Transcript BEE_RH13.ppt

THE PURE ROTATIONAL SPECTRA OF THE TWO
LOWEST ENERGY CONFORMERS OF n-BUTYL ETHYL
ETHER.
B. E. Long, G. S. Grubbs II, and S. A.
Cooke
RH13
Motivation
• Accurate prediction of thermochemical predictions is
an important goal in computational chemistry:
– G4 (Pople et al.)
– ccCA (Cundari, Wilson)
– W4 (Martin)
– HEAT (Stanton)
• Conformationally complex molecules can be
problematic.
n-Butyl ethyl ether:
Solvent,
B.p. 81 oC
O
The Search Accelerated Correct Intensity FTMW Spectrometer
“A Search Accelerated Correct Intensity Fourier Transform Microwave Spectrometer with
Pulsed Laser Ablation Source.” G. S. Grubbs II, C. T. Dewberry, K. C. Etchison, K. Kerr and S.
A. Cooke Rev.Sci. Instrum. 78, 096106 (2007).
Circuits
Acknowledgements:
Brooks Pate
Jens-Uwe Grabow
Fitting procedure:
1.
2.
3.
4.
5.
If possible identify harmonic series of a-types – get B + C
Or use other expertise, or QC Calcs
Make QN assignment in AABS
SPFIT
SPCAT – repeat from step 3 as necessary
6. When done, subtract assigned lines from observed spectra
AABS can do this!
7. Start over on step 1 for new conformer
Spectroscopic parameters for the two observed conformers
MP2/6-311G** conformers, relative energies in kJ mol-1
Conf-I = Conformer A
Conf-III = Conformer B
Where is Conf-II?
MP2/6-311G**
Conf-I = Conformer A
Conf-III = Conformer B
Where is Conf-II?
MP2 Conformer energies with different basis sets.
Conformer energies with different methods.
MP2:CC = E(CCSD(T)/aug-cc-pVTZ) + [E(MP2/CBS-ave)-E(MP2/aug-pVTZ)]
(Stefan Grimme)
MP2 relative zero-point energy corrected conformer energies.
So the observed conformers:
Missing conformer:
MP2/6-311G** potential scan of the CCCC dihedral angle
Conclusions
Spectra from two conformers of n-butyl ethyl ether observed
Only at very high levels of calculation is it revealed the these
Are the two lowest energy conformers.
Evidence is presented that a third low lying energy conformer is
relaxed into the lowest energy conformer.
Acknowledgements
• Laboratory Funding from the NSF
• An experimental study has been performed shedding light on the
conformational energies of n-butyl ethyl ether. Rotational spectroscopy
between 7.8 GHz and 16.2 GHz has identified two conformers of n-butyl
ethyl ether, C4H9OC2H5. In these experiments spectra were observed as
the target compound participated in an argon expansion from high to low
pressure causing molecular rotational temperatures to be below 4 K. For
one conformer, 95 pure rotational transitions have been recorded, for the
second conformer, 20 pure rotational transitions were recorded.
Rotational constants and centrifugal distortion constants are presented for
both butyl ethyl conformers. The structures of both conformers have been
identified by exploring the multi-dimensional, molecular potential energy
surface using ab initio calculations. From the numerous low energy
conformers identified using ab initio methods, the three lowest
conformers were pursued at increasingly higher levels of theory, i.e.
complete basis set extrapolations and also coupled cluster methods. The
two conformers observed experimentally are only revealed to be the two
lowest energy conformers when high levels of quantum chemical
methodologies are employed.