Waltraud Zummack

HCSiF and HCSiCl: The First Detection of Molecules with Formal C≡Si Triple Bonds

Neutralization of [C,H,Si,X]  .+ radical cations (X=F, Cl) in conjunction with electronic structure calculations provides the first experimental evidence for the formation of the neutral silynes HC≡SiF and HC≡SiCl, which have nonlinear structures (see picture).

Generation of α-acetolactone and the acetoxyl diradical •CH2COO• in the gas phase

Abstract The formation of neutral [C 2 ,H 2 ,O 2 ] has been investigated by tandem mass spectrometry in a sector instrument and by energy-resolved collision-induced dissociation in a flowing afterglow-triple quadrupole apparatus. The neutral species are generated by two different methods: (i) neutralization of the distonic anion radical • CH 2 COO − by collisional electron detachment and (ii) collision-induced loss of halides X − concomitant with formation of [C 2 ,H 2 ,O 2 ] from α-haloacetate ions XCH 2 COO − . The tandem mass spectrometry results suggest that neutralization of • CH 2 COO − and high-energy collisional activation of α-haloacetate ions lead to a mixture of α-acetolactone, c -(CH 2 C(O)O), and the acetoxyl diradical, • CH 2 COO • . Low-energy collisions with α-chloroacetate ions in the triple quadrupole analyzer produce α-acetolactone exclusively at the dissociation threshold. From the dissociation threshold measured for the appearance of Cl − from ClCH 2 COO − the heat of formation of acetolactone is determined to be ΔH f,298 = −47.3 ± 4.7 kcal/mol.

On the structure of the C2H4O2 neutrals (acetic acid versus 1,1-dihydroxyethene) generated from ionized n-hexanoic acid and n-butyl acetate in the gas phase. Implications for the mechanism of the McLafferty rearrangement

Abstract Results are reported of CIDI and NRMS experiments which show that the C 2 H 4 O 2 neutral co-generated with C 4 H + 8 from the metastable n -hexanoic acid ion and the n -butyl acetate molecular ion is acetic acid and not its stable enol, CH 2 C(OH) 2 . This is in marked contrast to the structure of the C 2 H 4 O + 2 ion, co-generated with C 4 H 8 from the metastable n -hexanoic acid ion, which was earlier shown to be exclusively the enol form of acetic acid. The implication of this and earlier findings for the mechanism of McLafferty-type processes is discussed and it is suggested that this ubiquitous reaction is more complex than hitherto expected. The reaction may well proceed via long-lived ion/dipole or hydrogen-bridged intermediates.

Reversible β-hydrogen transfer between Fe(C2H5)+ and HFe(C2H4)+: case of two-state reactivity?

Abstract The iron ethyl cation, Fe(C 5 H 5 ) + , and its tautomer, the ethene complex of the iron hydride cation HFe(C 2 H 4 ) + , have been examined computationally using a hybrid of density functional theory and the Hartree-Fock approach ( Becke 3LYP). The quintet Fe(C 2 H 5 ) + ( 5 A′) corresponds to the global minimum of the [Fe,C 2 ,H 5 ] + potential energy hypersurface. Fe(C 2 H 5 ) + can interconvert via β-hydrogen transfer into HFe(C 2 H 4 ) + ( 5 A′), which is ca. 13 kcal mol −1 less stable. The transition structure (TS) associated with their mutual interconversion on the quintet surface requires 36 kcal mol −1 relative to Fc(C 2 H 5 ) + . However, this barrier may be circumvented by a reaction path on the energetically low-lying triplet surface in which the corresponding transition structure for β-H transfer is 8 kcal mol −1 lower in energy than the quintet TS. Thus, the path of minimal energy requirement connects the quintet species Fe(C 2 H 5 ) + and HFe(C 2 H 4 ) + via the triplet surface such that spin inversion is part of the reaction coordinate. Agostic interaction, which is only possible in the low-spin system, constitutes an essential factor for this unprecedented reaction mechanism. Further support to this interpretation is providedby mass spectrontetric experiments which demonstrate that the interconversion Fe(C 2 H 5 ) + ⇄ HFe(C 2 H 4 ) + is facile and occurs well below the respective dissociation asymptotes.

Regio- and diastereoselective CH bond activation of valeramide and 3-methyl valeramide by bare Fe+ ions

Abstract Mass spectrometric studies of metastable complexes of valeramide ( n -C 4 H 9 CONH 2 ) with Fe + cations are used to probe selectivity in metal-mediated CH and CC bond activations. Extensive labeling studies reveal that the dehydrogenation occurs via regioselective activation of the remote CH bonds at C(4) and C(5). As a side reaction, C(3)C(4) bond activation leads to loss of ethene concomitant with the formation of propionamide/Fe + . Introduction of an additional methyl group at the C(3) position allows to probe stereochemical aspects of bond activation in 3-methyl valeramide/Fe + complexes by means of diastereospecific labeling. Quantitative analysis of the labeling distributions reveals that the steric effect (SE=2.0±0.1) due to diastereoselective discrimination is of similar magnitude as the kinetic isotope effect (KIE=2.3±0.1) associated with remote functionalization of the C(4) and C(5) positions.

