Microscopic dynamics of gas molecules confined in porous channel-like ice structure.

In the rich ice polymorphism landscape, ice XVII, metastable at ambient pressure and at temperatures below 130 K, is surely one of the most interesting from both fundamental and technological perspectives due to its porosity, i.e., its capability to repeatedly absorb and desorb molecular hydrogen by dosing the gas at pressures even below the ambient one. Here, owing to this exceptional key feature, we investigate the roto-vibrational dynamics of the H2 molecules trapped in the fully deuterated ice XVII structure. Making use of the high-resolution and brilliance of the TOSCA neutron vibrational spectrometer, combined with high-resolution Raman data, we are able to efficiently distinguish the center-of-mass translational bands from the rotational ones and to study them as a function of the guest filling of the ice structure, unraveling a peculiar behavior for the confined particle in a low-dimensional system. Moreover, we also report the study of the microscopic dynamics of confined nitrogen and oxygen, which are the most abundant molecular species in the atmosphere and are of paramount interest for technological applications. Finally, we show that the ice XVII porosity is a unique feature, especially in the low pressure regime, within the emptied-hydrate phases discovered to date.

Hydrogen Bond Dynamics of Cellulose through Inelastic Neutron Scattering Spectroscopy.

This work explores the dynamics of hydrogen bond networks in cellulose through inelastic neutron scattering (INS) and periodic CASTEP calculations. Estimated spectra were based on the crystal structure of cellulose Iα and Iß and replicate the INS spectrum of cellulose samples with remarkable similarity, allowing a reliable assignment of INS bands to vibrational modes of cellulose. Comparison of cellulose samples from varied sources, from bacterial to kraft pulp, allows the identification of characteristic INS bands, arising from C2-OH torsional motions, which easily identify which allomorph-Iα or Iß-is prevalent. A high crystallinity index is revealed by the presence of well-defined INS bands associated with highly cooperative CH bending modes along the chain. Hydrating kraft cellulose samples clearly affects those INS bands related with the hydroxymethyl group, identified as the preferred binding site for water molecules. At high humidity content level, a significant proportion of the water molecules is aggregated in clusters within the amorphous cellulose domains. The formation of ice microcrystals leads to a partial disruption of the hydrogen-bond network, as can be concluded from the observed red-shift of the torsional OH vibrational modes. The full assignment and interpretation of cellulose's INS spectra herein provided is a sound basis for future use of INS spectroscopy in the characterization of functionalized cellulose fibers and composite materials.

Inelastic neutron scattering study of reline: shedding light on the hydrogen bonding network of deep eutectic solvents.

The solids choline chloride and urea, mixed in a 1 : 2 molar proportion, form the iconic deep eutectic solvent "Reline". A combination of computational and vibrational spectroscopy tools, including inelastic neutron scattering (INS), have been used to probe intermolecular interactions in the eutectic mixture. Reline's experimental spectra were estimated using discrete and periodic ab initio calculations of a molecular aggregate with two choline chloride and four urea units. This is the minimum size required to achieve satisfactory agreement with experiment, as smaller clusters cannot represent all of reline's significant intermolecular interactions. The INS spectrum of reline, compared with that of pure choline chloride, reveals a displacement of chloride anions away from their preferred positions on top of choline's methyl groups, whose torsional movement becomes less hindered in the mixture. Urea, which adopts a planar (sp2) shape in the crystal, becomes non-planar (sp3) in reline, a feature herein discussed for the first time. In reline, urea molecules form a wide range of hydrogen bonds, from soft contacts to stronger associations, the latter being responsible for the deviation from ideality. The chloride's interactions with choline are largely conserved at the hydroxyl end while becoming weaker at the cationic headgroup. The interplay of soft and strong interactions confers flexibility to the newly formed hydrogen-bond network and allows the ensemble to remain liquid at room temperature.

Optical properties of micrometer size water droplets studied by cavity ringdown spectroscopy.

Optical extinction by homogeneous, pure water droplets of 30 to 70 microm diameter produced by a vibrating orifice aerosol generator has been studied by pulsed cavity ringdown (CRD) spectroscopy at lambda=560 nm under ambient conditions. Experimental sensitivity of better than 1% achieved in measurements of CRD times enabled detection of changes in laser light losses per pass due to changes in the number and size of particles within the laser beam volume. By systematically changing the droplet size in the cavity while recording the CRD time, a periodic modulation in the value of the loss per pass was observed. The modulation is caused by the oscillatory nature of the extinction efficiency, which was subsequently inferred and compared with the results of theoretical calculations based on Mie theory.

Infrared laser spectroscopy of the CH3-HCN radical complex stabilized in helium nanodroplets.

The CH3-HCN and CD3-HCN radical complexes have been formed in helium nanodroplets by sequential pickup of a CH3 (CD3) radical and a HCN molecule and have been studied by high-resolution infrared laser spectroscopy. The complexes have a hydrogen-bonded structure with C3v symmetry, as inferred from the analysis of their rotationally resolved nu = 1 <-- 0 H-CN vibrational bands. The A rotational constants of the complexes are found to change significantly upon vibrational excitation of the C-H stretch of HCN within the complex, DeltaA = A'-A" = -0.04 cm(-1) (for CH3-HCN), whereas the B rotational constants are found to be 2.9 times smaller than that predicted by theory. The reduction in B can be attributed to the effects of helium solvation, whereas the large DeltaA is found to be a sensitive probe of the vibrational averaging dynamics of such weakly bound systems. The complex has a permanent electric dipole moment of 3.1 +/- 0.2 D, as measured by Stark spectroscopy. A vibration-vibration resonance is observed to couple the excited C-H stretching vibration of HCN within the complex to the lower-frequency C-H stretches of the methyl radical. Deuteration of the methyl radical was used to detune these levels from resonance, increasing the lifetime of the complex by a factor of 2. Ab initio calculations for the energies and molecular parameters of the stationary points on the CN+CH4 --> HCN+CH3 potential-energy surface are also presented.

Infrared laser spectroscopy of CH3...HF in helium nanodroplets: The exit-channel complex of the F + CH4 reaction.

High-resolution infrared laser spectroscopy is used to study the CH3...HF and CD3...HF radical complexes, corresponding to the exit-channel complex in the F + CH4 --> HF + CH3 reaction. The complexes are formed in helium nanodroplets by sequential pickup of a methyl radical and a HF molecule. The rotationally resolved spectra presented here correspond to the fundamental v = 1 <-- 0 H-F vibrational band, the analysis of which reveals a complex with C(3v) symmetry. The vibrational band origin for the CH3...HF complex (3797.00 cm(-1)) is significantly redshifted from that of the HF monomer (3959.19 cm(-1)), consistent with the hydrogen-bonded structure predicted by theory [E. Ya. Misochko et al., J. Am. Chem. Soc. 117, 11997 (1995)] and suggested by previous matrix isolation experiments [M. E. Jacox, Chem. Phys. 42, 133 (1979)]. The permanent electric dipole moment of this complex is experimentally determined by Stark spectroscopy to be 2.4+/-0.3 D. The wide amplitude zero-point bending motion of this complex is revealed by the vibrational dependence of the A rotational constant. A sixfold reduction in the line broadening associated with the H-F vibrational mode is observed in going from CH3...HF to CD3...HF. The results suggest that fast relaxation in the former case results from near-resonant intermolecular vibration-vibration (V-V) energy transfer. Ab initio calculations are also reported (at the MP2 level) for the various stationary points on the F + CH4 surface, including geometry optimizations and vibrational frequency calculations for CH3...HF.