Rotational directionality

Control of Rotational Directionality with Chiral Pulse Trains... 

H202 (wt.%) Magnetic Field Axial Velocity (/iim/s) Directionality (0.1s) Directionality (2 s) Rotational Diffusion Coefficient C /s)... 

Femtosecond pulse trains offer another control knob to rotational excitation of molecules, namely, the control of its directionality. By applying a series of laser pulses, linearly polarized at an angle with respect to one another, either clockwise or counterclockwise rotation can be initiated in a molecular ensemble [27]. The effect has been experimentally demonstrated with a sequence of only two pulses [28], and longer pulse trains [29, 30]. The latter were dubbed chiral because of the directional rotation of light polarization from pulse to pulse shown in Figure 10.3a. Chiral pulse trains enable control of the sense of molecular rotation... 

All of these devices show a certain degree of restriction in the movement of their molecular components, but rotation occurs randomly with no control over directionality. In an attempt to achieve directional motion Kelly investigated the dynamic behavior of a so-called molecular ratchet , 6 [21,22],... 

A homodimer of a tetra-urea calix[4]arene consisting of identical phenolic units A is composed of two enantiomers with C4-symmetry, which results in overall S8-symmetry. Consequently, a heterodimer with a second calixarene consisting of four units B must be chiral, but this chirality is due only to the directionality of the hydrogen-bonded belt or (in other words) to the orientation of the carbonyl groups [42,43]. Rotation around the (four) aryl-NH bonds leads to the opposite enantiomer (conformational chirality). 

In Section IV,C,2 octa- and pentaphenyl-, as well as penta(isopropyl)-and penta(neopentyl)-Cp, were cited as examples for hindered rotations about substituent-C5 bonds. Half-sandwich metal complexes or sym-metallocenes with these ligands will be chiral when there is a barrrier for the interconversion of the enantiomers created by the phenyl canting or the isopropyl and neopentyl directionalities of the substituents (clockwise or counterclockwise) around the C5 ring. For penta(isopropyl)- and penta(neopentyl)-Cp this difference in directionalities is commonly sketched (24) by showing the methine proton only and omitting the methyl or ethyl groups for clarity (14). 

The results cast doubts as to whether a true H-bond exists between the acetylene molecules. The T-shape is precisely what one would expect based solely upon electrostatic con-siderations i-. The symmetry of HCCH yields a zero dipole moment, so the moment of lowest order is a quadrupole. A T-arrangement would best allow the approach of the two quadrupole moments. The interaction, probably less than 2 kcal/mol, is less than normally expected for a H-bond. Finally, the ease of rotation of one molecule around the other contrasts with the directionality of most H-bonds. 

The initial explanation of the strong directionality was in terms of the field ionization model and the molecule s elongated shape [26]. The potential difference created by the laser is largest when the molecule finds itself with its axis aligned with the laser electric field. The barrier to tunnelling is lower and the ionization easier. Since there is little time for molecular rotation, the ions are also ejected along the laser field. 

The chemist s sketches, which are typically drawn to emphasize directionality of the sp hybrid orbitals, and a contour plot of the actual shape, are shown in Figure 6.44. Each of these contours can be rotated about the x-y plane to produce a three-dimensional isosurface whose amplitude is chosen to be a specific fraction of the maximum amplitude of the wave function. These isosurfaces demonstrate that sp hybridization causes the amplitude of the boron atom to be pooched out at three equally spaced locations around the equator of the atom (see Fig. 6.42). The 2p orbital is not involved and remains perpendicular to the plane of the sp hybrids. The standard chemist s sketches of the sp hybrid orbitals and a contour plot that displays the exact shape and directionality of each orbital are shown in Figure 6.44. The isosurfaces shown in Figure 6.43 were generated from these contour plots. 

From the foregoing it is apparent that the 3 banana bonds are induced by the directionality of the sp2 GO s. By the same token the directionality of the sp2 GO s can be considered to induce the contributions to any of the three banana bonds on each of the N atoms. These contributions are, however, linear combinations of N valence p AO s and hence themselves are p AO s rotated in new directions. Thus the directionality of the sp2 GO s can be imagined to induce a set of three p orbitals 2px,... 

Two points should be added to these conclusions. First, if the role of H-bonds appears so fundamental in biophysics and biochemistry that H-bonds may be declared the bonds of life , mainly thanks to the presence of H2O molecules in aU biomedia and to the fundamental role they play there, their action is not limited to these media. The HjO molecule being ubiquitous, thanks to its exceptional possibilities to establish H-bonds, H-bonds are also often encountered in chemistry, where such terms as H-bonded solvents, hydrophilic or hydrophobic groups or molecules are currently encountered and well taken into account, even if a more precise understanding of the role that these HjO molecules play is often needed. They are, also often encountered in physics where H2O molecules are also currently met. Physicists, however, are less aware of their fundamental role. We have seen that the dynamics of HjO molecules in liquid water is yet not understood at all. It is studied by recent time-resolved nonlinear IR methods that are stiU the domain of physicists and also by theoretical methods of molecular dynamics (MD) that have up to now not succeeded in incorporating the directionality of H-bonds in the huge H-bond network of liquid water and consequently the fundamental role rapid rotations (librations) of these very small H2O molecules play. In another... 

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