No abstract

In this situation, the Q* approach can be applied as a useful alternative to a direct calculation of constants. We have shown that the Q* approach can be used in those cases in which nuclear quadrupole coupling constants are rather small ( |3 < 1 MHz).However, both HF/ and MP2/ small basis set calculations are not sufficient in those cases. One has to use at least MP3 in connection with a TZ+P basis set to get reasonable values, which has to do with the fact that for MP3 there is a fortuitous cancellation of basis set and correlation errors. Investigation of 10 isonitriles with g*(14N) = 16.9 (5) MHz determined at MP3/ 6-31 lG(d,p) leads to reasonable ß values, which can be used to correctly assign the microwave spectra of hitherto uninvestigated molecules.Acknowledgment. This work was supported by the Swedish Natural Science Research Council (NFR). M.K. thanks the Verband Chemischer Industrie for financial support. Calculations were carried out on the CRAY XMP/48 of the Nationellt Superdatorcentrum (NSC), Linkóping, Sweden. D.C. thanks the NSC for a generous allotment of computer time.

The infrared spectrum of ammonium fluoride and deutero-ammonium fluoride at —195°C and 25°C is shown to be explicable in terms of ``essentially'' Td symmetry requiring that the axial ratio (c/a) be 1.63. New x-ray-lattice parameter measurements show this to be true. A comparison of the spectrum of NH4F with other ammonium salts indicates that broad bands do not necessarily accompany strong hydrogen bonds.

Experimental results concerning the ascent of sap in the xylem are usually interpreted in terms of gradients of hydrostatic pressure in the xylem conduits. In this report an alternative model is proposed that is equally consistent with the experimental results: under static conditions the water column is supported by a gradient in the chemical activity of the water, and the hydrostatic pressure is constant throughout. Observations that support the new model are cited, and experiments are suggested that would permit a choice between the two models.