Thermodynamics of the transformation of spiranes of the perimidine series into quinonimines

Article abstract of DOI:10.1023/A:1012405229637

Thermodynamic equilibrium between the spirane and quinonimine structures of photochromic perimidine derivatives was studied by electron absorption spectroscopy and semiempirical quantum-chemical methods (AM1 and PM3). The experimental data showed that the reaction entropy ΔS₀ in octane weakly depends on the substituent structure (0.015-0.024 kJ mol^-1 K^-1) and that the enthalpy of the reaction for the compound with bulky isobutyl substituent (ΔW₀ = 7.1 kJ/mol) is lower than for those containing hydrogen and methyl group (12.9 and 13.1 kJ/mol, respectively). According to the calculations, in the spirane structure the orientation of lone electron pairs on the perimidine nitrogen atoms may be both cisoid or transoid for different orientations of the cyclohexadiene fragment which adopts a boat conformation. The quinonimine isomer can also exist in different stable conformations.

Full text of DOI:10.1023/A:1012405229637

Russian Journal of Organic Chemistry, Vol. 37, No. 3, 2001, pp. 437 441. Translated from Zhurnal Organicheskoi Khimii, Vol. 37, No. 3, 2001,
pp. 462 466.
Original Russian Text Copyright
2001 by Kharlanov.
Thermodynamics of the Transformation of Spiranes
of the Perimidine Series into Quinonimines*
V. A. Kharlanov
Research Institute of Physical and Organic Chemistry, Rostov State University,
pr. Stachki 194/2, Rostov-on-Don, 344104 Russia
e-mail: vlad@ipoc.rnd.runnet.ru
Received May 25, 1999
Abstract Thermodynamic equilibrium between the spirane and quinonimine structures of photochromic
perimidine derivatives was studied by electron absorption spectroscopy and semiempirical quantum-chemical
methods (AM1 and PM3). The experimental data showed that the reaction entropy S0 in octane weakly
1
depends on the substituent structure (0.015 0.024 kJ mol 1 K ) and that the enthalpy of the reaction for
the compound with bulky isobutyl substituent ( H0 = 7.1 kJ/mol) is lower than for those containing hydrogen
and methyl group (12.9 and 13.1 kJ/mol, respectively). According to the calculations, in the spirane structure
the orientation of lone electron pairs on the perimidine nitrogen atoms may be both cisoid or transoid for
different orientations of the cyclohexadiene fragment which adopts a boat conformation. The quinonimine
isomer can also exist in different stable conformations.
Thermodynamic equilibrium between the spirane
and quinonimine structures of photochromic perimi-
dine derivatives I (Scheme 1) [1 12] involves
spectroscopy and quantum-chemical calculations of
the geometry of tautomeric forms.
The experimental results are summarized in Table 1
and Fig. 1. The entropy of the reaction S0 deter-
mined in a nonpolar solvent (octane) only slightly
depends on the R substituent; it ranges from 0.015 to
3
cleavage of the N Csp bond and intramolecular
proton transfer [1, 4 6]. The detailed mechanism of
this process is difficult to elucidate because of the
lack of the corresponding quantitative thermodynamic
parameters and information on possible conformers
of the cyclic and open structures. In the present com-
munication we report the results of our study on the
thermodynamics of the transformation of the spirane
structure into quinonimine by electron absorption
1
0.024 kJ mol 1 K , being almost similar for com-
pounds Ia and Ic. The enthalpy of the reaction for
compound Ic having a bulky isobutyl group ( H0 =
7.1 kJ/mol) is considerably lower than H0 for
Scheme 1.
R = H (a), CH3 (b), CH2CH(CH3)2 (c).
____________
Fig. 1. Temperature dependences of the Gibbs energy G0
of the transformation of the spirane structure into quinon-
imine for compounds Ia Ic in octane (experimental data).
*
This study was financially supported by the Russian Founda-
tion for Basic Research (project no. 99-03-32485a).
1070-4280/01/3703-0437$25.00 2001 MAIK Nauka/Interperiodica

