Intrinsic acidities of carbon acids: the nitroalkanes. Solvation effects in water and DMSO

1977 ◽  
Vol 55 (19) ◽  
pp. 3474-3479 ◽  
Author(s):  
J. B. Cumming ◽  
T. F. Magnera ◽  
P. Kebarle
Keyword(s):  
Aqueous Solution ◽  
High Pressure ◽  
Gas Phase ◽  
Methyl Substitution ◽  
Charge Localization ◽  
Enthalpy Changes ◽  
Gas Phase Acidity ◽  
Protic Solvents ◽  
Carbon Acids ◽  
The Difference

The ion equilibria R1H + R2− = R1− + R2H involving nitroalkanes and compounds with known gas phase acidity were measured at 500 K with a pulsed high pressure mass spectrometer. The resulting ΔG0°= −RT ln K combined with calculated ΔS0 values lead to the corresponding ΔH changes. The enthalpy changes are used for the evaluation of the difference between the bond dissociation energy and the electron affinity D(R—H) – EA(R). The values obtained are: CH3NO2 44.0, CH3CH2NO2 43.7, (CH3)2CHNO2 43.0 kcal/mol. D(R—H) – EA(R) is a measure of the gas phase acidity. The nearly equal gas phase acidities of the nitroalkanes above are due to a decrease of D(R—H) and to a decrease of EA(R) with methyl substitution. In aqueous solution the acidities are known to increase substantially for the above nitroalkane order. Decrease of EA with methyl substitution indicates that there will be more charge localization on the O atoms in R− with methyl substitution. This leads to better H bonding and solvation of R− in protic solvents. This explains the higher acidity of the methyl substituted nitroalkanes in aqueous solution. The acidities in DMSO are closer to those in the gas phase since DMSO is not so sensitive to the charge distribution in R−.

Monomerie Metaphosphate: Does it Exist in Aqueous Solution?

1987 ◽  
Vol 42 (12) ◽  
pp. 1585-1587 ◽  
Author(s):  
R. G. Keesee ◽  
A. W. Castleman
Keyword(s):  
Aqueous Solution ◽  
Gas Phase ◽  
Water Molecules ◽  
The Third ◽  
Enthalpy Changes ◽  
Successive Addition ◽  
Dihydrogen Orthophosphate

AbstractEnthalpy changes for the successive addition of the first four water molecules onto monomeric metaphosphate anion in the gas phase have been determined to be -12.6, -11.4, -16.3, and - 11.0 kcal/mol, respectively. The results suggest that the first addition is a simple formation of the adduct PO3- · H2O as apposed to formation of the dihydrogen orthophosphate anion (HO)2PO2-, but that the third addition involves a transformation to the orthophosphate anion.

Relative methylmercury cation affinities in the gas phase determined by high pressure mass spectrometry

1981 ◽  
Vol 59 (12) ◽  
pp. 1779-1786 ◽  
Author(s):  
John A. Stone ◽  
Dena E. Splinter
Keyword(s):  
High Pressure ◽  
Gas Phase ◽  
External Rotation ◽  
Equilibrium Constants ◽  
Partition Functions ◽  
Pulsed Electron Beam ◽  
Nitrogen Bases ◽  
Relative Affinity ◽  
The Difference ◽  
Soft Acid

A pulsed electron beam, high pressure mass spectrometer has been used to determine equilibrium constants for the exchange of CH3Hg+ between bases; [Formula: see text] A series of aromatic, hydrocarbon bases has been studied at 417 K and several nitrogen bases have been studied at 580 K. There is a good linear correlation between differences in CH3Hg+ affinity (ΔG0) and H+ affinity for bases in each series. The single sulfur base examined ((C3H7)2S) shows anomalously high relative affinity for CH3Hg+ compared with H+ while two oxygen bases (CH3COOCH3 and C6H5NO2) show lesser relative affinity. These results are in qualitative agreement with the hard–soft acid base theory. ΔH0 and ΔS0 values have been obtained from Arrhenius plots. For a pair of aromatic bases (toluene–ethylbenzene) ΔH0 is of the same magnitude as that for H+ and ΔS0 may be calculated using partition functions for translation and external rotation. For toluene/methylacetate the difference in binding energy is much greater for H+ than for CH3Hg+ and a similar calculation of ΔS0 gives a result not consistent with the experimental value.

