Observation of the Low-Frequency Spectrum of the Water Trimer as a Sensitive Test of the Water-Trimer Potential and the Dipole-Moment Surface.

Intermolecular interactions in bulk water are dominated by pairwise and non-pairwise cooperative interactions. While accurate descriptions of the pairwise interactions are available and can be tested by precise low-frequency spectra of the water dimer up to 550 cm-1 , the same does not hold for the three-body interactions. Here, we report the first comprehensive spectrum of the water trimer in the frequency region from 70 to 620 cm-1 using helium-nanodroplet isolation and free-electron lasers. By comparison to accompanying high-level quantum calculations, the experimentally observed intermolecular bands can be assigned. The transition frequencies of the degenerate translation, the degenerate in-plane and the non-degenerate out-of-plane libration, as well as additional bands of the out-of-plane librational mode are reported for the first time. These provide a benchmark for state-of-the-art water potentials and dipole-moment surfaces, especially with respect to three-body interactions.

Observation of the Low-Frequency Spectrum of the Water Dimer as a Sensitive Test of the Water Dimer Potential and Dipole Moment Surfaces.

Using the helium nanodroplet isolation setup at the ultrabright free-electron laser source FELIX in Nijmegen (BoHeNDI@FELIX), the intermolecular modes of water dimer in the frequency region from 70 to 550 cm-1 were recorded. Observed bands were assigned to donor torsion, acceptor wag, acceptor twist, intermolecular stretch, donor torsion overtone, and in-plane and out-of-plane librational modes. This experimental data set provides a sensitive test for state-of-the-art water potentials and dipole moment surfaces. Theoretical calculations of the IR spectrum are presented using high-level quantum and approximate quasiclassical molecular dynamics approaches. These calculations use the full-dimensional ab initio WHHB potential and dipole moment surfaces. Based on the experimental data, a considerable increase of the acceptor switch and a bifurcation tunneling splitting in the librational mode is deduced, which is a consequence of the effective decrease in the tunneling barrier.

Acid solvation versus dissociation at "stardust conditions": Reaction sequence matters.

Chemical reactions at ultralow temperatures are of fundamental importance to primordial molecular evolution as it occurs on icy mantles of dust nanoparticles or on ultracold water clusters in dense interstellar clouds. As we show, studying reactions in a stepwise manner in ultracold helium nanodroplets by mass-selective infrared (IR) spectroscopy provides an avenue to mimic these "stardust conditions" in the laboratory. In our joint experimental/theoretical study, in which we successively add H2O molecules to HCl, we disclose a unique IR fingerprint at 1337 cm-1 that heralds hydronium (H3O+) formation and, thus, acid dissociation generating solvated protons. In stark contrast, no reaction is observed when reversing the sequence by allowing HCl to interact with preformed small embryonic ice-like clusters. Our ab initio simulations demonstrate that not only reaction stoichiometry but also the reaction sequence needs to be explicitly considered to rationalize ultracold chemistry.

Understanding the microsolvation of radicals: Infrared spectroscopy of benzyl radical water clusters.

The IR spectrum of benzyl radical:water clusters was recorded. Benzyl radicals were produced by vacuum flash pyrolysis and trapped in superfluid helium nanodroplets. The infrared spectrum of benzyl radical water aggregates in the range 2585-2820 cm-1 was recorded by subsequent addition of deuterated water. A total of seven peaks are observed at 2766, 2750, 2656, 2638, 2633, 2598, and 2593 cm-1. Based on pressure dependent measurements and comparison to accompanying ab initio calculations, five of these bands are assigned to distinct O-D stretch vibrations of benzyl:water clusters with one and two water molecules. In line with previous experiments on benzene:water clusters, we observe the formation of a water dimer-like motif that is attached to one face of the benzyl radical.

Helium droplet infrared spectroscopy of glycine and glycine-water aggregates.

Infrared absorption spectra of glycine and glycine-water aggregates embedded in superfluid helium nanodroplets were recorded in the frequency range 1000-1450 cm-1. For glycine monomer, absorption bands were observed at 1106 cm-1, 1134 cm-1, and 1389 cm-1. These bands were assigned to the C-OH stretch mode of the glycine conformers I, III and II, respectively. For glycine-water aggregates, we observed two bands at 1209 cm-1 and 1410 cm-1 which we assign to distinct conformers of glycine-H2O. In all cases, the water is found to preferentially bind to the carboxyl group of the glycine.

Infrared spectroscopy of the ν2 band of the water monomer and small water clusters (H2O)n=2,3,4 in helium droplets.

We have recorded infrared spectra in the frequency range of the ν2 band of water monomer and water clusters in superfluid helium droplets. In order to be able to map the chemically important fingerprint range, we have used an IR quantum cascade laser as a radiation source. We were able to observe three ro-vibrational transitions of the water monomer between 1590 and 1670 cm(-1). The lines were assigned to the 110 ← 101, 111 ← 000 and 212 ← 101 transitions of the ν2 vibration of H2O. Based upon the linewidths, we could deduce relaxation times of 1.9 to 4.2 ps for the monomer. Additional absorption bands could be assigned to ν2 vibrational bands of water clusters (H2O)n with n = 2, 3, 4. These experimental results are compared to theoretical calculations by Wang and Bowman which are reported in an accompanying paper [ref. 1, Y. Wang and J. M. Bowman, Phys. Chem. Chem. Phys., 2016, DOI: ]. We find a very good agreement of our results with both calculations and with previous results from gas phase cavity ringdown experiments.

Large scale polarizability calculations using the approximate coupled cluster model CC2 and MP2 combined with the resolution-of-the-identity approximation.

We present an implementation of static and frequency-dependent polarizabilities for the approximate coupled cluster singles and doubles model CC2 and static polarizabilities for second-order Mo̸ller-Plesset perturbation theory. Both are combined with the resolution-of-the-identity approximation for electron repulsion integrals to achieve unprecedented low operation counts, input-output, and disc space demands. To avoid the storage of double excitation amplitudes during the calculation of derivatives of density matrices, we employ in addition a numerical Laplace transformation for orbital energy denominators. It is shown that the error introduced by this approximation is negligible already with a small number of sampling points. Thereby an implementation of second-order one-particle properties is realized, which avoids completely the storage of quantities scaling with the fourth power of the system size. The implementation is tested on a set of organic molecules including large fused aromatic ring systems and the C(60) fullerene. It is demonstrated that exploiting symmetry and shared memory parallelization, second-order properties for such systems can be evaluated at the CC2 and MP2 level within a few hours of calculation time. As large scale applications, we present results for the 7-, 9-, and 11-ring helicenes.