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44177-AC6
Structure and Dynamics of Highly Excited Vibrational States of Complexes of Water and Hydrogen Fluoride
We have completed a detailed high resolution study of the complex of hydrogen with carbonyl sulfide (OCS).
In this study the complex is formed by supersonic expansion of a mixture of hydrogen, OCS and an inert gas, either He or Ne. The effective temperature of the jet is 3K. Thus the hydrogen is in its lowest accessible rotational state. The five forms of hydrogen used are p-H2, o-H2, HD, o-D2 and p-D2. The spectral resolution was high, thus hyperfine structure was observed for all species except for p-H2 in which I = 0. Few very high resolution studies of hydrogen complexes exist, and none of those have studied all available forms of hydrogen. The reasons why the study of the intermolecular interactions of hydrogen are important range from the dominance of hydrogen throughout the universe as well as the growing interest in it as a nonpolluting fuel.
The rotational angular momentum , j , of the hydrogen forms studied are
p-H2 (j =0), o-H2 (j =1), HD (j =0),, o-D2(j =0), and p-D2 (j =1). These then generate the hydrogen-OCS complexes. Perhaps the most interesting discovery in our study is that the rotational transitions of o-H2-OCS and p-H2-OCS are striking similar
Transition (MHz) pH2-OCS oH2-OCS
101- 000 10595.586(-1) 10218.255(-1)
110- 101 17773.021(2) 18378.973(3)
211- 202 19210.809(-1) 19539.715(-2)
312- 303 21519.761(-1) 21381.849(1)
212- 111 19791.114(1) 19304.742(-1)
202- 101 21108.051(1) 20385.663(5)
111- 000 26990.819(0) 27477.568(-1)
as are the derived rotational constants.
Rotational Constant (MHZ)
p-H2-OCS o-H2-OCS
A 22401.889(4) 22942.218(6)
B 5993.774(2) 5675.156(7)
C 4602.038(2) 4542.960(7)
This result was quite puzzling in terms of earlier work on weakly bound complexes of hydrogen. Our initial interest in hydrogen OCS was the p-H2-OCS complex, OCS in mixtures of p-H2 and liquid helium shows beautifully resolved rotational structure. We found transitions for the complex quite readily which fitted well what was expected for p-H2-OCS. It was noted , however, that the 101- 000 transition at 10595.586MHz showed hyperfine structure establishing that it originated fromo-H2-OCS. We had expected an entirely different spectrum for o-H2-OCS, in view of the rotation of o-H2. To in fact understand the system all of the hydrogen forms were studied.
This resulted in a much deeper understanding of the hydrogen complexes than had previously existed.
The production of p-H2-OCS using normal hydrogen is readily accomplished if the stagnation composition is low in hydrogen. Having the spectra of both p-H2-OCS and o- H2-OCS it is easy to monitor their ratio . We find that at H2 of 1-2% the
p-H2-OCS /o- H2-OCS ratio is 1/3, while at 10% it has dropped to 1/100. This behavior is typical of reactive systems
From the relative spectral line intensities we infer that binding energies of the hydrogen- OCS complexes are:
He-OCS < pH2-OCS < Ne-OCS < oH2-OCS < HD-OCS < oD2-OCS < pD2-OCS.
The relative binding of pH2-OCS and oH2-OCS is understood from first order perturbation theory. The average of the three j=1 (o-H2) components is that of the j=0 (p-H2). The threefold degeneracy of j=1 is lifted in every geometry of the complex. For H2-OCS the location of the center of mass of the hydrogen is nearest the C atom. The symmetry point group is Cs which has only one dimensional irreducible representations. oH2-OCS has two in-plane and one out-of-plane hydrogen orientational functions. The actual functions are the Cartesian j=1 hydrogen rotor functions. Since the excitation from j=1 to j=3 is 10B(H2) = 600 cm-1 there will be no distortion of these by the weak intermolecular potential. The lowest orientational state has the hydrogen in the plane of the complex aligned close to parallel with the OCS axis. This is shown from the hyperfine structure due to the nuclear spin-spin (dipole-dipole) interaction, as well as from high a level electron structure calculation of the intermolecular potential.
We are presently measuring the spectrum of the first excited orientational state of oH2-OCS, where the H2 unit is out of plane. The hyperfine structure splitting is much reduced in the 101- 000 transition. However, the intensity of the transitions are very low as a result of the very low population of the excited state in the jet.
Our understanding of complexes with hydrogen has changed quite markedly as a result of the hydrogen-OCS study. We note, as an example, that it is clear why helium droplets containing o-H2 will not support rotation of OCS, as is observed with p-H2. In the o-H2 case there will be a large number of low lying excitations possible, as a consequence of the effective quenching of the o-H2 free rotation.
