Intramolecular ordering transition

Figure 6 Examples of compounds that in selective solvents produce biomimetic supramolecular polymers that depending on the conditions exhibit a co-operature intramolecular ordering transition from random to highly ordered helical stacks or a nucleated polymerization transition from oligomeric prenuclei to very long, helical polymeric objects (ten Cate and Sijbesma, 2002 Dankers and Meijer, 2007 van Gorp et al., 2002 Brunsveld, 2001 Jonkheijm, 2005 Hirschberg, 2001).

Internal conversion involves intramolecular radiationless transitions between vibronic states of the same total energy (isoenergetic states) and the same multiplicity, for example S2(v = 0) -Wr Si(v = n) and T2(v = 0) -Wr Ti(v = n). Typical timescales are of the order of 10 14-10 11s (internal conversion between excited states) and 10 9-10 7s (internal conversion between Si and S0). 

Ptitsyn et al. (1968) argued that the collapse process was a first order transition (i.e. the first derivatives of the Gibbs free energy with respect to both the temperature and pressure are discontinuous at the transition). They used the terms coil and globule for the two different states and likened the transition to the first order condensation of a real vapour into liquid droplets when the forces of attraction are increased sufficiently. They were led to believe that experimental observation of the transition would be very difficult, if not impossible, because intermolecular condensation would overwhelm the intramolecular contraction, leading to phase separation. Until quite recently this pessimism seemed fully justified. 

Transitions and Relaxations. Only one first-order transition is observed, the melting point. Increasing the pressure raises the melting point. At low pressures, the rate of increase in the melting point is ca 1.74°C/MPa (0.012°C/psi) at high pressures this rate decreases to ca 0.725°C/MPa (0.005°C/psi). Melting increases the volume by 8%. In the presence of the HFP comonomer, crystal distortion occurs with an increase in intramolecular distance that, in turn, reduces the melting point (57). 

When individual, isolated molecules exist in helical, or other ordered forms, environmental changes, either in the temperature or solvent composition, can disrupt the ordered structure and transform the chain to a statistical coil. This conformational change takes place within a small range of an intensive thermodynamic variable and is indicative of a highly cooperative process. This reversible intramolecular order-disorder transformation is popularly called the helix-coil transition. It is an elementary, one-dimensional, manifestation of polymer melting and crystallization. 

In order to define how the nuclei move as a reaction progresses from reactants to transition structure to products, one must choose a definition of how a reaction occurs. There are two such definitions in common use. One definition is the minimum energy path (MEP), which defines a reaction coordinate in which the absolute minimum amount of energy is necessary to reach each point on the coordinate. A second definition is a dynamical description of how molecules undergo intramolecular vibrational redistribution until the vibrational motion occurs in a direction that leads to a reaction. The MEP definition is an intuitive description of the reaction steps. The dynamical description more closely describes the true behavior molecules as seen with femtosecond spectroscopy. 

In the event, treatment of a rapidly stirred solution of 3 and sodium acetate in MeOH-tbO at 38 °C with PdCl2 results in the fomation of carpanone (1) in 46% yield. The ordered unimolecular transition state for the oxidative coupling reaction furnishes putative bis(quinodimethide) 2 stereoselectively. Once formed, 2 readily participates in an intramolecular Diels-Alder reaction4 to give carpanone (1). Two new rings and all five contiguous stereocenters are created in this spectacular sequential transformation.5... 

In Chapters 4 and 5 we made use of the theory of radiationless transitions developed by Robinson and Frosch.(7) In this theory the transition is considered to be due to a time-dependent intramolecular perturbation on non-stationary Bom-Oppenheimer states. Henry and Kasha(8) and Jortner and co-workers(9-12) have pointed out that the Bom-Oppenheimer (BO) approximation is only valid if the energy difference between the BO states is large relative to the vibronic matrix element connecting these states. When there are near-degenerate or degenerate zeroth-order vibronic states belonging to different configurations the BO approximation fails. 

The metalloalkyne complex Ru ( )-CH=CH(CH2)4C CH Cl(CO)(P,Pr3)2 exhibits behavior similar to that of cyclohexylacetylene (Scheme 10).40 Thus, it reacts with OsHCl(CO)(P Pr3)2 to give the hydride-vinylidene derivative (P Pr3)2 (CO)ClRu ( )-CH=CH(CH2)4CH=C OsHCl(CO)(P,Pr3)2, which evolves in toluene into the heterodinuclear-pi-bisalkenyl complex (P Pr3)2(CO)ClRu (is)-CH=CH(CH2)4CH=CH-( ) OsCl(CO)(P,Pr3)2. Kinetic measurements between 303 and 343 K yield first-order rate constants, which afford activation parameters ofAH = 22.1 1.5, kcal-mol-1 andAS = -6.1 2.3 cal-K 1-mol 1. The slightly negative value of the activation entropy suggests that the insertion of the vinylidene ligand into the Os—H bond is an intramolecular process, which occurs by a concerted mechanism with a geometrically highly oriented transition state. 

Since the transition from dilute to semi-dilute solutions exhibits the features of a second-order phase transition, the characteristic properties of the single- chain statics and dynamics observed in dilute solutions on all intramolecular length scales, are expected to be valid in semi-dilute solutions on length scales r < (c), whereas for r > E,(c) the collective properties should prevail [90]. 

Ab initio Hartree-Fock and density functional theory (DFT) calculations were performed to study transition geometries in the intramolecular hetero-Diels-Alder cycloaddition reactions of azoalkenes 20 (LJ = CH2, NFI, O) (Equation 1). The order of the reactivities was predicted from frontier orbital energies. DFT calculations of the activation energies at the B3LYP level were in full agreement with the experimental results described in the literature <2001JST(535)165>. 

As the temperature is decreased, the chains become increasingly rigid zc then approaches 1 if we assume that there is only one fully ordered crystalline structure and Zconf for the liquid becomes smaller than 1. This means that, at this level of approximation, the disordered state becomes less favorable than the crystalline ground state. A first-order disorder-order phase transition is expected to occur under these conditions. Flory interpreted this phase transition as the spontaneous crystallization of bulk semiflexible polymers [12], However, since the intermolecular anisotropic repulsion essential in the Onsager model is not considered in the calculation, only the short-range intramolecular interaction is responsible for this phase transition. 

---