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NMR of 15N15N in nematic phases
Volume 106. number 5
NMR OF lsN15N
CHEhflCAL
PIIYSICS LE-I-I’ERS
4 May 1984
1N NEMATIC PHASES
E.E. BURNELL Depar~rmwt o/Cbenlisrry. Urrivcrsil~ of British Columbia. -7036 Mail: Mall. Vmrcouwr. EC, Cormdo V6T I Y6
and C.A. DE LANGE Dcparrnrenr of Physical CberGslry, Free Utliversity. De Boelelaarr 1083. 1 OS1 H V Anzsrerdarrl, Tire h’etlzerlands Received
28 February
1984
The chemical shielding anisotropy and the orientation parameters of 15N’s N dissolved in the nematic liquid crys~ais EBBA and 1132 are studied. For o ,, - oI a value of 590 ? 50 ppm is obtained. The signs of the order parameters ax related to the sign of ou - oI, and are positive in both solvents. This bthaviour is not consistent with the simple solutesolvent interaction mechanism found for hydrogen.
and provide a natural esplanation for the negative sign of the order parameters observed in several nematic solvents. In addition, in the liquid crystal p-ethoxybenzylidenep’-n-butylaniline (EBBA) an interesting correlation between order parameters and molecular quadrupole moments for a number of small solures is found [4] _For a better understanding 6f the solute-solvent interaction mechanisms operative in liquid crystals, more experimental information on small solute molecules whose physical properties are weU established is needed. In this letter we describe results for molecular nitrogen (lsN15N) dissolved in two nematic solvents.
1. Introduction A precise understanding
of the physical mechaof small solute molecules dissolved in liquidcrystalline phases remains an elusive goal. Recent detailed nuclear magnetic resonance (NMR) studies on hydrogen, methane and their deuterated analogues dissolved in various nematic iiquid crystals have provided new information on soluresolvent interactions [l-6]. The results are consistent with the presence of a secondarder tensorial interaction between the solvent mean “field(s)” and some electronic property(ies) of the solute. For the methanes it is impossible to explain all the results obtained in several different liquid crystals in terms of a single physical interaction mechanism. However, the hydrogen and methane results clearly demonstrate the presence of an appreciable average electric field gradient in liquid crystals, and values of this gradient have been determined for several nematic phases. The interaction between this gradient and the solute molecular quadrupole moment leads to solute orientation. The order parameters for molecular hydrogen predicted by this mechanism largely account for the values measured independently from NMR dipolar couplings
nisms causing partial orientation
0 009-2614/84/S INnrthJinllanA
03.00 0 Elsevier Science Publishers Phvcirc
Parhlirhino
lXGcinn\
2. Experimental The liquid crystals used as solvents were EBBA and Merck ZLI 1132 (a mixture of three phenylcyclohexanes and one biphenylcyclohexane). lsN15N from hlerck, Sharp and Dohme was condensed into 9 mm o-d. Pyrex glass tubes conmining the degassed liquid crystal. Dissolved IsN”N was in equilibrium with gaseous lsN15N at about 20 atm pressure at room temperature above solution. The waled 9 mm tubes B.V.
413
Volume
106, number
5
CHEMICAL
4 May 1984
PHYSICS LEITERS
were
placed inside 10 mm 0-d. NMR tubes. Fourier transform ‘5N NMR spectroscopy was performed at temperatures ranging from 303 to 375 K on a Bruker WH400 NMR spectrometer operating at 40.531 MHz. Samples were spun at ~20 Hz. Chemical shifts were measured relative to the absolute frequency of the spectrometer and no field-frequency lock was needed because of the inherent stability of the superconducting NMR system. The magnet drift was negligible over the time scale of a net of experiments. Similar experiments performed using 14N14N failed to yield results.
,-
P
/4 Il32-A0
0
1132-B
/
EBBA
3. Results and discussion The NMR spectrum of partially oriented 15NlsN consists of a doublet with spacing three times rhe dipolar coupling constant D, with [7] D = -(h&4n2)
S/R3 _
0)
In eq. (I), 7N is the gyromagnetic ratio of the 15N distance. Using the known equilibrium bond length R, = 1.0977 A [S], the orientation parameter S is obtained. Because for 15N15N the vibrational frequency we is rather large and the rotational constant B, is small, vibrational and centrifugal corrections [9] affect S by less than one percent. Hence, effects of intramolecular vibrations are neglected. The position of the centre of the
nucleus and R the internuclear
doublet,Ho(l - &), is related to the anisotropy in the chemical shielding, u,, - ul, through the relationship [7J ozz =u+$(u,
-ul)S,
ing tensor. Relative values of yzz have been measured at various temperatures. The relative value of u for lsNlsN has been measured in the isotropic phase of 1132. The latter measurements show that the isotropic shielding u is virtually constant in the temperature range studied. Eq. (2) indicates two independent means of obtaining u8 - uI_ First, in the gradient method, a plot of the relative values of czV versus orientation parameters S should yield a straight line with slope f (u,, uJ Secondly, if the relative isotropic shift u is known, every determination of Liz2gives a value of cq - al.
