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NMR spectra of partially deuterated acetonitrile dissolved in the nematic phase
JOURNAL
OF MAGNETIC
RESONANCE
1,
524-526 (1969)
NMR Spectra of Partially Deuterated Acetonitrile Dissolvedin the Nematic Phase P. Department
DIEHL
AND
of Physics,
C.
L.
University
KHETRAPAL* of Basel,
Switzerland
Received January 6, 1969; accepted March 28, 1969 The proton and the deuteron magnetic resonance spectra of partially deuterated acetonitrile are studied in the nematic phase of a mixture of 50% 4-methoxy benzylidene-4-amino-a-methyl cinnamic acid-n-propyl ester and 50% anisoleazophenyl-n-capronate at room temperature (Z 27°C). The proton spectra of the various isotopic species show direct couplings with deuterons. The isotopic substitution is shown to reduce the orientation parameter by about 0.5 % per deuteron substitution. The orientation parameter (weighted mean of the various species) obtained from the proton spectrum is used to determine the deuteron quadrupole coupling constant from the deuteron magnetic resonance spectrum of the same sample. The value is found to be 172.5 f 1.5 kHz.
INTRODUCTION
NMR spectra of partially deuterated molecules oriented in the nematic phase of liquid crystals may provide information about isotopic effects on the molecular geometry and the orientation. Furthermore, the quadrupole coupling constant may be measured (I, 2). Such studies have earlier been made in monodeuterobenzene. The present communication deals with the results on partially deuterated acetonitrile. EXPERIMENTAL
An approximately 18 mole percent solution of acetonitrile containing CH,CN, CH,DCN, CHD,CN, and CD,CN was studied in the nematic phase of 50% 4-methoxy benzylidene-4-amino-cl-methyl cinnamic acid-n-propyl ester (I) and 50 % anisole-azophenyl-n-capronate (II) at room temperature (Z 27°C). The proton spectrum was recorded on a Varian spectrometer HA60, operating in the field sweep mode without lock. The statistical error in the measurement of the line position was &-1.5 Hz. The average line-width was 8 Hz. The deuteron magnetic resonance (DMR) spectrum was studied in the same sample and at the same temperature at 7.65 MHz frequency of the spectrometer, in the absorption mode, with 40 Hz modulation. * Post-doctoral India.
fellow;
on leave from Tata Institute 524
of Fundamental
Research, Bombay-5
PARTIALLY
RESULTS PROTON MAGNETIC
525
DEUTERATED ORIENTED ACETONITRILE
RESONANCE
AND
DISCUSSION
SPECTRUM
The proton spectrum of the sample (Fig. 1) consists of a triplet (lines 1, 7, 13) due to the species containing the CH,-group, a doublet of triplets (lines 2-4; 10-12) 7
/
do
381io
M
FIG. 1. Proton magnetic resonance spectrum of partially deuterated acetonitrile in the nematic phase of 50% (I) and 50% (II). Solute concentration is 18 mole percent; temperature z 27°C.
due to the CHID and a quintet (lines 5-9) due to the CHD, group. The direct coupling Dij between the two nuclei i and j defined by Eq. [l] is reproduced in Table 1 for each of the species: Dij = - $$
. S,,, Q where Sij is the degree of orientation along the axis joining the two nuclei separated by a distance rij and y is the magnetogyric ratio. Several spectra were recorded and the mean D-values thus obtained are included in Table 1. The table indicates that the (Dnn) or [yH/yD(DHD)] values are largest for the species containing the CH, group and smallest for the molecule with the CHD, group. This value and consequently the orientation parameter (Sn, or Sn,) decreases by about 0.5% per substituted deuteron. The average Sn, and Sc, values obtained from the weighted mean of the various isotopic species using the microwave r,-values (rHH = 1.803 f 0.001 A or rCH = 1.104 + 0.001 A) of the inter-nuclear distances (3, 4), are included in Table 1. The direction and the order of magnitude of the isotopic effect agree with those observed for monodeuterobenzene (I). MAGNETIC RESONANCE (DMR) SPECTRUM The DMR spectrum of the same sample gives a quadrupole split doublet (2) the spacing (A) of which is given by Eq. [2]:
DEUTERON
526
DIEHL
AND TABLE
THE
AVERAGE
D,, AND Sfj VALUES DEUTERON-SUBSTITUTED
