**Table 1. Rate Constants^a 10² (h^-1)**
		k₁	k₂
Cephaloridine	HPLC	19	16
	¹H-NMR	22	10
Ceftazidime	HPLC	19	12
	¹H-NMR	15	8

a) The rate constants were determined in carbonate buffer solution (pH 10.5, µ= 0.1, 37°C) by HPLC and in deuteriated carbonate buffer solution (pD 10.5, µ= 0.5, 37°C) by ¹H-NMR.

Table 2. Calculated (AM1) heats of formation (in kcal/mol) for cephalosporin isomers and intermediate ions. Values in brackets are the heats of formation plus delta G solvation (in kcal/mol) obtained with the AMSOL solvation method.

Delta³-isomer (1) Carbanion (2) Delta²-isomers (3a / 3b)

Cefotaxime -222.5 -172.6 -227.2 (-298.9)/-224.7 (-293.9)

Cephaloridine -14.6 -31.7 -11.2 (-55.6)/ -11.7 (-54.8)

Cephalexine -133.4 -74.0 -137.3 (-213.6)/ -140.5 (-211.9)

	Delta³-isomer (1)	Carbanion (2)	Delta²-isomers (3a / 3b)
Cefotaxime	-222.5	-172.6	-227.2 (-298.9)/-224.7 (-293.9)
Cephaloridine	-14.6	-31.7	-11.2 (-55.6)/ -11.7 (-54.8)
Cephalexine	-133.4	-74.0	-137.3 (-213.6)/ -140.5 (-211.9)