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Research paper thumbnail of Solvation of negative ions by protic and aprotic solvents. Information from gas phase ion equilibria measurements

Faraday Discussions of the Chemical Society, 1977

Measurement of the gas phase ion equilibria between ions M and solvent molecules Sl provide bindi... more Measurement of the gas phase ion equilibria between ions M and solvent molecules Sl provide binding energies of the complexes M±(S1)n(for n= 1 to ∼6). Comparison of these data with single ion energies of solvation shows that differences in ion solvation in solution are reflected in the binding energies of ion-molecule complexes in the gas phase. The weaker solvation of negative ions (relative to positive ions) observed in liquid aprotic solvents is reflected in the binding energies of negative ion aprotic molecule complexes, a weaker binding being found for the first and subsequent few aprotic molecules. An analysis of the bonding in Cl–(CH3CN) and K+(CH3CN) shows that the weaker bonding to Cl– is due to the very diffuse distribution of the positive pole of the dipole in acetonitrile. In effect the dipole can not come close to the negative ion. Analysis shows a similar picture also for acetone. Experimental results for the bonding between Cl–HR are given for a variety of compounds HR. These show that for RH = protic compounds, like oxygen acids, the hydrogen bond in Cl–HR increases with the acidity of HR. For aprotic compounds, i.e., carbon acids, no relationship between the bond in the complex and the acidity of HR is found. An examination of the solvation of substituted phenoxide ions by protic and aprotic solvents shows that solvation by protic solvents is adversely affected by charge dispersal in the ion, while aprotic solvents are much less sensitive to charge dispersal. The reasons for this important difference in behaviour are examined.

Research paper thumbnail of Intrinsic acidities of carbon acids. Sulfones and sulfoxides

Journal of the American Chemical Society, 1978

Research paper thumbnail of Intrinsic acidities of ketones and aldehydes. Bond dissociation energies and electron affinities

Journal of the American Chemical Society, 1977

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

Canadian Journal of Chemistry, 1977

The ion equilibria R1H + R2− = R1− + R2H involving nitroalkanes and compounds with known gas phas... more 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...

Research paper thumbnail of Summary of gas phase acidity measurements involving acids AH. Entropy changes in proton transfer reactions involving negative ions. Bond dissociation energies D (A—H) and electron affinities EA(A)

Canadian Journal of Chemistry, 1978

The complete ladder of ΔG10 determinations obtained from measurements of some 110 gas phase proto... more The complete ladder of ΔG10 determinations obtained from measurements of some 110 gas phase proton transfer equilibria A1− + A2H = A1H + A2− involving some 60 acids AH and connecting to the standard acid HCl is given. Evaluation of the entropy changes leads to values for the deprotonation energies ΔHD0 and ΔGD0 (at room temperature) corresponding to the gas phase process AH = A− + H+; ΔHD0 = D(A—H) − EA(A) + 313.6 kcal/mol. Comparison of the present data with literature determinations of the bond dissociation energies and electron affinities shows agreement within 2 kcal/mol. Some experimentally determined entropy changes ΔS10 are compared with the theoretically calculated values.

Research paper thumbnail of Dr. John Cumming Profile

Research paper thumbnail of Solvation of negative ions by protic and aprotic solvents. Information from gas phase ion equilibria measurements

Faraday Discussions of the Chemical Society, 1977

Measurement of the gas phase ion equilibria between ions M and solvent molecules Sl provide bindi... more Measurement of the gas phase ion equilibria between ions M and solvent molecules Sl provide binding energies of the complexes M±(S1)n(for n= 1 to ∼6). Comparison of these data with single ion energies of solvation shows that differences in ion solvation in solution are reflected in the binding energies of ion-molecule complexes in the gas phase. The weaker solvation of negative ions (relative to positive ions) observed in liquid aprotic solvents is reflected in the binding energies of negative ion aprotic molecule complexes, a weaker binding being found for the first and subsequent few aprotic molecules. An analysis of the bonding in Cl–(CH3CN) and K+(CH3CN) shows that the weaker bonding to Cl– is due to the very diffuse distribution of the positive pole of the dipole in acetonitrile. In effect the dipole can not come close to the negative ion. Analysis shows a similar picture also for acetone. Experimental results for the bonding between Cl–HR are given for a variety of compounds HR. These show that for RH = protic compounds, like oxygen acids, the hydrogen bond in Cl–HR increases with the acidity of HR. For aprotic compounds, i.e., carbon acids, no relationship between the bond in the complex and the acidity of HR is found. An examination of the solvation of substituted phenoxide ions by protic and aprotic solvents shows that solvation by protic solvents is adversely affected by charge dispersal in the ion, while aprotic solvents are much less sensitive to charge dispersal. The reasons for this important difference in behaviour are examined.

Research paper thumbnail of Intrinsic acidities of carbon acids. Sulfones and sulfoxides

Journal of the American Chemical Society, 1978

Research paper thumbnail of Intrinsic acidities of ketones and aldehydes. Bond dissociation energies and electron affinities

Journal of the American Chemical Society, 1977

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

Canadian Journal of Chemistry, 1977

The ion equilibria R1H + R2− = R1− + R2H involving nitroalkanes and compounds with known gas phas... more 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...

Research paper thumbnail of Summary of gas phase acidity measurements involving acids AH. Entropy changes in proton transfer reactions involving negative ions. Bond dissociation energies D (A—H) and electron affinities EA(A)

Canadian Journal of Chemistry, 1978

The complete ladder of ΔG10 determinations obtained from measurements of some 110 gas phase proto... more The complete ladder of ΔG10 determinations obtained from measurements of some 110 gas phase proton transfer equilibria A1− + A2H = A1H + A2− involving some 60 acids AH and connecting to the standard acid HCl is given. Evaluation of the entropy changes leads to values for the deprotonation energies ΔHD0 and ΔGD0 (at room temperature) corresponding to the gas phase process AH = A− + H+; ΔHD0 = D(A—H) − EA(A) + 313.6 kcal/mol. Comparison of the present data with literature determinations of the bond dissociation energies and electron affinities shows agreement within 2 kcal/mol. Some experimentally determined entropy changes ΔS10 are compared with the theoretically calculated values.

Research paper thumbnail of Dr. John Cumming Profile

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