Table 1.
Solvation energies†, kcal/mol | Acetamide | N-methyl acetamide | N,N-dimethyl acetamide | Propionamide |
---|---|---|---|---|
ΔGobs‡ | −9.7 | −10.07 | −8.54 | −9.38 |
ΔGvdw+cavity‡ | 1.95 | 2.16 | 2.32 | 2.12 |
ΔGpol‡ | −11.65 | −12.23 | −10.86 | −11.50 |
ΔG*es‡ | −11.75 | −12.20 | ND | −11.57 |
ΔH*obs‡ | −16.32 | −17.07 | −16.56 | −17.45 |
ΔHvdw+cavity‡ | −4.67 | −5.58 | −6.25 | −5.41 |
ΔHpol‡ | −11.65 | −11.49 | −10.31 | −12.04 |
Polar ASA (Å2)§ | 95 | 51 | 32 | 88 |
Nonpolar ASA (Å2) | 97 | 173 | 217 | 130 |
ND, not determined. ASA, accessible surface area.
The units are kcal/mol, and the temperature is 25°C.
ΔG is the standard Gibbs free energy of transfer from the gas phase to the dilute aqueous solution (or the solvation free energy), and ΔH is the corresponding enthalpy of transfer, or solvation enthalpy; both are referred to the standard state of Ben-Naim (17), (see also refs. 15 and 16). ΔGpol and ΔHpol are the polar contributions to the solvation free energy and enthalpy, respectively. They are found by subtracting the sum of the cavity and van der Waals terms from the solvation free energy of enthalpy. ΔG*es is the calculated value, based on delphi and the parse parameter set, of the ESF. The transfer free energy data from amides are from Wolfenden (17) and Wolfenden et al. (18). The enthalpy data for amides are from Della Gatta et al. (19). The alkane transfer data, which are used to give the sum of the cavity and van der Waals terms, are from Cabani et al. (42) and Ben-Naim and Marcus (43). The equations used to fit the alkane data are ΔH = 0.101 − 0.0278x and ΔG = 0.727 + 0.0064x, where x is ASA in Ȧ2; the data point for methane was not used in fitting in both cases.
Values of polar and nonpolar ASA were calculated with naccess 2.1 (S. Hubbard and J. Thornton, University College, London) by using a probe radius of 1.4 A° and van der Waals radii from Chothia (44).