Torsion


HYORGN 0. 00 Nt TRGN 0.00 OXYGEN 0.00  [c.167]

This involves the determination of the damping of the oscillations of a torsion pendulum, disk, or ring such as illustrated in Fig. IV-8. Gaines [1] gives the equation  [c.119]

Fig. IV-8. Torsion pendulum surface viscometer. (From Ref. I.) Fig. IV-8. Torsion pendulum surface viscometer. (From Ref. I.)
The dynamics and surface rheology of polymeric monolayers is of both practical and fundamental interest. On the practical side, the process of Langmuir-Blodgett deposition (Section XV-8) is an important means of forming polymeric thin films and the nature of the film transfer depends on flow processes in the monolayer. One way to monitor the polymer orientation in a monolayer undergoing extensional or stretching flow has been introduced by Fuller and co-workers [18]. They measure the optical anisotropy in the mono-layer through the dichroism, or anisotropy in the imaginary (absorptive) part of the refractive index. With this in situ technique, they can monitor orientation of rodlike polymers along the extension axis during flow and their relaxation on cessation of flow. More conventional stress relaxation measurements have been performed on poly(vinyl stearate), PVS [19]. Dynamic measurements on PVS and PMMA with a torsion pendulum (see Section IV-3) showed shear moduli 100-500 times that of simple amphiphile monolayers and viscosities 3000-20,000 times greater [20]. TTiese viscoelastic properties are correlated with the development of surface pressure gradients on compression described above.  [c.541]

Brooks and Pethica [103] used a film balance type of trough such that the oil-water interface could be swept and interfacial films could be directly compressed. They used a hydrophobic Wilhelmy slide for measuring 7 and hence t values (as did Takenaka [92]). The procedure was claimed to be superior to the fixed area interface one, where film pressure is built up by successive addition of film-forming material, since spreading against an existing high surface pressure may not always be complete. Interfacial potentials have been measured by means of the vibrating electrode [104] with polar oils direct measurements with a high-impedance voltmeter are possible [105]. For film viscosity, a torsion pendulum may be used [106],  [c.552]

An important issue in the formation of LB films is their stability and deformation during the transfer process. This has motivated research in the areas of process feedback control, flow in Langmuir monolayers, and the structure of transferred films. In addition to the LB deposition technique illustrated in Fig. XV-15, freely suspended films (in effect, soap films) can be transferred from an aperture to a surface. The transfer of freely suspended films creates layers having fewer defects since they can equilibrate prior to deposition [140]. The control and optimization of the LB film transfer has been studied by Kober-stein and co-workers [190, 191]. They found the optimum control parameters to minimize pressure excursions on compression [190], and they investigated the effect of the monolayer properties on those parameters. The behavior of the monolayer during transfer can be related to their surface rheology (see Sections XV-2B and IV-3C). A new apparatus for measuring shear modulus has been developed by Ketterson and co-workers [192] this versatile torsion device has a large dynamic range and high sensitivity (capable of measuring moduli from 0.005 to 1000 dyn/cm ). Fuller and co-workers have used Brewster angle microscopy to directly visualize the anisotropy induced in a fatty-acid mono-layer by extensional flow [193]. This type of flow is important in the deposition process and can alter both domain and molecular orientation.  [c.562]

Figure Bl.19.23. Principle of simultaneous measurement of nomial and lateral (torsional) forces. The intensity difference of the upper and lower segments of the photodiode is proportional to the z-bending of the cantilever. The intensity difference between the right and left segments is proportional to the torsion, t, of the force sensor. (Taken from [110], figure 2.) Figure Bl.19.23. Principle of simultaneous measurement of nomial and lateral (torsional) forces. The intensity difference of the upper and lower segments of the photodiode is proportional to the z-bending of the cantilever. The intensity difference between the right and left segments is proportional to the torsion, t, of the force sensor. (Taken from [110], figure 2.)
Energy (in Kcal/mol] is given for the total, kinetic, potential (with respect to the initial values —1664.96 corresponding to a local minimum near the initial configuration), bond length, bond angle, torsion angle, van der Waals, and electrostatic components.  [c.254]

The variance percental of the mean emi ie shown as a subscript to the variance value recorded for the BBK scheme. Energy [in Kcal/moi] is given for the total, kinetic, potential (with respect to the initial values -4 37.8fi corresponding to s minimum near the initial configuration), bond length, bond angle, torsion angle, van der Waals, and electrostatic components. The speedup is relative to the reference trajectory, which takes 72 hours.  [c.255]

