1. ACCURATE EQUILIBRIUM STRUCTURES FOR trans-HEXATRIENE BY THE MIXED ESTIMATION METHOD AND FOR THE THREE ISOMERS OF OCTATETRAENE FROM THEORY; STRUCTURAL CONSEQUENCES OF ELECTRON DELOCALIZATION NORMAN C. CRAIG, Department of Chemistry and Biochemistry, Oberlin College, Oberlin, OH USA JEAN DEMAISON, Laboratoire de Physique des Lasers, Atomes et Molécules, Université de Lille 1, Villeneuve d’Ascq Cedex, France PETER GRONER, Department of Chemistry, University of Missouri–Kansas City, Kansas City, MO, USA HEINZ DIETER RUDOLPH and NATALJA VOGT, Chemieinformationssysteme, Universität Ulm, Ulm, Germany Norm.Craig@oberlin.edu

2. Isomers of hexatriene

3. Isomers of octatetraene

4. Mixed Estimation Method for Determining an Equilibrium Structure of trans-Hexatriene Structures good to 0.001 Å and 0.1° needed. Simultaneous fitting of bond parameters to a set of equilibrium moments of inertia and to bond parameters computed at a high level of theory. Each set of data has appropriate uncertainties attached.

5. Mixed estimation method Ground state rotational constants of trans-hexatriene from the analysis of the rotational structure in hi-res (0.0015 cm -1 ) IR spectra. For 2- 13 C 1 and 3- 13 C 1 from scaling QC predictions. Vibration-rotation interaction constants computed with the MP2/cc-pVTZ model and used to calculate equilibrium rotational constants from ground state rotational constants. Small corrections were made for the contribution of the electronic g factors.

6. Mixed estimation method Bond parameters at a high level of theory r e BO (I) = CCSD(T)_FC/cc-pVTZ + [MP2(FC)/cc-pVQZ – MP2(FC)/cc-pVTZ] + [MP2(AE)/cc-pwCVQZ – MP2(FC)/cc-pwCVQZ] Also by r e BO (II) = CCSD(T)_AE/cc-pwCVQZ Uncertainties ±0.002 Å, ±0.2°

7. Semiexperimental equilibrium structures of the isomers of hexatriene Butadiene: “C=C” 1.338 Å; “C–C” 1.454 Å. N. C. Craig, P. Groner, D. C. McKean J. Phys. Chem. A,, 110, 7461-7469 (2006). cis-Hexatriene structure. N. C. Craig, Y. Chen, H. A. Fuson, H. Tian, H. van Besien, A. R. Conrad, M. J. Tubergen, H. D. Rudolph, and J. Demaison, J. Phys. Chem. A, 117, 9391-9400 (2013).

8. Comparison of Estimates for the Structure of trans-Hexatriene

9. Isomers of Octatetraene Structures computed from the CCSD(T)_FC/cc-pVTZ model with the r e BO (I) method. trans-HTE C1=C2 1.339 Å C3=C4 1.346 Å C2–C3 1.449 Å

10. Conclusions Semiexperimental structure (r e SE ) for trans-hexatriene by the mixed estimation method. Good to 0.001 Å and 0.1°. Structure from high-level theory compares favorably with r e SE. cis-Hexatriene shows expansion of bond lengths (except C3– H3) and bond angles due to H3–H5 repulsion. Increased blurring between “C=C” and “C–C” bond lengths compared to butadiene – greater toward center of a molecule – due to π-electron delocalization. Structures for the three isomers of octatetraene determined by comparable high-level theory. The three isomers of octatetraene show more evidence of the structural consequences of π-electron delocalization.

11. Acknowledgments Dreyfus Foundation (Senior Scholar Mentor Grant to N.C.C.) Ohio Supercomputer Center Chemieinformationssysteme, U. Ulm Oberlin College