STUDIES ON THE CONFORMATIONAL LANDSCAPE OF TERT-BUTYL ACETATE USING MICROWAVE SPECTROSCOPY AND QUANTUM CHEMICAL CALCULATIONS

The tert -butyl acetate molecule was studied using a combination of quantum chemical calculations and molecular beam Fourier transform microwave spectroscopy in the 9.0 to 14.0 GHz range. Due to its rather rigid frame, the molecule possesses only two different conformers: one of C s and one of C 1 symmetry. According to ab initio calculations, the C s conformer is 46 kJ/mol lower in energy and is the one observed in the supersonic jet. We will report on the structure and dynamics of the most abundant conformer of tert -butyl acetate, with accurate rotational and centrifugal distortion constants. Additionally, the barrier to internal rotation of the acetyl methyl group was determined. Splittings due to the internal rotation of the methyl group of up to 1.3 GHz were observed in the spectrum. Using the programs XIAM and BELGI-Cs, we determine the barrier height to be about 113 cm -1 and compare the molecular parameters obtained from these two codes. Additionally, the experimental rotational constants were used to validate numerous quantum chemical calculations. This study is part of a larger project partly supported by the PHC PROCOPE 25059YB which aims at determining the lowest energy conformers of organic esters and ketones which are of interest for flavor or perfume synthetic applications.

Acetyl-and nitrogen containing substances play an important role in many chemical, physical, and especially biological systems.These both aspects are present in acetamides.In this work, N,N-diethylacetamide was investigated by a combination of molecular beam Fourier transform microwave (MB-FTMW) spectroscopy and quantum chemical calculations.This molecule undergoes internal rotation of the acetyl methyl group and quadrupole coupling due to the nitrogen atom, which cause fully resolved fine and hyperfine splittings in the spectrum.
The measurements have been carried out using a MB-FTMW spectrometer in Aachen, which has the experimental accuracy of about 2 kHz.In a scan from 9 to 15 GHz, more than 75 lines were observed.Each of these lines appears as a group of transitions in the high resolution measurements.
Quantum chemical calculations were carried out at the MP2/6-311++G(d,p) level of theory in order to assign the spectrum.The conformational analysis yielded five stable conformers with a maximal energy difference of 7 kJ/mol.The two lowest energy conformers (conformer I and II) could be assigned in the microwave spectrum.They both have C 1 symmetry, where the two ethyl groups are tilted out of the molecular plane.
The barriers to internal rotation of the acetyl methyl group were found to be approximately 517.1 cm -1 and 619.5 cm -1 for conformer I and II, respectively.The fine splittings due to the internal rotation are of a few tens to hundredths kHz, in the same order of magnitude as the 14 N quadrupole coupling splittings.For both conformers, global fits using the rotational constants A, B, and C, the centrifugal distortion constants, the internal rotation parameters as well as the quadrupole coupling constants χ aa and χ -were carried out with excellent standard deviations within the experimental accuracy of our spectrometer.
The rotational spectrum of organoarsenic compound, methyl arsine, CH 3 AsH 2 , was recorded in the frequency range 15 -80 GHz using the Stark-modulated spectrometer in Oslo, and in the frequency range 150 -660 GHz using the spectrometer in Lille.The experimental work was augmented by high-level ab initio calculations.The analysis of the rotational spectrum of CH 3 AsH 2 is complicated by several factors: (i) the asymmetry parameter of the molecule is very close to -1, which leads to a tight grouping of rotational transitions; (ii) whereas the barrier to internal rotation of the methyl top is of intermediate height, V 3 = 520 cm -1 , the internal rotation parameter  is high,  = 0.41, that results in relatively large A-E splittings in the spectra; and finally (iii) rather high value of the arsenic nuclear quadrupole moment,  aa = 35.5 MHz,  bb = 117 MHz, is leading additionally to relatively large hyperfine splittings.To fit the observed rotational transitions we use the RAM36 code modified to take nuclear quadrupole hyperfine structure into account.The analysis, which is in progress, includes the ground vibrational state as well as v t = 1 and v t = 2 excited torsional states.The latest results will be presented.

WD04 15:49 -16:06 MICROWAVE AND FAR-INFRARED SPECTRUM OF DIMETHYL SULFIDE
A. Jabri 1 , V. Van 2 , H. V. L. Nguyen 1 , F. Kwabia Tchana Dimethyl sulfide CH 3 SCH 3 (DMS) is a non-rigid molecule containing sulfur atom whose astronomical detection is considered as possible.DMS has two equivalent internal rotors with internal rotation barriers of about 720 cm -1 .The microwave spectrum of the DMS molecule has been re-measured in the 4-40 GHz frequency range, using the two MB-FTMW spectrometers in Aachen [1], with an instrumental uncertainty of a few kHz for unblended lines.
A part of the millimeter spectra has been also measured for the first time in the 50-100 GHz range using the millimeter spectrometer in Aachen with an uncertainty of 40 kHz.
A fit including the new measurements and previous transitions from the literature [2] for the ground torsional state v t = 0 has been performed using the XIAM and the BELGI codes.The results will be discussed.The far-infrared spectrum has also been recorded for the first time at high resolution using the Fourier-transform spectrometer and the newly built cryogenic cell at the French SOLEIL synchrotron [3] and the assignments for the v t =10 torsional band under course will be presented.