CALCULATED SPIN-SPIN COUPLING SURFACES IN THE WATER MOLECULE - PREDICTION AND ANALYSIS OF J(O, H), J(O, D) AND J(H, D) IN WATER ISOTOPOMERS

Ab initio symmetry and internal valence coordinate oxygen-proton and p roton-proton spin-spin coupling surfaces for the water molecule have b een computed for the first time. Calculations have been performed at t he SOPPA (CCSD) level using a large basis set and a grid of forty-nine geometries on the two surfaces. Equilibrium values differ significant ly from some other calculated values especially for the Fermi contact terms. The bond length dependence of J(O, H) is 'normal' i.e. J(O, H-1 ) is much more sensitive to stretching the O-H-1 bond than the O-H-2 b ond. This contrasts greatly with the corresponding situation in methan e. The surfaces have been averaged over the nuclear motion using a rec ent highly accurate force held to give values of J(O, H) and J(O, D) f or (H2O)-O-17, (HDO)-O-17 and (D2O)-O-17 and J(H, D) for (HDO)-O-16, ( HDO)-O-17 and (HDO)-O-18 over a range of temperatures. For J(O,H) and J(O, D) bond stretching at first order is the dominant part of the nuc lear motion correction with second order bending making an important c ontribution. For J(H, D) the second order bending is by far the larges t contribution to the nuclear motion corrections although the other te rms partially cancel this contribution. Non-additivity can be largely attributed to the bending term for J(O, H). As expected, the bending t erms also contribute relatively more to the temperature dependence of the couplings for J(O, H), J(O, D) and J(H, D). Our calculated J(O, H) in H-2 O-17 of -77.22 Hz at 293 K is in very good agreement with Wasy lishen and Friedrich's observed value of -78.70 (+/-0.02) Hz in cycloh exane at this temperature. Our calculated J(H, D) in (HDO)-O-16 at 323 K of -1.233 Hz is close to a recent experimental value of -1.114 (+/- 0.003) Hz in nitromethane-d(3) observed by Sergeyev et al. at that tem perature.