Christiane Aubry

Correlating thermochemical data for gas-phase ion chemistry

Abstract This article presents a brief review of correlation schemes which relate ionization energies, ionic heats of formation, proton affinities and heterolytic bond strengths with some simple property of the ion, most commonly its size. The schemes reviewed here have mainly been developed for homologous series and simple atom or group substitutions at charge-bearing sites. The utility of such schemes is illustrated by appropriate examples.

Group Additivity Values for Estimating the Enthalpy of Formation of Organic Compounds: An Update and Reappraisal. 2. C, H, N, O, S, and Halogens

This study extends a previous publication on group additivity values (GAVs) for the elements C, H, and O, to include the elements nitrogen, sulfur, and the halogens. The present state and utility of the Benson additivity schemes for estimating the enthalpy of formation (Δ(f)H(0)) of organic compounds are again described, extending them to include more elements. Old and new GAVs for a wide variety of compounds are provided and are revised where necessary. When new terms are proposed, or old ones are significantly altered, the rationale for so doing is presented. GAV derived ring strain values for benzene and pyridine indicate that the aromatic stabilization of each is essentially the same. As before, the thermochemical consequences of replacing one functional group by another are also shown, thus permitting quick shortcuts to the estimation of new Δ(f)H(0) values.

Group additivity values for estimating the enthalpy of formation of organic compounds: an update and reappraisal. 1. C, H, and O.

This study examines critically the present state and utility of the Benson additivity schemes for estimating the enthalpy of formation of organic compounds. Old and new group additivity values (GAV) for a wide variety of compounds containing C, H and O are described and are revised where appropriate. When new terms are proposed, or old ones significantly altered, the rationale for so doing is provided. Corrections for such items as cis-isomer effects, gauche interactions, ring strain energies, double-bond position, conjugation effects, steric hindrance in aromatic molecules, etc. are included and discussed. Also provided are the thermochemical consequences of functional group replacements, in which one group in a molecule is substituted by another, thus providing quick short cuts to estimating new Δ(f)H(0) values. Results derived from the new additivity terms are consistent with those produced by computational chemistry methods in general use.

The behavior of oxygen as a collision-induced dissociation target gas

The unusual and unique ability of O2 as target gas in kV collision-induced dissociations, to enhance a specific fragmentation of a mass selected ion, has been examined in detail. The affected dissociations studied were the loss of CH3. from CH3CH+X (X = OH, CH3, NH2, SH); CH3 and Cl. loss from CH3C+(Cl)CH3; C2H5 loss from CH3CH2CH+X (X = OH and NH2); H loss from Cl. CH2OH and+CH2NH2; O. loss from 1,2−, 1,3−, and 1,4−C6H4(NO2+.CH3NO2+.; C6H5NO2+.; C5H5NO+. (pyridine N-oxide); 3− and 4−CH3C5H4NO+.. A general explanation of the phenomena, which was semiquantitatively tested in the present work, can be summarized as follows: the ion − O2 encounter excites the target molecules to their3∑g−state which resonantly return this energy to electronic state(s) in the ion. The excited ion now contains a sharp excess of a narrow range of internal energies, thus significantly and only enhancing fragmentations whose activation energies lie within this small energy manifold.

Neutral and ion thermochemistry: Its present status and significance

This short account outlines the sources of thermochemical data that are important for gas phase ion chemistry. It describes some of the relationships that have been identified for the empirical estimation of enthalpies among neutral molecules, free radicals, and odd and even electron ions. For neutral species, the additivity principle works well and this has been developed to cover a very wide range of structures and isomers. Ionization energies of homologous species depend inversely on molecular size, allowing estimates to be made for missing members. For ions, the effect of a group substitution (such as replacing a hydrogen atom by, e.g., a methyl or hydroxyl group) can easily be estimated, but such results are strongly dependent upon the position of the charge site, relative to that of the substitution. Special emphasis is given to the reliability of data collections and simple directions are provided as to how critically to assess and identify less-than-satisfactory values.

