Conformational Landscape of Diisopropyl Ketone: Quantum Chemical Calculations Validated by Microwave Spectroscopy

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 117; no. 2; pp. 311 - 314
• Main Authors: Zhao, Yueyue, Mouhib, Halima, Stahl, Wolfgang
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 17.01.2013
• Subjects: Ab initio calculations; Atomic and molecular physics; Calculations and mathematical techniques in atomic and molecular physics (excluding electron correlation calculations); Constants; Density-functional theory; Distortion; Electronic structure of atoms, molecules and their ions: theory; Exact sciences and technology; General molecular conformation and symmetry; stereochemistry; Ketones; Landscapes; Mathematical analysis; Microwave spectroscopy; Molecular properties and interactions with photons; Molecular structure; Physics; Properties of molecules and molecular ions; Rotation, vibration and vibration-rotation constants; Symmetry; Inorganic compounds; Ketones; Molecular structure; Carbon Molecules; Rotational constant; Theoretical study; Diatomic molecules; Centrifugal distortion constant; Density functional method; Potential energy surfaces; Molecular beams; Ab initio calculations; Carbon monoxide; Gas phase; Microwave spectra; Organic compounds; Conformation
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp308943t

We report on the gas-phase structure of the most abundant conformer of diisopropyl ketone, (CH3)2HC–CO–CH(CH3)2, as observed by molecular beam Fourier transform microwave spectroscopy. The gas-phase structures of five conformers of diisopropyl ketone were optimized using ab initio calculations at the MP2/6-311++G(d,p) level of theory. The natures of the stationary points were verified using harmonic frequency calculations. The only conformer observed in the supersonic jet possesses C 2 symmetry and appears as an enantiomeric pair. From the microwave spectrum, a set of three highly accurate rotational constants, five centrifugal distortion constants, and three sextic centrifugal distortion constants were determined. The structure of the observed conformer was optimized again at different levels of theory using the HF, MP2, and B3LYP methods. The theoretical constants of the C 2 conformer were subsequently validated using the experimental constants. To understand the transitions of one conformer to the others, the isopropyl groups were rotated against each other. The resulting two-dimensional potential energy surface shows nicely the symmetry of the conformational landscape and also indicates the enantiomeric pairs of the conformers. The barriers to internal rotation of the methyl groups were determined to be 1052 and 905 cm–1 at the MP2/6-311++G(d,p) and the B3LYP/6-311++G(d,p) levels, respectively. In agreement with the theoretical predictions, no internal rotation patterns could be observed in the microwave spectrum.