Neutral and ion thermochemistry: Its present status and significance

Abstract

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.

Acid–base thermochemistry of gaseous aliphatic α-aminoacids

“…For this reason it may be estimated using an empirical procedure 49 or correlated with the size of the molecule, or part of it. 46 In order to assess this comparison we calculated D f H1 298 (AAA) using the Benson's incremental method. 49 For this purpose we assign a C-(N)(CO)(H 2 ) increment value equal to the C-(N)(C)(H 2 ) contribution.…”

Proton affinities and ion enthalpies

“… 2 , 3 Similarly, Δ f H 0 [CH 3 C + (OH)CH 3 ] has been derived from AE measurements and the PA of acetone, Δ f H 0 = 499 ± 3 kJ/mol. 4 Based on such measurements, it has been well established in a number of publications 5 – 13 that plots of ΔH f for a series of organic cations, in which an electron releasing group is successively added to the formal charge bearing site, are a simple exponential function of ion size [ln( n )], where n is the number of atoms in the ion and is commonly assumed to relate directly to ion size. These ΔH f versus ln( n ) plots have been shown to be linear for ethyl, 12 methyl, 5 – 8 hydroxy, 5 , 7 – 9 , methoxy 5 , 7 – 9 and amino 5 , 7 – 9 substitutions at the formal charge site in species such as methane, 5 , 12 the methyl cation, 5 , 7 , 8 , 13 olefinic, 5 , 7 , 8 , 13 and aromatic hydrocarbons 5 , 11 and compounds containing N, 5 , 7 , 8 , 10 Si, Ge, Sn and Pb.…”

“…4 Based on such measurements, it has been well established in a number of publications [5][6][7][8][9][10][11][12][13] that plots of ÁH f for a series of organic cations, in which an electron releasing group is successively added to the formal charge bearing site, are a simple exponential function of ion size [ln(n)], where n is the number of atoms in the ion and is commonly assumed to relate directly to ion size. These ÁH f versus ln(n) plots have been shown to be linear for ethyl, 12 methyl, [5][6][7][8] hydroxy, 5,[7][8][9] , methoxy 5,7-9 and amino 5,[7][8][9] substitutions at the formal charge site in species such as methane, 5,12 the methyl cation, 5,7,8,13 olefinic, 5,7,8,13 and aromatic hydrocarbons 5,11 and compounds containing N, 5,7,8,10 Si, Ge, Sn and Pb. 12 In addition to providing a method by which thermochemical data may be estimated, such plots provide physicochemical information.…”

Size Effects on Cation Heats of Formation. II. Methyl Substitutions in Oxygen Compounds

“…The present work on the heats of formation of the cations of methyl-substituted oxygen compounds is part II of a study that adopts the graphical approach of Holmes and Lossing in relating cation heats of formation to cation size. As in the case of nitrogenous molecules (Part I), the results of the present work provide tests of the validity of this graphical method and also point to the necessity of investigating or reinvestigating the ionization energies and the heats of formation of several oxygen-containing molecules.…”