Table 5. Effects of Substituents and pK_a on Quantum Yields^a for the Substituted pHP GABA 31 in Unbuffered H₂O;^b Entries Are Arranged in the Order of Decreasing pK_a of the Substituted pHP Chromophore⁹⁴.

31	pKa	Φdisc	ΦGABA	Φ (25)d	Φdis (Ac or DEP)d
3,5-CH3	8.2	0.15	0.14	0.13
3-CH3	8.1	0.15	0.14	0.13
2-CH3	8.0	0.11	0.1	0.1
3-OCH3	7.9	0.07	0.06	NDe	0.39 (DEP)
R1–R4 = H	7.8	0.20	0.19	0.16	0.30 (Ac)
					0.40 (DEP)
3,5-OCH3	7.8	0.03	0.03	ND	0.44 (DEP)
2-F	7.2	0.28	0.27	0.26
2,6-F	6.8	0.16	0.16	0.15
3-F	6.7	0.16	0.15	0.15
3-OCF3	6.5	0.09	0.09	0.07
2,3-diF	5.9	0.24	0.24	0.22
2,5-diF	5.7	0.22	0.21	0.2
3-CF3	5.5	0.17	0.16	0.14
3,5-F	5.3	0.11	0.11	0.1
3-CN	5.2	0.42	0.35	0.39	0.17 (Ac)
2,3,5-triF	4.5	0.08	0.07	0.06
tetra-F	3.9	0.11	0.1	0.1

^aAll runs were low conversions to products (<5%); standard deviations were < ±0.02.

^bUnbuffered 18 MΩ ultrapure H₂O.

^cDisappearance quantum yield when GABA is the leaving group.

^dQuantum yield for the substituted phenylacetic acid (25).

^eDisappearance quantum yield when Ac (acetate) or DEP (diethyl phosphate) is the leaving group.

^eND = not determined.⁹⁴