Cyanide Thermodynamics. III. Enthalpies and Entropies of Ionization of Water and Hydrogen Cyanide

1996 ◽  
Vol 49 (6) ◽  
pp. 651 ◽  
Author(s):  
JS Solis ◽  
PM May ◽  
G Hefter
Keyword(s):  
Aqueous Solution ◽  
Ionic Strength ◽  
Stability Constant ◽  
Hydrogen Cyanide ◽  
Infinite Dilution ◽  
Medium Effect ◽  
Titration Calorimetry ◽  
Enthalpy Changes ◽  
The Difference ◽  
Ionization Of Water

The heats (enthalpy changes) associated with the ionization of water and of hydrogen cyanide have been determined by titration calorimetry at 25�C as a function of ionic strength up to 5 M in both NaCl and NaClO4 media. The enthalpy changes for both reactions exhibit a 'medium effect' with ?H being more positive in NaCl than in NaClO4 and with the difference becoming more pronounced with increasing ionic strength. This is attributed to the greater solvation of Cl- cf. CN- in aqueous solution. The present ?H values are similar to previous published results at high ionic strengths, and are in excellent agreement with the well established literature values at infinite dilution. The present ?H values were combined with literature stability constant data to calculate the corresponding entropies for the ionization of H2O and HCN as a function of ionic strength.

Intrinsic Acidities of α, β, γ Chlorosubstituted Aliphatic Acids from Gas Phase Equilibrian Mesurements

1974 ◽  
Vol 52 (5) ◽  
pp. 861-863 ◽  
Author(s):  
R. Yamdagni ◽  
P. Kebarle
Keyword(s):  
Proton Transfer ◽  
Gas Phase ◽  
Equilibrium Constants ◽  
Transfer Reactions ◽  
Pulsed Electron Beam ◽  
Aliphatic Acids ◽  
Gas Phase Acidity ◽  
Proton Transfer Reactions ◽  
The Difference ◽  
Acidity Scale

Measurements of the proton transfer equilibria: A1− + A2H = A2− with a pulsed electron beam high pressure mass spectrometer were extended to α, β, γ chlorosubstituted aliphatic acids. The equilibrium constants were used to evaluate ΔG0 for the proton transfer reactions. Assuming ΔG ≈ ΔH and using standard acids AH for which the difference between the bond dissociation energy D(A—H) and the electron affinity of A, EA(A) was known one could evaluate the corresponding difference for the newly measured acids and place them on an absolute acidity scale. The gas phase acidity was observed to increase in the order: acetic, propionic, butyric, γ-Cl butyric, β-Cl butyric, β-Cl propionic, α-Cl butyric, α-Cl propionic, α-Cl acetic. The gas phase acidities are compared with those observed in aqueous solution. The effects of the Cl substituent parallel those in solution but are much larger. The attenuation occurring in solution is attributed to weaker hydrogen bonding of the chloro stabilized acid anions to water molecules.

Hydrogen bonding of O—H and C—H hydrogen donors to Cl−. Results from mass spectrometric measurements of the ion–molecule equilibria RH + Cl− = RHCl−

1982 ◽  
Vol 60 (15) ◽  
pp. 1907-1918 ◽  
Author(s):  
M. A. French ◽  
S. Ikuta ◽  
P. Kebarle
Keyword(s):  
Hydrogen Bond ◽  
Hydrogen Bonding ◽  
Gas Phase ◽  
Positive Charge ◽  
Equilibrium Constants ◽  
Basis Set ◽  
Molecular Orbital Calculations ◽  
Simple Relationship ◽  
Gas Phase Acidity ◽  
Carbon Acids

Equilibrium constants K1 for reaction [1] RH + Cl− = RHCl− in the gas phase were measured with a high pressure mass spectrometer under chemical ionization conditions. Data for some 40 compounds RH are presented. It is found that the binding free energies [Formula: see text] for RH = oxygen acids increase with the gas phase acidity of RH. The strongest bonds are formed with strong acids like HCO2H, CH3CO2H, and phenol. Water and alkyl alcohols give much weaker interactions. A simple relationship between gas phase acidity and binding free energy does not occur for RH = carbon acids. Carbon acids like cyclopentadiene, whose high gas phase acidity is largely due to charge derealization by conjugation in the completed anion, do not give Cl− adducts with stability commensurate with the acidity. A relationship between gas phase acidity and binding energy is found for carbon acids with carbonyl groups and for the substituted toluenes. Molecular orbital calculations with the STO-3G basis set provide insights to the bonding occurring in RHCl−. For all cases investigated, hydrogen bonding to Cl− provides the most stable structure. Generally the hydrogen bond occurs through the hydrogen which has the highest net positive charge. The hydrogen bond strength is found approximately proportional to this positive charge. Another proportionality is found between the charge transferred from Cl− to RH, on formation of RHCl−, and the strength of the hydrogen bond.

