Internal Rotation of the Acetyl Methyl Group in Methyl Alkyl Ketones: The Microwave Spectrum of Octan‐2‐one

Abstract

Methyl n-alkyl ketones form a class of molecules with interesting internal dynamics in the gas-phase. They contain two methyl groups undergoing internal rotations, the acetyl methyl group and the methyl group at the end of the alkyl chain. The torsional barrier of the acetyl methyl group is of special importance, since it allows for the discrimination of the conformational structures. As part of the series, the microwave spectrum of octan-2-one was recorded in the frequency range from 2 to 40 GHz, revealing two conformers, one with C 1 and one with C s symmetry. The barriers to internal rotation of the acetyl methyl group were determined to be 233.340(28) cm À 1 and 185.3490(81) cm À 1 , respectively, confirming the link between conformations and barrier heights already established for other methyl alkyl ketones. Extensive comparisons to molecules in the literature were carried out, and a small overview of general trends and rules concerning the acetyl methyl torsion is given. For the hexyl methyl group, the barrier height is 973.17(60) cm À 1 for the C 1 conformer and 979.62(69) cm À 1 for the C s conformer.

Conformational sampling and large amplitude motion of methyl valerate

“…However, a slightly decrease in the barrier height by longer alkyl chain can be recognized from the values given in Table 4. This chain length effect has also been reported for linear aliphatic ketones [22].…”

“…The structure of the alkyl chain is the same as that of the most stable conformer of heptan-2-one where the alkyl chain is bent at the  carbon position. Probably, the most stable conformer of methyl hexanoate, methyl heptanoate, and longer methyl alkanoate will also contain conformer I of methyl valerate as their sub-structures, similar to the observations found for pentan-2-one, hexan-2-one, heptan-2-one, and octan-2-one [19][20][21][22].…”

“…The angle θ1 of 146.5° is significantly different from 180°, indicating that the entire alkyl chain is tilted out of the OCOC plane by an angle  = 180°  θ1 = 34.5°, as can be recognized in the lower right sub-figure of Figure 2. Many previous studies on fruit esters like ethyl valerate [2] and alkylbutyrate [33] or ketones like the series of linear aliphatic ketones [18][19][20][21][22] have indicated the same tilt angle  caused by the soft degree of freedom when an alkyl chain is attached to a carbonyl group, whereby the angle  is reported to be much larger in esters (from 20 to 50°) than in ketones (from 5 to 25°). This seems to be the reason for disagreements between the predicted and experimental rotational constants with differences of up to 15%, and a more complicated spectral analysis.…”

“…This difference is remarkable, because deviations of less than 1% or even better are usually observed [50][51][52][53][54], and the MP2/6-311++G(d,p) level is therefore our most frequently used level due to the reasonable accuracy/calculation time ratio. On the other hand, recent investigations on the C1 conformers of a series of methyl n-alkyl ketone have indicated deviations of up to 5% between the experimental and calculated rotational constants, which can be traced back to the flexibility of up to 25° of the angle  upon the internal rotation of the acetyl methyl group which is also attached to the carbonyl bond [19][20][21][22]. An even larger deviation of 9 % was found for the C1 conformer of methyl butyrate for the same reason [24].…”

“…In contrast, the torsional barrier of the acetyl methyl group in linear aliphatic ketones are conformation-dependent. Two different groups of barrier heights around 180 cm −1 and 240 cm −1 are observed for the "pseudo-Cs" and the C1 molecular geometry, respectively [18][19][20][21][22]. In methyl alkanoates, the methoxy methyl group COOCH3 shows internal rotation with a higher barrier, which is, e.g., 424.581(56) cm −1 in methyl acetate (1) [12].…”