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Intramolecular O−Li+…ON⇌O−…Li+ON bonds with large Li+ polarizability. A FTIR study
CHEMICAL PHYSICS LETTERS
Volume 156, number 2,3
INTRAMOLECULAR A FTlR STUDY Bogumil BRZEZINSKI,
O-Li+...ONgO-...Li+ON
31 March 1989
BONDS WITH LARGE Li’ POLARIZABILITY.
Jerzy OLEJNIK
Department of Chemistry, A. Mickiewicz University, 60-780 Poznari. Poland Georg
ZUNDEL
and Rainer KR;iMER
Institute ofPhysical Chemistry University of Munich, D-8000 Munich 2, Federal Republic of Germany
Received 14 December 1988
Five Li+ salts of (4-R-Zpyridyl-N-oxide) acetic acid were studied by FTIR spectroscopy. Intramolecular O-Li...ON =O-...Li+ON bonds with large so-called Li+ polarizability are formed. The double minimum cation potential has changed in the -OCzH5 substituted compound to a broad flat single minimum, i.e. the nature ofthe bond to the O-...Li+...ON type.
1. Introduction
R
In heteroconjugated AH...B+A-...H+B hydrogen bonds in solutions and in the solid state double minimum proton potentials may be present since to the potential present in gas phase the potential of the reaction field is added [ 1,2]. Furthermore, specific interactions favor the creation of the second minimum. Such hydrogen bonds show very large so-called proton polarizabilities which are indicated by continua in the infrared spectra [ l-3 1. Intra- as well as inter-molecular homoconjugated OLi+...O~O...Li+O or NLi+...N*N...Li+N, (O...Li+...O) bonds, respectively, cause continua in the far-infrared spectra which show that the Li+ ions may fluctuate within these bonds and furthermore, that these bonds show large polarizabilities, so-called Li+ polarizabilities, since the charge can easily be shifted within these bonds. In the following we study whether heteroconjubonds may also gated O-Li+... ONSO-...Li+ON show large polarizabilities. 2. Results aad discussion Five Li’ complexes of (4-R-2-pyridyl-N-oxide) acetic acids are studied by FTIR spectroscopy: 0 009-2614/89/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
CL I
b
R=-NO2
$H2
COOH
-Br
-H -CH3
-OC2H5
With the tetrabutylammonium salts in the series of substituents from -NO, to -O&H5 the v,, ( COz) band shifts toward lower wave numbers (fig. 1). The same behavior is shown by the v( NO) vibration (fig. 1). These shifts show that the affinity of the -CO, and of the NO group for cations changes systematically within this series of compounds. Fig. 2 shows the carbonyl stretching vibration region (1780-1630 cm-‘) of the spectra of the Li’ complexes. In this region two bands are found. In the series of compounds from the -NO, to the -CHJ substituted ones, the intensity of the band at higher wave number decreases and the intensity of the band at lower wave number increases. Furthermore, the band at higher wave number shifts from 1708 to 1687 cm-’ . In the case of the compound with the -OC2H5 substituent only one broad band is observed. If two carbonyl bands are observed ( Y(C-O) and B.V.
213
Volume 156, number 2,3
31 March 1989
CHEMICAL PHYSICS LETTERS
0
WAVENUMBER [l/CM] Fig. I. The carbonyl stretching and v(NO)f regions of the FTIR spectra of acetonitrile solutions of the tetrabutylammonium salts of (4-R-2-pyridyl-N-oxide) acetic acids: _, -NOz; -- -, -Br; -.-.-, -H, .... -CH,, -*-a-, -OC2Hs. v,,( CO, ) ) this fact shows that a double minimum the (I) present in
Li+ potential is O-Li+...ON*O-...LiON
(II) bond, p”
a0
-C
\
-2: -c O-
Li+
ma *ON
q.
o-.
. *Li+ON
uu
(I)
If only one broad band is observed the Li+ ion is present in a broad flat singIe minimum and the Li+ bond must be represented by the structure O-...Li+...ON. The decrease of the intensity of the band at higher wave number and the increase of the lower one demonstrate that within the series of the -NO2 to the -CH3 substituted compounds the weight of limiting structure I decreases and that of structure II in-
0
0
'lt300
17ao
1760
1720
.1740
1700
1680
1680
1640
1020
1600
WAVENUMBER [l/CM] Fig. 2. Carbonyl region of the FIIR spectra of acetonitrile
- - -, -Br; -.-a-, -H, ,.., -CH,; -e-e-, -OC2H5.
