J. Tortajada

A theoretical study on the complexation of sp, sp2 and sp3 nitrogen-containing species by Cu+

Abstract A study of the cationization of NH 3 , NH 2 CH 3 , NHCH 2 and HCN with Cu + in the gas-phase is performed at the B3LYP, B3PW91 and G2 levels of theory. A comparison of the geometries, binding energies and harmonic vibrational frequencies indicate that these DFT methods are a good option for this kind of system when the size of the molecule makes higher levels of theory difficult. The nature of the CuN bond through the series, as well as the character of the hybridization of the orbitals involved have been analyzed by means of the Bader topological analysis and NBO method. As Cu + has a covalent interaction with N, the possible activation of the different CN vicinal bonds by the effect of copper cation complexation is discussed.

Complexes ion/molecule intermediaires dans la fragmentation des ethers protones

Abstract The experimental study of metastable ions [iso-C 3 H 7 O + H CH 3 ] 1 shows that the dissociation is preceded by H exchange, either by interconversion between [iso-C 3 H + 7 , CH 3 OH] “ α complex” and [C 3 H 6 , CH 3 O + H 2 “ β complex”, or by reversible isomerisation between [iso-C 3 H 7 O + H CH 3 ] 1 and [ n -C 3 H 7 O + H CH 3 ] 2 . More generally, ions [iso-C 3 H 7 O + HR] have been studied. The interconversion between [iso-C 3 H + 7 , ROH] “ α complexes” and [C 3 H 6 , ROH + 2 ] “ β complexes” is only observed if PA[ROH] — PA[C 3 H 6 ] −1 . Ab initio calculations indicate that the association 1α between the cation iso-C 3 H + 7 and CH 3 OH is stabilized by dipolar effects and by weak bonds. However 1α does not correspond to an energetic minimum and therefore to a stable form. On the contrary, the complex [C 3 H 6 , CH 3 O + H 2 ] 1β is a stable form. The stabilization energy Δ H s [ 1β ] is about 12 kcal mol −1 .

Experimental and theoretical study of carbon suboxide C3O2, protonated carbon suboxide C3HO2+ and C3HO2· radical in the gas phase

Abstract The proton affinity (PA) of carbon suboxide OCCCO has been determined by Fourier transform ion cyclotron resonance as 791 kJ mol−1. Ab initio calculations at the MP4(SDTQ)/6–31G∗∗//6–31G∗∗ + ZPE(6–31G∗∗) level of theory give a proton affinity for C3O2 of 789 kJ mol−1. The gas-phase reactivity of ions a [OCCHCO]+ and ions b [OCCCOH]+, generated from various precursors has been studied by mass spectrometry techniques (metastable ion kinetic energy (MIKE), collision induced decomposition (CID) and neutralization — reionization (NR) mass spectra). From the experimental and theoretical results it follows that the gas-phase protonation of carbon suboxide yields ion a. At high internal energies these ions show competing losses of H′ and CO. The radical [OCCHCO]·, the neutral counterpart of protonated carbon suboxide, has a lifetime of at least 1 μs.

REARRANGEMENT AND DISSOCIATIVE PROCESSES IN THE C3H6O+. POTENTIAL ENERGY SURFACE. RADICAL CATIONS WITH THE CCCO FRAME : A MODEL SYSTEM

The complete potential energy profile associated with the isomerization of ionized enol, [CH3CHCHOH]+, 1; distonic ion, [CH2CH2CHOH]+, 2; ionized propanal, [CH3CH2CHO]+, 3; ionized ally alcohol, [CH2CHCH2OH]+, 4; and their dissociation into C2H5CO+ + H., 5 has been constructed by means of molecular orbital calculations at the MP2/6−311 + G∗∗//MP2/6−31G∗ + ZPE level. Investigated reactions were: 1,2-, 1,3- and 1,4-hydrogen migrations, 1,2-HCOH migration 2 ⇄ 2′ and the sigmatropic 1,3-OH shift, 4 ⇄ 4′. It is found that the energy-determining step of the overall process 1 → 2 → 3 → 5 is the 1,4-hydrogen atom migration 2 → 3. Another illustration of the key role played by the distonic species 2 is provided by the calculation of a very low critical energy for its degenerate isomerization 2 ⇄ 2′. A rationalization of the experimental findings concerning the dissociation energetics and the metastable dissociations of 1–4 is given by RRKM calculations on the ab initio potential energy surface.

Gas phase catalyzed keto-enol isomerization of cations by proton transport☆

Abstract In the gas phase, the unimolecular isomerization of the H 3 COC(O)CH 2 CO + cation 1 ( m / z 101) into the H 3 CO(HO)CCHCO + enol ion 2 by a 1,3-H shift possesses a high energy barrier and is therefore not observed. In contrast, in the cell of a FT-ICR mass spectrometer, interaction with gaseous methanol catalyzes the isomerization of 1 into its more stable isomer 2 , which can be characterized by low energy collision with argon. This exothermic reaction is irreversible. Reaction with labeled methanol and ligand exchange experiments indicate the existence of two distinct reactions. By formation of a covalent bond, one reaction yields protonated dimethyl malonate while the second one leads to ion 2 by a 1,3-H transfer catalyzed by methanol. Conversely, loss of methanol from collisionally activated long-lived m / z 133 cations formed by protonation of dimethyl malonate yields some m / z 101 ions with structure 2 , which shows that methanol catalyzes the isomerization of ion 1 within a [ 1 , CH 3 OH] complex. The efficiency of different catalysts is studied in order to probe the mechanism of the isomerization processes.

