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42856-AC6
Microwave Studies of Open Shell Complexes
Work conducted under this grant has involved the observation and analysis of microwave spectra of both open and closed shell molecular complexes. In our annual report for Year 1, we reported on the observation and spectral analysis of the HF-O2 complex, isotopic work on the system OH-H2O, and spectroscopic studies of the acid complexes H2SO4-H2O, HNO3-(H2O)3, and HNO3-N(CH3)3. In this report, we describe new work on the DF-O2 system, a new measurement of the dipole moment of H2SO4, and additional isotopic studies on the HNO3-(H2O)3.
Our work on O2-HF involved the observation of six rotational transitions with complete resolution of the magnetic hyperfine structure arising from the interaction of the H and F nuclear spins with the electronic magnetic moment of the O2. The analysis was accomplished by coupling two spin angular momenta to total spin and rotational angular momentum of the molecule, and the result is two sets of hyperfine constants. Spectra of HF-O2 alone, however, do not unambiguously establish which set of constants belongs to which nucleus. To solve this problem, we recorded new spectra of DF-O2, which shares a common fluorine nucleus with HF-O2, but differs in the isotope of hydrogen. The analysis was modified to include the nuclear electric quadrupole interaction of the deuterium, and a clear correspondence between nuclei and hyperfine constants was established. We find the somewhat counterintuitive result that the Fermi contact parameter on the hydrogen is so near zero as to be indeterminate, while that for the fluorine is considerably larger. We argue that this implies that the interaction does not arise from direct overlap of the in-plane unpaired electron of the O2 with the HF, but rather involves an indirect transfer of spin polarization. The nuclear – electron spin dipolar interaction constants are more commensurate with the expected inverse r3 distance dependence. A manuscript has been submitted.
Following our work last year on the dipole moment of H2SO4-H2O,[1] we became interested in the experimental value of the dipole moment of H2SO4 itself. Sulfuric acid and its hydrates are important in the formation of atmospheric sulfate aerosol, and new models are emerging which incorporate charge-dipole interactions into the calculation of new particle formation rates.[2] We had observed a curious discrepancy between theoretical and experimental dipole moments for H2SO4, enough that we decided to re-examine a 25 year old measurement of this quantity. In doing so, we have obtained a revised value of 2.9643(67) D and have performed a series of ab initio and DFT calculations which concur with this result. A manuscript on this work has also been submitted.
Over the past year, we have also expanded our work on HNO3-(H2O)3 by locating and analyzing spectra of the DNO3 complex. Calculations in the literature[3] predict two stable minima in the potential surface of this complex. One involves a ten-membered ring comprised of all four moieties, while the other contains an eight-membered ring involving HNO3 and two of the waters, with the third water peripheral to the ring. The calculated energies, however, are very close, and it is of interest to know which is more stable. Our isotope shifts confirm the ten-membered ring structure is the one observed. We have also examined the 14N nuclear quadrupole coupling constants for the series HNO3-H2O,[4] HNO3-(H2O)2,[5] and HNO3(H2O)3 and have found evidence of increasing progress toward ionization of the acid, though all three systems are still best described as hydrogen bonded. A manuscript is in preparation.
1. Brauer, C.S.; Sedo, G.; Leopold, K.R. Geophys. Res. Lett. 2006, 33, L23805.
2. Nadykto, A.B.; Yu, F. Phys. Rev. Lett. 2004, 93, 016101-1-4.
3. Escribano, R.; Couceiro, M.; Gómez, P.C.; Carrasco, E.; Moreno, M.A.; Herrero, V.J.; J. Phys. Chem. A 2003, 107, 651.
4. Canagaratna, M.; Phillips, J.A.; Ott, M.E.; Leopold, K.R. J. Phys. Chem. A 1998, 102, 1489.
5. Craddock, M.; Brauer, C.S.; Leopold, K.R., J. Phys. Chem. A, submitted.
