Enthalpy and entropy control

Temperature is also an important parameter for controlling the resolution of enantiomers in HPLC. The enthalpy and entropy control of chiral resolution on antibiotic CSPs is similar to the case of polysaccharide-based CSPs (Chapter 2). Armstrong et al. [1] have studied the effect of temperature on the resolution behavior of proglumide, 5-methyl-5-phenylhydantoin and A-carbamyl-D-pheny-lalanine on the vancomycin column. The experiments were carried out from 0°C to 45°C. These results are given in Table 6 for three chiral compounds. It has been observed that the values of k, a, and Rs for the three studied molecules have decreased with the increase in temperature, indicating the enhancement of chiral resolution at low temperature. In another work, the same workers [22] have also studied the effect of temperature on the resolution of certain amino acid derivatives on the teicoplanin chiral stationary phase. They further observed poor resolution at ambient temperature, whereas the resolution increased at low... 

Recent reports emphasizing the role of 1,4-diradicals in triplet Paterno-Biichi reactions should also be mentioned. The first to be discussed concerns the chiral induction of photocycloadditions of various olefins to chiral phenylglyoxalates11 28,29, The high diastereoselectivity of these reactions often reaches de values of >96% and shows a characteristic temperature dependence with specific points of inversion. This behavior is a result of competition between enthalpy- and entropy-controlled partial selection steps. 

This effect will not be discussed in detail here, however, extensive investigations by Scharf and co-workers show that temperature is one of the controlling parameters11. Characteristic temperature dependencies and points of inversion indicate a competition between enthalpy- and entropy-controlled partial selection (see Section 1.6.1.4.3.1.). Details and instructions on how to utilize measurements of the temperature dependence in order to optimize a specific asymmetric reaction can be found in Scharf s review article11. 

Theoretically, the problem has been attacked by various approaches and on different levels. Simple derivations are connected with the theory of extrathermodynamic relationships and consider a single and simple mechanism of interaction to be a sufficient condition (2, 120). Alternative simple derivations depend on a plurality of mechanisms (4, 121, 122) or a complex mechanism of so called cooperative processes (113), or a particular form of temperature dependence (123). Fundamental studies in the framework of statistical mechanics have been done by Riietschi (96), Ritchie and Sager (124), and Thorn (125). Theories of more limited range of application have been advanced for heterogeneous catalysis (4, 5, 46-48, 122) and for solution enthalpies and entropies (126). However, most theories are concerned with reactions in the condensed phase (6, 127) and assume the controlling factors to be solvent effects (13, 21, 56, 109, 116, 128-130), hydrogen bonding (131), steric (13, 116, 132) and electrostatic (37, 133) effects, and the tunnel effect (4,... 

Electrostatically-controlled pre-association interactions have an important effect on rates for [Pd(dien)Cl]+ reacting with thione-containing nucleosides, nucleotides and oligonucleotides, as is often the case for reactions between metal complexes and this type of biological ligand. Interaction between the charged complex and the polyanionic oligonucleotide surface leads to an increase in both enthalpy and entropy of activation in the DNA or model environment (252). 

Self-assembly is a thermodynamically controlled process. The formation of the capsules and the exchange of guest molecules proceed within seconds to hours, sometimes days, but finally, an equilibrium is reached which is governed by a finely balanced interplay of enthalpy and entropy. A... 

The total energy of a system involves both enthalpy and entropy. Thus, whichever causes the greater change in overall energy during the reaction will be the one controlling the reaction and determining whether it is exothermic, endothermic, spontaneous, or not spontaneous [11]. 

The forces that control adsorption of nonionic contaminants on the solid phase were summarized by Yaron et al. (1996) in terms of enthalpy and entropy adsorption... 

K. N. Houk, N. G. Rondan, and J. Mareda, Theoretical Studies of Halocarbene Cycloaddition Selectivities. A New Interpretation of Negative Activation Energies and Entropy Control of Selectivities, Tetrahedron 1985, 41, 1555. Calculations on carbene addition reactions led to a general explanation of why it is possible for very exothermic, bimolecular reactions to have negative activation enthalpies. 

Petkov and Stoineva (12) have more recently reported that the relative rate of the o-chymotrypsin-catalyzed hydrolysis of p-alkoxycarbonyl anilide derivatives 35 of N-acetyl-(.-phenylalanine is enhanced with an increase in the size of the R alkyl group of the leaving group. This rate enhancement specificity appears to be entropy controlled the bulky alkyl groups increase both enthalpy and entropy of activation. These kinetic and thermodynamic data were interpreted in the following way the bulky -alkoxycarbonyl... 

With sodium borohydride and catalytic amounts of titanyl acetoacetonate, a,fi-unsaturated carbonyl compounds give allyl alcohols regioselectively, whereas a-diketones and acyloins are reduced to vicinal diols.325 Enantioselectivities in the reduction of acetophenone, catalysed by 1,3,2-oxazaborolidones, have been examined using the AM1-SCF MO method. The optimized geometries, thermal enthalpies, and entropies of R and S transition states in the stereo-controlling steps of the reduction have been obtained.326... 

Protein folding cannot occur in a random fashion but must follow distinct pathways a protein of 100 amino acids with diffusional control of encounter between different parts of the chain would take (210°/2)/lO10 s 1 = 1033 s = 5 x 1024 years, or longer than the age of the universe, to fold. Levinthal s paradoxon reaches the same conclusion a protein of 100 amino acids with side chains in three different states can feature 100 / 3 states diffusion of those states would take even longer than 1024 years. Proteins cannot be stable without hydrophobic interactions convergence temperatures for unfolding enthalpies and entropies (AHm and ASm) are around 112 °C, the presumed Tmax for proteins in aqueous solution. 

Standard enthalpies and entropies for the ketone to enol equilibrium have been determined from data obtained by the kinetic halogenation method (see Section 3) at 5, 15, 25 and 35°C. Since the keto-enol equilibrium constants depend on the absolute rate constant arbitrarily chosen for the diffusion-controlled halogen addition to the enol, only the differences in from one ketone to another must be considered... 

We can determine the specific enthalpies and the specific entropies from the temperature, pressure, and composition of the environment. Once we specify the environmental conditions, all enthalpy and entropy terms are fully defined regardless of the process within the control volume. The term 7 0Chemical exergy, Exch, is... 

Finally, a brief overview was presented of important experimental approaches, including GITT, EMF-temperature measurement, EIS and PCT, for investigating lithium intercalation/deintercalation. In this way, it is possible to determine - on an experimental basis - thermodynamic properties such as electrode potential, chemical potential, enthalpy and entropy, as well as kinetic parameters such as the diffusion coefficients of lithium ion in the solid electrode. The PCT technique, when aided by computational methods, represents the most powerful tool for determining the kinetics of lithium intercalation/deintercalation when lithium transport cannot be simply explained based on a conventional, diffusion-controlled model. 

With both acids 52 and 54 and the coacid 33, the diastereoselectivity is temperature independent. This means that the rate difference for the diastereofacial coupling of the intermediate ester radical is fully entropy controlled. In the corresponding heterocoupling with the radical from the coacid 35, the diastereoselectivity is controlled by activation enthalpy and entropy [102]. 

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