Table 1.

Gas → water transfer energetics for amides: Nonpolar and electrostatic contributions to the enthalpy and standard free energy

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). ΔG_pol and ΔH_pol 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).