Heterogeneous ice nucleation is an important mechanism for cloud formation in the upper troposphere. Recently, pores on atmospheric particles have been proposed to play a significant role in ice nucleation. To understand how ice nucleation occurs in idealized pores, we characterized the immersion freezing activity of various sizes of carbon nanotubes. Carbon nanotubes are used both as a model for pores and proxy for soot particles. We determined that carbon nanotubes with inner diameters between 2 and 3 nm exhibit the highest ice nucleation activity. Implications for the freezing behavior of porous materials and nucleation on soot particles will be discussed.
Microwave spectra of N,N'-dimethylaniline and 4,4'-dimethylaminobenzonitrile have been recorded in a pulsed supersonic jet using chirped pulse techniques. Experimental substitution structures have been determined for both molecules by detection of the spectra of all (13)C and (15)N isotopomers in natural abundance using a broadband spectrometer. Additionally, a narrowband spectrometer has been used to reveal the (14)N quadrupole splittings at high resolution, from which the orbital occupancy numbers of the amino- and cyano-nitrogen atoms have been determined. An apparent direct relationship between these values and the barriers to inversion of the amino groups is discussed.
Mineral dust is prevalent in the atmosphere due to emissions from natural and anthropogenic sources. As mineral dust particles undergo long distance transport, they are exposed to trace gases and water vapor. We have characterized the interactions of acetic acid on kaolinite using Diffuse Reflectance Infrared Fourier Transform Spectroscopy and molecular modeling to determine the chemisorbed species present. After the addition of acetic acid, gas phase water was introduced to explore how water vapor competes with acetic acid for surface sites. We have found that four chemisorbed acetate species are found on kaolinite after exposure to acetic acid in which acetate bonds through a monodentate, bidenatate, or bidentate bridging linkage with an aluminum atom. These species exhibit varying levels of stability after the introduction of water, indicating that water vapor affects the adsorption of organic acids. These results indicate that the type of chemisorbed species determines its stability toward competitive adsorption, which has potential implications for atmospheric composition and ice nucleation.
Errors appear in eqs 8−12, owing to a change in coordinates. (The correct coordinates are shown in the existing Figure 7.)Interchange χ yy and χ zz in eqs 8 and 9. Also, change 3 to 0.3 in the denominator of eq 9.Permute N x , N y , and N z in eqs 10−12, so that they give, respectively, the orbital occupancies N y , N z , and N x .The correct equations were used in all calculations. Therefore, the results reported in Tables 3, 4, and 5 and the conclusions of the paper remain unchanged.
■ ACKNOWLEDGMENTSWe thank Dr. Melanie Schnell for bringing these problems to our attention.
Reported here are CP-FTMW experiments on N-ethylformamide (NEF), previously studied by pure rotational spectroscopy in the gas phase by Hirota and co-workers [J. Mol. Struct. 744 (2005) 815-819]. Two conformers of the monomer (trans sc and cis ac), the trans-trans sc dimer, and several NEF-water complexes have been detected. (Only trans sc was observed in the earlier experiments.) Of these species, the trans-trans sc dimer is the most interesting since its three-dimensional structure bears a striking resemblance to β-turns in folded peptides. The well-known anti-parallel β-sheet structures would be expected to have near-zero dipole moments and are therefore discriminated against by the CP-FTMW method.
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