Energetic comparison

The potential energy barriers obtained by using the reaction coordinate and the micro- iterative methods for the racemization of mandelate and propargylglycolate are shown in Tables 3.9 and 3.10, respectively. No significant differences are observed among the two sets of energy barriers in the case of mandelate, no other than those that could be practically removed by performing a scanning with more intermediate points along the reaction coordinate. On the contrary, in the case of propargylglycolate important differences exist among some energy barriers obtained with the two methods. These differences are mainly centered in step 4, when stereogenic center C$_\alpha$ changes its configuration. Another significant discrepancy can be seen at the TS1 of the mechanism I. A geometric analysis of the corresponding transition state structures will shed some light to this point.

Table 3.9: Potential energies (kcal/mol) for the racemization of mandelate using the reaction coordinate method. In brackets, the corresponding values obtained from the micro-iterative method.

Structures	Mech. I	Mech. II	Mech. III
S	0.00	0.00	0.00
TS1	17.77 (18.24)	17.69 (17.78)	$\vert\$
TS2	19.52 (19.65)	14.77 (14.46)	$\vert\$
TS3	20.04 (20.06)	14.55 (14.95)	$\vert\$
TS4	22.54 (22.56)	$\vert\$	20.19 (19.50)
TS5	25.15 (25.75)	23.57 (23.83)	$\vert\$
TS6	27.22 (27.28)	28.14 (28.18)	$\vert\$
R	6.74	6.74	4.63

Table 3.10: Potential energies (kcal/mol) for the racemization of propargylglycolate using the reaction coordinate method. In brackets, the corresponding values obtained from the micro-iterative method.

Structures	Mech. I	Mech. II	Mech. III
S	0.00	0.00	0.00
TS1	15.90 (19.66)	11.83 (11.24)	$\vert\$
TS2	19.88 (19.92)	15.33 (15.32)	$\vert\$
TS3	22.18 (22.20)	16.96 (16.99)	$\vert\$
TS4	30.12 (22.20)	28.11 (20.68)	22.05 (21.98)
TS5	23.16 (22.60)	19.23 (20.08)	$\vert\$
TS6	23.89 (24.35)	26.45 (24.59)	$\vert\$
R	3.34	3.34	3.34