Acid dissociation and the subsequent solvation of the charged fragments at ultracold temperatures in nanoenvironments, as distinct from ambient bulk water, are relevant to atmospheric and interstellar chemistry but remain poorly understood. Here we report the experimental observation of a nanoscopic aqueous droplet of acid formed within a superfluid helium cluster at 0.37 kelvin. High-resolution mass-selective infrared laser spectroscopy reveals that successive aggregation of the acid HCl with water molecules, HCl(H2O)n, readily results in the formation of hydronium at n = 4. Accompanying ab initio simulations show that undissociated clusters assemble by stepwise water molecule addition in electrostatic steering arrangements up to n = 3. Adding a fourth water molecule to the ringlike undissociated HCl(H2O)3 then spontaneously yields the compact dissociated H3O+(H2O)3Cl- ion pair. This aggregation mechanism bypasses deep local energy minima on the n = 4 potential energy surface and offers a general paradigm for reactivity at ultracold temperatures.

Single-conformation ultraviolet and infrared spectra of a series of model gamma-peptides are reported, with the goal of providing new tests of amide stacking as an amide-amide binding motif. The data also serve to illustrate the power and challenges of carrying out single-conformation spectroscopy of neutral molecules of this size in the gas phase under jet-cooled conditions. Building on recent work on Ac-γ2-hPhe-NHMe (James et al., J. Am. Chem. Soc., 2009, 131, 14243), the effects of derivatization and H2O complexation on amide stacking are studied. Ac-γ2-hPhe-N(Me)2 shows only amide stacked structures, blocking the competing position for formation of an amide-amide H-bond. The Ac-γ2-hPhe-NHMe-H2O complex includes structures in which the H2O molecule forms a bridge between the two stacked amide planes, retaining and enhancing amide stacking. IR population transfer methods are also employed to study the dynamics of photodissociation of the amide stacked-H2O complex. Finally, IR ion-gain spectroscopy is introduced as a means of recording infrared spectra containing contributions from all conformers present, based on IR-induced broadening of the UV absorptions. Its role in estimating fractional abundances is tested on Ac-γ2-hPhe-NHMe.

Single-conformation spectroscopy of two diastereomers of 1-(4-hydroxy-3-methoxyphenyl)propane-1,2,3-triol (HMPPT) has been carried out under isolated, jet-cooled conditions. HMPPT is a close analog of coniferyl alcohol, one of the three monomers that make up lignin, the aromatic biopolymer that gives structural integrity to plants. In HMPPT, the double bond of coniferyl alcohol has been oxidized to produce an alkyl triol chain with chiral centers at C(α) and C(β), thereby incorporating key aspects of the β-O-4 linkage between monomer subunits that occurs commonly in lignin. Both (R,S)- and (R,R)-HMPPT diastereomers have been synthesized in pure form for study. Resonant two-photon ionization (R2PI), UV hole-burning (UVHB)/IR-UV hole-burning (IR-UV HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been carried out, providing single-conformation UV spectra in the S(0)-S(1) region (35200-35800 cm(-1)) and IR spectra in the hydride stretch region. Five conformers of (R,S)- and four conformers of (R,R)-HMPPT are observed and characterized, leading to assignments for all nine conformers. Spectroscopic signatures for α-β-γ, γ-β-α, and α-γ-β-π chains and two cyclic forms [(αβγ) and (αγβ)] of the glycerol side chain are determined. Infrared ion-gain (IRIG) spectroscopy is used to determine fractional abundances for the (R,S) diastereomer and constrain the populations present in (R,R). The two diastereomers have very different conformational preferences. More than 95% of the population of (R,R) configures the glycerol side chain in a γ-β-α triol chain, while in (R,S)-HMPPT, 51% of the population is in α-β-γ chains that point in the opposite direction, with an additional 21% of the population in H-bonded cycles. The experimental results are compared with calculations to provide a consistent explanation of the diastereomer-specific effects observed.

No abstract

We report on rotationally resolved IR spectra of dimers of HDO as a deuterium (d) donor with H(2)O, HDO, and D(2)O embedded in superfluid Helium nanodroplets in the 2650-2660 and 2725-2740 cm(-1) regions of the O-D donor stretch and symmetric acceptor stretch vibrations, respectively. By comparing spectra at different levels of deuteration we were able to unambiguously assign the donor stretch signals of H(2)O···DOH, HDO···DOH, and D(2)O···DOH. For H(2)O···DOH, three ΔK(a) = 0 sub-bands were found that were assigned to transitions from the lower and upper acceptor switching states of K(a) = 0 and the lower acceptor switching state of K(a) = 1. In addition, b- and c-type transitions in the acceptor stretch region of HDO···DOH were observed that allowed us to determine the acceptor switching splitting of Δṽ = 5.68 cm(-1) in the HDO···DOH vibrational ground state. We suggest that the dominating broadening mechanism is intervibrational relaxation due to coupling of the rovibrational levels of the chromophore via internal droplet excitations.