Rotational constants and internal rotation parameters in the ρ-axes-frame.
|Axes and anglesa||Parametersb||Our fit from refs.d||MP2/aug-cc-pVTZ||Our work||Units|
Axis frames in methyl ketene under the Ir representation: in black the principal axis frame and in red the rho-axis. In blue, it is indicated the internal rotation axis.
ARAM, BRAM, CRAM are the rotational constants in the rho-axis-frame. V3 is the height of the torsional barrier. ρ is the norm of the ρ vector described in Sect. 4 and represented in green in the figure. F is the internal rotational constant defined by F = h2/2rIα where and λg the direction cosines mentioned in Sect. 4. s is the reduced barrier parameter which is calculated as s = 4V3/9F. δ is the angle between the internal rotation and the aPAM axis, which is thus related to the direction cosines as cos(δ)=λa. θRAM is the angle of frame rotation and can be calculated based on the ρ projections: θ = arctan(ρx/ρy). Finally, Iα is the reduced mass of the methyl top.
Iα is, in case of the ab initio parameters, fixed to 3.2 which is its average value for a methyl top in the calculation from the ab initio structure. In the other cases it is calculated from the RAM rotational constants and either the ρ or F values as follows (Ilyushin 2011): and .
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