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Rotation of methyl radicals in a solid krypton matrix
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Image of FIG. 1.
FIG. 1.

The resonance portion of the ESR spectrum of in solid Kr at temperatures. The -line transition of the asymmetric nuclear spin state is labeled by E and the symmetric nuclear spin state -line by A above the top panel. The frequency fluctuation of the microwave source is compensated by shifting the resonance positions to the same magnetic field value.

Image of FIG. 2.
FIG. 2.

Linewidths for the -and -lines as obtained from the component are shown. The -lines exhibit strong broadening when temperature is decreased.

Image of FIG. 3.
FIG. 3.

The broad transition under the sharp pair of and resonance lines is shown at . The maximum and minimum of the broad transition are indicated by the arrows. The intensity, as well as the linewidth, is tenfold with respect to the signal originating from well-ordered lattice sites.

Image of FIG. 4.
FIG. 4.

Calculated pair potentials for the interaction in three orientations as illustrated in the inset: Rg approaching the C atom from above the molecular plane (solid), in-plane toward a H atom (dashed) or between two C–H bonds (dotted). The curves were obtained with the RHF-UCCSD(T) method using Dunning basis sets and counterpoise correction for the BSSE.

Image of FIG. 5.
FIG. 5.

Angular dependence of the caged interaction calculated after relaxing the nearest 18 Kr atom positions. The energy minimum in each contour plot panel locates the orientation of the rotation axis during the lattice geometry optimization. Color scales are quantified in and represent the low barriers in the case of nearest-neighbor distance set initially for the Kr lattice. The top left panel shows the dependence of the potential barrier height on the variation in .

Image of FIG. 6.
FIG. 6.

Dependence of the rotational energy level separation from the ground state on the crystal field strength parameter for gas phase fixed rotational constants.

Image of FIG. 7.
FIG. 7.

The intensity ratio between the spin-symmetric and antisymmetric states is constructed for several cases. The rotational energy level populations and for the and , respectively, appear as in the EPR spectrum. The nonlinearity in the plot against inverse temperature indicates the deviation from the two-level model. Top: The experimental solid Ar and Kr values are compared to curves obtained by downscaling of the rotational constants by 90% and 80%, respectively, corresponding to the pseudorotating cage model. The gas phase (unscaled and , ) result is given by the dashed curve. Bottom: Comparison to the crystal field model. The curves are plotted for the parameter or that best fit the experiments in Ar and Kr.


Generic image for table
Table I.

Experimental population ratios in solid Kr (present) and Ar (Ref. 12).

Generic image for table
Table II.

Summary of the pair interaction data used in the evaluations of potential barriers for rotation. The atom-molecule potentials are obtained from the present RHF-UCCSD(T) calculations, while the Rg-Rg is taken from Ref. 42. The Rg is assigned values of 1, 2, or 3 according to the number of equal H atoms within the molecule (see Fig. 4): Labels 1 and 2 are the in-plane components, while Rg3 aligns with the surface normal. is the plane averaged potential that accounts for the free rotation about the axis.

Generic image for table
Table III.

Temperature dependence of some computed population ratios (beyond the two-level approximation) as seen in the ESR spectra. The scalings correspond to the pseudorotating cage model, while CF labels the static crystal field model in Eq. (2).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Rotation of methyl radicals in a solid krypton matrix