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NMR investigation of the geometry of a pyridine—bromine complex in the nematic phase
CHEMICAL PHYSICS LE-ITERS
Volume 19, number 4
15 April 1973
NMR INVESTIGATION OF THE GEOMETRY OF A PYRIDINE-BROMINE COMPLEX IN THE NEMATIC PHASE C.A. VERACINI, Laboratorio di Clhica Istituto
di Chimico
M. LONGERI
Qnautistica ed Energetica Molecolare, Fisico dell
’ Urriwrsitb
di Fisa. Italy
and P.L. BARILI Istitctto di Chimica Organica della Facolth di Fanmcia Jell’Universita di Pisa, Italy Received 17 Janu;lry 1973
The NMR spectrum of a bromine-pyridine charge-transfer complex oriented in a nematic phase has been obtained and analyzed. The orientational parameters suggest relative positions of donor and acceptor molecules similar to those found in the solid state for analogous halogen-pyridine complcses.
1. Introduciion
2. Experimental
Although the structures of several molecular chargetransfer complexes have been eittensively studied in the solid state [ 11, their geometries in solution are still under investigation [2]. Because of the small enthalpies of formation, some doubt exists whether forces other than those present in solution or vapour contribute to the observed structure in crystals [2]. It is known that the alignment of solute molecules in a nematic liquid crystai appears to depend primarily on solute molecular shape [Z]. If data concerning the average shape of a charge-transfer complex in the nematic phase can be obtained from motional parameters, then conclusions about the relative positions of donor and acceptor moieties may be drawn. .h the following discussion, the NMR spectra of a pyddinebromine complex partially oriented in the nematic phase are studied and the results compared with those obtained from benzonitrile and pyrindine under similar experimental conditions.
The spectrum of a solution containing 20 mole per cent of pyridine-bromine complex [4] in “nematic phase IV” [S] (E. Merck) was recorded at probe temperature (~28°C) with a Jeol C-60HL spectrometer in the field sweep mode with external lock. The spectrunr (I, fig. 1) shows an AA’BB’C pattern. The starred line can be attributed to HBr resulting from partial reaction between complex and solvent molecules. It is indeed known that pyridine-bromine complex is a good brominating agent [4]. From comparison of the intensity of this signal with those of pyridine protons it can be roughly extimated that about 25% of the complex reacted before registration of the spectrum. From spectra subsequently recorded on the same sample, it was however deduced that only a very small fraction of bromine reacted with solvent during ragistration time (250 set). The spectra of benzonitrile and pyridine were also recorded in the same phase (see II and III, fig. 1).
Volume
19, number 4
CHEMICAL
PHYSICS LETTERS
15 April 1973
Br ji r
Fig. !
Evidence of a specific interaction, in the nematic solution! between bromine and pyridine molecules leading to a strongly oriented complex was obtained also by recording spectra of pyridine dissolved in the nematic phase in the presence of non-specifically interacting molecules such as CS,, CO,, C6D6. In these
cases only a slight contraction of the spectrum was ob-
served (probably a dilution effect). Moreover the spectrum of benzene. in the same nematic phase was not greatly affected by the addition of bromine: this shows that bromine has a small effect or! the orientat-
ing properties of the nematic phase (even if it slowly reacts with solvent); All spectra were anaIyzed by means of the ATHENA computer program [6]. Itera593
15 April 1973
CHEMICAL PHYSICS LETTERS
Volume 19, number 4
Table 1 Spectral parameters”) ---
Value (Hz) -3.Okl.S --79.0~1.5
v2 -v1 v3-y1
-1325.4~1.5
D12
-167.0r1.6
&3
-18.Ok2.5
014
Table 2
WI%4 r1&34
0.978*0.008 1.730+0.012
r14h
1.959cO.016
rl Jr34 r24lr34
1.698cO.022 1.696iO.005
58.Ok1.2 D15 -145.0*1.2 D23 58.Ot1.2 O24 --~-.--“)l-hCJ H_H values (J12 = 4.9, J, 3 = 1.9, Jz~ = 7.7 Hz) nssumtxi. tive calculations gave a best 5 Hz for I and 1.2 Hz for II couplings and other spectral dine-bromine complex are
were
fit with an rms error of and III. The direct dipolar parameters of the pyrireported in table 1.
3. Results and discussion Because of the C,, symmetry of the problem, only two motional constants are needed to describe the molecular orientation. With the coordinate axes shown in fig. 1, the dipolar couplings are given by [3]: gdir ij
= -2 x 5-“2K..
