Matthew B. Craddock

Microwave spectrum, structure, and internal dynamics of the nitric acid dihydrate complex.

A-type rotational spectra of the complex HNO3-(H2O)2 have been observed by rotational spectroscopy in a supersonic jet. Extensive isotopic substitution and analysis of the resulting moments of inertia reveals that the complex adopts a cyclic geometry in which a second water inserts into the weak secondary hydrogen bond of the (also cyclic) HNO3-H2O dimer. The complex is planar, except for one free proton from each water unit that lies above or below the plane. The primary hydrogen bond, formed between the HNO3 proton and the first water molecule in the trimer, is 1.643(76) A in length. All intermolecular distances are smaller than those of the constituent dimers. Internal motion, inferred from spectral doubling and studied by isotopic substitution experiments, likely corresponds to proton interchange involving the second water unit, but no such motion is revealed by the a-type spectrum for the first water unit. The degree of proton transfer across the hydrogen bond is discussed in terms of the proton-transfer parameter, rhoPT, which assesses the degree of ionization on the basis of interatomic distances. Measured in this way, the complex is best described as hydrogen bonded, in accord with numerous theoretical predictions. However, an increase in the degree of ionization relative to that in HNO3-H2O is discernible. Using rhoPT as a metric, two water molecules do less to ionize nitric acid than one water does to ionize sulfuric acid.

Microsolvation of a partially bonded complex by non-polar and weakly polar microsolvents: a microwave and ab initio study of HCN–SO3–Ar and HCN–SO3–CO

Rotational spectra of HCN–SO3–Ar and HCN–SO3–CO, and several of their isotopically substituted derivatives, have been observed by Fourier transform microwave spectroscopy. Both complexes are symmetric tops with the Ar or CO weakly bound to the SO3 on the side opposite the HCN. The N–S and S–Ar distances in HCN–SO3–Ar are 2.5905(19) and 3.4465(50) Å, respectively. In HCN–SO3–CO, the N–S and S–C distances are 2.6563(14) and 3.0109(48) Å, respectively, representing a marked increase relative to the analogous distances in HCN–SO3 and SO3–CO. These results are in sharp contrast to previous work on HCN–HCN–SO3, in which the second HCN attaches to the HCN side of the HCN–SO3 and causes a large contraction of the partially formed N–S bond. Stark effect measurements have been performed and the resulting dipole moments are 4.230(10) D for HC15N–SO3–Ar and 3.678(11) D for HC15N–SO3–CO. These values are very near the differences between the dipole moments of the constituent dimers, suggesting that the dipole moments of the trimers are the vector sums of the dimer moments. Ab initio calculations for both complexes in the HCN–SO3–X and X–HCN–SO3 configurations are reported.

Microsolvation of a partially bonded complex by non-polar and weakly polar microsolvents: a microwave and ab initio study of HCN–SO3–Ar and HCN–SO3–CO – ARTICLE WITHDRAWN

WITHDRAWN TO APPEAR IN SPECIAL ISSUE IN 2007