Studies Of Thermal Ion Molecule Equilibria Determinations Of Gas Phase Basicities And Proton Affinities By High Pressure Mass Spectrometry

Author : Stuart Kenneth Searles
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (144 download)

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Download or read book Mass Spectrometric Investigation of the Equilibrium Gas Phase Solvation Reactions Involving (a) Hydration of the Proton, (b) Hydration of the Potassium Ion, and (c) Solvation of the Ammonium Ion by Ammonia Molecules written by Stuart Kenneth Searles and published by . This book was released on 1968 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: A specially designed mass spectrometer was used to analyze the equilibrium concentrations of gas phase solvated ions arising from: 1. the irradiation of water vapor, 2. the irradiation of ammonia vapor, and 3. the thermal ionization of potassium in water vapor. The analyses proved the occurrence of the general reactions 1-3 respectively. The notation n-l,n is used to denote specific reactions. (For example, reaction 1,2 of the general reaction 1 is H[superscript +](H[subscript 2]O) + H[subscript 2]O [double arrows] H[superscript +](H[subscript 2]O)[subscript 2]). (1) H[superscript +](H[subscript 2]O)[subscript n-1] + H[subscript 2]O [double arrows] H[superscript +](H[subscript 2]O)[subscript n] n=2-7 (2) NH[subscript 4][superscript +](NH[subscript 3])[subscript n-1] + BG[subscript 3] [double arrows] NH[subscript 4[superscript +(NH[subscript 3])[subscript n] n=1-6 (3) K[superscript +](H[subscript 2]O)[subscript n-1] + H[subscript 2]O [double arrows] K[superscript +](H[subscript 2]O)[subscript n] n=1-6. Measurement of the gaseous solvent pressure led to determination of the equilibrium constants K[subscript n-1,n] for reactions 1-3. Typically the pressure was varied from 0.2 to 3 torr to prove the necessary condition that K[subscript n-1,n] is pressure independent. A temperature study of the reactions over the range -60° to 600°C allowed determination of AG°[subscript n-1,n], [triangle]H°[subscript n-1,n], and [triangle]S°[subscript n-1,n] from van't Hoff type plots. With one exception these values monotonically increased with increasing cluster size. The ranges for AH°[subscript n-1,n] (298°K, 1 atm) were -36 to -11.7, -27 to -7.5, and -17.9 to -10.0 kcal/mole for reactions 1-3 respectively. The experimental entropy values for [triangle]S°[subscript 1,2] for reaction 1 and [triangle]S°[subscript 0,1] for reactions 2 and 3 were in agreement with those calculated from statistical mechanics. The values of [triangle]H°[subscript n-1,n] for reaction 3 were in approximate n-l,n agreement with the theoretical values obtained from classical electrostatic calculations. These calculations took into account ion-dipole, ion-polarizability, ion-atom repulsion, and dipole-dipole repulsion energy terms. The calculations were performed with the assumption that all K[superscript +] -- H[subscript 2]O distances are equal, i.e. the cluster has a single shell of water molecules. The conclusion from these calculations was that the increase in dipole-dipole repulsion energy with cluster size primarily accounted for the increase in AH°[subscript n-1,n] with increasing n. A special electrostatic n-l,n calculation indicated that the formation of an outer shell of water molecules becomes energetically favorable for reaction 3 with n = 7 i.e. reaction 6,7. The clusters NH[subscript 4][superscript +](NH[subscript 3])[subscript n] and H[superscript +](H[subscript 2]O)[subscript n] are more complicated since these clusters could also be H[superscript +](NH[subscript 3])[subscript n+1] and H[subscript 3]O[superscript +](H[subscript 2]O)[subscript n-1] respectively. The smooth change of the thermodynamic values with n for reaction 1 was interpreted to mean that the appropriate formula for the hydrates is H[superscript +](H[subscript 2]O)[subscript n]. In contrast to the thermodynamic values for H[superscript +](H[subscript 2]O)[subscript n], the results for NH[subscript 4][superscript +](NH[subscript 3])[subscript n] had a break in the thermodynamic 4 3 n values. The break appeared between reactions 3,4 and 4,5 as as a sharp increase in the values. This suggested the existence of an inner cluster NH[subscript 4][superscript +](NH[subscript 3])[subscript 4]. Since a more distantly bound ammonia molecule will be more weakly bound, the values of [triangle]H°[subscript 4,5] and [triangle]S°[subscript 4,5] are greater. From the present results, some comparisons are made to ionic solvation in the liquid phase. With the experimental value of AH°[subscript 0,1] for reaction 3 and known thermodynamic values, the proton affinity of KOH was calculated to be 262 kcal/mole.