Experimental Fourier-transform infrared spectra and DFT calculated infrared spectra are compared to investigate the effect of adsorbed nitrogen on the OH-stretch band complex of water clusters. Using a collisional cooling experiment, pure as well as partially and completely N(2)-covered water clusters consisting of 20-200 water molecules have been generated in thermal equilibrium in the aerosol phase within the temperature range of 5-80 K. Computational IR-spectra simulations have been performed for discrete pure and N(2)-covered water clusters including 10, 15, 20, and 30 water molecules. The adsorbed N(2) molecules especially affect the three-coordinated water molecules at the cluster surface which could be observed as a blue shift of the companion O-H band at 2900 cm(-1) and a red shift of the dangling O-H band at 3700 cm(-1) by about 20 cm(-1) in both cases. The most striking effect of the N(2) adsorbate is an intensity increase of the dangling O-H band by a factor of 3-5. Furthermore, the onset temperature of nitrogen adsorption at the water cluster surface was experimentally found to be roughly 30 K for cluster sizes of about 100 water molecules. Experimental and computational results are in good agreement. The presented results are based on and support the work of V. Buch, J. P. Devlin, and co-workers (e.g., J. Phys. Chem. B, 1997; J. Phys. Chem. A, 2003; Int. Rev. Phys. Chem., 2004).
"ering our experimental results and the literature, the band at 3725 cm −1 is positively attributed to a water monomer interacting with the surface via its two electronic doublets; its large intensity compared to the other dH bands is explained by the magnifying effect of nitrogen, as extensively investigated by Hujo et al.. We suggest that nitrogen molecules, present as a lowlevel pollutant in the chamber, serendipitously complex the water molecule, as illustrated in Figure 4, stabilising the molecule, preventing any further adsorption, and magnifying the OH stretching bands. "
[Show abstract][Hide abstract] ABSTRACT: In the quest to understand the formation of the building blocks of life,
amorphous solid water (ASW) is one of the most widely studied molecular
systems. Indeed, ASW is ubiquitous in the cold interstellar medium (ISM), where
ASW-coated dust grains provide a catalytic surface for solid phase chemistry,
and is believed to be present in the Earth's atmosphere at high altitudes. It
has been shown that the ice surface adsorbs small molecules such as CO, N$_2$,
or CH$_4$, most likely at OH groups dangling from the surface. Our study
presents completely new insights concerning the behaviour of ASW upon selective
infrared (IR) irradiation of its dangling modes. When irradiated, these surface
H$_2$O molecules reorganise, predominantly forming a stabilised monomer-like
water mode on the ice surface. We show that we systematically provoke
"hole-burning" effects (or net loss of oscillators) at the wavelength of
irradiation and reproduce the same absorbed water monomer on the ASW surface.
Our study suggests that all dangling modes share one common channel of
vibrational relaxation; the ice remains amorphous but with a reduced range of
binding sites, and thus an altered catalytic capacity.
[Show abstract][Hide abstract] ABSTRACT: A simple method has been developed for the measurement of high quality FTIR spectra of aerosols of gas-hydrate nanoparticles. The application of this method enables quantitative observation of gas hydrates that form on subsecond timescales using our all-vapor approach that includes an ether catalyst rather than high pressures to promote hydrate formation. The sampling method is versatile allowing routine studies at temperatures ranging from 120 to 210 K of either a single gas or the competitive uptake of different gas molecules in small cages of the hydrates. The present study emphasizes hydrate aerosols formed by pulsing vapor mixtures into a cold chamber held at 160 or 180 K. We emphasize aerosol spectra from 6 scans recorded an average of 8 s after "instantaneous" hydrate formation as well as of the gas hydrates as they evolve with time. Quantitative aerosol data are reported and analyzed for single small-cage guests and for mixed hydrates of CO(2), CH(4), C(2)H(2), N(2)O, N(2), and air. The approach, combined with the instant formation of gas hydrates from vapors only, offers promise with respect to optimization of methods for the formation and control of gas hydrates.
The Journal of Chemical Physics 10/2011; 135(14):141103. DOI:10.1063/1.3652756 · 2.95 Impact Factor
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