In an atomic level simulation, the bond stretch vibrations are usually the fastest motions in the molecular dynamics of biomolecules, so the evolution of the stretch vibration is taken as the reference propagator with the smallest time step. The nonbonded interactions, including van der Waals and electrostatic forces, are the slowest varying interactions, and a much larger time-step may be used. The bending, torsion and hydrogen-bonding forces are treated as intermediate time-scale interactions.  [c.309]

Triaceiin is about 90% glycerol triacetate and 10% diacetate. Used as a plasticizer for lacquers and as a solvent for certain gums and resins.  [c.11]

AZ/trogen content in crude oil is typically less than 0.1% by weight, but can be as high as 2%. The nitrogen compounds in crude oil are complex, and remain largely unidentified. Gaseous nitrogen reduces the calorific value and hence sales price of the hydrocarbon gas. Natural gas containing significant guantities of nitrogen must be blended with high calorific value gas to maintain a uniform product quality.  [c.94]

This film pressure (or two-dimensional pressure) has the units of dynes per centimeter and can be measured directly. As illustrated in Fig. Ill-14, if one has a trough divided by a thin rubber membrane into two compartments, one filled with solvent and the other with solution, then a force will be observed to act on a float attached to the upper end of the membrane. In the PLAWMf [124, 125], the rubber membrane was very thin, and the portion below the surface was so highly convoluted that it could easily buckle so as to give complete equalization of any hydrostatic differences between the two solutions. The force observed on the float was thus purely surface tensional in origin and resulted from the fact that a displacement in the direction of the surface of higher surface tension would result in a lower overall surface free energy for the system. This force could be measured directly by determining how much opposing force applied by a lever attached to a torsion wire was needed to prevent the float from moving.  [c.81]

Kosloff R, Rice S A, Gaspard P, Tersigni S and Tannor D J 1989 Wavepacket dancing achieving chemical selectivity by shaping light pulses Chem. Phys. 139 201-20  [c.281]

Maeda K, Terazima M, Azumi T and Tanimoto Y 1991 CIDNP and CIDEP studies on intramolecular hydrogen abstraction reaction of polymethylene-linked xanthone and xanthene. Determination of the  [c.1620]

Z-matrix coordinates are widely used to define molecular geometries. A Z matrix specifies the molecular geometry in a treelike maimer, by coimecting each new atom in the system to those that have been defined previously. The first tliree atoms in the Z matrix are unique, with the first atom at die origin, the second lying on the Z axis (connected to the first by a single stretch) and the third lying in the XZ plane (coimected to either the first or second atom via a stretch and defining a bend with the uncoimected atom). Each new atom after the third is defined witli respect to atoms previously defined in the Z matrix using, for example, one stretch, one bend and one torsion. An example of a typical Z matrix (for fluoroethylene) is shown in figure B3.5.2.  [c.2342]

Tarazona A, Kreisig S, Koglin E and Schwuger M J 1997 Adsorption properties of two cationic surfactant classes on silver surfaces studied by means of SERS spectroscopy and ab initio calculations Prog. Colloid Polym. Sol. 103 181-92  [c.2607]

Much of tills chapter concerns ET reactions in solution. However, gas phase ET processes are well known too. See figure C3.2.1. The Tiarjioon mechanism by which halogens oxidize alkali metals is fundamentally an electron transfer reaction [2]. One might guess, from tliis simple reaction, some of tlie stmctural parameters tliat control ET rates relative electron affinities of reactants, reactant separation distance, bond lengtli changes upon oxidation/reduction, vibrational frequencies, etc.  [c.2972]

The third major category of processes are dynamics after bound-bound transitions, such as photoexcitation to a bound state. In Figure 4, the system dynamics of the butatriene radical cation are shown after excitation from the neutral molecule ground state to a simple model of the cationic first excited state, a process related to the first excited band in the photoelectron spectrum. The dynamics are dominated by two vibrational modes, the central C—C stretch, labeled Qu and the torsion, Q. These coordinates are shown in Figure Ic. In this simple model, the PES is taken as a haiinonic approximation around the minimum energy point, which is found to be shifted along the 2i4 mode. Here, non-adiabatic effects have been ignored. As will be shown in Section ni.D, there is in fact strong vibronic coupling to the cationic ground state via the torsional mode, and the true dynamics after excitation into this state is radically altered. This model is, however, a reasonable representation of a bound state in which vibronic coupling does not play a role. The systems dynamics in the space of the two normal modes shown is fairly simple. The initial Gaussian shaped wavepacket representing the neutral ground-state wave function moves back and forth across the well, driven by the initial force due to the shifted energy minimum.  [c.264]

