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Proton Affinity of SO3

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Proton Affinity of SO3 View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Proton Affinity of SO3 Cynthia Ann Pommerening, Steven M. Bachrach, and Lee S. Sunderlin Department of Chemistry, Northern Illinois University, DeKalb, Illinois, USA ϩ Collision-induced dissociation (CID) of the radical cation H2SO4 gives the product pairs ϩ ϩ ϩ ϩ H2O SO3 and HO HSO3 with a 1:3 ratio that is essentially independent of collision energy. Statistical analysis of the two channels indicates that the proton affinity of HO is 3 Ϯ ϭ Ϯ 4 kJ/mol lower than that of SO3. This can be used to derive PA(SO3) 591 4 kJ/mol at 0 K and 596 Ϯ 4 kJ/mol at 298 K. Previously, Munson and Smith bracketed the proton affinity as PA(HBr) ϭ 584 kJ/mol Ͻ PA(SO ) Ͻ PA(CO) ϭ 594 kJ/mol. The threshold of 152 Ϯ 16 ϩ 3 kJ/mol for formation of H O ϩ SO indicates that the barrier to CID is small or nonexistent, 2 3 ϩ in contrast to the substantial barriers to decomposition for H3SO4 and H2SO4. (JAmSoc Mass Spectrom 1999, 10, 856–861) © 1999 American Society for Mass Spectrometry he development of extensive scales of proton this work provide an independent measurement of the ⌬ affinity (PA), gas basicity (GB), and acidity ( Ha) PA of SO3. values has provided a framework for the quanti- The gas-phase addition of H2OtoSO3 to form T ϭϪ⌬ tative understanding of ion properties. (PA H for sulfuric acid has a substantial barrier, as indicated by addition of a proton, GB ϭϪ⌬G for addition of a reaction rate measurements [4–6] and computational ⌬ ϭ⌬ proton, and Ha H for deprotonation.) The history results [7, 8]. A similar barrier is seen for the reaction of and current status of the PA and GB scales has been protonated water with SO3 [9]. This was determined by reviewed recently [1]. The bulk of the measurements of measuring the activation barrier for the reverse reac- ϩ these properties use equilibrium constants to determine tion, collision-induced dissociation (CID) of H3SO4 , the relative thermodynamics of a pair of compounds. and corroborated by further computational results. The Such measurements are then anchored to absolute present work explores CID of the similar molecule ϩ measurements, which are available for a limited num- H2SO4 to determine the effect of the removal of an ber of compounds. Some molecules are not easily electron on the potential energy surface of sulfuric acid. compared to standard references, but can be studied by less conventional means. An excellent example is the Experimental use of the energetics of decarboxylation reactions to determine the acidities of alkanes, a technique pio- The flowing afterglow tandem mass spectrometer used neered by Graul and Squires [2]. in these experiments consists of an ion source, a flow The proton affinity of sulfur trioxide is not well reactor, and a tandem mass spectrometer comprising a known or easily predicted by comparison with other quadrupole mass filter, an octopole ion guide [10], a molecules. One study [3] has determined the proton second quadrupole mass filter, and a detector. This affinity of SO3 by noting that in an ion source, reaction instrument has been described in detail previously [11]; 1 is fast (although apparently reversible), whereas the a brief description follows. equilibrium for reaction 2 lies to the right. This suggests The ion source used in these experiments is a dc that the proton affinity of SO3 lies discharge that typically operates at 1000 V with 1 mA of emission current. The ions studied in this work were ϩ ϩ 3 ϩ ϩ produced by addition of fuming sulfuric acid to the ion HSO3 CO HCO SO3 (1) source. This produced ions of m/z 98 and 99, which ϩ ϩ ϩ ϩ H Br ϩ SO 3 HSO ϩ HBr (2) correspond to H2SO4 and H3SO4 [9, 12]. Other ions 2 3 3 qualitatively consistent with those seen in the electron impact ionization spectra of sulfuric acid [13] were also between those of HBr (584 kJ/mol) and CO (594 kJ/ seen. Isotope intensity patterns were consistent with mol) [1]. Interpreting the results of such studies is these assignments. difficult because of the uncertain temperature and neu- The flow tube is a 92 cm ϫ 7.3 cm i.d. stainless steel tral concentrations in the ion source. The results from pipe with five neutral reagent inlets. The buffer gas is 95% He and 5% Ar. The pressure in the flow tube is 0.4 torr and the buffer gas flow velocity is 100 m/s, giving Address reprint requests to Lee Sunderlin, Department of Chemistry, 5 Northern Illinois University, DeKalb, IL 60115. E-mail: [email protected] approximately 