214
solutions of the Li+ salts of (4-R-2-pytidyl-N-oxide) acetic acids: y1
-NO,;
b
500
450
400
350
300
250
WAVENUMBER
p
500
450
400
350
300
250
WAVENUMBER
I+
3 I March 1989
CHEMICAL PHYSICS LETTERS
Volume 156, number 2,3
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WAVENUMBER
200
150
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50
150
100
50
150
100
50
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50
[l/CM]
200
[l/CM]
200
[l/CM]
~~~.-I-:”
4.50
400
950
300
250
WAVENUMBER
200
[l/CM]
~~-:-::r_;_: 450
400
350
300
250
WAVENUMBER
200
[l/CM]
Fig. 3. FIR spectra of chloroform solutions of the Li+ salts of (4-R-2-pyridyl-N-oxide) acetic acids p, butylammonium salts, - - -. (a) -NO,, (b kBr, (c)-H, (d )-CHa, (c) -0CzHs.
and for comparison the tetra-
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CHEMICALPHYSICSLETTERS
creases, i.e. the Li* potential well at the carboxylic acid group is raised and that at the NO group lowered. Furthermore, the shift of the band at higher wave number from 1708 to 1687 cm-’ within this series of compounds shows a change in the nature of limiting structure I. The relatively high position ( 1708 cm-‘) of the compound with the -NO2 substituent shows that this vibration has large v(C-0) character. The shift of the band of structure I toward smaller wave numbers within this series of compounds proves that the v( C=O) character of this vibration in limiting structure (I) decreases. This band gains more and more v’as(-CO, ) character. Fig. 3 shows the spectra of these compounds in the far-infrared region. The ion motion band observed in the pure LiC104 solution at 407 cm- ’ (see ref. [ 41, fig. 2) has completely vanished in the spectra in fig. 3. This result confirms that the Li+ bonds are formed. Instead of this band a far-infrared continuum is found. The intensity distribution of this continuum changes characteristically within this series of compounds. In the case of the compound with the -NO2 substituent the continuum begins at 370 cm-’ and extends down to about 30 cm-‘, and the intensity of the continuum is higher in the region of higher wave numbers. In the series of compounds the intensity of the continuum decreases in the higher wave number region and increases in the lower wave number region. In the case of the compound with the -OC*H, substitutent the continuum shows a broad band-like structure in the region 150-20 cm-‘. These continua demonstrate that the Li+ ions fluctuate within these bonds and that these bonds interact strongly with their environments due to their large Li+ polarizability. The intensity distribution of the continua confirms the result that the doubleminimum cation potential present in the O-Li+...ON tiO-...LiON bonds has changed in the compound with the -O&H5 substituent to a broad flat single minimum in which the Li+ ion fluctuates [5-7 1. Hence, the bond is now represented by the formula O-...Li+...ON. 3. Conclusions In the Lit salts of (4-R-2-pyridyl-N-oxide) acetic acids heteroconjugated intramolecular Li+ bonds are 216
31 March 1989
formed. In these (I) O-Li+...ON*)O-...Li+ON (II) bonds double-minimum cation potentials are present. In the series of compounds from -NO2 to CH, the weight of limiting structure I decreases and that of limiting structure II increases. Furthermore, the v (C-O) character of the band of limiting structure I decreases. The nature of this Li+ bond has changed in the compound with the -O&H5 substituent from a double-minimum Li+ potential to a broad flat single minimum. The Li+ ion fluctuates now in a O-...Li+...ON bond. Far-infrared continua show that the Li+ ions can easily be shifted within these bonds and show Li+ polarizabilities. Thus, these bonds interact strongly with their environments caused by their large so-called Li+ polarizability.
