Bond order, excited state

Bitopic, 190, 357, 379. See also Topicity Blue shift. 133 Boltzmann s Law, 6 Bond dissociation. See Dissociation Bond order-hond distance relation, 45 Bond order, excited state, 441 Born-Oppenheimer approximation, 10, 34. 179,328... 

An alternative to the conventional methods of quanfum chemistry is the state-specific theory, useful especially for excited states [1,42,131,132]. Such a theory is based on the choice and optimization of the function spaces for each excited state of interest, both at the zeroth-order and at the correlation level. This allows the systematic inclusion of relaxation and correlation effects to a very good degree of accuracy and the reliable description of phenomena and calculation of properties with small wavefunctions. Furthermore, physical and chemical concepts become more transparent while aspects of transferability of parfs of fhe energy or of the wavefunctions and their distinct effects on spectroscopy, on properties, and on chemical bonding in excited states may be systematized. 

The factor i produces the observed Dx. It is noted that another molecular form, the familiar H, has Dx = 269 1,389rQ/5 = 261 kJ, and with d = 1.09, bond order b < 0. The intermediate first-order excited state with d = 0.84 A and D = 1,389a-q/4 = 327kJmol has not been observed. 

The disadvantage of molecular mechanics is that there are many chemical properties that are not even defined within the method, such as electronic excited states. Since chemical bonding tenns are explicitly included in the force field, it is not possible without some sort of mathematical manipulation to examine reactions in which bonds are formed or broken. In order to work with extremely large and complicated systems, molecular mechanics software packages often have powerful and easy-to-use graphic interfaces. Because of this, mechanics is sometimes used because it is an easy, but not necessarily a good, way to describe a system. 

F. 1-26. (a) ir-Bond order of the C-S bonds in the ground state, (fc) ir-Bond order of the C-S bonds in the first excited state, (c) Free-valence number of the intermediate diradicaf. (Most probable bicyclic intermediate resulting from the ring closure of the diradicai. 

The bond orders in the polymethine chain are equalized in the ground and excited states. If Tg = 45°, the bond equalization is maximum. This is the ideal polymethine state (1) of the polymethine chain. Any deviation from this state (ie, Oq is greater than or less than 45°) causes the bond to alternate from the polymethine chain center to its ends. The alternation ampHtude is found to be proportional to the absolute value 45° — 4>g. ... 

Because aH bonds within the polymethine chain of symmetrical PMDs are significantly equalized and change slightly on excitation, relatively smaH Stokes shifts (500 600 cm ) are observed in their spectra. In unsymmetrical PMDs, the essential bond alternation exists in the ground state. However, bond orders in the excited state are found to be insensitive to the symmetry perturbation. As a result, the deviations of fluorescence maxima, are much lower than those of absorption maxima, (3,10,56—58). The vinylene shifts of fluorescence maxima of unsymmetrical PMDs are practicaHy constant and equal to 100 nm (57). 

Indazoles have been subjected to certain theoretical calculations. Kamiya (70BCJ3344) has used the semiempirical Pariser-Parr-Pople method with configuration interaction for calculation of the electronic spectrum, ionization energy, tt-electron distribution and total 7T-energy of indazole (36) and isoindazole (37). The tt-densities and bond orders are collected in Figure 5 the molecular diagrams for the lowest (77,77 ) singlet and (77,77 ) triplet states have also been calculated they show that the isomerization (36) -> (37) is easier in the excited state. 

We have said that when a molecule absorbs a quantum of light, it is promoted to an excited state. Actually, that is not the only possible outcome. Because the energy of visible and UV light is of the same order of magnitude as that of covalent bonds (Table 7.3), another possibility is that the molecule may cleave into two parts, a process known as photolysis. There are three situations that can lead to cleavage ... 

The symmetries of the lowest excited states listed in Table 1 are nothing but the symmetries to which the most soft second-order bond distortions belong. It is seen that the types of symmetry reduction predicted using the symmetry rule are in complete agreement with those obtained on the basis of the dynamic theory. 

Using the same method as described in II.B, Binsch and Heil-bronner have examined the second-order bond distortion in the lowest excited states of nonalternant hydrocarbons (I, IV—VII, X, XI, XIII — XV and XVII), and have shown that, of the molecules examined, only VI and XVII suffer a molecular-symmetry reduction in the lowest... 

It is interesting to note that the reactivity of the excited states of (25), (26), (27), and (28) in Table 8.4 increases in this order as stabilizing terminal substitution is increased. Zimmerman suggests that vinyl-vinyl bridging (the start of bond formation between 2 and 4) controls the reaction rate. 

An important feature of the mechanism suggested by Hammond and co-workers was the increased bond order of the carbon-carbon single bond in the excited state. This is shown in the accompanying scheme. 

The bond orders for both n- n and ir - n singlet and triplet states (PPP Cl calculation) indicate (Table 7.1) that the one excited state which is not ft/ bonding is the triplet state. Such bonding is predicted to be... 

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