1I
{C~~2_,2 [((hij)2/A
Pyridinebromine
Motional constants
Distance ratio
c3t
0.352+0.004 0.111+0.001
-rl
G-J+
--
Benzonitrile
Pyridine
0.356*0.003
0.112kO.003
0.127+0.001
0.144s0.001
Owing to the rather poor quality of the pyridinebromine complex spectrum (the rms error between observed and calculated spectrum is 5 Hz, as compared with 1.2 Hz both in the case of benzonitrile and of pyridine), it was not possible to make a meaningful comparison of the geometries of all these molecules. However, a comparison of the motional constants appears quite interesting. In fact, the data reported in table 2 permit to deduce that the extent of orientation of the pyridine-bromine complex is nearly the same as that of benzonitrile (spectra I and II are indeed very similar), and much higher than that of pyridine. From the similarity in motional behaviour between the complex and benzonitrile it should be possible to distinguish between the two structures I and II (fig. 2), proposed on theoreiical gounds for halogen-pyridine complexes [S]. In fact, if only dispersion forces are considered and a dipole-dipole approximation is used [9], the orientational potental energy of a molecule in the liquid crystal phase may be expressed in terms of polarizability anisotropies [9] :
13BV
-f <(A+‘/$.
>iy - +( (&,)‘@J
+31/k
[~c(~,)~/~~~‘~~-~c(4~ij)z/~‘~~l~,
Cl) + aYY sin’8 sin29 + aZZcos%),
sr2_y2
where conventional notation is used and where C,Z~_,z and C,2_,,2 are the motional constants which defme the orientation of the solute molecule witi respect to the optical axis of the liquid crystal. From eq. (1) both proton geometry and molecular orientation can be obtained by assuming one interproton distance [3]. The interproton distance ratios of the complex are collected in table 1; those of benzonitrile and pyridine do not appreciably differ from the values already observed in different nematic phase [6, 73. The motional constants obtained for the three molecules are reported in table 2. 594
(2)
where Q’ depends on the solvent and I, and 1, are the ionization potentials of solute and solvent respectively. Unfortunately the polarizability ellipsoids are not known for the molecule under study. However, a rough estimate can be made approximating the ratio +: cyYY: %z with the ratio of the corresponding molecular dimension &: LY: L, [ 10, 1 l] ; within the further ap-
x
=%I
Fig. 2.
Volume 19, number 4
CHEMICAL
proximation of neglecting the differences in the ionization potential (the experimental conditions, i.e., solvent, temperature, concentration being equal), it can be shown from eq. (2) that it is the molecular shape that governs the orientation in the anisotropic media [3]. Therefore the motional constants of table 2 show that the shape of benzonitrile molecule and the average shape of bromine-pyridine complex are nearly the same, i.e., they are consistent with a structure like I (fig. 2).
Thus the mean relative donor acceptor positions are similar at least in the liquid crystal phase to those observed in solid state for analogous halogen-pyridine complexes [I], Although our approximations are rather crude, the results show that studies of this type of charge-transfer complexes in the nematic phase can give valuable information. However, in many other cases the solvent can be reasonably expected to act as a complexing agent [12] and no information would be obtained. Finally, although it was not possible to obtain useful information about the geometric distortion of the py ridine moiety in the present case, it can be presumed that such information may be obtainable with a less reactive complex. The extension of this work to the iodine-pyridine complex is now in progress.
Acknowledgement The authors would like to thank Professor E.
PHYSICS
LETI-ERS
15 April
1973
Scrocco for encouragement and the Consiglio Nazionale delle Ricerche for financial support.
References [l] 0. Hassel and C. Romming, Quart. Rev. 16 (1962) 1. (Academic Press, New York, 1969) p. 216. [ 3 j P. Diehl and C.L. Khetrapnl, NM R basic principles and progress, Vol. 1 (Springer, Berlin. 1969). [4] H.E. Xfertel, The chemistry of heterocyctic compounds, pyridine and its derivatives, Part 2, cd. E. Kiingsberg (Intcraience, New York, 1961) ch. 6, p. 32I. [5] R. Steintnsser and L. Pobl,Tetrahcdron Letters (1969) 1921. [6] C.A. Veracini, P. Bucci and P.L. Barili, Mol. Phys. 23 (1972) 59. [ 71 P. Diehl. CL. Khetrapal and H.P. Kellerhals, MoI. Phys. 15 (1968) 333. [S] C. Reid and R.S. hfullikcn, I. Am. Chem. Sot. 76 (1954) 3869. [9] E. Sackmann and H. Mijhwald, Chcm. Phys. Letters 12 (1972) 461. [ 101 J.C. RoSertson, CT. Yim and D.F.R. Gilson,Can. J. Chem. 49 (1971) 2325. [ 111 C.G. Ie Fevre and R.J.W. le F&e, Rev. Pure Appl. Chem. 5 (1955) 261. [ 121 P.L. Barili and CA. Veracini, Chcm. Phys. Letters 8 (1971) 229.
