Millimetre-wave spectrum of (HCO)-O-17+. Experimental and theoretical determination of the quadrupole coupling constant of the O-17 nucleus

The millimetre-wave spectrum of (HCO+)-O-17 has been analyzed up to 348.2 G Hz by recording the J = 2 <-- 1 and J = 4 <-- 3 rotational transitions. Pre sent measurements and the previous detection of the J = 1 <-- 0 transition carried out in this laboratory allowed us to determine accurate values of t he rotational and centrifugal distortion constants, and of the nuclear quad rupole coupling (<chi>) and spin-rotation constants. Moreover, chi has been evaluated from the electric field gradient at the oxygen nucleus calculate d by using the multiconfiguration self-consistent field approach plus subse quent multireference configuration interaction computation, employing basis sets of quadruple zeta quality. Excellent agreement with experiment has be en obtained. In addition, the molecular dipole moment has been calculated a t the same level of accuracy.