Ionization and appearance potentials in organic chemistry. II—Appearance potentials and enthalpy differences for the stereoisomers of methylbicyclo[4.4.0]decanes

The ionization potentials for the stereoisomers of trans-fused 2- and 3-methylbicyclo[4.4.0]decanes and the appearance potentials for the ions at m/e 137 [M–CH₃]⁺, 109, 96, 95 and 82 were measured by the electron impact method. The ionization potentials and appearance potentials of the [M–CH₃]⁺ ions appeared to be equal for each of the epimers. Appearance potentials of the other ions were always lower for the less stable epimer. No quantitative correlation was observed between the difference in the appearance potentials for any ions and the differences in enthalpies of the ground states.     

Ionization and appearance potentials in organic chemistry. I—energetic aspects of the mass spectra of stereoisomeric 4-substituted N-alkyl-2-methyldecahydroquinol-4-ols

The ionization potentials for the stereoisomers of trans-fused 1,2-dimethyl- and 1-ethyl-2-methyl-4-R-decahydroquinol-4-ols (R?C?CH, CH?CH₂ or C₂H₅) and the appearance potentials for the [M–CH₃]⁺ and [M–C₂H₅]⁺ ions (loss of 2-CH₃ and 4-C₂H₅ groups potential, respectively) were measured by using the electron impact method. The ionization and appearance potential for [M–CH₃]⁺ are always lower for the isomers with the axial 2-CH₃ group. For the C-2 epimers, the difference between the appearance potentials for the [M–CH₃]⁺ ion values is likely to be equal to the enthalpy differences between the ground states of the epimers and the dissociation energy differences between the axial and equatorial C₂–CH₃ bonds. The appearance potentials for [M–C₂H₅]⁺ for the C-4 epimers possessing the 4-C₂H₅ group were very similar. At the same time, the appearance potentials for the [M–CH₃]⁺ ions were lower for less stable epimers which had an axial 4-C₂H₅ group.     

Potentiel d'ionisation,potentiel d'apparition et courbes d'ionisation differentielle pour les 1-2 dimethylcyclohexanes CIS et Trans

The molecular ionisation and appearance potentials of several ions have been determined for the two cis and trans 1-2 dimethylcyclohexanes using the Energy Distribution Difference (EDD) technique. The differences between the ionization efficiency curves for the ions [M—CH₃]^+. and [M—2CH₃]^+. tend to prove that the rearrangements give rise to cyclobutenic ions.     

Appearance-ionization potentials differences as a guide to the relative stabilities of the cis- and trans-A/B-steroids

The ionization potentials of cis- and trans-A/B-steroids of the androstane and pregnane series, as well as the appearance potentials of the M⁺-Me ions have been determined.The approximate activation energies (obtained as the AP-IP differences) for the loss of an angular Me from the molecular ion were found to be higher in the case of the more stable trans-A/B (5α)-steroids as compared to their cis (5β)-isomers.     

Factors determining relative ionic abundances in competing fragmentation reactions

For the competing fragmentation reactions [la] and [Ib], it is shown that the relative abundances of [R₁]⁺ and [R₁]⁺ are determined by the relative values of the ionization potentials of the corresponding free radicals R₁^. and R₂^. In most cases the appearance potentials of [R]⁺ from RH follow the trends shown by the radical ionization potentials with the result that the appearance potentials can be used to correlate relative ionic abundances. The potential usefulness of relative abundances to estimate radical ionization potentials is illustrated.     

Mass Spectrometric Studies of Mono- and Di-Haloacetylenes

The mass spectra of the four monohaloacetylenes X-C?C-H with X = F, Cl, Br, I and the six dihaloacetylenes X-C?C-Y with X, Y = Cl, Br, I have been recorded. The dissociation energies of the carbon-carbon triple bonds of these compounds have been determined from the appearance potentials of the CH⁺ and CX⁺ ions. The appearance potentials of the singly- and doubly-charged molecular ions are also reported.     