414
from
three
independent
0
I
I
_/
I
I
.Ol
.02
03
.04
.05
S
Fig. 1. Relative chemical shift (arbitrary zero, escept for 1132-A where the isotropic shift was also measured) versus order parameter. The lines are linear least-squares fits to the points. The slopes of the lines give the values of ou - oI listed in table 1. The series of experiments were performed in the following temperature ranges: 1132-A, 305-335 K in the nematic phase and 345-375 K in the isotropic phase; 1132-B, 303-330 K; EBBA, 303-345 K.
have been obtained in the liquid crystals 1132 and EBBA. In fig. 1 the gradient method is demonstrated. A least-squares analysis of the results leads to values for ull - uL which are summarized in table 1. These values, obtained in two different liquid crystals, are
(3
where u is the isotropic value of the chemical shield-
Results
0
sets of experiments
Table 1 Experimental values of a,, - o1 in t5Nt5N with a previous literature value [ 131 01
1132-A
a)
1132-B a)
-
oL
and comparison
(ppm)
575 + 18 575 * 6
EBBA a)
621 + 14
1132-A b)
597?31
ref.
662 + 30
[ 131
a) From gradient method, errors are least-squares standard errors. b) Average of values obtained using the isotropic-nematic shift method; error is 1 standard deviation.
Volume 106, number 5
CHEMICAL
PHYSICS LETTERS
consistent. In the series of measurements in 1132 where the isotropic shielding was measured, values for (I,, - a, were obtained from each experiment in the nematic phase. These values range from
reasonably
577 to 659 ppm, their average being 597 + 3 1 ppm, again in good agreement with the results in table 1. Since the relative chemical shielding values were all determined with respect to an external reference (viz. the spectrometer frequency), both the effects of bulk susceptibility and local solvent anisotropies are important [lo-121. Hence the experimental values ob-
tained for Liz,-could well contain a systematic error of typically 0.5 ppm. Since the observed chemical shifts are rather large, such systematic errors add to the experimental uncertainty in uu - a, but do not affect the conclusions to any significant extent. For us - u1 we obtain a value of 590 ppm with an estimated uncertainty of 50 ppm. No other direct experimental determination of (I,, - o1 for 15N15N is available. However, the diamagnetic contributions to u,, and u1 have been calculated using quantum chemical methods [ 131. A value for the paramagnetic contribution to u, has been obtained from the well-known relationship between Pm and the experimentally determined spin-rota?m constant for 15K15N [ 141. Since u{~ is zero for a diatomic molecule, a combination of theoretical and experimental considerations leads to the value ou - uI = 662 + 30 ppm [ 13 J. Our direct experimental result is in agreement with this value. The sign of u,, - uI obtained on the basis of the above experimental and theoretical considerations [13,14] is undoubtedly positive. Knowledge of the sign of u,, - u1 provides a means of determining the absolute sign of the orientation parameters of tsN15N in EBBA and 1132. Order parameters are found to be positive in both solvents. From a previous study on hydrogen dissolved in the same nematic phases it is known that its orientation parameter is positive in EBBA and negative in 1132 [lJ_ Also, an interaction between the solvent electric field gradient and the solute molecular quadrupole moment is capable of explaining sign and approximate magnitude of the orienta:ion parameters of hydrogen in a satisfactory fashion [4]. However, the present results for tjN15N cannot be explained in terms of an electric field gradient/molecular quadrupole moment mechanism. Using a nitrogen molecular quadrupole moment of
1 \13)’ 1984
-(I.4 + 0.1) X 10mZ6 esu [15] and electric field gradient values obtained in the hydrogen case, in EBBA a positive orientation parameter is predicted as observed. The predicted values are a little smaller than those obtained experimentally. However, in 1132 the predicted sign is opposite to that observed. Apparently this single mechanism, although reasonable for hydrogen, does not suffice to explain the results in l5 N I5 N. A similar conclusion was reached for the deuterated methanes dissolved in EBBA and 1132 [3]. In conclusion, the precise orienting mechanisms of solutes dissolved in anisotropic phases are not understood in detail, even for small molecules. In
order to shed more light on thus problem a sysrematic experimental study of small molecules whose physical properties (e.g. electric dipole moment. electric polarizability anisotropy, molecular quadrupole moment, etc.) are well known, is required.
