A SEMIEXPERIMENTAL EQUILIBRIUM STRUCTURE OF cis-HEXATRIENE FROM

MICROWAVE SPECTROSCOPY

NORMAN C. CRAIG, YIHUI CHEN, HANNAH A. FUSON, HENGFENG TIAN, and HERMAN VAN BESIEN, Department of Chemistry and Biochemistry, Oberlin College, Oberlin, OH 44074

ANDREW R. CONRAD and MICHAEL J. TUBERGEN, Department of Chemistry, Kent State University, Kent, OH 44242

HEINZ DIETER RUDOLPH, Department of Chemistry, University of Ulm, D-89069 Germany

JEAN DEMAISON, Laboratoire de Physique des Lasers, Atomes et Molécules,

Université de Lille I, 59655 Villeneuve d’Ascq Cedex, France

Preliminaries

Preliminaries - continued

• Last year at this conference, we reported GS rotational constants for a full set of isotopologues (normal, three 13C, five 2H) of cis-hexatriene.

• Equilibrium rotational constants were derived from GS rotational constants with calculated 1/2alpha sums.

• A semiexperimental (SE) equilibrium structure based on the Kraitchman substitution method was given. Parameters for the C2H2 bonds were in doubt.

Initial Structure of cis-Hexatriene

Potential Problems

• Contributions of -electrons to g-factors affecting angular momenta.

• C2H2 bonds out of molecular plane.

• Faulty rotational constants for the 2-d1 species.

Conversion of Ground State Rotational Constants to Equilibrium Rotational Constants

cis-hexatriene

Similar small adjustments for all isotopologues.

Corrections B0/MHz rovib/MHz electronic/MHz Be(se)/MHz

A 14651.229 183.291 1.312 14835.832 B 1583.181 7.640 0.021 1590.843 C 1429.462 7.340 -0.003 1436.799

a -0.166 -0.0052 a Inertial defect, = Ic - Ia - Ib, in amu Å2. 0.0134 amu Å2 without electronic correction.

Planarity

• Very small inertial defect for SE rotational constants.• MP2/cc-pVQZ(FC) with all torsion angles free (<0.001º).• CCSD(T)/cc-pVTZ(FC) C-C=C-C free (0.005º).

C-C=C-C, C=C-C=C, C=C-C-H free (~0.1º).

Conclusion: molecule is planar.

Flawed SE Rotational Constants Revealed with Mixed Estimation Method

• Determining predicate bond parameters CCSD(T)/cc-pwCVTZ(AE) + MP2/cc-pwCVQZ(AE) - MP2/cc-pwCVTZ(AE)

• Mixed estimation method: fit SE rotational constants and predicates concurrently, each with appropriate uncertainties.

• Consequences for 2-d1 and 3-d1 species.

Fit to Rotational Constants Be(SE)/MHz (obs-calc)/MHz normal Ae 14835.832 -0.037 Be 1590.843 -0.012 Ce 1436.799 0.012

e -0.0052 -2-d1 Ae 14047.317 -1.930 Be 1585.252 0.102 Ce 1424.515 0.082

e -0.0047 -3-d1 Ae 13667.298 0.512 Be 1583.810 -0.030 Ce 1419.352 0.001

e -0.0048

H…H Bonding in Support of Planarity

• H2-H2 bond distance is 2.138 Å, shorter than twice the van der Waals’ radius (2.4 Å). In the range for energy lowering despite repulsion.

• C2CH2 bond length is short by 0.002 Å, as expected for C-H…H-C bonding.

• Could this effect cause a flaw in the 1/2alpha sums for the 2-d1 species?

Conclusions

Equilibrium rotational constants, derived from MW spectra, and predicate bond parameters, computed from quantum chemistry, were used in the mixed estimation method to determine a semiexperimental equilibrium structure of cis-hexatriene.

The structure of cHTE shows greater effects of -electron delocalization than does butadiene.

cHTE is planar despite one H…H separation that is less than the sum of van der Waals’ radii. C-H…H-C bonding outweighs van der Waals’ repulsion.

Acknowledgements

Peter Groner, University of Missouri at Kansas City.

Dreyfus Foundation, Senior Scientist Mentor grant.

Oberlin College, Department of Chemistry and Biochemistry.

Kent State University, Department of Chemistry.

National Science Foundation for support of the Beowulf computer cluster at Oberlin College.