ISSN:
0951-4198
Keywords:
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Physics
Notes:
Interpretation of 252Cf plasma-desorption mass spectra of glycine and alkali salt mixtures is correlated to thermochemical values. The interpretation of direct spectra is based on a model considering competitive fragmentations among the ionic bonds involved in the crystalline structure; the desorption yield of Li, Na, K, Rb and Cs alkali-glycine adduct ions is shown to depend upon the relative lattice energies of the different salts bound in the solid mixture. Ab initio calculations were performed at the 6-31G*//3-21G level to evaluate the dissociation energies associated with the in-flight decomplexation reactions GlyCat+ → Gly + Cat+ (1) for Cat+ = Li+, Na+ and K+ whether we consider that the GlyCal+ desorbed species enter the field-free area in their ‘preformed geometry’ or that they rearrange during the acceleration step. The experimental metastable transition rates associated with these reactions were determined to be in good agreement with the theoretical values for both GlyCat+ geometries under consideration. We calculated the cation affinities of glycine to be 271 kJ/mol, 188kJ/mol and 127kJ/mol for Li+, Na+ and K+, respectively. The fragmentation of GlyCat+ species into immonium ions, GlyCat+ → [H2N-CH2]+ + CO + HOCat (2), is also experimentally and theoretically investigated. The competition between reactions (1) and (2) is favourable to (2) for Cat+ = H+ and to (1) for Cat+ = Li+, Na+ and K+. From both competitive reactions, the comparison between theoretical values and experimental observations allows one to estimate the average internal energy of the GIyCat+ desorbed species at about 300 kJ/mol in the field-free area of the mass spectrometer.
Additional Material:
5 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/rcm.1290070614
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