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Gas-phase Acidity (gas-phase + acidity)

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Chemistry	100%

Selected Abstracts

Gas-phase acidity of proline from density functional computations

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2001
T. Marino
Abstract Density functional computations using the B3LYP hybrid potential coupled with the extended 6-311++G** basis set were performed to evaluate gas-phase acidity of proline ,-amino acid. The computed value of gas-phase acidity is 1403.8 kJ/mol in good agreement with the experimental counterpart (1395.0±13.8 kJ/mol) and is referred to the deprotonated species deriving from the first relative minimum of neutral proline characterized by the exo ring conformation with the carboxyl group in cis position. The most stable neutral proline isomer gives rise to a species for which the gas-phase acidity value (1384.5 kJ/mol) is also close to the experimental data. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 264,268, 2001 [source]

The Ever-Surprising Chemistry of Boron: Enhanced Acidity of Phosphine,Boranes

CHEMISTRY - A EUROPEAN JOURNAL, Issue 18 2009
Marcela Hurtado Dr.
Abstract The acidity-enhancing effect of BH3 in gas-phase phosphine,boranes compared to the corresponding free phosphines is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. Thus, the enhancement of the acidity of protic acids by Lewis acids usually observed in solution is also observed in the gas phase. For example, the gas-phase acidities (GA) of MePH2 and MePH2,BH3 differ by about 118,kJ,mol,1 (see picture). The gas-phase acidity of a series of phosphines and their corresponding phosphine,borane derivatives was measured by FT-ICR techniques. BH3 attachment leads to a substantial increase of the intrinsic acidity of the system (from 80 to 110,kJ,mol,1). This acidity-enhancing effect of BH3 is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. This indicates that the enhancement of the acidity of protic acids by Lewis acids usually observed in solution also occurs in the gas phase. High-level DFT calculations reveal that this acidity enhancement is essentially due to stronger stabilization of the anion with respect to the neutral species on BH3 association, due to a stronger electron donor ability of P in the anion and better dispersion of the negative charge in the system when the BH3 group is present. Our study also shows that deprotonation of ClCH2PH2 and ClCH2PH2,BH3 is followed by chloride departure. For the latter compound deprotonation at the BH3 group is found to be more favorable than PH2 deprotonation, and the subsequent loss of Cl, is kinetically favored with respect to loss of Cl, in a typical SN2 process. Hence, ClCH2PH2,BH3 is the only phosphine,borane adduct included in this study which behaves as a boron acid rather than as a phosphorus acid. [source]

Gas-phase acidity of proline from density functional computations

Determination of the electron affinities of ,- and ,-naphthyl radicals using the kinetic method with full entropy analysis.

JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 6 2001
H bond dissociation energies of naphthalene, The C
Abstract The C , H bond dissociation energies for naphthalene were determined using a negative ion thermochemical cycle involving the gas-phase acidity (,Hacid) and electron affinity (EA) for both the ,- and ,-positions. The gas-phase acidity of the naphthalene ,- and ,-positions and the EAs of the ,- and ,-naphthyl radicals were measured in the gas phase in a flowing afterglow,triple quadrupole apparatus. A variation of the Cooks kinetic method was used to measure the EAs of the naphthyl radicals by collision-induced dissociation of the corresponding ,- and ,-naphthylsulfinate adducts formed by reactions in the flow tube portion of the instrument. Calibration references included both , and , radicals, and full entropy analysis was performed over a series of calibration curves measured at collision energies ranging from 3.5 to 8 eV (center-of-mass). The measured EAs are 33.0 ± 1.4 and 31.4 ± 1.0 kcal mol,1 (1 kcal = 4.184 kJ) for the ,- and ,-naphthyl radicals, respectively. The gas-phase acidities for naphthalene were measured by the DePuy silane cleavage method, which utilizes the relative abundances of aryldimethylsiloxides and trimethylsiloxide that result from competitive cleavages from a proposed pentacoordinate hydroxysiliconate intermediate. The measured acidities are 394.0 ± 5.0 and 397.6 ± 4.8 kcal mol,1 for the ,- and ,- positions, respectively. The C , H bond dissociation energies calculated from the thermochemical cycle are 113.4 ± 5.2 and 115.4 ± 4.9 kcal mol,1 for the ,- and ,-positions, respectively. These energies are, to within experimental error, indistinguishable and are approximately the same as the first bond dissociation energy for benzene. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Ab initio and density functional theory studies of the structure, gas-phase acidity and aromaticity of tetraselenosquaric acid

CHINESE JOURNAL OF CHEMISTRY, Issue 6 2000
Li-Xin Zhou
Abstract Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure, acidity (free energy of deprotonation, ,G*) and aromaticity of tetraselenosquaric acid (3, 4-diselenyl-3-cyclobutene-1,2-diselenone, H2 C4 Se4)are reported. The global minimum found on the potential energy surface of tetraselenosquaric acid presents a planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very dose in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference compounds, cyclobutanediselenone, and cyclobutanediselenol. The computed aromatic stabilization energy (ASE) by homodesmotic reaction is -77.4 (MP2(fu)/6,311+G** /RHF/6 - 311 + G** ) and - 54.8 kJ/mol (B3LYP/6 - 311 + G** //B3LYP/6 -311 + G**). The aromaticity of tetraselenosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation (A) -19.13 (CSGT(IGAIM)-RHF/6,311 + G**// RHF/6,311 + G** and -32.91 (4,·10,6 m,3/mol)(CSGT(I-GAIM)-B3LYP/6 - 311 + G* * //B3LYP/6 - 311 + G**). Thus, tetraselenosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The calculated gas-phase acidity is ,G1*(298k) = 1257.7 and ,G*2 (298k) = 1617.1 kJ/mol. Hence, tetraselenosquaric acid is the strongest acid among the three squaric acids (3, 4-dihydroxy-3-cyclobutene-1, 2-dione, H2 C4 3,4-dithiohydroxy-3-cyclobutene-1,2-dithione, H2C4 S4, 3, 4-diselenyl-3-cyclobutene-1,2-diselenone, H2C4Se4). [source]