Rotational effects

C) ROTATIONAL EFFECTS STERIC HINDRANCE AND CENTRIFUGAL ENHANCEMENT... 

McCormack D A and Kroes G J 1999 A classical study of rotational effects in dissociation of H2 on Cu(IOO) Phys. Chem. Chem. Phys. 1 1359... 

Under most circumstances the equations given in Table 10.4 accurately calculate the thermodynamic properties of the ideal gas. The most serious approximations involve the replacement of the summation with an integral [equations (10.94) and (10.95)] in calculating the partition function for the rigid rotator, and the approximation that the rotational and vibrational partition functions for a gas can be represented by those for a rigid rotator and harmonic oscillator. In general, the errors introduced by these approximations are most serious for the diatomic molecule." Fortunately, it is for the diatomic molecule that corrections are most easily calculated. It is also for these molecules that spectroscopic information is often available to make the corrections for anharmonicity and nonrigid rotator effects. We will summarize the relationships... 

A lifetimes comparison for each excited state (Table 8) shows that they are smaller for the A n state except for v =0. The vibrational decays occur by means of cascade processes. Rotational effects (v =l - 2 i =l - 2) appear to be even more intense than for the corresponding transition bands and in either case these effects disappear for j =4. For... 

Rotation effects (slower rotation, i.e., larger molecule yields improved relaxivity)... 

Peter Zulch, Mark Davis, Larry Adzima, Robert Hancock, Sid Theis, The Earth Rotation Effect on a LEO L-Band GMTI SBR and Mitigation Strategies, IEEE Radar Conference, Philadelphia, PA, April 2004. 

Five isotopomers of Sia were studied in Ref (20), and are labeled as follows Si- Si- Si (I) Si- Si- Si (II) Si- Si- Si (III) Si- "Si- Si (IV) Si- Si- °Si (V). Rotational constants for each (both corrected and uncorrected for vibration-rotation interaction) can be found towards the bottom of Table I. Structures obtained by various refinement procedures are collected in Table II. Two distinct fitting procedures were used. In the first, the structures were refined against all three rotational constants A, B and C while only A and C were used in the second procedure. Since truly planar nuclear configurations have only two independent moments of inertia (A = / - 4 - 7. = 0), use of B (or C) involves a redundancy if the other is included. In practice, however, vibration-rotation effects spoil the exact proportionality between rotational constants and reciprocal moments of inertia and values of A calculated from effective moments of inertia determined from the Aq, Bq and Co constants do not vanish. Hence refining effective (ro) structures against all three is not without merit. Ao is called the inertial defect and amounts to ca. 0.4 amu for all five isotopomers. After correcting by the calculated vibration-rotation interactions, the inertial defect is reduced by an order of magnitude in all cases. 

Barberi, P. and Lo Cascio, B. 2001. Long-term tillage and crop rotation effects on weed seed-bank size and composition. Weed Research 41 325-340. 

In Chap. 5, further consideration is given to the influence of rotational effects on reactions in liquids. 

Finally, since the long-range transfer probabilities are all, to some degree, dependent on mutual orientation of donor and acceptor, it is necessary to retain the facility to average over all orientations. Further attention is given in Chap. 5 to rotational effects during reactions between anisotropic molecules in solution. In Chap. 5, Sect. 4.2, the experimental evidence currently available indicates that molecules re-orient more rapidly than they react. 

The factors which influence the rate of rotational relaxation are briefly discussed in the next two sections. These are then considered in relation to experimental studies of chemical reaction in which rotational effects may be of importance. 

The rotational effect on the correction term for diffusion-limited rate coefficients is shown as the intercepts on the ordinate of Fig. 17. Here, B is a small particle and only A can be re-oriented. As the size of A decreases, it can re-orient more quickly, but the mutual diffusion coefficient decreases till rA = rB. Hence, the very fact that the correction term increases with increase of rA shows that rotational diffusion is very important. 

To answer this question, we should consider reactions where rotational effects are most pronounced. Fast translational diffusion but slow rotational diffusion are two important criteria and eqn. (118) indicates a large (hydrogen-bonding) molecule to be suitable. The discussion in Sect. 

The rotational relaxation times of these nitrocompounds have not been measured. Comparison with the studies of perylene by Klein and Haar [253] suggests that most of these nitrocompounds have rotational times 10—20 ps in cyclohexane. For rotational effects to modify chemical reaction rates, significant reaction must occur during 10ps. This requires that electron oxidant separations should be <(6 x 10-7x 10-11)J/2 2 nm. Admittedly, with the electron—dipole interaction, both the rotational relaxation and translational diffusion will be enhanced, but to approximately comparable degrees. If electrons and oxidant have to be separated by < 2 nm, this requires a concentration of > 0.1 mol dm-3 of the nitrocompound. With rate coefficients 5 x 1012 dm3 mol-1 s 1, this implies solvated electron decay times of a few picoseconds. Certainly, rotational effects could be important on chemical reaction rates, but extremely fast resolution would be required and only mode-locked lasers currently provide < 10 ps resolution. Alternatively, careful selection of a much more viscous solvent could enable reactions to show both translational and rotational diffusion sufficiently to allow the use of more conventional techniques. 

T. P. Softley At the present time, we do not say that there are no rotational effects on the cross section of the H + H2 reaction but that our signal-to-noise levels at the lowest collision energies (where these effects might occur) are not sufficiently good to draw any conclusions (see the current chapter). 

D, M. Neumark Didn t Chupka investigate rotational effects in the H2 + H2 reaction many years ago ... 

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