IR Spectroscopy of the Cesium Iodide-Water Complex

Structural Versatility and Energy Difference of Salt-Water Complex NaCl(H2O) Encoded in Cryogenic Photoelectron Spectroscopy

A weakly bound complex usually has multiple structural isomers with small energy differences. The sophisticated ab initio calculations are the main workhorse for providing theoretical results of different isomers. In contrast, the experimental determination of the energy difference is very rare. We report the energy-difference measurement of a model complex: salt-water complex NaCl(H₂O). We measured the energy difference among the structural isomers of the negatively charged NaCl(H₂O) complex and the neutral counterpart using cryogenic photoelectron spectroscopy. The temperature-dependent photoelectron spectra (15-300 K) revealed that the negatively charged NaCl(H₂O) and the neutral counterpart both have three isomers. The two higher-lying isomers are 186(22) and 481(48) cm^-1, respectively, above the most stable isomer for the negatively charged and 123(10) and 1821(24) cm^-1 for the neutral. These results provide a benchmark for the development of theoretic methods of weakly bound complexes. The experimental technique demonstrated here can be employed to investigate other weakly bound complexes with multiple isomers.

Stepwise dissociation of ion pairs by water molecules: cation-dependent separation mechanisms between carboxylate and alkali-earth metal ions

Microhydrated H₂-tagged ion pairs (Ca²⁺, AcO^-)(H₂O)_n=0-8 and (Ba²⁺, AcO^-)(H₂O)_n=0-5 are investigated by IR photodissociation laser spectroscopy and DFT-D frequency calculations. The detailed picture of the first steps of ion dissociation reveals two mechanisms, where water molecules promote dissociation either directly or indirectly depending on the nature of the cation.

Comparison of the Microsolvation of CaX2 (X = F, Cl, Br, I) in Water: Size-Selected Anion Photoelectron Spectroscopy and Theoretical Calculations

To understand the microsolvation of alkaline-earth dihalides in water and provide information about the dependence of solvation processes on different halides, we investigated CaBr₂(H₂O)_n^-, CaI₂(H₂O)_n^-, and CaF₂(H₂O)_n^- (n = 0-6) clusters using size-selected anion photoelectron spectroscopy and conducted theoretical calculations on these clusters and their neutrals. The results are compared with those of CaCl₂(H₂O)_n^-/0 clusters reported previously. It is found that the vertical detachment energies (VDEs) of CaCl₂(H₂O)_n^-, CaBr₂(H₂O)_n^-, and CaI₂(H₂O)_n^- show a similar trend with increasing cluster size, while the VDEs of CaF₂(H₂O)_n^- show a different trend. The VDEs of CaF₂(H₂O)_n^- are much lower than those of CaCl₂(H₂O)_n^-, CaBr₂(H₂O)_n^-, and CaI₂(H₂O)_n^-. A detailed probing of the structures shows that a significant increase of the Ca-X distance (separation of Ca²⁺-X^- ion pair) in CaCl₂(H₂O)_n^-/0, CaBr₂(H₂O)_n^-/0, and CaI₂(H₂O)_n^-/0 clusters occurred at about n = 5. However, for CaF₂(H₂O)_n^-/0, no abrupt change of the Ca-F distance with the increasing cluster size has been observed. In CaCl₂(H₂O)₆^-/0, CaBr₂(H₂O)₆^-/0, and CaI₂(H₂O)₆^-/0, the Ca atom coordinates directly with 5 H₂O molecules. However, in CaF₂(H₂O)_n^-/0, the Ca atom coordinates directly with only 2 or 3 H₂O molecules. The similarity or differences in the structures and coordination numbers are consistent with the fact that CaCl₂, CaBr₂, and CaI₂ have similar solubility, while CaF₂ has much lower solubility.

Proton Transfer in Nitromethane-Ammonia Clusters under VUV Single-Photon Ionization Explored by Infrared Spectroscopy and Theoretical Calculations

It is known that the acidity and reactivity of the CH bond can be enhanced after ionization. Also, this property plays a pivotal role in proton transfer reaction and in the formation of new molecules. Herein, infrared spectroscopy and high-precision quantum chemical calculations are used to study the neutral and cationic clusters of nitromethane-ammonia (CH₃NO₂-NH₃). It is found that in the neutral cluster, CH₃NO₂ and NH₃ are mainly bonded by three intermolecular hydrogen bonds, in which electrostatic contribution plays a major role. After vacuum ultraviolet (VUV) single-photon ionization of CH₃NO₂-NH₃, the positive charge redistributes from the ionized nitrogen atom of NH₃ to the CH₃NO₂ molecule immediately. Then, the proton of CH₃NO₂ transfers to NH₃ to form a proton-transferred type structure CH₂NO₂-NH₄⁺, without any effective energy barrier, due to the positive hyperconjugation of cationic nitromethane. A closed loop of positive charge transfer takes place in the CH₃NO₂-NH₃ cluster after VUV ionization. The present work demonstrates that both the proton transfer reaction and charge transfer process have occurred in the ionized CH₃NO₂-NH₃ cluster. Moreover, it is found that the proton transfer reaction is a result of the highly acidic CH bond caused by hyperconjugation between the σ (CH) bond and π orbital.