FT-IR and NMR study of tris(oxaalkyl) borates and their complexes with HAuCl4

Article abstract of DOI:10.1016/S0022-2860(99)00281-1

Three tris(oxaalkyl) borates and their complexes with HAuCl₄ were studied by FT-IR, ¹H and ¹³C NMR spectroscopy. Continuous absorption in the FT-IR spectra of the complexes testifies the formation of proton channels showing large proton polarizability. The shape of the proton potential is discussed on the basis of the NM -data. (C) 2000 Elsevier Science B.V.

Full text of DOI:10.1016/S0022-2860(99)00281-1

Journal of Molecular Structure 519 (2000) 119–123
www.elsevier.nl/locate/molstruc
FT-IR and NMR study of tris(oxaalkyl) borates and their
complexes with HAuCl4
a,
a
a
a
a
*
B. Brzezinski , B. Gierczyk , B. R o´ z˙ alski , G. Wojciechowski , G. Schroeder ,
b
G. Zundel
a
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, PL-60 780 Poznan, Poland
b
Institute of Physical Chemistry, University of Munich, Theresienstr. 41, D-80333 Munich, Germany
Received 19 April 1999; accepted 20 May 1999
Abstract
1
13
Three tris(oxaalkyl) borates and their complexes with HAuCl were studied by FT-IR, H and C NMR spectroscopy.
4
Continuous absorption in the FT-IR spectra of the complexes testifies the formation of proton channels showing large proton
polarizability. The shape of the proton potential is discussed on the basis of the NMR data. ᭧ 2000 Elsevier Science B.V. All
rights reserved.
1
13
Keywords: Tris(oxaalkyl) borates; Proton polarizability; Hydrogen bond; Collective hydrogen bond; FT-IR spectroscopy; H and C NMR
1. Introduction
[5]. These systems can be considered as models for
biological proton channels [6–8].
Systems with very fast proton fluctuations in multi-
New systems such as polyethylene glycol esters of
boric(III) acid including many acceptor oxygen atoms
in oxaalkyl chains, which are able to form proton
channels, are studied in this paper by FT-IR and
NMR spectroscopy.
minima potentials show the so-called proton polariz-
abilities. These proton polarizabilities are two or three
orders of magnitude larger than polarizabilities due to
the distortions of electron systems. These proton
polarizabilities are demonstrated by intense contin-
uous absorption in the infrared spectra [1–3].
Recently, we have studied trisalkyl phosphates in
which the fluctuation of proton is screened by three
hydrophobic alkyl chains [4]. If in such chains oxygen
atoms are present the hydrogen-bonded channels are
formed which show large proton polarizability due to
the proton motion in multiminima proton potentials
2
. Experimental
Tris(oxaalkyl) borates were obtained by the heating
of boric(III) acid with HO–[CH –CH –O] –CH
3
2
2
n
(
where n ˆ 1–3) alcohol for about 20 h in Dean–
Stark apparatus. After this time the solvent was
evaporated under reduced pressure and the tris(oxa-
alkyl) borates were distilled. B2 (yield, 65%; b.p.,
1
2
79–180ЊC/20 mmHg); B3 (yield, 82%; b.p., 202–
03ЊC/3 mmHg); B4 (yield, 88%; b.p., 256–262ЊC/
*
Corresponding author. Tel.: ϩ 48-61-869-9181; fax: ϩ 48-61-
8
65-8008.
E-mail address: bbrzez@main.amu.edu.pl (B. Brzezinski).
0.5 mmHg).
0
022-2860/00/$ - see front matter ᭧ 2000 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(99)00281-1

