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Molecular motion is solid tropylium salts
CHEMICAL PHYSICS LETTERS
Volume 2. number 3
MOLECULAR
MOTION
IS
SOLID
July 1968
TROPYLIUM
SALTS
C. A. FYFE and C. A. MCDOWELL P:panr::wt
of Clremistry.
Univavsily
of British
CoZumbia,
Vancouver
8, Canada
Received 1’7 May 1968
Broad line nuclear magnetic resonance studies have been carried out over the temperature range ‘7’7OKto 300°K on a representative series of tropylium compounds. The second moments of the line widths indicate that reorientation of the tropylium ions occurs in these solids. Estimates of the activation energies for the reorientation processes have been made from the temperature dependence of the spin-lattice relaxation times. 1. INTRODUCTION
Since the early work of Andrew [l, 21 on the molecular reorientation of benzene in the solid state, broad-line NMR has %en widely used in the investigation
of molecular
motion in solids
[3,4). In general, the occurrence ~1 motion in a crystal lattice can cause profound changes in both the NMR signal shape and the spin-lattice relaxation time (~1) of nuclei involved in the motion. In the present paper broad-line NMR is used to investigate possible motion in the aromatic tropylium ion ring system * C7H$ in the solid state. [Tr+J2 SnCli, [Tr+] BF4 and [Tr+] Brwere chosen as representative of the various types of tropylium ions. 2. EmERIMENTAL Spectra were recorded at 30 MHz on a crossedcoil, wide line NMR spectrometer constructed from VA: lan units and employing a Varian V-4007
six-inch water-cooled
electromagnet.
Signals
were rccsrded as derivatives of the absorption made &titer phase-sensitive detection. Care was taken to avoid saturation by using sufficiently low r.f. power levels. The second moments of the experiment:Ll spectra were calculated using an IBM 7044 Foriran IV computer programme and were corrected for modulation broadening [S]. T1 measurements were made by the “adiabatic fast-passage” method as described recently by Janzen, Cyr and Dune11 [6] at 8 MHz using a square wave to modulate the main magnetic field. * Hereafter referred to as [Tr?. 170
Spectra at 77’K were obtained by immersion of the sample in a dewar containing liquid nitrogen. Temperatures intermediate between liquid nitrogen and room temperatures were obtained using a flow of cooled nitrogen gas. Temperatures were recorded using a thermocouple placed close to the sample. Samples in all cases were in powder form in 10 mm O.D. tubes. The tropylium ions were prepared by methods described in the literature [7-91 and gave satisfactory micro-analyses and melting points in agreement with published values. 3. SECOND MOMENT MEASUREMENTS All the compounds investigated showed smooth, relatively narrow, derivative curves with no fine structure at both room and liquid nitrogen temperatures. Second moments calculated from these spectra are shown in table 1. An X-ray investigation of the crystal structures of [Tr+]I- and [Tr+]CIO4 by Kitaigorodskii et al. [lo] found a value of 1.69 A for the ring radius in both cases. Assuming a value for the C-H bond length and the value of the ring radius Table 1 Experimental second moments (gauss2) of tropylium ions at 293OK and at WoKa) -~___-293oK
77oK
[Tr+]BFi
1.5, * 0.2
1.64 r 0.2
[Tr+]2SnCli
1.07 i 0.2
3.4
[Tr+]Br-
o-g3 * 0.2
1.25 * 0.2
1011
+ 0.6
a) Average of several measurements: error gives range of observed results.