Acknowledgement We wish to thank Dr. C.N. Patsy for stimulating discussions and Mr. LJ. Muenster for synthesizing EBBA. Financial support from the North .4tlantic Treaty Organization (grant No. 1954) and from the Natural Sciences and Engineering Research Council of Canada (EEB) is gratefully acknowledged.
References E.E. BurnelJ, C..A. de Lange and J.G. Snijders, Phys. Rev. X25 (1982) 2339. E.E. BurnelI and CA. de La+, J. Chem. Phys. 76 (1982) 3171. J.C. Snijders,C.A. de Lange and E.E. BurnsU. J. Chem. Phys. 77 (19SZ) 53S6. GN. Palsy, E.E. BumelI, J.C. Snijdcrs snd CA. de Lange,Chem. Phys. Letters 99 (19S3) 271. J.G. SniJdcrs, CA de Lange and E.E. BumelI, J. Chrm. Phys. 79 (1983) 2961. J.G. Snijders,C.A. de Lange and E.E. Bums& Isue J. Chem.. IO be published. P. Diehl znd CL. Khetmpd. N\lR b&sicprinciples and pro_ercss,Vol. 1 (Springer, Berlin, 1969). J. Bendtsen, J. Raman Sprctry. 2 (1971) 133. AD. Buckin:ham. J. Chcm. Phys. 36 (1961) 3096. AD. Buckingham and E.E. BurnelI, J. Am. Chem. Sot. 69 (1967) 3311.
415
Volume 106. number 5
[ 111 AJ). Buckingham. E.E. BumelI and CA. de Lange, J. Am. Chem. Soc:90 (1968) 2972. [ 121 AD. Buckingham. E.E.BumelIand CA. de Lange, Ji Chem. Phys. 54 (1971) 3242. [ 131 R.D. Amos, Mol. Phys. 39 (1980) 1.
416
4 May 1984
CHEMICAL PHYSICS LETTERS
[ 141 S.I. Ghan, M.R. Baker and N.F. Ramsey, Phys. Rev. 136 (1964) A1224.
[ 151 A.D. Buckingham. R.L. Disch and D.A. Dunmur. J. Am.Chem.
Sot. 90 (1968) 3104.
NMR OF lsN15N
CHEhflCAL
PIIYSICS LE-I-I’ERS
4 May 1984
1N NEMATIC PHASES
E.E. BURNELL Depar~rmwt o/Cbenlisrry. Urrivcrsil~ of British Columbia. -7036 Mail: Mall. Vmrcouwr. EC, Cormdo V6T I Y6
and C.A. DE LANGE Dcparrnrenr of Physical CberGslry, Free Utliversity. De Boelelaarr 1083. 1 OS1 H V Anzsrerdarrl, Tire h’etlzerlands Received
28 February
1984
The chemical shielding anisotropy and the orientation parameters of 15N’s N dissolved in the nematic liquid crys~ais EBBA and 1132 are studied. For o ,, - oI a value of 590 ? 50 ppm is obtained. The signs of the order parameters ax related to the sign of ou - oI, and are positive in both solvents. This bthaviour is not consistent with the simple solutesolvent interaction mechanism found for hydrogen.
and provide a natural esplanation for the negative sign of the order parameters observed in several nematic solvents. In addition, in the liquid crystal p-ethoxybenzylidenep’-n-butylaniline (EBBA) an interesting correlation between order parameters and molecular quadrupole moments for a number of small solures is found [4] _For a better understanding 6f the solute-solvent interaction mechanisms operative in liquid crystals, more experimental information on small solute molecules whose physical properties are weU established is needed. In this letter we describe results for molecular nitrogen (lsN15N) dissolved in two nematic solvents.
1. Introduction A precise understanding
of the physical mechaof small solute molecules dissolved in liquidcrystalline phases remains an elusive goal. Recent detailed nuclear magnetic resonance (NMR) studies on hydrogen, methane and their deuterated analogues dissolved in various nematic iiquid crystals have provided new information on soluresolvent interactions [l-6]. The results are consistent with the presence of a secondarder tensorial interaction between the solvent mean “field(s)” and some electronic property(ies) of the solute. For the methanes it is impossible to explain all the results obtained in several different liquid crystals in terms of a single physical interaction mechanism. However, the hydrogen and methane results clearly demonstrate the presence of an appreciable average electric field gradient in liquid crystals, and values of this gradient have been determined for several nematic phases. The interaction between this gradient and the solute molecular quadrupole moment leads to solute orientation. The order parameters for molecular hydrogen predicted by this mechanism largely account for the values measured independently from NMR dipolar couplings
nisms causing partial orientation
0 009-2614/84/S INnrthJinllanA
03.00 0 Elsevier Science Publishers Phvcirc
Parhlirhino
lXGcinn\
2. Experimental The liquid crystals used as solvents were EBBA and Merck ZLI 1132 (a mixture of three phenylcyclohexanes and one biphenylcyclohexane). lsN15N from hlerck, Sharp and Dohme was condensed into 9 mm o-d. Pyrex glass tubes conmining the degassed liquid crystal. Dissolved IsN”N was in equilibrium with gaseous lsN15N at about 20 atm pressure at room temperature above solution. The waled 9 mm tubes B.V.