[ 21 R. Foster, Organic charge transfer compleses
Volume 19, number 4
15 April 1973
NMR INVESTIGATION OF THE GEOMETRY OF A PYRIDINE-BROMINE COMPLEX IN THE NEMATIC PHASE C.A. VERACINI, Laboratorio di Clhica Istituto
di Chimico
M. LONGERI
Qnautistica ed Energetica Molecolare, Fisico dell
’ Urriwrsitb
di Fisa. Italy
and P.L. BARILI Istitctto di Chimica Organica della Facolth di Fanmcia Jell’Universita di Pisa, Italy Received 17 Janu;lry 1973
The NMR spectrum of a bromine-pyridine charge-transfer complex oriented in a nematic phase has been obtained and analyzed. The orientational parameters suggest relative positions of donor and acceptor molecules similar to those found in the solid state for analogous halogen-pyridine complcses.
1. Introduciion
2. Experimental
Although the structures of several molecular chargetransfer complexes have been eittensively studied in the solid state [ 11, their geometries in solution are still under investigation [2]. Because of the small enthalpies of formation, some doubt exists whether forces other than those present in solution or vapour contribute to the observed structure in crystals [2]. It is known that the alignment of solute molecules in a nematic liquid crystai appears to depend primarily on solute molecular shape [Z]. If data concerning the average shape of a charge-transfer complex in the nematic phase can be obtained from motional parameters, then conclusions about the relative positions of donor and acceptor moieties may be drawn. .h the following discussion, the NMR spectra of a pyddinebromine complex partially oriented in the nematic phase are studied and the results compared with those obtained from benzonitrile and pyrindine under similar experimental conditions.
The spectrum of a solution containing 20 mole per cent of pyridine-bromine complex [4] in “nematic phase IV” [S] (E. Merck) was recorded at probe temperature (~28°C) with a Jeol C-60HL spectrometer in the field sweep mode with external lock. The spectrunr (I, fig. 1) shows an AA’BB’C pattern. The starred line can be attributed to HBr resulting from partial reaction between complex and solvent molecules. It is indeed known that pyridine-bromine complex is a good brominating agent [4]. From comparison of the intensity of this signal with those of pyridine protons it can be roughly extimated that about 25% of the complex reacted before registration of the spectrum. From spectra subsequently recorded on the same sample, it was however deduced that only a very small fraction of bromine reacted with solvent during ragistration time (250 set). The spectra of benzonitrile and pyridine were also recorded in the same phase (see II and III, fig. 1).
Volume
19, number 4
CHEMICAL
PHYSICS LETTERS
15 April 1973
Br ji r
Fig. !
Evidence of a specific interaction, in the nematic solution! between bromine and pyridine molecules leading to a strongly oriented complex was obtained also by recording spectra of pyridine dissolved in the nematic phase in the presence of non-specifically interacting molecules such as CS,, CO,, C6D6. In these
cases only a slight contraction of the spectrum was ob-
served (probably a dilution effect). Moreover the spectrum of benzene. in the same nematic phase was not greatly affected by the addition of bromine: this shows that bromine has a small effect or! the orientat-
ing properties of the nematic phase (even if it slowly reacts with solvent); All spectra were anaIyzed by means of the ATHENA computer program [6]. Itera593
15 April 1973
CHEMICAL PHYSICS LETTERS
Volume 19, number 4
Table 1 Spectral parameters”) ---
Value (Hz) -3.Okl.S --79.0~1.5
v2 -v1 v3-y1
-1325.4~1.5
D12
-167.0r1.6
&3
-18.Ok2.5
014
Table 2
WI%4 r1&34
0.978*0.008 1.730+0.012
r14h
1.959cO.016
rl Jr34 r24lr34
1.698cO.022 1.696iO.005
58.Ok1.2 D15 -145.0*1.2 D23 58.Ot1.2 O24 --~-.--“)l-hCJ H_H values (J12 = 4.9, J, 3 = 1.9, Jz~ = 7.7 Hz) nssumtxi. tive calculations gave a best 5 Hz for I and 1.2 Hz for II couplings and other spectral dine-bromine complex are
were
fit with an rms error of and III. The direct dipolar parameters of the pyrireported in table 1.
3. Results and discussion Because of the C,, symmetry of the problem, only two motional constants are needed to describe the molecular orientation. With the coordinate axes shown in fig. 1, the dipolar couplings are given by [3]: gdir ij
= -2 x 5-“2K..