Gas-phase unimolecular reactivity of C3H7O+cations : a combined mass spectrometric-molecular orbital study

The unimolecular dissociations of the two isomeric ions [CH 3 CH 2 CHOH] + (1) and [CH 3 CH 2 BCH 2 ] + (2) were re-examined. Molecular orbital calculations conducted at the MP2/6-31G * //HF/6-31G * + ZPE level were used to characterize the corresponding potential energy profile. The experimental data were completed by a Fourier transform ion cyclotron resonance spectrometric investigation on the system [CH2OH] + + C 2 H 4 and by a study of various metastable [C 3 H 7 O] + ions the isomerization pathway of lowest energy connecting 1 and 2 involves two ion-neutral complexes between protonated formaldehyde and ethene. The isomerization 1=2 is typically a complex mediated reaction since the key step consists simply of the reorientation of the two partners [CH 2 OH] + and C 2 H 4 inside the ion-neutral cage. The model is demonstrated to account for the H-D exchange observed during the dissociation of variously deuterated species.

Experimental and theoretical studies of the gas-phase reactivity of the (HO)2PO+ phosphonium ions towards methanol

Abstract Ion–molecule reactions between the (HO)2PO+ phosphonium ions and methanol were performed in a quadrupole ion trap mass spectrometer. The (HO)2PO+ phosphonium ions, formed by electron impact from neutral trimethyl phosphite ions were found to react with methanol according to three consecutive reactions, via sequential methanol addition/water elimination, to yield protonated trimethyl phosphate. To confirm the experimental results, and to state the mechanism for the formation of the ionic species, a theoretical study by using the density functional theory (DFT) approach has been carried out. According to calculations performed at the B3LYP/6-311+G(2df,p) over B3LYP/6-31G∗ optimized geometries, the overall reaction leading to protonated trimethyl phosphate occurs by an exothermic process of 365 kJ/mol. The isomerization barriers connecting the different intermediates have been also calculated in order to have a more complete description of the reaction processes. In addition, the proton affinity (PA) and the gas-phase basicity (GB) of the molecular species related to the reactions of the (HO)2PO+ cations with methanol namely: monomethyl phosphate, dimethyl phosphate, and trimethyl phosphate (TMP) have been evaluated to be 855, 875, and 892 kJ/mol (for PAs) and 823, 843, and 862 kJ/mol (for GBs), respectively. The excellent agreement between the theoretical (892 kJ/mol) and the experimental value (891 kJ/mol) of the PA of TMP shows the reliability of our DFT calculations.

Analysis of alkenes by copper ion chemical ionization gas chromatography/mass spectrometry and gas chromatography/tandem mass spectrometry

A novel chemical ionization/fast atom bombardment (CI/FAB) source was used to analyse alkenes by chemical ionization mass spectrometry (CI-MS) using copper ions as the ionizing agent. The Cu(+)-CI mass spectra showed abundant pseudomolecular adduct ions [alkene-Cu](+) and characteristic fragment ions. Mass-analysed ion kinetic energy spectroscopy was used to study the product ions resulting from the decomposition of adduct ions and to eliminate background interferences derived from the copper ions. The major fragmentations permitted the localization of double bonds and minor fragments allowed the differentiation of alkene isomers. The CI/FAB source was coupled to a gas chromatograph and simple and complex mixtures of octene isomers were analysed by gas chromatography (GC)/Cu(+)-CI-MS and GC/Cu(+)-CI-MS/MS. Copyright 1999 John Wiley & Sons, Ltd.

Cu+ chemical ionization for analysis of hydrocarbons by gas chromatography/ mass spectrometry

The use of copper ions for chemical ionization (CI) coupled with gas chromatography/mass spectrometry (GC/MS) of hydrocarbons is reported. Cu+−CI was performed in a high-pressure, fast atom bombardment ion source coupled with both a gas chromatograph and a mass spectrometer. The suitability of the Cu+−CI method is illustrated by the analysis of pure alkylbenzenes, alkylthiophenes, octenes, and by the analysis of a light mixture of aromatic hydrocarbons. The Cu+−CI/GC mass spectra display an abundant [M+63Cu]+ ion, together with fragmentations, that are of structural interest. The detection limit for isobutylbenzene, taken as model compound, is 100 times lower than that for electron ionization.

Experimental and theoretical studies of the gas-phase protonation of orthophosphoric acid

The gas-phase proton affinity (PA) and the gas-phase basicity (GB) of orthophosphoric acid (H3PO4) have been determined by the kinetic method to be 833 and 800 kJ/mol, respectively. High level ab initio calculations at the G2(MP2) level of theory give a PA of 827 kJ/mol. A good agreement is observed between the G2(MP2) and density functional theory (using B3LYP) results. B3LYP/6-311+G(2df,p) calculations and chemical ionization (CI) experiments have been performed in order to clarify the loss of water from protonated H3PO4. The oxo–oxygen atom is confirmed as the most basic site, the hydroxy–oxygen protonated isomer O=P(OH)2(OH2)+ being predicted to be 126 kJ/mol less stable than P(OH)4+. The isomerization barrier connecting both isomers is calculated as 200 kJ/mol and the dissociation products, water and protonated metaphosphoric acid O=P(OH)2+, are found to be 284 kJ/mol with regard to P(OH)4+. The proton affinity of metaphosphoric acid (HPO3) is also evaluated to be 712 kJ/mol at the G2(MP2) level of t...