Alkyl substituent effect on the polarity of phenols-tri-n-alkylamine complexes

Article abstract of DOI:10.1139/v03-107

The formation constants and the dipole moments of the H-bonded adducts of 1:1 and 2:1 stoichiometries formed between three different phenols (phenol, 2,4,6-trichlorophenol, and 2,4-dinitrophenol) and different tri-n-alkylamines are determined in solvents

Full text of DOI:10.1139/v03-107

1012
Alkyl substituent effect on the polarity of phenols–
tri-n-alkylamine complexes
Zbigniew Pawe»ka and Therese Zeegers-Huyskens
Abstract: The formation constants and the dipole moments of the H-bonded adducts of 1:1 and 2:1 stoichiometries
formed between three different phenols (phenol, 2,4,6-trichlorophenol, and 2,4-dinitrophenol) and different tri-n-
alkylamines are determined in solvents of weak polarity. The polarity of the 1:1 complexes of 2,4,6-trichlorophenol
with tri-n-alkylamines markedly increases with increasing degree of amine alkylation, in contrast with the complexes
involving the two other phenols. The influence of the basicity and steric hindrance of the tri-n-alkylamines on the
proton-transfer constant is discussed and quantitative correlations are deduced.
Key words: phenols, tri-n-alkylamines, H-bonded complexes, proton transfer, polarity, steric effect.
Résumé : Les constantes de formation et les moments dipolaires de complexes à liaison hydrogène formés entre trois
différents phénols (phénol, 2,4,6-trichlorophénol et 2,4-dinitrophénol) et différentes tri-n-alkylamines sont déterminés
dans des solvants de faible polarité. La polarité des complexes de 1:1 stoechiométrie formés entre le 2,4,6-
trichlorophénol et les tri-n-alkylamines augmente avec le degré d’alkylation, contrairement aux complexes des deux au-
tres phénols. L’effet de la basicité et de l’encombrement stérique des tri-n-alkylamines sur la constante de transfert de
proton est discuté et des correlations quantitatives sont présentées.
Mots clés : phénols, tri-n-alkylamines, complexes à liaison hydrogène, transfert de proton, polarité, effect stérique.
Pawe»ka and Zeegers-Huyskens 1018
Introduction
shift, etc. Low-barrier, strong hydrogen bonds (LBHBs)
were proposed to explain the above particularities (18–20).
The H-bonds formed by groups with similar pKas possess
some partial covalent character, as they are the result of or-
bital overlapping between the matched energy states of AH
and B. This covalent character contributes to the strength of
the H bond, in addition to the pure electrostatic forces. The
dependence of the strength and polarity of the AH···B sys-
tem on ∆pKa can be demonstrated by changing the H-bond
donating ability of AH in the families of phenols or
carboxylic acids complexed with triethylamine or pyridines
(1–5, 21–23).
One of the most spectacular manifestations of hydrogen
bond (H-bond or HB) interactions (AH···B) is the enhance-
ment of the dipole moment µAHB in comparison with the
vectorial sum of the dipole moments of the isolated mole-
r
r
cules, µAH and µB. The dependence of the H-bond polarity
r
r
r
r
∆µ (= µAHB – µAH – µB) on the aqueous ∆pKa (= pK
–
+
pKAH) of the interacting species AH and B is very charBaHcter-
istic (1–5). For homologous series of complexes, e.g.,
phenols–triethylamine (1), the ∆µ vs. ∆pKa curve presents a
sigmoidal shape with a strong increase of ∆µ in the inver-
r
r
r
sion region. The ∆µ/∆pKa gradient in that region is so high
that an increase of ∆pKa by two units can lead to a new
protonic state of the complex, i.e., to hydrogen-bonded ion
pair AΘ···HB . The polarity of the adducts in the inversion
region can be explained in terms of the protomeric equilib-
rium (1–7)
The alkyl substitution effect on the basicity of amines has
been discussed for a long time (24–40). There is much less
information about this effect on the polarity of the molecular
complexes. The sensitivity to N-alkylation is manifested in
charge transfer complexes (41–46). For instance, the dipole
moment of the triethylamine–I2 complex is higher by about
0.5 D than that of trimethylamine adduct, irrespective of the
solvent (45, 46).
