James T. Francis

Experimental and theoretical studies of the (C 1s−1,π*)3Π state of CO: Momentum transfer dependence and vibrational structure

The intensity of the X 1Σ+→(C 1s−1,π*)3Π transition of CO has been measured by electron energy loss spectroscopy using a range of scattering angles (0°–45°) and impact energies (376 to 1806 eV) in order to investigate the momentum transfer dependence of a spin forbidden inner‐shell excitation. A Franck–Condon factor analysis of the vibrational structure of the singlet and triplet (C 1s−1,π*) states was used to quantify differences in the potential energy curves of these states. Ab initio self‐consistent field configuration interaction (SCF‐CI) calculations were carried out to generate the potential curves of the 1Π and 3Π(C 1s−1,π*) states. The electronic and vibrational energies and Franck–Condon factors are in good agreement with the experimental results. The calculations indicate that the difference in the 1Π and 3Π potential curves are related to differences in relaxation of both the (active) π* and other (passive) valence electrons.

Generalized oscillator strengths for inner-shell excitation of SF6 recorded with a high-performance electron energy loss spectrometer

The generalized oscillator strength profiles for S 2p, S 2s and F 1s excited and ionized states of sulfur hexafluoride (SF6) are reported up to very high momentum transfer. These have been measured with a variable impact energy, variable scattering angle electron energy loss spectrometer, which is dedicated to studies of electric dipole (optically allowed) and non-dipole (optically forbidden electric quadrupole and spin-exchange) inner shell electronic transitions of gases, and systematic measurements of their angular and impact energy distributions in order to derive generalized oscillator strength profiles. In addition to presenting the SF6 results, we describe the design, construction and performance of the instrument, as well as data acquisition and analysis procedures. ” 2000 Published by Elsevier Science B.V. All rights reserved.

The intriguing behaviour of (ionized) oxalacetic acid investigated by tandem mass spectrometry

Abstract Tandem mass spectrometry based experiments on commercial oxalacetic acid (OAA) samples confirm the indirect evidence from the elegant study of Flint et al. (J. Org. Chem. 57 (1992) 7270) that solid OAA exists solely in the (Z)-enol form of HOOCC(H)C(OH)COOH. It is further shown that: (1) the samples contain a minor impurity in the low percent range, assigned as a dehydration product of 4-hydroxy-4-methyl-2-ketoglutaric acid; (2) careful evaporation of solid OAA samples yields mass spectra representative of the (Z)-enol form. The (E)-enol is not present in the solid, nor is its ion generated in the gas phase via isomerization of the ionized (Z)-enol form. However, even under gentle sample introduction conditions, a partial ketonization of the neutral (Z)-enol takes place. The resulting keto OAA molecules are either ionized intact or decarboxylate to yield a mixture of α-hydroxyacrylic acid, CH2C(OH)COOH and its keto isomer pyruvic acid, CH3C(O)COOH. From computational quantum chemistry (CBS-4) and experimental data, ionic enthalpies of formation of 95 and 109 kcal/mol were derived for CH2C(OH)COOH·+ and CH3C(O)COOH·+, respectively; and (3) under less controlled sample introduction conditions, thermal dehydration may take place to yield C4H2O4 molecules whose mass spectral characteristics are indistinguishable from those of ionized hydroxymaleic anhydride.

Electron impact core excitation of molecules: non-dipole spectroscopy and generalized oscillator strengths

Abstract A status report on the field of non-dipole core excitation electron energy loss spectroscopy is given together with a summary of recent studies of S2 s generalized oscillator strengths (GOS) of SF 6 and C1 s excitation in isomeric xylenes.

Reactions of metastable ionized γ, γ-dialkylallyl methyl ethers: relative rates for elimination of alkyl radicals by formal γ-cleavage

Abstract The reactions of seven metastable ionised allylic alkenyl methyl ethers of general structure, R1R2C= CHCH2OCH3+ [R1=CH3, R2=C2H5, n-C3H7, n-C6H13, or iso-C3H7; R1=C2H5, R2=n-C3H7 or iso-C3H7; R1 = n-C3H7, R2 = n-C4H9] with two different γ-alkyl substituents are reported and discussed. Two common reactions are observed: loss of a molecule of methanol or elimination of an alkyl radical. The second fragmentation is interpretable by a mechanism in which two consecutive 1,2-hydrogen shifts lead to an ionised enol ether, R1R2CHCH  CHOCH3+, which then undergoes γ-cleavage to give either R1CH  CHCH  O+CH3 or R2CH  CHCH  O+CH3. Competition experiments reveal that the relative rate of alkyl radical loss from these metastable radical-cations follows the order (CH3)2CH· > CH3CH2· > CH3CH2CH2· ≥ CH3CH2CH2CH2·⪢ CH3· Metastable ionised ethers containing a γ-isopropyl group differ from their isomers with an n-propyl substituent in expelling propane at an appreciable rate. A unique process apparently involving loss of a neutral with a mass of 74 Da is observed from metastable CH3CH2CH2CH2CH2CH2(CH3)C  CHCH2OCH3+, this fragmentation is attributed to consecutive elimination of two species, C3H6 followed by CH3OH, rather than direct expulsion of C4H10O.

The chemistry of ionized ethyl glycolate, HOCH 2 CO 2 C 2 H 5 •+ , and its enol isomer, HOCH=C(OH)OC 2 H 5 •+ . Formation of ionized and neutral trihydroxyethylene

The unimolecular reactions of ionized ethyl glycolate, HOCH2CO2C2H5•+, and its enol isomer, HOCH=C(OH)OC2H5•+, have been studied by a variety of techniques including 2H-, 13C- and 18O-labelling experiments, kinetic energy release information, analysis of collision-induced dissociation and neutralization–reionization mass spectra and thermochemical measurements. The metastable ionized enol eliminates C2H4 essentially exclusively by β-hydrogen transfer to give (HO)2C=CHOH•+, which itself expels H2O with very high selectivity (∼99%). The metastable ionized keto isomer also eliminates C2H4, but minor amounts (∼5%) of competing fragmentations resulting in expulsion of H2O, HOCO• and HCO• are also observed. Moreover, in this case, loss of C2H4 no longer involves specific β-hydrogen transfer. This behavior is interpreted in terms of rearrangement of the ionized keto form via a 1,5-H shift to the distonic ion HOCH2C+(OH)OCH2CH2•, which undergoes a further 1,5-hydrogen shift to form HOCH=C(OH)OC2H5•+, from which C2H4 is lost to give (HO)2C=CHOH•+. The chemistry of these C4H8O3•+ species, in which unidirectional tautomerism of HOCH2CO2C2H5•+ to HOCH=C(OH)OC2H5•+ via two 1,5-H shifts is important, contrasts sharply with the behavior of the lower homologues, HOCH2CO2CH3•+ and HOCH=C(OH)OCH3•+, for which the analogous tautomerism via 1,4-H shifts does not occur. The mechanism for loss of C2H4 from HOCH=C(OH)OC2H5•+ is essentially the same as that observed for ionized phenyl ethyl ether in that the reaction proceeds via an exothermic proton transfer in an ethyl cation–radical complex. Neutralization–reionization experiments show that neutral trihydroxyethylene in the gas phase is a remarkably stable species which does not tautomerize to glycolic acid, HO–CH2–COOH. Using G2(MP2) theory, the enthalpy of formation, ΔHf298, of the neutral molecule (most stable conformer) was found to be −449.4 kJ mol−1 while that of the corresponding ion was calculated to be 299.6 kJ mol−1.