Modes, electronic rotational

In the production mode the rotation rate of the P-CAC can be varied between 0°/h and 5000°/h. By comparison, the drivers used in the CAC units throughout the ORNL studies were only able to rotate the column between 2°/h and 1000°/h. The housing of the drive is made of stainless steel coated with polyethylene and protects the drive as well as the electronic parts against environmental influences. 

The most convincing physical model [63] explains electron spin in terms of spherical rotation, another way of rotating a solid object, different from the well-known mode of rotation about an axis. It starts as a slight wobble, which, by continuous exaggeration of the motion, develops into a double... 

The electronic spectrum of a nonlinear polyatomic molecule is very complicated. In addition to three modes of rotation with distinct moments of inertia, there are 3N — 6 modes of vibration. While some of these may be forbidden in the infrared or Raman spectrum on the basis of symmetry, there is no rule to forbid their appearance in the electronic spectrum, which is extraordinarily complex as a consequence. For our purposes here, we mention only a few fundamental points and present one example. 

In the previous section, we treated rr-electron rotation within a frozen-nuclei approximation. However, the effects of nonadiabatic coupling should not be ignored when the duration of n-electron rotations becomes close to the period of molecular vibrations. Therefore, in this section, we explicitly take into account vibrational degrees of freedom and perform nuclear WP simulations in a model chiral aromatic molecule irradiated by a linearly polarized laser pulse. The potentials of the vibrational modes were determined by ab initio MO methods [12]. For reducing computational time, while maintaining properties of jt-electronic structures, we used 2,5-dichloropyrazine (DCP, Fig. 6.4) instead of 2,5-dichloro[n](3,6)pyrazinophane (DCPH), in which the ansa group is replaced by hydrogen atoms. 

Thermodynamic data that are suitable for tabulation include standard enthalpies, entropies, and free energies and can be regarded as universally applicable for systems at specified temperature when all participants are at thermal equilibrium. Though such data can also be obtained without thermal equilibrium, compensating experiments, or mathematical corrections are required, sometimes creating difficulties in practice and/or interpretation. A chemical system in the gas phase can reach thermal equilibrium, at a defined temperature, when a sufficient number of intermolecular collisions produce a Boltzmann distribution of energies in all modes, electronic, vibrational, rotational, and translational. In measurements made with an ion trap instrument or Fourier Transform Ion Cyclotron Resonance (FT-ICR) spectrometer at low pressure, hot ions must be cooled, commonly with a pulse of buffer... 

Many of the ideas that are essential to understanding polyatomic electronic spectra have already been developed in the three preceding chapters. As in diatomics, the Born-Oppenheimer separation between electronic and nuclear motions is a useful organizing principle for treating electronic transitions in polyatomics. Vibrational band intensities in polyatomic electronic spectra are frequently (but not always) governed by Franck-Condon factors in the vibrational modes. The rotational fine structure in gas-phase electronic transitions parallels that in polyatomic vibration-rotation spectra (Section 6.6), except that the rotational selection rules in symmetric and asymmetric tops now depend on the relative orientations of the electronic transition moment and the principal axes. Analyses of rotational contours in polyatomic band spectra thus provide valuable clues about the symmetry and assignment of the electronic states involved. 

It has been empirically established [2] that the presence of torsional modes (intramolecular rotations about bonds) accelerates Sq IC in dyes. For example, the dye phenolphthalein has essentially zero fluorescence yield, while fluorescein has 90% yield (Fig. 9.6) The torsional modes in phenolphthalein are frozen out by the presence of the oxygen bridge in fluorescein. In the absence of such torsional modes, most of the <- Sq electronic energy difference in IC is... 

In order to calculate q (Q) all possible quantum states are needed. It is usually assumed that the energy of a molecule can be approximated as a sum of terms involving translational, rotational, vibrational and electronical states. Except for a few cases this is a good approximation. For linear, floppy (soft bending potential), molecules the separation of the rotational and vibrational modes may be problematic. If two energy surfaces come close together (avoided crossing), the separability of the electronic and vibrational modes may be a poor approximation (breakdown of the Bom-Oppenheimer approximation. Section 3.1). 

A noteworthy feature of the photoacoustic spectra shown in Figure 2 Is the presence of water librations. These are frustrated rotations and have been observed for ice (24) by infrared spectroscopy, as well as for water adsorbed on Ft and Ag surfaces by electron energy loss spectroscopy (25-27). The three libration modes have been associated with the bands at 600, 538 and 468 cm" > this set of peaks occurs for water adsorbed on both the hydroxylated and methoxylated silica. 

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