Hydration of Alkylammonium Ions in the Gas Phase

Abstract

Electrospray ionization-high-pressure mass spectrometry was used to determine the enthalpy, entropy, and Gibbs free energy for the equilibrium gas-phase ion-molecule association reactions of C m H 2m+1 NH 3 + (m ) 1-8), (CH 3 ) 2 NH 2 + , and (CH 3 ) 3 NH + with one to three water ligands. A subset of these ions has been studied previously, and the thermochemical properties determined in this investigation compare favorably with the previously published results, providing validation of our experimental technique and method. In addition, the present measurements show that the addition of a water ligand to the monohydrated alkylammonium ion results in an increased stabilization in its structure and binding.

Hydration of gas-phase protonated alkylamines, amino acids and dipeptides produced by electrospray

“…A weighted least-squares fit of the plots yields the thermochemical quantities for these systems listed in Table 2. Generally, the hydration enthalpies of the protonated amino acids and dipeptides obtained in the present work are similar to those found for a large number of protonated alkylamines whose values are in the range of 14-16 kcal/mol for the first, and 12-15 for the second water molecule (Table 1) [6,[9][10][11]17]. There is an obvious disagreement between the H • 0,1 and S • 0,1 values obtained in the present work and earlier data reported for ValH + , but good agreement in G • 0,1 ( Table 2) [14].…”

“…These systems were chosen because they have been well characterized by several investigators [9][10][11][12] and the results are of interest in connection with the hydration of the protonated amino acids and peptides. A comparison of the present results with previous measurements using the high-pressure mass spectrometric techniques with pulsed-electron [10,11] and ESI [9] ionization methods is shown in Table 1. The van't Hoff plot for the hydration of C 2 H 5 NH 3 + shown in Fig.…”

“…However, the present ESI source contains two important features which represent a distinct improvement over the others [5,9]: (1) the pressure reducing capillary, PRC (0.5 mm i.d. ; 118 mm long), which introduces the ions generated by electrospray into the forechamber, FC, is resistively heated (80-200 • C) and serves as the desolvation capillary.…”

“…Several laboratories developed mass spectrometric techniques with ESI to study the stepwise hydration of polyatomic ions such as amines and biomolecules under high-pressure equilibrium conditions [5][6][7][8][9]. Thermochemical data on the binding of water molecules to protonated alkylamines [6,[9][10][11][12][13], amino acids [5,14,15], peptides [5,[16][17][18][19] and proteins [7,20] have been reported. This paper describes an ion source in which the ion beam generated by electrospray is pulsed into a temperature-variable reaction chamber.…”

Protonated primary amines induced thymine quintets studied by electrospray ionization mass spectrometry and density functional theory calculations

“…In order to ensure the aforementioned assumption approximately valid, we have to choose those amines with their alkyl chains long enough (containing four or more carbon atoms) to perform the competition experiments. Such a judicious choice is based on both the finding that the primary alkyl amines share a similar solvation energy when the number of carbon in the alkyl chain reaches four or larger, [33] and the notion that the solvation energy of the compound parallels its ion production rate according to the ion evaporation model of ESI, which is mainly accounted for the ionization of the small molecules. [34] In fact, Tang and Kebarle have reported that n-heptylamine and n-decylamine have nearly the same ESI-MS response because of their similar solvation energies.…”