Molecular spectral interpretation steps

Crosscheck proposed molecular structure and mass spectral data. This is also recommended between the single steps of mass spectral interpretation. 

After high quality mass spectra are obtained, the next step in data interpretation is to deduce the atomic composition of mass spectral features. Since mass spectra relate ion abundances with their mjz values, in order to find the molecular weight (m) of a compound, it is necessary to predict the charge state (z) of recorded ions. The number of electric charges acquired by ions in the gas phase is influenced by the ionization method. This problem used to be less important because old ionization techniques [e.g., electron ionization (El), chemical ionization (Cl), fast atom bombardment (EAB), secondary ion mass... 

Applying these steps mm/ allow assignment of the structure of an unknown compound. However, without additional information about a substance, such as its molecular formula or the chemical reaction(s) by which it was formed, only a partial structural determination may be possible. Other chemical or spectral data may be required to make a complete assignment. In any case, structural assignments based exclusively on interpretation of spectra must be confirmed by comparing the spectra of the unknown with those of an authentic specimen, if the compound is known, or by chemically converting the unknown to a known compound. 

The small step rotational diffusion model has been employed to extract rotational diffusion constants Dy and D from the measured deuterium spectral densities in liquid crystals [7.25, 7.27, 7.46, 7.49 - 7.53]. Both the single exponential correlation functions [Eq. (7.54)] and the multiexponential correlation functions [Eq. (7.60)] have been used to interpret spectral densities of motion. However, most deuterons in liquid crystal molecules are located in positions where they are rather insensitive to motion about the short molecular axis. Thus, there is a large uncertainty in determining D or Tq (t o) because of Dy > D and the rather small geometric factor [doo( )] for most deuterons in liquid crystal molecules. For 5CB, it is necessary to fix [7.52] the value of D using the known activation... 

Today the spectral profiles can be simulated for any motional regime by a numerical integration of the stochastic Liouville equation, as discussed in Chapter 12 and in the references therein. The noticeable improvement in the techniques of calculation of the magnetic parameters and their dependence on the solvent, and of the minimum energy conformation of the molecules, have opened the possibility of an integrated computational approach. Since it gives calculated spectral profiles completely determined by the molecular and physical properties of the radical and of the solvent at a given temperature, this method is a step forward in the direction of a sound interpretation of complex spectra. 

---