Conformational preferences and internal rotation of methyl butyrate by microwave spectroscopy

Abstract

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Favored Conformations of Carbonyl Compounds: A Structural Study of n‐Octanal

“…[18] The preferred chain conformationo ft hese aldehydes also agreesw ith the most abundant chain conformations of the aliphatic ester ethyl valerate (green apple flavor), [13] and methyl butyl ketone. [16] In all five cases, no stabilizing short-range intramolecular hydrogen bonding, which could account for the energetically favored structure of the C 1 conformer,w as found. The bend of the carboxyl group would be induced only by weakh ydrogen bonds of 2.639 and 2.654 .H owever,t his may also only be due to an overestimation of the intramolecular van-der-Waals interaction in the MP2 method.…”

“…[31] In agreement with the experimental results, the ab initio calculationsa tt he MP2 level (see Table 1a nd 2) showt hat the two lowest energyc onformers were observed in the molecular beam. [16] In all five cases, no stabilizing short-range intramolecular hydrogen bonding, which could account for the energetically favored structure of the C 1 conformer,w as found. The structures of both observed conformersc alculated using the MP2 method are depicted in Figure 5.…”

“…[12] We were interested in studying the most abundant conformers of octanal within the scope of investigating the chain effect in different carbonyl compounds, e.g., ethyl valerate, [13] n-pentyl acetate, [14] n-hexyl acetate, [15] and methylb utyl ketone. [16] The resultsc an also be directly compared to the gas-phase structures of hexanal and heptanal, which were previously studied using rotational spectroscopy. [17,18] The number of conformational isomers increases with the number of rotatable bonds in am olecule (3 N ,w ith N = the number of rotatableb onds).…”

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

“…For example, deviations of up to 9 % have been reported for methyl butyrate. [16] This is not surprising, since the structure of methyl butyrate is very similar to that of pentan-2-one, [2] where the acetyl methyl group is exchanged with a methoxy group. In the study on methyl butyrate, Hernandez-Castillo et al pointed out that the bond between the carbonyl group and the rest of the carbon chain causes most of the problems for the calculations with a wide variety of values found for the corresponding dihedral angle at different levels of theory.…”

“…In methyl alkynoates the two examples are methyl propionate (429.324(23) cm À 1 ) [15] and methyl butyrate (420.155(71) cm À 1 for the (a,a) and 419.447(59) cm À 1 for the (g � ,a) conformer). [16] For alkynyl alcohols, there are the examples of 3-pentyn-1-ol (9.46 cm À 1 ) [17] and 4-hexyn-3-ol (7.16 cm À 1 ). [18] 2.…”