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.

β-Hydrogen Elimination of Ionic Butylzinc Complexes

“…A comparison of the present value with KIEs reported for β-H eliminations of deuterated transition-metal complexes is complicated by the fact that the latter almost completely refer to inter molecular assays. Intra- and intermolecular KIEs are not necessarily equivalent, however …”

Gas-Phase Halide Affinity of Aliphatic Alcohols Estimated by the Kinetic Method

“…The data in the figure suggests that CH 3 OH possesses a higher fluoride affinity than its isotopologue CD 3 OH, a result that corresponds to a normal secondary kinetic isotope effect (KIE). 18,[60][61][62][63][64] A value for the KIE of 1.3 was calculated from the ratio of the abundances of the product ions ([CH 3 OH + F] -/ [CD 3 OH + F] -). Based on this result and supposing an effective temperature of 437 K for the dissociation of such fluoride dimer, the fluoride affinity of CD 3 OH was calculated and found to be 94.5 kJ mol -1 .…”

Energy‐dependent normal and unusually large inverse chlorine kinetic isotope effects of simple chlorohydrocarbons in collision‐induced dissociation by gas chromatography‐electron ionization‐tandem mass spectrometry

“…Extremely large Cl-KIEs of CHCl4 − during CID were observed and inferred to be owing to the near-threshold centrifugal effect [14], while Petersen et al concluded the Cl-KIEs were caused by symmetry-induced degeneracy of vibrational modes [19]. Augusti et al observed large normal and inverse Cl-KIEs of chloride adducts of aliphatic alcohols, benzaldehyde, and 2,4-pentanedione during CID [16], and deduced that the Cl-KIEs were triggered by differences of zero-point energies (∆ZPEs) between isotopomers, which was in agreement with the conclusions of Gozzo et al [15]. However, Lehman et al indicated that KIEs in CID could not ascribe to ∆ZPEs based on the observed KIEs of diatomic ions [20].…”

“…By means of MS, KIEs can be readily determined and applied to probing reaction mechanisms of molecules and ions in gaseous phase [8][9][10]. So far, most studies relevant to on-MS KIEs focused on hydrogen/deuterium (H/D) KIEs [5,[11][12][13], whereas a few concerned heavy-atom KIEs such as chlorine KIEs (Cl-KIEs) [14][15][16].…”

“…Cl-KIEs can take place in electron ionization-MS (EI-MS) during both ionization and metastable-ion dissociation [17]. In addition, some studies have reported the Cl-KIEs of some mono-chlorinated organic compounds [18], chloroform [19], tetrachloromethane [20], and chloride adducts of some chlorinated/non-chlorinated organic compounds [14,16,19] taking place during CID in MS/MS. Both normal and inverse Cl-KIEs were observed in these studies with very large magnitudes in comparison with those during reactions in solution.…”