[1]
A—H···B
AΘ···H—B
To discuss the influence of N-alkylation on the proton ac-
ceptor abilities of tri-n-alkylamines, we have determined in
the present work the dipole moments of hydrogen-bond
complexes involving phenols and aliphatic tertiary amines
characterized by different degrees of substitution. The stud-
ied phenols are unsubstituted phenol, 2,4,6-trichlorophenol,
governed by the proton transfer (PT) constant KPT. Such H-
bonded systems are very sensitive to the intrinsic basicity or
acidity of the two partners and also to environmental effects
(8–17). Moreover, these complexes show anomalous spectro-
scopic phenomena, e.g., low isotopic ratio, large shift of the
νAH vibration frequencies, highly downfield proton chemical
Received 6 February 2003. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 1 September 2003.
Z. Pawe»ka.1 Faculty of Chemistry, University of Wroc»aw, 14 Joliot-Curie, 50-385 Wroc»aw, Poland.
Th. Zeegers-Huyskens. Department of Chemistry, University of Leuven, 200F Celestijnenlaan, B-3001 Heverlee, Belgium.
1Corresponding author (e-mail: zp@wchuwr.chem.uni.wroc.pl).
Can. J. Chem. 81: 1012–1018 (2003)
doi: 10.1139/V03-107
© 2003 NRC Canada

Pawe»ka and Zeegers-Huyskens
1013
Table 1. Parameters of eq. [10] and dipole moments of 2,4,6-trichlorophenol and tri-n-alkylamines in
tetrachloroethylene (or carbon tetrachloride).
Compound
Range of x2
αε1
2.533
β
γ n12
0.3859
P (cm3)
µ (D)
2,4,6-Trichlorophenol
Diethylmethylamine (DMA)
Triethylamine (TEA)
Tri-n-propylamine (TPrA)
Tri-n-butylaminea (TBA)
Tri-n-pentylamine (TPeA)
Tri-n-octylamine (TOA)
0.008–0.017
0.012–0.086
0.022–0.071
0.008–0.059
0.019–0.040
0.012–0.086
0.011–0.063
0.009–0.0033
0.031–0.079
0.016–0.046
–0.0232
–0.6391
–0.7385
–0.9702
–1.1681
–1.3615
–2.0315
–2.0882
–2.8441
–2.9172
36.7
24.7
14.4
11.0
9.7
9.6
9.9
13.3
9.9
14.8
1.33
1.09
0.83
0.73
0.69
0.68
0.69
0.80b
0.69
0.85b
1.072
0.407
0.157
0.070
0.087
0.214
0.292
–0.475
0.237
–0.3682
–0.4133
–0.4695
–0.4440
–0.4322
–0.7064
–0.0774
–0.9228
–0.0373
Tri-n-dodecylamine (TDA)
aFrom ref. (15).
bIn carbon tetrachloride.
and 2,4-dinitrophenol, and the tertiary aliphatic amines are
diethylmethyl-, triethyl-, tri-n-propyl-, tri-n-butyl-, tri-n-
pentyl-, tri-n-octyl-, and tri-n-dodecylamine. These systems
were chosen to cover a broad ∆pKa range.
