Resistance to rotation

Section 3.7 describes the basis for the resistance to rotation of the C=C bond. 

In neutral polyenes a clear distinction can be made between essential single bonds (which can be twisted fairly easily) and double bonds (which offer much more resistance to rotation). It has become customary to distinguish stable isomers (classified E/Z with regard to the configuration around the double bonds) and easily interconvertible conform-ers (classified as s-cis, s-trnns or gauche with regard to the substituents on the essential single bonds) in polyenes. 

On account of this overlap there is considerable resistance to rotation about a double bond and it produces a rigid molecule, hi other words the disposition of groups attached to the carbon atom can be shown in different ways in space, giving rise to isomers. Therefore geometrical isomerism is a consequence of restricted rotation about double bonds. 

The dislodged residues may not remain in the pump chamber. After cleaning is completed check the pump for operability by slowty turning the impellers by hand. There may be no resistance to rotation. It is generally not necessary to dismantle the roots pump. If this should nevertheless be required due to heavy soiling, then it is highly advisable to have this done by the manufacturer. 

Problem 4.7 Give two factors that account for the resistance to rotation through the high-energy eclipsed conformation. 

The intramolecular flexibilities of poly(1,4-phenylene oxide), polyi2,6-dimethyl-1,4-phenylene oxide), poly(2-methyl-6-phenyl-1,4-phenylene oxide), and poly 2,6-diphenyl-1,4-phenylene oxide) are evaluated through estimation of the resistance to rotation about the Cj 4—0 bonds in their backbones. A 6-12 potential is used to account for the van der Waals interactions between nonbonded atoms and groups encountered during the backbone rotations, while the twofold intrinsic potential to rotation about the C14—0 bonds resulting from the -electron delocalization is also included. 

The measure of a body s resistance to rotation is its Moment Of Inertia. 

The spinning rotor of a horizontal-head centrifuge offers considerable resistance to rotation and generates heat as a result of air friction. This resistance is lessened if the swinging buckets are enclosed in a windshield. 

The unit of absolute viscosity is the pascal second (Pa.s), but centipoise (cP) is generally used as the alternative unit, where 1 Pa.s = 10 cP. Absolute viscosity is usually measured with rotary viscometers where a rotor spins in a container of the fluid to be measured and the resistance to rotation, torque, is measured. Absolute viscosity is an important measurement for the lubricating properties of oils used... 

As described in Section 11,1, hydrolysis of pyranosides involves, as the rate-determining step, heterolysis of the conjugate acid (2) to a cyclic, carbonium-oxonium ion (3) which has the half-chair conformation (6) having C-2, C-1, 0, and C-5 in one plane. Therefore, the formation of (3) involves a net counterclockwise rotation about the C-2 to C-3 and C-4 to C-5 bonds. According to Feather and Harris, the rate of hydrolysis is controlled by the resistance to rotation counter to the net effect. They stated that it is probable that transient, structural distortions are responsible for this behavior, and that the conjugate acid might assume a... 

The nonbonded interactions term. This represents the change in potential energy with distance apart of atoms A and B that are not directly bonded (as in A-B) and are not bonded to a common atom (as in A-X-B) these atoms, separated by at least two atoms (A-X-Y-B) or even in different molecules, are said to be nonbonded (with respect to each other). Note that the A-B case is accounted for by the bond stretching term stretch> and the A-X-B term by the angle bending term bend. but the nonbonded term nonbond is, for the A-X-Y-B case, superimposed upon the torsional term torsion we can think of lorsion as representing some factor inherent to resistance to rotation about a (usually single) bond X-Y (MM does not attempt to explain the theoretical, electronic basis of this or any other effect), while for certain atoms attached to X and Y there may also be nonbonded interactions. 

Weber, G., Scarlata, S. and Rholam, M, 1984, Thermal coefficient of the frictional resistance to rotation in simple fluorophores determined by fluorescence polarization. Biochemistry 23, 6785-6788. 

Construction of the vane with a very small shearing diameter would seem the ideal way of achieving a high H D ratio, but caution needs to be taken with small diameter vanes as it is likely that torque contributions from the resistance to rotation at the exposed shaft smfaces, above the vane, may then become important. 

In Chapter 9, we will look at another similarly defined property of an object, mass moment of inertia, which provides a measure of resistance to rotational motion. 

Material carried within the cross-section of the feed screw is deterred from rotating with the hlade by the boundary conditions at the outside radius of the screw flight. Frictional resistance to rotation is offered over the area of the screw periphery that is not exposed to the inlet flow from the hopper, hy material in contact with the outer casing surface. This restraining surface is radially displaced from the tip of the screw flight, by the working clearance of the screw in the casing. Products which shear thin can tend to log in the screw due to the low restraint offered to... 

Si-O-Si links, contained in silicones, are extremely flexible, followed by CH2-CH2 and CH2-CH=CH-CH2. The great flexibility of the latter, despite the rigidity of the double bond it contains, stems from the very weak resistance to rotation around the two CH2-CH bonds. Introducing a ring into the main chain considerably increases the value of Tg. 

G Shear stiffiiess gf/cm degree Resistance to rotational movement of the warp and weft threads when subjected to low shear deformation. The lower the value of G, the more readily the fabric will conform to three-dimensional curvatures. 

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