Rotation molecular spectra

In an analysis of a molecular spectrum, the primary task is, for purpose of characterisation, to assign each narrow spectral feature to a transition between two molecular states specified with rotational, vibrational and electronic quantum numbers or other indices. Nearly as important as the former, another task is to... 

Under normal conditions the catacondensed hydrocarbons of molecular formula C4n+2H2n+4 adopt a crystal lattice104 shown schematically as Type A in Figure 13 in which the (rotational and translational) displacement of adjacent molecular planes to produce a symmetric sandwich configuration is prohibited by the interaction of neighboring molecules. These crystals exhibit a structured ( molecular ) fluorescence spectrum red-shifted by 100 cm-1 from the molecular spectrum observed in dilute solutions. 

In general, if a bond has a dipole moment, its stretching frequency causes an absorption in the IR spectrum. If a bond is symmetrically substituted and has zero dipole moment, its stretching vibration is weak or absent in the spectrum. Bonds with zero dipole moments sometimes produce absorptions (usually weak) because molecular collisions, rotations, and vibrations make them unsymmetrical part of the time. Strongly polar bonds (C=0 groups, for example) may absorb so strongly that they also produce overtone peaks, which are relatively small peaks at a multiple (usually double) of the fundamental vibration frequency. 

Legon, A. C., Lister, D. G., and Thom, J. C., Non-reactive interaction ofantmonia and molecular chlorine Rotational spectrum of the charge-transfer complex NHf ClJ. Chem. Soc. Faraday Trans. 90, 3205-3212 (1994). 

Four new systems in the molecular spectrum of ThO(g) were rotationally analysed. The molecular constants for the new states are given. However, two of these, the interrelated Y and W states, cannot as yet be placed in the term scheme. 

In the molecule, the energy of a given state is determined not only by the distribution of the electrons but also by the particular state of the atomic nuclei building up the molecule. In addition to the energy of electron transition, there are energies involved in changing the states of oscillation and of rotation. This explains why the spectrum of a molecule consists of very many more lines than that of an atom. A mere glance shows that this is so, if one compares an atomic spectrum with a molecular spectrum the former can properly be described as a line spectrum, the latter as a band spectrum. 

Analysis of the rotation structure of the molecular spectrum (infra-red) + + ... 

A pure rotation spectrum may be observed using microwaves (radar waves or, more correctly, EM waves in the wavelength range 1 mm < k < 30 cm). Rotational spectra can be seen in the gas phase for molecules with a dipole moment and for molecular groups rotating around a bond in larger molecules, for example, in proteins or transition metal complexes. 

Although the above equations imply that a lot of symmetry analyses must be performed, that is not always the case. Equations 14.2 and 14.3 allow for the possibility of broad statements about which transitions will and will not be allowed for particular atomic or molecular systems. Such general statements, ultimately based on quantum mechanics and symmetry, are called selection rules. Selection rules allow us to easily determine which transitions will occur. When one is faced with a spectrum to interpret, knowledge of the selection rules is an indispensable tool in deriving physical information from the spectrum. Rotational and vibrational spectroscopy, in this chapter, are simplified to a large extent thanks to selection rules. 

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