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 o C 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, (2007).Rev.Sci. Instrum. 78, (2007).

Circuits Acknowledgements: Brooks Pate Jens-Uwe Grabow

Fitting procedure: 1.If possible identify harmonic series of a-types – get B + C 2.Or use other expertise, or QC Calcs 3.Make QN assignment in AABS 4.SPFIT 5.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?

Conf-I = Conformer A Conf-III = Conformer B Where is Conf-II? MP2/6-311G**

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, C 4 H 9 OC 2 H 5. 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.