-
Volume 2. number 3
CHEMICAL PHYSICS LETTERS
Table 2 intramolecular second moments (gaussa) for rigid and rotating tropylium ion ring
petted). This conclusion is in harmony with the results of the X-ray study on [Trf]E’ and [Tr+]CIOi where it was found [lo] that “the tropylium ion occupied a statistically disordered po-
Calculated
Second moments (G2) Compound
Tropylium
tic-w
ion
Benzene
x
Rigid ring
sition
Rotzitionabout axis 1 to ring plane
axis
“rotation”
3.95
0.99
C-C
I.08
3.86
0.97
1.18
3.78
0.94
1.08”)
3.10
0.78
the theoretical
1. Plots of lolog
inrramole-
T1 against l/T
about
within
an
of the ring”.
the determination the ring.
in the experimental
the value
study,
prevented lengths
it “rotated”
second
This
of the
There
is Little
moments of
tion still OCCUFSfreely for these ions at this temperature. There is however, a change in the value for [Tr+]$ZnClg, although it does not reach
cular contribution to the second moment can be calculated from the formulation of Van Vleck [ 111. These are shown in table- 2 for a vsriation on the C-H bond length of 0.98 A to 1.18 A (an eiectron diffraction study [lz] gives the C-H bond length in benzene as 1.08 A). Included in the table are the corresponding results from the work of Andrew [2] on benzene. The experimental second moments at room temperature of all three species are considerably less than the intramolecular contribution calculated on the basis of a stationary ring, and reorientation of the tropylium ring is thought to take place freely at this temperature. Further, the values observed are close to those calculated for a rotation about the ring axis (unlike the case of benzene, very large intermclecular contributions to the second moment would not be ex-
Fig.
bond
i.e.
to the plane
[Tr+]BF;i and [Tr+]I’ when the temperature is lowered to 77’K and it is thought that reorienta-
a) From ref. [2]. from the X-ray
in the crystal. perpendicular
change
0.98
July 1968
(K-l)
calculated
for
a stationary
ion and it is
thought that this molecule is in a transition between free rotation and stationary state at this temperature. In the case
of tropylium
fluoFboFate,
the possibility tion from
of some intermolecular the lgF nuclei in the anion.
there
is
contribcrSecond mo-
ments of the IgF resonance in this crystal at room and liquid nitrogen temperatures are 0.76 & * 0.1 G2 and 2.2 + 0.3 G2 respectively. Assuming the BFi ion to be jetrahedral and the B-F bond length to be 1.43 A as determined by X-ray diffraction [ 13]! the intramolecular contribution to the second moment is 14.55 G2_ Rotation of the ion about a three-foid 3r a two-fold symmetry axis would reduce this value to 3.98 G2 and 0.75 G2 respectively_ The experimental values are considerably less than the value for a stationary ion, and the BF: ion is thought to be rotating, possibly in a random fashion, at all temperatures studied.
fc.r [TrTBBFi (curve A) and ITrr12SnC1z (curve 3). 171
Volume 2, number 3
CHEMICAL PHYSICS LETTERS
4. Tl MEASUREMENTS Measurements of Tl were carried out on the proton resonances of [Tr+]BFi and [Tr+]SnCl~. If the motions of the nuclei can be characterised by a single correlation time 2’c, and if this obeys the Arrhenius relation Tc = T exp(Ea/kT). (Where To is a limiting correlation time and E, is the activation energy for the reorientation process) then a plot of log T1 again& l/T will be a curve with limiting slopes -E,/R and da/R when W,T >> 1 and JVoTc << 1. respectively [ 141. Plots of 18 log Tl against l/T for the compounds studied are shown in fig. 1. Assuming in the case of [Tr+]BFi that there is no significant relaxation through the fluorine nuclei and approximating the limiting slopes by straight lines ‘as shown; activation energies for the rotation process of -2.0 * f 0.2 kcal/mole (fluoborate) and -2.3 f 0.2 kcal/ mole (hexachlnrostannate) are obtained. That the minimum in the T1 curve for the hexachlorostannate occurs at a much higher temperature than the fluoborate may mean that this compound stops rotating at a higher temperature, as indicated from the second moment measurements. The activation energy for rotation in sol-
id benzene [2] is found to be -3.7 f 0.2 kcal/‘mole, so that the results obtained in the present work are reasonable in view of the much looser structures expected for the compounds
studied here.
