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Proton transfer and correlations between the C-O, O-H, N-H and O⋯N bond lengths in amine phenolates
8 August 1997
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 274 (1997) 361-364
Proton transfer and correlations between the C - O , O-H, N - H and O . . . N bond lengths in amine phenolates I. Majerz, Z. Malarski, L. Sobczyk Facul~" of Chemist~', University of Wroctaw, Joliot-Curie 14, 50-383 Wroctaw, Poland
Received 13 March 1997; in final form 20 March 1997
Abstract
A relationship between the geometrical parameters of the O - H - • • N bridge based on the results of X-ray diffraction studies of a number of amine-phenol hydrogen-bonded complexes covering a broad ApK a range from -3.91 to 5.93 are proposed. The analysis shows that the shortest bridges are realized when the proton transfer degree, reflected in the C-O bond length, reaches about 50%. The position of the hydrogen atom is then close to the centre of the bridge. Such quasi-symmetric bridges behave critically, being sensitive to temperature, pressure and deuteration. © 1997 Elsevier Science B.V. 1. Introduction O - H proton donors form hydrogen-bonded adducts with amines that have interesting properties both in solution and in the solid state. There is a certain inversion (critical) range of A p K a ( A p K a = pKa(NH ÷) - p K a ( O H ) ) where proton transfer takes place. In this critical range of A p K , the adducts show a number of anomalies. Thus in solution a stepwise increase in the dipole moment takes place [1], a maximum in the chemical shift of the proton magnetic resonance for the bridge proton is reached [2,3] and a maximum shift in the stretching vibration band towards low frequencies is observed [4,5]. A review of the properties of such critical quasi-sym55+ metric O . . . H - . - N systems has been reported [6,7]. In the solid state a stepwise shift in the nuclear quadrupole resonance frequency of chlorophenol or chloroacetic acid complexes takes place [5,8] analogous to the shift in the protonic vibration band in solution [9,10].
It seemed of some interest to prove a correlation between the properties of hydrogen-bonded systems from the inversion range and the geometry of the O-H..-N bridge length. Therefore, systematic Xray diffraction studies were undertaken on the complexes of phenols with amines [11]. Pentachlorophenol and 2,6-dichloro-4-nitrophenol were selected as proton donors while substituted pyridines were selected as proton acceptors. In all cases good quality crystals were grown from non-aqueous solutions.
2. Discussion
Data concerning the adducts are collected in Table 1. Excepting 3, 6, 13 and 20 all were taken from Ref. [11]. In Table 1 and in further correlations only complexes with tertiary amines were taken into consideration. The secondary and primary amines form in the lattice additional hydrogen bonds which substantially affect the charge distribution and, hence,
0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0009-2614(97)00674-X
362
I. Majerz et al. / Chemical Physics Letters 274 (1997) 361-364
the geometry of the main hydrogen bonds [16,17]. From the correlations pentachlorophenolates, which undergo an additional strong specific interaction, e.g. with the phenol molecules themselves, are also omitted [18-20]. Thus we tried to select systems in which the O - H . . . N hydrogen bonds are isolated from the environment as much as possible. For the 2,6-dichloro-4-nitrophenol-3,4-dimethylpyridine complex there are in the crystal lattice two symmetry independent molecules and thus two different geometries of molecules. The average value of the C - O bond lengths was taken for correlation. The charge transfer degree (and as a consequence the proton density distribution) in the O - H - . . N bridge is most clearly reflected in the C - O bond length. It is known that such a length for the phenol itself equals about 1.36 A while for the phenolate anion it is 1.26 ,~ [21,22]. For pentachlorophenol1,8-bis(dimethylaminomethyl)naphthalene, where a free phenolate anion exists [14], this length is 1.255(5) A.
Assuming formally that the proton transfer equilibrium exists in a complex expressed by [23] lg Kvr = x A p K a + C',
(1)
one can relate the dc_ o length to the A p K a parameter. The fitting leads to the following values: x = 0.78, C ' = -1.056, d c _ o ( H B ) = 1.330 A, dc_o(PT) = 1.268 A. dc_o(HB) and dc_o(PT) mean the C - O bond length without proton transfer and for the ionic state. Fig. 1 shows the experimental points compared with the fitted curve, which is similar to plots of the increase in the dipole moment in solution [1] or the change in NQR frequency in the solid state versus A p K a [8]. The scattering of experimental points is as large as in the case of the NQR frequency plot which is due to packing effects. The critical (deflection) point on the dc_ o vs. A p K a plot corresponds exactly to that found from NQR measurements. Assuming the dc_ o parameters to be most objective for expressing the charge transfer degree and the
Table 1 The ApK a values and geometrical parameters (in .~ and °) of O - H • • • N hydrogen bonds in pentachlorophenolates and 2,6-dichloro-4nitrophenolates of tertiary amines No.