1
20
B. Brzezinski et al. / Journal of Molecular Structure 519 (2000) 119–123
1
3
5
7
1
3
5
O
O
O
B
O
4
B
O
4
2
6
8
2
3
3
B2
B3
1
3
5
7
9
11
O
O
B
O
4
O
10
2
6
8
3
B4
Scheme 1.
2
.1. Preparations of complexes with HAuCl4
sw ˆ 19 000 Hz; at ˆ 1.8 s; T ˆ 293 K and TMS as
internal standard.
The corresponding 1:1 complexes of tris(oxaalkyl)
borates with HAuCl were obtained by mixing of the
4
equimolar amounts of the ethanol solutions of these
substances. The solvent was evaporated under
reduced pressure at room temperature and the
complexes were dissolved in CHCl3 for FT-IR
3
. Results and discussion
We synthesised and studied three borates: tris(1,4-
dioxa-pentane) (B2), tris(1,4,7-trioxaoctane) (B3) and
tris(1,4,7,10-tetraoxaundecane) (B4). The formulae of
these compounds are shown in Scheme 1.
measurements and in CDCl for NMR measurements.
3
The concentration of the complexes was always
Ϫ3
0
.1 mol dm . All preparations and transfers of solu-
The IR spectra of these compounds and their
tions were carried out in a carefully dried glovebox
under nitrogen atmosphere.
complexes with HAuCl are given in Fig. 1. In the
4
IR spectra of these complexes intense continua are
observed. The wavenumber region in which the
continua are found as well as the structure of these
continua depends strongly on the length of the chains,
i.e. on the number of O-atoms in the chains. This
result demonstrates that the proton fluctuates in multi-
minima potentials along the O-atoms in the channel
formed by the three chains as shown in Scheme 2.
The continuous absorption observed in the spectra
of these three systems demonstrates that these chan-
nels show large proton polarizability in multiminima
proton potentials. With increasing number of the O-
atoms in the chains, the absorption of the continuum
shifts toward higher wavenumbers. The reason of this
is an increase of the barriers in the multiminima
potentials. This increase implies that the distance
between the E and E energy levels increases, and
2
.2. IR measurements
A cell with Si windows and a wedge-shaped layer
was used to avoid interferences (mean layer thickness
.176 mm). The IR spectra were taken with an FT-IR
0
spectrophotometer IFS 113v from Bruker, using an
MCT detector (125 scans, resolution 2 cm ).
Ϫ1
2.3. NMR measurements
NMR spectra were recorded using a Varian Gemini
1
300 spectrometer. All H NMR measurements in
CDCl3 were carried out at operating frequency
sfrq ˆ 300.075 MHz; pw ˆ 45Њ; sw ˆ 4500 Hz;
at ˆ 2.0 s; T ˆ 293 K and TMS as the internal
standard.
0
1
therefore the continuous absorption shifts toward
higher wavenumbers.
13
C NMR spectra were recorded for CDCl at the
3
operating frequency sfrq ˆ 75.454 MHz; pw ˆ 60Њ;
Further information about the nature of the proton

B. Brzezinski et al. / Journal of Molecular Structure 519 (2000) 119–123
121
4
Fig. 1. FT-IR spectra of: (- - - -) tris(oxaalkyl) borates and (—) their 1:1 HAuCl complexes. (a) B2; (b) B3; (c) B4.
fluctuation in the channels is obtained from NMR
studies. The H NMR data on the three borates and
proton is strongly hydrogen-bonded in the channel
and these hydrogen bonds are of comparable strength.
All signals of the CH protons are shifted slightly
with the complex formation toward lower fields. This
result indicates that all O-atoms in the channel
become slightly positive due to the influence of the
1
their complexes with HAuCl are collected in Table 1.
4
In the NMR spectra of the borate complexes the
signals of the excess protons are found as singlets in
the region 9.07–9.93 ppm, indicating that the excess

