Partial Proton Transfer in the Nitric Acid Trihydrate Complex

Partial Proton Transfer in the Gas Phase: A Spectroscopic and Computational Analysis of the Trifluoroacetic Acid - Trimethylamine Complex

The 1:1 complex formed from trifluoroacetic acid (TFA) and trimethylamine (TMA) has been observed in the gas phase by rotational spectroscopy and further investigated by DFT and MP2 methods. Spectra of both the parent form and the -OD isotopologue have been obtained. The complex is structurally similar to a hydrogen bonded system, with the O-H bond directed toward the nitrogen of the TMA. However, both the spectroscopic and computational results indicate that it is intermediate between a hydrogen bonded complex and a proton-transferred ion pair. Two metrics are used to assess the degree of proton transfer from the acid to the base. The first is based on experimental 14N nuclear quadrupole coupling constants. Specifically, the component of the 14N nuclear quadrupole coupling tensor along the c-inertial axis of the complex, χcc, is 31% of the way between that of free TMA (no proton transfer) and that of TMAH+ (complete proton transfer). A second metric, adapted from that of Kurnig and Scheiner [Int. J. Quantum Chem. Quantum Biol. Symp. 1987, 14, 47-56], is based on calculated O-H and H-N distances and corroborates this description. These results indicate that the degree of proton transfer in TFA-TMA is very similar to that in the TMA complex of HNO3, which has been previously studied and for which the proton affinity of the conjugate anion (NO3-) is almost identical to that of CF3COO-. While the solid salt, TMAH+·CF3COO-, is an ionic plastic above 307 K and exhibits free rotation of the ions, no such motion is observed in the cold 1:1 gas phase adduct.

Microwave Study of Triflic Acid Hydrates: Evidence for the Transition from Hydrogen-Bonded Clusters to a Microsolvated Ion Pair

Rotational spectra of the mono-, di-, and trihydrates of triflic acid, CF₃SO₃H···(H₂O)_n=1-3, have been recorded by pulsed nozzle Fourier transform microwave spectroscopy and spectroscopic constants obtained have been compared with values calculated at several levels of theory. The experimental results are consistent with the theoretical predictions presented here and elsewhere, indicating that with only one or two water molecules, triflic acid remains un-ionized in a cold molecular complex. The experiments further concur with theoretical predictions that the addition of a third water molecule transforms the system into what is best regarded as a hydrated hydronium triflate ion pair. Thus, only three water molecules are needed to induce ionization of triflic acid in a cold molecular cluster. This number is somewhat low compared with that for other simple protic acids and likely reflects the superacidity of triflic acid. Simple energetic arguments can be used to rationalize this result.

Proton Transfer in a Bare Superacid-Amine Complex: A Microwave and Computational Study of Trimethylammonium Triflate

The complex formed from trimethylamine ((CH₃)₃N) and trifluoromethanesulfonic acid (triflic acid, CF₃SO₃H) has been observed by Fourier transform microwave spectroscopy in a supersonic jet. Spectroscopic data, most notably ¹⁴N nuclear quadrupole coupling constants, are combined with computational results at several levels of theory to unambiguously demonstrate complete or near-complete proton transfer from the triflic acid to the trimethylamine upon complexation. Thus, the system is best regarded as a trimethylammonium triflate ion pair in the gas phase. The formation of an isolated ion pair in a 1:1 complex of a Brønsted acid and base is unusual and likely arises due to the strong acidity of triflic acid. Simple energetic arguments based on proton affinities and the Coulomb interaction energy can be used to rationalize this result.

