Photodissociation and formation of an ion-pair in CH2 FCl (HCFC-31)

Although CH2 FCl (HCFC-31) recently became of great atmospheric importance, studies concerning its excited states are almost nonexistent. Several excited singlet states were studied (valence nσ* and Rydberg n3s, n3p, σ3s, and σ3p) through highly correlated multireference configuration interaction with singles and doubles, including extensivity correction. Comparison with the states of CH3 Cl indicates a strong influence of the F atom. Potential energy curves suggest formation of an electrostatically bound complex that relaxes to a hydrogen-bonded contact ion-pair (HBCIP) which can decay yielding CH2 F + Cl or to the ground state minimum of CH2 FCl. The HBCIP has a dipole moment of 9.57 D, a CI wavefunction described as 0.65ionic + 0.20biradical and it is strongly bonded by 4.72 eV. Its H bond has characteristics of moderate and strong H bonds. The simulated absorption spectrum confirms the nσ* assignment for the first and suggests the n3s + n3pσ assignment for the second band.

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

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.

Photochemistry of CH3Cl: Dissociation and CH···Cl Hydrogen Bond Formation

State-of-the-art electronic structure calculations (MR-CISD) are used to map five different dissociation channels of CH3Cl along the C-Cl coordinate: (i) CH3(X̃(2)A2″) + Cl((2)P), (ii) CH3(3s(2)A1') + Cl((2)P), (iii) CH3(+)((1)A1') + Cl(-)((1)S), (iv) CH3(3p(2)E') + Cl((2)P), and (v) CH3(3p(2)A2″) + Cl((2)P). By the first time these latter four dissociation channels, accessible upon VUV absorption, are described. The corresponding dissociation limits, obtained at the MR-CISD+Q level, are 3.70, 9.50, 10.08, 10.76, and 11.01 eV. The first channel can be accessed through nσ* and n3s states, while the second channel can be accessed through n(e)3s, n(e)3p(σ), and σ3s states. The third channel, corresponding to the CH3(+) + Cl(-) ion-pair, is accessed through n(e)3p(e) states. The fourth is accessed through n(e)3p(e), n(e)3p(σ), and σ3p(σ), while the fifth through σ3p(e) and σ(CH)σ* states. The population of the diverse channels is controlled by two geometrical spots, where intersections between multiple states allow a cascade of nonadiabatic events. The ion-pair dissociation occurs through formation of CH3(+)···Cl(-)and H2CH(+)···Cl(-) intermediate complexes bound by 3.69 and 4.65 eV. The enhanced stability of the H2CH(+)···Cl(-) complex is due to a CH···Cl hydrogen bond. A time-resolved spectroscopic setup is proposed to detect those complexes.