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Negatively charged hydrogen-bonded chains formed by tetrazole
Journal of Molecular Structure 508 (1999) 175–180
Negatively charged hydrogen-bonded chains formed by tetrazole B. Brzezinski a,*, G. Wojciechowski a, G. Zundel b, L. Sobczyk c, E. Grech d a Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan˜, Poland Institute of Physical Chemistry, University of Munich, Theresienstr. 41, 80333 Munich, Germany c Faculty of Chemistry, Wroclaw University, Joliot-Curie 14, 50-383 Wroclaw, Poland d Institute of Fundamental Chemistry, Technical University, Al.Piasto´w 42, 71-065 Szczecin, Poland b
Received 14 December 1998; accepted 22 January 1999
Abstract The complexes formed by tetrazole with MTBD, an N-base of guanidin-like character, were studied by FTIR spectroscopy. In the chloroform solution of 1:1 ratio, a 2N…H 1MTBD asymmetrical complex is formed and no dissociation of this complex is visible. With increasing tetrazole–MTBD ratio, the formation of chains of tetrazole molecules is observed. In acetonitrile solution of tetrazole with MTBD, the amount of protonated and dissociated H 1MTBD species increases with increasing concentration of tetrazole. In these mixtures, negatively charged chains are observed to be formed with tetrazole molecules only; however, some amount of free tetrazole molecules is also detected in this case. All these chains show large proton polarizability q 1999 Elsevier Science B.V. All rights reserved. Keywords: Tetrazole; MTBD; Hydrogen-bonded chains; Collective hydrogen bonds
1. Introduction Recently, very strong bases such as 7-methyl-1,5,7triazabicyclo[4.4.0]dec-5-ene (MTBD) and phosphazenes have been synthesised and used in the deprotonation reactions of N–H and C–H acids. In the semi-deprotonated N–H acids, negatively charged (NH…N) 2 structurally symmetrical hydrogen bonds showing large proton polarisability, are formed [1–3]. The interest in such homoconjugated anions is continuously increasing as a number of crystalline adducts containing [NHN] 2 bridges were isolated [4–9]. The geometry of [NHN] 2 bridges, IR spectra in solid state and 1H and 15N NMR spectra were discussed in Ref. [10]. A strong tendency of tetrazole to form homoconjugated anions was reported [11]. * Corresponding author. Tel.: 1 43-662-642311; fax: 1 43662-64231176.
Particularly interesting, however, that negatively charged hydrogen-bonded chains are formed by MTBD and N–H acids [12] or phenols [13–16]. These systems are models of biological proton conductors such as those present in the L550 intermediate of bacteriorhodopsin [17] as well as in the F0 subunit of ATP synthases. In these model systems only chains with maximum three hydrogen bonds are formed [12–18]. In this article we would like to show that longer chains of large proton polarizability also can be formed with participation of tetrazole, especially in non-polar solvents.
2. Experimental MTBD and tetrazole were purchased from Fluka. Tetrazole was purified by crystallisation from acetonitrile. Chloroform and acetonitrile were stored over
0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00064-2
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B. Brzezinski et al. / Journal of Molecular Structure 508 (1999) 175–180
N
N
N
N
N H
N
N
CH3
Tetrazole
MTBD Scheme 1.
