A new experimental method has been developed to conduct surface chemistry and extract surface kinetic rates on size-selected nanoparticles. The method utilizes a tandem differential mobility analyzer (TDMA) technique in which monodisperse particles are selected from a polydisperse aerosol input stream and then subjected to chemical processing. The change in particle size is measured and used to determine kinetic information for the relevant surface reaction. The method has been applied to measure the oxidation rate of soot in air over the temperature range 800-1120°C. Soot was generated in situ using an ethylene diffusion flame and sent to a differential mobility analyzer (DMA) to extract monodisperse particles. Three initial particle sizes of 40, 93, and 130 nm mobility diameter were subjected to oxidation in a high-temperature flow reactor, and the resulting change in particle size was measured with a second DMA. The measured size decreases were fit to a model utilizing a modified Arrhenius expression for the rate of decrease: Ḋ p ) -A nm T 1/2 exp(-E a /(RT)). The fit yielded an activation energy of E a ) 164 kJ mol -1 with a different preexponential factor, A nm , for each initial particle size. The size-decrease rates, and therefore the preexponential factors, differed by a factor of 1.7 between the 40 and 130 nm particles, with the 130 nm particles decreasing faster than the 40 or 93 nm particles. This may be the result of several factors including different effective densities or different soot particle compositions. The current experiments are the first measurements of the soot oxidation rate to be performed on size-selected, freshly generated soot particles using online aerosol techniques. Our results agree well with previous work over the temperature range covered, which is somewhat surprising given the wide range of techniques and materials previously studied in the effort to understand soot oxidation.

The nitric acid‐water complex is studied by high level ab initio calculations. The equilibrium structure of the complex has an asymmetric doubly hydrogen‐bonded ring. A strong hydrogen bond is donated by the hydroxyl group of nitric acid to the oxygen atom of water, and a much weaker hydrogen bond is donated by an OH bond of water to a second oxygen of nitric acid. The HNO3 unit in the complex is considerably distorted with the OH stretching frequency red‐shifted by over 300 cm−1 from the isolated molecule and the infrared intensity enhanced by an order of magnitude. The binding energy of the complex is calculated as De = −9.5 kcal/mol. The equilibrium fraction of nitric acid complexed with water is predicted to be near 1% at the Earth's surface and falls with the increase of altitude.

The dynamics of molecules trapped inside or bound to the surface of liquid helium droplets is a most exciting new development. As an aid toward the understanding of the spectrum of OCS in liquid helium, we report the intermolecular potential between He and OCS studied by ab initio calculations and high-resolution microwave spectroscopy. The potential is found to have three minima, corresponding to a T-shaped configuration for the global minimum and secondary minima at either end of the OCS molecule. The three lowest calculated bound states are loosely localized in each of the minima, with the ground state being T-shaped. Ten rotational transitions of the ground state are observed in the frequency region 1.5–45.0 GHz. Comparison of theory with experiment shows good agreement. The agreement improves substantially if the calculated He–OCS intermolecular potential is made uniformly 10% deeper and the He–OCS separation is reduced by 0.05 Å.

The technique of high-temperature oxidation tandem differential mobility analysis has been applied to the study of diesel nanoparticle oxidation. The oxidation rates in air of diesel nanoparticles sampled directly from the exhaust stream of a medium-duty diesel engine were measured over the temperature range of 800-1140 degrees C using online aerosol techniques. Three particle sizes (40, 90, and 130 nm mobility diameter) generated under engine load conditions of 10, 50, and 75% were investigated. The results show significant differences in the behavior of the 10% load particles as compared to the 50 and 75% load particles. The 10% load particles show greater size decrease at temperatures below 500 degrees C and significant size decrease at temperatures between 500 and 1000 degrees C in a non-oxidative environment, indicating release of adsorbed volatile material or thermally induced rearrangement of the agglomerate structure. Activation energies determined are 114, 109, and 108 kJ mol(-1) for the 10, 50, and 75% load particles, respectively. These activation energies are lower than for flame soot (Higgins et al. J. Phys. Chem. A 2002, 106, 96), but the preexponential factors are lower by 3 orders of magnitude, and the overall oxidation rates are slower by up to a factor of 4 over the temperature range studied. Possible reasons for the differences are discussed in the text.

Rotational spectroscopy and ab initio calculations have been used to characterize the complexes H(3)N-HF and H(3)N-HF-HF in the gas phase. H(3)N-HF is a C(3v) symmetric, hydrogen bonded system with an NF distance of 2.640(21) A and an N...H hydrogen bond length of 1.693(42) A. The H(3)N-HF-HF complex, on the other hand, forms a six-membered HN-HF-HF ring, in which both the linear hydrogen bond in the H(3)N-HF moiety and the F-H-F angle of (HF)(2) are perturbed relative to those in the corresponding dimers. The N...F and F...F distances in the trimer are 2.4509(74) A and 2.651(11) A, respectively. The N...H hydrogen bond length in H(3)N-HF-HF is 1.488(12) A, a value which is 0.205(54) A shorter than that in H(3)N-HF. Similarly, the F...F distance, 2.651(11) A, is 0.13(2) A shorter than that in (HF)(2). Counterpoise-corrected geometry optimizations are presented, which are in good agreement with the experimental structures for both the dimer and trimer, and further characterize small, but significant, changes in the NH(3) and HF subunits upon complexation. Analysis of internal rotation in the spectrum of H(3)N-HF-HF gives the potential barrier for internal rotation of the NH(3) unit, V(3), to be 118(2) cm(-1). Ab initio calculations reproduce this number to within 10% if the monomer units and the molecular frame are allowed to fully relax as the internal rotation takes place. The binding energies of H(3)N-HF and H(3)N-HF-HF, calculated at the MP2/aug-cc-pVTZ level and corrected for basis set superposition error are 12.3 and 22.0 kcal/mol, respectively. Additional energy calculations have been performed to explore the lowest frequency vibration of H(3)N-HF-HF, a ring-opening motion that increases the NFF angle. The addition of one HF molecule to H(3)N-HF represents the first step of microsolvation of a hydrogen bonded complex and the results of this study demonstrate that a single, polar near-neighbor has a significant influence on the extent of proton transfer across the hydrogen bond. As measured using the proton-transfer parameter rho(PT), previously defined by Kurnig and Scheiner [Int. J. Quantum Chem., Quantum Biol. Symp. 1987, 14, 47], the degree of proton transfer in H(3)N-HF-HF is greater than that in either (CH(3))(3)N-HF or H(3)N-HCl but less than that in (CH(3))(3)N-HCl.