Transcript RK02 cHTE MW struct Cols '13 PC.ppt

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, D89069 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 cishexatriene.
• 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.
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
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.