Flowing afterglow spectroscopy

Note The acronyms used here are OSPED (optical spectroscopy in a pulsed electrical discharge), FAMS (flowing afterglow mass spectrometry), SIFT (selected ion flow tube), TRAPI (time-resolved atmospheric pressure ionization mass spectrometry), PHPMS (pulsed high-pressure ionization mass spectrometry), ICRMS (ion cyclotron resonance mass spectrometry), and ADO (averaged dipole orientation collision rate theory). 

Gas-phase transfers of hydride from methoxide to C02, CS2 and S02 have been observed by the flowing afterglow technique (Bierbaum et al., 1984) and by Fourier transform ion cyclotron resonance spectroscopy (FT-ICR) (Sheldon et al., 1985). With aldehydes and ketones, the normal gas-phase reaction with methoxide is enolate formation, but FT-ICR methods have been used to demonstrate reduction of non-enolizable aldehydes including benzaldehyde, pivalaldehyde, and 1-adamantylaldehyde. 

Gas-phase acid-base studies are usually performed by using one of the following techniques high-pressure mass spectrometry (HPMS), chemical ionization mass spectroscopy (CIMS) with mass-analysed ion kinetic energy spectroscopy/collision induced dissociation (MIKES/CID), flowing afterglow (FA) or ion cyclotron resonance (ICR) spectrometry. For a brief description of all methods, Reference 8 should be consulted. 

The electronic structure and gas-phase thermolysis of 4-substituted 3,3,5,5-tetramethyl-3,5-dihydro-4//-pyrazoles has been studied by photoelectron spectroscopy and the first evidence for an alkylideneselenirane was obtained <1996T1965>. The 351.1 nm photoelectron spectrum of the 1-pyrazolide anion has been measured <2006PCA8457>. The 1-pyrazolide ion 29 is produced by hydroxide deprotonation of pyrazole in a flowing afterglow ion source and a small amount of the 5-pyrazolide ion 30 was also detected and studied by photoelectron spectroscopy. 

A number of proton-transfer equilibrium constants for reactions similar to those shown in Eq. (3) have been measured by ion cyclotron resonance, high-pressure mass spectroscopy, flowing afterglow, MIKES, and MIKES/CID techniques. These studies allowed the relative proton affinities of a variety of bases to be determined with an accuracy of better than +0.2 kcal mol" and compared with related thermodynamic data measured in solution. 

Platinum and palladium were among the first metals that were investigated in the molecular surface chemistry approach employing free mass-selected metal clusters [159]. The clusters were generated with a laser vaporization source and reacted in a pulsed fast flow reactor [18] or were prepared by a cold cathode discharge and reacted in the flowing afterglow reactor [404] under low-pressure multicollision reaction conditions. These early measurements include the detection of reaction products and the determination of reaction rates for CO adsorption and oxidation reactions. Later, anion photoelectron spectroscopic data of cluster carbonyls became available [405, 406] and vibrational spectroscopy of metal carbonyls in matrices was extensively performed [407]. Finally, only recently, the full catalytic cycles for the CO oxidation reaction with N2O and O2 on free clusters of Pt and Pd were discovered and analyzed [7,408]. 

In the gas phase, the negative of the enthalpy of the reaction (equation 8) is defined experimentally as the proton affinity (PA) of B, indicated by equation 9. This quantity represents the intrinsic basicity of the base B in the absence of solvent . Experimental techniques for determination of gas-phase proton affinities are ion cyclotron resonance , high-pressure mass spectroscopy ", the flowing afterglow technique and molecular beam experiments ... 

Ion cyclotron resonance (ICR) and flowing afterglow experiments can also be used to derive relative affinities. Neutral beam experiments, where a beam of alkali atoms such as Cs is crossed with a beam of molecules such as PCI3 or (012)2 have been used to derive thermochemistry for anions such as PCli" and Cli", but proper analysis of this type of data is difficult. High-resolution negative ion photoelectron spectroscopy (NIPES) experiments can provide otherwise unobtainable information on hypervalent anions, including precise electron affinities and vibrational frequencies.This technique has limited applicability to hypervalent species with more than three atoms because of vibrational congestion from low-frequency modes. 

Reactions with neutral molecules have been investigated by the flowing afterglow (FA) technique (see [2, 3]) and earlier by ICR spectroscopy. The reaction usually starts with an initial nucleophilic attack of PHg on the neutral, followed by Intramolecular proton transfer and/or expulsion of a neutral fragment [4]. The table on p. 110 lists the rate constants k at 298 K (with an estimated error of 25% for the FA measurements [4]), efficiencies k/k oo (with kADo calculated by the average-dipole-orlentation theory of [5]), products (neutral products were not detected), and branching ratios, k and the branching ratio depend on the total pressure, when adducts are formed. Molecules, for which no reaction could be observed, are listed below the table. 

The flowing afterglow (FA) is a flow reactor tube. Ions are produced by an ion source at the upstream end of the tube. These ions are carried by a buffer gas (He or Ar) and thermahzed to room temperature down the flow tube. On their way down, they react with neutral molecules added downstream in the tube. The (ionic) reaction products can be moiutored in a number of ways, including optical spectroscopy and MS. In the latter case, the resulting swarm of ions is sampled through an orifice into a high-vacuum chamber where they are mass analyzed and detected. 

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