Zwitterionic state

It has been suggested that the elusive zwitterionic state [75], or a novel nucleophilic addition/elimination mechanism at the central carbon of the exocyclic bridge [79], or solvent-solute H-bonding interactions [76, 80] might play a role in modulating cis-trans interconversion. Cis-trans isomerization gives rise also to a remarkable intrinsic photochromism of HBI, as it can be easily and reversibly induced upon light absorption [74—76, 79, 80]. 

Ehrenfest dynamics with the MMVB method has also been applied to the study of intermolecular energy transfer in anthryl-naphthylalkanes [85]. These molecules have a naphthalene joined to a anthracene by a short alkyl —(CH)n— chain. After exciting the naphthalene moiety, if n = 1 emission is seen from both parts of the system, if n = 3 emission is exclusively from the anthracene. The mechanism of this energy exchange is still not clear. This system is at the limits of the MMVB method, and the number of configurations required means that only a small number of trajectories can be run. The method is also unable to model the zwitterionic states that may be involved. Even so, the calculations provide some mechanistic information, which supports a stepwise exchange of energy, rather than the conventional direct process. 

In this section, compounds are described that differ from 6 by the replacement of one or more methylene groups by heteroatoms or heteroatom groups. Quantum-chemical calculations on such species have not been carried out. However, on the basis of the results discussed in the above sections and depicted in Schemes 6.42 and 6.64, there is no doubt that all reactive intermediates under consideration are genuine allenes. After all, the tether across the allene subunit is larger in the corresponding compounds in this section than in 3<52-lH-pyridine (179), 3<52-pyran (180) and 3<52-thiopyran (299) owing to the absence of a double bond. Thus, compared with these models, the allene structures in this section suffer from less strain and are hence stabilized relative to their zwitterionic states. 

Theoretical calculations predict that, compared to other amino acids, arginine may dimerize and trimeiize in the zwitterionic state. Soft-sampling ESI of the racemate of arginine, with one of the enantiomer isotopically labeled, reveals the formation of stable trimers with NOj" present as counterion. No preference for... 

At a pH where the protons from the carboxyl group have been completely removed but significant protons have not yet been released from the amino group, the charges on an amino acid balance, so the overall charge is zero, which defines the zwitterion state. 

Here, Tj and 2 correspond to the reactant and product state, respectively, and T3 is a zwitterionic state having two chloride anions separated by a carbocation. 

Mention should be made of the 9-phenylhomocub-l(9)-ene (5) to l-phenylhomocub-9-ylidene (6) rearrangement, which is believed to result from a Wagner-Meerwein-type rearrangement of the zwitterionic state of the alkene.3,4... 

It is well-known that amino acids are in the zwitterionic state in essentially every crystalline solid, not in a mixed neutral/zwitterionic form. Therefore the equilibrium strongly favors the reverse direction, i.e. logK 0. Thus, we conclude... 

The AR mechanism for rectification [121], showing a proposed D-molecule electron flow from the excited zwitterion state D+-ground state D°-metal electrodes M-, and M2. Here = 0 is the vacuum level, (j> is the work function of the metal electrodes, V is the potential applied on the left electrode (the right electrode is grounded), lD is the ionization potential of the donor moiety D, Aa is the electron affinity of the acceptor moiety A, and fF1 and F2 are the Fermi levels of the metal electrodes. HOMO (LUMO) levels are the highest occupied (lowest unoccupied) molecular orbitals of D-tr-A. 

It should be noted that the effect does not require the addition of an extra proton donor-acceptor system. In fact, the proton exchange may take place between a zwitterion and a non-zwitterion state of different orientation as indicated by the reaction scheme... 

The structural dependence of biradicaloid minima discussed in Section 4.3.3 on an example of twisting of a double bond A=B can be extended to take solvent effects into account. Not only the nature of the atoms A and B but also polar solvents and counterions affect the stability of zwitterionic states and states of charged species. Then, depending on the solvent, a biradicaloid minimum can represent either an intermediate or a funnel for a direct reaction. 

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