Kinetic Isotope and Collision Energy Effects in the Dissociation of Chloride and Bromide Adducts of Aliphatic Alcohols, Benzaldehyde, and 2,4-Pentanedione

Abstract

A tandem-in-space triple quadrupole mass spectrometer was used to measure kinetic isotopic effects (KIEs) for the dissociation of chloride and bromide adducts of several compounds that bind halide anions via either hydrogen bonds or by nucleophilic attachment. Two isotopomers of each adduct were simultaneously mass-selected in the first quadrupole and dissociated by collision with argon in the second quadrupole. The KIEs were measured by comparing the extents of dissociation of the lighter versus the heavier isotopomeric adducts. In most cases, lower collision energies and multiple collision conditions favoured larger KIE values, an expected feature of easily dissociated cluster ions considering zero-point energies (ZPEs). The larger chloride adduct of cyclohexanol gave greater KIEs compared with the smaller alcohols, a consequence of slower dissociation due to the larger number of degrees of freedom. Dissociation of the chloride adducts gave greater KIEs than the corresponding bromide adducts, a result that is also consistent with expectations based on ZPEs. Both the chloride and bromide adducts of 2,4-pentanedione, when dissociated at 6 eV collision energy under single-collision conditions, displayed normal KIEs (1.0460 ± 0.0012 and 1.0092 ± 0.0035 respectively). These and the alcohol results were correctly predicted by the ZPEs calculated using commonly applied ab initio Hartree–Fock (HF) and B3LYP density functional theory (DFT) methods with large basis sets (6–311 containing both polarization and diffuse functions). Geometry optimization calculations for the 2,4-pentanedione chloride adduct using either the Restricted Hartree–Fock (RHF) method with a 6–31G* basis set or using the more accurate 6–31++G** method showed that, in the most stable form, the chloride is bonded at multiple sites by a molecule of 2,4-pentanedione. In this structure, chloride binds weakly to both the methylene and the methyl hydrogen atoms. Collision-induced dissociation furnishes chloride and 2,4-pentanedione anion ([M – H]^–) as competitive negatively charged products, which is consistent with the proposed structure. It is interesting that the intermolecular KIEs in this study tend to be normal, while intramolecular isotope effects in halides, notably of the type M₁Cl⁺M₂ are inverse, as a consequence of the lower ZPEs associated with the heavier isotopomers. The difference in the two systems is that the stronger bonds are found in the products in the case of M₁Cl⁺M₂ dissociation but in the reactants in the case of MCl^– dissociation.