Processing induced transitions

We have so far reviewed the thermodynamic concept of phase transitions and introduced a classification system for hydrates. It remains to explore where in the dosage form development process such transitions are most likely to occur and what we can say about them in light of the preceding discussion. The following discussion will be divided into situations where processing induces transitions, and transitions taking place in the final product. When appropriate, polymorphic systems are also illustrated for contrast and completeness. 

Let us return to the nonadiabatic chemical processes. When a PES has been built, a part of the total Hamiltonian may remain unaccounted for, and this part, acting as a perturbation, induces transitions from the initial to the final state. There are several types of such a perturbation, namely (i) an unaccounted part of the electronic interaction (ii) non-adiabaticity (iii) spin-orbit coupling. 

Because non-adiabatic collisions induce transitions between rotational levels, these levels do not participate in the relaxation process independently as in (1.11), but are correlated with each other. The degree of correlation is determined by the kernel of Eq. (1.3). A one-parameter model for such a kernel adopted in Eq. (1.6) meets the requirement formulated in (1.2). Mathematically it is suitable to solve integral equation (1.2) in a general way. The form of the kernel in Eq. (1.6) was first proposed by Keilson and Storer to describe the relaxation of the translational velocity [10]. Later it was employed in a number of other problems [24, 25], including the one under discussion [26, 27]. 

During the past few years, increasing numbers of reports have been published on the subject of domino reactions initiated by oxidation or reduction processes. This was in stark contrast to the period before our first comprehensive review of this topic was published in 1993 [1], when the use of this type of transformation was indeed rare. The benefits of employing oxidation or reduction processes in domino sequences are clear, as they offer easy access to reactive functionalities such as nucleophiles (e. g., alcohols and amines) or electrophiles (e. g., aldehydes or ketones), with their ability to participate in further reactions. For that reason, apart from combinations with photochemically induced, transition metal-catalyzed and enzymatically induced processes, all other possible constellations have been embedded in the concept of domino synthesis. 

Using the same method that led to Eq. (5.27), it is easy to establish the rule of multiplication of depolarization factors when several processes inducing successive rotations of the transition moments (each being characterized by cos2 C,) are independent random relative azimuths, the emission anisotropy is the product of the depolarization factors (3 cos2 c, — l)/2 ... 

Electron transfer processes induce variations in the occupancy and/or the nature of orbitals which are essentially localized at the redox centers. However, these centers are embedded in a complex dielectric medium whose geometry and polarization depend on the redox state of the system. In addition, a finite delocalization of the centers orbitals through the medium is essential to-promote long-range electron transfers. The electron transfer process must therefore be viewed as a transition between two states of the whole system. The expression of the probability per unit time of this transition may be calculated by the general formahsm of Quantum Mechanics. 

We consider a collision between two atoms. A chemical bond between the atoms can be formed, during the collision, via an electromagnetically induced transition to a bound electronic state. This process is called photo-association. 

Figure 8. A DSC record (a thermogram) from a thermally induced transition of lysozyme in dilute aqueous solution. Tt = transition temperature ACp = change in heat capacity accompanying the unfolding process. The hatched area is proportional to the enthalpy of transition. Adapted from Privalov (1980).

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