Ab Initio Structures for 90°-Twisted s-trans1,3-Butadiene and Cyclooctatetraene; the Naked sp2-sp2 Bond and Other Disclosures about Equilibrium Structures of Butadiene David Feller, Department of Chemistry, Washington State University, Pullman, WA 99164 Norman C. Craig and Albert R. Matlin, Department of Chemistry and Biochemistry, Oberlin College, Oberlin, OH 44074 Norm.Craig@oberlin.edu Outline • Structures of various 1,3-butadiene rotamers from ab initio calculations. • Lengths of the naked sp2-sp2 single bond and the localized C=C bond. • Comparison with the structure of cyclooctatetraene from ab initio calculations. • Comparison with the structure of a dimer of butadiene ([4]-dendralene) from electron diffraction. trans | t.s. gau. t.s.(cis) | | | - trans. st ate (t-to-g) 24.8 kJ/mol 77° - trans. st ate (cis) 16.5 kJ/mol 180° - gauche 12.0 kJ/mol 142° (38°) From analysis of the structure in multiple transiti ons of the torsional overtone in the gasphase Raman spectrum. R. Engeln, D. Consalvo, J. Reuss, Chem. Phys. 1992, 160, 427-433. See also J. R. Durig, W. E. Bucy, and A. R. H. Cole, Can. J. Phys. 1975, 53, 1832-1837. Method for ab initio Calculations • CCSD(T)/aug-cc-pVnZ, n = 2,3,4. Frozen core. • Complete basis set extrapolation of the geometric parameters. • Corrections for core/valence effects with CCSD(T)/cc-pCVQZ applied to geometric parameters. • Corrections for scalar relativistic effects from Douglas-Kroll-Hess CCSD(T)/cc-pVTZ_DK calculations applied to geometric parameters. Equilibrium (re) Structures for Butadiene r(C=C)/Å r(C–C)/Å r(C–Ha)/Å r(C–H c)/Å r(C–H t)/Å (C=C–C)/Å (C=C–Ha)/Å (C=C–Hc)/Å (C=C–Ht)/Å a trans-butadiene 90°-twisted SemiCCSD(T) CCSD(T) a expt. Lit. extrapol.b extrapol.b 1.338 1.338 1.333c 1.454 1.455 1.482d 1.085 1.085 1.085 1.082 1.082 1.082 1.079 1.080 1.081 123.6 123.5 123.8 119.9 119.8 119.3 121.0 121.0 120.2 121.5 121.5 121.2 N. C. Craig; P. Groner; D. C. McKean, J. Phys. Chem. A 2006, 110, 7461. b D. Feller, N. C. Craig, A. R. Matlin, J. Phys. Chem. A 2008, 112, 2131-2133. c Compare with ethylene, re = 1.330 Å. d Compare with an sp2-sp2 single bond of 1.470 Å estimated from near-equilibrium structures of ethane and propene. Rotamers of 1,3-Butadiene ab initio trans r(C1=C2)/Å r(C2-C3)/Å (C1C2C3)/deg (C1C2C3C4)/deg E/kJ/mol 1.338 1.454 123.6 0 0 Transition State (t-to-g) 1.333 1.483 123.8 78.3 26.0 90°twisted 1.333 1.482 123.8 90.0 25.5 gauche Transition State (cis) 1.336 1.468 124.4 144.5(35.5) 180.0 12.1 Experimentala (C1C2C3C4)/deg 0 0 E/kJ/mol a 77 24.8 90 23.1 142(38) 180.0 b 12.0/12.2 16.5 From evaluating the potential function reported by R. Engeln, D. Consalvo, J. Reuss, Chem. Phys. 1992, 160, 427-433. b rH° from analysis of the temperature dependence of the UV spectrum by J. Saltiel, D. F. Sears Jr., A. M. Turek J. Phys. Chem. A 2001, 105, 7569-7578. 90°-twisted butadiene Boat form of cyclooctatetraene [4]Dendralene Dimer of 1,3-Butadiene Dihedral angle = 71.7°; r(C3-C4) = 1.497 Å (long).a Compare sp2-sp3 bond length in propene = 1.496 Å.b a Electron diffraction: P. T. Brain, B. A. Smart, H. E. Robertson, D. W. H. Rankin, W. J. Henry, I. Gosney, J. Org. Chem. 1997, 62, 2767-2773. b Equil. struct.: J. Demaison, H. D. Rudolph, J. Mol. Spectrosc. 2008, 248, 66-76. Funding National Science Foundation (Beowulf cluster supercomputer) Dreyfus Foundation Department of Chemistry and Biochemistry, Oberlin College Additional Geometric Parameters for [4]-Dendralenea r(C1=C2)/Å 1.349(1) r(C2-C3)/Å 1.478(2) r(C3=C7)/Å 1.346(1) 124.4(3) (C1C2C3)/deg (123.2) (C2C3C7)/deg 117.6(7) (C4C3C7)/deg 119.2(5) (C2C3C4)/deg -174.8(28) (C1C2C3C7)/deg a Electron diffraction: P. T. Brain, B. A. Smart, H. E. Robertson, D. W. H. Rankin, W. J. Henry, I. Gosney, J. Org. Chem. 1997, 62, 2767-2773 Comparison of re Structures of Butadiene and Propene