413
Volume
106, number
5
CHEMICAL
4 May 1984
PHYSICS LEITERS
were
placed inside 10 mm 0-d. NMR tubes. Fourier transform ‘5N NMR spectroscopy was performed at temperatures ranging from 303 to 375 K on a Bruker WH400 NMR spectrometer operating at 40.531 MHz. Samples were spun at ~20 Hz. Chemical shifts were measured relative to the absolute frequency of the spectrometer and no field-frequency lock was needed because of the inherent stability of the superconducting NMR system. The magnet drift was negligible over the time scale of a net of experiments. Similar experiments performed using 14N14N failed to yield results.
,-
P
/4 Il32-A0
0
1132-B
/
EBBA
3. Results and discussion The NMR spectrum of partially oriented 15NlsN consists of a doublet with spacing three times rhe dipolar coupling constant D, with [7] D = -(h&4n2)
S/R3 _
0)
In eq. (I), 7N is the gyromagnetic ratio of the 15N distance. Using the known equilibrium bond length R, = 1.0977 A [S], the orientation parameter S is obtained. Because for 15N15N the vibrational frequency we is rather large and the rotational constant B, is small, vibrational and centrifugal corrections [9] affect S by less than one percent. Hence, effects of intramolecular vibrations are neglected. The position of the centre of the
nucleus and R the internuclear
doublet,Ho(l - &), is related to the anisotropy in the chemical shielding, u,, - ul, through the relationship [7J ozz =u+$(u,
-ul)S,
ing tensor. Relative values of yzz have been measured at various temperatures. The relative value of u for lsNlsN has been measured in the isotropic phase of 1132. The latter measurements show that the isotropic shielding u is virtually constant in the temperature range studied. Eq. (2) indicates two independent means of obtaining u8 - uI_ First, in the gradient method, a plot of the relative values of czV versus orientation parameters S should yield a straight line with slope f (u,, uJ Secondly, if the relative isotropic shift u is known, every determination of Liz2gives a value of cq - al.
414
from
three
independent
0
I
I
_/
I
I
.Ol
.02
03
.04
.05
S
Fig. 1. Relative chemical shift (arbitrary zero, escept for 1132-A where the isotropic shift was also measured) versus order parameter. The lines are linear least-squares fits to the points. The slopes of the lines give the values of ou - oI listed in table 1. The series of experiments were performed in the following temperature ranges: 1132-A, 305-335 K in the nematic phase and 345-375 K in the isotropic phase; 1132-B, 303-330 K; EBBA, 303-345 K.
have been obtained in the liquid crystals 1132 and EBBA. In fig. 1 the gradient method is demonstrated. A least-squares analysis of the results leads to values for ull - uL which are summarized in table 1. These values, obtained in two different liquid crystals, are
(3
where u is the isotropic value of the chemical shield-
Results
0
sets of experiments
Table 1 Experimental values of a,, - o1 in t5Nt5N with a previous literature value [ 131 01
1132-A
a)
1132-B a)
-
oL
and comparison
(ppm)
575 + 18 575 * 6
EBBA a)
621 + 14
1132-A b)
597?31
ref.
662 + 30
[ 131
a) From gradient method, errors are least-squares standard errors. b) Average of values obtained using the isotropic-nematic shift method; error is 1 standard deviation.
