Pseudorotation

Pseudorotation.—A number of spirocyclic phosphoranes possess square-pyramidal structures rather than the trigonal-bipyramidal structures previously assumed, and this could have important consequences on the interpretation of their variable-temperature spectra. There is, as yet, no evidence that acyclic or monocyclic phosphoranes favour the square-pyramidal geometry, and the variable-temperature XH 

Pseudorotation.—An alternative intramolecular exchange process to Berry pseudorotation has been suggested, which also occurs with conservation of angular momentum. It has been called a turnstile-rotation process because it involves the rotation of an apical-radial pair of ligands 

Glonek, M. Lunde, M. Mudgett, and T. C. Myers, Arch. Biochem. Biophys., 1971, 142, 508. 

Pseudorotation.—Ab initio calculations on the hypothetical phosphorane (60) indicated that the relative tendency of ligand Y to occupy an apical site is OR R 0,  

Pseudorotation.— The methoxyphosphorane (39 X = OMe) is sufficiently stable at — 80 °C to exhibit non-equivalent methyl groups and PH coupling to the methoxy protons. However, upon raising the temperature, dissociation occurs before any pseudorotation effects can be observed. The corresponding fluorophosphorane (39 X = F) is dissociated at —85 °C. Studies of a 

Pseudorotation.—LCAO-MO calculations on the hypothetical molecule PHs confirm (a) that the trigonal bipyramid (t.b.p.) is the most stable configuration, (6) that the apical orbitals of the t.b.p, are more electropositive than the radial orbitals, (c) that there would be an equilibrium of t.b.p. structures, but those with the most electronegative groups in apical orientations will be more stable and predominate extensively over other t.b.p. configurations, and (d) that the lowest fundamental frequency of a t.b.p. corresponds to the equatorial in-plane bending motion as shown in (37), the force constants being nearly four times smaller than those of the 

The evidence for Berry pseudorotation and turnstile rotation has been reviewed. Further evidence for the turnstile rotation for spiro-penta-oxyphosphoranes has been presented, based on the lowering of the exchange barrier as the distortion of the t.b.p. of the phosphorane increases. A 

Simple octahedral compounds, e.g. CeHs SFs, are rigid even at high temperatures, but several octahedral iron compounds, e.g. 

HaFe[P(OEt)3]4 and H2Fe[PPh(OEt)2]4, are non-rigid at high temperatures. This observation, which is effectively of pseudorotation in an octahedral 

Several modes of rearrangement of the compounds Rh(diolefin)(LL), where diolefin = nor or cod, and LL = dibenzoylmethane (bzbz) or its monothio-derivative (S-bzbz), are possible. Rates of rearrangement of, for instance, Rh(nor)(S-bzbz) can be followed by n.m.r. as the protons in nor are not equivalent when the other ligand is the unsymmetrical (S-bzbz) [see formula (59)]. Equilibrium loss of diolefin or (S-bzbz) is not a 

There have been four investigations of intramolecular cis trans isomerisation in the formally seven-co-ordinate compounds (60) to (63) -  

That isomerisation is intramolecular has been demonstrated by observing C and W satellite features in the n.m.r. spectra of (60) as a function of temperature, and by noting the lack of dependence of isomerisation rates 

A general graphical topological approach to representing intramolecular processes of trigonal-bipyramidal molecules has been presented, and these processes have been used to illustrate a general topological treatment of polytopal isomerizations.  

Miscellaneous. It is possible to monitor several reactions of silyltriazenes (13) by n.m.r. spectroscopy. 1 3 migration of the silyl groups occurs by an associative intramolecular mechanism with a transition state (14). The order of migration rates for various SiRg groups is R = Et Me OMe Cl. ° At lower temperatures it is possible to isolate cis- and ra/25-isomers. Isomerization of these compounds occurs by inversion at nitrogen. Activation parameters for the isomerization of (15) are strongly solvent dependent. Thus, for 

Several cyclopentadienyl derivatives of Group IV elements have long been known to behave as fluxional molecules. Now F n.m.r. spectroscopy of P(A -C5H5)F2 shows that this Group V derivative is also a fluxional molecule. No quantitative kinetic data were quoted, but a qualitative impression can be 

As already said, in the NMR spectrum all ligands appear equivalent, since they quickly pass between axial and equatorial positions of the trigonal bipyramid (of D3h symmetry). Theory shows that when two nuclei in different positions, of frequencies Vj and V2, and their difference Av, exchange positions at a frequency larger than Av, then only one frequency will be observed, amounting to 0.5 (Vj + V2). 

Analysis shows that there are only two ways of intramolecular exchange  

Energies of the square pyramid and trigonal bipyramid stereo structures differ only slightly. Berry has shown than even a small deformation of the oscillation angles can enable an interconversion. 

In principle, the trigonal bipyramid has slightly lower energy than the square pyramid. 