It is well recognized that in nonpolar solvents (used in our
dielectric measurements), the tertiary amines form both 1:1
(AHB) and 2:1 [(AH)2B] adducts (15, 16, 47, 48). Thus the
starting point of the dielectric investigations was the deter-
ent constant (Kapp), based on a model of 1:1 stoichiometry
(47)
1
[2]
=
2CAH + KappC2AH
Kapp
1
KAHB 2CAH + KappCA2 H
− K
(AH)2 B
mination of the KAHB and K(AH) B formation constants by IR
2
The intensity of the ν(OH) absorption at ca. 3600 cm–1
was used in the determination of the concentration of the
nonbonded phenol derivative. It is worth mentioning that
this method gives the total concentration of complexes in
their neutral (OH···N), as well in their ionic (O–···HN+),
forms. At the low concentrations used in this work, second-
ary interactions such as stacking effects are unlikely to oc-
cur.
spectrometry. Knowledge of both stability constants allowed
us to attribute the dielectric polarization to adducts of a
given stoichiometry. In the present work we focused our at-
tention on the H-bond polarities and on the proton transfer
degrees in the 1:1 and 2:1 complexes formed between 2,4,6-
trichlorophenol and various trialkylamines.
Results and discussion
The equilibrium constants are indicated in Table 2. It may
be seen that the formation constants K(AH) B are larger than
AHB, which indicates a strong tendency to2form 2:1 species.
Both equilibrium constants are used for the simultaneous
Dipole moments of the components
K
The dipole moments of amines and 2,4,6-trichlorophenol in
tetrachloroethylene (or carbon tetrachloride) are reported in
Table 1. The values obtained for amines are of the expected
magnitudes (see values in other solvents (49)) and will not be
discussed hereafter. The increase of the dipole moments in
CCl4 as compared with tetrachlorethylene may be accounted
for by a charge-transfer interaction ϵN···Cl-Cϵ (50, 51). In
the case of amines with a lower degree of substitution, their
photochemical reaction with CCl4 leads, after some time, to a
turbidity of the solution. This was the reason for which — in
the time-consuming dielectric measurements — CCl4 was re-
placed by tetrachloroethylene. In the next section, the dielec-
tric polarizations of amines are employed in the calculation of
dielectric polarizations of the 1:1 and 2:1 complexes. The di-
pole moments of amines and 2,4,6-trichlorophenol are also
necessary for assessing the dipole moments of the molecular
HB (µHB) and ion pair PT (µPT) forms of the further discussed
1:1 complexes of 2,4,6-trichlorophenol with trialkylamines.
estimation of the dielectric polarizations of the 1:1 (PAHB
)
and 1:2 (P(AH) B) species. It is worth noticing that the K val-
2
ues reported in Table 2 indicate that in an excess of amine
larger than 5, the 1:1 complex dominates in the solution.
These conditions were therefore taken for the determination
of the dipole moment of the 1:1 complexes. In excess of
amine the following relation is valid:
°
P − P x
xAH
1 1
[3]
= PB + PAHB
xB
xB
where P, P1, PB, and PAHB are the dielectric polarizations of
solution, solvent, amine, and the 1:1 complex, respectively,
°
°
°
and xB (xB = xB − xAH) and xAH are the molar fractions of
the amine and 2,4,6-trichlorophenol; the superscript ° indi-
cates the initial molar fractions.
°
Figure 1 shows the plot of (P – P1x1)/xB against xAH/xB
for selected complexes. Similar linear relationships were
Dielectric polarizations and dipole moments of the
complexes
The formation constants KAHB and K(AH) B of the 2,4,6-
trichlorophenol–trialkylamine complexes in 2a given solvent
were estimated simultaneously by the method of the appar-
-
-
also observed for other systems. The molar dielectric polar
izations and dipole moments obtained in this way are col
lected in Table 3.
One should note the very good coincidence of the PB val-
ues found from the linear equation, eq. [3] (Fig. 1), and from
© 2003 NRC Canada

1014
Can. J. Chem. Vol. 81, 2003
Table 2. Formation constants of 1:1 (KAHB) and 2:1 (K(AH) B
complexes between 2,4,6-trichlorophenol and tri-n-alkylamines in
carbon tetrachloride.
