Single bond, dissociation stretching

Single bond, dissociation. 188-90 stretching, 205 Singlet. 13. 180 impure, 28... 

Although the symmetric stretch can dissociate (by simultaneously breaking both bonds) similarly to a single bond local stretch, the antisymmetric stretch cannot dissociate to a knowable dissociation asymptote, thus there is no physical basis for treating an antisymmetric stretch (or any bend) as a Morse oscillator. 

Two results appear immediately from the study of CO on Mo2C. First, the TPD data show that CO only partially dissociates on Mo2C. Dissociation on Mo2C leads to high temperature recombinative desorption of CO. However, the ratio of the area of the latter peak to the saturation area of the 325 K feature is 0.2. Second, the RAIRS spectra display a single v(CO) stretching mode with a frequency characteristic of on-top bonded... 

Figure 11.3 illustrates the behaviour of the single determinant wave functions, RHF, UHF and PUHF (Projected UHF, Section 4.4). The RHF energy continues to increase as the bond is stretched since it has the wrong dissociation limit, while the UHF converges to a value of 87 kcal/mol. At the equilibrium geometry the two electrons in the O-H... 

As expected, the activation enthalpy, AH, is close to the (CO)3Ni-CO BDE of 25 2 kcal/mol. Also as predicted for a dissociative mechanism, the activation entropy, AS, is positive (8 1 eu in toluene and 13 1 eu in hexane). The positive AS lies in the range expected for a reaction in which the M-L bond is stretched to two or three times the length of the bond in the ground state " and experiences additional rotational freedom in the transition state. " Because the starting complex and the transition state are both single particles, the contribution of translational entropy to AS is zero," and the absolute value of the entropy term is smaller than that for an associative process. 

In 1998 scientists using a special type of electron microscope were able to measure the force needed to break a single chemical bond. If 2.0 X 10 N was needed to break a C—Si bond, estimate the bond dissociation energy in kJ mol. Assume that the bond had to be stretched by a distance of 2 A (2 X 10 m) before it was broken. 

The restricted Mqller-Plesset description of the dissociation process is improved as we go to higher orders in perturbation theory, in particular at the MP4 level. However, as for all methods based on the dominance of a single electronic configuration, the description deteriorates as the OH bonds are stretched, although the MP4 curve is quite satisfactoiy for bond distances up to 3.5oo- For sufficiently large distances, the restricted potential-energy curves diverge at all levels of theory. 

An interesting application of these principles is the prediction of CO dissociation routes on the closed-packed (111) surface of rhodium (see Fig. A.17). Two factors determine how the dissociation of a single CO molecule proceeds. First, the geometry of the final situation must be energetically more favorable than that of the initial one. This condition excludes final configurations with the C and the O atom on adjacent Rh atoms, because this would lead to serious repulsion between the C and O atoms. A favorable situation is the one sketched in Fig. A.17, where initially CO occupies a threefold hollow site, and after dissociation C and O are in opposite threefold sites. The second requirement for rupture of the CO molecule is that the C-0 bond is effectively weakened by the interaction with the metal. This is achieved when the C-O bond stretches across the central Rh atom. In this case there is optimum overlap between the d-electrons of Rh in orbitals, which extend vertically above the surface, and the empty antibonding orbitals of the CO molecule. Hence, the dissociation of CO requires a so-called catalytic ensemble of at least 5 Rh atoms [8,21,22]. 

---