Determination of the gas-phase acidity of methylthioacetic acid using the Cooks’ kinetic method

Ren, Jianhua; Patel, Chirag

doi:10.1016/j.jasms.2004.12.012

Cited by 5 publications

(6 citation statements)

References 38 publications

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“…We tested the suitability of the computational procedure by calculating the enthalpies of deprotonation of CH 3 NHCH 2 CO 2 H (sarcosine) and CH 3 SCH 2 CO 2 H (MTA). The calculated values for sarcrosine (339.3 kcal/mol) and MTA (338.9 kcal/mol) agree well with the experimental values obtained from the literature, 341.5 ± 2.1 kcal/mol , and 340.0 ± 1.7 kcal/mol, respectively. The calculated theoretical enthalpies of deprotonation of the series of carboxylic acids are summarized in Table .…”

Section: Computational Resultssupporting

confidence: 87%

“…The uncertainty is ±1.9 kcal/mol for the expt value. c Obtained from ref . d Obtained from refs and .…”

Section: Computational Resultsmentioning

confidence: 99%

“…The values of Δ H acid and Δ G acid are obtained as the ratios of the negative intercepts to the slopes. The entropy contribution (the differences in activation entropies between the two dissociation channels) can be very small and negligible if structurally similar carboxylic acids are used as the reference acids and the structures of the reference acids are also similar to the unknown acid. − In our previous studies of the gas-phase acidity of methylthioacetic acid, we found that the entropy contribution was negligible by using a set of acetic acid derivatives as reference acids . Other studies of the proton affinities of nitriles using a flowing afterglow-triple quadrupole instrument also show small entropy effects (0.6−1.5 kcal/mol) providing that a suitable set of reference molecules are chosen. , …”

Section: Experimental Methodsmentioning

confidence: 90%

“…[32][33][34][35][36] In our previous studies of the gasphase acidity of methylthioacetic acid, we found that the entropy contribution was negligible by using a set of acetic acid derivatives as reference acids. 37 Other studies of the proton affinities of nitriles using a flowing afterglow-triple quadrupole instrument also show small entropy effects (0.6-1.5 kcal/mol) providing that a suitable set of reference molecules are chosen. 38,39 The experiments with the kinetic method were carried out at room temperature using a flowing afterglow-triple quadrupole apparatus that has been described extensively elsewhere.…”

Section: Gas-phase Acidity Measurementmentioning

confidence: 99%

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Polar Group Enhanced Gas-Phase Acidities of Carboxylic Acids: An Investigation of Intramolecular Electrostatic Interaction

Ren

2006

J. Phys. Chem. A

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We studied the effects of polar groups on the gas-phase acidities of carboxylic acids experimentally and computationally. In this connection, the gas-phase acidities (DeltaH(acid), the enthalpy of deprotonation, and DeltaG(acid), the deprotonation free energy) of borane-complexed methylaminoacetic acid ((CH(3))2N(BH(3))CH(2)CO(2)H) and methylthioacetic acid (CH(3)S(BH(3))CH(2)CO(2)H) were measured using the kinetic method in a flowing afterglow-triple quadrupole mass spectrometer. The values of DeltaH(acid) and DeltaG(acid) of (CH(3))2N(BH(3))CH(2)CO(2)H were determined to be 328.8 +/- 1.9 and 322.1 +/- 1.9 kcal/mol, and those of CH(3)S(BH(3))CH(2)CO(2)H were determined to be 325.8 +/- 1.9 and 319.2 +/- 1.9 kcal/mol, respectively. The theoretical enthalpies of deprotonation of (CH(3))2N(BH(3))CH(2)CO(2)H (329.2 kcal/mol) and CH(3)S(BH(3))CH(2)CO(2)H (325.5 kcal/mol) were calculated at the B3LYP/6-31+G(d) level of theory. The calculated enthalpies of deprotonation of N-oxide-acetic acid (CH(3)NOCH(2)CO(2)H, 329.4 kcal/mol) and S-oxide-acetic acid (CH(3)SOCH(2)CO(2)H, 328.6 kcal/mol) are comparable to the experimental results for borane-complexed methylamino- and methylthioacetic acids. The enthalpy of deprotonation of sulfone-acetic acid (CH(3)SO2CH(2)CO(2)H, 326.1 kcal/mol) is about 2 kcal/mol lower than the S-oxide-acetic acid. The calculated enthalpy of deprotonation of sulfoniumacetic acid, (CH(3))2S+CH(2)CO(2)H, is 243.0 kcal/mol. Compared to the corresponding reference molecules, CH(3)NHCH(2)CO(2)H and CH(3)SCH(2)CO(2)H, the dipolar group and the monopolar group substituted carboxylic acids are stronger acids by 11-14 and 97 kcal/mol, respectively. We correlated the changes of the acidity upon a polar group substitution to the electrostatic free energy within the carboxylate anion. The acidity enhancements in polar group substituted carboxylic acids are the results of the favorable electrostatic interactions between the polar group and the developing charge at the carboxyl group.