˚ molecular sieves. All preparations and transfers of 3A the solutions were carried out in a carefully dried glove box. The samples were prepared by addition of pure MTBD to achieve the concentration ratios of 1:1, 2:1, 4:1 and 6:1. The spectra of the samples were taken with a FTIR spectrophotometer (Bruker IFS 113v) using a cell with Si windows (sample thickness 0.176 mm, detector DTGS, resolution 2 cm 21). The temperature of the samples was 293 K and the concentration of the base was 0.1 mol/dm 3. 3. Results and discussion The structures of the compounds studied are shown in Scheme 1. 3.1. Chloroform solutions The FTIR spectra of various mixtures of MTBD with tetrazole in chloroform are shown in Fig. 1. In
the spectrum of the 1:1 mixture no continuous absorption but only a broad band in the region 3250– 2600 cm 21 is observed. This broad band indicates the formation of an asymmetrical MTBDH 1…N 2 hydrogen bond shown in Scheme 2. In the spectrum of the 2:1 mixture of MTBD and tetrazole an intense continuum is observed beginning at about 3200 cm 21 and extended over whole of the region studied. This continuous absorption shows a band-like structure with two broad maxima at about 2600 and 1980 cm 21. Recently we have shown that the band-like structure depends on the nature of the ring of the molecules involved in the [NHN] 2 bridges [3]. With increasing concentration of tetrazole in the mixture of tetrazole and MTBD up to the 6:1 ratio, the continuum intensity increases, which indicates the formation of hydrogen-bonded chains with large proton polarizability. Fig. 2 presents the spectra of mixtures of tetrazole with MTBD in the region 3500–3250 cm 21. For the sake of comparison the spectrum of 1:1 MTBD– HAuCl4 complex is shown as the dashed line. The band at 3377 cm 21 originates from the free NH 1 groups of protonated MTBD molecules. This band is not observed in the spectra of the mixtures of tetrazole and MTBD in chloroform, which demonstrates that all MTBD molecules are bonded to the chain formed by tetrazoles in this case. Fig. 3 shows the same spectra as in Fig. 2 in the region of 1780–1550 cm 21. The spectral feature, shown in Fig. 3, is characteristic of protonated
Fig. 1. FTIR spectra of chloroform solutions of tetrazole–MTBD mixtures: (– – –) 1:1, (—) 2:1, (–·–·–) 4:1, and (……) 6:1.
B. Brzezinski et al. / Journal of Molecular Structure 508 (1999) 175–180 N
N N CH3
177
... H
N
N N
N N
N
N
CH3
H
.. .
N
N
N
N
Scheme 2.
The spectra of the tetrazole–MTBD mixtures in acetonitrile are shown in Fig. 4. In the spectrum of the 1:1 mixture, the band of the protonated MTBD at 3377 cm 21 is observed indicating that after proton transfer in the hydrogen-bonded complex, it is partially dissociated. In the spectrum of the 2:1 mixture (solid line) a continuum appears in the whole region but it is less intense at smaller wavenumbers as compared with the chloroform solution. The intensity of the band at 3377 cm 21 increases with increasing concentration of tetrazole indicating that almost all protonated
MTBD molecules exist in the 6:1 mixture as free ions in the acetonitrile solution. The intensity of the continuous absorption increases with increasing concentration of tetrazole in the mixtures up to the 6:1 ratio. The n (N–H) stretching vibration band of tetrazole is observed at 3195 cm 21. In the mixtures of tetrazole with MTBD the intensity of this band also increases with increasing concentration of tetrazole, which means that not all tetrazole molecules are involved in the chains. The quantitative evaluation of this band shows that in 6:1 mixture, almost five tetrazole molecules form a negatively charged chain with large proton polarisability due to a collective proton motion (Scheme 4). In Fig. 5 the spectra of the mixtures are compared with the spectra of the MTBD–HAuCl4 salt in acetonitrile. As follows from this comparison, with increasing concentration of tetrazole in the mixtures the protonated MTBD molecules are often removed from the hydrogen-bonded complexes and exist as
Fig. 2. FTIR spectra in the region 3500–3200 cm 21 of chloroform solutions of tetrazole–MTBD mixtures: (– – –) 1:1, (—) 2:1, (–·–·–) 4:1, and (……) for comparison of the spectrum of 1:1 complex of MTBD with HAuCl4.
Fig. 3. FTIR spectra in the region 1700–1550 cm 21 of chloroform solutions of tetrazole–MTBD mixtures: (– – –) 1:1, (—) 2:1, (–·–·–) 4:1, and (……) for comparison of the spectrum of 1:1 complex of MTBD with HAuCl4.
