The effects of two internal rotations in the microwave spectrum of ethyl methyl ketone

Abstract

The rotational spectra of ethyl methyl ketone, CH3CH2COCH3, were measured in the microwave region from 2 to 40 GHz using two molecular beam Fourier transform microwave spectrometers. Splittings due to internal rotations of both, the acetyl methyl group -COCH3 and the ethyl methyl group CH3CH2CO-, could be completely resolved. All measured transitions were fitted using two different codes, XIAM and BELGI-Cs-2Tops. Molecular parameters like the rotational constants and the centrifugal distortion constants were determined with very high accuracy. The barrier to internal rotation of the acetyl methyl group was fitted to 181.502(98) cm(-1), much lower than the value of 763.87(65) cm(-1) found for the ethyl methyl group. The splittings in the spectrum due to internal rotation of the acetyl methyl group are accordingly much larger, up to 1.2 GHz, whereas for the ethyl methyl group only splittings from a few hundreds of kHz up to 4 MHz were observed.

The Structure and Torsional Dynamics of Two Methyl Groups in 2‐Acetyl‐5‐methylfuran as Observed by Microwave Spectroscopy

“…. Atypical spectrum of the 6 16 ! 5 15 transitiono fthe trans conformer of AMF with its (00), (10), (01), (11), and (12) torsionalspecies.…”

“…We found av alue of 307.78(59) cm À1 fort he trans and 212.71(30) cm À1 for the cis conformer.F rom many previous investigations, it is known that the torsional barriero ft he acetyl methyl group in ketones depends strongly on the substitution at the other side of the carbonylg roup (Figure8). [15][16][17][18][19][20][21] This behavior in ketones,w hereby the substituent on the other side of the carbonyl group causes the acetyl-methyl barrier to vary over aw ider ange without any apparent trends, is in strong contrast to acetates, which can be divided into classes with predictable barrier heights. [22] For example in a,b-saturated alkyl acetates,t he barrier to internal rotationi sa pproximately 100 cm À1 and remains largely invariant.…”

Laboratory microwave, millimeter wave and far-infrared spectra of dimethyl sulfide

“…This method sets up the rotation-torsion Hamiltonian in the rho axis system for each top, and then converts the rho axis wave functions into the principal axis system. The XIAM code has shown its efficiency for treating internal rotation problems with high potential barriers for molecules with two internal rotors as well (Nguyen et al 2014).…”

“…As explained in (Tudorie et al 2011), terms in this Hamiltonian can be constructed by taking symmetry-allowed and Hermitian products of a rotational factor (chosen from operators of the form J The BELGI-C s -2Tops program has been applied for fitting high resolution torsion-rotational spectra of molecules with: (i) two nonequivalent methyl rotors; (ii) two different three-fold barriers and (iii) a plane of symmetry at equilibrium, e.g. methyl acetate (Tudorie et al 2011), methyl propionate (Nguyen et al 2012), three isomers of dimethyl benzaldehyde (Tudorie et al 2013) and ethyl methyl ketone (Nguyen et al 2014). In the case of DMS, we modified the code slightly so that all internal rotor parameters corresponding to one methyl group, referred as top 1, possess the same magnitudes as those of the other methyl top, referred as top 2.…”

Acetyl Methyl Torsion in N‐Ethylacetamide: A Challenge for Microwave Spectroscopy and Quantum Chemistry

“…However,s ince only af ew parameters can be fitted with the program XIAM and the barriert oi nternal rotation is quite low,t he fit is already much better than in many other cases with comparably low barriers. [21,22] In the case of N,N-diethylacetamide, two conformers were assigned whose structures differed only by the orientationso f the two ethyl groups. Quite different V 3 potentials of 517.04(13) cm À1 and 619.48(91) cm À1 were observed.…”