Inversion transition state

When Wiberg and Pracht (1972b) synthesized 3,3-di-(trimethylsilyl)-l-phenyltri-azene by reacting benzenediazonium chloride with sodium di-(trimethylsilyl)amide they found a faintly yellow compound if the reaction was carried out at -78 °C and an orange form at — 20 °C. NMR spectra were consistent with (Z)/( )-stereoiso-merism. Measurement of the isomerization rates at various temperatures in ether and in pentane indicates that the mechanism involves an inversion transition state (13.5) and not a rotation, because the free reaction enthalpies are independent of the polarity of the solvent. 

According to Bartell (1961a), the relative motion of the interacting non-bonded atoms is described by means of a harmonic oscillator when the two atoms are bonded to the same atom, and by means of two superimposed harmonic oscillators when the atoms are linked to each other via more than one intervening atom. It is the second case which is of interest in connection with the biphenyl inversion transition state. The non-bonded interaction will of course introduce anharmonicity, but since a first-order perturbation calculation of the energy only implies an... 

Recently, Zewail and co-workers have combined the approaches of photodetachment and ultrafast spectroscopy to investigate the reaction dynamics of planar COT.iii They used a femtosecond photon pulse to carry out ionization of the COT ring-inversion transition state, generated by photodetachment as shown in Figure 5.4. From the photoionization efficiency, they were able to investigate the time-resolved dynamics of the transition state reaction, and observe the ring-inversion reaction of the planar COT to the tub-like D2d geometry on the femtosecond time scale. Thus, with the advent of new mass spectrometric techniques, it is now possible to examine detailed reaction dynamics in addition to traditional state properties." ... 

The inversion barrier for syn/anti isomerization of H2Si=NH is only 5.6 kcal mol-1, whereas the internal rotation energy is 37.9 kcal mor1 (SOCI level of calculation). The rotation barrier can be equated to the ir-bond strength. The inversion transition state has an even shorter SiN bond length of 153.2 pm. The symmetry is C2V.9,10... 

In amines, dialkoxyl subsfilufion resulls in much higher barriers to inversion fhan in alky famines a fad that has also been explained in terms of an electronegativity effect increased p-character in the a bonds results in more s-character for the electron pair on nitrogen. The planar-inversion transition state is therefore destabilized since, in it, the lone pair must develop pure p-character. This transition state is also destabilized by a six-electron anti-bonding interaction between heteroatom lone pairs and, additionally, the better anomeric overlap that is possible in sp rather than sp systems may also play a role (Figure 4) . These phenomena have also been rationalized theoretically ... 

FIGURE 3. (a) Fadfe amide isomerism in anomeric amides (b) stabilization of inversion transition state... 

R = Me, Ph, t-Bu, CF3, and CC13) does not correlate with the Es parameters of the groups.424 This is probably because CF3 and CC13 have lower activation energies than predicted on the basis of Es due to negative hyperconjugation stabilizing the inversion transition state. 

The influence of carbon group substituents ER3 (E = C to Sn R = H, Me, f-Bu) on the energy barrier of bond shift and electrochemical reduction of substituted cyclooc-tatetraenes (COT-ER3) has been studied with experimental and theoretical methods by Staley et al.164. The ring inversion transition state (Figure 45) was taken as a model for the steric interactions in the bond shift transition state which could not be calculated... 

TABLE 38. Calculated energies of the ring inversion transition state (A.Eri) shown in Figure 45 for substituted cyclooctatetraenes. a... 

FIGURE 2. Schematic representation of the retention (192, TSret, a) and inversion transition states (193, TSjnv, b) in the 1,3-silyl migration of CH2=CHCH2SiH3... 

The more important structural data for all minima and inversion transition states of HEC4H4, HE(C2H3)2, and HE(C2H5>2 (E =N, P, As, Sb, and Bi) are listed in Tables 14 and 15. The inversion transition states of all 5-ring systems are of C2 symmetry and exhibit a perfectly planar arrangement. The tendency to nonplanarity around the C2EH unit can be expressed by the angle ft (Equation 2) ... 

Table 15 Selected molecular structure parameters (In A and degrees) for the Inversion transition state structures of HEC4H4 ...

The formation of trisubstituted double bonds from Cope substrates with a quaternary stereo-genic center is less selective, because nonbonding diaxial interactions arise in both ring-inversed transition states. The equatorial/axial preference of both substituents depends on the size of the substituent, and the larger one usually occupies the equatorial position, e.g., II - 12803. 

Table 1.1 Central-atom NPA charges Q(E) and NAO/NLMO hybridization ratios of bonds (BD) and lone pairs for minima and inversion transition states of ammonia homologues .

The modem chemist could not reasonably have expected either planar or pyramidal CH4 to be potentially isolable molecules, i.e. to be potential energy surface minima, and indeed as we have seen calculation indicates that they are not (and the inversion transition state for tetrahedral methane is not planar Chapter 1). Consider however the simple artifice of anchoring the basal bonds of pyramidal methane to a... 

Jensen calculated the energy surface for the walk reaction in the parent case at various levels of theory and found the inversion transition state to be 8 kcal/mol lower than a retention transition state with CASSCF/3-21G. Subsequent calculations at the CASPT2N/CASSCF(8/8)/c-31G(d) level revealed 11.1 kcal/mol difference in the inversion/retention transition states. 

Similar results were obtained using the 6-3IG basis set in multiconfiguration SCF with dynamic correlation (MROPT2) and zero point energy corrections. These revealed a small preference, 1.8 kcal/mol, for retention over inversion in transition states linking norcaradienes. At the B3LYP (UHF) the two transition states were of equal energy, and at the CASSCF (8/8) level, the inversion transition state was favored by 1.2 kcal/mol. The calculations, therefore, make no clear prediction about stereochemistry. 

Keywords Edge inversion Vertex inversion Transition state Recoupled pair bond Recoupled pair bond dyad Generalized valence bond (GVB) theory... 

Table 2 The geometries, frequencies and total energies for the inversion transition states (TS) of the FJSrH 3 ) (n = 0-3) molecules from CCSD(T) calculations with the indicated basis sets...

In the last section, we posed three questions. In this section, we show that the answers to all of these questions center on the ability of the P atom to form recoupled pair bonds with F and, more specifically, p-recoupled pair bond dyads with two F atoms. In molecules other than F2PH and PF3, the inversion transition states involve formation of s-recoupled pair bond dyads. 

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