Energy levels of molecules

Photon Quantum of electromagnetic radiation with energy proportional to the observed frequency i/ of the light E = his (where h is Planck s constant). 

Atom An extremely small particle of matter that retains its identity during chemical reactions. 

Nucleus Central core of an atom containing one or more positive charges. 

Proton A particle found in the nucleus of an atom. Its electric charge is equal in magnitude, but opposite in sign, to that of an electron and its mass is 1836 times that of the electron. 

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Experimental information about tire energy levels of molecules is obtained from spectroscopic studies, in the infra-red for the rotational states and in the ultra-violet for die vibrational and most of the dissociation energies. Some thermodynamic data are also obtained for the dissociation energies using mass spectroscopy. 

The most accurate theories of reaction rates come from statistical mechanics. These theories allow one to write the partition function for molecules and thus to formulate a quantitative description of rates. Rate expressions for many homogeneous elementary reaction steps come from these calculations, which use quantum mechanics to calculate the energy levels of molecules and potential energy surfaces over which molecules travel in the transition between reactants and products. These theories give... 

These energy spacings are of relevance to microwave spectroscopy which probes the rotational energy levels of molecules. 

Any external electric field is minute in comparison with the internal field generated by the system of electrons and nuclei inside a molecule. The effect of the operator (8.4) is therefore always much smaller than the electronic energy of the molecule. In most cases, the effects of electric-field perturbations are also much smaller than the vibrational energy of the molecule. The interaction with an external DC field can thus be treated as a perturbation to the vibronic energy levels of molecules. 

Differences between rotational energy levels of molecules correspond to frequencies lying in the microwave region of the electromagnetic spectrum see Chapter 5. 

The significance of light absorption in biochemical studies lies in the great sensitivity of electronic energy levels of molecules to their immediate environment and to the fact that spectrophotometers are precise and sensitive. The related measurements of circular dichroism and fluorescence also have widespread utility for study of proteins, nucleic acids, coenzymes, and many other biochemical substances that contain intensely absorbing groups or chromophores.58... 

Infrared (ir) 100 to 10,000 Rotational-vibrational excitation Rotational and vibrational energy levels of molecules (Section 9-7)... 

Ultraviolet 25,000 to 50,000 Electronic excitation accompanied by vibration-rotation changes Electronic energy levels of molecules (Sections 9-9 and 28-1)... 

Figure 4.1 Electronic energy levels of molecules with associated vibrational and, rotational levels.

In view of the different ways in which a molecule may absorb quantized energy, spectroscopists often describe the energy levels of molecules using quantum numbers (only very roughly analogous to the quantum... 

The use of in situ STM for the identification of structurally similar molecules on the basis of tlreir redox properties was reported by Tao."" By adjusting the substrate potential, he aligned its Fermi level to different energy levels of molecules, which resulted in a nearly ten-fold increase in the tunneling current flowing through molecules due to resonant tuimehng. This phenomenon can be used to follow the reaction of individual molecules under in situ conditions in the future. 

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