The kinetic and thermodynamic properties of the concerted double proton transfer reaction in GC: the reaction energy, ΔG, the respective forward and reverse barrier heights, ΔG^‡_f and ΔG^‡_r (kcal mol⁻¹), and the equilibrium constant, K. Mean values are calculated from the QM/MM-ensemble using the B3LYP-XDM/aug-cc-pvdz/AMBER method at 300 K. The respective forward and reverse rate coefficients k_f and k_r (s⁻¹), and the half-life (t_1/2) of the G*C* tautomer (s) are calculated using only the replicas from the QM/MM ensemble that have positive reverse Gibbs energy barriers (E_−x, 7 out of 17 replicas; E₀, 2 out of 3 replicas; and E_+x, 0 out of 1 replica). The applied field strengths are at 1.00 × 10⁹ V m⁻¹. The standard deviation is denoted by σ (where σ = ‘—’, the sample size consists of a single replica).

Concerted DPT	ΔG‡f		ΔG‡r		ΔG		K × 10−9		t 1/2 × 10−12		k f × 104		k r × 1012
	Mean	σ	Mean	σ	Mean	σ	Mean	σ	Mean	σ	Mean	σ	Mean	σ
E −x	12.79	1.18	0.38	1.08	12.42	1.52	26.2	68.3	1.42	1.68	6.81	11.2	2.41	2.07
E 0	12.46	1.86	1.01	1.45	11.44	0.61	7.99	8.09	4.57	6.29	0.53	0.71	2.75	3.78
E +x	11.33	—	−0.86	—	12.19	—	1.32	—	—	—	—	—	—	—