Rho Axis Method Research Articles

Rotational spectrum, internal rotation barrier and structure of 3,3,3-trifluoropropene

The rotational spectrum of 3,3,3-trifluoropropene (CF3CH=CH2) in the ground and the first three excited states of the CF3 torsion has been analysed up to 75 GHz. Accurate rotational and centrifugal distortion constants have been determined. A-E splittings due to the internal rotation of the trifluoromethyl group have been observed in the second torsionally excited state using Fourier transform microwave spectroscopy. The torsional data have been analysed using the internal axis method (IAM) of Woods and the rho axis method (RAM). Both approaches lead to similar values of 612(2) cm-1 and 653·06(83)CM-1, respectively, for the IAM and RAM methods for the barrier height. An ab initio structure has been calculated and a near equilibrium structure has been determined using offsets derived empirically. Finally, an approximate equilibrium structure has been obtained by a combined use of the experimental moments of inertia and the ab initio results.

Rotational spectrum, internal rotation barrier and structure of 3,3,3-trifluoropropene

The rotational spectrum of 3,3,3-trifluoropropene (CF3CH=CH2) in the ground and the first three excited states of the CF3 torsion has been analysed up to 75 GHz. Accurate rotational and centrifugal distortion constants have been determined. A-E splittings due to the internal rotation of the trifluoromethyl group have been observed in the second torsionally excited state using Fourier transform microwave spectroscopy. The torsional data have been analysed using the internal axis method (IAM) of Woods and the rho axis method (RAM). Both approaches lead to similar values of 612(2) cm-1 and 653·06(83)CM-1, respectively, for the IAM and RAM methods for the barrier height. An ab initio structure has been calculated and a near equilibrium structure has been determined using offsets derived empirically. Finally, an approximate equilibrium structure has been obtained by a combined use of the experimental moments of inertia and the ab initio results.

Selection Rules and Intensity Calculations for a Cs Asymmetric Top Molecule Containing a Methyl Group Internal Rotor

A detailed discussion is presented of the relationships between four different molecular symmetry groups. i.e.. D 2, C s , C 3 v , and C ( m) 3 v commonly used to discuss energy level symmetry species and electric-dipole selection rules for internal rotor problems in molecules with a symmetric rotor top and a frame with a plane of symmetry. paying particular attention to confusion arising from the fact that the smaller groups are not always subgroups of the larger, and the groups are applied to different parts of the internal rotation problem (i.e., D 2 to pure rotation, C s to vibration-rotation, C 3 v and C ( m) 3 v to torsion-rotation). The meaning in this context of the traditional K a , K c labels for rotational energy levels in molecules with internal rotation is examined in detail. A discussion is also given of the relationship between three different schemes for defining a molecule-fixed axis system in internal rotation problems. i.e., the principal axis method, the internal axis method, and a hybrid method referred to here as the "rho axis method," paying particular attention to the meaning of a-type, b-type, and c-type transitions when using each of these axis systems. Some of the above ideas were helpful in adding intensity calculations to our earlier computer program treating internal rotation and overall rotation in acetaldehyde-like molecules, and these intensity calculations are also briefly discussed.