Steric strain and electron-impact. The behaviour of some n,n'-dimethyl-1,1-binaphthyls,some n,n'-dimethylbiphenyls and model compounds

For the title compounds I(M), A[M ? H]⁺ and A[M ? Me]⁺ have been determined. In the biaryl compounds, unlike the xylene and the two dimethylnaphthalenes studied, A[M ? Me]⁺ is noticeably dependent on the position of the methyl group. Deuterium labelling of the methyl groups in the biaryl compounds reveals that more than one process is involved in the formation of the [M ? Me]⁺ ion. In contrast, only in the case of 2,2′-dimethylbiphenyl is the appearance potential of the [M ? H]⁺ ion position dependent. The labelling results suggest complete scrambling of the hydrogen atoms before formation of the [M ? H]⁺ ions. Some comments are made on the problem of trying to relate differences in strain energies and ionisation and appearance potentials in alkyl substituted aromatics.     

Mass spectrum and heat of formation of isocyanic acid. Production of [hco]+ from discrete electronic state of molecular ion

The appearance potentials for the transition ([HNCO]^+·→ [HCO]⁺ + N), determined for the reaction in the ion source and in the first field free region (15·84 and 15·52 eV) correspond, respectively, to the vertical and adiabatic third ionisation potential of HNCO, as determined by photoelectron spectroscopy. The formyl ion and nitrogen radical are formed in the ground state, which requires predissociation of a quartet molecular ion of HNCO. A heat of formation δHf(HNCO)g = ?25 ± 3 kcal mol^·1 was determined from the appearance potential and kinetic energy release for the metastable ion [HCO]⁺, and from the difference in appearance potentials for the ion [NH]⁺· produced from the isoelectronic molecules HNCO and HN₃.     

Ionization and appearance potential measurements in arene chromium tricarbonyls

The ionisation potentials and the appearance potentials for ions arising from the loss of carbonyl groups as well as certain other low-energy fragmentation ions have been measured for a series of substituted arene chromium tricarbonyls. The values obtained are found to correlate very well, for the loss of one and two carbonyl groups, with both the CO force constant k and the Hammett σ_p function and substituent on the benzene ring. The appearance potentials of the ions arising by the loss of three carbonyl groups do not exhibit a high correlation. Arene⁺ ions are found to occur at energies intermediate between those for the loss of two and three carbonyl groups.The various effects observed are discussed in terms of a model which involves definite contributions from both σ- and π- bonding of carbonyl groups in the gaseous cations.     

Lithium redox-responsive ferrocene bis-tertiary amide derivatives

The addition of Li⁺ to ferrocene bis-tertiary amide derivatives in acetonitrile results in a shift of the ferrocene oxidation wave to more positive potentials and the appearance of a new redox couple associated with a Li⁺ complex.     

High energy photophysics and photochemistry: Assignment of initial electronic states of fragmentation processes in CO2+2

The assignment of the electronic states of gas-phase molecular dications presents a number of difficulties. After discussing these problems a strategy for assignment, which collates the results of several experimental and theoretical techniques, is presented. The use of information from the fragmentation dynamics of dication states is proposed. Three states, g̃a, g̃b and g̃g, of CO²⁺₂, whose appearance potentials were previously observed by photoion-photoion coincidence techniques at 37.9, 40.5 and 45 eV, respectively, are assigned using the methods presented. The results also strongly support the value 37.7 eV for the appearance potential of the metastable ground state X̃³∑^-_g of CO²⁺₂. Present limitations and future desirable progress of the assignment techniques are discussed.     

Appearance potentials determined by the electron-impact method as an analytical aid in the evaluation of conformational energies and clarification of ring conformation—I: Appearance potentials of the [M?R]+· ions formed in the primary fragmentation of stereo-isomeric 1,3-dioxans. A direct route to conformational energies

The appearance potentials for the major primary fragmentation ions of three pairs of isomeric methyl-1,3-dioxans were measured. The correlation between the differences in these appearance potentials for each pair and the differences in the standard heats of formation for the corresponding isomer pairs is discussed.     

Determination of the appearance potentials of [C3H7]+ ions by a computerized electron distribution difference procedure

A simple computerized electron distribution difference method has been developed using a conventional mass spectrometer and an on-line computer. Using the system, ionization potentials for various propyl halides were measured and found to agree well with photoionization data. The appearance potentials for [C₃H₇]⁺ agreed well with literature values, except for 1-propyl bromide and 1- propyliodide which gave values approximately 0.7 eV lower than previous reports. The results are interpreted to indicate threshold formation of the sec-propyl ion from all propul halides.     

