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ABSTRACT: Fourier transform ion cyclotron resonance mass spectrometry, combined with modern ionization (fast atom bombardment , electrospray ionization, matrix-assisted laser desorption-ionization), fragmentation (collision-induced dissociation, surface-induced dissociation, one-photon ultraviolet photodissociation, infrared multiphoton dissociation, blackbody infrared radiative dissociation, electron-capture dissociation), and separation (high-performance liquid chromatography, liquid chromatography, capillary electrophoresis) techniques is now becoming one of the most attractive and frequently used instrumental platforms for gas-phase studies of biomolecules such as amino acids, bioamines, peptides, polypeptides, proteins, nucleobases, nucleosides, nucleotides, polynucleotides, nucleic acids, saccharides, polysaccharides, etc. Since it gives the possibilities to trap the ions from a few seconds up to thousands of seconds, it is often applied to study ion/molecule reactions in the gas phase, particularly proton-transfer reactions which provide important information on acid-base properties. These properties determine in part the three-dimensional structure of biomolecules, most of their intramolecular and intermolecular interactions, and consequently their biological activity. They also indicate the form (unionized, zwitterionic, protonated, or deprotonated) which the biomolecule may take in a nonpolar environment.
Analytical and Bioanalytical Chemistry 12/2007; 389(5):1365-80. · 3.78 Impact Factor
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ABSTRACT: By building a relative basicity ladder, the current basicity scale (Lias et al., J. Phys. Chem. Ref. Data, 17 Suppl. No. 1 (1988)) has been reexamined in its upper part, and extended for organic compounds (amenable to proton transfer measurements) up to proton affinity, PA = 1050 kJ mol−1. Structural effects involved in superbasicities are briefly discussed and routes to further extension of the gas-phase basicity scale for organic compounds are proposed.
Rapid Communications in Mass Spectrometry 04/2005; 7(7):599 - 602. · 2.79 Impact Factor
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ABSTRACT: The superbase gas-phase scale has been further extended up to proton affinities of ca 1080 kJ mol−1 by use of cyclic and acyclic guanidines and vinamidines. Structural features such as Y-conjugation, vinylogy and intramolecular ionic hydrogen bonding leading to their superbasic behaviour are analysed. Solvation effects by water and acetonitrile on basicity are discussed. From a correlation pKa(acetonitrile) vs gas-phase basicity, proton affinity values in the range 1070–1410 kJ mol−1 are predicted for Schwesinger phosphazene compounds.
Journal of Physical Organic Chemistry 10/2004; 7(12):725 - 733. · 1.96 Impact Factor
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ABSTRACT: Experimental gas-phase acidities of 2-oxopropanaloximes, XCH2COCHNOH (X H, CH3S, CH3SO, CH3SO2; compounds 1–4), were determined by Fourier transform ion cyclotron resonance (FT-ICR) spectrometry. The values are δ Gacid° = 1401, 1381, 1360 and 1351 kJ mol−1 for 1, 2, 3, and 4, respectively. Molecular orbital calculations using the semi-empirical AM1 method provided information on the geometry and relative energy of neutrals species 1–4 and their conjugate bases, together with charge distributions and entropies of deprotonation. It is demonstrated that the proton abstraction occurs preferentially at the oxime function; the formation of an enolate as a conjugate base is unfavourable by 70–140 kJ mol−1. The large variation of the gas-phase acidities for 1–4 is explained in terms of the field/inductive empirical substituent constant σF. The variation of solution acidities appears to be comparatively strongly attenuated. This attenuation is attributed mainly to charge delocalization in the anion, which was confirmed by charge density calculations.
Journal of Physical Organic Chemistry 10/2004; 4(5):285 - 292. · 1.96 Impact Factor
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ABSTRACT: The gas-phase acidity of ethyl-, vinyl-, ethynyl-, and phenyl-substituted silanes, germanes, and stannanes has been measured by means of FT-ICR techniques. The effect of unsaturation on the intrinsic acidity of these compounds and the corresponding hydrocarbons was analyzed through the use of G2 ab initio and DFT calculations. In this way, it was possible to get a general picture of the acidity trends within group 14. As expected, the acid strength increases down the group, although the acidity differences between germanium and tin derivatives are already rather small. As has been found before for amines, phosphines, and arsines, the carbon, silicon, germanium, and tin alpha,beta-unsaturated compounds are stronger acids( )than their saturated analogues. The acidifying effect of unsaturation is much larger for carbon than for Si-, Ge-, and Sn-containing compounds. The allyl anion is better stabilized by resonance than its Si, Ge, and Sn analogues, [CH(2)(-)(delta)--CH(+)(delta)(') --CH(2)(-)(delta)](-) vs [CH(2)(-)(delta)()II = CH(-)(delta)()III - XH(2)(-)(delta)()IV](-) (X = Si, Ge, Sn). The enhanced acid strength of unsaturated compounds is essentially due to a greater stabilization of the anion with respect to the neutral, because the electronegativity of the alpha,beta-unsaturated carbon group increases with its degree of unsaturation. The phenyl derivatives are systematically weaker acids than the corresponding ethynyl derivatives by 15-20 kJ mol(-)(1). Experimentally, toluene acidity is very close to that of propyne, because the deprotonation of propyne takes place preferentially at the =CH group rather than at the -CH(3) group.
