Galen Sedo

Rotational spectrum of 1,1,1-trifluoro-2-butanone using chirped-pulse Fourier transform microwave spectroscopy.

The pure rotational spectra of 1,1,1-trifluoro-2-butanone and its four (13)C isotopologues have been studied using the new chirped-pulsed Fourier transform microwave spectrometer at the University of Manitoba in combination with a conventional Balle-Flygare-type instrument. Quantum chemical calculations, at the MP2/6-311++G(d,p) level, were carried out to obtain information about the structure, relative stability, and difference in populations of the three lowest energy conformers corresponding to dihedral angles of 0°, 82.8°, and 119.2° along the carbon backbone. The observed spectra are that of conformer I (dihedral angle 0°), and, based on analysis of the observed splitting, the V(3) barrier to internal rotation of the methyl group has been determined to be 9.380(5) kJ mol(-1). The spectroscopic constants of the five isotopologues were used to precisely derive the r(s) and partial r(0) geometries of this conformer based on an assumed planar carbon backbone (as supported by the spectra and ab initio calculations).

Partial Proton Transfer in the Nitric Acid Trihydrate Complex

Four isotopologues of the gas-phase complex HNO(3)-(H(2)O)(3) have been observed by microwave spectroscopy in a supersonic jet. Rotational and nuclear electric quadrupole coupling constants have been obtained and the experimentally derived inertial defect has been used to infer a near-planar geometry for the complex. The data identify the observed species from among several structures predicted by theory, favoring a 10-membered ring geometry with the HNO(3) hydrogen-bonded to the first water, a series of water-water hydrogen bonds, and ring completion with the third water acting as a hydrogen-bond donor to an unprotonated HNO(3) oxygen. This structure corresponds to the lowest energy form predicted computationally in several prior studies as well as in this work using the MP2/6-311++G(2df,2pd) level/basis set. Although its observation does not rigorously establish its status as the lowest energy form, the concurrence between the predicted low-energy conformer and that observed in the ultracold supersonic jet strongly suggests that it is indeed the minimum-energy structure. The a-type spectra show evidence of internal dynamics, likely resulting from large amplitude motion of one or more of the water subunits. This complex represents the third step in the sequential hydration of HNO(3), and both the theoretical structure and experimental (14)N quadrupole coupling constants have been used to track the degree of ionization of the acid as function of hydration number. Based on (14)N quadrupole coupling constants, transfer of the HNO(3) proton to its nearest water molecule is about one-third complete in the trihydrate.

Microwave spectra of O2-HF and O2-DF : Hyperfine interactions and global fitting with infrared data

Spectra of the open shell complexes O(2)-HF and O(2)-DF were recorded using Fourier transform microwave spectroscopy. A complete analysis of the hyperfine structure and a global fit including microwave and infrared frequencies [W. M. Fawzy, C. M. Lovejoy, D. J. Nesbitt, and J. T. Hougen, J. Chem. Phys. 117, 693 (2002)] are reported. The Fermi contact interaction between the electron and nuclear spins, the electron spin-nuclear spin dipolar interaction, the nuclear spin-nuclear spin dipolar interaction, and the nuclear electric quadrupole interaction (for O(2)-DF) were considered in the analysis. The correspondence between the magnetic hyperfine constants and the two nuclei of the H(D)F is unambiguously established. In both O(2)-HF and O(2)-DF, the Fermi contact parameter is larger for the fluorine than for the hydrogen, while for the nuclear spin-electron spin dipolar hyperfine constants, the reverse is true. The effective angle between the HF bond and the a axis of the complex, determined from the nuclear spin-nuclear spin interaction constant, is 38(4) degrees. The same angle for the DF complex, derived from the deuterium nuclear quadrupole coupling constant, is 31(4) degrees.

Partial Proton Transfer in a Molecular Complex: Assessments From Both the Donor and Acceptor Points of View

Microwave spectra have been observed for the gas phase complexes (CH(3))(3)(14)N-H(14)NO(3) and (CH(3))(3)(15)N-H(14)NO(3) and rotational and nuclear quadrupole coupling constants are reported. The structure and binding energy have also been calculated at the MP2 level of theory using the 6-311++G(d,p) and 6-311++G(2df,2pd) basis sets both with and without corrections for basis set superposition error. The HNO(3) forms a near-linear hydrogen bond to the amine nitrogen with a rather short hydrogen bond distance of about 1.5-1.6 Å (depending on the basis set and method of computation). The C(3) axis of the trimethylamine lies in the plane of the nitric acid. For both the H(14)NO(3) and the (CH(3))(3)(14)N moieties of the parent species, the component of the nuclear quadrupole coupling tensor perpendicular to the molecular symmetry plane, χ(cc), is sensitive to the electronic structure at the corresponding nitrogen but independent of relative orientation within the plane. Its value, therefore, provides a convenient experimental measure of the degree of proton transfer within the complex. For the HNO(3), χ(cc) lies 62% of the way between those of free HNO(3) and aqueous NO(3)(-), indicating a substantial degree of proton transfer. A similar comparison of the quadrupole coupling constant of (CH(3))(3)N in the (CH(3))(3)N-HNO(3) complex with those of free (CH(3))(3)N and (CH(3))(3)NH(+) indicates only about 31% proton transfer, about half that determined from the HNO(3) coupling constant. Though surprising at first, this disparity is to be expected if the quadrupole coupling constants vary nonlinearly with the position of the proton relative to the donor and acceptor atoms. Calculations of the (14)N nuclear quadrupole coupling constants as a function of proton position using density functional theory are reported and confirm that this is the case. We suggest that when proton transfer is assessed according to changes in individual monomer molecular properties, the overall process may be best described in terms of a dual picture involving proton release by the acid and proton acquisition by the base.

THE COMPLETE HEAVY-ATOM STRUCTURE OF A CP-FTMW CHIRAL TAG PRECURSOR, VERBENONE

FRANK E MARSHALL, Department of Chemistry, Missouri University of Science and Technology, Rolla, MO, USA; CHANNING WEST, GALEN SEDO, Department of Natural Sciences, University of Virginia’s College at Wise, Wise, VA, USA; BROOKS PATE, Department of Chemistry, The University of Virginia, Charlottesville, VA, USA; G. S. GRUBBS II, Department of Chemistry, Missouri University of Science and Technology, Rolla, MO, USA.