)
concentration on µAHB, we carried out additional measure-
ments in an excess of tri-n-dodecylamine ((concentration =
c) cB/cAH ≈10) at concentrations of 2,4,6-trichlorophenol
ranging from 0.4 × 10–3 to 1 × 10–2 mol dm–3. The dipole
moments at xAHB = 4.23 × 10–4 and 1.06 × 10–2 are equal to
6.08 and 6.01 D, respectively. Moreover, the mean deviation
from the average value of µAHB for the whole concentration
range is only ±0.04 D, which indicates the lack of self-
association of the complex.
Finally, to estimate the alkylation effect on the polarity of
the (AH)2B complex, we have compared the dipole moments
of the 2:1 adduct of 2,4,6-trichlorophenol with triethylamine
and tri-n-dodecylamine. The equilibrium concentrations of
all species in the solution were evaluated using the forma-
2
KAHB
K
(AH)2 B
Amine
(dm3 mol–1)
(dm3 mol–1)
Triethylamine
55
54
Tri-n-propylamine
Tri-n-butylamine
Tri-n-pentylamine
Tri-n-octylamine
Tri-n-dodecylamine
20.5
23
28.5
36.5
45
44
57
84.5
77
64
Fig. 1. Dielectric polarization (P – P1x1)/xB as a function of the
xAH/xB ratio (eq. [3]). Complexes of 2,4,6-trichlorophenol with ꢀ
tri-n-dodecylamine (CCl4), ᭿ tri-n-butylamine, and ᭜
triethylamine.
tion constants KAHB and K(AH) B. In the next step, the dielec-
2
tric polarizations PAHB and P(AH) B were estimated according
2
to the linear equation
P − P x − PAHxAH − PBxB
1 1
[4]
P* =
xAHB
x
(AH)2 B
= PAHB + P
(AH)2 B
xAHB
To allow the 1:1 or 2:1 species to be in excess, the mea-
surements were carried out within a wide range of initial
x
AH/xB ratios. For these ratios, the corresponding
x(AH) B/xAHB values comprise between 0.02 and 2.45.
2
The µAHB and µ(AH) B values of the 2,4,6-trichlorophenol–
2
TEA adduct in tetrachloroethylene were obtained by us ear-
lier (16). Equation [4], applied to all investigated complexes
of 2,4,6-trichlorophenol, can be expressed as:
[4.1] P*(TEA, C2Cl4)
= 610 + 1830 (x(AH) B/xAHB
N = 7
independent measurements in binary solutions (indicated in
parentheses). The data reported in Table 3 reveal that the di-
pole moment markedly increases on going from diethyl-
methyl- to tri-n-butylamine and that only smooth changes
are observed for longer alkyl chains. This effect was com-
pared with analogous results for much weaker (phenol–
trialkylamines) and much stronger (2,4-dinitrophenol–
trialkylamines) interactions. The unsubstituted phenol–
trialkylamine complex belongs to the typical molecular hy-
drogen-bonded form OH···N, in which the polarity enhance-
ment of the order of 0.7–1 D is caused by induction and
charge transfer effects. In contrast, the 2,4-dinitrophenol–
trialkylamine adduct is the hydrogen-bonded ion pair
OΘ···HN , in which the proton is transferred to the nitrogen
atom of the amine. The results obtained from eq. [3] are
summarized in Table 4.
