Journal of Molecular Structure (Theochem), 151(1987) 341-354 zyxwvutsrqponmlkjihgfedcbaZYXW Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands zyxwvutsrqponmlkjihg SCALED QUANTUM MECHANICAL (SQM) FORCE FIELD AND VIBRATIONAL ASSIGNMENT FOR HEXATRIENE GEZA FOGARASI and PETER G. SZALAY Eiitviis Lorhnd University, Laboratory of Theoretical Chemistry, H-1088 Budapest, Muzeum krt. 6-8, (Hungary) PHILLIP P. LIESCHESKI and JAMES E. BOGGS University of Texas, Department of Chemistry, Austin, TX 78 712 (U.S. A.) PETER PULAY University of Arkansas, Department of Chemistry, Fayetteville, AR 72701 (U.S.A.) (Received 18 November 1986) ABSTRACT The geometry and complete harmonic force field for the all-trans form of 1,3,5-hexa- triene have been determined from ab initio Hartree-Fock calculations. To account for systematic errors, the theoretical force field was scaled by empirical factors taken over fixed from butadiene. The scaled quantum mechanical (SQM) force field obtained this way reproduces the fundamental vibrational frequencies of hexatriene within 20 cm-’ in most cases, which gives a good basis for analyzing the spectral assignment. In the light of a slight discrepancy between theory and experiment for the C=C stretching frequencies and bond lengths, the question of how different the two kinds of double bonds in hexa- triene are is raised. INTRODUCTION zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1,3,5-hexatriene (referred to in the following simply as hexatriene) is an interesting prototype of conjugated n-electron systems which has been the subject of numerous structural studies. The molecule has two stable confor- mers, the all-trans (tTt) form and a cis (Et) form. Only the all-truns confor- mer is considered in the present study. Among previous studies, those on the geometry and vibrational spectrum are of interest here. For the geometry, electron diffraction studies have been reported by Traetteberg [ 1,2] , The vibrational spectrum has been measured and analyzed in detail by Lippincott et al. [3, 41. Several later works dealt with the spectrum using normal coordinate analyses based on the original data of Lippincott without new measurements [5-lo]. On the deuterated isotopomer, the only experimental spectroscopic study is due to Panchenko et al. [ll] . A quantum chemical calculation of the force field, using the semiempirical CNDO method has also been performed recently [12]. Molecules of medium complexity are now accessible to calculations at the 0166-1280/87/$03.50 o 1987 Elsevier Science Publishers B.V.