ACKNOWLEDGEMENTS
The authors would like to thank Mr. P. P. Borda for performing the micro- analyses and
172
July 1968
acknowledge helpful discussions with Mr. 3. Ripmeester and Dr. P. Raghunathan. One of the authors (C.A.F.) wished to acknowledge the award of a Killam Postdoctoral Fellowship by the University of British Columbia. REFERENCES [l] E. R. Andrew, J. Chem. Phys. 18 (1950) 607. [2] E. R. Andrew and R. G. Eades. Proc. Roy. Sot. (London) A218 (1953) 537. [3] J. G. Powles. Arch.Sci. (Geneva) 12 (1959) 87. i4] (a) E. R. Andrew, J. Phys. Chem:Solids 18.(1961) 9: (b) E. R. Andrew and P.S. Allen, J. Chim. Phys. 85
i1966).
[5] E. R.Andrew. Phys. Rev. 91 (1953) 425. [6] W. R. Janzen. T. J. R. Cyr and B. A.Dunell, J. Chem. Phys. 48 (1968) 1246. [7] H. J. Dauben Jr.. F. A. Gadecki. K. M. Hannon and D. L. Pearson, J. Am. Chem.Soc. 79 (1957) 4557. (31 II. J. Dauben Jr., L. R. Hannen and K. M. Hannon. J. Org. Chem. 25 (1960) 1442. [SJ K. M. Hannon, A. B.Hannon and F. E.Cummings, J. Am. Chem. Sot. 86 (1964) 5511. [lo] A. I.Kitaigorodskii, ?w_ T:Struchkov, T. L. Khotsyanova, M. E. Volrpin and D. N.Kursanov, Izv. Akad.Nauk SSSR. Otd.Khim.Nauk 39 (1960) (C.A. 56. 11028 c.). [Ill J.H.Van Vleck. Phys. Rev. ‘74 (1948) 1168. 1121 I. L. Dnrle. J_ Chem. Phys. 20 (1952) 65. [13] J. L-Hoard and V. Blair, J-Am. Chem.Soc. 57 (1935) 1985. [14] E. 0. Etejskal, D. E. Woessner, T. C. Farrar and H. S. Gutowsky, J. Chem. Phys. 31 (1959) 55: J. E. r\nderson and 1%‘.P. Slichter, J. Chem. Phys. 41 (lS64) 1922.
Volume 2. number 3
MOLECULAR
MOTION
IS
SOLID
July 1968
TROPYLIUM
SALTS
C. A. FYFE and C. A. MCDOWELL P:panr::wt
of Clremistry.
Univavsily
of British
CoZumbia,
Vancouver
8, Canada
Received 1’7 May 1968
Broad line nuclear magnetic resonance studies have been carried out over the temperature range ‘7’7OKto 300°K on a representative series of tropylium compounds. The second moments of the line widths indicate that reorientation of the tropylium ions occurs in these solids. Estimates of the activation energies for the reorientation processes have been made from the temperature dependence of the spin-lattice relaxation times. 1. INTRODUCTION
Since the early work of Andrew [l, 21 on the molecular reorientation of benzene in the solid state, broad-line NMR has %en widely used in the investigation
of molecular
motion in solids
[3,4). In general, the occurrence ~1 motion in a crystal lattice can cause profound changes in both the NMR signal shape and the spin-lattice relaxation time (~1) of nuclei involved in the motion. In the present paper broad-line NMR is used to investigate possible motion in the aromatic tropylium ion ring system * C7H$ in the solid state. [Tr+J2 SnCli, [Tr+] BF4 and [Tr+] Brwere chosen as representative of the various types of tropylium ions. 2. EmERIMENTAL Spectra were recorded at 30 MHz on a crossedcoil, wide line NMR spectrometer constructed from VA: lan units and employing a Varian V-4007
six-inch water-cooled
electromagnet.