)
2
R = 1
[4.2] P*(TEA, CCl4)
= 610 + 1740 (x(AH) B/xAHB
N = 6
)
)
2
R = 0.99
[4.3] P*(TDA, C2Cl4)
= 770 + 1820 (x(AH) B/xAHB
N = 10
2
R = 0.99
The corresponding dipole moments of the triethylamine
adducts are µAHB = 5.42 ± 0.02 D and µ(AH) B = 9.42 ±
2
0.05 D in tetrachloroethylene, and µAHB = 5.43 ± 0.07 D
A rough inspection of the data in Table 4 shows a moder-
ate increase in the dipole moment when the triethyl group is
replaced by the tri-n-dodecyl one. Our measurements were
carried out at low concentration of phenols. Nevertheless, in
the case of 2,4,6-trichlorophenol complexed with long chain
amines, one could admit association of the following type:
and µ(AH) B = 9.17 ± 0.14 D in CCl4. Equation [4.3] gives
2
values of µAHB = 6.10 ± 0.06 D and µ(AH) B = 9.38 ± 0.14 D
2
for the tri-n-dodecylamine adduct. The good agreement be-
tween the µAHB value determined in excess of amine (see Ta-
ble 3) and that resulting from eq. [4] is worth noticing.
Earlier data based on infrared spectrometry or dipole mea-
surements (15–17, 48) have demonstrated that the adducts of
2:1 stoichiometry between trihalogenophenols and tributyl-
amine are hydrogen-bonded ion pairs. The similar value of
the dipole moment of the tridodecylamine complexes indi-
cate that this conclusion can be extended to more hindered
amines. The 2:1 complexes are characterized by the struc-
monomer
dimer
n-mer. As a final step, an inverted mi-
celle may be formed with the polar O-H···N group in its in-
terior and the lipophilic alkyl chains directed towards the
nonpolar solvent. In principle such an aggregation should
enlarge the effective dipole moment resulting from specific
dipole orientation. To establish the effect of the proton donor
© 2003 NRC Canada

Pawe»ka and Zeegers-Huyskens
Table 3. Dielectric polarizations and dipole moments of the 1:1 complexes formed between 2,4,6-
1015
trichlorophenol and tri-n-alkylamines in tetrachloroethylene (or carbon tetrachloride).
Amine
Range of xAH/xB
PAHB(cm3)
PB (cm3)
a
µAHB (D)
Diethylmethylamine
Triethylamine
0.022–0.158
0.021–0.338
0.023–0.215
0.030–0.108
0.037–0.327
0.019–0.271
0.024–0.226
0.054–0.233
0.026–0.118
475
580
600
600
700
680
745
765
750
24.5 (24.7)
24.7
14.9 (14.4)
11.7 (11.0)
9.5 (9.7)
9.6 (9.6)
13.3 (13.3)
9.6 (9.8)
4.79±0.07
5.28±0.04b
5.40±0.02
5.38±0.06
5.81±0.03
5.74±0.05
6.00±0.05b
6.08±0.04
6.02±0.08b
Tri-n-propylamine
Tri-n-butylamine
Tri-n-pentylamine
Tri-n-octylamine
Tri-n-dodecylamine
14.9 (14.8)
aValues in parentheses are determined in a binary solution.
bIn carbon tetrachloride.
Table 4. Dielectric polarizations and dipole moments of the 1:1 H-bonded complexes formed between some
phenols and trialkylamines in tetrachloroethylene.
System
Range of xAH/xB
PB (cm3)
a
PAHB (cm3)
µAHB (D)
190
210
2640
2720
3.04±0.02
3.18±0.05
11.29±0.03
11.46±0.02
Phenol–triethylamine
0.007–0.242
0.059–0.206
0.027–0.263
0.051–0.318
14.5 (14.9)
9.9 (9.8)
14.8 (14.9)
9.5 (9.8)
Phenol–tri-n-dodecylamine
2,4-Dinitrophenol–triethylamine
2,4-Dinitrophenol–tri-n-dodecylamine
aValues in parentheses are determined in a binary solution.
ture OH···O–···HN+. In this structure, both OH···O– and
O–···HN+ hydrogen bonds become stronger than the OH···O
and OH···N hydrogen bonds in the corresponding phenol
dimer and in the 1:1 complex. This σ-bond cooperative ef-
fect results in a large charge redistribution and proton trans-
fer in the O-H···N bridge. Despite a relatively large error
(±0.15 D) in the determination of µ(AH) B, the very low sen-
sitivity to the lengthening of the alkyl 2groups in the amine
moiety is worth mentioning. This behaviour is apparently
analogous to that found in the 1:1 complexes of 2,4-
dinitrophenol with the same tri-n-alkylamines (see Table 4).
