The test of an analogy: Are intramolecular hydrogen bonds in .beta.-diols and .beta.-hydroxy ethers preserved in their molecular ions?

10–100 eV Ar+ ion induced damage to d-ribose and 2-deoxy-d-ribose molecules in condensed phase

We report that 10-100 eV Ar+ ion irradiation induces severe damage to the biologically relevant sugar molecules D-ribose and 2-deoxy-D-ribose in the condensed phase on a polycrystalline Pt substrate. Ar+ ions with kinetic energies down to 15 eV induce effective decomposition of both sugar molecules, leading to the desorption of abundant cation and anion fragments, including CH3+, C2H3+, C3H3+, H3O+, CHO+, CH3O+, C2H3O+, H-, O-, and OH-, etc. Use of isotopically labelled molecules (5- 13C D-ribose and 1-D D-ribose) reveals the site specificity for some of the fragment origins, and thus the nature of the chemical bond breaking. It is found that all of the chemical bonds in both molecules are vulnerable to ion impact at energies down to 15 eV, particularly both the endo- and exocyclic C-O bonds. In addition to molecular fragmentation, several chemical reactions are also observed. A small amount of O-/O fragments abstract hydrogen to form OH-. It is found that the formation of the H3O+ ion is related to the hydroxyl groups of the sugar molecules, and is associated with additional hydrogen loss from the parent or adjacent molecules via hydrogen abstraction or proton transfer. The formation of several other cation fragments also requires hydrogen abstraction from its parent or an adjacent molecule. These fragmentations and reactions are likely to occur in a real biomedium during ionizing radiation treatment of tumors and thus bear significant radiobiological relevance.

Hydrogen bonding in transient bifunctional hypervalent radicals by neutralization-reionization mass spectrometry

Neutralization-reionization mass spectrometry is used to generate hypervalent 9-N-4 (ammonium) and 9-O-3 (oxonium) radicals derived from protonated α,ω-bis-(dimethylamino)alkanes and α,ω-dimethoxyalkanes, which exist as cyclic hydrogen-bonded structures in the gas phase. Collisional neutralization with dimethyl disulfide, trimethylamine, and xenon of the hydrogen-bonded onium cations followed by reionization with oxygen results in complete dissociation. Bond cleavages at the hypervalent nitrogen atoms are found to follow the order CH2-N>CH3-N>N-H, which differs from that in the monofunctional hydrogen-n-heptyldimethylammonium radical, which gives CH2-N>N-H>CH3-N. No overall stabilization through hydrogen bonding of the bifunctional hypervalent ammonium and oxonium radicals is observed. Subtle effects of ring size are found that tend to stabilize large ring structures and are attributed to intramolecular hydrogen bonding.

Characterization of the C3H 6O (+·) ion from 2-methoxyethanol. Mixture analysis by dissociation and neutralization-reionization

The C3H6O(+·) ion formed upon the dissociative ionization of 2-methoxyethanol is identified by a combination of several tandem mass spectrometry methods, including metastable ion (MI) characteristics, collisionally activated dissociation (CAD), and neutralization-reionization mass spectrometry (NRMS). The experimental data conclusively show that 2-methoxyethanol molecular ion, namely, HOCH2CH2OCH 3 (+·) , loses H2O to yield mainly the distonic radical ion ·CH2CH2OCH 2 (+) along with a smaller amount of ionized methyl vinyl ether, namely, CH2=CHOCH 3 (+·) . Ring-closed products, such as the oxetane or the propylene oxide ion are not observed. The proportion of ·CH2CH2OCH 2 (+) increases with decreasing internal energy of the 2-methoxyethanol ion, which indicates a lower critical energy for the pathway leading to this product than for the competitive generation of CH2=CHOCH 3 (+·) . The present study also uses MI, CAD, and NRMS data to assess the structure of the distonic ion(+) (CH3)CHOCH2· (ring-opened ionized propylene oxide) and evaluate its isomerization proclivity toward the methyl vinyl ether ion.