MTBD molecules [13–16]. Hence, one proton from the chain is always localised at the MTBD molecule. The other five protons fluctuate very fast in the sixmembered chain. This negatively charged tetrazole chain (given in Scheme 3) shows large proton polarisability due to collective charge fluctuations. 3.2. Acetonitrile solutions
178
B. Brzezinski et al. / Journal of Molecular Structure 508 (1999) 175–180 N N
N
CH3
H
... N N
... H N
N
N
N
N
... H
N
N
H ... N
N
N
N
N
N
N
... H
N
H ... N
N
N
N
N
N
N
N
N
N
N
N N
N
CH3
H
N
. ..N
... N
N
N
N
H
N
N
N
...N
H
N
... H
N
N
N
N
H ... N
N
N
N
N
...
H
Scheme 3.
free solvated cations in solutions. An additional proof that the MTBD molecules in the mixtures are always protonated is the spectral feature shown in Fig. 6.
4. Conclusion In the chloroform solutions six tetrazole molecules form a hydrogen-bonded chain with MTBD. One proton in this chain is localised at MTBD and the
other five protons undergo fast fluctuations between six tetrazole molecules. This negatively charged chain shows large proton polarisability. In contrast to the spectrum in chloroform solution, the spectra of the 6:1 mixture in acetonitrile demonstrate that protonated MTBD is completely removed from the hydrogen-bonded complex and exists as a free solvated cation. The negatively charged hydrogen bonded chain is formed by almost five tetrazole molecules, while one tetrazole molecule is not attached to the polarisable chain.
Fig. 4. FTIR spectra of acetonitrile solutions of (……) tetrazole and tetrazole-MTBD mixtures: (– – –) 1:1, (—) 2:1, (–·–·–) 4:1, and (—·—·) 6:1.
B. Brzezinski et al. / Journal of Molecular Structure 508 (1999) 175–180 H
N
+ N
6
N
N
N
N
N
179
CH3
N
N
...H N
N
N
N
N
N
N
N
N. . .
N
N
H
...H
...
H
N
N
N
N
N
N
N
N
+
N
N
CH3
H
+ N
N
...N
N
H N
N
N
N
. ..
H
N
N
N
N
N
N
...N
N
H
... N
N
N
N
N
N
N
N
H
H
Scheme 4.
Fig. 5. FTIR spectra in the region 3500–3000 cm 21 of acetonitrile solutions of tetrazole–MTBD mixtures: (– – –) 1:1, (—) 2:1, (–·–·–) 4:1, (—·—·—) 6:1, and (……) for comparing the spectrum of 1:1 complex of MTBD with HAuCl4.
Fig. 6. FTIR spectra in the region 1700–1550 cm 21 of acetonitrile solutions of tetrazole–MTBD mixtures: (– – –) 1:1, (—) 2:1, (–·–·–) 4:1, (—·—·—) 6:1, and (……) for comparing the spectrum of 1:1 complex of MTBD with HAuCl4.
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B. Brzezinski et al. / Journal of Molecular Structure 508 (1999) 175–180
Acknowledgements This article is written in memory of Professor Zbigniew Malarski and his profound achievements. References [1] B. Brzezinski, G. Zundel, J. Chem. Soc. Faraday Trans. II 79 (1983) 1249. [2] B. Brzezinski, E. Grech, Z. Malarski, L. Sobczyk, J. Chem. Soc. Perkin Trans. 2 (1991) 1267. [3] B. Brzezinski, R. Bauer, G. Zundel, J. Mol. Struct. 436–437 (1997) 103. [4] K. Wijaya, D. Henschel, O. Moers, A. Blaschette, R.G. Jones, Z. Naturforsch. 52b (1997) 1219. [5] Z. Malarski, T. Lis, E. Grech, J. Nowicka-Scheibe, K. Majewska, J. Mol. Struct. 221 (1990) 227. [6] T. Glowiak, E. Grech, Z. Malarski, L. Sobczyk, J. Mol. Struct. 403 (1997) 73. [7] T. Glowiak, E. Grech, Z. Malarski, J. Nowicka-Scheibe, L. Sobczyk, J. Mol. Struct. 381 (1996) 169.
