NMR study of the complex formation
Table 1 The thermodynamic parameters of complex formation between (CH3)3C–O–OH and Alk4NBr
System
KC/dm3 mol-1
d
comp/ppm
D2co9m8KpG /kJ mol-1
1H NMR
13C NMR
1H NMR
13C NMR
1H NMR
13C NMR
ROOH ? Et4NBr
ROOH ? Pr4NBr
ROOH ? Bu4NBr
4.4 0.4
3.4 0.4
2.3 0.1
4.6 0.6
3.5 0.4
2.4 0.2
10.63 0.08
10.72 0.09
10.95 0.03
26.6 0.2
26.6 0.3
26.9 0.8
-3.64
-3.02
-2.09
-3.78
-3.11
-2.12
The equilibrium constants of the complex formation
between hydroperoxide and Alk4NBr (KC) and the chem-
ical shift of the –CO–OH group proton of complexed
hydroperoxide (dcomp) were determined and are listed in
Table 1.
protons of Alk4NBr in the spectra of (CH3)3C–O–OH–
Alk4NBr solutions. However, the KC value depends on the
quaternary cation structure and limiting equivalent con-
ductivity (k1i ), namely it decreases with increasing of the
intrinsic volume of the salt cation (V) and with ki1
decreasing (Fig. 4).
The effect of the Alk4NBr on tert-butyl hydroperoxide
was investigated by 13C NMR spectroscopy. There are two
signals in the 13C NMR spectra of tert-butyl hydroperox-
ide: the first one at 26.06 ppm corresponds to the methyl
group carbon atoms and the second one at 80.44 ppm to the
–CO–OH moiety carbon. Addition of Alk4NBr to the
hydroperoxide solution leads to a shift of both signals. The
methyl group’s carbon atom signal is used for systematic
study because of high intensity and short experiment time
for each sample as compared with the signal of –CO–OH
moiety carbon.
So, dependence of the KC value on the structure of the
quaternary ammonium cation indicates cation participation
in the complex formation, which is consistent with a key role
of the anion [3] and regulating action of the cation [15, 16] in
hydroperoxide molecule activation. A similar effect has
already been observed in CDCl3 solution for the hydroper-
oxide—Alk4NBr systems [20]. The equilibrium constant
values also decrease with increasing intrinsic cation volume,
and this dependence effect remains over the temperature
range 297–313 K. DcompH values of the complex formation
are within -20 to -9 kJ mol-1 in CDCl3 solution, which
corresponds to a weak hydrogen bond. Thus, the cation effect
is not due to the solvent-specific properties only, but is also
caused by the complex structure.
The fact that the tetraalkylammonium bromides affect
the signal of the methyl group carbon and do not affect the
corresponding proton signal may indicate that the presence
of salt varies slightly the distribution of electron density in
–CH3 groups of the hydroperoxide molecule, but change of
the molecular conformation is possible. A similar effect is
shown for the binging of tetraalkylammonium salts with
meso-octamethylcalix [4] pyrrole [14].
One of the possible structure models of the complex
under consideration is a substrate-separated ion pair. This
model is characterized by the location of hydroperoxide in
the space between the cation and anion of the onium salt.
The solvent molecule is also taken into account (Fig. 5a).
We can propose an alternative model for the complex,
wherein the cation is not directly bonded with the hydro-
peroxide, but regulates the anion reactivity (Fig. 5b). In
this case, the hydroperoxide molecule is attacked by the
solvent-separated ion pair.
The relative chemical shifts in 13C NMR also provide a
tool for obtaining the stabilization constant of the complex
experimentally. 13C NMR spectroscopic studies also were
1
carried out in the same conditions as in the H NMR ones.
Increasing the Alk4NBr concentration in the system leads to a
shift of the –CH3 group carbon signal to the side of the weak
fields without splitting or significant broadening. This char-
acter of signal changing of the methyl group in the presence of
Alk4NBr (Fig. 3) indicates the formation of a complex
between hydroperoxide and Alk4NBr in the system.
The semi-empirical AM1 calculations showed the struc-
tural reorganization of the -COOH fragment geometry in the
complex. This process requires energy; thus, the complex
stability is not high. This is in accordance with the experi-
mental facts. Such reorganization leads to peroxide bond
activation and increasing of the hydroperoxide reactivity.
Further investigations to detail the structure and stability
of the complex in the framework of DFT methods are
required.
These experimental dependences of the Dd on the
Alk4NBr initial concentration are linear (Fig. 3) in the
Foster-Fyfe equation coordinates. This enables determining
the equilibrium constant of complexation (KC) and the
chemical shift of the –CH3 group carbon atoms of complex
bonded hydroperoxide (dcomp). The corresponding values
are listed in Table 1. These Kc values coincide with those
Thus, complex formation between tert-butyl hydroper-
1
oxide and Alk4NBr has been demonstrated by H and 13C
1
determined by H NMR spectroscopy.
NMR spectroscopy. The thermodynamic characteristics of
(CH3)3C–O–OH–Alk4NBr complex formation were deter-
mined. It is shown that the stability of these associates
It should be noted that there are no significant changes
in the chemical shifts of the methyl and methylene groups’
123