The first study in which a full CASSCE treatment was used for the non-adiabatic dynamics of a polyatomic system was a study on a model of the retinal chromophore [86]. The cis-trans photoisomerization of retinal is the primary event in vision, but despite much study the mechanism for this process is still unclear. The minimal model for retinal is l-cis-CjH NHj, which had been studied in an earlier quantum chemisti7 study [230]. There, it had been established that a conical intersection exists between the Si and So states with the cis-trans defining torsion angle at approximately a = 80° (cis is at 0°). Two  [c.305]

Four trajectories were run, starting at the Franck-Condon point, varying the torsion angle a from 0-20°. In all cases, the same behavior was seen. Initial motion away from the Franck-Condon region involved stretching motion along the molecular backbone. After 60 fs, the motion then changes, with energy being tiansfered to the torsional mode. This motion then takes the system to the intersection, and the resulting (diabatic) hop takes the system to the bans isomer. This dynamic behavior is consistent with calculations on retinal using semiempirical surfaces [241], and using adiabatic direct dynamics on the excited state [242], It also supports the use of low-dimensional models that have been used in quantum mechanical calculations on retinal [196].  [c.306]

Model systems for cyanine dyes have also been studied [87]. In this case, it is important to understand the mechanism by which relaxation to the ground-state occurs so as to design efficient dye molecules, that is, without fast internal conversion. The simplest model is tians-NH2—(CH)3-NHJ. Although this molecule has a structural similarity to the retinal model investigated above, the dynamics after photoexcitation are quite different. A trajectory starting from near the Franck-Condon point is sketched in Figure 9. The initial motion is dominated hy conrotatory torsional motion around the C—C bonds, which after 50 fs changes to disrotatory motion. This last only 20 fs until the molecule reaches the minima on the 5i surface. Here, the torsion remains twisted at 104°, and large amplitude motion involving skeletal stietching and pyra-midalization of a terminal nitrogen atom. The system oscillates in the minima for 50 fs, before crossing to the ground-state near the conical intersection. This crossing leads to the cis confonner, and so isomerization has taken place.  [c.306]

A major motivation for the study of conical intersections is their assumed importance in the dynamics of photoexcited molecules. Molecular dynamics methods are often used for this purpose, based on available potential energy surfaces [118-121]. We briefly survey some methods designed to deal with relatively laige molecules (>5 atoms). Several authors combine the potential energy surface calculations with dynamic simulations. A relatively stiaightfor-ward approach is illustrated by the work of Ohmine and co-workers [6,122]. Ab initio calculations of the ground and excited potential surfaces of polyatomic molecules (ethylene and butadiene) were performed. Several specific nuclear motions were chosen to inspect their importance in inducing curve crossing. These included torsion, around C=C and C-C bonds, bending, stretching and hydrogen-atom migration. The ab initio potentials were parametrized into an analytic form in order to solve the dynamic equations of motion. In this way, Ohmine was able to show that hydrogen migration is important in the radiationless decay of ethylene.  [c.385]

Iodine occurs to a minute extent (less than 0.001 %) in sea water, but is found in greater concentration, combined in organic form, in certain seaweeds, in oysters and in cod livers. Crude Chile saltpetre, or caliche contains small amounts of sodium iodate, NalOj. from which iodine can be obtained (see below). Some insoluble iodides, for example liiose of silver and mercury(II), occur in Mexico. Iodine is found in the human body in the compound thyroxin in the thyroid gland deficiency of iodine in diet causes enlargement of this gland (goitre).  [c.319]

In complex systems the set of fast degrees of freedom arises both from vibrations of stiff bonds or particles with small mass. An example of the latter is the fast vibrational motions of the C-H and 0-H bonds in biomolecules and the 0-H bonds of water. In systems with multiple time scales it is necessary to choose a time step much smaller than the periods of the fastest motions and to recalculate the forces after each small time step. It then requires very long runs to sample the conformational space of the slower degrees of freedom. To bypass this problem some fast degrees of freedom can be eliminated by constraining the length of the stiff bonds. [11] Constrained molecular dynamics suffers from several problems (a) bond constraints introduce additional angular correlations is torsion angle distribution functions that are not found in the flexible systems in nature (b) constraints cannot be used to eliminate problems like the fast librational motion of water (c) the integrators often used in constrained MD are neither reversible nor symplectic. This latter problem means that constrained dynamics cannot be wed to Monte Carlo  [c.298]


See pages that mention the term Torsion : [c.163]    [c.171]    [c.267]    [c.23]    [c.119]    [c.119]    [c.120]    [c.224]    [c.1016]    [c.2143]    [c.2144]    [c.2473]    [c.2474]    [c.2480]    [c.2481]    [c.2481]    [c.37]    [c.91]    [c.216]    [c.394]    [c.756]    [c.756]    [c.308]   
Chemoinformatics (2003) -- [ c.343 ]

Computational chemistry using the PC (2003) -- [ c.118 ]