Isomerization barrier

MeSSMe and inhibited to isomerise by a significantly larger isomerization barrier of 340 kj mol Thus, both H2S=S and Me2S=S are predicted to be kinetically stable toward unimolecular isomerization and should be experimentally observable in the gas phase or after matrix isolation. HF/b-SlG calculations indicate that the allyl analogue has a similar R2S=S/RSSR energy difference but a substantially lower activation barrier (110 kJ mol ) for rearrangement [25]. 

With the availability of stable geometric isomers of doubly bonded germanium compounds, experimental determinations of the 7r-bond strength can be made. The enthalpy of activation for double bond isomerization in Mes(Tip)Ge=Ge(Tip)Mes (Tip = 2,4,6-triisopropylphenyl) has been determined for the Z-E conversion, 22.2 . 3 kcal/mol and for the E-Z conversion, 20.0 0.3 kcal/mol.15 These values agree well with recent theoretical estimations.7 The isomerization barrier in germaphos-... 

The isomerization barrier of 15.0-20.0 kcal mol-1 (AG ) can be considered to be large enough to allow isolation and characterization of bis(q3-<2 /),A- nms-dodecatrienediyl-Nin stereoisomers of 7b41 as reactive intermediates in the stoichiometric cyclotrimerization process. Furthermore, the trans orientation of the two allylic groups gives rise to an insurmountable barrier for reductive elimination for these cases, which prevents these species from readily leaving the thermodynamic sink via a facile reductive elimination. The isolated intermediates clearly constitute dead-end... 

The isomerization reactions are first-order in concentration of the initial alkyl complex, [p0] (eqs 6, 7 of Figure 11). Thus, the relative probability for the two isomerizations (eq.3) is given by the ratio of their rate constants, kisoj/kiso.2 (as equal to the relative rate, risoj/risoJ), and at given temperature it can be calculated from the isomerization barriers AG i,0,/ and AG <,./, as in eq. 3. 

In tolnene-rfg, below 217 K, the benzyl aromatic signal resolved into two and the ben-zylic protons became diastereotopic. The exchange process, which was characterized by 217 = 246 s and AG = 10.2 kcalmoU, is a complex process involving both rotation aronnd the O—N bond and inversion at nitrogen, bnt since barriers to the former process are small the barrier best reflects that for rotation away from the anomeric conformation (Fignre 13). The amide isomerization barrier is even lower and both energies are in accordance with theoretical calcnlations (10.7 and 7.7 kcalmoG, respectively, for A—O rotation and amide isomerism in A-chloro-A-methoxyformamide) . 

These results indicate that barriers to all isomerization processes in A-acyloxy-A-alkoxyamides are likely to be less than about 8 kcalmol. Like that for its A-chloro analogue, the amide isomerization barrier in 100 is too low to be observed by H NMR. While there is dehnihve X-ray and theoretical evidence for anomeric effects in A-acyloxy-A-alkoxyamides, in the case of 100 the barrier to isomerization about the A-OBn bond must be lower than 10.3 kcalmol, the barrier in the corresponding A-chlorohydroxamic ester (Section in.B.2). The no-Oj Q anomeric interaction in A-chloroadducts is predicted to be stronger than the uo-Oj oAcyi interaction on perturbation arguments . 

Estimated from the experimental cis-trans isomerization barrier in a substituted disilene (84MI6). 

Figure 2.5. Isomerization barrier and coalescence temperature for silyl nitronates.

TABLE 1. Experimental and theoretical binding energies De and Do in cm ), equilibrium distances Re and Ro in A), for the linear and T-shaped isomers together with the theoretical predictions for the isomerization barrier of the HeBr2 complex. 

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