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Section: Computational Resultssupporting

confidence: 87%

“…The uncertainty is ±1.9 kcal/mol for the expt value. c Obtained from ref . d Obtained from refs and .…”

Section: Computational Resultsmentioning

confidence: 99%

Section: Experimental Methodsmentioning

confidence: 90%

Section: Gas-phase Acidity Measurementmentioning

confidence: 99%

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Polar Group Enhanced Gas-Phase Acidities of Carboxylic Acids: An Investigation of Intramolecular Electrostatic Interaction

Ren

2006

J. Phys. Chem. A

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“…For example, 'proton affinity', 'gas phase basicity' and 'gas phase acidity' values that are typically estimated by Cooks' kinetic method are considered as indicators of degree of ionization of an analyte [1]. Cooks' kinetic method involves a standard reference compound (SRC), wherein proton bound dimer formed by the analyte under study and the SRC, would be subjected to collision induced dissociation (CID) and the abundances of resulting product ions are used to estimate proton affinity or gas phase acidity of the analyte of interest [1][2][3]. In this study, a new parameter called 'molecular ESI index (MESII)' is defined, which also can be useful to comprehend, how different analytes respond to ESI.…”

Section: Introductionmentioning

confidence: 99%

Understanding the Response of the Molecule to Electrospray Ionization (ESI) by Selected Reaction Monitoring (SRM) Tandem Mass Spectrometry: Defining Molecular ESI Index (MESII)

Guleria¹,

Pasha²,

Makharia³

2015

Mass Spectrom Purif Tech

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In an attempt to understand the response of the molecule to electrospray ionization (ESI), a new parameter called 'molecular electrospray ionization index (MESII)' is defined. Selected reaction monitoring (SRM) data acquired on an ESI tandem quadrupole mass spectrometer is utilized for calculating MESII, which we denote as ε SRM : ε SRM =-log (SRM intensity/Population of ions or molecules in-solution). Population of ions or molecules in-solution is estimated using molecular mass of the respective compound and Avogadro's constant. Simvastatin acid (SVA), lovastatin (LV) and simvastatin (SV) are chosen as model compounds. SRM experiments in positive ion mode were performed on singly protonated ions ([M+H] +) of SVA, LV and SV. In negative ion mode, only SVA was investigated by SRM, using singly deprotonated ion ([M-H]-) as precursor ion. Thus estimated MESII values in positive ion mode are: ε + SRM (SVA)=7.4288, ε + SRM (LV)=7.4541 and ε + SRM (SV)=8.6833 and in negative ion mode, ε-SRM (SVA)=7.2253. This newly defined index not only gives an idea about ionization potential (i.e., degree of ionization), but can also be an indicator of limit of detection (LOD) of an analyte. When utilizing SRM data recorded from different type of instruments for a compound, the variations that may arise in MESII values could help in understanding the influence of different instrument configurations/methods on the degree of ionization of that compound. Matrix effects on the extent of analyte's ionization too can be understood from the differences in the MESII values. Further, it may be possible to utilize MESII for quantitation as well.

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Molecular recognition by mass spectrometry

Tullio¹,

Reale²,

Angelis³

2005

J. Mass Spectrom.

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A recent major advance in the field of mass spectrometry in the biomolecular sciences is represented by the study of the supramolecular interactions among two or more partners in the gas phase. A great deal of chemistry and most of biochemistry concerns molecular interactions taking place in solution. The electrospray technique, which allows direct sampling from solution, and soft ionization of the solute without deposition into the analyte of large amounts of energy, guarantees in many cases the survival of noncovalent bondings and, hence, the direct analysis of the supramolecular complexes present in the condensed phase. The proper preparation of the solution to be studied and also the expert and accurate setting and use of the instrumental parameters are the prerequisites for gaining results as to the specific interactions between, for instance, a protein conformationally modified by its specific metal ion, eventually, and a ligand molecule. The analysis of the charge state of the protein itself and of the modifications of the complex integrity by activating collisions are also methods for studying the biomolecule-molecule interactions. Accordingly, this new mass spectrometric approach to the supramolecular chemistry, which could be also defined as 'supramolecular mass spectrometry', allows the study of ion-protein, protein-protein, protein-ligand and DNA-drug interactions. Chiral recognition can also be performed in the gas phase, studying by electrospray mass spectrometry the fragmentation of diastereomeric complex ions. Not the least, a deep insight can also be obtained into the formation and nature of inclusion complexes like those formed with crown ethers, cyclodextrins and calixarenes as host molecules. All these topics are treated to a certain extent in this special feature article.

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Determination of the gas-phase acidity of methylthioacetic acid using the Cooks’ kinetic method

Cited by 5 publications

References 38 publications

Polar Group Enhanced Gas-Phase Acidities of Carboxylic Acids: An Investigation of Intramolecular Electrostatic Interaction

Polar Group Enhanced Gas-Phase Acidities of Carboxylic Acids: An Investigation of Intramolecular Electrostatic Interaction

Understanding the Response of the Molecule to Electrospray Ionization (ESI) by Selected Reaction Monitoring (SRM) Tandem Mass Spectrometry: Defining Molecular ESI Index (MESII)

Molecular recognition by mass spectrometry

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