[8] B. Brzezinski, E. Grech, J. Nowicka-Scheibe, T. Gowiak, Z. Malarski, L. Sobczyk, J. Mol. Struct. 327 (1994) 71. [9] E. Grech, Z. Malarski, W. Sawka-Dobrowolska, L. Sobczyk, J. Mol. Struct. 406 (1997) 107. [10] T. Glowiak, E. Grech, T. Lis, J. Nowicka-Scheibe, Z. Malarski, W. Sawka-Dobrowolska, L. Stefaniak, L. Sobczyk, J. Mol. Struct. 448 (1998) 121. [11] E. Grech, L. Stefaniak, I. Ando, H. Yoshimizu, G.A. Webb, L. Sobczyk, Bull. Chem. Soc. Jpn. 63 (1990) 2716. [12] B. Brzezinski, G. Zundel, J. Mol. Struct. 446 (1998) 199. [13] B. Brzezinski, P. Radziejewski, G. Zundel, J. Chem. Soc. Faraday Trans. 91 (1995) 3141. [14] B. Brzezinski, G. Zundel, J. Mol. Struct. 380 (1996) 195. [15] B. Brzezinski, G. Schroeder, E. Grech, Z. Malarski, M. Rospenk, L. Sobczyk, J. Chem. Research (S) 151 (1997) 1021–1040. [16] B. Brzezinski, G. Schroeder, E. Grech, Z. Malarski, M. Rospenk, L. Sobczyk, J. Chem. Research (M) 00 (1997) 1021. [17] J. Olejnik, B. Brzezinski, G. Zundel, J. Mol. Struct. 271 (1992) 157. [18] F. Bartl, G. Deckers-Heberstreit, K. Altendorf, G. Zundel, Biophys. J. 68 (1995) 1004.
Negatively charged hydrogen-bonded chains formed by tetrazole B. Brzezinski a,*, G. Wojciechowski a, G. Zundel b, L. Sobczyk c, E. Grech d a Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan˜, Poland Institute of Physical Chemistry, University of Munich, Theresienstr. 41, 80333 Munich, Germany c Faculty of Chemistry, Wroclaw University, Joliot-Curie 14, 50-383 Wroclaw, Poland d Institute of Fundamental Chemistry, Technical University, Al.Piasto´w 42, 71-065 Szczecin, Poland b
Received 14 December 1998; accepted 22 January 1999
Abstract The complexes formed by tetrazole with MTBD, an N-base of guanidin-like character, were studied by FTIR spectroscopy. In the chloroform solution of 1:1 ratio, a 2N…H 1MTBD asymmetrical complex is formed and no dissociation of this complex is visible. With increasing tetrazole–MTBD ratio, the formation of chains of tetrazole molecules is observed. In acetonitrile solution of tetrazole with MTBD, the amount of protonated and dissociated H 1MTBD species increases with increasing concentration of tetrazole. In these mixtures, negatively charged chains are observed to be formed with tetrazole molecules only; however, some amount of free tetrazole molecules is also detected in this case. All these chains show large proton polarizability q 1999 Elsevier Science B.V. All rights reserved. Keywords: Tetrazole; MTBD; Hydrogen-bonded chains; Collective hydrogen bonds
1. Introduction Recently, very strong bases such as 7-methyl-1,5,7triazabicyclo[4.4.0]dec-5-ene (MTBD) and phosphazenes have been synthesised and used in the deprotonation reactions of N–H and C–H acids. In the semi-deprotonated N–H acids, negatively charged (NH…N) 2 structurally symmetrical hydrogen bonds showing large proton polarisability, are formed [1–3]. The interest in such homoconjugated anions is continuously increasing as a number of crystalline adducts containing [NHN] 2 bridges were isolated [4–9]. The geometry of [NHN] 2 bridges, IR spectra in solid state and 1H and 15N NMR spectra were discussed in Ref. [10]. A strong tendency of tetrazole to form homoconjugated anions was reported [11]. * Corresponding author. Tel.: 1 43-662-642311; fax: 1 43662-64231176.
