Correlation of mass spectra with molecular structure

Correlation of Mass Spectra with Molecular Structure 7.123... 

Correlation of mass spectra with molecular structure... 

Not until the late 1950s, however, were significant att pts made to correlate these fragment ions with molecular structures. And, in the ensuing years, mass spectra of literally millions of organic compounds containing various functional groups have been examined and snccessfnlly related to molecular structure. 

The spectrum of a compound represents the molecular structure in the form of a complex code. Therefore, a relationship may be expected between spectrum and biological activity. Such a spectra-activity relationship assumes that the mechanism of biological activity is similar to the physical and chemical processes which produce spectra. This similarity is at least doubtful and the direct way of structure-activity relationships seems to be more promising. An attempt to correlate mass spectra with biological activity has led to violent replies in the literature. Controversies on this theme made many chemists very suspicious against applications of pattern recognition. 

Like any other problem involving the correlation of spectral data with structure, having a weU-defmed strategy for analyzing mass spectra is the key to success. It is also true that chemical intuition plays an important role as well, and of course there is no substitute for practical experience. Before diving into the mass spectrum itself, take an inventory of what is known about the sample. Is the elemental composition known Has the molecular formula been determined by exact mass analysis What functional groups are present in the compound What is the sample s chemical history For example, how has the sample been handled From what sort of chemical reaction was the compound isolated And the questions can continue. 

Since the reference mass spectra of known compounds have been run previously for a number of years, correlations of SI mass spectra with structure can be made for many of the common classes of organic compounds. Most of these correlations emphasize the spectral pattern or simple decomposition pathways to be expected for a particular molecular structure. This has led to the tabulation of mass spectral correlations to provide empirical and structural formulas of ions that might be found at a particular m/z in a mass spectrum, plus an indication of how each such ion might have arisen. Such a table has been reported elsewhere [10]. 

SALI compares fiivorably with other major surface analytical techniques in terms of sensitivity and spatial resolution. Its major advantj e is the combination of analytical versatility, ease of quantification, and sensitivity. Table 1 compares the analytical characteristics of SALI to four major surfiice spectroscopic techniques.These techniques can also be categorized by the chemical information they provide. Both SALI and SIMS (static mode only) can provide molecular fingerprint information via mass spectra that give mass peaks corresponding to structural units of the molecule, while XPS provides only short-range chemical information. XPS and static SIMS are often used to complement each other since XPS chemical speciation information is semiquantitative however, SALI molecular information can potentially be quantified direedy without correlation with another surface spectroscopic technique. AES and Rutherford Backscattering (RBS) provide primarily elemental information, and therefore yield litde structural informadon. The common detection limit refers to the sensitivity for nearly all elements that these techniques enjoy. 

Obviously, in the case of PS these discrepancies are more and more reduced if the probed dimensions, characterized by 2ti/Q, are enlarged from microscopic to macroscopic scales. Using extremely high molecular masses the internal modes can also be studied by photon correlation spectroscopy [111,112], Corresponding measurements show that - at two orders of magnitude smaller Q-values than those tested with NSE - the line shape of the spectra is also well described by the dynamic structure factor of the Zimm model (see Table 1). The characteristic frequencies QZ(Q) also vary with Q3. Flowever, their absolute values are only 10-15% below the prediction. 

Fig. 11.3. Electron ionization and methane Cl mass spectra of toluene. The key features of the respective mass spectra are labeled. Spectral interpretation is based on recognition and understanding of these key features and how they correlate with structural elements of the analyte molecule of interest. The signal representing the most abundant ion in a mass spectrum is referred to as the base peak, and may or may not be the molecular ion peak (which carries the molecular mass information). Cl spectra provide confirmation of molecular mass in situations where the El signal for the molecular ion (M+ ) is weak or absent. The Cl mass spectrum provides reliable molecular mass information, but relatively little structural information (low abundance of the fragment ions). Compare with Fig. 11.4.

Abstract. Variations in the chemical composition of surfactants from natural sea slicks are compared to variations in surface elasticity using mass spectrometry, Langmuir film balance measurements, and multivariate statistical techniques. It is shown that the information on chemical class and molecular structure contained in the mass spectra is strongly correlated with measured static elasticity and can be used to estimate film elasticity at a given surface pressure. 

The crystal and molecular structures of (-)-28 hydrochloride have been determined by X-ray crystallography ( )-28 possesses the R configuration at C-3, and the pyrrolidine ring is in the envelope conformation (67). C-NMR spectra of 28, vasicinone (29), and related compounds have been reported (68), and the mass spectral fragmentations of 28, vasicinone (29), deoxyvasicine (32), and deoxyvasicinone (33) (Fig. 2) have been discussed (69,70). The mass spectral behavior of 2,3-polymethylene-3,4-dihydroquinazolin-4-ones (71) and of 2,3-polymethylene-1,2,3,4-tetrahydroquinazolin-4-ones (72) have been studied. Integrated intensities of the 1480-1630-cm IR bands of the skeletal vibrations of the heteroaromatic rings of pyrroloquinazoline alkaloids have been correlated with electron densities and substituent effects (73). 

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