The kinetic method of making thermochemical determinations

135

A rigorous analysis of the kinetic method is carried out using Rice-Ramsperger-KasselMarcus (RRKM) theory of microcanonical statistical unimolecular dissociation rates. The model employs a kinetics treatment appropriate for metastable ion dissociation. Protonbound alkoxide dimer anions are used as model systems, with realistic vibrational and rotational parameters calculated by ab initio methods for the cluster ion and transition states leading to the competitive dissociation channels. The numerical simulations show that the kinetic method plots of ln(I 2 /I 1 ) versus ⌬⌬H are nearly linear but can exhibit significant curvature. The apparent entropy obtained in the extended kinetic method is not approximately equal to the thermodynamic entropy difference for dissociation, ⌬⌬S(T), or for activation, ⌬⌬S ‡ (T), either at the effective temperature or at any fixed equilibrium temperature. Instead, the apparent entropy term can be related to the ratio of the microcanonical sum of states of the dissociation transition states for the kinetically selected internal energy of the dissociating ions. (J Am Soc Mass Spectrom 2002, 13, 435-452)

mentioning

confidence: 99%

mentioning

confidence: 99%

Microcanonical analysis of the kinetic method. The meaning of the “apparent entropy”

Ervin

2002

135

“…Various mass spectrometry methods have been applied to probe the interactions between ions and neutral analyte molecules [13][14][15]. These methods typically evaluate enthalpies of ion-molecule reactions or unimolecular dissociations of ionic adducts to derive gasphase thermodynamic properties such as bond energies and ion affinities.…”

mentioning

confidence: 99%

Evaluation of the role of multiple hydrogen bonding in offering stability to negative ion adducts in electrospray mass spectrometry

Cai

Concha

Murray

et al. 2002

An experimental approach, electrospray mass spectrometry (ES-MS), and a theoretical approach employing computer modeling, have been used to characterize the interaction between small inorganic anions and neutral analyte molecules that form anionic adduct species in negative mode ES mass spectrometry. Certain anionic adducts of small saccharides (e.g., ␣-D-glucose, sucrose) have shown exceptional stability in ES mass spectra even when internal energies are raised at high "cone" voltages. Computer modeling studies reveal that multiple hydrogen bonding strengthens the interaction between these neutral molecules and the attaching anion. The equilibrium structures and stabilization energies of these anionic adducts have been evaluated by semi-empirical, ab initio, and density functional theory (DFT) methods. Chloride anion is found to be capable of forming "bridging" hydrogen bonds between monosaccharide rings of polysaccharides resulting in the stabilization of chloride adducts, thus reducing the tendency for the glycosidic bond to decompose. Moreover, the tendency for various hydroxyl hydrogens on saccharide molecules to dissociate in the form of HA ( [5][6][7][8], have been used to generate anionic adducts for ES-MS studies. The formation and decomposition of chloride adducts have been further investigated from a thermodynamic standpoint [9 -12]. However, the conditions that favor the formation and survival of anionic adducts have not been extensively defined and are not fully understood. Moreover, the interactions between neutral analyte molecules and the attaching small inorganic anions have not been fully characterized.Various mass spectrometry methods have been applied to probe the interactions between ions and neutral analyte molecules [13][14][15]. These methods typically evaluate enthalpies of ion-molecule reactions or unimolecular dissociations of ionic adducts to derive gasphase thermodynamic properties such as bond energies and ion affinities. In some cases, the nature of the ion-molecule interaction could be further elucidated by determining the difference in the entropy changes for competing decomposition pathways [16,17]. To date, the vast majority of ion-molecule interaction studies performed by mass spectrometry focus on positive ion studies of cationic adduct species, especially protonated molecules [14] and alkali metal adducts [18 -21]. Dication adduct species, such as alkaline earth metal [22] and transition metal [23] adducts of oligosaccharides have also been explored. Small inorganic anions attaching to neutral analyte molecules, on the other hand, have received far less research attention. However, a few studies employing anion attachment in negative ion ES-MS have appeared, such as reports on iodide attachment to acetone [24], chloride attachment to saccharides [1, 10 -12], and cyanide attachment (including covalent) to fullerenes [6,8]. In order to gain insight into the exact nature of the interactions between neutral analyte molecules and attaching small inorganic anions that are responsi...

“…The effective temperature is an empirical parameter that depends on several experimental variables and properties of the proton-bound dimers [15][16][17][18][19][20]. The term ⌬(⌬S) is the difference of the activation entropies between the two competing dissociation channels, ⌬S (with k) -⌬S (with k i ).…”

Section: Methodsmentioning

confidence: 99%

Determination of the gas-phase acidities of cysteine-polyalanine peptides using the extended kinetic method

Tan

Ren

2007

We determined the gas-phase acidities of two cysteine-polyalanine peptides, HSCA 3 and HSCA 4 , using a triple-quadrupole mass spectrometer through application of the extended kinetic method with full entropy analysis. Five halogenated carboxylic acids were used as the reference acids. The negatively charged proton-bound dimers of the deprotonated peptides with the conjugate bases of the reference acids were generated by electrospray ionization. Collision-induced dissociation (CID) experiments were carried out at three collision energies. The enthalpies of deprotonation (⌬ acid H) of the peptides were derived according to the linear relationship between the logarithms of the CID product ion branching ratios and the differences of the gas-phase acidities. The values were determined to be ⌬ acid H(HSCA 3 ) ϭ 317.3 Ϯ 2.4 kcal/mol and ⌬ acid H (HSCA 4 ) ϭ 316.2 Ϯ 3.9 kcal/mol. Large entropy effects (⌬(⌬S) ϭ 13-16 cal/mol K) were observed for these systems. Combining the enthalpies of deprotonation with the entropy term yielded the apparent gas-phase acidities (⌬ acid G app ) of 322.1 Ϯ 2.4 kcal/mol (HSCA 3 ) and 320.1 Ϯ 3.9 kcal/mol (HSCA 4 ), in agreement with the results obtained from the CID-bracketing experiments. Compared with that in the isolated cysteine residue, the thiol group in HSCA 3,4 has a stronger gas-phase acidity by about 20 kcal/mol. This increased acidity is likely due to the stabilization of the negatively charged thiolate group through internal solvation. (J Am Soc Mass Spectrom 2007, 18, 188 -194)