ApK a
1 2 3 4 5 6 7 8 9 10 11 12 13
pentach|orophenolates 3-cyanopyridine -3.91 4-acetylpyridine - 1.75 acridine [12] 0.34 4-methylpyridine 0.77 the same at 80 K 0.77 5,6,7,8-tetrahydroquinoline [13] 1.39 2,4-dimethylpyridine 1.6 3-oxoazabicyclooctane 1.87 2,4,6-trimethylpyddine 1.99 N-methylmorpholine 2.12 4-N,N-dimethylaminopyridine 4.35 azabicyclo[2,2,2]octane 5.69 1,8-bis(dimethylaminomethyl) naphtha~l~e [14] a
14 15 16 17
2,6-dichloro-4-nitrophenolates 3-methylpyridine 2-ethylpyridine 4-methylpyridine 3,4-dimethylpyridine
18 19 20
2,4,6-trimethylpyridine 4-N,N-dimethyloaminopyridine 4-dihydroxymethylpyridine [15] b
1.54 2.21 2.35 2.78 3.45 5.93
a In this case almost isolated ions are present in the lattice. This gem-diol compound in solution does not exist.
N "'" O
O-C
O-H
N-H
/_OHN
2.703(5) 2.653(4) 2.565(3) 2.552(4) 2.515(4) 2.602(4) 2.604(3) 2.514(5) 2.659(4) 2.564(4) 2.579(4) 2.553(5) 3.612(5)
1.335(5) 1.331(4) 1.293(3) 1.314(4 1.306(3) 1.320(4) 1.278(3) 1.298(6) 1.264(4) 1.292(4) 1.273(4) 1.271(5) 1.255(5)
0.67(5) 0.78(4) 1.45(3) 1.09(6) 1.22(4) 0.96(5) 1.75(4) 1.32(6) 1.79(4) 1.48(6) 1.64(3) 1.35(4)
2.08(5) 1.91(4) 1.13(3) 1.47(6) 1.29(4) 1.70(5) 0.88(4) 1.21(7) 0.88(4) 1.13(6) 0.95(3) 1.23(4)
155(6) 156(4) 168(2) 170(5) 176(5) 155(5) 163(4) 166(5) 173(4) 159(5) 165(3) 164(4)
2.544(4) 2.613(2) 2.604(3) 2.532(3) 2.529(3) 2.686(4) 2.631(3) 2.683(3)
1.285(4) 1.281(2) 1.275(3) 1.287(3) 1.261(3) 1.250(4) 1.266(2) 1.276(3)
1.60(3) 1.71(3) 1.42(4) 1.45(3) 1.47(3) 1.65(5) 1.74(3) 1.97(4)
0.97(3) 0.91(3) 1.20(4) 1.09(3) 1.09(3) 1.04(5) 0.93(3) 0.76(4)
165(3) 173(3) 170(3) 170(2) 160(2) 173(4) 160(3) 156(4)
I. Majerz et al. / Chemical Physics Letters 274 (1997) 361-364
d(C-O)~,
363
d(NO) ~2~rl[ a
"
/
1.30
o
1.25
I
-.4
0
-2
2
4
i
2.45 L
o
6 ~.pl~
.
Fig. I. Correlation between the C - O bond length and A p K a value for complexes of pentachlorophenol and 2,6-dichloro-4nitrophenol.
/~,75 1 .............................................................................
d(N'"O)
2.7!~
2.5 ~
1.24
/
,
i
I
1.26
1.28
1.3
1.32
1.34
d(C-O) ~, Fig. 2. Correlation between the C - O and O . - . N distances.
.
.
.
.