Journal of the American Chemical Society 08/2001; 123(26):6353-9. · 9.91 Impact Factor
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ABSTRACT: Gas-phase basicities (GB) of strong organic bases containing the imino group were re-examined in the light of the re-evaluated GB values for the reference bases given in a recent compilation of Hunter and Lias. Structural (internal) effects which influence the basicity are discussed and general relations for the GB prediction are proposed for simple alkyl amidines and guanidines. These relations were used for estimation of cyclization and intramolecular H-bonding effects. Copyright © 2000 John Wiley & Sons, Ltd.
Journal of Physical Organic Chemistry 12/2000; 14(1):25 - 34. · 1.96 Impact Factor
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ABSTRACT: Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT ICR) has revealed the existence of the P4Li+ ion as a stable species in the gas phase. High-level ab initio calculations show that P4Li+ appears as a “planetary system” wherein the lithium cation can easily move around the P4 moiety along paths (or “orbits”) connecting the points above the middle points of the P−P edges with points above the center of the PPP faces, the movement toward the corners of the tetrahedron being the less favored. Due to symmetry, this orbiting is 4-fold degenerate.
Journal of the American Chemical Society 01/2000; 122:4451-4454. · 9.91 Impact Factor
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Biological Mass Spectrometry 05/1999; 33(10):1029 - 1031. · 3.41 Impact Factor
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ABSTRACT: Semiempirical calculations (AM1) together with experimental mass spectrometric (FT-ICR) data indicate the imino nitrogen atom as the favoured site of protonation and the amino nitrogen atom as the site of deprotonation of the amidine group in the gas phase. For tautomerizing N-methyl-N'-phenylbenzamidine the tautomer with the phenyl group at the imino nitrogen atom weakly predominates in tautomeric mixture.
Analytical and Bioanalytical Chemistry 07/1996; 355(3-4):412-4. · 3.78 Impact Factor
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ABSTRACT: The gas-phase lithium cation basicities (LCBs) were obtained for histamine (HA) and its agonist 2-(β-aminoethyl)-pyridine (AEP) from collision-induced dissociation of lithium adducts using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). For measurements, MeO(CH2)2OMe, Et3PO and (Me2N)3PO (HMPA) were used as the reference compounds. The experimental LCB of AEP was located between those of Et3PO and (Me2N)3PO. The experimental LCB of HA was found to be higher than those of AEP and HMPA by more than 2 kcal mol−1 clearly indicating that the LCB of HA is higher than any LCB for a neutral base yet measured (crown-ethers excepted). The experimental LCBs of the parent bases (pyridine and imidazole) are lower by more than 10 kcal mol−1. In parallel, DFT calculations {B3LYP/6-31G*//B3LYP/6-31G* and B3LYP/6-311+G**//B3LYP/6-31G*} were performed for HA, AEP and their lithium adducts. Among the 22 reasonable conformations of the HA-Li+ adduct, only one appears to be significantly more stable than the others. This is also the case for one structure among seven conformations of the AEP-Li+ adduct. These two stable structures have the ‘scorpion’ conformation, in which the Li+ cation is almost equally chelated by two basic nitrogen atoms, the ring N-aza and the chain N-amino. Other HA-Li+ and AEP-Li+ conformations have noticeably higher energies than the ‘scorpion’ structures. The difference between the DFT calculated LCBs of HA and AEP (about 4 kcal mol−1) is in agreement with that experimentally obtained (>2 kcal mol−1). The high experimental and theoretical values of LCB for HA and AEP militate in favor of a strong chelation of Li+ by both ligands in the gas-phase. This chelation effect was also evidenced previously for the proton gas-phase basicity.
International Journal of Mass Spectrometry. 267:315-323.
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ABSTRACT: The gas-phase basicities of the title compounds have been determined by Fourier transform ion cyclotron resonance mass spectrometry. Arsines are systematically weaker bases than phosphines. Basicity decreases in both series in the order ethyl > ethenyl > ethynyl. On the basis of their ionization energies, the phosphorus atom is recognized as the preferred site of protonation in the phosphines studied. A similar reasoning applied to arsines does not allow a clear-cut conclusion to be made. As it appears from the experimental results of this study and the few data on alkyl and phenyl derivatives available in the literature, substituent effects are attenuated by a factor of approximately 1.15 when going from phosphines to arsines.
International Journal of Mass Spectrometry and Ion Processes 175:27-33.
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ABSTRACT: The gas-phase lithium-cation basicities of a series of monosubstituted benzene derivatives, namely C6H5X (X=H, Me, CHCH2, OH, OMe, SH, Cl, Br) have been measured by means of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The structures of the corresponding complexes and their relative stabilities were investigated with B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) density functional theory calculations. In all cases, the π-complexes are favored with respect to those in which the metal monocation interacts with the substituent. These latter kind of complexes, which are entropically favored with respect to the π-complexes, are found to be chelated species, in which Li+ bridges the heteroatom of the substituent and the ipso carbon atom. The Li+ basicity of the benzene derivatives investigated reflects the electron-donor ability of the aromatic moiety as a function of the substituent. Consistently, there is a linear correlation between the Li+ basicity and the frequency of the vertical displacement of Li+ with respect to the aromatic ring.
International Journal of Mass Spectrometry. 219(3):445-456.
