-X	σm	σp
-NH2	-0.16	-0.66
-CH3	-0.07	-0.17
-H	0	0
-OH	+0.12	-0.37
-F	+0.34	+0.06
-COCH3	+0.38	+0.50
-NO2	+0.71	+0.78

Hansch, C. and Leo, A.
Substituent Constants for Correlation
Analysis in Chemistry and Biology,
Wiley, 1979.