15min:

NORMAN C. CRAIG AND MICHAEL C. MOORE, Department of Chemistry, Oberlin College, Oberlin, OH 44074; ROBERT L. SAMS, ESML Pacific Northwest National Laboratory, Richland, WA 99352; DONALD C. MCKEAN, School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U. K..

This report on the high-resolution (0.002 cm^-1) infrared spectroscopy of butadiene-2,3-¹³ C ₂ is a culmination of the investigation of rotational constants of a series of isotopomers of butadiene (BDE). The goal is an equilibrium structure of butadiene with which to answer a question. Does the structure of butadiene reflect pi-electron delocalization? Ground state rotational constants are now available for BDE, BDE-2,3-d₂,\footnoteN.~C.~Craig, J.~L.~Davis, K.~A.~Hanson, M.~C.~Moore, K.~ J. ~Weidenbaum, M.~Lock, J. Mol. Struct. \textbf2004, 695-696 , 59. BDE-1,1-d₂,\footnoteW.~Caminati, G.~Grassi, A.~Bauder, Chem. Phys. Lett. \textbf1988, 148 , 13. BDE-trans,trans-1,4-d₂, BDE-cis,trans-1,4-d₂, BDE-cis,cis-1,4-d₂,\footnoteN.~C.~Craig, K.~A.~Hanson, R.~W.~Pierce, S.~D.~Saylor, R.~L.~Sams, J. Mol. Spectrosc. \textbf2004, 228 , 401. and BDE-1-¹³C₁. BDE-2,3-¹³C₂ has been synthesized and its ground state rotational constants have been derived from the analysis of a C-type band arising from CH₂ flapping at 907.1722 (2) cm^-1. These values, which are fit to 1184 ground state combination differences, include A = 1.3545028 (6), B = 0.1469410 (2), and C = 0.1325846 (2) cm^-1. Medium-resolution infrared and Raman spectroscopy have led to a complete assignment of vibrational fundamentals, which are useful in assessing a new force field for butadiene. The new force field depends on selectively scaled force constants in symmetry coordinate space. The final step in this project is obtaining good vibration-rotation alphas predicted by quantum chemical calculations for use in extracting equilibrium rotational constants and an equilibrium structure.