Table 4.

Geometric and electronic structural parameters for the transition state between 1a and 2a (TS¹) and for the transition state between 1a and 2b (TS²), as well as for the products 2a and 2b.¹

	TS1	2a2	TS2	2b
energy (kcal/mol)
ΔE‡/ΔE0	8.8	8.3	7.8	6.1
ΔH‡/ΔH0	6.1	5.3	5.4	4.6
ΔG‡/ΔG0	7.5	10.53	8.2	7.0
bond length (A)1
Fe—O(1)	2.102	2.065	2.247	2.376
O—O	1.429	1.463	1.432	1.470
C2—O(2)	1.753	1.518	1.812	1.505
Fe—O(C1)	2.112	2.137	2.070	2.050
Fe—O(C2)	2.011	1.990	2.016	2.000
C1—O	1.281	1.279	1.279	1.276
C2—O	1.331	1.370	1.321	1.361
C1—C2	1.504	1.531	1.499	1.531
C2(deviation from ring)	0.261	0.380	0.239	0.385
hydrogen bonds (Å)1
O(1)-H	1.414	1.523	1.059	1.049
O(1)-N(His200)	2.556	2.621	2.606	2.642
spin density1
Fe	4.02	4.03	4.01	3.98
O(1)	0.03	0.06	0.00	0.01
O(2)	−0.11	−0.02	−0.13	−0.02
C6(4NC)	−0.31	−0.43	−0.30	−0.36

¹The numbering of atoms (the numbers in parentheses in the left column) is as shown in Figure 8.

²Single-point energy using ε = 4.0 with thermal corrections with ε = 20.0 were performed.

³2a in ε = 20.0 has a small entropy (390.0 cal/mol K) relative to TS¹ at ε = 4.0 (402.5 cal/mol K), because the number of imaginary modes irrelevant to the reaction barrier, resulting from the steric constraint imposed on the geometry optimization, in TS¹ is less than in 2a. As a result, this entropy effect lowers TS¹ more than 2a with respect of Gibbs free energy. This entropy effect arises from a tight electrostatic interaction between protonated H200 and the bare anionic O₂ moiety, and because of this we consider enthalpy in energetic comparisons involving 2a below.