Methyl torsional interactions in acetone

Abstract

The ground-state potential surface for methyl torsional interactions in acetone has been obtained by ab initio, semiempirical, and empirical methods. Frequencies acquired from recent Rydberg spectroscopy measurements on the a2 ground-state fundamental and overtone torsional vibrations and infrared measurements on b2 fundamentals in acetone-A6 and acetone-d6 are well simulated by purely theoretical ab initio HF/6-31G(d,p) and scaled MP2/6-31G(d,p) (i.e., semiempirical) fully-relaxed model potentials. This model incorporates skeletal flexing during the methyl rotation, causing the CCC angle to vary by 3°. The single unique empirical surface obtained from the above-mentioned measured frequencies is in good agreement with the ab initio and scaled ones. The four empirically determined constants in the two (equivalent C3") top potential function, eq 3, are V3 = 370 cm"1, V33 = 136 cm"1, V'33 = -156 cm"1 and V6 = 0 cm"1. The torsional barrier, Keff, is found to be 240 cm'1, significantly lower than the microwave value. We are unable to predict potential constants that simulate the frequency-generated empirical ones at any level of ab initio calculation [i.e., HF/DZ, HF/6-31G(d,p), and MP2/6-31G(d,p)] within the rigid-frame model (i.e., geometry parameters at equilibrium are held fixed during the methyl top rotation).

Ab InitioDetermination of the Roto-Torsional Energy Levels oftrans-1,3-Butadiene

“…The derivatives of the cartesian coordinates with respect to the internal coordinates (Eqs. [2] and [3]) have been evaluated analytically. From the MP2/6 -31G(d, p) geometries, the following relations are obtained: The variation of ␣ is overestimated at the optimization process.…”

“…Previous experiences show that a rigid model (1)(2)(3), where the neglected coordinates are considered constant during the variation of the independent coordinates, does not permit the reproduction of experimental data as barriers, energy differences between conformers, and frequencies. On the contrary, the approximation to the experimental data may be extremely good using relaxed models (1)(2)(3), where the dependent coordinates are considered as a function of the independent coordinates. From the point of view of the ab initio calculations, the analytic relations between dependent and independent coordinates may be obtained optimizing the geometry (2,3).…”

“…On the contrary, the approximation to the experimental data may be extremely good using relaxed models (1)(2)(3), where the dependent coordinates are considered as a function of the independent coordinates. From the point of view of the ab initio calculations, the analytic relations between dependent and independent coordinates may be obtained optimizing the geometry (2,3). This model has been employed successfully in the analysis of vibrational spectra (2, 4 -9).…”

Microwave spectra of mono-13C substituted acetone, (CH3)2CO

“…The results for the structural parameters are shown in Table 3 in comparison with the optimized structures from theoretical MP2 calculations [24][25][26][27]. The difference between the angle between the internal rotation axes, x, derived from the data in Table 2, and the angle a(CCC), indicates that the methyl groups are tilted away from each other towards the carbonyl group by 1.75°.…”

Influence of Protonation on Internal Rotation of Dimethyl Ether

“…For example, it is much higher than in acetone (∼2.3 kcal/mol). 22 On the basis of energy partitioning 10 developed in a natural bond orbital (NBO) formalism, 23 we found that the reason for such a high barrier energy lies in the prominent role played by the oxygen σ lone pair in the barrier energetics. During synchronous rotation, two C-H ip bonds come close to each other, and they repel each other.…”