Explaining the magnetism of gold

2017 ◽  
Author(s):  
Robson de Farias
Keyword(s):  
Gas Phase ◽  
Computational Study ◽  
Formation Enthalpy ◽  
Gold Atom ◽  
Orbital Energy ◽  
Knudsen Effusion ◽  
Energy Diagram ◽  
Unpaired Electrons ◽  
The Difference ◽  
Good Agreement

<p>In the present work, a computational study is performed in order to clarify the possible magnetic nature of gold. For such purpose, gas phase Au<sub>2</sub> (zero charge) is modelled, in order to calculate its gas phase formation enthalpy. The calculated values were compared with the experimental value obtained by means of Knudsen effusion mass spectrometric studies [5]. Based on the obtained formation enthalpy values for Au<sub>2</sub>, the compound with two unpaired electrons is the most probable one. The calculated ionization energy of modelled Au<sub>2</sub> with two unpaired electrons is 8.94 eV and with zero unpaired electrons, 11.42 eV. The difference (11.42-8.94 = 2.48 eV = 239.29 kJmol<sup>-1</sup>), is in very good agreement with the experimental value of 226.2 ± 0.5 kJmol<sup>-1</sup> to the Au-Au bond<sup>7</sup>. So, as expected, in the specie with none unpaired electrons, the two 6s<sup>1</sup> (one of each gold atom) are paired, forming a chemical bond with bond order 1. On the other hand, in Au<sub>2</sub> with two unpaired electrons, the s-d hybridization prevails, because the relativistic contributions. A molecular orbital energy diagram for gas phase Au<sub>2</sub> is proposed, explaining its paramagnetism (and, by extension, the paramagnetism of gold clusters and nanoparticles).</p>

2-Deoxyribose Radicals in the Gas Phase and Aqueous Solution. Transient Intermediates of Hydrogen Atom Abstraction from 2-Deoxyribofuranose

2005 ◽  
Vol 70 (11) ◽  
pp. 1769-1786 ◽  
Author(s):  
Luc A. Vannier ◽  
Chunxiang Yao ◽  
František Tureček
Keyword(s):  
Aqueous Solution ◽  
Hydrogen Atom ◽  
Gas Phase ◽  
Computational Study ◽  
Hydrogen Atom Abstraction ◽  
Oh Radical ◽  
Free Energies ◽  
Intramolecular Hydrogen ◽  
Solvation Free Energies ◽  
Transient Intermediates

A computational study at correlated levels of theory is reported to address the structures and energetics of transient radicals produced by hydrogen atom abstraction from C-1, C-2, C-3, C-4, C-5, O-1, O-3, and O-5 positions in 2-deoxyribofuranose in the gas phase and in aqueous solution. In general, the carbon-centered radicals are found to be thermodynamically and kinetically more stable than the oxygen-centered ones. The most stable gas-phase radical, 2-deoxyribofuranos-5-yl (5), is produced by H-atom abstraction from C-5 and stabilized by an intramolecular hydrogen bond between the O-5 hydroxy group and O-1. The order of radical stabilities is altered in aqueous solution due to different solvation free energies. These prefer conformers that lack intramolecular hydrogen bonds and expose O-H bonds to the solvent. Carbon-centered deoxyribose radicals can undergo competitive dissociations by loss of H atoms, OH radical, or by ring cleavages that all require threshold dissociation or transition state energies >100 kJ mol-1. This points to largely non-specific dissociations of 2-deoxyribose radicals when produced by exothermic hydrogen atom abstraction from the saccharide molecule. Oxygen-centered 2-deoxyribose radicals show only marginal thermodynamic and kinetic stability and are expected to readily fragment upon formation.

scholarly journals Experimental and Computational Studies on the Gas-Phase Acidity of 5,5-Dialkyl Barbituric Acids

2021 ◽  
Author(s):  
Juan Z. Dávalos-Prado ◽  
Javier González ◽  
Josep M. Oliva-Enrich ◽  
Emma J. Urrunaga ◽  
Alexsandre F. Lago
Keyword(s):  
Gas Phase ◽  
Computational Studies ◽  
Gas Phase Acidity

Simulation of UV Absorption Spectra and Relaxation Dynamics of Uracil and Uracil-Water Clusters

2020 ◽  
Author(s):  
Branislav Milovanović ◽  
Jurica Novak ◽  
Mihajlo Etinski ◽  
Wolfgang Domcke ◽  
Nadja Doslic
Keyword(s):  
Aqueous Solution ◽  
Gas Phase ◽  
Absorption Spectra ◽  
Absorption Spectroscopy ◽  
Water Clusters ◽  
Uv Absorption ◽  
Relaxation Dynamics ◽  
Nonradiative Relaxation ◽  
Uv Absorption Spectra ◽  
Uv Absorption Spectroscopy

Despite many studies, the mechanisms of nonradiative relaxation of uracil in the gas phase and in aqueous solution are still not fully resolved. Here we combine theoretical UV absorption spectroscopy...

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