Mass spectrometry of aerosol particle analogues in molecular beam experiments

Nanometer-size particles such as ultrafine aerosol particles, ice nanoparticles, water nanodroplets, etc, play an important, however, not yet fully understood role in the atmospheric chemistry and physics. These species are often composed of water with admixture of other atmospherically relevant molecules. To mimic and investigate such particles in laboratory experiments, mixed water clusters with atmospherically relevant molecules can be generated in molecular beams and studied by various mass spectrometric methods. The present review demonstrates that such experiments can provide unprecedented details of reaction mechanisms, and detailed insight into the photon-, electron-, and ion-induced processes relevant to the atmospheric chemistry. After a brief outline of the molecular beam preparation, cluster properties, and ionization methods, we focus on the mixed clusters with various atmospheric molecules, such as hydrated sulfuric acid and nitric acid clusters, N_x O_y and halogen-containing molecules with water. A special attention is paid to their reactivity and solvent effects of water molecules on the observed processes.

Disulfuric acid dissociated by two water molecules: ab initio and density functional theory calculations

We have studied geometries, energies and vibrational spectra of disulfuric acid (H2S2O7) and its anion (HS2O7(-)) hydrated by a few water molecules, using density functional theory (M062X) and ab initio theory (SCS-MP2 and CCSD(T)). The most noteworthy result is found in H2S2O7(H2O)2 in which the lowest energy conformer shows deprotonated H2S2O7. Thus, H2S2O7 requires only two water molecules, the fewest number of water molecules for deprotonation among various hydrated monomeric acids reported so far. Even the second deprotonation of the first deprotonated species HS2O7(-) needs only four water molecules. The deprotonation is supported by vibration spectra, in which acid O-H stretching peaks disappear and specific three O-H stretching peaks for H3O(+) (eigen structure) appear. We have also kept track of variations in several geometrical parameters, atomic charges, and hybrid orbital characters upon addition of water. As the number of water molecules added increases, the S-O bond weakens in the case of H2S2O7, but strengthens in the case of HS2O7(-). It implies that the decomposition leading to H2SO4 and SO3 hardly occurs prior to the 2nd deprotonation at low temperatures.

Unusual crystallographic existence of a hydrated zinc(ii) bisulphate complex: experimental and theoretical observations

Crystallographic and theoretical existence of a unprecedented hydrated zinc(ii) bisulphate, [Zn(H₂O)₆](HSO₄·H₂O)₂ in the presence of 4,4′-bipyridine and ammonium thiocyanate is reported.

Nucleation of Mixed Nitric Acid-Water Ice Nanoparticles in Molecular Beams that Starts with a HNO3 Molecule

Mixed (HNO3)m(H2O)n clusters generated in supersonic expansion of nitric acid vapor are investigated in two different experiments, (1) time-of-flight mass spectrometry after electron ionization and (2) Na doping and photoionization. This combination of complementary methods reveals that only clusters containing at least one acid molecule are generated, that is, the acid molecule serves as the nucleation center in the expansion. The experiments also suggest that at least four water molecules are needed for HNO3 acidic dissociation. The clusters are undoubtedly generated, as proved by electron ionization; however, they are not detected by the Na doping due to a fast charge-transfer reaction between the Na atom and HNO3. This points to limitations of the Na doping recently advocated as a general method for atmospheric aerosol detection. On the other hand, the combination of the two methods introduces a tool for detecting molecules with sizable electron affinity in clusters.

Proton transfer and autoionization in HNO3·HCl·(H2O)n particles

The structure and spectroscopic properties of clusters of HNO(3)·HCl·(H(2)O)(n), with n = 1 to 6, have been calculated at the MP2/aug-cc-pVDZ level of theory. Altogether 22 different clusters have been found as stable structures, with minima in their potential energy surfaces. The clusters can be grouped in families with the same number of water molecules, and with close aggregation energies within each family. The addition of each new water molecule increments the aggregation energy of the clusters by a nearly constant value of 76.2 ± 0.1 Hartree. The proton transfer parameter and the coordination number of HNO(3) and HCl in each cluster have been evaluated, and the wavenumber shifts for the X(-)-H(+) vibration from the corresponding mode in the isolated molecules have also been predicted. These values allow classification of the acidic species in the clusters into three types, characterized by the strength of the hydrogen bond and the degree of ionization. A correspondence is found between the coordination number of HNO(3) and the magnitude of the X(-)-H(+) vibrational shift.