Relative Proton Affinities from Kinetic Energy Release Distributions for Dissociation of Proton-Bound Dimers

Comparison of the electrospray ionization (ESI) responses of penicillins with ESI responses of their methanolysis products

The electrospray ionization (ESI) responses, defined as the area of chromatographic peak of ion [M+H]⁺ obtained upon HPLC/ESI-MS analysis, of three β-lactam antibiotics, namely penicillin G, ampicillin and carbenicillin have been compared with the ESI responses of their methanolysis products. It has been found that methanolyzed penicillin G has much higher ESI response than the penicillin G. Methanolyzed ampicillin also has higher ESI response than ampicillin; however, the effect is less pronounced than for penicillin. Methanolyzed carbenicillin does not have pronouncedly higher ESI response than carbenicillin.

IR Spectroscopy of Protonated Acetylacetone and Its Water Clusters: Enol-Keto Tautomers and Ion→Solvent Proton Transfer

Protonated ions of acetylacetone, H⁺(Hacac), and their argon-tagged analogues are produced via a pulsed discharge and cooled in a supersonic expansion. These ions are mass analyzed, selected in a time-of-flight spectrometer, and studied with infrared laser photodissociation spectroscopy using the method of rare-gas atom tagging. Computational studies at the DFT/B3LYP level are employed to elucidate the structures and spectra of these ions, which are expected to exist as either enol- or keto-based tautomers. The protonated acetylacetone ion is found to form a single enol-based isomer. Adding one or two water molecules to this ion, for example, H⁺(Hacac)(H₂O)_1,2, produces primarily enol-based structures, although a small concentration of keto structures also contribute to the spectra. The vibrational patterns resulting from hydrogen bonding in these systems are not well-described by theory. Addition of a third water molecule to form the H⁺(Hacac)(H₂O)₃ ion causes a significant change in the spectroscopy, attributed to proton transfer from the H⁺(Hacac) ion into the water solvent.

Communication: Trapping a proton in argon: Spectroscopy and theory of the proton-bound argon dimer and its solvation

Ion-molecule complexes of the form H⁺Ar_n are produced in pulsed-discharge supersonic expansions containing hydrogen and argon. These ions are analyzed and mass-selected in a reflectron spectrometer and studied with infrared laser photodissociation spectroscopy. Infrared spectra for the n = 3-7 complexes are characterized by a series of strong bands in the 900-2200 cm^-1 region. Computational studies at the MP2/aug-cc-pVTZ level examine the structures, binding energies, and infrared spectra for these systems. The core ion responsible for the infrared bands is the proton-bound argon dimer, Ar-H⁺-Ar, which is progressively solvated by the excess argon. Anharmonic vibrational theory is able to reproduce the vibrational structure, identifying it as arising from the asymmetric proton stretch in combination with multiple quanta of the symmetric argon stretch. Successive addition of argon shifts the proton vibration to lower frequencies, as the charge is delocalized over more ligands. The Ar-H⁺-Ar core ion has a first solvation sphere of five argons.

Microcanonical Modeling of the Thermokinetic Method

A microcanonical analysis of the thermokinetic method is performed using statistical rate calculations based on orbiting transition state theory in order to model a proton transfer process: MH(+) + B(i) --> M + B(i)H(+). The reaction efficiency is calculated as a function of the difference in zero point energy of reactants and products. Several models of reactions were investigated in order to simulate situations where the base of interest M exhibits loss of entropy upon protonation of up to approximately 40 J mol(-1) K(-1). It is shown that the standard thermokinetic method would predict correct 298 K gas phase basicities, GB(298)(M), even for polydentate molecules M, if experiments are conducted at this temperature. Proton affinity, PA(298)(M), and protonation entropy may be obtained by the thermokinetic method only in special circumstances such as, for example, experiments conducted at various temperatures.

Kinetic energy release of protonated methanol clusters using the low-temperature fast-atom bombardment: experiment and theory combined

Low-temperature fast-atom bombardment was found to be an excellent method for generating large protonated methanol clusters, (CH(3)OH)(n)H(+) (n = 2 to 15). Metastable dissociations of these clusters, involving elimination of one methanol molecule, were studied using mass-analyzed ion kinetic energy spectra (MIKES). From metastable peak profiles kinetic energy release (KER) distributions were obtained, even for clusters as large as (CH(3)OH)(15)H(+). The results were analyzed by a simple thermal model, by the finite heat bath theory (FHBT) and by the RRKM-based MassKinetics algorithm. The KER distribution was shown to correspond to a three-dimensional translational energy distribution, implying statistical energy partitioning in the transition state. The mean KER values and transition state temperatures were found to increase with cluster size, reaching 25 meV and approximately 210 K for large clusters (n = 10).

Entropy evaluation using the kinetic method: is it feasible?

The kinetic method is one of the most widely used experimental techniques for the measurement of thermochemical parameters by mass spectrometry. Recently it has been realized that it can also be used to determine reaction entropies, but the validity of this approach has not been established. This Perspective evaluates kinetic method plots in cases where there is a significant entropy difference between the competing fragmentation channels (i.e. between sample and reference compounds in the dissociating cluster ion). The concept underlying this study is to calculate mass spectra theoretically, based on known thermochemical parameters and as a function of experimental conditions. This can be done accurately using the RRKM-based MassKinetics software. The resulting mass spectra are then interpreted by the kinetic method, yielding DeltaH and DeltaS values. These values are, in turn, compared with the true values used to generate the calculated mass spectra. The results show that the reaction entropy difference between sample and reference has a very large influence on kinetic method plots. This should always be considered when studying energy-dependent mass spectra (using metastable ions or low- or high-energy collision-induced dissociation (CID)), even if only DeltaH is to be determined. Kinetic method plots are not strictly linear and this becomes a serious issue in the case of small molecules showing a large entropy effect. In such cases, results obtained at a low degree of excitation are more accurate. Energy and entropy effects can be evaluated in a relatively straightforward manner: first, the apparent Gibbs energy (DeltaG(app)) and effective temperature (T(eff)) are determined from kinetic method plots (intercept and slope, respectively), obtained from experiments using various degrees of excitation. Second, the resulting DeltaG(app) is plotted against T(eff), the slope yielding DeltaS while the intercept (extrapolation to zero temperature) yields DeltaH. This data evaluation yields more accurate results than alternative methods used in the literature. The resulting DeltaH values are fairly accurate, with errors, in most cases, <4 kJ mol(-1). On the other hand, DeltaS is systematically underestimated by 20-40%. Empirically scaling DeltaS values determined by the kinetic method by 1.35 results in a DeltaS value within 20% (or 10 J mol(-1) K(-1)) of the theoretical value.