1I
{C~~2_,2 [((hij)2/A
Pyridinebromine
Motional constants
Distance ratio
c3t
0.352+0.004 0.111+0.001
-rl
G-J+
--
Benzonitrile
Pyridine
0.356*0.003
0.112kO.003
0.127+0.001
0.144s0.001
Owing to the rather poor quality of the pyridinebromine complex spectrum (the rms error between observed and calculated spectrum is 5 Hz, as compared with 1.2 Hz both in the case of benzonitrile and of pyridine), it was not possible to make a meaningful comparison of the geometries of all these molecules. However, a comparison of the motional constants appears quite interesting. In fact, the data reported in table 2 permit to deduce that the extent of orientation of the pyridine-bromine complex is nearly the same as that of benzonitrile (spectra I and II are indeed very similar), and much higher than that of pyridine. From the similarity in motional behaviour between the complex and benzonitrile it should be possible to distinguish between the two structures I and II (fig. 2), proposed on theoreiical gounds for halogen-pyridine complexes [S]. In fact, if only dispersion forces are considered and a dipole-dipole approximation is used [9], the orientational potental energy of a molecule in the liquid crystal phase may be expressed in terms of polarizability anisotropies [9] :
13BV
-f <(A+‘/$.
>iy - +( (&,)‘@J
+31/k
[~c(~,)~/~~~‘~~-~c(4~ij)z/~‘~~l~,
Cl) + aYY sin’8 sin29 + aZZcos%),
sr2_y2
where conventional notation is used and where C,Z~_,z and C,2_,,2 are the motional constants which defme the orientation of the solute molecule witi respect to the optical axis of the liquid crystal. From eq. (1) both proton geometry and molecular orientation can be obtained by assuming one interproton distance [3]. The interproton distance ratios of the complex are collected in table 1; those of benzonitrile and pyridine do not appreciably differ from the values already observed in different nematic phase [6, 73. The motional constants obtained for the three molecules are reported in table 2. 594
(2)
where Q’ depends on the solvent and I, and 1, are the ionization potentials of solute and solvent respectively. Unfortunately the polarizability ellipsoids are not known for the molecule under study. However, a rough estimate can be made approximating the ratio +: cyYY: %z with the ratio of the corresponding molecular dimension &: LY: L, [ 10, 1 l] ; within the further ap-
x
=%I
Fig. 2.
Volume 19, number 4
CHEMICAL
proximation of neglecting the differences in the ionization potential (the experimental conditions, i.e., solvent, temperature, concentration being equal), it can be shown from eq. (2) that it is the molecular shape that governs the orientation in the anisotropic media [3]. Therefore the motional constants of table 2 show that the shape of benzonitrile molecule and the average shape of bromine-pyridine complex are nearly the same, i.e., they are consistent with a structure like I (fig. 2).
Thus the mean relative donor acceptor positions are similar at least in the liquid crystal phase to those observed in solid state for analogous halogen-pyridine complexes [I], Although our approximations are rather crude, the results show that studies of this type of charge-transfer complexes in the nematic phase can give valuable information. However, in many other cases the solvent can be reasonably expected to act as a complexing agent [12] and no information would be obtained. Finally, although it was not possible to obtain useful information about the geometric distortion of the py ridine moiety in the present case, it can be presumed that such information may be obtainable with a less reactive complex. The extension of this work to the iodine-pyridine complex is now in progress.
Acknowledgement The authors would like to thank Professor E.
PHYSICS
LETI-ERS
15 April
1973
Scrocco for encouragement and the Consiglio Nazionale delle Ricerche for financial support.
References [l] 0. Hassel and C. Romming, Quart. Rev. 16 (1962) 1. (Academic Press, New York, 1969) p. 216. [ 3 j P. Diehl and C.L. Khetrapnl, NM R basic principles and progress, Vol. 1 (Springer, Berlin. 1969). [4] H.E. Xfertel, The chemistry of heterocyctic compounds, pyridine and its derivatives, Part 2, cd. E. Kiingsberg (Intcraience, New York, 1961) ch. 6, p. 32I. [5] R. Steintnsser and L. Pobl,Tetrahcdron Letters (1969) 1921. [6] C.A. Veracini, P. Bucci and P.L. Barili, Mol. Phys. 23 (1972) 59. [ 71 P. Diehl. CL. Khetrapal and H.P. Kellerhals, MoI. Phys. 15 (1968) 333. [S] C. Reid and R.S. hfullikcn, I. Am. Chem. Sot. 76 (1954) 3869. [9] E. Sackmann and H. Mijhwald, Chcm. Phys. Letters 12 (1972) 461. [ 101 J.C. RoSertson, CT. Yim and D.F.R. Gilson,Can. J. Chem. 49 (1971) 2325. [ 111 C.G. Ie Fevre and R.J.W. le F&e, Rev. Pure Appl. Chem. 5 (1955) 261. [ 121 P.L. Barili and CA. Veracini, Chcm. Phys. Letters 8 (1971) 229.
[ 21 R. Foster, Organic charge transfer compleses