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Conformations equilibrium populations

A4) Bond angle bending makes a nonnegligible contribution to conformational entropy and can affect computed equilibrium populations [11]. [Pg.118]

The equilibrium population is said to have a structure when a substantial fraction of the molecules adopts similar conformations. But the phrase lacking structure does not imply that individual molecules comprising the ensemble lack a conformation rather, the population is too heterogeneous to be readily characterized using a coherent, structure-based descriptor. The unfolded state resists ready characterization because it is so diverse. Typical biophysical methods report ensemble-... [Pg.16]

Unlike 1,3,5-dioxaphosphorinanes the relative population of the con-former with the axial phenyl group at phosphorus decreases in 1,3,5-diazaphosphorinanes both for P(III) and for P(IV) compounds. Conformational analysis of 5-phenyl-1,3,5-diazaphosphorinanes and their oxides, sulfides, and selenides showed that the conformational equilibrium shifted toward the conformer with the axial phenyl group on changing from P(III) to P(1V) derivatives. The same conclusion made for 5-phenyl-l, 3,5-dioxaphosphorinanes, but it is contrary to that made for 1-phenyl-phosphorinane-4-ones and their derivatives (83MI1). [Pg.82]

The examples cited above are only two of the many possible cases of H-bond isomerization. Because of the low kinetic barriers separating these species, equilibration of H-bonded isomer populations to limiting thermodynamic values is generally expected to be much faster than for covalent isomers. Methods of quantum statistical thermodynamics can be used to calculate partition functions and equilibrium population distributions for H-bonded isomers,41 just as in the parallel case for covalent isomers and conformers. [Pg.607]

Analogously, the fluorescence quantum yield of an extrinsic fluorescent probe contained in a peptide can be measured by comparison with an appropriate standard. If the fluorescent peptide exists in a conformational equilibrium, the fluorophore may be located in a number of different environments and may have a distinct quantum yield (ip,) in each environment. In this case the determined fluorescence quantum yield represents a population-weighted average of the individual [Pg.700]

Furthermore, the observed relative intensity of the 625 cm band to that of the 603 cm i band should be correctable with the AA/GA population ratio of the conformation equilibrium. The observed ratios depended slightly on the anion For the halides, it seems to increase in the order [BFJ [PF ]" == Cl < Br- < r [50]. [Pg.320]

Measurements of dipole moments, Kerr constants, and dielectric absorption have been employed (81RCR336) widely to obtain information on the conformational equilibrium in acyl heterocycles. Details on conformer structures and populations depend on the choice of additive scheme, group moments, or polarizability tensor in the case of Kerr constants. Several early conclusions, especially for furan- and thiophene-2-carboxaldehyde, appeared contradictory, owing to the choice of these quantities. A more precise definition of polarizability tensors for several heterocycles and a choice of group moments and additive schemes tested on a large amount of available experimental results and supported by accurate theoretical calculations have led to more confidence in the use of experimental dipole moments and Kerr constants in conformational analysis. A limitation of the method is that the... [Pg.80]

In order to find the relevant conformers, the authors employed a Monte Carlo/minimization strategy that is described in more detail in the next chapter - in practice, (2/ , 4S )-l-hydroxy-2,4-dimethylhex-5-ene is sufficiently small that one could survey every possible torsional isomer by brute force, but it would be very tedious. Table 2.2 shows, for the nine lowest energy conformers, their predicted energies, their contribution to the 300 K equilibrium population, their individual 1 Jcc coupling constants between atoms C(2)C(5), C(2)C(8), C( 1 )C(4), and C(4)C(7), and the mean absolute error in these coupling... [Pg.58]

Two empirical increment systems (Table 4.22) derived from experimental data as collected in Table 4.23 permit prediction of alkanol carbon-13 shifts. One is related to the shift value B of the hydrocarbon R —H and involves, as usual, addition of the increments Z, = <5,(r oh) — r)(- R > according to eq. (4.8 a) [268]. The other employs a linear equation (4.8 b), correlating the shifts of an alkanol R — OH and the corresponding methyl-alkane R —CH3 by a constant bk and a slope ak, which is 0.7-0.8 for a and about 1 for ji and y positions [269]. Specific parameter sets characterize primary, secondary, and tertiary alcohols (Table 4.22). The magnitudes of Z) increments in eq. (4.8 a) decrease successively from primary to tertiary alcohols (Table 4.22), obviously as a result of reduced populations of conformers with yliauche interactions in the conformational equilibrium when the degree of alkylation increases. [Pg.206]

It was impossible to measure the coupling constants JAx and JBx for 7 in D20 however, for 6 (see Table V) these constants were almost equal at 7.0 Hz. The magnitudes of these constants require almost equal populations for 6a and 6c and relatively low abundance of 6b. The difference in chemical shifts for HA and HB, as the chemical shifts, are temperature independent. The following conformational equilibrium is expected to approximate that which actually exists since it agrees well with the NMR and the rotational data and general expectations based on conformational analysis,... [Pg.144]

Studies on conformational equilibrium, described below, establish that for tri-0-acetyl-/ -D-xylopyranosyl chloride the all-axial 1C (d) conformation comprises about 80% of the total conformational population, in an equilibrium wherein approximately 20% of the all-equatorial Cl (d) form is present. [Pg.168]

The qualitative interpretation is that the aa form is reduced near -2.3 V and the ee at more negative potentials. In the low-temperature limit, the two peak heights simply reflect the equilibrium populations of the two conformations that are present in comparable amounts. The fact that the first peak is much larger than the second at higher temperatures is explained by the conversion of significant amounts of ee to the aa form, which can then be reduced at the first peak. [Pg.495]

The SM2/AM1 model was used to examine anomeric and reverse anomeric effects and allowed to state that aqueous solvation tends to reduce anomeric stabilization [58]. Moreover, SM2/AM1 and SM3/PM3 models were accounted for in calculations of the aqueous solvation effects on the anomeric and conformational equilibria of D-glucopy-ranose. The solvation models put the relative ordering of the hydroxymethyl conformers in line with the experimentally determined ordering of populations. The calculations indicated that the anomeric equilibrium is controlled primarily by effects that the gauche/trans 0-C6-C5-0 hydroxymethyl conformational equilibrium is dominated by favorable solute-solvent hydrogen bonding interactions, and that the rotameric equilibria were controlled mainly by dielectric polarization of the solvent [59]. On the other hand, Monte Carlo results for the effects of solvation on the anomeric equilibrium for 2-methoxy-tetrahydropyran indicated that the AM1/SM2 method tends to underestimate the hydration effects for this compound [60]. [Pg.194]

The indirect influence of the solvent on the optical rotation due to the change in the conformational equilibrium upon solvation was studied by Polavarapu et al. [63] for (R)-epichlorhydrin. No solvent model was used, and the conformer populations in... [Pg.212]

The stereochemistry of the adduct 113a, obtained as major under thermal conditions, indicates that it is derived from endo-approach to conformation A. It suggests that rotamers with the sulfinyl oxygen in s-cis arrangement (B in Fig. 10) are not the most populated in the conformational equilibrium, even though they are favored upon electrostatic grounds. The higher stability of con-... [Pg.65]


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See also in sourсe #XX -- [ Pg.91 ]




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Conformational equilibrium

Conformational populations

Conformic equilibrium

Equilibrium populations

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