O(N"H)~s
.
d(NO)A 27 ~b.~ ,°°; \ 275 ~
2G 4
H-atom localization in the bridge, we tried to correlate this parameter with the O . . - N bridge length. Such a correlation is presented in Fig. 2. From the plot it follows that the shortest bridges are formed in the critical (inversion) region at a C - O bond length of about 1.30 ~,, i.e. exactly corresponding to the situation when the proton is equally shared between the bridge O and N atoms (quasi-symmetric bridge). The O • • • N bridge length is then about 2.52 A. The two parts of the curve in Fig. 2 corresponding to non-proton-transfer (HB) and proton transfer (PT) species are of different slopes. This reflects the fact that after proton transfer additional Coulomb forces start to play an increasing role. Nevertheless the results clearly show that the covalent character of interactions in the critical region is dominant. Although X-ray diffraction studies do not yield sufficiently precise information about the location of hydrogen atoms, the results obtained allow us to formulate some conclusions. In Fig. 3a,b we have correlated the O - H and N . . . H distances with the
.
. . . . . . .
•
•
25~
0.5
•
1
•
•
~5
2
d(O-H) A Fig. 3. Correlations between the O - H (a) and N - H (b) and O - -. N distances. Fitting by means of parabolic functions: d o . N = 0.3553dr~_H --0.9599dN_ n +3.181, R e = 0.7742; d o . . N = 0.4567do H - 1.1856do-H +3.3, R 2 = 0.9084. The values presented as squares have not been included in the correlations.
bridge length. One can see that for the shortest bridges the bridge hydrogen atom is, as a role, shifted to the centre of the bridge. This is well seen in the case of pentachlorophenolates of 3-oxoazabicyclooctane and 4-methylpyridine at 80 K [24] for which the O - ' . N distance equals 2.514(5) and 2.515(4) .~, respectively. These O - H . . . N hydrogen bridges from the critical (inversion) region are the shortest known so far in the literature. They show several interesting properties reflecting their unusual sensitivity to the environment. Thus 4-methylpyridine pentachlorophenolate which is characterized at room temperature by the O • • • N distance of 2.552(4) A with asymmetric location of the H-atom (closer to oxygen atom) at 80 K shows a decrease in that distance down to 2.515(4) ~, with nearly symmetric location of the H-atom [24]. The complexes from the critical region show a particularly high sensitivity to pressure [25] and deuteration [26] expressed by means of NQR frequencies which reflect the change in charge distribution within the bridges. One could also anticipate a strong influence of deuteration on the geometry of such critical hydrogen bonds. In fact for the most representative complex composed of 4-methylpyridine and pentachlorophenol a giant deu-
364
L Majerz et al. / Chemical Physics Letters 274 (1997) 361-364
terium isotope effect was found. On deuteration the elongation of the OHN bridge proceeds from 2.552(4) to 2.638(3) ~,. At the same time a change in symmetry occurs from PT (Z = 2) to Cc (Z = 4) [27].
Acknowledgements We express our gratitude to Professor T. Lis and Dr W. Sawka-Dobrowolska for fruitful help in crystal structure investigations shown over many years. Financial support from the Committee for Scientific Research (grant KBN 3TOA05910) is acknowledged.
References [1] L. Sobczyk, K. Bunzl, H. Engelhardt, in: The Hydrogen Bond, Vol. 3, P. Schuster, G. Zundel, C. Sandorfy (Eds.), North-Holland, Amsterdam, 1976, p. 937. [2] B. Brycki, M. Szafran, J. Chem. Soc. Perkin Trans II, 223 (1984). [3] M. Ilczyszyn, H. Ratajczak, K. Skowronek, Magn. Res. Chem. 26 (1988) 445. [4] V.P. Glazunov, A.A. Mashkovsky, S.E. Odinokov, J. Chem. Soc. Faraday Trans. I 85 (1979) 629. [5] B. Nogaj, E. Dulewicz, B. Brycki, A. Hrynio, P. Barczyfiski, Z. Dega-Szafran, P. Koziol, A.P. Katritzky, J. Phys. Chem. 94 (1990) 1279. [6] L. Sobczyk, Chem. Phys. Rep. 14 (1996) 19. [7] Th. Zeegers-Huyskens, L. Sobczyk, J. Mol. Liq. 46 (1990) 263. [8] E. Grech, J. Kalenik, L. Sobczyk, J. Chem. Soc. Faraday Trans. I 75 (1979) 1587.
