Ground state complexes

The high degree of orientational specificity which controls the cycloadditions to (267) of allene [(273) (274) 30 1 ] and acetoxybutenone [rz t/-adducts (278) and (279)] is suggestive of being meaningful in mechanistic terms. Several proposals have been advanced to account for these observations, inter alia a polar ground-state complex of the reactants, (281), which undergoes photoexcitation followed by concerted bond formation to products... 

If it is assumed that the ground-state complex is nonfluorescent and the complexation follows a single equilibrium... 

The values of K0 and Ksv obtained by using eq 9 are given in Table 4. The ground-state complex considered in eq 7 includes not only the CT complex but all kinds of complexes that may lead to apparent static quenching. Therefore, usually K0 is larger than KCT as can be seen from Table 4 (although there are a few exceptions). 

Scheme 1 represents the kinetics of a photoinduced ET system including ground-state complexation. Within the DA complex an almost simultaneous back-reaction would occur (step 1). Therefore, the CT complexation causes the yield of the photoproducts to decrease. In this scheme, (Dsf. .. As" denotes a... 

The higher energy features can indeed be associated with transitions of He lCl(K,v" = 0) ground-state complexes with rigid He I—Cl linear geometries. In contrast to the T-shaped band that is associated with transitions to the most strongly bound intermolecular vibrational level in the excited state without intermolecular vibrational excitation, n = 0, the transitions of the linear conformer access numerous excited intermolecular vibrational levels, n > 1. These levels are delocalized in the angular coordinate and resemble hindered rotor levels with the He atom delocalized about the l Cl molecule. 

The photocycloaddition of maleic anhydride to cyclohexene has been found to occur through ground state complexes of charge-transfer type<99> ... 

The stereospecificity of these reactions is surprising in light of the large energies absorbpd by these molecules. Indeed, the major photochemical product of these photolyses was the alternate olefin isomer (1-butene was also observed). These results indicate that free rotation about the photo-excited double bond does not occur in those molecules that dimerize. This suggests the participation of ground state complexes or excimers in the photodimerization. This view is supported by the observations that dilution of cw-2-butene with neopentane (1 1) decreased the yield of dimers and a 1 4 dilution almost completely suppressed dimerization. 

Fig. 6.21. Principle of detection of lipopolysaccharide (LPS) with the CD14-derived probe. It relies on the formation of a ground state complex between fluorescein and rhodamine in aqueous solution with quenching of donor and acceptor fluorescence. Spectrum A shows hypothetical fluorescence emission spectra of this complex. After LPS binding, the peptide sequence gets straightened prohibiting the close contact between the two fluorophores and leading to the recovery of red fluorescence (Spectra B).

Exciplexesn6,nl) can be formed if the excitation energy B - B is higher than the one for A -> A in (1.8). Such an excited complex is associative in the excited state only, the corresponding ground state complex between A and B being dissociative (Fig. 9). Such exciplexes are important intermediates in e.g. cycloaddition reactions as precursors of diradicals 118) which are themselves precursors of the cyclized photoproducts. 

Table 10 pBP/DN //HF/6-31G energies of reactants, ground state complex, transition state and products for reaction of azide with /V-I ormyloxy-/V-methoxyformamide 76ba... 

Fig. 23 HF/6-31G geometries for (a) the ground state complex and (b) the transition state in the reaction of azide anion with iV-formyloxy-iV-methoxyformamidc 76b.

Following an external perturbation, the fluorescence quantum yield can remain proportional to the lifetime of the excited state (e.g. in the case of dynamic quenching (see Chapter 4), variation in temperature, etc.). However, such a proportionality may not be valid if de-excitation pathways - different from those described above - result from interactions with other molecules. A typical case where the fluorescence quantum yield is affected without any change in excited-state lifetime is the formation of a ground-state complex that is non-fluorescent (static quenching see Chapter 4). 

It is interesting to note that the first demonstration of tyrosinate fluorescence in a protein was made by Szabo et al.au> with two cytotoxins from the Indian cobra Naja naja. While exhibiting different relative amounts of the two emission bands, both toxins had fluorescence at 304 and 345 nm, with the 304-nm band being greatly reduced on excitation at 290 nm. Since these proteins have three tyrosine residues and no tryptophan, it was concluded that the 345-nm emission band was due to tyrosinate. Furthermore, tyrosinate appeared to be formed in the excited state from a hydrogen-bonded ground-state complex based on the absorption spectra. Szabo subsequently reexamined these peptide samples and found that they were contaminated with tryptophan (A. G. Szabo, personal communication). While Szabo s approach to the demonstration of tyrosinate fluorescence was correct based on his initial data, his subsequent finding exemplifies an important caution if tyrosinate emission is suspected, every effort must be made to demonstrate the... 

Prereaction ground-state complex for 4o -flavinhydroperoxide with p-hydroxybenzoate complexed to guanidine (ARG214) and phenol (TYR201) optimized at B3LYP/6-31+G(d,p)... 

---