The role of water in stabilizing sites of protonation in small gaseous ions is investigated using electrospray ionization (ESI) coupled with infrared photodissociation spectroscopy and computational chemistry. Protonation of p-aminobenzoic acid (PABA) and p-aminobenzoic acid methyl ester (PABAOMe) occurs at the carbonyl oxygen atom both in isolation and when one water molecule is attached. However, protonation occurs at the amine nitrogen atom, which is the most favorable site in aqueous solution, for PABAOMeH(+)·(H(2)O)(3) and for a significant fraction of PABAH(+)·(H(2)O)(6). Fewer water molecules are necessary to stabilize the solution-phase isomer of PABAOMeH(+) (3) than for PABAH(+) (≥6), indicating that the favorable hydrogen bonding in PABAH(+) is a more important factor than the higher gas-phase basicity of PABAOMeH(+) in stabilizing protonation at the carbonyl oxygen atom. Relative Gibbs free energies (133 K) calculated using B3LYP and MP2 with the 6-311++G** basis set were significantly different from each other, and both are in poor agreement with results from the experiments. ωB97X-D/6-311++G**, which includes empirical dispersion corrections, gave results that were most consistent with the experimental data. The relative stabilities of protonating at the carbonyl oxygen atom for PABAH(+)·(H(2)O)(0-6) and PABAOMeH(+)·(H(2)O)(0-2) can be rationalized by resonance delocalization. These findings provide valuable insights into the solvent interactions that stabilize the location of a charge site and the structural transitions that can occur during the ESI desolvation process.
An attosecond pulse is used to create a wavepacket in molecular nitrogen composed of multiple bound and autoionizing electronic states of Rydberg and valence character between 12 and 16.7 eV. A time-delayed, few-femtosecond, near-infrared (NIR) laser pulse is used to couple individual states in the wavepacket to multiple neighboring states, resulting in time-dependent modification of the absorption spectrum and revealing both individual quantum beats of the wavepacket and the energy shifts of the excited states in the presence of the strong NIR field. The broad bandwidth of the attosecond pulse and high energy resolution of the extreme ultraviolet spectrometer allow the simultaneous observation of time-dependent dynamics for many individual vibrational levels in each electronic state. Quantum beating with periods from 1.3 to 12 fs and transient line shape changes are observed among vibrational levels of a progression of electronically autoionizing Rydberg states leading to the excited A (2)Πu N2(+) ion core. Vibrational levels in the valence b (1)Πu state exhibit 50 fs oscillation periods, revealing superpositions between individual vibrational levels within this state. Comparisons are made to previous studies of electronic wavepackets in atoms that highlight similarities to atomic behavior yet illustrate unique contributions of the diatomic molecular structure to the wavepacket, including the influence of different electronic potentials and vibrational-level-specific electronic dynamics.
Electronic wavepackets composed of multiple bound excited states of atomic neon lying between 19.6 and 21.5 eV are launched using an isolated attosecond pulse. Individual quantum beats of the wavepacket are detected by perturbing the induced polarization of the medium with a time-delayed few-femtosecond near-infrared (NIR) pulse via coupling the individual states to multiple neighboring levels. All of the initially excited states are monitored simultaneously in the attosecond transient absorption spectrum, revealing Lorentzian to Fano lineshape spectral changes as well as quantum beats. The most prominent beating of the several that were observed was in the spin-orbit split 3d absorption features, which has a 40 femtosecond period that corresponds to the spin-orbit splitting of 0.1 eV. The few-level models and multilevel calculations confirm that the observed magnitude of oscillation depends strongly on the spectral bandwidth and tuning of the NIR pulse and on the location of possible coupling states.
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