[9] Z. Malarski, M. Rospenk, L. Sobczyk, E. Grech, J. Phys. Chem. 86 (1982) 401. [10] J. Kalenik, I. Majerz, L. Sobczyk, E. Grech, M.M. Habeeb, Collect. Czech. Chem. Commun. 55 (1990) 81. [11] I. Majerz, Habilitation Thesis, Wroc{aw University Press, 1997. [12] K. Wo~'niak, T.M. Krygowski, B. Kariuki, W. Jones, J. Mol. Struct. 248 (1991) 331. [13] Z. Malarski, T. Lis, E. Grech, J. Crystallogr. Spectrosc. Res. 21 (1991) 255. [14] W. Sawka-Dobrowolska, E. Grech, B. Brzezifiski, Z. Malarski, L. Sobczyk, J. Mol. Struct. 356 (1995) 117. [15] I. Majerz, W. Sawka-Dobrowolska, L. Sobczyk, J. Mol. Struct., in press [16] I. van Bellingen, G. Gerrnain, P. Piret, M. van Meersche, Acta Crystallogr. B 27 (1971) 560. [17] I. Majerz, Z. Malarski, T. Lis, J. Mol. Struct. 161 (1987) 165. [18] J.A. Kanters, E.H. ter Horst, E. Grech, Acta. Crystallogr. C 48 (1992) 1345. [19] J.A. Kanters, E.H. ter Horst, E. Grech, Acta. Crystallogr. C 48 (1992) 328. [20] E. Grech, T. Lis, K. Majewska, Z. Malarski, Pol. J. Chem. 67 (1993) 1317. [21] K. Maartman-Moe, Acta Crystallogr. B 25 (1969) 1452. [22] I. Van Bellingen, G. Germain, P. Piret, M. Van Meersche, Acta Crystallogr. B 27 (1971) 7566. [23] P. Huyskens, Th. Zeegers-Huyskens, J. Chim. Phys. 61 (1964) 81. [24] Z. Malarski, I. Majerz, T. Lis, J. Mol. Struct. 380 (1996) 249. [25] M. Ma6kowiak, P. Koziol, J. Stankowski, Z. Naturforsch. A 41 (1986) 225. [26] J. Kalenik, I. Majerz, Z. Malarski, L. Sobczyk, Chem. Phys. Lett. 165 (1990) 15. [27] I. Majerz, Z. Malarski, T. Lis, J. Mol. Struct. 240 (1990) 47.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 274 (1997) 361-364
Proton transfer and correlations between the C - O , O-H, N - H and O . . . N bond lengths in amine phenolates I. Majerz, Z. Malarski, L. Sobczyk Facul~" of Chemist~', University of Wroctaw, Joliot-Curie 14, 50-383 Wroctaw, Poland
Received 13 March 1997; in final form 20 March 1997
Abstract
A relationship between the geometrical parameters of the O - H - • • N bridge based on the results of X-ray diffraction studies of a number of amine-phenol hydrogen-bonded complexes covering a broad ApK a range from -3.91 to 5.93 are proposed. The analysis shows that the shortest bridges are realized when the proton transfer degree, reflected in the C-O bond length, reaches about 50%. The position of the hydrogen atom is then close to the centre of the bridge. Such quasi-symmetric bridges behave critically, being sensitive to temperature, pressure and deuteration. © 1997 Elsevier Science B.V. 1. Introduction O - H proton donors form hydrogen-bonded adducts with amines that have interesting properties both in solution and in the solid state. There is a certain inversion (critical) range of A p K a ( A p K a = pKa(NH ÷) - p K a ( O H ) ) where proton transfer takes place. In this critical range of A p K , the adducts show a number of anomalies. Thus in solution a stepwise increase in the dipole moment takes place [1], a maximum in the chemical shift of the proton magnetic resonance for the bridge proton is reached [2,3] and a maximum shift in the stretching vibration band towards low frequencies is observed [4,5]. A review of the properties of such critical quasi-sym55+ metric O . . . H - . - N systems has been reported [6,7]. In the solid state a stepwise shift in the nuclear quadrupole resonance frequency of chlorophenol or chloroacetic acid complexes takes place [5,8] analogous to the shift in the protonic vibration band in solution [9,10].
It seemed of some interest to prove a correlation between the properties of hydrogen-bonded systems from the inversion range and the geometry of the O-H..-N bridge length. Therefore, systematic Xray diffraction studies were undertaken on the complexes of phenols with amines [11]. Pentachlorophenol and 2,6-dichloro-4-nitrophenol were selected as proton donors while substituted pyridines were selected as proton acceptors. In all cases good quality crystals were grown from non-aqueous solutions.