There are several factors influencing the geometry of these two structures  

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Figure Bl.4.9. Top rotation-tunnelling hyperfine structure in one of the flipping inodes of (020)3 near 3 THz. The small splittings seen in the Q-branch transitions are induced by the bound-free hydrogen atom tiiimelling by the water monomers. Bottom the low-frequency torsional mode structure of the water duner spectrum, includmg a detailed comparison of theoretical calculations of the dynamics with those observed experimentally [ ]. The symbols next to the arrows depict the parallel (A k= 0) versus perpendicular (A = 1) nature of the selection rules in the pseudorotation manifold.

Viant M R, Cruzan J D, Luoas D D, Brown M G, Liu Kand Saykaiiy R J 1997 Pseudorotation in water trimer isotopomers using terahertz iaser speotrosoopy J. Phys. Chem. 101 9032-41... 

The vibronic motion may be described by using the (p, 0, (p) coordinates. In particular, the wave functions for the pseudorotational motion along the hyperangle (p that encircles the origin in a X3 system may assume the form [11]... 

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]. 

In this section, we extend the above discussion to the isotopomers of X3 systems, where X stands for an alkali metal atom. For the lowest two electronic states, the permutational properties of the electronic wave functions are similar to those of Lij. Their potential energy surfaces show that the baniers for pseudorotation are very low [80], and we must regard the concerned particles as identical. The Na atom has a nuclear spin " K, and K have nuclear... 

Total Nuclear Spin s Rovibronic Rotational Pseudorotational... 

The furanose rings of the deoxyribose units of DNA are conformationally labile. All flexible forms of cyclopentane and related rings are of nearly constant strain and pseudorotations take place by a fast wave-like motion around the ring The flexibility of the furanose rings (M, Levitt, 1978) is presumably responsible for the partial unraveling of the DNA double helix in biological processes. 

Fig. 1. Berry pseudorotation about pentacoordinate ( ) phosphorus, where (Q) represent fluorine atoms, (a) Original trigonal bipyramid (b) square...

The F nmr spectrum of this compound gives only one signal over a wide range of temperatures, a result attributed to Berry pseudorotation (144). No alkyl- or aryltetrabromoarsorane has been reported. There is, however, an early report on the preparation of tetraiodomethylarsorane from methylarsonic acid and hydriodic acid (145). 

Dioxolane also pseudorotates essentially freely in the vapor phase. 2,2 -Bi-l,3-dioxolane (128) has been shown by X-ray crystallography to have a conformation midway between the half-chair and envelope forms. The related compound 2-oxo-l 3-dioxolane (129) shows a half-chair conformation. This result is confirmed by microwave spectroscopy and by NMR data. Analysis of the AA BB NMR spectra of the ring hydrogen atoms in some 1,3-dioxolane lerivatives is in agreement with a puckered ring. Some 2-alkoxy-l,3-dioxolanes (130) display anti and gauche forms about the exocyclic C(2)—O bond. 

Phosphonium hexafluorophosphate, benzotriazolyl-N-hydroxytris(dimethylamino)-in peptide synthesis, 5, 728 Phosphonium salts chromene synthesis from, 3, 753 reactions, 1, 531 Phosphonium salts, vinyl-in pyrrole synthesis, 4, 343 Phosphonium ylides in heterocyclic synthesis, 5, 165 Phosphoramide, triethylene-as pharmaceutical, 1, 157 Phosphoramide, triethylenethio-as pharmaceutical, 1, 157 Phosphorane, pentaphenyl-synthesis, 1, 532 Phosphoranes, 1, 527-537 Berry pseudorotation, 1, 529 bonding, 1, 528... 

Cyclopentane is nonplanar, and the two minimum-energy geometries are the envelope and half-chair. In the envelope conformation, one carbon atom is displaced from the plane of the other four. In the half-chair conformation, three carbons are coplanar, vdth one of the remaining two being above the plane and the other below. The energy differences between the conformers are very small, and interconversion is rapid. All of the carbon atoms r idly move through planar and nonplanar positions. The process is called pseudorotation. 

The total spread in energies calculated for the four conformations is only 2.7 kcal/mol. The individual twist-chair conformations interconvert rapidly by pseudorotation. 

Concept of " pseudorotation introduced by R. S. Berry to interpret the stereochemical non-rigidity of trigonal bipyramidal PF5 (and SF4, ClFi) the 5 F atoms are equivalent (1953) due to interconversion via a square pyramidal intermediate. 

Figure 12.13 Interchange of axial and equatorial positions by Berry pseudorotation (BPR).

P-F 153 pm). However, the F nmr spectrum, as recorded down to — 100°C, shows only a single fluorine resonance peak (split into a doublet by P- F coupling) implying that on this longer time scale (milliseconds, as distinct from instantaneous for electron diffraction) all 5 F atoms are equivalent. This can be explained if the axial and equatorial F atoms interchange their positions more rapidly than this, a process termed pseudorotation by R. S. Berry (1960) indeed, PF5 was the first compound to show this effect. The proposed mechanism is illustrated in Fig. 12.13 and is discussed more fully in ref. 91 the barrier to notation has been calculated as 16 2kJmol". ( ... 

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