Metal Ion Hydrocarbon Bidentate Bonding in Alkyl Acetates, Methyl Alkanoates, Alcohols and 1-Alkenes: A Comparative Study

Abstract

The relative affinity of the monovalent metal ions Li, Na, Cu and Ag towards a series of aliphatic alkyl acetates and some selected 1-alkenes (P) was examined using the kinetic method. A detailed analysis of the dissociation characteristics of a series of mixed metal-bound dimer ions of the type P-M-P and the evaluated proton affinities (PAs) of the monomers shows that the affinity of the cation towards long-chain alkyl acetates and alkenes (having a chain length ≤ C) is markedly enhanced. In line with recent studies of nitriles, alcohols and methyl alkanoates, this is attributed to a bidentate interaction of the metal ion with the functional group or double bond and the aliphatic chain. In particular, the longer chain alkyl acetates, methyl alkanoates and alcohols show a remarkably similar behaviour with respect to silver ion hydrocarbon bonding. The Ag adducts of the alkyl acetates dissociate by loss of CHCOOH. This reaction becomes more pronounced at longer chain lengths, which points to metal ion bidentate formation in [Ag···1-alkene] product ions having a long hydrocarbon chain. In the same vein, the heterodimers [1- hexene···Ag···1-heptene] and [1- heptene···Ag···1-octene] dissociate primarily into [Ag···1-heptene] and [Ag···1-octene] ions, respectively. Hydrocarbon bidentate formation in [Ag···1-octene] also reveals itself by the reluctance of this ion to react with water in an ion trap, as opposed to [Ag···1-hexene] which readily undergoes hydration.

Proton affinities and ion enthalpies

“…In a preliminary experiment we generated the proton bound dimer [CD 3 C(=O)OH]•••H + •••[HC(=O)OCH 3 ] and observed that it dissociated to m/z 64 ([CD 3 C(OH) 2 ] + ) and to m/z 61 ([HC(OH)OCH 3 ] + ) in a ratio of 2.12 ± 0.20. It has been established that in such kinetic method experiments for these classes of compounds, the equation ΔPA (kJ/mol) = 2lnR holds, 16 – 18 where R is the ratio of the product ions, in this case 2.12. This leads to a difference in PAs of acetic acid and methyl formate of only 1.5 ± 0.4 kJ/mol and so the PAs of acetic acid and methyl formate are indeed closely similar.…”

“…butyl acetate]•••Na + •••[iso propyl acetate] using the lnR versus PA curves for the proton bound and sodium bound dimers as references. 17 , 18 Since the slope of the lnR versus PA curve for the sodium bound dimers is only ¼ of the slope of the lnR versus PA curve for the proton bound dimers, 16 – 18 the experimental error for the derived PA of tert butyl acetate is concomitantly larger, see Table 1 .…”

“…Product ion intensities (R) from proton bound heterodimers were obtained at 50% survival yield 21 and the obtained lnR values were plotted against the known PA values of reference ester molecules (methyl acetate and octyl acetate 17 ) to obtain the new PA values. We 16 – 18 and others 22 have observed that the product ion ratio observed in ion trap experiments depends only slightly on the amplitude and so using low amplitudes (corresponding to high survival yields) does not give more accurate relative PA data, but leads to loss of signal strength only. Further details can be found in Jobst et al.…”

“… 17 and Burgers et al. 18 The D-labelled esters CD 3 C(=O)–O–R and CH 3 C(=O)–O–CD 3 were prepared by small scale esterification of CD 3 C(=O)–OH with ROH and of CH 3 C(=O)–OH with CD 3 OH, respectively.…”

“…Collision-induced dissociation (CID) experiments were performed using a Bruker Esquire ESI ion trap mass spectrometer as described in Jobst et al, 16 van Huizen et al 17 and Burgers et al 18 The esters were dissolved in methanol at a concentration of 0.01 M. The proton bound ester dimers were generated by infusion of the ester solutions with an acidified (0.1% trifluoroacetic acid) water/methanol (50/50) mixture in a ratio of 10: 190 at an infusion rate of 240 mL h À1 . The kinetic method 19,20 was employed to determine relative PA values of the esters.…”