Ionization and dissociation of C6F6 isomers under electron impact

The mass spectra of hexafluorobenzene, hexafluorobicyclo[2.2.0]hexa-2,5-diene (perfluoro-Dewar benzene) and 1,1,1,6,6,6-hexafluorohexa-2,4-diyne, and the fragmentation mechanisms of their parent ions are reported. The behaviour of the two cyclic isomers under electron impact is very similar; the linear one behaves quite differently. The ionization potentials of the molecules and the appearance potentials of the fragment ions (both normal and metastable) have been measured. The heats of formation of [C₆F₅]⁺ and [C₅F₃]⁺ are calculated. A value for the heat of formation of 1,1,1,6,6,6-hexafluorohexa-2,4-diyne is proposed.     

Models for the theory of the hydrogen bond: Linear H3/−

An analogy is drawn between the hydrogen bond and the interaction of H₂ with H^– as a prototype. The energy surface for linear H^3/– is calculated using a minimal basis set of 1s orbitals and complete configuration interaction. The appearance of single and double minimum potentials on this surface is discussed.Contribution No. 359 from the Department of Chemistry, Tufts University.     

Massenspektrometrische Untersuchung von Amiden. IX—Nachbargruppenwechselwirkung Statt Ringkontraktion bei der Fragmentierung von Carbonsäurepiperididen

Ring contractions which are typical for the electron-impact-induced fragmentation of N-acetylpiperidine play only a small part in the fragmentation of piperidides from higher, saturated carboxylic acids. The investigation of ²H labelled compounds as well as energetic measurements (appearance potentials) show that the elimination of CH₃^˙ and C₂H₅^˙ is best rationalised by the assumption of a neighbouring group participation of the amide function.     

Elektronenstossinduzierte Fragmentierung von Acetylenverbindungen. IX—Struktur der Stabilen [C9H11]+-Ionen aus Isomeren C9H11-Chloriden

The structure of stable [C₉H₁₁]⁺ ions from different model compounds with the molecular formula C₉H₁₁Cl has been estimated by energy measurements. It has further been shown that acetylenic compounds cyclise to form aromatic ions in the range of the appearance potentials.     

Mass spectrometric studies of structural isomers—II: Mono-and bicyclic C6H10 molecules

The electron-impact-induced ionization and fragmentation of six C₆H₁₀ structural isomers have been studied in order to determine the effect of isomerism upon their mass spectrometric behavior. The 70 eV mass spectra, metastable transitions and appearance potentials of the principal ions are reported. Significant differences between the mass spectra of the six isomers were observed; however, metastable transition and appearance potential data indicate that the fragmentation path-ways are the same for all the C₆H₁₀ molecules. Experimentally determined ionization potentials for the structural isomers are presented and compared to ionization potentials calculated by the bond orbital method. Utilizing fragmentation pathways deduced from general features in the mass spectra and from observed metastable transitions, we calculated heats of formation (ΔH_f) for the observed principal ions and compared these values to ΔH_f values for isomeric ions from other molecules.     

Surface enhanced raman scattering from silver-halide and silver-pyridine vibrations and the role of silver ad-atoms

Surface enhanced Raman scattering has been observed for silver-halide vibrations at 246 (Ag-Cl), 166 (Ag-Br) and 117 cm^?1 (Aa-I). At potentials cathodic to ?0.6 V, addition of pyridine leads to the appearance of two enhanced bands around 173 and 252 cm^?1. Two requirements are needed simultaneously for large enhancement factors, large-scale surface roughness and ad-atoms.     

Mass spectra of aryl substituted N-t- butylbenzamides: Substituent effects on unimolecular ion decomposition

The major reactions of aryl substituted N-t-butylbenzamides upon electron-impact involve direct cleavage of a methyl radical, the loss of a butene molecule with the transfer of one hydrogen, or the loss of a butenyl radical with the transfer of two hydrogens. The last of these processes parallels the mass spectral behavior of aliphatic amides. Substituent effects indicate that electron-withdrawing groups on the aromatic ring enhance the two hydrogen transfer process, while electron-donating groups enhance the single hydrogen transfer process. Ion abundances, ionization potentials and appearance potentials are discussed with respect to correlation with σ⁺ values.