Bond order alternation

It has been found (7,9) that Z) > 0, ie, the maximum of an asymmetrical dye, X, is shifted to the short-wavelength region with respect to the arithmetical mean of the parent dye maxima. The phenomenon has been named a deviation (7). The positive deviations in PMDs are explained by the bond order alternation within the polymethine chain caused by different contributions of both end groups to the dye energetic stabiUty (7,9,10). The deviation reaches its maximum at Tqi > 45° and Tqi < 45° if the end groups have > 45° and 4>q2 > 45°, or < 45° and 4>q2 < 45°, then the... 

Dynamic nuclear polarisation (DNP) enhanced 15N CP MAS NMR has been exploited by Mark-Jurkauskas et al.79 in the studies of intermediates of the bacteriorhodopsin photocycle. The data for L intermediate were similar to those found for 13-ds,15-anti retylidene chloride, while those for K intermediate were similar to those of acid blue bacteriorhodopsin in which the Schiff base counterion was neutralised (Table 3). The 15N chemical shifts observed have shown that for bacteriorhodopsin, the Schiff base in K intermediate state loses contact with its counterion and establishes a new one in L intermediate state. The proton energy stored at the beginning in the electrostatic modes has been transformed to torsional modes. The transfer of energy is facilitated by the reduction of bond order alternation in the polyene chain when the counterion interaction is initially broken and is driven by the attraction of the Schiff base to a new counterion. 3D CP MAS experiments of NCOCX, NCACX, CONCA and CAN(CO)CA types have been used in studies of proteorhodopsin.71... 

Fig. TI.5 Schematic figure showing how /30 (solid line) varies with bond-order alternation (BOA) for a donor-acceptor polyene from a neutral ground-state structure (left) to a fully zwitterionic ground-state structure (right), along with the variation of the constituent terms according to the two-state Eq. (16) ...

Furthermore, Lee et al. used a four-state model to describe the NLO properties of an octopolar molecule such as crystal violet [98]. In addition, they showed that one can use the concept of bond order alternation (BOA) to analyze the NLO structure-property relationship in octopolar molecules. 

Figure 3.6. Plot of y versus bond length alternation (a) and contribution of each individual term of Eq. (18) to y (b) for the structure (CH3)2N—(CH=CH)4—CHO. Some authors prefer to use the term bond order alternation (BOA) that is somehow related with bond length alternation that is, change of bond length results in change of bond order. The triangles represent dipolar contributions (first term in Eq. (18)) diamonds are for the coupling term (second part of Eq. (18)) and the squares are related to the negative term (third part in Eq. (18)). (From Ref. [318] with permission of the American Chemical Society.)...

Quantum mechanical analysis based on a simple two-level model [22] and bond-order alternation (BOA) principle exploiting aromaticity [23] have worked surprisingly well in providing useful structure/property relationships for the design of chromophores with ever improving molecular hyperpolarizability. Table 1 provides some representative examples with improved molecular optical nonlinearity developed over the past decade. It has been shown that very large nonlinearities... 

The diene ligands transform to the cisoid form because of the thermodynamic stability of the complex. Because the bond distances of Cj—and C2—Cy in the butadiene ligand are 1.45 and 1.46 A, respectively, the double bond character between C2—C3 and the bond order alternation is still insignificant [43]. This may be due to the strong K-acidity of three carbonyl ligands which reduces n-back donation from iron to the butadiene ligand. (Cyclohexadiene)tricarbonyliron complexes are also noteworthy. Fe(l,3-cyclo-hexadiene)(CO)3 was prepared by the reaction of 1,3-cyclohexadiene with Fe(CO)5 [44] (eq (13)) and more stable Fe(l,4-cyclohexadiene)(CO)3 was also prepared [45]. 

Bredas et al. studied theoretical TPA properties of the 4-dimethylamino-4 -formylstilbene (DAFS) with the degree of tt bond-order alternation (BOA) by the application of an external field [54]. This work shows two possible strategies to optimize TPA response in stilbenic molecules, as shown in Fig. 5 (1) In the case of TPA resonance in the lowest excited state at oi energy, the D term of Eq. 5 presents the main contribution and ajpA varies in the same way as and Afioi as a function of BOA, with a maximum for an intermediate regime between the neutral Brl and the cyanine Br2 Umits. (2) For a TPA resonance in the specific two-photon state S2, the structure presents huge ajpA values for the cyanine structure near the double resonance at Eoi =. ... 

Figure 2 Dependence of the first and second hyperpolarizabilities on bond-order alternation (BOA) calculated for a donor-acceptor polyene under the influence of an external electric field. The left-hand side of each plot corresponds to a structure dominated by the neutral resonance structure, whereas the right-hand side corresponds to a zwitterionic structure. In a two-state model BOA = 0 corresponds to an equal mixture of the two resonance forms.

It has been shown that the translational symmetry in the RHF solution for poljfmeis is broken and that the symmetry of the electron density distribution in poljmers exhibits a unit cell twice as long as that of the nuclear pattern [Bond-Order Alternating Solution (BOAS) J. Paldus and J. Ci k, J. Polym. ScL, Bm C, 29, 199 (1970) also J.-M. Andre, J. Delhalle, J.G. Fripiat, G. Hennico, J.-L. Calais, and L. Piela, J. MoL Struct. (Theochem), 179, 393 (1988)]. The BOAS represents a feature related to the Jahn-TeUer effect in molecules and to the Peierls effect in the sohd state (see Chapter 9). 

BOAS Bond-Order Alternating Solution The electronic density distribution that... 

BOAS stands for Bond-Order Alternating Solution. It has been shown, that the translational sym-... 

Figure 4. The dipole moment (aX polarizability anisotropy Aa Q>), first hypeipolarizability P (cX and figure of merit FOM (d), as a fimction of bond order alternation fi>r the molecule (CI%)2 N(CHK304 (see (27 and...

Fig. 1.10 Left. Canonical resonance structures for the l-dimethylamino-l,3,5,7-octatetraen-8-al molecule, DAO. (Top) Neutral polyene limit (middle) cyanine limit (bottom) zwitterionic charge-separated limit. Right Corresponding evolution of bond length alternation BLA (in A) (x) and bond order alternation BOA ( ), as a function of the applied external electric field F.

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