Signals
were rccsrded as derivatives of the absorption made &titer phase-sensitive detection. Care was taken to avoid saturation by using sufficiently low r.f. power levels. The second moments of the experiment:Ll spectra were calculated using an IBM 7044 Foriran IV computer programme and were corrected for modulation broadening [S]. T1 measurements were made by the “adiabatic fast-passage” method as described recently by Janzen, Cyr and Dune11 [6] at 8 MHz using a square wave to modulate the main magnetic field. * Hereafter referred to as [Tr?. 170
Spectra at 77’K were obtained by immersion of the sample in a dewar containing liquid nitrogen. Temperatures intermediate between liquid nitrogen and room temperatures were obtained using a flow of cooled nitrogen gas. Temperatures were recorded using a thermocouple placed close to the sample. Samples in all cases were in powder form in 10 mm O.D. tubes. The tropylium ions were prepared by methods described in the literature [7-91 and gave satisfactory micro-analyses and melting points in agreement with published values. 3. SECOND MOMENT MEASUREMENTS All the compounds investigated showed smooth, relatively narrow, derivative curves with no fine structure at both room and liquid nitrogen temperatures. Second moments calculated from these spectra are shown in table 1. An X-ray investigation of the crystal structures of [Tr+]I- and [Tr+]CIO4 by Kitaigorodskii et al. [lo] found a value of 1.69 A for the ring radius in both cases. Assuming a value for the C-H bond length and the value of the ring radius Table 1 Experimental second moments (gauss2) of tropylium ions at 293OK and at WoKa) -~___-293oK
77oK
[Tr+]BFi
1.5, * 0.2
1.64 r 0.2
[Tr+]2SnCli
1.07 i 0.2
3.4
[Tr+]Br-
o-g3 * 0.2
1.25 * 0.2
1011
+ 0.6
a) Average of several measurements: error gives range of observed results.
-
Volume 2. number 3
CHEMICAL PHYSICS LETTERS
Table 2 intramolecular second moments (gaussa) for rigid and rotating tropylium ion ring
petted). This conclusion is in harmony with the results of the X-ray study on [Trf]E’ and [Tr+]CIOi where it was found [lo] that “the tropylium ion occupied a statistically disordered po-
Calculated
Second moments (G2) Compound
Tropylium
tic-w
ion
Benzene
x
Rigid ring
sition
Rotzitionabout axis 1 to ring plane
axis
“rotation”
3.95
0.99
C-C
I.08
3.86
0.97
1.18
3.78
0.94
1.08”)
3.10
0.78
the theoretical
1. Plots of lolog
inrramole-
T1 against l/T
about
within
an
of the ring”.
the determination the ring.
in the experimental
the value
study,
prevented lengths
it “rotated”
second
This
of the
There
is Little
moments of
tion still OCCUFSfreely for these ions at this temperature. There is however, a change in the value for [Tr+]$ZnClg, although it does not reach
cular contribution to the second moment can be calculated from the formulation of Van Vleck [ 111. These are shown in table- 2 for a vsriation on the C-H bond length of 0.98 A to 1.18 A (an eiectron diffraction study [lz] gives the C-H bond length in benzene as 1.08 A). Included in the table are the corresponding results from the work of Andrew [2] on benzene. The experimental second moments at room temperature of all three species are considerably less than the intramolecular contribution calculated on the basis of a stationary ring, and reorientation of the tropylium ring is thought to take place freely at this temperature. Further, the values observed are close to those calculated for a rotation about the ring axis (unlike the case of benzene, very large intermclecular contributions to the second moment would not be ex-
Fig.
bond
i.e.
to the plane
[Tr+]BF;i and [Tr+]I’ when the temperature is lowered to 77’K and it is thought that reorienta-
a) From ref. [2]. from the X-ray
in the crystal. perpendicular
change
0.98
July 1968
(K-l)
calculated
for
a stationary
ion and it is
thought that this molecule is in a transition between free rotation and stationary state at this temperature. In the case
of tropylium
fluoFboFate,
the possibility tion from
of some intermolecular the lgF nuclei in the anion.
there
is
contribcrSecond mo-
ments of the IgF resonance in this crystal at room and liquid nitrogen temperatures are 0.76 & * 0.1 G2 and 2.2 + 0.3 G2 respectively. Assuming the BFi ion to be jetrahedral and the B-F bond length to be 1.43 A as determined by X-ray diffraction [ 13]! the intramolecular contribution to the second moment is 14.55 G2_ Rotation of the ion about a three-foid 3r a two-fold symmetry axis would reduce this value to 3.98 G2 and 0.75 G2 respectively_ The experimental values are considerably less than the value for a stationary ion, and the BF: ion is thought to be rotating, possibly in a random fashion, at all temperatures studied.