Fig. 2. Dipole moment increment of the 1:1 complexes between
phenol, 2,4,6-trichlorophenol, and 2,4-dinitrophenol with tri-n-
alkylamines as a function of ∆pKa. The dipole moments of phe-
nol and 2,4-dinitrophenol in carbon tetrachloride are 1.45 and
3.31 D, respectively; their pKa are 9.95 and 4.10.
These results indicate that the effect of N-alkyl substitu-
tion on the polarity is weak in complexes characterized by
normal OH···N hydrogen bonds and in proton-transfer com-
plexes characterized by O–···HN+ hydrogen bonds. For both
types of complexes, only a very small increase in ∆µAHB
with ∆pKa is observed. This feature is displayed in Fig. 2,
which also illustrates the sigmoidal shape found for other
systems (1–5).
Proton transfer constant in the 1:1 complexes between
2,4,6-trichlorophenol and tri-n-alkylamines
As discussed in the previous section, the polarity of the of
2,4,6-trichlorophenol–trialkylamine complexes increases
with increasing extent of alkylation of the amine. This com-
plex lies in the inversion region of the ∆µAHB vs. ∆pKa curve
(Fig. 2). The polarity of such systems can be discussed in
terms of the protomeric equilibrium (1). To discuss the
alkylation effect more quantitatively, we calculated the H-
µ2AHB − µ2HB
[5]
xPT =
µ2PT − µ2HB
The proton transfer constant KPT was taken as the ratio
r
r
r
r
r
xPT/xHB
.
bond polarity ∆µ, defined as ∆µ = µAHB – (µAH + µB), as
well as the proton transfer constant KPT. For the calculation
The dipole moments of the molecular AH···B (µHB) and
r
of ∆µ, the vectorial scheme proposed in ref. (2) was applied.
The degree of proton transfer, xPT, was calculated from the
relation
proton-transfered AΘ···HB (µPT) forms were estimated as-
r
r
suming ∆µ = 0.8 D (3) in the former and ∆µ = 9.3 D (1) in
the latter one. These data are summarized in Table 5.
© 2003 NRC Canada

1016
Can. J. Chem. Vol. 81, 2003
r
Table 5. The pKa and molar volumes of the tri-n-alkylamines, polarity of the hydrogen bond (∆µ), pro-
ton transfer degree (% PT), and logKPT in the 2,4,6-trichlorophenol–tri-n-alkylamines complexes.
Amine
pKa
V (dm3 mol–1)
∆µ (D)
% PT
–log KPT
Diethylmethylamine
Triethylamine
10.29
10.65
0.120
0.139
2.46
3.21a
3.34
3.42
3.89
3.83
4.08a
4.16
4.10
10.5
16.2a
17.3
17.8
22.2
21.5
21.5a
24.9
24.3
0.931
0.714a
0.679
0.664
0.545
0.562
0.498a
0.479
0.498
Tri-n-propylamine
Tri-n-butylamine
Tri-n-pentylamine
Tri-n-octylamine
10.65
10.89
11.00
11.19
0.192
0.242
0.292
0.462
Tri-n-dodecylamine
11.3b
0.697
aIn carbon tetrachloride.
bObtained from extrapolation of the pKa values as a function of the number of CH2 groups in the aliphatic chain.
r
The increase in ∆µ or KPT may be rationalized in terms of
the increasing proton-accepting abilities of amine. The pro-
ton affinity is a direct measure of the intrinsic basicity, unaf-
fected by solvation effects. Unfortunately, the proton
affinities are known only for the aliphatic tertiary amines
characterized by a low degree of substitution, and they in-
crease from 232.3 to 234.8 kcal mol–1 on going from
triethylamine to tri-n-butylamine (52). Therefore, we de-
cided to correlate the KPT values with the basic properties of
amines in water (24, 27) (see Table 5). The classical relation
Fig. 3. log KPT as a function of ∆pKa for the 1:1 complexes be-
tween 2,4,6-trichlorophenol and trialkylamine.