4. Experimental The acids l-5 were synthesized by the procedure described in ref. [ 8 1. The tetrabutylammonium salts of the acids were obtained by addition of equimolar amounts of 1 mol/dm3 methanol solutions of tetrabutylammonium hydroxide to dried methanol solutions of the respective compounds. The solvent was removed under reduced pressure and the residue dissolved in acetonitrile. The 0.1 mol/dm’ lithium complex solutions were obtained by addition of equimolar amounts of 0.2 mol/dm3 acetonitrile solutions of LiCIOs to 0.2 mol/ dm3 acetonitrile solutions of the respective acids. The chloroform solutions of the lithium complexes were obtained from acetonitrile solutions of the Li+ complexes. The acetonitrile was removed under reduced pressure and the residue dissolved in chloroform. All manipulations with the substances were performed in a carefully dried glove box. The IR spectra were recorded from acetonitrile (mid-infrared) and from chloroform (far-infrared) solutions (0.1 mol/dn?). A cell was used with Si windows and wedge-shaped layer to avoid interferences (mean layer thickness 0.4 mm). The spectra were taken with a FTIR spectrophotometer IFS 113 V of Bruker, JSarlsruhe with a DTGS detector. The spectra in the far-infrared were registered with a Hecooled bolometer.
Volume 156, number 2,3
CHEMICAL PHYSICS LETTERS
Acknowledgement Our thanks are due to the Deutsche Forschungsgemeinschaft and to the Fonds der Chemischen Industrie for providing the facilities for this work.
I31 G. Zundel and J. Fritsch, in: The chemical physics of
141 ISI
References [ 1]
M. Eckert and G. Zundel, J. Phys. Chem. 9 1 ( 1987) 5 170.
[2] G. Zundel, J. Mol. Stntct. 177 (1988) 43.
31 March 1989
[61 [71
[81
salvation, Vol. 2, eds. R.R. Dogonadze, E. Kalman, A.A. Komyshev and J. Ulstrup (Elmvier, Amsterdam, 1986) pp. 21-96. B. Brzezinski, G. Zundel and R. Rriimer, Chem. Phys. Letters81 (1985) 138. B. Brzezinski and G. Zundel, J. Chem. Phys. 8 1 (1984) 1600. B. Brzezinski and G. Zundel, J. Chem. Sot. Faraday Trans. 181 (1985) 2375. B. Brzezinski, G. Zundel and R. Kramer, J. Phys. Chem., in press. 8. Brzezinski and J. Olejnik, Polish J. C&em., in press.
217
Volume 156, number 2,3
INTRAMOLECULAR A FTlR STUDY Bogumil BRZEZINSKI,
O-Li+...ONgO-...Li+ON
31 March 1989
BONDS WITH LARGE Li’ POLARIZABILITY.
Jerzy OLEJNIK
Department of Chemistry, A. Mickiewicz University, 60-780 Poznari. Poland Georg
ZUNDEL
and Rainer KR;iMER
Institute ofPhysical Chemistry University of Munich, D-8000 Munich 2, Federal Republic of Germany
Received 14 December 1988
Five Li+ salts of (4-R-Zpyridyl-N-oxide) acetic acid were studied by FTIR spectroscopy. Intramolecular O-Li...ON =O-...Li+ON bonds with large so-called Li+ polarizability are formed. The double minimum cation potential has changed in the -OCzH5 substituted compound to a broad flat single minimum, i.e. the nature ofthe bond to the O-...Li+...ON type.
1. Introduction
R
In heteroconjugated AH...B+A-...H+B hydrogen bonds in solutions and in the solid state double minimum proton potentials may be present since to the potential present in gas phase the potential of the reaction field is added [ 1,2]. Furthermore, specific interactions favor the creation of the second minimum. Such hydrogen bonds show very large so-called proton polarizabilities which are indicated by continua in the infrared spectra [ l-3 1. Intra- as well as inter-molecular homoconjugated OLi+...O~O...Li+O or NLi+...N*N...Li+N, (O...Li+...O) bonds, respectively, cause continua in the far-infrared spectra which show that the Li+ ions may fluctuate within these bonds and furthermore, that these bonds show large polarizabilities, so-called Li+ polarizabilities, since the charge can easily be shifted within these bonds. In the following we study whether heteroconjubonds may also gated O-Li+... ONSO-...Li+ON show large polarizabilities. 2. Results aad discussion Five Li’ complexes of (4-R-2-pyridyl-N-oxide) acetic acids are studied by FTIR spectroscopy: 0 009-2614/89/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
CL I
b
R=-NO2
$H2
COOH
-Br
-H -CH3
-OC2H5
With the tetrabutylammonium salts in the series of substituents from -NO, to -O&H5 the v,, ( COz) band shifts toward lower wave numbers (fig. 1). The same behavior is shown by the v( NO) vibration (fig. 1). These shifts show that the affinity of the -CO, and of the NO group for cations changes systematically within this series of compounds. Fig. 2 shows the carbonyl stretching vibration region (1780-1630 cm-‘) of the spectra of the Li’ complexes. In this region two bands are found. In the series of compounds from the -NO, to the -CHJ substituted ones, the intensity of the band at higher wave number decreases and the intensity of the band at lower wave number increases. Furthermore, the band at higher wave number shifts from 1708 to 1687 cm-’ . In the case of the compound with the -OC2H5 substituent only one broad band is observed. If two carbonyl bands are observed ( Y(C-O) and B.V.
