we can also calculated the equilibrium distribution of the two at room temperature using the formula[26]:
Nketo/Nenol = exp [(-1060)( Eketo- Eketo) (5)
Where Ni the number of molecules in a tautomer i, and E is the energy of the tautomer in au. Using this formula we
get the equilibrium constant for Keto to enol (Z) is 1.5 x 102 and keto to enol (E) is 6.64x 104. This is number of
keto formed with respect to one enol, which clearly shows that enol (Z) isomer formed and stable.
Thermodynamic calculations of ΔH of the isodesmic reaction for keto-enol (Z) are 3.6439 kcal/mol and keto-enol
(E) is 7.0067 kcal/mol. This explains the shift in equilibrium to the keto side. It can be seen that the ΔG (Table 4) for
the keto-enol (Z) system show that 1.4225 kcal/mol, and more positive for keto-enol (E) system shows that 4.2620
kcal/mol, which means that the reaction is non spontaneous and more stable form is enol (Z) isomer. This is
explaining the intramolecular hydrogen bond generates a five member ring [27] in enol (Z) isomer. Intramolecular
H-bonding is between O8-H17 donor and N11 acceptor of the enol (Z) isomer, but in case of the enol (E) no
intramolecular hydrogen bond generate.
Table 1: Optimized parameters Calculated bond lengths in (Å) and bond Angles ( 0)and Dihedral angles ( 0) of Cathinone for Keto, (Z)-
isomer ,( E)- isomer and their TS structures ,using B3LYP/6-311G (d,p) .