Volume 106, number 5
CHEMICAL
PHYSICS LETTERS
consistent. In the series of measurements in 1132 where the isotropic shielding was measured, values for (I,, - a, were obtained from each experiment in the nematic phase. These values range from
reasonably
577 to 659 ppm, their average being 597 + 3 1 ppm, again in good agreement with the results in table 1. Since the relative chemical shielding values were all determined with respect to an external reference (viz. the spectrometer frequency), both the effects of bulk susceptibility and local solvent anisotropies are important [lo-121. Hence the experimental values ob-
tained for Liz,-could well contain a systematic error of typically 0.5 ppm. Since the observed chemical shifts are rather large, such systematic errors add to the experimental uncertainty in uu - a, but do not affect the conclusions to any significant extent. For us - u1 we obtain a value of 590 ppm with an estimated uncertainty of 50 ppm. No other direct experimental determination of (I,, - o1 for 15N15N is available. However, the diamagnetic contributions to u,, and u1 have been calculated using quantum chemical methods [ 131. A value for the paramagnetic contribution to u, has been obtained from the well-known relationship between Pm and the experimentally determined spin-rota?m constant for 15K15N [ 141. Since u{~ is zero for a diatomic molecule, a combination of theoretical and experimental considerations leads to the value ou - uI = 662 + 30 ppm [ 13 J. Our direct experimental result is in agreement with this value. The sign of u,, - uI obtained on the basis of the above experimental and theoretical considerations [13,14] is undoubtedly positive. Knowledge of the sign of u,, - u1 provides a means of determining the absolute sign of the orientation parameters of tsN15N in EBBA and 1132. Order parameters are found to be positive in both solvents. From a previous study on hydrogen dissolved in the same nematic phases it is known that its orientation parameter is positive in EBBA and negative in 1132 [lJ_ Also, an interaction between the solvent electric field gradient and the solute molecular quadrupole moment is capable of explaining sign and approximate magnitude of the orienta:ion parameters of hydrogen in a satisfactory fashion [4]. However, the present results for tjN15N cannot be explained in terms of an electric field gradient/molecular quadrupole moment mechanism. Using a nitrogen molecular quadrupole moment of
1 \13)’ 1984
-(I.4 + 0.1) X 10mZ6 esu [15] and electric field gradient values obtained in the hydrogen case, in EBBA a positive orientation parameter is predicted as observed. The predicted values are a little smaller than those obtained experimentally. However, in 1132 the predicted sign is opposite to that observed. Apparently this single mechanism, although reasonable for hydrogen, does not suffice to explain the results in l5 N I5 N. A similar conclusion was reached for the deuterated methanes dissolved in EBBA and 1132 [3]. In conclusion, the precise orienting mechanisms of solutes dissolved in anisotropic phases are not understood in detail, even for small molecules. In
order to shed more light on thus problem a sysrematic experimental study of small molecules whose physical properties (e.g. electric dipole moment. electric polarizability anisotropy, molecular quadrupole moment, etc.) are well known, is required.
Acknowledgement We wish to thank Dr. C.N. Patsy for stimulating discussions and Mr. LJ. Muenster for synthesizing EBBA. Financial support from the North .4tlantic Treaty Organization (grant No. 1954) and from the Natural Sciences and Engineering Research Council of Canada (EEB) is gratefully acknowledged.
References E.E. BurnelJ, C..A. de Lange and J.G. Snijders, Phys. Rev. X25 (1982) 2339. E.E. BurnelI and CA. de La+, J. Chem. Phys. 76 (1982) 3171. J.C. Snijders,C.A. de Lange and E.E. BurnsU. J. Chem. Phys. 77 (19SZ) 53S6. GN. Palsy, E.E. BumelI, J.C. Snijdcrs snd CA. de Lange,Chem. Phys. Letters 99 (19S3) 271. J.G. SniJdcrs, CA de Lange and E.E. BumelI, J. Chrm. Phys. 79 (1983) 2961. J.G. Snijders,C.A. de Lange and E.E. Bums& Isue J. Chem.. IO be published. P. Diehl znd CL. Khetmpd. N\lR b&sicprinciples and pro_ercss,Vol. 1 (Springer, Berlin, 1969). J. Bendtsen, J. Raman Sprctry. 2 (1971) 133. AD. Buckin:ham. J. Chcm. Phys. 36 (1961) 3096. AD. Buckingham and E.E. BurnelI, J. Am. Chem. Sot. 69 (1967) 3311.
415
Volume 106. number 5
[ 111 AJ). Buckingham. E.E. BumelI and CA. de Lange, J. Am. Chem. Soc:90 (1968) 2972. [ 121 AD. Buckingham. E.E.BumelIand CA. de Lange, Ji Chem. Phys. 54 (1971) 3242. [ 131 R.D. Amos, Mol. Phys. 39 (1980) 1.
416
4 May 1984
CHEMICAL PHYSICS LETTERS
[ 141 S.I. Ghan, M.R. Baker and N.F. Ramsey, Phys. Rev. 136 (1964) A1224.
[ 151 A.D. Buckingham. R.L. Disch and D.A. Dunmur. J. Am.Chem.
Sot. 90 (1968) 3104.