36
13C14	0	1.33	2.66	0	0.34	0.67	0.91	0.9736
18O218O3	0	1.27	2.54	0	0.32	0.63	0.86	0.87,41 0.8343
D1	0	0.082	0.16	0	0.015	0.031	0.042	0.05136,43
18O218O3 D1	0	0.078	0.15	0	0.014	0.028	0.038	0.043 (estimated)43
18O2	1.02	1.28	3.27	0.89	0.32	1.89	1.98	1.70 (estimated)41,44
13C4	–0.55	1.32	2.86	–0.62	0.33	1.41	1.54	0.89 (outlier)36
13C6	0.36	1.33	2.75	0.34	0.34	0.95	1.14	1.1336
13C8	0.10	1.33	2.67	0.11	0.34	0.71	0.94	0.9836
13C8 D1	0.10	0.082	0.26	0.042	0.015	0.091	0.095

^aThe subscript numbers for the isotopomers indicate the atom(s) that were isotopically substituted, using the numbering from Figure 1, whereas the superscript numbers indicate the substituted atomic mass. Δ_Δ-ML^corr corresponds to the splittings corrected with MTA-CCSD(T) points along the instanton path.