Centrifugal effects molecules

Data from microwave spectra on the centrifugal effect of rotational transitions of selenophene and its deuterium-substituted derivatives have been determined experimentally and compared with the calculated theoretical values of the centrifugal stretching constants by means of the force constants determined from the solution of the inverse vibrational problem.26 The two sets of values show good agreement, indicating that the system of force constants obtained for selenophene correctly reflects the characteristic features of the force field of the molecule. 

Formation of the complex is, however, associated with changes of the moments of inertia of the molecule. Consequently, part of the energy of total rotation is exchanged with that of vibration and internal rotation. In the case where the molecule is stretched in the complex, the overall rotational energy is decreased and hence +, IF( +) and the derived k( ) are enhanced. This is known as the centrifugal effect. The equation for the energy balance now has to be modified to... 

As a molecule rotates faster, the bond between the two atoms stretches this centrifugal effect increases the moment of inertia so that there is a reduction in the rotational constant which increases with increasing J. This effect can be described by the addition of an extra term to the effective Hamiltonian for the rotational kinetic energy ... 

Equilibria. As a worked out example, I wish to present here the dynamics and relative TS associated with a very well known system of molecular physics, the van der Waals molecule. Simply stated, a van der Waals molecule AB is bound not by the creation of electronic orbitals extending over both fragments A and B but rather by the attraction due to multipolar electric forces between the two fragments A and B. In addition to this attraction, there also exists a chemical repulsion, responsible for the nonreactivity of A and B. Since the bonding is multipolar, it is weak, of the order of tens to hundreds cm (1 atomic unit of energy 2.19 x 10 cm ). Because the bonding is so weak, it is to be expected that centrifugal effects will be important, as has often been experimentally and theoretically observed. 

Just as we corrected the expressions for the rigid rotor to allow for the centrifugal effect and an interaction with the vibration, we also must adjust the expression for the harmonic oscillator to account for the anharmonicity in the oscillation. The potential energy surface for the molecule is not symmetrical (Fig. 25.2). The parabola (dotted figure) represents the potential energy of the harmonic oscillator. The correct potential energy is shown by the full lines the vibration is anharmonic. The vibrational energy levels for such a system can be approximated by a series ... 

FIGURE 14.17 Centrifugal effect on a molecule of HjO. Because water has three different rotations, it will have three different D, values. 

Actually, symmetrical tetrahedral molecules like methane do have extremely small dipole moments, caused by centrifugal distortion effects these moments are so small that they can be ignored for all practical purposes. For CH4, p is 5.4 x 10 D Ozier, I. Phys. Rev. Lett., 1971, 27, 1329 Rosenberg, A. Ozier, I. Kudian, A.K. J. Chem. Phys., 1972, 57, 568. 

This is a method for the separation of molecules with very similar densities and depends upon the formation of a density gradient by the effect of centrifugal force on the supporting solute molecules but because of their low mass the technique demands a long period of centrifugation, e.g. 2-3 days. The test compounds are... 

The dipole moment is a fundamental property of a molecule (or any dipole unit) in which two opposite charges are separated by a distance . This entity is commonly measured in debye units (symbolized by D), equal to 3.33564 X 10 coulomb-meters, in SI units). Since the net dipole moment of a molecule is equal to the vectorial sum of the individual bond moments, the dipole moment provides valuable information on the structure and electrical properties of that molecule. The dipole moment can be determined by use of the Debye equation for total polarization. Examples of dipole moments (in the gas phase) are water (1.854 D), ammonia (1.471 D), nitromethane (3.46 D), imidazole (3.8 D), toluene (0.375 D), and pyrimidine (2.334 D). Even symmetrical molecules will have a small, but measurable dipole moment, due to centrifugal distortion effects. Methane " for example, has a value of about 5.4 X 10 D. 

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