Particularly interesting, however, that negatively charged hydrogen-bonded chains are formed by MTBD and N–H acids [12] or phenols [13–16]. These systems are models of biological proton conductors such as those present in the L550 intermediate of bacteriorhodopsin [17] as well as in the F0 subunit of ATP synthases. In these model systems only chains with maximum three hydrogen bonds are formed [12–18]. In this article we would like to show that longer chains of large proton polarizability also can be formed with participation of tetrazole, especially in non-polar solvents.
2. Experimental MTBD and tetrazole were purchased from Fluka. Tetrazole was purified by crystallisation from acetonitrile. Chloroform and acetonitrile were stored over
0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00064-2
176
B. Brzezinski et al. / Journal of Molecular Structure 508 (1999) 175–180
N
N
N
N
N H
N
N
CH3
Tetrazole
MTBD Scheme 1.
˚ molecular sieves. All preparations and transfers of 3A the solutions were carried out in a carefully dried glove box. The samples were prepared by addition of pure MTBD to achieve the concentration ratios of 1:1, 2:1, 4:1 and 6:1. The spectra of the samples were taken with a FTIR spectrophotometer (Bruker IFS 113v) using a cell with Si windows (sample thickness 0.176 mm, detector DTGS, resolution 2 cm 21). The temperature of the samples was 293 K and the concentration of the base was 0.1 mol/dm 3. 3. Results and discussion The structures of the compounds studied are shown in Scheme 1. 3.1. Chloroform solutions The FTIR spectra of various mixtures of MTBD with tetrazole in chloroform are shown in Fig. 1. In
the spectrum of the 1:1 mixture no continuous absorption but only a broad band in the region 3250– 2600 cm 21 is observed. This broad band indicates the formation of an asymmetrical MTBDH 1…N 2 hydrogen bond shown in Scheme 2. In the spectrum of the 2:1 mixture of MTBD and tetrazole an intense continuum is observed beginning at about 3200 cm 21 and extended over whole of the region studied. This continuous absorption shows a band-like structure with two broad maxima at about 2600 and 1980 cm 21. Recently we have shown that the band-like structure depends on the nature of the ring of the molecules involved in the [NHN] 2 bridges [3]. With increasing concentration of tetrazole in the mixture of tetrazole and MTBD up to the 6:1 ratio, the continuum intensity increases, which indicates the formation of hydrogen-bonded chains with large proton polarizability. Fig. 2 presents the spectra of mixtures of tetrazole with MTBD in the region 3500–3250 cm 21. For the sake of comparison the spectrum of 1:1 MTBD– HAuCl4 complex is shown as the dashed line. The band at 3377 cm 21 originates from the free NH 1 groups of protonated MTBD molecules. This band is not observed in the spectra of the mixtures of tetrazole and MTBD in chloroform, which demonstrates that all MTBD molecules are bonded to the chain formed by tetrazoles in this case. Fig. 3 shows the same spectra as in Fig. 2 in the region of 1780–1550 cm 21. The spectral feature, shown in Fig. 3, is characteristic of protonated
Fig. 1. FTIR spectra of chloroform solutions of tetrazole–MTBD mixtures: (– – –) 1:1, (—) 2:1, (–·–·–) 4:1, and (……) 6:1.
B. Brzezinski et al. / Journal of Molecular Structure 508 (1999) 175–180 N
N N CH3
177
... H
N
N N
N N
N
N
CH3
H
.. .
N
N
N
N
Scheme 2.
