Energy barrier to pseudorotation

It is believed that the energy barriers to pseudorotation are much higher for phosphoranyl radicals than for normal phosphorane molecules. Calculated values for PF4 and PFj are 25.2 and 3.6 kcals, respectively. There is also evidence that cleavage processes (13.180) take place preferentially from apical positions, and p cleavage from equatorial positions. Some spirophosphoranyl radicals of type (13.195) are remarkably stable towards a or p scission. 

If pseudorotation should occur, at least one CH3 group would come into an axial position, and two F into equatorial positions, thus causing high electron pair repulsions, i.e., a high energy barrier to pseudorotation. This is the... 

Experimental work, described in Section VII, shows that the barriers to pseudorotation are highly dependent on the nature of the CX2 group, but unfortunately, no strain energy calculations are available on this point. 

Table 9 gives the free energy barriers for conformational interconversions in cyclooctanone. This table also gives the barrier to pseudorotation in... 

Microwave [90,91,92] and far-IR [93,94] spectra indicate that the barrier to pseudorotation in tetrahydrofuran is small (0.8 to 2.0kJmol ). Modeling the potential energy surface has... 

We point out that similar analyses and results have been performed and obtained also by other authors [33, 35, 38 0]. The spectral lines at 86meV and 123 meV excitation energy in the theoretical spectrum correspond to excitation of the modes V6 and vi, respectively. The first spacing deviates from the harmonic frequency of mode V6 in Table 3 because of the JT effect, while the second coincides with that of mode vi because of the linear coupling scheme adopted. For higher excitation energies the lines represent an intricate mixture of the various modes because of the well-know nonseparability of modes in the multi-mode dynamical JT effect. Overall, the excitation of the various modes can be characterized as moderately weak. The total JT stabilization energy amounts to 930 cm and is dominated by the contribution of mode ve- The barrier to pseudorotation is of the order of 10 cm only, consistent with the fact that the theoretical spectrum of Fig. 3 is obtained within the LVC scheme (see Sect. 2.1 above). 

Pseudorotation about pivot ligand 1 is illustrated by Eq. (9). To be operationally detectable, it is necessary that the energy barriers for pseudorotation are accessible, and that the phosphorane has a sufficient lifetime, relative to its tetracoordinate reaction partners. 

Distortion from planarity to relieve torsional strain also applies to cyclopentane (Figure 3.19). Experimental evidence indicates that the molecule exhibits ten different envelope conformations in which one carbon atom at a time is above or below the plane defined by the other four, as well as ten twist or half-chair conformations in which three carbon atoms at a time define a plane, with the fourth carbon atom above this plane and the fifth below it. The molecule is continually changing its conformation, however, so that each carbon atom is 0.458 A above the molecular plane one-fifth of the time, with the barrier to planarity of 5.16 kcal/mol. In the absence of isotopic labeling, the effect of this shifting permutation is indistinguishable from a rotation of the molecule about an axis through its center. For that reason, the process is called pseudorotation. The energy barrier for pseudorotation is so small that the process is described as essentially barrierless. ... 

All the homoleptic, hypervalent compounds of the Group 15 elements described in this chapter are fluxional. The activation energies are not accurately known, but have been estimated to about 20 kJ mol in PF5 and PCI5 13 kJ mol in AsFs and about 8 kJ mol in SbCls and Sb(CH3)5 the activation energies appear to decease as the group is descended. The magnirnde of these barriers to pseudorotation is comparable to the magnitude of the rotational barrier in ethane, 12 kJ mol, or the barrier to inversion in ammonia, 21 kJ mol . ... 

There appear to be two important types of pseudorotation—these are Berry type (BPR) [18] and Turnstile type (TR) [19]. The energy barrier to the latter process is believed generally to be the smaller. 

Oxygen-containing Rings.—Some calculations of the relative energies of the possible conformations of THF derivatives have been presented. The authors used low-temperature n.m.r. spectra to estimate barriers to pseudorotation in several systems by measurements of line-broadening. A barrier of 7.6 kcal mol is claimed for 3,3,4,4-tetramethyltetrahydrofuran but, as no corrections for line-broadening due to viscosity at low temperatures appear to have been made, this value must remain in question. 

Similar to the case without consideration of the GP effect, the nuclear probability densities of Ai and A2 symmetries have threefold symmetry, while each component of E symmetry has twofold symmetry with respect to the line defined by (3 = 0. However, the nuclear probability density for the lowest E state has a higher symmetry, being cylindrical with an empty core. This is easyly understand since there is no potential barrier for pseudorotation in the upper sheet. Thus, the nuclear wave function can move freely all the way around the conical intersection. Note that the nuclear probability density vanishes at the conical intersection in the single-surface calculations as first noted by Mead [76] and generally proved by Varandas and Xu [77]. The nuclear probability density of the lowest state of Aj (A2) locates at regions where the lower sheet of the potential energy surface has A2 (Ai) symmetry in 5s. Note also that the Ai levels are raised up, and the A2 levels lowered down, while the order of the E levels has been altered by consideration of the GP effect. Such behavior is similar to that encountered for the trough states [11]. 

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