2. Discussion
Data concerning the adducts are collected in Table 1. Excepting 3, 6, 13 and 20 all were taken from Ref. [11]. In Table 1 and in further correlations only complexes with tertiary amines were taken into consideration. The secondary and primary amines form in the lattice additional hydrogen bonds which substantially affect the charge distribution and, hence,
0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0009-2614(97)00674-X
362
I. Majerz et al. / Chemical Physics Letters 274 (1997) 361-364
the geometry of the main hydrogen bonds [16,17]. From the correlations pentachlorophenolates, which undergo an additional strong specific interaction, e.g. with the phenol molecules themselves, are also omitted [18-20]. Thus we tried to select systems in which the O - H . . . N hydrogen bonds are isolated from the environment as much as possible. For the 2,6-dichloro-4-nitrophenol-3,4-dimethylpyridine complex there are in the crystal lattice two symmetry independent molecules and thus two different geometries of molecules. The average value of the C - O bond lengths was taken for correlation. The charge transfer degree (and as a consequence the proton density distribution) in the O - H - . . N bridge is most clearly reflected in the C - O bond length. It is known that such a length for the phenol itself equals about 1.36 A while for the phenolate anion it is 1.26 ,~ [21,22]. For pentachlorophenol1,8-bis(dimethylaminomethyl)naphthalene, where a free phenolate anion exists [14], this length is 1.255(5) A.
Assuming formally that the proton transfer equilibrium exists in a complex expressed by [23] lg Kvr = x A p K a + C',
(1)
one can relate the dc_ o length to the A p K a parameter. The fitting leads to the following values: x = 0.78, C ' = -1.056, d c _ o ( H B ) = 1.330 A, dc_o(PT) = 1.268 A. dc_o(HB) and dc_o(PT) mean the C - O bond length without proton transfer and for the ionic state. Fig. 1 shows the experimental points compared with the fitted curve, which is similar to plots of the increase in the dipole moment in solution [1] or the change in NQR frequency in the solid state versus A p K a [8]. The scattering of experimental points is as large as in the case of the NQR frequency plot which is due to packing effects. The critical (deflection) point on the dc_ o vs. A p K a plot corresponds exactly to that found from NQR measurements. Assuming the dc_ o parameters to be most objective for expressing the charge transfer degree and the
Table 1 The ApK a values and geometrical parameters (in .~ and °) of O - H • • • N hydrogen bonds in pentachlorophenolates and 2,6-dichloro-4nitrophenolates of tertiary amines No.
ApK a
1 2 3 4 5 6 7 8 9 10 11 12 13
pentach|orophenolates 3-cyanopyridine -3.91 4-acetylpyridine - 1.75 acridine [12] 0.34 4-methylpyridine 0.77 the same at 80 K 0.77 5,6,7,8-tetrahydroquinoline [13] 1.39 2,4-dimethylpyridine 1.6 3-oxoazabicyclooctane 1.87 2,4,6-trimethylpyddine 1.99 N-methylmorpholine 2.12 4-N,N-dimethylaminopyridine 4.35 azabicyclo[2,2,2]octane 5.69 1,8-bis(dimethylaminomethyl) naphtha~l~e [14] a
14 15 16 17
2,6-dichloro-4-nitrophenolates 3-methylpyridine 2-ethylpyridine 4-methylpyridine 3,4-dimethylpyridine
18 19 20
2,4,6-trimethylpyridine 4-N,N-dimethyloaminopyridine 4-dihydroxymethylpyridine [15] b
1.54 2.21 2.35 2.78 3.45 5.93
a In this case almost isolated ions are present in the lattice. This gem-diol compound in solution does not exist.
