Odd-electron Cations Research Articles

Gas phase behavior of radical cations of perfluoroalkyl‐1,2,4‐triazines: an experimental and theoretical study

Electron ionization mass spectrometry and low-energy collision-induced decomposition reactions occurring in a tridimensional ion trap, together with density functional theory (DFT) calculations on neutrals, even- and odd-electron cations, have been used to study the gas-phase ion chemistry of a series of perfluoroalkyl-1,2,4-triazines. Loss of oxygen, due to thermal degradation occurring before ionization, likely involving the hydroxylamino group, has been observed. Compounds having a carbonyl group at position 6 of the triazine ring fragment in the source by elimination of NO followed by HF or CO. The decomposition pathways occurring due to CID experiments have shown interesting features depending on the nature and structure of precursor ions. Most of them involve elimination of endocyclic atoms, thereby producing contraction of the original six-membered ring or formation of acyclic structures. DFT (B3LYP/6-31G(d,p)) calculations have been used for evaluating structure, stability and properties of neutral and ionic species involved in gas-phase processes. In particular, it has been calculated that in the molecular ion the unpaired electron is mainly located on the exocyclic nitrogen, while the positive charge is on the C(6) carbon atom.

The influence of structural features on facile McLafferty‐type, even‐electron rearrangements in tandem mass spectra of carboxylate anions

In contrast to the well-described McLafferty rearrangement in odd-electron cations, relatively little has been reported on comparable rearrangements in even-electron ions, especially negative ions. This work reports a systematic study using tandem mass spectrometry (MS/MS) fragment ion spectra of carboxylate anions having a suitably acidic proton in the gamma position. The rearrangement process was studied in both ion trap and triple quadrupole mass spectrometers; characteristic enolate anions and stable neutral products were formed at low collision energies. The process has diagnostic and analytical potential in, for example, the analysis of peptides having C-terminal serine residues and of 3-hydroxy- or 3-aminocarboxylic acids in complex mixtures.

Effects of alkyl substitution on ionization energies of alkanes and haloalkanes and on heats of formation of their molecular cations Part 2.* Alkanes and chloro-, bromo- and iodoalkanes

The adiabatic ionization energies (IEs) of alkanes and their halogen derivatives have been correlated using the following equation: IE(RX) = IE(XC(CH 3) m H 3− m ) − f(γ +). Here f(γ +) = 0.14γ + + 0.25 eV for X = H and m = 2 or 3, f(γ +) = 0.14γ + for alkanes containing a quaternary carbon atom, and f(γ +) = 0.16γ +, 0.10γ + or 0.05γ + for X = Cl, Br or I respectively, and γ + is the total number of carbon atoms at and beyond the γ-position with respect to the α-carbon atom closest to the site of the formal charge in the molecular cation. The average deviations between estimated and observed values are less than 0.08 eV for 22 hydrocarbons and 0.04 eV for 34 monohaloalkanes. The heats of formation these odd-electron cations have been estimated and are compared with values from the equations of Holmes . [Can. J. Chem., 59 (1981) 80]. The differences in the localization of the formal charges in the different cations have been discussed.

Stereochemistry of organic ions in the gas phase: A review

The stereochemistry of organic ions in the gas phase can be regarded from two different points of view: (i) stereoselectivity in ion formation and (ii) stereospecifity of ion fragmentations. Fast ionization by electron or photon impact shows little stereoselection. Differences in the ionization energies and cross sections between stereoisomers are generally small, save for a few exceptions. Proton or larger ion transfer, as employed in chemical ionization mass spectrometry, gives more possibilities for stereoselection. Bi- or polyfunctional molecules can capture the proton in a hydrogen-bond stabilized [M + H]+ ion, which is feasible only with a favourable spatial orientation of the chelating groups. Adduct ions [M + R]+ can also be formed stereoselectively. The use of a chiral ionizing medium adds a new dimension, since enantiomers can be distinguished, or even independently identified. The stereochemistry of even-electron cations in the gas-phase is most pronounced with polyfunctional species. The stereochemical behaviour is ruled by two reactivity principles, i.e. the geometry-dependent stabilization of [M + H]+ ions by chelation, and the anchimeric assistance by neighbouring groups in elimination of small molecules (water, ammonia, alcohols, acetic acid, etc.). The stereochemistry of odd-electron cations seems to be governed by three principles, i.e. the thermochemistry of decompositions proceeding with simple-bond cleavage, stereoelectronic effects on bond dissociations in the presence of a control orbital, and long-range interactions resulting in transfer of a hydrogen atom or a larger group. All these three reaction classes have limited areas of application. The stereochemistry of even-electron anions has been developing rapidly. The reactivity of gas-phase anions finds numerous analogies in their chemistry in solution, e.g. hydride transfer reactions and nucleophilic substitution. The applications of mass spectrometry to configurational assignment and structure elucidation remain restricted to selected classes of organic compounds.

Towards a general scheme for estimating the heats of formation of organic ions in the gas phase. Part I. Odd-electron cations

It is shown that gas-phase heats of formation of homologous odd-electron organic cations can be well represented by an equation of the form ΔHf[ion]+• = A − Bn + C/n, where A, B, and C are constants and n is the total number of atoms in the molecule. Values for A, B, and C have been determined for the linear homologous series: alkanes, olefins, alkynes, alkanols, aliphatic ethers, aliphatic aldehydes, aliphatic ketones, alkanoic acids, alkyl chlorides, bromides, and iodides. For each of these series, correction terms for chain branching, double-bond position, and asymmetry effects are also given. These equations can be applied to molecules containing 40 or more atoms. The data base for this work is also presented.