Isomeric characterization via ion neutralization and dissociation. Experimental variables

Abstract

Major deficiencies of mass spectrometry for characterizing isomeric molecules, and of collisionally activated dissociation for characterizing isomeric ions, can be alleviated by complementary information from new techniques of neutralization-reionization (NR) mass spectrometry. Mass data can be obtained from most fragments of the original species, irrespective of their abdity to retain the charge; dissociation of fast neutrals prepared from isomeric ions can involve novel reaction pathways and can minimize competing isomerization reactions; isomeric neutrals undergoing similar dissociations can be differentiated by forming them with different internal energies; reionization of the neutral products to negative as well as positive ions can provide increased selectivity; and structural information on the resulting ions can be derived using MS/MS/MS. Dissociation by novel non-isomerization pathways can also be effected by a second addition (or subtraction) of an electron to produce an unstable ion of opposite charge. Special techniques can yield neutralized products in favorable dissociative states by collisional activation, by using neutralization targets of selected ionization energy, or through Franck-Condon factors. Optimum excitation of the neutral is important, as this should be high enough to minimize rearrangement, to maximize the differences in the dissociation pathways of isomers, and to minimize the further dissociation of the characteristic primary products of the neutral. NR experiments can, thus, also provide information on the energy surfaces for unimolecular dissociations of neutrals that are difticult to study by conventional techniques. Dissociations of the neutrals can be differentiated from those occurring after reionization by separate collisional activation of the neutrals, by changing the ionization energy of the neutralization agent, or by reionization to ions of opposite charge. INTRODUCTIONCharacterization of molecular isomers by electronionization (EI) mass spectrometry,' or of ionic isomers by collisionally activated dissociation (CAD)2 has been successfully applied to a wide variety of important problems. However, in a number of cases EI or CAD spectra do not provide sufficiently extensive or different information to distinguish all possible isomers. For example, if the isomeric ions are separated by an isomerization barrier that is low relative to the respective individual dissociation thresholds, equilibration of the isomers can occur before fragmentation, leading to EI or CAD spectra in which most, or all, peaks exhibit very similar abundances. , Fig. 1); although ABCD" and ABDC" ions of sufficiently low internal energy will retain their isomeric identity indefinitely, increasing this energy to their dissociation thresholds will make their rates of isomerization much faster than those of dissociation. Ion-molecule reactions' offer one method of t Dedicated to Professor Allan Maccoll on the occasion of his 75th$ Authors to whom correspondence should be addressed. characterizing such io...

Optically active amines. 34. Application of the benzene chirality rule to ring-substituted phenylcarbinamines and carbinols

“…After cooling the mixture was extracted with ether (2 x 50 mL), and the ether was dried (MgSG4) and evaporated. The residue was distilled to obtain (S)-2I (2.68 g, 32%): a24D -23.3°( neat, 0.5 dm); 94% ee; NMR (300 MHz) 1.46 (d, 3 H, J = 6.3 Hz, CH3), 2.18 (s, 1 H, OH), 5.31 (q, 1 H, J = 6.2 Hz, CH), 7.35 (t, 1 H, J = 7.6 Hz, Ar), 7.58 (m, 2 H, Ar), 7.81 ppm (d, 1 H, J = 7.9 Hz, Ar); EA max (CH3OH) 271 nm (< 940), 264 (970), 258 (670), 251 (360), 244 (170); CD (CH3OH, c 0.0315) [8)2,5 ±0, [6)270 +430, [6)2*7 +340, [8)2*3 +480, [6)25, +360 (sh), [8)250 +220 (sh), [6)235 ±0, [8)222 ±0.…”

“…Asymmetric reduction of 3,5-bis(trifluoromethyl)acetophenone gave (R)-2o (74%), which was purified by sublimation: mp 48-50 °C; [a]26D +21°( c 1.01, CHjOH), 94% ee; NMR (300 MHz) 1.55 (d, 35 H, J = 6.5 Hz, CHj), 2.05 (s, 1 H, OH), 5.05 (q, 1 H, J = 6.4 Hz, CH), 7.79 (s, 1 H, Ar), 7.85 ppm (s, 2 H, Ar); EA max (CH3OH) 272 nm (e 580), 265 (640), 259 (480); CD (CH3OH, c 0.0300) [6] (S) -2-Amino-3-methyl-1,1-diphenylbutan-l-ol [($)-3) was prepared from L-valine methyl ester hydrochloride and phenylmagnesium bromide according to the procedure used by Itsuno.54 The crude product was recrystallized from 90% ethanol to give (S)-3 (49%): mp 94-95 °C; (S)-a-Phenylneopentyl Alcohol [(S)-4a). Asymmetric reduction of pivalophenone gave (S)-4a (24%), which was purified by sublimation: mp 36-40 °C; [a]24D -13.3°( c 9.00, benzene), 49% ee [lit.19 [a]20578 -27.3°( c 9, benzene) for 100% ee); >H NMR (90 MHz) 0.91 (s, 9 H, CCHj), 1.88 (s, 1 H, OH), 4.38 (s, 1 H, CH), 7.29 ppm (s,5 H,Ar).…”

“…H, J = 7.7 and 1 6. Hz, Ar); EA max (CH3OH) 273 nm (e 160),265 (210), 263(200), 259 (180); CD (CH3OH, c 0.0576)[9]277 ±0,[8]273+ 190, [9]269 +150,[8)26, +170,[8)258 +150 (sh),[9]236 ±0,[9]231 -300;EA max (CH3OH-HCl) 274 nm (i 220), 267 (280), 262 (230); CD (CH3OH-HCl, c 0.0660)[9)2*1 ±0,[9]277 -70,53[8]275 ±0,[8]273 +180,[9) 268 +40, [9)266 +180,[8]262 +95, [9)258+130, [9]253 +60 (sh),[8]244 ±0,[9)228 -1 10.…”

Angle-resolved neutralization-reionization mass spectrometry

“…A second method is analogous to the collisionally activated dissociation (CAD) of ions [11]; subjecting the newly formed neutral to collisions with a gas such as helium can produce CAD of these neutrals, with subsequent ionization of the products. The NCR mass spectra thus produced have provided key information for a variety of problems [3,6,7,9,12,13].…”

Studies of Unusual Simple Molecules by Neutralization-Reionization Mass Spectrometry

“…Similarly, the anion of hydroxycarbene, :CH-OH, is stable, whereas the anion of its common isomer formaldehyde, H2C=O, is not (although both its neutral and cation are stable), so that reionization clearly distinguishes these isomers (21). For such reasons NRMS is particularly useful for the structural characterization of ions (10,16,22,23); in addition, such tandem mass spectrometry methods (MS/MS or MS') have many exciting analytical applications (24). Reactively generated radicals have also been studied by several other mass spectrometric methods (18,25).…”