[6]
logKPT = ξ∆pKa + C
takes for the present complexes the following form:
N = 7
[7]
logKPT = –2.67 + 0.42∆pKa
R = 0.944
As illustrated in Fig. 3, marked deviations from this linear
correlation are observed for tri-n-pentyl-, tri-n-octyl-, and
tri-n-dodecylamine, where the bulky substituents restrict the
approach of the proton. This steric hindrance must be larger
in the 2,4,6-trichlorophenol complexes than in monohydrate
B H···OH2. This comparison is justified since it appears that
steric hindrance to ion solvation in bulk water is comparable
in magnitude to steric hindrance of solvation by only one
H2O molecule (36). To estimate quantitatively the effect of
steric hindrances on KPT, we have added to eq. [6] a term de-
pendent on the molar volume of the amine in tetrachloro-
ethylene (V in dm3 mol–1, see Table 5). The two-parameter
linear regression now gives the following relation:
duced for the present complexes because, as discussed in
this section, both OH···N and O–···HN+ species are present in
solution. The O(H)···N distance is considerably shorter
(2.50–2.55 Å) than the O–···HN+ one (2.75–2.85 Å) (27),
and as a consequence, the normal OH···N hydrogen bonds
are expected to be more sensitive to steric interactions than
the ionic O–···HN+ ones.
[8]
logKPT = –3.24 – 0.55∆pKa + 0.03(logV)–1
N = 7
R = 0.989
Comparison with the crystal structures
In the solids, the inflection point, where 50% PT is for-
mally reached, occurs with the weaker bases. This is in
agreement with expectations since the crystal lattice favors
the polar PT state (53). When 2,4-dinitrophenol forms com-
plexes with diamines or tertiary amines, in all the published
structures where the hydrogen atoms have been located, the
phenol completely transfers the proton to the amine (54–59).
The structure of the (phenol)2-1,4-diazabicyclo[2,2,2]oc-
tane (DABCO) complex, determined at –130°, is worth men-
tioning. In this complex, the hydrogen atoms stay firmly
attached to the phenol oxygen atoms. Strong hydrogen
bonds are formed, the two O···N distances being 2.694 and
2.681 Å, respectively, and the corresponding OH···N angles
characterized by a better correlation coefficient than that of
eq. [7]. It may be concluded that both the C and ξ parame-
ters in eq. [6] are to some degree connected with the steric
effects.
A last remark concerns the influence of steric hindrance
on the formation constants of the 1:1 complexes. For com-
plexes between phenols and aliphatic tertiary amines charac-
terized by normal OH···N hydrogen bonds, the formation
constants have been shown to depend on the polar and steric
parameters of Taft (27). Similar correlations could not be de-
© 2003 NRC Canada

Pawe»ka and Zeegers-Huyskens
1017
167° and 165°.2 No PT structure is observed in this case.
This may result from the fact that DABCO is less basic than
triethylamine; its pKa is indeed equal to 8.8. A second factor
that plays a determinant role is the anticooperativity in the
hydrogen bond. The formation of the OH···N hydrogen bond
on one of the N atom of DABCO decreases the proton-
acceptor ability of the second N atom (60).
Acknowledgements
This work was done in the framework of the cooperation
between the Catholic University of Leuven and the Univer-
sity of Wroc»aw. The authors thank the anonymous referee
who provided the structure of the (phenol)2–DABCO com-
plex. Z.P. thanks the University of Leuven for a post-
doctoral fellowship.
Experimental
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