213
Volume 156, number 2,3
31 March 1989
CHEMICAL PHYSICS LETTERS
0
WAVENUMBER [l/CM] Fig. I. The carbonyl stretching and v(NO)f regions of the FTIR spectra of acetonitrile solutions of the tetrabutylammonium salts of (4-R-2-pyridyl-N-oxide) acetic acids: _, -NOz; -- -, -Br; -.-.-, -H, .... -CH,, -*-a-, -OC2Hs. v,,( CO, ) ) this fact shows that a double minimum the (I) present in
Li+ potential is O-Li+...ON*O-...LiON
(II) bond, p”
a0
-C
\
-2: -c O-
Li+
ma *ON
q.
o-.
. *Li+ON
uu
(I)
If only one broad band is observed the Li+ ion is present in a broad flat singIe minimum and the Li+ bond must be represented by the structure O-...Li+...ON. The decrease of the intensity of the band at higher wave number and the increase of the lower one demonstrate that within the series of the -NO2 to the -CH3 substituted compounds the weight of limiting structure I decreases and that of structure II in-
0
0
'lt300
17ao
1760
1720
.1740
1700
1680
1680
1640
1020
1600
WAVENUMBER [l/CM] Fig. 2. Carbonyl region of the FIIR spectra of acetonitrile
- - -, -Br; -.-a-, -H, ,.., -CH,; -e-e-, -OC2H5.
214
solutions of the Li+ salts of (4-R-2-pytidyl-N-oxide) acetic acids: y1
-NO,;
b
500
450
400
350
300
250
WAVENUMBER
p
500
450
400
350
300
250
WAVENUMBER
I+
3 I March 1989
CHEMICAL PHYSICS LETTERS
Volume 156, number 2,3
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WAVENUMBER
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[l/CM]
200
[l/CM]
200
[l/CM]
~~~.-I-:”
4.50
400
950
300
250
WAVENUMBER
200
[l/CM]
~~-:-::r_;_: 450
400
350
300
250
WAVENUMBER
200
[l/CM]
Fig. 3. FIR spectra of chloroform solutions of the Li+ salts of (4-R-2-pyridyl-N-oxide) acetic acids p, butylammonium salts, - - -. (a) -NO,, (b kBr, (c)-H, (d )-CHa, (c) -0CzHs.