fc.r [TrTBBFi (curve A) and ITrr12SnC1z (curve 3). 171
Volume 2, number 3
CHEMICAL PHYSICS LETTERS
4. Tl MEASUREMENTS Measurements of Tl were carried out on the proton resonances of [Tr+]BFi and [Tr+]SnCl~. If the motions of the nuclei can be characterised by a single correlation time 2’c, and if this obeys the Arrhenius relation Tc = T exp(Ea/kT). (Where To is a limiting correlation time and E, is the activation energy for the reorientation process) then a plot of log T1 again& l/T will be a curve with limiting slopes -E,/R and da/R when W,T >> 1 and JVoTc << 1. respectively [ 141. Plots of 18 log Tl against l/T for the compounds studied are shown in fig. 1. Assuming in the case of [Tr+]BFi that there is no significant relaxation through the fluorine nuclei and approximating the limiting slopes by straight lines ‘as shown; activation energies for the rotation process of -2.0 * f 0.2 kcal/mole (fluoborate) and -2.3 f 0.2 kcal/ mole (hexachlnrostannate) are obtained. That the minimum in the T1 curve for the hexachlorostannate occurs at a much higher temperature than the fluoborate may mean that this compound stops rotating at a higher temperature, as indicated from the second moment measurements. The activation energy for rotation in sol-
id benzene [2] is found to be -3.7 f 0.2 kcal/‘mole, so that the results obtained in the present work are reasonable in view of the much looser structures expected for the compounds
studied here.
ACKNOWLEDGEMENTS
The authors would like to thank Mr. P. P. Borda for performing the micro- analyses and
172
July 1968
acknowledge helpful discussions with Mr. 3. Ripmeester and Dr. P. Raghunathan. One of the authors (C.A.F.) wished to acknowledge the award of a Killam Postdoctoral Fellowship by the University of British Columbia. REFERENCES [l] E. R. Andrew, J. Chem. Phys. 18 (1950) 607. [2] E. R. Andrew and R. G. Eades. Proc. Roy. Sot. (London) A218 (1953) 537. [3] J. G. Powles. Arch.Sci. (Geneva) 12 (1959) 87. i4] (a) E. R. Andrew, J. Phys. Chem:Solids 18.(1961) 9: (b) E. R. Andrew and P.S. Allen, J. Chim. Phys. 85
i1966).
[5] E. R.Andrew. Phys. Rev. 91 (1953) 425. [6] W. R. Janzen. T. J. R. Cyr and B. A.Dunell, J. Chem. Phys. 48 (1968) 1246. [7] H. J. Dauben Jr.. F. A. Gadecki. K. M. Hannon and D. L. Pearson, J. Am. Chem.Soc. 79 (1957) 4557. (31 II. J. Dauben Jr., L. R. Hannen and K. M. Hannon. J. Org. Chem. 25 (1960) 1442. [SJ K. M. Hannon, A. B.Hannon and F. E.Cummings, J. Am. Chem. Sot. 86 (1964) 5511. [lo] A. I.Kitaigorodskii, ?w_ T:Struchkov, T. L. Khotsyanova, M. E. Volrpin and D. N.Kursanov, Izv. Akad.Nauk SSSR. Otd.Khim.Nauk 39 (1960) (C.A. 56. 11028 c.). [Ill J.H.Van Vleck. Phys. Rev. ‘74 (1948) 1168. 1121 I. L. Dnrle. J_ Chem. Phys. 20 (1952) 65. [13] J. L-Hoard and V. Blair, J-Am. Chem.Soc. 57 (1935) 1985. [14] E. 0. Etejskal, D. E. Woessner, T. C. Farrar and H. S. Gutowsky, J. Chem. Phys. 31 (1959) 55: J. E. r\nderson and 1%‘.P. Slichter, J. Chem. Phys. 41 (lS64) 1922.