Molecular rings

The second major mechanism of VCD is the production of vibrationally generated electronic current density by the vibrational motion of a local oscillator. Usually, such currents are induced in molecular rings, or fragments of rings, and the local oscillator may either be attached to the ring or contained in it. The resulting VCD per oscillator is biased and monosignate. 

Shortly after iproniazid was shown to have antidepressant properties, imipramine was introduced as the first tricyclic antidepressant. These drugs received the name tricyclic because their structure contains three molecular rings. At first, imipramine was investigated as a possible treatment for the psychotic episodes associated with schizophrenia, a severe mental disorder that causes hallucinations and delusions, because it was chemically similar to another effective anti-schizophrenia drug. Imipramine did not reduce the severity of psychotic episodes, but it did elevate the mood of the patients who took it. In the late 1950s, it was released in the United States under the name Tofranil for the treatment of depression. 

The tendency of atoms of certain elements to form chains with themselves (homoatomic catenation) or in alternation with other atoms (heteroatomic catenation) is of extreme importance in chemistry. The immense subject of organic chemistry and, indeed, life as we know it depend on the special ability of carbon to catenate from the chemical engineering standpoint, catenation and the associated ability to form molecular rings and cages provide opportunities to make materials of desired mechanical, electrical, thermal, chemical, or catalytic properties. 

A pseudorotaxane is the supramolecular product of the self-assembly of two components (i.e., a [2]pseudorotaxane), a molecular ring (macrocycle) and a molecular wire (thread) threaded into it. The occurrence of specific interactions between the two components lowers the free energy of the assembled system with respect to the separate components and drives the self-assembly of the pseudorotaxane (threading process). Threading occurs generally in solution and, upon changes of solvent... 

When a rotaxane contains two different recognition sites in its dumbbell component, it can exist as two different equilibrating coconformations, the populations of which reflect their relative free energies as determined primarily by the strengths of the two different sets of noncovalent bonding interactions. In the schematic representation shown in Fig. 13.8, it has been assumed that the molecular ring... 

Controlled rotation of the molecular rings has also been achieved in catenanes composed of three interlocked macrocycles. For example, catenane 42H26+ (Fig. 13.37) is made up of two identical macrocycles 2 interlocked with a cyclophane containing two bipyridinium and two ammonium units.44 Because of the type of the macrocycles used, the stable coconformation of 42H26+ is that in which the two rings surround the bipyridinium units (Fig. 13.37a, state 0). Upon addition of one electron in each of the bipyridinium units, the two macrocycles move on the ammonium stations (Fig. 13.37b, state 1) and move back to the original position when the bipyridinium units are reoxidized. 

Scheme 2. Possible mechanisms of the rapid slippage of two molecular rings a, conventional mechanism b, transmetallation mechanism. Note that these schemes give only the topology of the pathways. Thus the second transmetalation in mechanism b might happen between any Pd-N bond.

Experimentally, sodium nitrate was added to an aqueous solution of molecular ring 16, and the solution was heated at 100 °C. Then, the equilibrium between 16 and its catenated dimer 15 is pushed by the polar media toward the catenane. After self-assembling in a high yield, catenane 15 was isolated as a CIO4 salt in a high yield (Eq. 3). It was confirmed that catenane 15 thus obtained did not dissociate into two rings in aqueous solution because its framework had been locked. 

Single-bonded six-membered molecular rings, such as those in cyclohexane and in sugars, most often assume a chair (C) conformation. For sugars in pyra-nose ring forms, there are two possible chair conformations,4 25 26 which are designated 4Cj(or Cl) and C4 (or 1C). The superscript and subscript numbers on the designations indicate which atoms are above and... 

Hydrophobic forces are also important in the assemblies of metallo-supramolecular catenanes. One of the most interesting examples is formed when one of the unpolar bipyridine ligands of one macrocycle is included spontaneously in the other macrocycle s internal cavity [39]. Here, the benzene unit of the one macrocycle serves as a guest molecule for the other macrocycle, and the cyclization is favored by n-n interactions. In addition, the minimization of hydrophobic surfaces in polar medium constitutes the second driving force for the catenane formation. The quantitative formation of the [2]catenanes 31a and 31b based on this principle are depicted in Figure 13. Formation of catenane 31b was found to be reversible. Even at room temperature, two monomeric ring structures equilibrate quickly due to the labile nature of Pd-N bond and interlocked molecular ring system 31b is formed. 

J. Leszczynski, Are the Amino Groups in the Nucleic Acid Bases Coplanar with the Molecular Rings Ab Initio HF/6-31G and MP2/6-31G Studies. Int. J. Quantum Chem. 19, 43-55 (1992)... 

Adding up the 18 bond lengths around the ring, we obtain a value of 2510 pm. If we take this to be the circumference of the molecular ring, the radius R of the ring is approximately 400 pm. Thus the lowest electronic transition of this molecule requires an energy of... 

An important feature common to members of the MDA cluster is that substitutions on the molecular ring can be made fairly readily the process is expensive and requires sophisticated chemistry. Still, the chemistry is much simpler than for the four psychedelic groups already discussed, which is one reason why a tremendous number of these MDA-like substances have been synthesized. Many people feel that the number and variety of MDA analogues will enable researchers to make systematic comparisons of mental characteristics and chemical structures, thus providing an important key for understanding more about the nature of the human mind. 

Controlled rotation of the molecular rings has also been achieved in catenanes composed of three interlocked macrocycles. For example, catenane 19H26+ (Fig. 21) is made up of two identical dioxybenzene-based macrocycles interlocked with a cyclophane containing two bipyridinium and two ammonium units [87],... 

Rotaxanes-Threading Molecular Rings Rotaxanes are obtained by threading linear polymers through molecular rings such as cyclodextrins, crown ethers and cyclophanes. Molecular shuttles based on the rotaxane structure have been proposed. 

Figure 6 Schematic representation of the displacement vectors corresponding to the nontotally symmetric postdiagonalization nuclear distortions that show the most marked anti-GMPP character of phenanthrene and pentacene obtained at the HF/6-31G level. The depicted displacement vectors of the non-hydrogen atoms have been multiplied by two in the representation for seek of clarity. The numbers inside the molecular rings are the NICS values of these rings calculated at the HF/6-31+G(d) level with the HF/6-31G optimized geometry...

Fig. 3.8 The gradient approach of Geissler et al.55 A PDMS stamp is used to ink a mixture of MHA (white) and ODT (black) onto a 2D array of silica beads immobilized on a gold substrate (1). Molecules are moving from the stamp to the gold surface with different speed (2). By reaching the gold, the molecules assemble into molecular rings whose composition creates a gradient from the inside to the outside (3)...

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