KHETRAPAL 1
FOR THE SAMPLE CONTAINING SPECIES OF ACETONITIZILE
Species
THE VARIOUS
Dij W)
-
Dm = 635.3 & 0.3 D Hn = 632.2 -I: 0.5 D ED = 97.0 rt 0.5; (y&)DED = 632 + 3 CHDZCN DnD = 96.5 * 0.5 DH&&D) = 629 xt 3 The average S,, value = 0.0309 f 0.00015 The average ScD value = 0.0206 + 0.0001. This assumes Ca-symmetry of the molecule. Microwave r,-values of rEH and rCH distances have been used to obtain the value of Se,. This implies that any difference in geometry of the molecule in the liquid or gas phase is neglected. The values of the indirect H-H-couplings do not affect the spectra and those of H-D couplings are too small because of the small magnetogyric ratio of the deuteron. CH&N CHzDCN
where q is the electric field gradient experienced by the deuteron with quadrupole moment Q and Scn is the degree of orientation of the C-D bond. Each component of the doublet is an envelope of several unresolved lines due to H-D direct couplings. The doublet spacing (A) in the present case was found to be 5.33 f 0.03 kHz. Using this value of (A) along with the Scn-value equal to 0.0206 + 0.0001 (Table l), the value of the deuteron quadrupole coupling constant (e2qQ/pD) was determined as 172.5 + 1.5 kHz. The error takes into account the inaccuracy of the interproton distances, the uncertainty of the orientation parameter as the weighted mean of the various species and the inaccuracy of the measurement of (A) as well as of the direct couplings. The value of the quadrupole coupling constant is in agreement with that obtained for the deuteron bonded to sp3-hybridized carbon (2). ACKNOWLEDGMENT The authors are grateful to the Swiss National Foundation financial support.
for the Advancement
of Science for
REFERENCES 1. P. DIEHL AND C. L. KHETRAPAL, Can. J. C/rem. 47, 1411 (1969). 2. J. C. ROWELL, W. D. PHILLIPS, L. R. MELBY, AND M. PANAR, J. Gem. 3. G. ENGLERT AND A. SAUPE, 2nd liquid crystal conference, Kent, Ohio 4. C. C. COSTAIN, J. Chem. Phys. 29, 864 (1958).
P&s.
(1968).
43, 3442 (1965).
OF MAGNETIC
RESONANCE
1,
524-526 (1969)
NMR Spectra of Partially Deuterated Acetonitrile Dissolvedin the Nematic Phase P. Department
DIEHL
AND
of Physics,
C.
L.
University
KHETRAPAL* of Basel,
Switzerland
Received January 6, 1969; accepted March 28, 1969 The proton and the deuteron magnetic resonance spectra of partially deuterated acetonitrile are studied in the nematic phase of a mixture of 50% 4-methoxy benzylidene-4-amino-a-methyl cinnamic acid-n-propyl ester and 50% anisoleazophenyl-n-capronate at room temperature (Z 27°C). The proton spectra of the various isotopic species show direct couplings with deuterons. The isotopic substitution is shown to reduce the orientation parameter by about 0.5 % per deuteron substitution. The orientation parameter (weighted mean of the various species) obtained from the proton spectrum is used to determine the deuteron quadrupole coupling constant from the deuteron magnetic resonance spectrum of the same sample. The value is found to be 172.5 f 1.5 kHz.
INTRODUCTION
NMR spectra of partially deuterated molecules oriented in the nematic phase of liquid crystals may provide information about isotopic effects on the molecular geometry and the orientation. Furthermore, the quadrupole coupling constant may be measured (I, 2). Such studies have earlier been made in monodeuterobenzene. The present communication deals with the results on partially deuterated acetonitrile. EXPERIMENTAL
An approximately 18 mole percent solution of acetonitrile containing CH,CN, CH,DCN, CHD,CN, and CD,CN was studied in the nematic phase of 50% 4-methoxy benzylidene-4-amino-cl-methyl cinnamic acid-n-propyl ester (I) and 50 % anisole-azophenyl-n-capronate (II) at room temperature (Z 27°C). The proton spectrum was recorded on a Varian spectrometer HA60, operating in the field sweep mode without lock. The statistical error in the measurement of the line position was &-1.5 Hz. The average line-width was 8 Hz. The deuteron magnetic resonance (DMR) spectrum was studied in the same sample and at the same temperature at 7.65 MHz frequency of the spectrometer, in the absorption mode, with 40 Hz modulation. * Post-doctoral India.