Methods for critically assessing old and for estimating new organic gas-phase neutral and ion thermochemical data. A user’s guide

A broad, general understanding of the relationships between chemical structures and their various energies is of primary importance when assessing the validity of an energy derived from experiment or computation. This review is intended to provide guidance and advice for scientists seeking data for standard enthalpies of formation (ΔfH0) and ionisation energies (IE) for a wide variety of organic compounds and ΔfH0 values for their odd- and even-electron cations. The major reference sources are critically reviewed and methods are described for the accurate estimation of ΔfH0 of neutral compounds and correlation schemes for IE, proton affinities (PA) and ΔfH0 values for odd- and even-electron positive ions. ΔfH0 data for neutral organics are well reproduced by the additivity method and up-to-date tables of group values are provided, as well as directions for their application. Free radical ΔfH0 values are also reported. The situation for odd-electron cations (molecular ions) is not as simple, but reliable correlations exist between adiabatic ionisation energies (IEa) and molecular size, particularly for closely related species such as homologues, where IEa falls linearly with 1/n, n being the number of atoms. The same relationship obtains for PA. The ΔfH0 values for even-electron cations (ionised free radicals) also display useful correlations, chiefly based on the effects of substitution at a charge site, localised or delocalised. Here the relationship is exponential, with ΔfH0(Ion) being a linear function of ln(n). A separate brief guide, intended for users of the NIST WebBook seeking molecular ΔfH0 values and IE, is appended.

A tandem mass spectrometry study of [C,H 2 ,X 2 ,Si] +• ions (X = H, D, Cl) and the generation of their neutral counterparts

Two [Si,C,H4]+• isomers, having similar stabilities and a relatively low barrier for their interconversion, have been separately generated in mass spectrometric experiments. CH2SiD2+• and D2-CH3SiH+• ions were produced from the same ionic precursor, namely ionized 1,1-dideutero-silacyclobutane (D2-I). Dissociation of the latter in the ion source resulted in the silaethene structure, whereas metastable dissociation gave rise to the dideuterated methylsilylene. Appearance energy measurements confirmed the formation of the two [Si,C,H2,D2]+• isomers, whose heats of formation were estimated to be 1017 and 1038 kJ mol−1 for CH3SiH+• and CH2SiH2+•, respectively. Both ion-source and metastable dissociations of ionized D2-I resulted in [Si,C,H3,D]+• and [Si,C,H4]+• ions possessing the methylsilylene structure. The collision-induced dissociation (CID) and neutralization–reionization (NR) mass spectra of CH2SiD2+• and D2-CH3SiH+• ions were distinctively different. The characteristic dissociation channel for the CH2SiD2 structure was the loss of CH2. Some dissociations of the ionized silaethene involved the formation of the methylsilylene intermediate. However, CH3SiH+• ions produced by this isomerization appeared to be unstable and did not survive neutralization–reionization. As a result, the CID mass spectra of CH2SiD2+• ions prior to neutralization and surviving the NR event were almost identical. Strong recovery signals in the NR mass spectra were consistent with the intrinsic stability of neutral silaethene and methylsilylene. The interconversion of these structures generated by neutralization of their positively-charged counterparts seems to be very unlikely. The CID, NR and NR/CID mass spectra of [Si,C,H2,Cl2]+• ions produced from various precursors and in different time frames were indistinguishable. They corresponded to the CH2SiCl2 connectivity of the atoms. The heat of formation for CH2SiCl2+• ions was estimated to be ∼732 kJ mol−1, based on appearance energy measurements. The presence of a strong recovery signal in the NR mass spectra was consistent with the formation of stable neutral 1,1-dichlorosilaethene in the gas phase on the microsecond time frame.

The effect of phenyl substitution on the thermochemistry of gas-phase ions and their neutral counterparts

The available data for the heats of formation of phenyl-substituted organics have been critically evaluated. Phenyl substitution is always thermochemically destabilizing in that ΔfH0[R–C6H5] – ΔfH0[R–H] is a positive quantity. The magnitude of the effect in neutrals depends on the substitution site, e.g. from + 79 kJ mol−1 for HCOR or HCCR to + 155 kJ mol−1 for RCO2H. Multiple phenyl substitution (where possible) at the same site is generally simply additive. For the corresponding cationic species, the first phenyl substitution is stabilizing when the ionization energy of the substrate is lowered thereby and for even-electron ions resonance-stabilization energies may also come into play. Additional phenyl substitution at the same site is, however, always destabilizing. Phenyl substitutions were compared with vinyl substitution: for neutral species the effects are similar, whereas for even-electron ions, multiple vinyl substitution is only weakly stabilizing or destabilizing.