Geminal protons

Figure 13 20 there are two other vinylic protons Assuming that the coupling constant between the two geminal protons in ArCH=CH2 is 2 Hz and the vicinal coupling constants are 12 Hz (cis) and 16 Hz (trans) describe the splitting pattern for each of these other two vinylic hydrogens... 

The relative configuration is derived from the NOE difference spectra. Significant NOEs are detected owing to cis relationships within the neighbourhood of non-geminal protons ... 

The NOESY spectrum of 7-hydroxyfrullanolide reveals the spatial proximities between the various protons. Cross-peak D arises from the gemi-nal coupling between the exomethylenic geminal protons (8 5.71 and 6.06). Dipolar interaction between the 06 proton (8 4.97) and the allylic methyl protons (8 1.64) is inferred from cross-peak C. This interaction is possible only when the C-6 proton is a-oriented. The C-1/3 and 02/8 protons (8 1.31 and 1.45, respectively) exhibit cross-peaks... 

The first step of the structure refinement is the appHcation of distance geometry (DG) calculations which do not use an energy function but only experimentally derived distances and restraints which follow directly from the constitution, the so-caUed holonomic constraints. Those constraints are, for example, distances between geminal protons, which normally are in the range between 1.7 and 1.8 A, or the distance between vicinal protons, which can not exceed 3.1 A when protons are in anti-periplanar orientation. 

Central ion-proton distance in A b For direction cosines, see60,188) c Geminal protons d For notation, seeI 8) e Dipolar part Af - a 189) f From an EHT-SCCC calculation189 8 No hf data available... 

Let us consider the possible events following excitation of an acid AH that is stronger in the excited state than in the ground state (pK < pK). In the simplest case, where there is no geminate proton recombination, the processes are presented in Scheme 4.6, where t0 and Tq are the excited-state lifetimes of the acidic (AH ) and basic (A- ) forms, respectively, and ki and k i are the rate constants for deprotonation and reprotonation, respectively, kj is a pseudo-first order rate constant, whereas k i is a second-order rate constant. The excited-state equilibrium constant is K = k /k 7 ... 

Fig. 2 a Symmetric model of ligand-protein complex. LI to L5 represent the ligand, while PI to P5 represent the protein. All the distances between the nearest neighbor protons in the complex are assumed to be 2.5 A. b Asymmetric model, with the L4-L5 distance set at 1.8 A (to mimic the geminal protons)... 

The STDs and, hence, the apparent epitope map can exhibit significant dependence on intraligand relaxation rates, thus necessitating caution in simple qualitative interpretations. In particular, geminal protons on a ligand can show smaller STDs despite their proximity to the protein [36]. 

Related to these electronic factors, some examples of stereospecific coupling are known. In 7 the geminal protons on phosphorus are non-equivalent and exhibit slightly different couplings to phosphorus, both of which are temperature and solvent dependent.(1969,51) A clearer example is given by the recent work of Mikolajczyk(1969,52) on the two isomers 8 and 9. 

Solution studies of malonate complexes include the unusual observation that [(H20)sCr(Hmal)]2+ has considerable kinetic stability.924 The mechanism of ring closure has been the subject of detailed study.925 The methylenic protons of [Cr(mal)3]3- undergo bromination when a suspension of the potassium salt is treated with bromine in ether (2 h). Legg and co-workers927 have recently used a number of malonate complexes to demonstrate that the 2H NMR spectra are both readily observed and useful for solving structural problems. The resolution that may be obtained by this method may be appreciated by the fact that in the complex [Cr(mal-d2)2(bipy)]2 the geminal protons of the malonate are clearly resolved. 

It should be stressed that the reversible chemical reactions give us better chance to observe many-particle effects since there is no need here to monitor vanishing particle concentrations over many orders of magnitude. Indeed, the fluctuation-controlled law of the approach to the reaction equilibrium similar to (2.1.61) was observed recently experimentally [85] for the pseudo-first-order reaction A + B AB of laser-excited ROH dye molecules which dissociate in the excited state to create a geminate proton-excited anion pair. The solvated proton is attracted to the anion and recombines with it reversibly. After several dissociation-association cycles it finally diffuses to long distances and further recombination becomes unobservable. 

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