N "'" O
O-C
O-H
N-H
/_OHN
2.703(5) 2.653(4) 2.565(3) 2.552(4) 2.515(4) 2.602(4) 2.604(3) 2.514(5) 2.659(4) 2.564(4) 2.579(4) 2.553(5) 3.612(5)
1.335(5) 1.331(4) 1.293(3) 1.314(4 1.306(3) 1.320(4) 1.278(3) 1.298(6) 1.264(4) 1.292(4) 1.273(4) 1.271(5) 1.255(5)
0.67(5) 0.78(4) 1.45(3) 1.09(6) 1.22(4) 0.96(5) 1.75(4) 1.32(6) 1.79(4) 1.48(6) 1.64(3) 1.35(4)
2.08(5) 1.91(4) 1.13(3) 1.47(6) 1.29(4) 1.70(5) 0.88(4) 1.21(7) 0.88(4) 1.13(6) 0.95(3) 1.23(4)
155(6) 156(4) 168(2) 170(5) 176(5) 155(5) 163(4) 166(5) 173(4) 159(5) 165(3) 164(4)
2.544(4) 2.613(2) 2.604(3) 2.532(3) 2.529(3) 2.686(4) 2.631(3) 2.683(3)
1.285(4) 1.281(2) 1.275(3) 1.287(3) 1.261(3) 1.250(4) 1.266(2) 1.276(3)
1.60(3) 1.71(3) 1.42(4) 1.45(3) 1.47(3) 1.65(5) 1.74(3) 1.97(4)
0.97(3) 0.91(3) 1.20(4) 1.09(3) 1.09(3) 1.04(5) 0.93(3) 0.76(4)
165(3) 173(3) 170(3) 170(2) 160(2) 173(4) 160(3) 156(4)
I. Majerz et al. / Chemical Physics Letters 274 (1997) 361-364
d(C-O)~,
363
d(NO) ~2~rl[ a
"
/
1.30
o
1.25
I
-.4
0
-2
2
4
i
2.45 L
o
6 ~.pl~
.
Fig. I. Correlation between the C - O bond length and A p K a value for complexes of pentachlorophenol and 2,6-dichloro-4nitrophenol.
/~,75 1 .............................................................................
d(N'"O)
2.7!~
2.5 ~
1.24
/
,
i
I
1.26
1.28
1.3
1.32
1.34
d(C-O) ~, Fig. 2. Correlation between the C - O and O . - . N distances.
.
.
.
.
O(N"H)~s
.
d(NO)A 27 ~b.~ ,°°; \ 275 ~
2G 4
H-atom localization in the bridge, we tried to correlate this parameter with the O . . - N bridge length. Such a correlation is presented in Fig. 2. From the plot it follows that the shortest bridges are formed in the critical (inversion) region at a C - O bond length of about 1.30 ~,, i.e. exactly corresponding to the situation when the proton is equally shared between the bridge O and N atoms (quasi-symmetric bridge). The O • • • N bridge length is then about 2.52 A. The two parts of the curve in Fig. 2 corresponding to non-proton-transfer (HB) and proton transfer (PT) species are of different slopes. This reflects the fact that after proton transfer additional Coulomb forces start to play an increasing role. Nevertheless the results clearly show that the covalent character of interactions in the critical region is dominant. Although X-ray diffraction studies do not yield sufficiently precise information about the location of hydrogen atoms, the results obtained allow us to formulate some conclusions. In Fig. 3a,b we have correlated the O - H and N . . . H distances with the
.
. . . . . . .
•
•
25~
0.5
•
1
•
•
~5
2
d(O-H) A Fig. 3. Correlations between the O - H (a) and N - H (b) and O - -. N distances. Fitting by means of parabolic functions: d o . N = 0.3553dr~_H --0.9599dN_ n +3.181, R e = 0.7742; d o . . N = 0.4567do H - 1.1856do-H +3.3, R 2 = 0.9084. The values presented as squares have not been included in the correlations.
bridge length. One can see that for the shortest bridges the bridge hydrogen atom is, as a role, shifted to the centre of the bridge. This is well seen in the case of pentachlorophenolates of 3-oxoazabicyclooctane and 4-methylpyridine at 80 K [24] for which the O - ' . N distance equals 2.514(5) and 2.515(4) .~, respectively. These O - H . . . N hydrogen bridges from the critical (inversion) region are the shortest known so far in the literature. They show several interesting properties reflecting their unusual sensitivity to the environment. Thus 4-methylpyridine pentachlorophenolate which is characterized at room temperature by the O • • • N distance of 2.552(4) A with asymmetric location of the H-atom (closer to oxygen atom) at 80 K shows a decrease in that distance down to 2.515(4) ~, with nearly symmetric location of the H-atom [24]. The complexes from the critical region show a particularly high sensitivity to pressure [25] and deuteration [26] expressed by means of NQR frequencies which reflect the change in charge distribution within the bridges. One could also anticipate a strong influence of deuteration on the geometry of such critical hydrogen bonds. In fact for the most representative complex composed of 4-methylpyridine and pentachlorophenol a giant deu-
364
L Majerz et al. / Chemical Physics Letters 274 (1997) 361-364
terium isotope effect was found. On deuteration the elongation of the OHN bridge proceeds from 2.552(4) to 2.638(3) ~,. At the same time a change in symmetry occurs from PT (Z = 2) to Cc (Z = 4) [27].
