Extended Data Table 1.

Quantum chemical calculations of ELVOC cluster formation energies

Formation Gibbs free energies at 278 K, ΔG_{278 K}, for neutral, negatively charged and positively charged ELVOC clusters. The cluster processes indicate the incident E₁/E₂ vapour molecule + the target cluster. Quantum chemical calculations made at other temperatures (not shown) indicate that the binding energies strengthen by −1.0 kcal mol⁻¹ per 20 K reduction in temperature. The uncertainty in the calculated energies is less than 2 kcal mol⁻¹. Our calculations indicate the following approximate order for different functional groups to contribute to the cluster binding energies involving or H₂SO₄ (starting with the strongest): (i) carboxylic acids, R–C(=O)–OH; (ii) hydroxyls, R–OH; (iii) hydroperoxy acids, R–C(=O)–O–OH; (iv) hydroperoxides, R–O–OH; and (v) carbonyls, R–(R′–)C=O. In the case of , the main interacting group is carbonyl, independently of which other groups are attached to it; therefore ammonium will form stronger clusters with carboxylic acids, hydroperoxy acids or carbonyls than it will with hydroxyls or hydroperoxides.