10 collisions with the buffer gas to In memory of Robert R. Squires, mentor and friend. thermalize the ions. The helium flows through a molec- © 1999 American Society for Mass Spectrometry. Published by Elsevier Science Inc. Received January 14, 1999 1044-0305/99/$20.00 Revised May 10, 1999 PII S1044-0305(99)00055-0 Accepted May 10, 1999 J Am Soc Mass Spectrom 1999, 10, 856–861 PROTON AFFINITY OF SO3 857 ular sieve trap that is cooled by liquid nitrogen to To derive CID threshold energies, the threshold remove condensible impurities. The relative ratio of region of the data is fitted to the model function given ϩ ϩ ␴ H2SO4 and H3SO4 is strongly dependent on the condi- in eq 3 [14], where (E) is the cross section for forma- tion of the flow tube, with wet conditions resulting in tion of the product ions ϩ more H3SO4 (presumably from proton transfer) and ϩ dry conditions resulting in more H SO (presumably ␴͑ ͒ ϭ ␴ ⌺ ͓ ͑ ͒͑ ϩ Ϫ ͒n ͔ 2 4 E 0 i giPD E,Ei E Ei ET /E (3) from direct ionization). Ions are sampled from the flow tube into the main ␴ at CM energy E, ET is the desired threshold energy, 0 chamber, which contains the tandem mass spectrome- is a scaling factor, n is an adjustable parameter, PD is the ter. This chamber is differentially pumped to pressures probability of an ion with a given amount of energy sufficiently low that further collisions of the ions with dissociating within the experimental window (ϳ30 ␮s), the buffer gas are unlikely. The operating conditions for and i denotes vibrational states having energy Ei and the first quadrupole were set to ensure that only ions of population g (⌺g ϭ 1). P and the branching fractions m/z 98 were allowed to pass into the octopole, which i i D for multiple dissociation pathways were calculated us- passes through a gas cell that is filled with argon for the ing the RRKM formalism. The transition states were CID experiments. The intensities of the products and assumed to be at the centrifugal barriers, where most of unreacted ions were measured by the second quadru- the degrees of freedom are equal to those in the pole and the electron multiplier detector. The resolution products. The crunch program written by Armentrout of the second quadrupole was usually left low to and co-workers was used in this threshold analysis; the improve collection efficiency and reduce mass discrim- ϩ statistical modeling procedures have been extensively ination. In some cases, the cross section for H2O was ϩ discussed recently [15]. Rotational energy was handled corrected for overlap with the peak for H3O . This ion is formed by proton transfer to adventitious water in using the equipartitioning approximation [15]. The ef- the gas cell. The cross section for this reaction declines fects of the energy distribution of the ion beam, Doppler Ϫ with translational energy roughly as E 1.3, and this motion of the neutral target gas, and the kinetic energy interference is not significant above 2 eV (1 eV ϭ 96.49 distribution of the reactant ion are accounted for by the kJ/mol). crunch program. The collision gas pressure can influence the observed cross sections because of secondary collisions. This is Threshold Analysis accounted for by linear extrapolation of data taken at The threshold energy for a reaction is determined by several pressures to a zero pressure cross section [16]. modeling the intensity of product ions as a function of The uncertainty in the reaction thresholds because of the reactant ion kinetic energy in the center-of-mass the internal energy of the reactant ions and kinetic shifts (CM) frame, ECM. The translational energy zero of the in the thresholds is estimated by determining the reactant ion beam is measured using the octopole as a threshold with the calculated frequency sets multiplied retarding field analyzer [10, 14]. The first derivative of by 0.9, 1.0, and 1.1. Also, the uncertainty in the energy the beam intensity as a function of energy is approxi- scale is 0.15 eV in the lab frame. The uncertainty mately Gaussian, with a full-width at half-maximum of associated with a factor of 3 change in the 30 ␮s time typically 1.0 eV for these experiments. A small fraction window for dissociation is 0.001 eV. These uncertainties of the ions can be translationally excited in excess of this are combined with the standard deviation of the thresh- distribution by the rf fields of the first quadrupole. This olds derived from different data sets to give the overall results in additional tailing at the lowest energies in the uncertainty in reaction energetics. cross section data.
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