Ketone physics – structure, conformations, and dynamics of methyl isobutyl ketone explored by microwave spectroscopy and quantum chemical calculations

Abstract

The microwave spectrum of methyl isobutyl ketone was recorded using a molecular beam Fourier transform microwave spectrometer and analyzed to determine the rotational constants A = 4.7517(17) GHz, B = 1.496115(79) GHz, C = 1.324364(39) GHz, and centrifugal distortion constants. A-E splittings from 250 MHz up to 1 GHz were observed due to internal rotation of the acetyl methyl group with a potential barrier of 250.3(19) cm -1 . 59 A and 57 E species lines were fitted to experimental accuracy. The experimental rotational constants were compared with results from quantum chemical calculations to validate the molecular structure.

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

“…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.…”

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

“…The second class encloses all C 1 conformers of the methyl alkyl ketone series, [2][3][4] therefore called the "C 1 class", as well as allyl acetone (8) [47] and methyl isobutyl ketone (9). [48] The barrier height is approximately 240 cm À 1 , which connects to a structure with a synclinal γcarbon. In summary, the barrier to internal rotation of the acetyl methyl group can be linked to structural characteristics of the alkyl chain, making the acetyl methyl group a spectroscopic "detector" of the molecular conformation.…”

“…Conformational structures of ketones in the C s and C 1 class, as well as their respective barrier to internal rotation of the acetyl methyl group (rounded to an accuracy of 1 cm À 1 ): (1) butan-2-one, [1] (2) pentan-2-one, [2] (3) hexan-2-one, [3] (4) heptan-2-one, [4] (5) octan-2-one (this work), (6) methyl neopentyl ketone, [37] (7) cat ketone, [38] (8) allyl acetone, [47] and (9) methyl isobutyl ketone. [48] [b] 259 cm À 1 [59] Acetone + Ar [b] 260 cm [c] 275 cm À 1 [57] Mesomeric Class sp-β-Ionone [d] 341-343 cm À 1 [53] sp-α-Ionone [d] 356-360 cm [52] ap-α-Ionone [d] 439-455 cm À 1 [53] Phenyl Class…”

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

“…At first, the calculations were performed at the MP2/6-311++G(d,p) level of theory using the Gaussian 09 package. 15 This method and basis set was chosen, since it yielded reasonable rotational constants for other ketones such as methyl isobutyl ketone 16 and methyl neopentyl ketone. 17 It is also widely used in the microwave spectroscopic community.…”