Lee S. Sunderlin

A tandem selected ion flow tube—triple quadrupole instrument

Abstract The design, operation and calibration of a selected ion flow tube (SIFT)—triple quadrupole instrument are described. A detailed examination of the gas-phase reaction between ClCH+2 and CH3Cl has been carried out in order to demonstrate some of the unique experimental capabilities of the new instrument. The primary reactions at room temperature and 0.6 Torr total pressure are thermoneutral Cl-isotope exchange and termolecular association to yield [ClCH2ClCH3]+, with apparent bimolecular rate coefficients of 6.6 x 10−11 and 4.1 x 10−11 cm3 s−1, respectively. Double-labelling experiments with ClCD+2 as the reactant ion identify hydride transfer as the mechanism for the observed Cl-isotope exchange. Collision-induced dissociation (CID) of the addition product yields ClCH+2 with a threshold energy of 31.1 ± 3.0 kcal mol−1. The relative yields of the35ClCH+2 and37ClCH2+ product ions produced by CID of the mass-selected (35Cl,37Cl) isotopomeric adduct have been measured as a function of the CH3Cl concentration in the flow reactor. Analysis of these data with a simple kinetic model indicates that approximately one third of the adduct-forming collisions are accompanied by Cl-exchange via hydride transfer within the collision complex. When the [ClCH2ClCH3]+ ions are formed in the flow tube by a “switching” reaction between ClCH+2(SCO) and CH3Cl, Cl-exchange does not occur, as shown by the complete retention of the original Cl-isotope in the ClCH+2 fragment ion produced by CID.

A new flowing afterglow-guided ion beam tandem mass spectrometer. Applications to the thermochemistry of polyiodide ions

A new flowing afterglow-guided ion beam tandem mass spectrometer has been constructed. The tandem mass spectrometer has a linear quadrupole-octopole-quadrupole geometry. The apparatus has been successfully tested for the measurement of reaction rates and endothermic reaction thresholds. The new instrument has been used to determine 0 K bond strengths in two polyiodide ions: D(I2−I−)=126±6 kJ/mol and D(I2−I3−)=49±6 kJ/mol. These values compare well to recent computational results. Electron affinity (EA)(I3)=4.15±0.12 eV can be derived from this work and values in the literature.

Determination of the H2ONO+2 and CH3O(H)NO+2 bond strenghts and the proton affinities of nitric acid and methyl nitrate

Abstract The binding energies of water and methanol to NO + 2 have been measured to be 14.8 ± 2.3 and 19.2 ± 2.3 kcal/mol, respectively, using energy-resolved collision-induced dissociation of H 2 ONO + 2 and CH 3 O(H)NO + 2 in a flowing afterglow triple quadrupole apparatus. These values are used with literature thermochemistry to derive proton affinities for nitric acid and methyl nitrate; PA(HONO 2 ) = 177.7 ± 2.3 kcal/mol and PA(CH 3 ONO 2 ) = 175.0 ± 2.5 kcal/mol. These results are in good agreement with recent calculations by Lee and Rice, but only the methyl nitrate result is in agreement with experimental results of Cacace and co-workers.

COMBINING ELECTROSPRAY IONIZATION AND THE FLOWING AFTERGLOW METHOD

The design and implementation of a simple electrospray ionization source for a flowing afterglow/triple quadrupole device are described. Ions can be electrosprayed directly into the room temperature flow tube through a heated capillary without the need for differential pumping or ion focusing. Detected ion currents at the detector sampling orifice as high as 3 pA have been achieved, and the mass spectra indicate little or no re-clustering of the desolvated ions with the background solvent vapor in the flow tube. Spatially and temporally resolved ion/molecule reactions of electrosprayed ions can be carried out in the flow reactor under thermal energy conditions. Sufficient ion densities can be achieved for tandem mass spectrometric experiments in the triple quadrupole analyzer, including energy-resolved collision-induced dissociation. Selected chemical applications illustrating these features are described, including proton transfer reactions with aromatic polysulfonate dianions and multiply protonated polypeptides and threshold CID of a doubly charged transition metal coordination complex.

