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.

Can a gas phase contact ion pair containing a hydrocarbon carbocation be formed in the ground state?

So far, no conclusive evidence of a ground-state contact ion-pair containing a hydrocarbon carbocation has been given in the gas phase. Due to the very high stability of the 1,2:4,5-dibenzotropylium (or dibenzo[a,d]tropylium) carbocation, we suggest (supported by DFT and MP2 calculations) the formation of a contact ion pair between this carbocation and chloride, occurring during the reaction between 1,2:4,5-dibenzotropyl (also named dibenzo[a,d]tropyl or dibenzo[a,d]cycloheptenyl) radical and chlorine atom at very low temperatures, through the harpoon mechanism. This is the first modeling study to find computational evidence for the possibility of a gas-phase contact ion pair (containing a hydrocarbon carbocation) formed in the ground state. Identification of this metastable species can be carried out by trapping it in He nanodroplets, along with infrared laser spectroscopy routinely coupled with this technique.

Photochemistry of CF3Cl: Quenching of Charged Fragments Is Caused by Nonadiabatic Effects.

For the first time, high-level multireference electronic structure calculations have been performed to study the photochemistry of CF3Cl, allowing a comprehensive interpretation and assignment of experimental data concerning fluorescence, ion-pair formation, and generation of CF3 fragments in several electronic states. All studied dissociation channels correlate either with Cl or Cl- in the ground state. On the other hand, a CF3 fragment can be generated either in the ground or excited state. A rationalization for the nonadiabatic relaxation of CF3Cl, including the formation of an (n4s) stable state and internal conversion at multiple-state intersections, has been provided. Our results explain the anomalous quenching of a charged fragment after low-energy excitation, a fact experimentally observed by separate groups. We show that the CF3+···Cl- ion pair undergoes an internal conversion to the ground state, producing neutral CF3 and Cl fragments. The results also allow understanding as to why CF3Cl is usually a nonemitting species and how UV emission could be induced.

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.

Ab initio and DFT conformational study on N-nitrosodiethylamine, (C2H5)2N-N=O.

Ab initio (MP2) and DFT (B3LYP) calculations, using the cc-pVTZ and aug-cc-pVTZ basis sets, have been performed to characterize some stationary points on the ground state potential energy surface of the title molecules. Several properties as, for instance, relative energies, the barriers for NO rotation around the NN bond, NBO charges on O and amino N atoms, as well as the dipole moments, have been calculated and analyzed in the light of the structures found. Both computational levels here employed yield three minima, in which the C(2)NNO frame is 'planar' or 'quasi-planar'. Important correlations between NBO charges and geometric parameters, as well as between some structural features and dipole moments are also discussed. A total of 17 structures have been found for the (C(2)H(5))(2)N-N=O molecule. Two ranges of values have been obtained for the dipole moment, with the largest values occurring for the structures in which the nitrogen lone pair is parallel to the NO group π system. For instance, these two ranges are from ~4.1 to 4.5 D, and from ~1.6 to 2.1 D, at the MP2/cc-pVTZ level. These ranges are consistent with a larger and a smaller contribution of a dipolar resonance structure, respectively. As the method or basis set changes the values of the dipole moments change by at most ~0.23 D.