Bond weakening effect

The hydrogen moleeule has been investigated in detail. As will be seen in Chapters 6 and 10 the theory ehallenges some very subtle experiments. Let us approximate the most aceurate theoretieal potential energy curve (as a function of the internuclear distanee R) by a Morse eurve. 

Is such an approximation reasonable Let us see. From Wolniewicz s calculations we may take the parameter D = 109.52 kcal/mol= 38293 cm , while the 

3777 cm . The first value agrees very well with the experimental value 4161 cm . Comparison of the second value with the measured value 3926 cm is a little bit worse, although it is still not bad for our simple theory. The quantity 

The above example pertains to a chemical bond. Let us take, in the same way, a quite different situation where we have relatively weak intermolecular interactions, namely the hydrogen bond between two water molecules. The binding energy in such a case is of the order of D = 6 kcal/mol = 0.00956 a.u. = 2097 cm , i.e. about twenty times smaller as before. Tb stay within a single oscillator model, let us treat each water molecule as a point-like mass. Then, fji = 16560 a.u. Let us stay with the same value of o = 1. We obtain (p. 171) a = 17.794 and hence bo = 17.294, b = 16.294. bn = 0.294, bn i 0. Thus, (accidentally) we also have 18 vibrational levels. 

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The results (Figs. 21-25) show that bond-strengthening and bond-weakening effects of the substituents are also at work here. The most heavily substituted bond is broken most easily. This is specially evident in the case of [13], where Figs. 23—25 show the energetics of the rupture of each of the three bonds. Moreover, the double rotation at the reacting centers is... 

The stereoelectronic bond-weakening effect on A—B can be further promoted by additional vicinal nD donors in molecules of the form... 

The spin-spin coupling data indicate that the platinum s character tends to be concentrated in the bonds to the more polarizable phosphine ligands. Pidcock et al. (46) adopted the term "trans influence to describe the trans bond weakening effect observed with ligands such as phosphines. In the cis complex, the metal d and s character can be maximized in the bonds to the two phosphines, leaving the trans bonds with predominantly 6p character consistent with their ease of substitution and long bond lengths (56). 

Compound VIII thus formed would then undergo ring contraction as postulated before, followed by a now logical cyclobutane ring opening that would occur under the auspices of acetate and the C-C bond weakening effect of the sulfonium unit. 

If we deal with a pure a LUMO (or a HOMO), electron injection (or removal) will entail the strongest bond weakening effect. ET activation of such systems will always lead to bond dissociation. 

It is now well established in platinum(II) chemistry that Pt—Cl bonds are substantially weaker when trans to ligands such as H and CHf than when they are tram to Cl . The bonds cis to H in trans-[PtHCl(EtPh2P)2] (9) are of very similar length to those cis to Cl in tram-[PtCl2(Et3P)2] (10), so the bond weakening effect of H compared with Cl is directed specifically at the ligand trans to H . Since neither H or Cl Tt-bonds effectively with platinum, this trans influence of H must stem from its effect on the (t-electrons of the complex. 

Finally, transition states for hydroxide attack and methoxide departure, corresponding to each of the rotameric TBP intermediates [56]-[58] were partially geometry-optimized. Population analysis of these putative penta-coordinate transition states does indeed demonstrate that the bond-weakening effects observed in the intermediate and ground state are accentuated in the transition state . The percentage decreases in overlap population in P—O bonds subject to an app lone-pair interaction in the ground, intermediate and transition states are 4-5%, 6-8% and 7-12% respectively. 

This anomeric (n->CT ) interpretation, employing an electron-delocalization model, is the basis of the phosphorus app lone-pair hypothesis (PAPH). This hypothesis is completed by integration of two observations from the theoretical calculations discussed above (I) bond-weakening effects are calculated to be greater for apical than equatorial P—O bonds and (2) effects on conformational energy and electron distribution are amplified in pentacoordinate transition-state structures. 

Lowe et al 1988). In addition, the failure to recognize any n->-n interactions in phosphoryl species is disconcerting. However, the results of such interactions may be minimized in the pentacoordinate transition state since the major bond-weakening effects of the n->n and n->a interactions are on double and single bonds respectively. 

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