The spectra of the tetrazole–MTBD mixtures in acetonitrile are shown in Fig. 4. In the spectrum of the 1:1 mixture, the band of the protonated MTBD at 3377 cm 21 is observed indicating that after proton transfer in the hydrogen-bonded complex, it is partially dissociated. In the spectrum of the 2:1 mixture (solid line) a continuum appears in the whole region but it is less intense at smaller wavenumbers as compared with the chloroform solution. The intensity of the band at 3377 cm 21 increases with increasing concentration of tetrazole indicating that almost all protonated
MTBD molecules exist in the 6:1 mixture as free ions in the acetonitrile solution. The intensity of the continuous absorption increases with increasing concentration of tetrazole in the mixtures up to the 6:1 ratio. The n (N–H) stretching vibration band of tetrazole is observed at 3195 cm 21. In the mixtures of tetrazole with MTBD the intensity of this band also increases with increasing concentration of tetrazole, which means that not all tetrazole molecules are involved in the chains. The quantitative evaluation of this band shows that in 6:1 mixture, almost five tetrazole molecules form a negatively charged chain with large proton polarisability due to a collective proton motion (Scheme 4). In Fig. 5 the spectra of the mixtures are compared with the spectra of the MTBD–HAuCl4 salt in acetonitrile. As follows from this comparison, with increasing concentration of tetrazole in the mixtures the protonated MTBD molecules are often removed from the hydrogen-bonded complexes and exist as
Fig. 2. FTIR spectra in the region 3500–3200 cm 21 of chloroform solutions of tetrazole–MTBD mixtures: (– – –) 1:1, (—) 2:1, (–·–·–) 4:1, and (……) for comparison of the spectrum of 1:1 complex of MTBD with HAuCl4.
Fig. 3. FTIR spectra in the region 1700–1550 cm 21 of chloroform solutions of tetrazole–MTBD mixtures: (– – –) 1:1, (—) 2:1, (–·–·–) 4:1, and (……) for comparison of the spectrum of 1:1 complex of MTBD with HAuCl4.
MTBD molecules [13–16]. Hence, one proton from the chain is always localised at the MTBD molecule. The other five protons fluctuate very fast in the sixmembered chain. This negatively charged tetrazole chain (given in Scheme 3) shows large proton polarisability due to collective charge fluctuations. 3.2. Acetonitrile solutions
178
B. Brzezinski et al. / Journal of Molecular Structure 508 (1999) 175–180 N N
N
CH3
H
... N N
... H N
N
N
N
N
... H
N
N
H ... N
N
N
N
N
N
N
... H
N
H ... N
N
N
N
N
N
N
N
N
N
N
N N
N
CH3
H
N
. ..N
... N
N
N
N
H
N
N
N
...N
H
N
... H
N
N
N
N
H ... N
N
N
N
N
...
H
Scheme 3.
free solvated cations in solutions. An additional proof that the MTBD molecules in the mixtures are always protonated is the spectral feature shown in Fig. 6.
4. Conclusion In the chloroform solutions six tetrazole molecules form a hydrogen-bonded chain with MTBD. One proton in this chain is localised at MTBD and the
other five protons undergo fast fluctuations between six tetrazole molecules. This negatively charged chain shows large proton polarisability. In contrast to the spectrum in chloroform solution, the spectra of the 6:1 mixture in acetonitrile demonstrate that protonated MTBD is completely removed from the hydrogen-bonded complex and exists as a free solvated cation. The negatively charged hydrogen bonded chain is formed by almost five tetrazole molecules, while one tetrazole molecule is not attached to the polarisable chain.
Fig. 4. FTIR spectra of acetonitrile solutions of (……) tetrazole and tetrazole-MTBD mixtures: (– – –) 1:1, (—) 2:1, (–·–·–) 4:1, and (—·—·) 6:1.
B. Brzezinski et al. / Journal of Molecular Structure 508 (1999) 175–180 H
N
+ N
6
N
N
N
N
N
179
CH3
N
N
...H N
N
N
N
N
N
N
N
N. . .
N
N
H
...H
...