The thermochemistry of Group 15 tetrachloride anions

Bond strengths for a series of Group 15 tetrachloride anions AC14− (A = P, As, Sb, and Bi) have been determined by measuring thresholds for collision-induced dissociation of the anions in a flowing afterglow-tandem mass spectrometer. The central atoms in these systems have ten electrons, which violates the octet rule: the bond dissociation energies for ACl4− help to clarify the effect of the central atom on hypervalent bond strengths. The 0 K bond energies in kJ mol−1 are D(Cl3A-Cr−1) = 90 ± 7, 115 ± 7, 161 ± 8, and 154 ± 15, respectively. Computational results using the B3LYP/LANL2DZpd level of theory are higher than the experimental bond energies. Calculations give a geometry for BiCl4− that is essentially tetrahedral rather than the see-saw observed for the other tetrachlorides. NBO calculations predict that the phosphorus and arsenic systems have 3C-4E bonds, while the antimony and bismuth systems are more ionic.

Proton affinity of SO3

Collision-induced dissociation (CID) of the radical cation H2SO4+ gives the product pairs 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±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±4 kJ/mol at 298 K. Previously, Munson and Smith bracketed the proton affinity as PA(HBr)=584 kJ/mol<PA(SO3)<PA(CO)=594 kJ/mol. The threshold of 152±16 kJ/mol for formation of H2O++SO3 indicates that the barrier to CID is small or nonexistent, in contrast to the substantial barriers to decomposition for H3SO4+ and H2SO4.

Bond strengths in cyclopentadienyl metal carbonyl anions

Abstract Energy-resolved collision-induced dissociation of metal cyclopentadienyl carbonyl anions CpM(CO) x − (Cp c -C 5 H 5 , MV, Cr, Mn, Fe, Co) is used to determine metal–carbonyl bond energies in these systems. These bond energies are, in general, slightly stronger than those for the corresponding homoleptic metal carbonyl anions. The bond strength in CpCo(CO) 2 − , a 19-electron complex, is notably weaker than most of the others. D[CpMn − -CO] is also weak; this is attributed to a mismatch in the electronic ground states of CpMn − and CpMnCO − . D[CpCo − -CO], on the other hand, is substantially larger than the others, and is comparable to the bond energy measured in solution for CpMn(CO) 3 .

THE THERMOCHEMISTRY OF FORMIC ACID-HALIDE ANION CLUSTERS

Abstract Bond dissociation energies for formic acid–halide ion clusters have been measured using energy-resolved collision-induced dissociation in a flowing afterglow-tandem mass spectrometer. The resulting 0 K bond energies (in kJ mol −1 ) are D(HCOOH–X − ) = 114 ± 9, 70 ± 7, and 52 ± 9 for X = Cl, Br, and I, respectively. The second bond energies are D[(HCOOH)X − –HCOOH] = 50 ± 9, 43 ± 7, and 45 ± 9, respectively. These results are compared to empirical correlations, semiempirical calculations, and previously determined values where available.

Bond Strengths in Transition Metal Carbonyl Anions

Translationl energy thresholds for collision-induced dissociative loss of carbonyl ligands from metal carbonyl anions M(CO)n - (M = V, Cr, Mn, Fe, Co, and Ni) have been measured in a flowing afterglow-triple quadrupole apparatus. These thresholds can be used to derive metal-carbonyl bond strengths for these species. The sequential M-CO bond strengths vary widely, emphasizing the importance of measuring sequential rather than average bond strengths. The bond energies can be combined with other thermochemistry to give neutral metal-carbonyl bond strengths, ionization potentials, metal-alkene bond energies, and limits on other metal-ligand bond strengths. The 14- and 16-electron metal carbonyl anions display systematically weaker bonds than the 13-, 15-, and 17-electron species.