18 February 2000 Ž . Chemical Physics Letters 318 2000 222–231 www.elsevier.nlrlocatercplett The nuclear quadrupole moment of 91 Zr from molecular data for ZrO and ZrS Vladimir Kello a,b,1 , Pekka Pyykko c, ) , Andrzej J. Sadlej a,d,2 , Peter Schwerdtfeger e , ¨ ¨ Jørn Thyssen e,f a Department of Theoretical Chemistry, Chemical Centre, UniÕersity of Lund, Box 124, S-221 00 Lund, Sweden b Department of Physical Chemistry, Faculty of Sciences, Comenius UniÕersity, Mlynska dolina, SK-842 15 BratislaÕa, SloÕakia c ( ) Department of Chemistry, UniÕersity of Helsinki, P.O. Box 55 A. I. Virtasen aukio 1 , FIN-00014 Helsinki, Finland d Department of Quantum Chemistry, Institute of Chemistry, Nicolaus Copernicus UniÕersity, 7 Gagarin Street, PL-87 100 Torun, Poland ´ e Department of Chemistry, UniÕersity of Auckland, PriÕate Bag 92019, Auckland, New Zealand f Department of Chemistry, UniÕersity of Southern Denmark, DK-5230 Odense M, Denmark Received 10 November 1999; in final form 4 January 2000 Abstract Ž. Quasirelativistic Douglas–Kroll calculations at levels up to CCSD T , combined with the recent experimental zirconium Ž 91 . Ž. Ž y28 nuclear quadrupole coupling constants for diatomic ZrO and ZrS, yield a Q Zr of y176 3 millibarn mb; 1 barn is 10 2 . Ž. Ž . m . This value is supported by fully relativistic CCSD T calculations using a slightly smaller basis. The new Q Zr is much smaller than the available ‘atomic’ values of y206 to y257 mb. q 2000 Elsevier Science B.V. All rights reserved. 1. Introduction Over the years, primarily owing to developments in the accurate calculations of electric field gradients in molecules, the determination of nuclear quadrupole wx moments eQ from molecular data 1 has become one of the most important and reliable sources of this w x nuclear property 2,3 . This method has already con- tributed to the resolution of discrepancies between ) Corresponding author. Fax: q358-91-914-0169; e-mail: pekka.pyykko@helsinki.fi 1 Department of Physical Chemistry, Faculty of Sciences, Comenius University, Mlynska dolina, SK-842 15 Bratislava, Slovakia. 2 Department of Quantum Chemistry, Institute of Chemistry, Nicolaus Copernicus University, 7 Gagarin Street, PL-87 100 Torun, Poland. ´ their values obtained from different theoretical and w x experimental sources 4,5 . Equally important is the improvement over the accuracy of the existing nu- clear quadrupole moment data. From these points of view, new accurate measurements of the nuclear quadrupole coupling constants in small, preferably diatomic, molecules are welcome. High-precision microwave spectra of ZrO and ZrS have recently been recorded by Beaton and Gerry wx 91 6 . The hyperfine structure due the Zr nucleus has Ž 1 q . been observed in the spectra of the ground S electronic state of both molecules and accurate val- Ž ues of the nuclear quadrupole coupling constant B 2 . s e qQrh are given. The uncertainty of these data, which follow from a rather sophisticated fitting scheme, is of the order of less than 0.005%. Thus, these recent experimental data for ZrO and ZrS may 0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 00 00031-2