JOURNAL OF MOLECULAR SPECTROSCOPY 149, 3 17-328 ( I99 1) zyxwvutsrqponmlkjihgfedcbaZYXWVUTS The Centimeter and Millimeter Microwave Spectra of Butadiene Sulfone and q&-D4 Butadiene Sulfone ALBERTO G. LESARRI, M. ELENA CHARRO, ROSA M. VILLAMAF&N, DAVID G. LISTER,’ JUAN C. LOPEZ, AND Jose L. ALONSO zyxwvutsrqponmlkjihg Departamento de Quimica-Fisica, Fan&ad de Ciencias. Universidad de Ihlladolid, E-47005 Ihlladolid. Spain Microwave measurements on the ground and first eight excited states of the ring-puckering vibration of butadiene sulfone have been extended to millimeter wavelengths. The microwave spectra of the same vibrational states of u.(Y’-D~ butadiene sulfone have been observed. For both isotopomers the Coriolis interaction between the u = 0 and u = 1 states has been analyzed to give the energy separation between these two states. These data and the variation of the rotational constants have been used to derive reduced potential functions for the ring-puckering vibration. The barrier to ring inversion is 49(2) cm-’ for butadiene sulfone and 44( 2) cm-’ for the a,<~‘- Da isotopomer. The ring-puckering vibrational dependence of the quartic centrifugal distortion constants, including a small dependence of hJ and 6,. has been accounted for. cc IYYI Aademlr Press. Inc. INTRODUCTION The microwave spectrum of butadiene sulfone (BDS, see Scheme 1) (I, 2) shows the molecule to have a nonplanar ring equilibrium conformation with a barrier to ring inversion of 50( 11) cm-‘. Ab initio computations using ST0 3-2 1 G* orbitals (3) show some structural relaxation involving the sulfone and methylene groups during the ring-puckering motion (2). Microwave measurements have now been extended to the ~,cx’-D~ isotopomer (BDS-D4, see Scheme 2) of butadiene sulfone in order to see whether experimental evidence for the structural relaxation of the methylene groups can be obtained from rotational spectroscopy. A value of = 1 D has been estimated for the pLc transition moment connecting the II = 0 and u = 1 states of BDS (2) and this indicates the possibility of observing transitions between these states. As in molecules such as propargyl mercaptan (4) and hydroxyacetonitrile (5) the high J pclc Q-branch transitions would be expected to give a strong bandhead close to A&, , the energy separation of the two vibrational states. In order to facilitate the search for such transitions the accuracy of A,!& for both isotopomers has been improved by extending measurements into the millimeter-wave region. This has been done in order to observe transitions with larger Coriolis pertur- bations. A further motive for making such measurements has been to improve the precision of the quartic centrifugal distortion constants in order to search for a ring- puckering dependence of the Watson centrifugal distortion constants (6) A, and 6J. ’ Permanent address: lstituto di Chimica Industriale, Cassella Postale 29,1-98 166 Sant’Agata di Messina. Italy. 317 0022-285219 I $3.00 Cop~nghl9 1991 by Academic Press. Inc. All nghts of reproduction I” anv form rrxrvcd.