Large Amplitude Motions in Fruit Flavors: The Case of Alkyl Butyrates

Abstract

To accurately characterize the large amplitude motions and soft degrees of freedom of isolated molecules, sampling their conformational landscape by molecular mechanics and quantum chemical calculations may provide a valuable insight into the structure and dynamics. However, the resulting models need to be validated by a reliable experimental counterpart. For ethyl pentanoates, which belong to the family of fruit esters, benchmark calculations at different levels of theory showed that the C−C bond in proximity to the ester carbonyl group exhibits a large amplitude motion that is extremely sensitive to the choice of quantum chemical method and basis set. In such cases, insights from high‐resolution molecular jet techniques are ideal to accurately identify and characterize soft degrees of freedom. Here, we report on the most abundant conformer of ethyl 2‐ethyl butyrate using Fourier‐transform microwave spectroscopy. We show that – unlike other structurally related pentanoates for which gas‐phase and crystallographic data is available – ethyl 2‐ethyl butyrate possesses a Cs symmetry plane under molecular jet conditions.

Benchmarking quantum chemical methods for accurate gas-phase structure predictions of carbonyl compounds: the case of ethyl butyrate

“…This is consistent with the observation in several molecules that exhibit a soft-degree of freedom around the S7 in the ESI † to test the performance of dispersion functions and methods, respectively. (1) Methyl butyrate, 32 (2) methyl valerate, 20 (3) methyl hexanoate, 21 (4) ethyl butyrate (this work), (5) ethyl valerate, 18 (6) ethyl isovalerate, 19 (7) pentan-2-one, 33 (8) hexane-2-one, 34 (9) heptan-2-one, 35 (10) octan-2-one, 36 (11) ethyl 2-methylbutyrate, 25 (12) ethyl 2-methylpentanoate, 13 (13) ethyl 2-ethylbutyrate, 50 (14) octanal. 51 angle Y (cf.…”

“…Diffuse functions are necessary for all Pople basis sets, since omitting them leads to a tremendous decrease of the prediction accuracy on the A rotational constant. However, if substituting the hydrogen atoms in the CH 2 –(CO)–O center moiety results in a C s configuration, as in the case of ethyl 2-ethylbutyrate ( 13 ), 50 the MP2/6-311++G(d,p) level delivers very satisfactory results, as it does in the C s conformer cases of many other molecules including all those presented in Fig. 6.…”

“…, with X = A, B, C. B3LYP/cc-pVDZ and B3LYP-D3BJ/6-311++G(d,p) calculations were also carried out with values in TableS7in the ESI † to test the performance of dispersion functions and methods, respectively. (1) Methyl butyrate,32 (2) methyl valerate,20 (3) methyl hexanoate,21 (4) ethyl butyrate (this work), (5) ethyl valerate,18 (6) ethyl isovalerate,19 (7) pentan-2-one,33 (8) hexane-2-one,34 (9) heptan-2-one,35 (10) octan-2-one, 36 (11) ethyl 2-methylbutyrate, 25 (12) ethyl 2-methylpentanoate,13 (13) ethyl 2-ethylbutyrate,50 (14) octanal 51.…”

“…(C=O)-O center moiety results in a Cs configuration, as in the case of ethyl 2-ethylbutyrate(13) [50], the MP2/6-311++G(d,p) level delivers very satisfactory results, as it does in the Cs conformer cases of manyOpen Access Article. Published on 09 February 2023.…”

Conformational sampling and large amplitude motion of methyl valerate

“…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.…”