Picosecond Proton Transfer Kinetics in Water Revealed with Ultrafast IR Spectroscopy

Aqueous proton transport involves the ultrafast interconversion of hydrated proton species that are closely linked to the hydrogen bond dynamics of water, which has been a long-standing challenge to experiments. In this study, we use ultrafast IR spectroscopy to investigate the distinct vibrational transition centered at 1750 cm^–1 in strong acid solutions, which arises from bending vibrations of the hydrated proton complex. Broadband ultrafast two-dimensional IR spectroscopy and transient absorption are used to measure vibrational relaxation, spectral diffusion, and orientational relaxation dynamics. The hydrated proton bend displays fast vibrational relaxation and spectral diffusion timescales of 200–300 fs; however, the transient absorption anisotropy decays on a remarkably long 2.5 ps timescale, which matches the timescale for hydrogen bond reorganization in liquid water. These observations are indications that the bending vibration of the aqueous proton complex is relatively localized, with an orientation that is insensitive to fast hydrogen bonding fluctuations and dependent on collective structural relaxation of the liquid to reorient. We conclude that the orientational relaxation is a result of proton transfer between configurations that are well described by a Zundel-like proton shared between two flanking water molecules

Reactions of Fe⁺ and FeO⁺ with C₂H₂, C₂H₄, and C₂H₆: Temperature-Dependent Kinetics

We present the first temperature-dependent rate constants and branching ratios for the reactions of Fe⁺ and FeO⁺ with C₂H₂, C₂H₄, and C₂H₆ from 170 to 700 K. Fe⁺ is observed to react only by association with the three hydrocarbons, with temperature dependencies of <i>T</i>^–2 to <i>T</i>^–3. FeO⁺ reacts with C₂H₂ and C₂H₄ at the collision rate over the temperature range, and their respective product branchings show similar temperature dependences. In contrast, the reaction with ethane is collisional at 170 K but varies as <i>T</i>^–0.5, while the product branching remains essentially flat with temperature. These variations in reactivity are discussed in terms of the published reactive potential surfaces. The effectiveness of Fe⁺ as an oxygen-transfer catalyst toward the three hydrocarbons is also discussed

Persistence of Dual Free Internal Rotation in NH₄⁺(H₂O)·He_{<i>n</i>=0–3} Ion–Molecule Complexes: Expanding the Case for Quantum Delocalization in He Tagging

To explore the extent of the molecular cation perturbation induced by complexation with He atoms required for the application of cryogenic ion vibrational predissociation (CIVP) spectroscopy, we compare the spectra of a bare NH₄⁺(H₂O) ion (obtained using infrared multiple photon dissociation (IRMPD)) with the one-photon CIVP spectra of the NH₄⁺(H₂O)·He_1–3 clusters. Not only are the vibrational band origins minimally perturbed, but the rotational fine structures on the NH and OH asymmetric stretching vibrations, which arise from the free internal rotation of the −OH₂ and −NH₃ groups, also remain intact in the adducts. To establish the location and the quantum mechanical delocalization of the He atoms, we carried out diffusion Monte Carlo (DMC) calculations of the vibrational zero point wave function, which indicate that the barriers between the three equivalent minima for the He attachment are so small that the He atom wave function is delocalized over the entire −NH₃ rotor, effectively restoring <i>C</i>₃ symmetry for the embedded −NH₃ group

Comment on “Role of (NO)₂ Dimer in Reactions of Fe⁺ with NO and NO₂ Studied by ICP-SIFT Mass Spectrometry”

Comment on “Role of (NO)₂ Dimer in Reactions of Fe⁺ with NO and NO₂ Studied by ICP-SIFT Mass Spectrometry

Isomer-Specific IR–IR Double Resonance Spectroscopy of D₂-Tagged Protonated Dipeptides Prepared in a Cryogenic Ion Trap

Isomer-specific vibrational predissociation spectra are reported for the gas-phase GlySarH⁺ and SarSarH⁺ [Gly = glycine; Sar = sarcosine] ions prepared by electrospray ionization and tagged with weakly bound D₂ adducts using a cryogenic ion trap. The contributions of individual isomers to the overlapping vibrational band patterns are completely isolated using a pump–probe photochemical hole-burning scheme involving two tunable infrared lasers and two stages of mass selection (hence IR²MS²). These patterns are then assigned by comparison with harmonic (MP2/6-311+G­(d,p)) spectra for various possible conformers. Both systems occur in two conformations based on cis and trans configurations with respect to the amide bond. In addition to the usual single intramolecular hydrogen bond motif between the protonated amine and the nearby amide oxygen atom, <i>cis</i>-SarSarH⁺ adopts a previous unreported conformation in which both amino NH's act as H-bond donors. The correlated red shifts in the NH donor and CO acceptor components of the NH···OC linkage to the acid group are unambiguously assigned in the double H-bonded conformer