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

Electric nuclear quadrupole coupling reveals dissociation of HCl with a few water molecules

Investigating the dissociation of acids in the presence of a limited number of water molecules is crucial for understanding various elementary chemical processes. In our study, focusing on HCl(H2O)n clusters (where HCl is hydrogen chloride and H2O is water) formed in a cold and isolated jet expansion, we used the nuclear quadrupole coupling tensor obtained through rotational spectroscopy to decipher the nature of the hydrogen-chlorine (H-Cl) chemical bond in a microaqueous environment. For n = 1 to 4, the H-Cl bond is covalent. At n = 5 and 7, the contact ion pair of H3O+Cl- is spontaneously formed within the hydrogen bond networks of book and cube acid-water clusters, respectively.

Role of Explicit Solvation in Computational Modeling of Chemical Reactions: Mechanism of Cu(I) Transfer Between Thiolate-Based Chelators in Water

Solvation plays important roles in controlling the thermodynamic and kinetic aspects of chemical reactions. The conventional approaches to treat solvation via electronic structure methods are likely to become inadequate, when the reacting solutes have strong electrostatic and hydrogen bonding interactions with the solvent and undergo significant structural changes during the course of the reaction. In this article, we present evidence of such solvent and structural effects in the computational study of the Cu(I) transfer reaction between thiolate-based chelators dithiobutylamine (DTBA) and dithiotheritol (DTT) in water, inspired from biological copper trafficking phenomena. We propose a general solution to the problem by combining classical molecular dynamics (MD) simulations of the bulk system and static quantum chemistry calculations. The fluctuating solvation shell was estimated from MD, and energetics was assessed by averaging QM energies of a series of molecular clusters constructed from the MD snapshots. Applying this approach, we propose a reaction pathway with estimates of relative intermediate stabilities and barriers, which suggest the overall reaction to be reversible in nature and likely to go through both two and three coordinated intermediates, confirming previous studies of similar protein analogues. An interesting fact that emerged from our study was the strong indication that the rate-determining step is the deprotonation of initial thiol bound Cu(I) complex, without involving any Cu(I)-S bonds. The proposed method will lead to a better treatment of solvations, and these mechanistic insights will aid our understanding of biological copper(I) trafficking.

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.

Wetting vs Droplet Aggregation: A Broadband Rotational Spectroscopic Study of 3-Methylcatechol⋅⋅⋅Water Clusters

Two competing solvation pathways of 3-methylcatechol (MC), an atmospherically relevant aromatic molecule, with up to five water molecules were explored in detail by using a combination of broadband rotational spectroscopy and computational chemistry. Theoretically, two different pathways of solvation emerge: the commonly observed droplet pathway which involves preferential binding among the water molecules while the solute serves as an anchor point for the formation of a water cluster, and an unexpected wetting pathway which involves interactions between the water molecules and the aromatic face of MC, i.e., a wetting of the π-surface. Conclusive identification of the MC hydrate structures, and therefore the wetting pathway, was facilitated by rotational spectra of the parent MC hydrates and several H2 18 O and 13 C isotopologues which exhibit splittings associated with methyl internal rotation and/or water tunneling motions. Theoretical modelling and analyses offer insights into the tunneling and conversion barriers associated with the observed hydrate conformers and the nature of the non-covalent interactions involved in choosing the unusual wetting pathway.

Microwave and Computational Study of Methanesulfonic Acid and Its Complex with Water

Spectra of methanesulfonic acid (CH₃SO₃H, MSA) and its complex with water have been studied by microwave spectroscopy and density functional theory calculations. For the monomer, spectra were obtained for both the parent and -OD isotopologues and, in each case, revealed a pair of tunneling states that are attributed to large amplitude motion of the hydroxyl hydrogen about the S-O(H) bond. Transitions crossing between tunneling states were not found in the parent spectrum and are estimated to be outside the range of the spectrometer, thus precluding the direct determination of the tunneling energy. For the -OD form, however, the tunneling energy was determined to be ΔE = 6471.9274(18) MHz from direct measurement of the cross-state c-type transitions. In its complex with water, the acidic hydrogen of the MSA forms a hydrogen bond with the water oxygen. A secondary hydrogen bond involving the water hydrogen and an SO₃ oxygen completes a six-membered ring, forming a cyclic structure typical of hydrated oxyacids. No evidence of internal motion was observed. Rotational spectra of the CH₃SO₃H···D₂O and CH₃SO₃D···D₂O isotopologues were also obtained and analyzed. Comparison with theoretical calculations confirms the cyclic structure, though the orientation of the unbound water hydrogen is ambiguous.

Stepwise hydrations of anhydride tuned by hydrogen bonds: rotational study on maleic anhydride-(H2O)1-3

The rotational spectra of maleic anhydride-(H₂O)_1-3 have been investigated for the first time by using pulsed jet Fourier transform microwave spectroscopy with complementary computational analyses. The experimental evidence points out that water tends to self-aggregate with hydrogen bonds and form homodromic cycles. Differences in bond lengths and charge distribution between the two carbonyl sites have been observed upon stepwise hydrations, which might further introduce a selectivity on the nucleophilic attack sites of hydrolysis. This study provides an important insight into the incipient solvation process (microsolvation) of maleic anhydride in water by understanding the cooperation and rearrangement of intermolecular hydrogen bonds in its stepwise hydrates.