Tunable farhyphen;infrared laser spectroscopy of hydrogen bonds: TheKa=0(u)rarr;1(g) rotationndash;tunneling spectrum of the HCl dimer

摘要

The ground stateKa=0(u)rarr;1(g)bhyphen;type subband of the rotationndash;tunneling spectrum of the symmetric35Clndash;35Cl,37Clndash;37Cl, and the mixed35Clndash;37Cl hydrogen chloride dimers have been recorded near 26.3 cmminus;1with subhyphen;Doppler resolution in a continuous twohyphen;dimensional supersonic jet with a tunable farhyphen;infrared laser spectrometer. Quadrupole hyperfine structure from the chlorine nuclei has been resolved. From the fitted rotational constants a (H35Cl)2centerhyphen;ofhyphen;mass separation of 3.81 Aring; is derived for theKa=1(g) levels, while the nuclear quadrupole coupling constants yield a vibrationally averaged angular structure for both tunneling states of approximately 20ndash;25 deg for the hydrogen bonded proton and at least 70ndash;75 deg for the external proton. This nearly orthogonal structure agrees well with that predicted byabinitiotheoretical calculations, but the observed splittings and intensity alterations of the lines indicate that the chlorine nuclei are made equivalent by a large amplitude tunneling motion of the HCl monomers. A similar geared internal rotation tunneling motion has been found for the HF dimer, but here the effect is much greater. The ground state tunneling splittings are estimated to lie between 15ndash;18 cmminus;1, and the selection rules observed indicate that thetranstunneling path dominates the large amplitude motion, as expected, provided the dimer remains planar. From the observed hyperfine constants, we judge the dimer and its associated tunneling motion to be planar to within 10deg;.