Acknowledgements We express our gratitude to Professor T. Lis and Dr W. Sawka-Dobrowolska for fruitful help in crystal structure investigations shown over many years. Financial support from the Committee for Scientific Research (grant KBN 3TOA05910) is acknowledged.
References [1] L. Sobczyk, K. Bunzl, H. Engelhardt, in: The Hydrogen Bond, Vol. 3, P. Schuster, G. Zundel, C. Sandorfy (Eds.), North-Holland, Amsterdam, 1976, p. 937. [2] B. Brycki, M. Szafran, J. Chem. Soc. Perkin Trans II, 223 (1984). [3] M. Ilczyszyn, H. Ratajczak, K. Skowronek, Magn. Res. Chem. 26 (1988) 445. [4] V.P. Glazunov, A.A. Mashkovsky, S.E. Odinokov, J. Chem. Soc. Faraday Trans. I 85 (1979) 629. [5] B. Nogaj, E. Dulewicz, B. Brycki, A. Hrynio, P. Barczyfiski, Z. Dega-Szafran, P. Koziol, A.P. Katritzky, J. Phys. Chem. 94 (1990) 1279. [6] L. Sobczyk, Chem. Phys. Rep. 14 (1996) 19. [7] Th. Zeegers-Huyskens, L. Sobczyk, J. Mol. Liq. 46 (1990) 263. [8] E. Grech, J. Kalenik, L. Sobczyk, J. Chem. Soc. Faraday Trans. I 75 (1979) 1587.
[9] Z. Malarski, M. Rospenk, L. Sobczyk, E. Grech, J. Phys. Chem. 86 (1982) 401. [10] J. Kalenik, I. Majerz, L. Sobczyk, E. Grech, M.M. Habeeb, Collect. Czech. Chem. Commun. 55 (1990) 81. [11] I. Majerz, Habilitation Thesis, Wroc{aw University Press, 1997. [12] K. Wo~'niak, T.M. Krygowski, B. Kariuki, W. Jones, J. Mol. Struct. 248 (1991) 331. [13] Z. Malarski, T. Lis, E. Grech, J. Crystallogr. Spectrosc. Res. 21 (1991) 255. [14] W. Sawka-Dobrowolska, E. Grech, B. Brzezifiski, Z. Malarski, L. Sobczyk, J. Mol. Struct. 356 (1995) 117. [15] I. Majerz, W. Sawka-Dobrowolska, L. Sobczyk, J. Mol. Struct., in press [16] I. van Bellingen, G. Gerrnain, P. Piret, M. van Meersche, Acta Crystallogr. B 27 (1971) 560. [17] I. Majerz, Z. Malarski, T. Lis, J. Mol. Struct. 161 (1987) 165. [18] J.A. Kanters, E.H. ter Horst, E. Grech, Acta. Crystallogr. C 48 (1992) 1345. [19] J.A. Kanters, E.H. ter Horst, E. Grech, Acta. Crystallogr. C 48 (1992) 328. [20] E. Grech, T. Lis, K. Majewska, Z. Malarski, Pol. J. Chem. 67 (1993) 1317. [21] K. Maartman-Moe, Acta Crystallogr. B 25 (1969) 1452. [22] I. Van Bellingen, G. Germain, P. Piret, M. Van Meersche, Acta Crystallogr. B 27 (1971) 7566. [23] P. Huyskens, Th. Zeegers-Huyskens, J. Chim. Phys. 61 (1964) 81. [24] Z. Malarski, I. Majerz, T. Lis, J. Mol. Struct. 380 (1996) 249. [25] M. Ma6kowiak, P. Koziol, J. Stankowski, Z. Naturforsch. A 41 (1986) 225. [26] J. Kalenik, I. Majerz, Z. Malarski, L. Sobczyk, Chem. Phys. Lett. 165 (1990) 15. [27] I. Majerz, Z. Malarski, T. Lis, J. Mol. Struct. 240 (1990) 47.