From the results of the quantum-chemical calculation of the heats of formation (Table 2), the isomers
can be placed in the following order of increasing thermodynamic stability relative to the simple substance:
6b (-61.8 kcal/mol) < 6a (-63.6 kcal/mol)
7b (-54.6 kcal/mol) < 7a (-55.2 kcal/mol)
8b (-16.0 kcal/mol) ~ 8a (-16.2 kcal/mol)
9b (-55.4 kcal/mol) < 9a (-56.5 kcal/mol)
10b (-93.9 kcal/mol) < 10a (-96.9 kcal/mol)
Thus among the o-isomers conformers 6a and 9a are somewhat more stable. The same is true for the
1,6-disubstituted compound 7 and the 2,4-disubstituted compound 10. There is practically no difference in
thermodynamic stability in the conformers of the m-isomer 8. Since the differences in thermodynamic stability
of the isomers a and b for compounds 6-10 do not exceed 3 kcal/mol, then, considering the accuracy of the
calculation, it is not possible to conclude that one or other of the isomers predominates to any extent in the
Schiff base formation reaction.
It may be noted from analysis of the geometric structures of the substituted thiazoles that the bond
lengths and valence angles in molecules with different substituents change very little. Nevertheless some
regularities can be noticed. Changes in bond lengths with different substituents are observed only in the
benzylidene component: the lengths of the bonds C(18)–C(19) and C(19)–C(20) increase with increasing electron
donor properties of the substituents in the p-position. For conformer a, the presence of an o-substituent in the
benzylidene component increases the length of the C(16)–C(17) and C(17)–C(18) bonds, whereas there is no change
in conformer b. On the other hand, the length of the N(14)–C(15) bond is decreased in conformer b but is
unchanged in conformer a. In the presence of a m-substituent, the C(18)–C(19) and N(14)–C(15) bond lengths are
decreased and the C(20)–C(21) bond length is increased in conformer b, while in conformer a the C(17)–C(18) bond
is lengthened and the C(19)–C(20) bond is shortened. Among the molecules calculated the angle N(10)–C(11)–S(12) is
decreased only for compounds 9a and 10a with a CH3O substituent at position 2. In comparison with the
p-substituted, the angle C(15)–C(16)–C(17) is larger in the o-isomers of form a and smaller in form b, whereas the
reverse is true for the m-isomers.
The effects of substituents on the torsion and dihedral angles are shown in Fig.2. We note that the size
(φ1), C
(φ2), and C
(φ3) are effectively
of the torsion angles O(22)–C(1)–C(2)–C(7)
(4)–C(5)–N(8)–C(9)
(13)–C(9)–N(8)–C(5)
unchanged by the introduction of a substituent into the benzylidene residue, whereas the values of the torsion
(φ4) and N (φ5) change considerably and depend on the nature
angles N(10)–C(11)–N(14)–C(15)
(14)–C(15)–C(16)–C(21)
and position of the substituent. For example for compounds with substituents in the m- and p- positions the size
of the torsion angles φ4 and φ5 change within the ranges from -13.8° to -8.0° and from -22.8° to -11.8°
-isomers the values of the torsion angles φ4 and φ5 for the form
respectively. For o
a lie within the limits from
-5.6° to -3.2° and -25.4° to -14.9° respectively; in the form b the thiazole and benzene rings are rotated more
considerably: φ4 and φ5 are found within the limits from -2.4° to 10.2° and from -45.3 to -43.0° respectively.
Maximum rotation of the rings occurs in the compounds with two o-substituents (from 15.1° to 16.8° and from
-59.1° to -57.2° for φ4 and φ5 respectively). Introduction
of two o-substituents into the molecule leads to
rotation of these rings in opposite directions, the amplitude of the rotation being greater for torsion angle φ5
than for angle φ4.
We have observed a relation between the electronic properties of the substituents in the benzene ring of
the benzylidene unit and the values of the torsion angles φ4 and φ5: the increase in electron acceptor properties
of the substituents in the series N(CH3)2 < OCH3 < OH < Br < NO2 leads to an increase in the amplitudes of the
deviation of the size of the angles (Fig. 2).
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