K. Levsen

Study of the kinetics of fast unimolecular decomposition processes and of organic rearrangement reactions by field ionization mass spectrometry

Abstract It is shown that extremely fast unimolecular decomposition processes of organic ions in the gas phase can be studied by means of the field ionization method. The shortest decomposition time which can be attributed to a process that remains virtually unmodified by the electrical field is about 10 −11 sec. A retarding potential technique was developed permitting measurement of the distribution of the average rate constants between 10 −8 and 10 −6 sec. A continuous distribution of rate constants was found for a number of large molecular ions in the range of ion times-of-flight between several 10 −11 and 10 −6 sec, in accordance with the quasi-equilibrium theory of the mass spectra of large molecules. Organic rearrangement reactions have been studied by combined electron impact and field ionization mass spectrometry. Three different types of rearrangement reactions were studied: 1. the formation of smaller rings from larger ones; 2. the McLafferty and other hydrogen rearrangement reactions; 3. skeletal rearrangement reactions. It is postulated that these reactions have lower rates than the single-step direct single-bond ruptures of the same molecules (the rearrangement reactions have large negative entropies of activation). This is confirmed by the field ionization mass spectra where the intensities of these rearrangement-process peaks are extremely small as compared with other fragment peaks. This is due to the extremely short time (several 10 −14 −10 −12 sec) available for “normal” fragment formation after field ionization.

On-line liquid chromatography-mass spectrometry analysis of non-ionic surfactants

Abstract Non-ionic surfactants of the alkylphenolethoxylate and oxoalcoholethoxylate classes were analysed by on-line liquid chromatography-mass spectrometry using a mechanical transport interface. Information on the distribution of homologues the presence of impurities was obtained. The method can be used to study the primary biodegradation of non-ionic surfactants.

The gas-phase chemistry of C4H4+−

Abstract Ion/molecule reactions, unimolecular dissociations and collisional activation spectra of the C 4 H 4 +− , cation have been studied to obtain information about the chemistry, and possibly structure (or structures), of this ion. Precursor molecules employed included 1-buten-3-yne (vinyl acetylene), pyridine, benzene and 1,5-hexadiyne. Some ion reactions involving neutral 1-buten-3-yne were also followed. The reactivity of C 4 H 4 +− formed by ionization of neutral C 4 H 4 was observed to be the same as that of C 4 H 4 +− cations formed as fragments following dissociative ionization.

Hydrogen rearrangements in ionized alkanoic acids. An ion lifetime study

Abstract Single and double hydrogen rearrangements involving β-bond cleavage have been studied in ionized butanoic, pentanoic and hexanoic acids specifically deuterium labelled at the hydroxy group and all carbon atoms of the side chains. The reactions have been studied in the range 6×10 −11 -10 −5 s after ionization using the field ionization kinetic technique. At short ion lifetimes ( r −10 s) n -butanoic acid shows exclusively a single hydrogen rearrangement which specifically involves the γ-hydrogen and which is followed by β-bond cleavage. At longer ion lifetimes ( t >10 −8 s) this rearrangement is preceded by hydrogen exchange between the hydroxylic hydrogen and the β- and γ-hydrogens as found previously. In ionized n -pentanoic and n -hexanoic acids, double hydrogen transfer with β-bond cleavage competes effectively with single hydrogen transfer at long ion lifetimes. At short ion lifetimes ( −10 s) single hydrogen rearrangement in n -pentanoic and n -hexanoic again involves predominantly the γ-hydrogen atom, the double hydrogen transfer involving those atoms from the γ- and δ-positions. In the acids extensive hydrogen exchange which is observed even on the picosecond time scale leads to participation of the β- and ϵ- (for n -hexanoic acid) positions in the hydrogen transfer reactions, although γ- and δ-hydrogens are predominantly involved even at the longest time (∼ 10 −5 s).

