Effect of skeletal relaxation on the methyl torsion potential in acetaldehyde

Abstract

Fully-relaxed model ab initio calculations at Hartree–Fock/6–31G(d,p) and Mo/ller–Plesset (MP2)/6–31G(d,p) levels for acetaldehyde methyl conformers indicate significant skeletal flexing (e.g., the CH3 –C bond length changes by 0.006 Å) and methyl hydrogen folding. Thirteen methyl conformer energies at 15° intervals are used to assess the magnitudes of the torsional potential function expansion terms. Only two terms V3=373.8 and V6=3.4 cm−1 (both significantly different from those obtained from microwave and infrared analyses) are found to be important. These calculations clearly show that relaxation during methyl rotation (i.e., skeletal flexing and methyl hydrogen folding) is an important determinant of the torsional potential. Energy levels obtained from internal rotation potentials which include flexing simulate infrared torsional fundamental frequencies in CH3CHO and CD3CHO to within 1–2 cm−1 of the experimental values. In the absence of relaxation infrared torsional fundamental frequencies are poorly simulated by ab initio calculations.

Symmetry and Fourier analysis of the ab initio-determined torsional variation of structural and Hessian-related quantities for application to vibration–torsion–rotation interactions in CH3OH

“…(i) The large-amplitude torsion motion can conveniently be defined as motion along the path of steepest descent in mass-weighted Cartesian coordinates, following the recommendations of [12] and the computational options offered by Gaussian [4], but Fourier expansions can be carried out using the average dihedral angle c [9,10] at points along the steepest-descent path, since, to a good approximation [6], c is linearly related to distance along the steepest-descent path. (ii) The Gaussian suite of programs [4] is capable of delivering: (a) structural information a i (c), (b) potential surface information V, $V, $$V, and (c) projected small-amplitude vibrational frequency information m j (c) at points c along the steepestdescent path to levels of self-consistency and absolute accuracy that make this information potentially useful as an aid to highresolution spectroscopic analyses in energy regions exhibiting extensive vibration-torsion-rotation interactions.…”

“…For example, Refs. [9,10] discuss problems that arise in ab initio calculations when a largeamplitude internal-rotation coordinate is chosen which does not satisfy the usual threefold symmetry requirements. Ref.…”

“…Several papers (e.g., [9][10][11]) have advocated using the average of the three dihedral angles A-B-C-H i for the large-amplitude internal rotation coordinate, where C-H i with i = 4, 5, 6 represents the methyl top and A and B are two atoms chosen from the frame. While such a coordinate is simple, has good symmetry properties, and is very close in value to the angular coordinate defined along the path of steepest descent for methanol and acetaldehyde (see Fig.…”

“…(21); b 3 = 10.246 (9), where one standard uncertainty from the least squares fit (k = 1, type A, Ref. [27]) is given in parentheses.…”

The microwave spectrum of methyl chlorodifluoroacetate: Methyl internal rotation and chlorine nuclear electric quadrupole coupling

“…A rough calculation shows that the distance from the methyl carbon to the hydrogen in the ortho position of toluene is 2.7 Å. For o-chlorotoluene, a similarly rough [42,43]. b Ref.…”