H
N
N
N
N
N
N
N
N
+
N
N
CH3
H
+ N
N
...N
N
H N
N
N
N
. ..
H
N
N
N
N
N
N
...N
N
H
... N
N
N
N
N
N
N
N
H
H
Scheme 4.
Fig. 5. FTIR spectra in the region 3500–3000 cm 21 of acetonitrile solutions of tetrazole–MTBD mixtures: (– – –) 1:1, (—) 2:1, (–·–·–) 4:1, (—·—·—) 6:1, and (……) for comparing the spectrum of 1:1 complex of MTBD with HAuCl4.
Fig. 6. FTIR spectra in the region 1700–1550 cm 21 of acetonitrile solutions of tetrazole–MTBD mixtures: (– – –) 1:1, (—) 2:1, (–·–·–) 4:1, (—·—·—) 6:1, and (……) for comparing the spectrum of 1:1 complex of MTBD with HAuCl4.
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B. Brzezinski et al. / Journal of Molecular Structure 508 (1999) 175–180
Acknowledgements This article is written in memory of Professor Zbigniew Malarski and his profound achievements. References [1] B. Brzezinski, G. Zundel, J. Chem. Soc. Faraday Trans. II 79 (1983) 1249. [2] B. Brzezinski, E. Grech, Z. Malarski, L. Sobczyk, J. Chem. Soc. Perkin Trans. 2 (1991) 1267. [3] B. Brzezinski, R. Bauer, G. Zundel, J. Mol. Struct. 436–437 (1997) 103. [4] K. Wijaya, D. Henschel, O. Moers, A. Blaschette, R.G. Jones, Z. Naturforsch. 52b (1997) 1219. [5] Z. Malarski, T. Lis, E. Grech, J. Nowicka-Scheibe, K. Majewska, J. Mol. Struct. 221 (1990) 227. [6] T. Glowiak, E. Grech, Z. Malarski, L. Sobczyk, J. Mol. Struct. 403 (1997) 73. [7] T. Glowiak, E. Grech, Z. Malarski, J. Nowicka-Scheibe, L. Sobczyk, J. Mol. Struct. 381 (1996) 169.
[8] B. Brzezinski, E. Grech, J. Nowicka-Scheibe, T. Gowiak, Z. Malarski, L. Sobczyk, J. Mol. Struct. 327 (1994) 71. [9] E. Grech, Z. Malarski, W. Sawka-Dobrowolska, L. Sobczyk, J. Mol. Struct. 406 (1997) 107. [10] T. Glowiak, E. Grech, T. Lis, J. Nowicka-Scheibe, Z. Malarski, W. Sawka-Dobrowolska, L. Stefaniak, L. Sobczyk, J. Mol. Struct. 448 (1998) 121. [11] E. Grech, L. Stefaniak, I. Ando, H. Yoshimizu, G.A. Webb, L. Sobczyk, Bull. Chem. Soc. Jpn. 63 (1990) 2716. [12] B. Brzezinski, G. Zundel, J. Mol. Struct. 446 (1998) 199. [13] B. Brzezinski, P. Radziejewski, G. Zundel, J. Chem. Soc. Faraday Trans. 91 (1995) 3141. [14] B. Brzezinski, G. Zundel, J. Mol. Struct. 380 (1996) 195. [15] B. Brzezinski, G. Schroeder, E. Grech, Z. Malarski, M. Rospenk, L. Sobczyk, J. Chem. Research (S) 151 (1997) 1021–1040. [16] B. Brzezinski, G. Schroeder, E. Grech, Z. Malarski, M. Rospenk, L. Sobczyk, J. Chem. Research (M) 00 (1997) 1021. [17] J. Olejnik, B. Brzezinski, G. Zundel, J. Mol. Struct. 271 (1992) 157. [18] F. Bartl, G. Deckers-Heberstreit, K. Altendorf, G. Zundel, Biophys. J. 68 (1995) 1004.