1
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B. Brzezinski et al. / Journal of Molecular Structure 519 (2000) 119–123
B
O
O
O
O
B
O
O
O
O
O
O
O
+
H
B
+
O
O
H
O
O
O
O
O
O
O
O
+
H
O
O
O
O
O
O
H C
3
CH3
CH3
H3C
CH3
H C
3
H C
3
H C
3
H C
3
B2
B3
B4
Scheme 2.
fluctuating proton and therefore all CH protons
become slightly deshielded.
The C NMR data on the three borates and their
groups in the complex are shifted toward higher fields.
7
9
The C and C signals are particularly strongly shifted
indicating that the residence time of the fluctuating
13
7
10
complexes with HAuCl are shown in Table 2. A
proton at the O and O is longer, i.e. the multimi-
nima of the proton potentials at these O-atoms are
deeper.
4
1
3
comparison of the C NMR spectra of B2 and its
:1 HAuCl complex shows that the signals of the
1
4
2
3
C and C atoms are shifted toward higher fields.
This result indicates that the proton fluctuates between
all O-atoms. A comparable result is obtained from the
4. Conclusions
spectra of compound B3 and its 1:1 HAuCl complex.
4
3
5
In this case, however, the signals of the C and C
atoms are much more shifted when compared with
In the IR spectra of HAuCl complexes of tris(ox-
4
aalkyl) borates a continuous absorption is observed
due to the formation of proton channels with large
proton polarizability. The intensity distribution of
the continua depends on the length of the channels
since with increasing length the barriers in the proton
potentials become higher. The NMR data give the
information on the structure of the proton potentials.
2
6
the signals of the C and C atoms. This result demon-
strates that the residence time of the fluctuating proton
4
at the O oxygen atoms is much longer than that at the
1
7
4
O and O oxygen atoms, i.e. the minima at the O
atoms are deeper. A comparison of the signals in the
13
C NMR spectra of compound B4 and its HAuCl
complex shows that all these C signals of the CH2
4
1
3
For the B2–HAuCl complex both minima potentials
4
Table 1
1
H NMR chemical shifts (ppm) of tris(oxaalkyl) borates and their 1:1 complexes with HAuCl
triplet, m—multiplet)
4
in CDCl
3
(s—singlet, bs—broad singlet, t—
2
3
5
6
8
9
11
Compounds
C H
C H
C H
C H
C H
C H
C H
H
B2
3.86(t)
3.92(bs)
3.95(t)
4.00(bs)
3.93(t)
3.96(m)
3.44(t)
3.30(s)
3.35(s)
3.55(m)
3.73(bs)
3.65(m)
3.73(m)
B2–HAuCl
B3
B3–HAuCl
B4
B4–HAuCl
4
3.52(bs)
3.63(m)
3.73(bs)
3.65(m)
3.73(m)
9.24(s)
9.93(s)
9.07(s)
3.55(m)
3.73(bs)
3.65(m)
3.73(m)
3.39(s)
3.51(s)
3.58(m)
3.73(m)
4
3.56(m)
3.62(m)
3.38(s)
3.40(s)
4

B. Brzezinski et al. / Journal of Molecular Structure 519 (2000) 119–123
123
Table 2
Polish State Committee for Scientific Research KBN,
Grant 3 TO9A01817.
1
3
C NMR chemical shifts (ppm) of tris(oxaalkyl) borates and their
1
:1 complexes with HAuCl
4
in CDCl
3
2
3
5
6
8
9
11
Compounds
C
C
C
C
C
C
C
B2
63.42 73.54 58.83
63.17 72.85 58.75
62.58 70.28 71.53 71.87 58.89
62.48 69.02 70.07 71.69 58.40
62.55 70.37 70.41 70.53 71.47 71.81 58.86
62.54 69.40 70.22 70.30 70.49 71.09 59.70
References
B2–HAuCl
B3
B3–HAuCl
B4
4
[
1] G. Zundel, in: P. Schuster, G. Zundel, C. Sandorfy (Eds.), The
Hydrogen Bond—Recent Developments in Theory and Experi-
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4
B4–HAuCl
4
[
[
[
4] B. Brzezinski, F. Bartl, G. Zundel, J. Phys. Chem. 101 (1997)
5
607.
are equally deep. For the B3–HAuCl complex the
4
4
[5] B. Brzezinski, B. R o´ z˙ alski, G. Schroeder, F. Bartl, G. Zundel, J.
Chem. Soc. Faraday Trans. 94 (1998) 2093.
well at O atoms is the deepest and in the case of
7
10
B4–HAuCl complex the wells at the O and O
4
[
6] J. Olejnik, B. Brzezinski, G. Zundel, J. Mol. Struct. 271 (1992)
57.
7] G. Zundel, in: A. M u¨ ller, H. Ratajczak, W. Junge, E. Diemann
Eds.), Electron and Proton Transfer in Chemistry and Biology,
1
4
atoms are deeper than those at O and O atoms.
1
[
(
Acknowledgements
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8] F. Bartl, G. Deckers-Hebersheit, K. Altendorf, G. Zundel,
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[
The authors are grateful for financial support of the