Conformational sampling and large amplitude motion of methyl valerate

Abstract

Θ-Problem: the interplay of microwave spectroscopy and quantum chemistry to explore the soft degree of freedom around a carbonyl bond and methyl internal rotation.

The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

“…In the class of methylpyrrole, the potential of N-methylpyrrole (17) possesses a pure V 6 term with a barrier of about 60 cm −1 [111]. The C 2v symmetry is broken in the cases of 2-(18) and 3-methylpyrrole (19), leading to V 3 potentials of 280 cm −1 [112] and 246 cm −1 [113], respectively, without significant V 6 contributions. In methylimidazole, the symmetry is also broken in the case of the N-isomer (20) due to the presence of an additional nitrogen atom at the third position of the ring, and only V 3 potentials were reported for all four isomers of this class [114].…”

“…For many intermediate cases between the free internal rotor and the harmonic oscillator, resolvable A-E splittings in the microwave spectra have been reported, and the focus is often on the molecular structures and internal dynamics. Among them, the most extensively studied molecules are those with one methyl rotor, i.e., methacrylic acid [14], methyl vinyl ketone [15], 2-butynoic acid [16], N-methylformamide [17], methyl cyanoacetate [18], methyl valerate [19], and methyl hexanoate [20]. The number of studies significantly decreases if multiple methyl internal rotors are present in the molecule.…”

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

“…An overview of the results is depicted in Figure 6 and summarized in Table S-7 in the ESI. Note that the choice of these levels is not random, but based on exhaustive benchmarking already performed for methyl valerate (2) [20], methyl hexanoate (3) [21], and the series of methyl alkyl ketones (7-10) [33][34][35][36].…”

“…The bond connecting these two moieties is extremely flexible and predictions become very sensitive to the applied Please do not adjust margins Please do not adjust margins quantum chemical method and basis set. Although this applies foremost to esters, as shown for a series of methyl alkynoate like methyl butyrate [32], methyl valerate [20], methyl hexanoate [21] and many ethyl esters like ethyl valerate [18] and ethyl isovalerate [19], it also concerns ketones like the series of linear aliphatic ketones [33][34][35][36] and 2-propionyl thiophene [37]. However, the effect seemingly is much stronger in esters, with predicted values of  ranging from 20° to 50°, than it is in ketones where values between 5° and 25° are reported.…”

“…Please do not adjust margins Please do not adjust margins exhibit some soft-degree of freedom, e.g., within a broad potential energy well that cannot be properly captured by routine quantum chemical methods. Well-documented cases, for instances, are ethyl and methyl alkynoates [18][19][20][21]. These fruit esters have been shown to possess an extreme soft-degree of freedom around the C-C single bond in proximity to the CO carbonyl group.…”