438
KHARLANOV
Table 1. Gibbs energies G0 (kJ/mol, 293 K), enthalpies
compounds Ia and Ib (12.9 and 13.1 kJ/mol, respec-
tively). These data show that there is no correlation
between thermodynamic parameters of the transforma-
1
H0 (kJ/mol), and entropies S0 (kJ mol 1 K ) of the
transformation of the spirane structure of compounds Ia Ic
into quinonimines IIa IIc
tion I
II and electronic properties of the R substit-
uent {the donor effect increases in the series H <
CH3 < CH2CH(CH3)2 [13]}. Presumably, the main
factor affecting H0 is steric interaction between the
substituent and spirane framework. In order to check
this assumption we performed quantum-chemical cal-
culations by the AM1 and PM3 methods of possible
structures of compounds Ia and Ib. The results are
presented in Tables 1 3 and Figs. 2 and 3.
According to the calculations, molecules Ia and Ib
can exist as spirane conformers A D. Conformers A
and C of Ia are enantiomeric (Fig. 2). Compound Ic
gives rise to a larger number of conformers due to
nonrigidity of the isobutyl group. The PM3 heats of
formation increase in the series A < B < C < D
(Fig. 2) for compound Ib, the energy difference
between conformers A and D being 8.8 kJ/mol.
According to the AM1 calculations, the heats of
formation change in the series B < A < D < C, and
the energy difference between the most and least
stable conformers is 6.5 kJ/mol. The opposite results
were also obtained by the AM1 and PM3 calculations
of compound Ia: the most stable conformers are D
(AM1) and B (PM3), but the differences between the
most and least stable conformers are considerably
lesser, 0.9 (AM1) and 1.3 kJ/mol (PM3).
Calculationa
Experimental
Reaction Parameter
data (octane)
AM1
8.8
PM3
57.7
56.6
Ia
Ib
Ic
IIa
IIb
IIc
G0
H0
S0
G0
H0
S0
G0
H0
S0
5.7
12.9 0.3
0.024 0.001
8.6
13.1 0.5
0.015 0.002
0.2
7.4
7.1 0.6
0.024 0.002
a
For most stable conformers.
Table 2. Experimental (X-ray diffraction data [4]) and
calculated (AM1, PM3) bond lengths and bond and tor-
sional angles of compound Ib (conformer C)
Calculation
Experiment
Bond (angle)
AM1
PM3
[4]
All conformers of Ia and Ib are characterized by
a folding angle of 20 30 along the N N axis in the
perimidine fragment, and the cyclohexane fragment
has a boat conformation with a folding angle of
25 35 (Table 2). The calculated data are well con-
sistent with those obtained by X-ray analysis of com-
pound Ib [4] (Table 2). However, the calculations
showed that the spirane structure can be characterized
by not only cisoid orientation of lone electron pairs
on the nitrogen atoms in the perimidine fragment
(conformers A and C; it is observed experimentally
[4]) but also transoid orientation (coformers B and D).
Moreover, the presence of tert-butyl groups is
responsible for the boat conformation of the cyclo-
hexadienone fragment. Analogous calculations of
model compounds having no tert-butyl groups predict
a planar structure of this fragment.
The quinonimine isomer of compounds Ia and Ib
can exist as conformers A and B shown in Fig. 3
(Table 3). The cyclohexadienone fragment in these
conformers also adopts a boat conformation, which
was found experimentally for some compounds with
electron-acceptor substituents in the crystalline phase
[6]. Both AM1 and PM3 data indicate that conformer
Bond lengths,
C1 C2
1.3928
1.4098
1.3730
1.4211
1.4123
1.4052
1.4041
1.4814
1.4820
1.5343
1.3410
1.4880
1.5366
1.2354
1.3806
1.4108
1.3698
1.4203
1.4075
1.4312
1.4323
1.5027
1.5007
1.5153
1.3409
1.4969
1.5181
1.2150
1.369(3)
1.400(3)
1.364(4)
1.402(3)
1.413(3)
1.382(2)
1.394(2)
1.458(2)
1.480(2)
1.506(2)
1.331(2)
1.488(3)
1.492(3)
1.219(3)
C2 C3
C3 C4
C4 C10
C9 C10
C1 N11
C8 N13
N11 C12
N13 C12
C12 C14
C14 C15
C15 C16
C18 C12
C16 O19
Angles, deg
N11C12N13
C14C12C18
112.7
108.7
23.5
28.3
32.4
83.8
109.2
109.7
27.7
29.6
31.7
92.1
106.8
112.0
41.6
10.8
21.3
87.8
1
2
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 3 2001

THERMODYNAMICS OF THE TRANSFORMATION OF SPIRANES
439
Fig. 2. Spirane structure of compound Ib (conformers A, B, C, and D), according to AM1 amd PM3 quantum-chemical calcula-
tions; tert-butyl groups are not shown. Conformers of compound Ia have similar structures; conformers A and C are enantiomers.
Fig. 3. Quinonimine structure of compound Ia (conformers A and B), according to AM1 amd PM3 quantum-chemical
calculations; tert-butyl groups are not shown. Conformers of compound Ib have similar structures.
A of structures IIa and IIb is more favorable than B:
the energy difference is 3.1 3.2 kJ/mol.
most stable conformers of the spirane and quinon-
imine structures. The calculated enthalpies were com-
pared with the experimental data (Table 1). Both AM1
and PM3 calculations show that spirane structure Ia
is more stable than quinonimine isomer IIa, but the
Using the results of quantum-chemical calculations,
we estimated the enthalpies of reactions I
II as
the difference between the heats of formation of the
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 3 2001

440
KHARLANOV
Table 3. Calculated (AM1, PM3) bond lengths and
bond and torsional angles in quinonimine structure IIa
(conformer A; for atom numbering, see Fig. 3)
dynamic parameters and equilibrium constant Keq.
The equilibrium concentrations of the quinonimine
([Q]) and spirane structures ([S]) were determined
spectrophotometrically from the optical density at the
long-wave absorption maximum of quinonimine II,
its molar absorption coefficient, and overall concentra-
Bond (angle)
AM1
PM3
tion. Below are given
( ) for the quinonimine
max
Bond lengths,
isomers of the compounds under study: IIa, 584 nm
C1 C2
1.3928
1.4098
1.3730
1.4211
1.4123
1.4052
1.4041
1.4814
1.4820
1.5343
1.3410
1.4880
1.5366
1.2354
1.3806
1.4108
1.3698
1.4203
1.4075
1.4312
1.4323
1.5027
1.5007
1.5153
1.3409
1.4969
1.5181
1.2150
1
1
(5330 l mol 1 cm ); IIb, 620 nm (4670 l mol 1 cm );
C2 C3
IIc, 630 nm (4200 l mol 1 cm ) [7]. A correction
1
C3 C4
was introduced, which took into account variation of
the concentration due to thermal expansion.
AM1 and PM3 quantum-chemical calculations of
the spirane (I) and quinonimine structures (II) were
performed using Gaussian 94W software [14].
C4 C10
C9 C10
C1 N11
C8 N13
N11 C12
N13 C12
C12 C14
C14 C15
C15 C16
C18 C12
C16 O19
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112.7
108.7
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28.3
32.4
109.2
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=
=
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THERMODYNAMICS OF THE TRANSFORMATION OF SPIRANES
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 3 2001

G0, kJ/mol
Temperature, K