The study of rearrangement reactions by field ionization mass spectrometry: III. Experimental results for different types of reactions☆

Abstract The kinetic behaviour of rearrangement reactions and direct bond cleavages as derived qualitatively from FI mass spectra is summarized. At low excitation energies (at 12 eV EI, or under FI conditions) skeletal rearrangements are slow, and hydrogen migration relatively fast, decomposition processes (decomposition times corresponding to the peak maxima: several times 10 −6 sec, and several times 10 −11 sec, respectively). Among direct bond fissions alkyl radical eliminations are especially slow processes. An unambiguous differentiation between rearrangements and direct bond cleavages from the kinetic data is difficult. The kinetic study of the tropylium ion formation demonstrates that this rearrangement starts with a field-induced benzyl ion formation in an FI source (in contrast to EI conditions).

Field ionization mass spectrometry of organic compounds

Abstract A review is given on field ionization mass spectrometry of organic compounds. Four different subjects are treated and illustrated by means of significant examples: Experimental techniques, surface reactions induced by high electric fields, the kinetics of fast unimolecular decompositions of ions, and qualitative and quantitative analyses of organic compounds by field desorption methods.

Gaseous odd- and even-electron ions

Abstract The principal collision-induced fragmentations of simple protonated ketones, aldehydes, ethers, amines, sulphides, alcohols, acids, nitriles and halides are discussed. These protonated molecules decompose mainly by loss of alkane, alkene and RX (R = alkyl, H; X = OH, SH, NH 2 , Br, I). Substantial radical losses are only observed for small protonated molecules. Deuterium-labelling demonstrates that the XH bond is particularly strong. The fragmentation of (MH) + ions is compared with that of the corresponding (M) +− ions. The spectra of the (M) +− ions are dominated by direct bond cleavages, in particular α-cleavages, as a result of both the stability of the ionic fragment and the loose transition state. In (MH) + ions direct bond cleavages lead to energetically less favourable products. Thus rearrangement reactions play a more important role in the decomposition of these ions. (MH) + ions are more stable relative to fragmentation than (M) +− ions.

Kinetic differentiation between non-specific hydrogen rearrangement and hydrogen scrambling in phenyl n-propyl ether. A field ionisation and collisional activation study

Abstract The different kinetics of non-specific hydrogen rearrangements on the one hand, and hydrogen scrambling processes on the other, can be used to distinguish between these two processes as demonstrated for C3H6 elimination from the phenyl n-propyl ether molecular ion. The kinetic results support the conclusion, reached previously in an electron-impact study [2], that the rearrangement in phenyl alkyl ethers is a non-specific hydrogen transfer involving transition states of different ring sizes. Collisional activation spectra, reported in this study, show that the [C6H6O]+− ion formed by this rearrangement has the structure of the ionised phenol, which suggests that the hydrogen atoms from all positions of the side chain are transferred to the oxygen and not to the ortho-position of the phenyl ring.

Multi-step reaction pathways in the decomposition of ionised n-hexanoic acid

Abstract Metastable molecular ions of n-hexanoic acid ( 1 ) decompose unimolecularly to C2H5 and protonated methacrylic acid ( 5 -H+). Investigation of the mechanism reveals that 1) the branched cation radical 11 must be regarded as the essential intermediate in the course of the rearrangement/dissociation reaction and 2) the process commences with intramolecular hydrogen transfer from either C-3 or C-5 to the ionised carbonyl oxygen. Hydrogen transfer from C-4, which would correspond to the well-known McLafferty rearrangement, is of no importance in the C2 H5 elimination from 1 . The same conclusion applies for various alternative mechanisms, as for example a SR i type reaction, e.g. 1 + → 2 -H+. The gas phase chemistry of the cation radical of 1 , and in particular the hydrogen exchange processes between the methylene groups C -2 C -3 and C -5 C -6 , is in surprisingly close correspondence to the chemistry of alkyl radicals.

The study of rearrangement reactions by field ionization mass spectrometry: I. Theoretical aspects

Abstract The peak shapes of fragment ions caused by rearrangement reactions in a field ionization ( fi ) mass spectrometer are derived theoretically. The quasi-equilibrium theory ( qet ) is used and different distributions of rate constants are assumed. The peak shapes agree qualitatively with those observed in the fi mass spectra. The shift of the rearrangement peak maxima from the normal fragment peak position is discussed for the case of a single-focusing magnetic sector field mass spectrometer. The peak shifts are studied as a function of the emitter geometry and radius of curvature, surface structure, ion retardation, emitter to cathode distance and emitter temperature. The rate constants as a function of energy are compared for the case of electron impact and fi .