Potential energy curves, ethylene

Potential energy curves, ethylene. 66. 373 diatomic molecule. 36. 239 formaldimine, 374-75 formaldiminium ion. 373 molecular oxygen. 478-79 SiH, elimination, 394... 

FIGURE 1. Schematic potential energy curve for a case 1 push-pull ethylene. E 3>. Ester... 

Figure 1. Potential energy curve and geometrical and charge transfer parameters of the Pd-ethylene complex.

Figure 46. Potential energy curve for internal rotation of ethylene derivatives...

Figure 2.2. Potential energy curves of the ground state and some excited states of ethylene as a function of the torsional angle (by permission from Michl and Bona(ii( -Kouteck, 1990).

Olson was the first to postulate that optical excitation of the ethylenic double bond involves rotation around a double bond in its excited state and that this rotation leads to an observable photoisomerization process [8-10]. Olson dealt with this aspect in terms of potential energy curves and mentioned the possibility of adiabatic photoisomerization process. Later, Lewis and co-workers [11] studied the photoisomerization process of tronj-stilbene with great interest but could not detect the cw-stilbene fluorescence. More recently, more detailed fluorescence studies carried out by Saltiel and co-workers [12-15] revealed that cw-stilbene fluoresces very weakly (Oa 0.(X)01) and shows an inefficient adiabatic isomerization process. The singlet mechanism currently proposed by Saltiel [16] is supported by quenching studies [17-20]. The extensive studies carried out on stil-bene and its analogs have already been reviewed [21-23]. Here the nature of the singlet excited state involved in the trans-cis isomerization process is dealt with. 

With BH2 kept in the ethylene plane, we get a 4-electron trans-butadiene (fig. 11 (II)), and a large rotational barrier (Ei 10 kcal.mol-1). This is not a surprise, because all the occupied MO s are bonding along CO. When the BH2 group is frozen perpendicular to the conjugation plane, the empty p orbital cannot conjugates, and we obtain a potential energy curve very similar to the one of the parent enol (E w 5.3 kcal.mol-1). 

Potential Energy Curves. The MO calculations for ethylene, as performed by Mulliken [384, 385], represent a basis for potential energy curves of various classes of ofefins. The potential surfaces of excited states of stilbene have been calculated by several researchers [38, 40, 42, 385-396], The question of whether tram - cis photoisomerization of stilbene proceeds via a singlet or a triplet mechanism has been discussed by several groups (see Section VI.A. 1). It seems to us that on the basis of theoretical calculations this question could not have been answered unequivocally. 

Figure 4.2 Potential-energy curves for the Sq and Si electronic states of 90° twisted ethylene as a function of CH2 monopyramidal-ization. TDDFT and ab initio [SA-2-CAS(2/2)] results are compared. The 6-3IG basis set was used with both methods. The zero of energy is chosen as in Figure 4.1. All coordinates not involved in pyrami-dalization were fixed at the values obtained by optimizing the geometry on Si at the respective level of theory subject to the constraint that it be twisted by 90° with no pyramidalization. The Sq/Si energy gap is unaffected by pyramidalization in the TDDFT method, in contrast to CAS(2/2), where pyramidalization is a dominant coordinate in tuning the energy gap to reach a conical intersection.

Fig. 2. Potential energy curves for C=C bond twisting in polyene triplets ethylene A), 3.4 bond and 1.2 bond in trans—irons hexatriene B and D, respectively), irons butadiene (C)...

Fig. 23. Potential energy curve for the addition of triplet carbenc to ethylene 30)...

Figure 2.15 Computed potential energy curves of low-energy states of (a) and ethylene (b), as a function of intemuclear separation R and of twist angle 0, respectively. Rydberg states of ethylene are indicated by letter R. (Source Wen 2015 (27). Reproduced with permission of American Chemical Society.)...

Two possible potential-energy curves for the decomposition of cyclobutane to ethylene... 

Figure 1.10 Zero-point energy-corrected potential energy curve along the reaction path in mass-weighted intrinsic coordinate for the complex [CpCrlPHjlCHj]. II designates the structure of the methyl complex before the coordination of ethylene.

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