Photochemistry of Monohydrated Chloromethane: Formation of Free and Hydrated Cl- and CH3+ Ions from a Solvent-Shared Semi-Ion-Pair

Mariana G Bezerra; Ezequiel F V Leitão; Railton B de Andrade; Elizete Ventura; Silmar A do Monte

doi:10.1021/acs.jpca.1c05704

Photochemistry of Monohydrated Chloromethane: Formation of Free and Hydrated Cl^- and CH₃⁺ Ions from a Solvent-Shared Semi-Ion-Pair

J Phys Chem A. 2021 Oct 7;125(39):8603-8614. doi: 10.1021/acs.jpca.1c05704. Epub 2021 Sep 28.

Authors

Mariana G Bezerra¹, Ezequiel F V Leitão², Railton B de Andrade¹, Elizete Ventura¹, Silmar A do Monte¹

Affiliations

¹ Departamento de Química, CCEN, Universidade Federal da Paraíba, João Pessoa, PB 58-059-900, Brazil.
² Unidade Acadêmica de Ciências Exatas e da Natureza, Universidade Federal de Campina Grande, Cajazeiras, PB 58900-000, Brazil.

PMID: 34582197
DOI: 10.1021/acs.jpca.1c05704

Abstract

The effect of water molecule on the excited states of CH₃Cl(H₂O), as compared to those of the isolated chloromethane, has been studied at the multireference configuration interaction with singles and doubles (MR-CISD), including extensivity corrections. Eight new Rydberg states are due to the water molecule but the common states of both systems are not severely altered. Potential energy curves of 23 singlet states along the C-Cl coordinate have also been computed at the MR-CISD level. The dissociation energy of the C-Cl bond decreases from ∼0.4 to 0.5 eV due to the water molecule. As for CH₃Cl (de Medeiros, V. C., J. Am. Chem. Soc. 2016, 138, 272-280), a stable ion-pair has also been characterized. However, for CH₃Cl(H₂O), this ion-pair is better described as a solvent-shared semi-ion-pair, CH₃^+δ(H₂O)Cl^-δ. This species is connected with three ionic dissociation channels, with two being due to the water molecule. The presence of these new ionic channels, particularly the lowest energy one, [H₃C-O]⁺ + Cl^-, raises a very important question of atmospheric relevance: can the interaction of chloroalkanes with water decrease its deleterious effect on the ozone layer? Several potentially new competing dissociation channels are also studied. The latter results can help to set up the most important states to be included in nonadiabatic dynamic calculations to study how the yields of the ionic channels change due to the water molecule.