Combined quantum chemical and mass spectrometric study of [Si,C,H3,O]+ isomers

The potential energy surface of [Si,C,H3,O]+ has been explored by means of ab initio MO calculations at the G2 level of theory as well as by mass spectrometric techniques. The silicon–methoxide cation H3COSi+ has been identified as the global minimum (ΔfH= 151 kcal mol–1), followed by the silaacetyl cations H3SiCO+(ΔfH= 172 kcal mol–1) and H3CSiO+(ΔfH= 180 kcal mor–1). A number of other intermediates, the transition structures associated with the mutual interconversion reactions and the energetics of possible fragmentation channels are explored computationally. It turns out that the energy demands for the unimolecular isomerizations of most of the isomers are substantially lower than those of fragmentations. Consequently, an identification of the different isomers based solely on mass spectrometric information is made difficult by intramolecular rearrangements which precede the structure-indicative fragmentation reactions, even though H3SiCO+ is clearly distinguishable from H3CSiO+ and H3COSi+. However, when the experimental findings are combined with the computationally predicted [Si,C,H3,O]+ potential-energy surface and with thermochemical information, this difficulty can be overcome, and a coherent picture of this complex system emerges. In addition, the existence of the neutral radicals H3COSi˙, H3CSiO˙, as well as H3SiCO˙ and/or H3SiOC˙ is explored by means of neutralization–reionization mass spectrometry.

Nachweis von HCSiF und HCSiCl als die ersten Moleküle mit formalen C≡Si‐Bindungen

Die Neutralisierung von [C,H,Si,X]-Radikalkationen (X=F, Cl) liefert den ersten experimentellen Beleg fur die Existenz der neutralen Siline HC≡SiF und HC≡SiCl, die nach Ab-initio-MO-Rechnungen nichtlineare Strukturen aufweisen (siehe Bild).

Charge inversion as a structural probe for C6H5+ and C6H6+· cations

Charge reversal (+CR−) of cations to anions can be used to structurally differentiate isomeric C6H5+ and C6H6+· hydrocarbon ions by means of tandem mass spectrometry. In view of the manifold of possible isomers, only a few prototype precursors are examined. Thus, charge inversion demonstrates that electron ionization of 2,4-hexadiyne yields an intact molecular ion, whereas the charge inversion spectra of C6H6+· obtained from benzene, 1,5-hexadiyne, and fulvene are identical within experimental error. Similarly, the +CR− spectrum of the C6H5+ cation generated by dissociative ionization of 2,4-hexadiyne is significantly different from the +CR− spectrum of C6H5+ obtained from iodobenzene, suggesting the formation of a 2,4-hexadiynyl cation from the former precursor. Although charge inversion of cations to anions has a low efficiency and requires large precursor ion fluxes, the particular value of this method is that the spectra may not just differ in fragment ion intensities, but these differences can directly be related to the underlying ion structures.

Isotope‐Sensitive Degenerate [1,3]‐Hydrogen Migration versus Competitive Enol–Keto Tautomerization

The question of intramolecular energy distribution in polyatomic molecules or, more precisely, whether reacting molecules display ergodic or non-ergodic behavior, is of central importance in reaction dynamics. For ionic gas-phase systems, the unimolecular behavior of the enol and keto forms of ionized acetone, 1 and 2, respectively, constitutes one of the best studied systems. Detailed Hand C-labeling experiments, energetic measurements, analysis of kinetic energy release distributions as well as electronic-structure calculations of the potential-energy surface with the inclusion of trajectory studies reveal the following (Scheme 1): Direct dissociation of 1 to generate the hydroxyvinyl cation 4 does not occur; rather, irreversible isomerization to a highly excited, short-lived acetone ion 2 takes place, which decomposes to the acylium ion 3 concomitant with loss of a CH3 radical at a time-scale of 5 10 13 s. From 2, the newly formed methyl group is eliminated preferentially as compared to the one which is already present in 1, and the kinetic energy release associated with elimination of this process is smaller than that for the loss of the newly generated methyl group. All these findings clearly point to a non-statistical (i.e., “non-ergodic”) behavior of the chemically activated acetone ion 2. Inspired by the detailed mechanistic information which can be achieved from kinetic analysis of multiple labeling data, here we report a re-investigation of the dissociation of 1 for a wide set of isotopologues, generated via dissociative electron ionization of the corresponding labeled 2-hexanones in a McLafferty reaction. To our surprise, however, it turned out that the unimolecular dissociation of ionized acetone and its enol form offers additional mechanistic puzzles which have not been recognized before.

Detection and characterization of competitive processes giving rise to isomeric products in the reactions of Fe+, Co+ and Ni+ with n-butyl cyanide

Abstract Two independent processes lead to loss of C 2 H 4 upon reaction of Co + with n -C 4 H 9 CN. A “marriage” of deuterium labelling with collision-induced dissociation (CID) spectrometry, viz. CID spectra taken from isotopomeric product ions generated from the same labelled precursor, helps to uncover mechanistic details of this particular system. While part of the C 2 H 4 is produced via “remote functionalization”, as is exclusively the case for Ni + , part is generated from internal positions in a manner analogous to the reaction of Fe + . The isomeric complexes Co(CH 3 CH 2 CN) + and CH 3 Co + CH 2 CN are formed, which can be distinguished and characterized by virtue of their CID spectra. Dehydrogenation proceeds for all three metal ions by remote functionalization, but this reaction is preceded by two different equilibria which give rise to different labelling distributions in the cases of Fe + and Ni + respectively.