Molecular rigidity

The fluorescence quantum yield of trans-stilbene is 0.75 when measured in a rigid glass at 77 K, showing that the rigid medium results in more efficient fluorescence. 

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The necessary molecular rigidity of polybenzamide undoubtedly results from the hindered bond rotation within the planar amide group. 

Although liquid crystalline phases can be formed from entirely rigid units such as rods and ellipsoids, all liquid crystal molecules are partially flexible objects which can change their shape in a variety of ways. The extension of rigid-rod theories to account for flexibility is complex and has been the object of considerable effort for several years. It is now clear that simply relaxing the assumption of molecular rigidity is expected to have profound effects on phase behaviour. 

In molecularly rigid systems, the direct (through-space) dipole-dipole interaction between nuclear spins I = fe is normally the dominant source of broadening of the NMR lineshape. For a pair of similar nuclear spins i and j in a magnetic field Hq the dipolar splitting in their spectrum is given by... 

Assign differences in fluorescence quantum yield to differences in the electronic configuration of SI, substituent effects, molecular rigidity and the presence or absence of heavy atoms. 

Table 4.2 The effect of molecular rigidity on fluorescence quantum yield measured in solution at room temperature...

One may compare this result with that of Section 1.2. The vibrational part of (1.13) is again identical to Eq. (1.68). The rotational part is, however, missing in the one-dimensional problem. It is worth commenting on this special feature of the vibrational problem. It arises from the fact that molecular potentials usually have a deep minimum at r = re. For small amplitude motion (i.e., for low vibrational states) one can therefore make the approximation discussed in the sentence following Eq. (1.13) of replacing r by re in the centrifugal term. In this most extreme limit of molecular rigidity, the vibrational motion is the same in one, two and three dimensions. 

The triplet quantum yield tplsc is for the helicenes nearly (1 — radiationless decay approximates klsc. The low value of kfl shows that the fluorescence of all the compounds occurs from the Lb state. The fluorescence rate (kfj of the helicenes with an odd number of benzene rings decreases more slowly than of the helicenes with an even number of rings. This is explained by the difference in molecular rigidity of the two groups. The even ring helicenes are symmetric about a naph-... 

Podsiadlo P, Tang Z, Shim BS et al (2007) Counterintuitive effect of molecular strength and role of molecular rigidity on mechanical properties of layer-by-layer assembled nanocomposites. Nano Lett 7 1224-1231... 

The formation of bi- and tricyclic compounds by transannular photochemical cyclization of monocyclic reactants mostly follows special rules that are caused by the molecular rigidity, equilibria of ring conformers and distinct stereoelectronic effects and, therefore, will be discussed in a separate section. Naturally, it is no longer possible to strictly structure the reactions according to the size and the type of the ring and, furthermore, vastly different examples are known. Therefore, only some representative examples will be discussed in this section and the selection does not claim to be complete. 

The unique chemistry was adapted to use in combinatorial synthesis and was complemented by in silico technologies for the rational design of libraries, in order to optimize diversity of building blocks, obtain molecules with remarkable enzymatic stability, but also to get information from analysis of i.e. conformational constrains, molecular rigidity, size, charge, and hydrophobicity. 

Modulus and creep-resistance of plastics are still inferior to metals, ceramics, and glass, which means there are areas where they cannot compete. We know that molecular rigidity, crystallinity, polarity, and reinforcing fibers can all go a long way toward closing the gap. 

Barrier properties of organic polymers cannot equal metals and glass. Plastics offer so many other advantages that we often try to compromise or laminate to optimize overall balance of properties. It is known that molecular rigidity and crystallinity improve barrier... 

Figure 26 shows the formula, conformation, and absolute configuration of veratridine (52). The structure and conformation of the C27-steroidal base is very similar to that determined for zygacine (C32H49N08) another Veratrum alkaloid (53). Variations in the pharmacological effects of Veratrum alkaloids appear to be dependent on the substituents attached to the essentially rigid molecular framework. The numerous intramolecular hydrogen bonds that enhance the molecular rigidity are indicated in Fig. 26. The positions of all the hydrogen atoms have been located for the veratridine molecule and the intramolecular donors and acceptors are identified as...

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