Intramolecular equilibria

If Q-symmetric ligands are employed in asymmetric hydrogenation instead of the corresponding C2-symmetric ligands, there coexist principally four stereoiso-meric substrate complexes, namely two pairs of each diastereomeric substrate complex. Furthermore, it has been shown that, for particular catalytic systems, intramolecular exchange processes between the diastereomeric substrate complexes should in principle be taken into account [57]. Finally, the possibility of non-estab-hshed pre-equilibria must be considered [58]. The consideration of four intermediates, with possible intramolecular equilibria and disturbed pre-equihbria, results in the reaction sequence shown in Scheme 10.3. This is an example of the asymmetric hydrogenation of dimethyl itaconate with a Rh-complex, which contains a Q-symmetrical aminophosphine phosphinite as the chiral ligand. 

For purely intramolecular equilibria the system of differential equations (72) is already linear and the procedure described above transforms the system into the form of equation (104) exactly without any approximations. For such equilibria further simplification of equations (72) and (104) is possible by deleting all those quantities which refer to empty sets of nuclei. 

For non-mutual intramolecular equilibria, if compounds and Aj are converted into one another in only one way, the basis sets can be chosen such that the superoperators Q become unit matrices. Thus, the superoperators Xy take the form ... 

Figure 10.15 Schematic showing competing intermolecular and intramolecular equilibria. The rectangles represent binding sites.

The values of 0(ASD) /2.3O3 R listed in Table 5 are the entropic components of log EM. These are the log EM- alues for ideal strainless cyclisation reactions, i.e. reactions where 0AH° = 0. It is of interest to note that, as far as the entropic component is concerned, symmetry corrected effective molarities on the order of 102 106M are found. This observation leads to the important conclusion that cyclisation reactions of chains up to about 7 skeletal bonds are entropically favoured over reactions between non-connected 1 M end-groups. The intercept of 33 e.u. corresponds to an effective molarity of exp(33/R) or 107 2M, which may be taken as a representative value for the maximum advantage due to proximity of end-groups in intramolecular equilibrium reactions. It compares well with the maximum EM of about 108M estimated by Page and Jencks (1971). 

The conformational entropies of copolymer chains are calculated through utilization of semiempirical potential energy functions and adoption of the RIS model of polymers. It is assumed that the glass transition temperature, Tg, is inversely related to the intramolecular, equilibrium flexibility of a copolymer chain as manifested by its conformational entropy. This approach is applied to the vinyl copolymers of vinyl chloride and vinylidene chloride with methyl acrylate, where the stereoregularity of each copolymer is explicitly considered, and correctly predicts the observed deviations from the Fox relation when they occur. It therefore appears that the sequence distribution - Tg effects observed in many copolymers may have an intramolecular origin in the form of specific molecular interactions between adjacent monomer units, which can be characterized by estimating the resultant conformational entropy. 

Intermolecular interactions and complexes Dielectric measurements on interacting solutes in inert solvents provide information about molecule complex formation. Some such dipoles induced by intermolecular interactions and molecular complexes in benzene solution are listed in Table 1.3. The dipole moment of the complex is a function of the relative strengths of the acid and base and the intramolecular equilibrium is described by Eq. (49) ... 

In the presence of an aromatic ligand such as bipyridyl or o-phenanthroline ternary complexes of the type nucleotide—M" —ligand form in aqueous solution with enhanced thermodynamic stability. The enhanced stability arises from rc-stacking interactions between the aromatic ligand and the heterocyclic base. In solution these interactions are evident from induced H NMR shifts and in the solid state the two bases are aligned parallel within van der Waals contact distances. Recently the extent of these stacking interactions for a number of Mn" and Zn" complexes of nucleoside triphosphates have been estimated in terms of intramolecular equilibrium constants. ... 

Tabie 1 Intramolecular equilibrium Isotope effects for M-H to a-CHx exchange reactions... 

The total of the dimensionless intramolecular equilibrium constant, Kl/tot, is defined by equation (13) (see also below eq. (21)),... 

The connection between the overall intramolecular equilibrium constant Kl/ tot already introduced in Section 3.4, and the accessible stability enhancement (eq. (12)) is given by equations (21a) -(21e) ... 

Figure 9 The competing intramolecular equilibrium between 24 and 25, in which the substituent may be in either the d- or L-form, and the intermolecular equilibria between 24, 26, 27, 28, and 29 where 26 and 27 may be in either the d- or L-form.

Table I, Results regarding the position of the intramolecular equilibrium...

Among the three pyrimidine-nucleobase residues shown in Figure 1 only the cytosine moiety is not protonated at N3 in the physiological pH range and hence, freely available for metal ion coordination. Therefore, this residue will be discussed first. The cytosine residue is an ambivalent ligating site as follows from crystal structure studies e.g., Pt " coordinates to N3, Ba " to (C2)0, and Cu " binds to both sites [35,40]. Thus, in the latter instance chelate formation occurs and for aqueous solution then the intramolecular equilibrium (13) needs to be considered ... 

The position of equilibrium (13) for Cyd = L is defined by the dimension-less intramolecular equilibrium constant Ki (eq. 16),... 

For purine-nucleoside 5 -phosphates the formation of macrochelates was proposed nearly 60 years ago [131] and more than 50 years ago it was concluded that they actually exist [132-135], i.e., a metal ion coordinated to the phosphate residue of a purine nucleotide may also interact in the dominating anti conformation with N7 of the purine moiety. Nowadays formation of macrochelates in complexes formed by purine nucleotides with various metal ions including Cd " is well established [117,120,136-138]. Clearly, the formation of such a macrochelate must give rise to the intramolecular equilibrium (22) ... 

A number of issues need to be addressed before this method will become a routine tool applicable to problems as the conformational equilibrium of protein kinase. E.g. the accuracy of the force field, especially the combination of Poisson-Boltzmann forces and molecular mechanics force field, remains to be assessed. The energy surface for the opening of the two kinase domains in Pig. 2 indicates that intramolecular noncovalent energies are overestimated compared to the interaction with solvent. 

Thirdly, the intramolecular assodation of a solvent affects the Lewis acid - base equilibrium Upon... 

As supporting evidence, rapid isomerization of the ds- and maui-Tr-allylpal-ladium complexes 27 and 28 is catalyzed by Pd(Ph3P)4 in THF even at -15 C to give a 45 55 equilibrium mixture from either 27 or 28[29-31].. Actually, in the intramolecular reaction of soft nucleophiles of 29 and 30, a trans-ds mi.xttire (31 and 32) (1 1) was obtained from /raiw-allylic acetate 29. On the... 

The next important phenomena that the result of supramolecular effect are the concentration and proximity effects concerning the components of analytical reaction, even through they are considerably different in hydrophobicity, charge of the species, complexing or collisional type of interaction. The concentration and proximity effects determine the equilibrium of analytical reaction, the efficiencies of intramolecular or intermolecular electronic energy or electron transfer and as a result the sensitivity of analytical reactions. 

Here Zint is the intramolecular partition function accounting for rotations and vibrations. However, in equilibrium, the chemical potential in the gas phase is equal to that in the adsorbate, fi, so that we can write the desorption rate in (I) as... 

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