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M. Tabrizchi, S. Shooshtari / J. Chem. Thermodynamics 35 (2003) 863–870
Thermodynamic data for solvent free reactions are required to determine the in-
trinsic reactivity of the substances.
Gas phase basicities and proton affinities have been obtained from measurements
of the equilibrium constants of the proton transfer reaction:
MHþ þ N ꢀ M þ NHþ:
ðIIÞ
The enthalpy change of reaction (II) is referred to as the relative proton affinity.
Absolute proton affinities defined by (I) can be determined from (II) provided a
ÀDH0 of (I) is known for either M or N.
Ion molecule reactions and clustering equilibria have been studied extensively
with high pressure mass spectrometric techniques [5–7]. This method involves gener-
ation and reaction of ions in a field-free high pressure region {(10À2 to 102) Pa} of the
spectrometer and diffusion of the ions through a slit into a low pressure region where
the ions are accelerated and mass determined [8]. When the ratio of the pressure of M
and N is kept constant but the total pressure is increased, the relative intensities of
the MHþ and NHþ ions approach an asymptote. This is interpreted as an indication
that the proton transfer reaction between M and N has reached equilibrium. The
equilibrium constant K for the proton transfer reaction (II) defined by
½Mꢀ½NHþꢀ
K ¼
;
ð1Þ
½Nꢀ½MHþꢀ
is then determined.
If the pressures of M and N are known, then the equilibrium constant of the re-
action can be calculated from the peak area of the ion mass-response and the pres-
sure ratio [8]. Determinations of K over a temperature range can be used to give the
enthalpy change of the reaction which is equal to the difference in proton affinities of
species M and N. Techniques such as ion cyclotron resonance (ICR) spectroscopy
have also been used to study ion–molecule reaction equilibria [4,9].
In Ion Mobility Spectrometry (IMS) [10,11] ions, that are driven by an electric
field through an inert gas at ambient pressure are separated according to their indi-
vidual velocities. We report here, for the first time, the use of an IMS to determine
the relative proton affinities from measurements of ion equilibrium concentration at
atmospheric pressure as a function of temperature.
2. Experimental
The ion mobility spectrometer used in this study was fitted with a corona discharge
ionization source and a detailed description of this instrument can be found elsewhere
[12]. In the present work, the corona needle was at a distance of 1cm from and at an
electric potential of 3.5 kV above the target electrode. The IMS cell was housed in a
thermostated oven in which temperature was controlled within Æ1K. The corona cur-
rent increased with increasing temperature while the potential required for corona
discharge decreased. The ion mobility spectrometer was operated with a field of
3:14V Á mÀ1. An ion injection pulse time of 100 ls was used during the measurements.