and for comparison the tetra-
215
Volume 156, number 2.3
CHEMICALPHYSICSLETTERS
creases, i.e. the Li* potential well at the carboxylic acid group is raised and that at the NO group lowered. Furthermore, the shift of the band at higher wave number from 1708 to 1687 cm-’ within this series of compounds shows a change in the nature of limiting structure I. The relatively high position ( 1708 cm-‘) of the compound with the -NO2 substituent shows that this vibration has large v(C-0) character. The shift of the band of structure I toward smaller wave numbers within this series of compounds proves that the v( C=O) character of this vibration in limiting structure (I) decreases. This band gains more and more v’as(-CO, ) character. Fig. 3 shows the spectra of these compounds in the far-infrared region. The ion motion band observed in the pure LiC104 solution at 407 cm- ’ (see ref. [ 41, fig. 2) has completely vanished in the spectra in fig. 3. This result confirms that the Li+ bonds are formed. Instead of this band a far-infrared continuum is found. The intensity distribution of this continuum changes characteristically within this series of compounds. In the case of the compound with the -NO2 substituent the continuum begins at 370 cm-’ and extends down to about 30 cm-‘, and the intensity of the continuum is higher in the region of higher wave numbers. In the series of compounds the intensity of the continuum decreases in the higher wave number region and increases in the lower wave number region. In the case of the compound with the -OC*H, substitutent the continuum shows a broad band-like structure in the region 150-20 cm-‘. These continua demonstrate that the Li+ ions fluctuate within these bonds and that these bonds interact strongly with their environments due to their large Li+ polarizability. The intensity distribution of the continua confirms the result that the doubleminimum cation potential present in the O-Li+...ON tiO-...LiON bonds has changed in the compound with the -O&H5 substituent to a broad flat single minimum in which the Li+ ion fluctuates [5-7 1. Hence, the bond is now represented by the formula O-...Li+...ON. 3. Conclusions In the Lit salts of (4-R-2-pyridyl-N-oxide) acetic acids heteroconjugated intramolecular Li+ bonds are 216
31 March 1989
formed. In these (I) O-Li+...ON*)O-...Li+ON (II) bonds double-minimum cation potentials are present. In the series of compounds from -NO2 to CH, the weight of limiting structure I decreases and that of limiting structure II increases. Furthermore, the v (C-O) character of the band of limiting structure I decreases. The nature of this Li+ bond has changed in the compound with the -O&H5 substituent from a double-minimum Li+ potential to a broad flat single minimum. The Li+ ion fluctuates now in a O-...Li+...ON bond. Far-infrared continua show that the Li+ ions can easily be shifted within these bonds and show Li+ polarizabilities. Thus, these bonds interact strongly with their environments caused by their large so-called Li+ polarizability.
4. Experimental The acids l-5 were synthesized by the procedure described in ref. [ 8 1. The tetrabutylammonium salts of the acids were obtained by addition of equimolar amounts of 1 mol/dm3 methanol solutions of tetrabutylammonium hydroxide to dried methanol solutions of the respective compounds. The solvent was removed under reduced pressure and the residue dissolved in acetonitrile. The 0.1 mol/dm’ lithium complex solutions were obtained by addition of equimolar amounts of 0.2 mol/dm3 acetonitrile solutions of LiCIOs to 0.2 mol/ dm3 acetonitrile solutions of the respective acids. The chloroform solutions of the lithium complexes were obtained from acetonitrile solutions of the Li+ complexes. The acetonitrile was removed under reduced pressure and the residue dissolved in chloroform. All manipulations with the substances were performed in a carefully dried glove box. The IR spectra were recorded from acetonitrile (mid-infrared) and from chloroform (far-infrared) solutions (0.1 mol/dn?). A cell was used with Si windows and wedge-shaped layer to avoid interferences (mean layer thickness 0.4 mm). The spectra were taken with a FTIR spectrophotometer IFS 113 V of Bruker, JSarlsruhe with a DTGS detector. The spectra in the far-infrared were registered with a Hecooled bolometer.
Volume 156, number 2,3
CHEMICAL PHYSICS LETTERS
Acknowledgement Our thanks are due to the Deutsche Forschungsgemeinschaft and to the Fonds der Chemischen Industrie for providing the facilities for this work.
I31 G. Zundel and J. Fritsch, in: The chemical physics of
141 ISI
References [ 1]
M. Eckert and G. Zundel, J. Phys. Chem. 9 1 ( 1987) 5 170.
[2] G. Zundel, J. Mol. Stntct. 177 (1988) 43.
31 March 1989
[61 [71
[81
salvation, Vol. 2, eds. R.R. Dogonadze, E. Kalman, A.A. Komyshev and J. Ulstrup (Elmvier, Amsterdam, 1986) pp. 21-96. B. Brzezinski, G. Zundel and R. Rriimer, Chem. Phys. Letters81 (1985) 138. B. Brzezinski and G. Zundel, J. Chem. Phys. 8 1 (1984) 1600. B. Brzezinski and G. Zundel, J. Chem. Sot. Faraday Trans. 181 (1985) 2375. B. Brzezinski, G. Zundel and R. Kramer, J. Phys. Chem., in press. 8. Brzezinski and J. Olejnik, Polish J. C&em., in press.
217