fellow;
on leave from Tata Institute 524
of Fundamental
Research, Bombay-5
PARTIALLY
RESULTS PROTON MAGNETIC
525
DEUTERATED ORIENTED ACETONITRILE
RESONANCE
AND
DISCUSSION
SPECTRUM
The proton spectrum of the sample (Fig. 1) consists of a triplet (lines 1, 7, 13) due to the species containing the CH,-group, a doublet of triplets (lines 2-4; 10-12) 7
/
do
381io
M
FIG. 1. Proton magnetic resonance spectrum of partially deuterated acetonitrile in the nematic phase of 50% (I) and 50% (II). Solute concentration is 18 mole percent; temperature z 27°C.
due to the CHID and a quintet (lines 5-9) due to the CHD, group. The direct coupling Dij between the two nuclei i and j defined by Eq. [l] is reproduced in Table 1 for each of the species: Dij = - $$
. S,,, Q where Sij is the degree of orientation along the axis joining the two nuclei separated by a distance rij and y is the magnetogyric ratio. Several spectra were recorded and the mean D-values thus obtained are included in Table 1. The table indicates that the (Dnn) or [yH/yD(DHD)] values are largest for the species containing the CH, group and smallest for the molecule with the CHD, group. This value and consequently the orientation parameter (Sn, or Sn,) decreases by about 0.5% per substituted deuteron. The average Sn, and Sc, values obtained from the weighted mean of the various isotopic species using the microwave r,-values (rHH = 1.803 f 0.001 A or rCH = 1.104 + 0.001 A) of the inter-nuclear distances (3, 4), are included in Table 1. The direction and the order of magnitude of the isotopic effect agree with those observed for monodeuterobenzene (I). MAGNETIC RESONANCE (DMR) SPECTRUM The DMR spectrum of the same sample gives a quadrupole split doublet (2) the spacing (A) of which is given by Eq. [2]:
DEUTERON
526
DIEHL
AND TABLE
THE
AVERAGE
D,, AND Sfj VALUES DEUTERON-SUBSTITUTED
KHETRAPAL 1
FOR THE SAMPLE CONTAINING SPECIES OF ACETONITIZILE
Species
THE VARIOUS
Dij W)
-
Dm = 635.3 & 0.3 D Hn = 632.2 -I: 0.5 D ED = 97.0 rt 0.5; (y&)DED = 632 + 3 CHDZCN DnD = 96.5 * 0.5 DH&&D) = 629 xt 3 The average S,, value = 0.0309 f 0.00015 The average ScD value = 0.0206 + 0.0001. This assumes Ca-symmetry of the molecule. Microwave r,-values of rEH and rCH distances have been used to obtain the value of Se,. This implies that any difference in geometry of the molecule in the liquid or gas phase is neglected. The values of the indirect H-H-couplings do not affect the spectra and those of H-D couplings are too small because of the small magnetogyric ratio of the deuteron. CH&N CHzDCN
where q is the electric field gradient experienced by the deuteron with quadrupole moment Q and Scn is the degree of orientation of the C-D bond. Each component of the doublet is an envelope of several unresolved lines due to H-D direct couplings. The doublet spacing (A) in the present case was found to be 5.33 f 0.03 kHz. Using this value of (A) along with the Scn-value equal to 0.0206 + 0.0001 (Table l), the value of the deuteron quadrupole coupling constant (e2qQ/pD) was determined as 172.5 + 1.5 kHz. The error takes into account the inaccuracy of the interproton distances, the uncertainty of the orientation parameter as the weighted mean of the various species and the inaccuracy of the measurement of (A) as well as of the direct couplings. The value of the quadrupole coupling constant is in agreement with that obtained for the deuteron bonded to sp3-hybridized carbon (2). ACKNOWLEDGMENT The authors are grateful to the Swiss National Foundation financial support.
for the Advancement
of Science for
REFERENCES 1. P. DIEHL AND C. L. KHETRAPAL, Can. J. C/rem. 47, 1411 (1969). 2. J. C. ROWELL, W. D. PHILLIPS, L. R. MELBY, AND M. PANAR, J. Gem. 3. G. ENGLERT AND A. SAUPE, 2nd liquid crystal conference, Kent, Ohio 4. C. C. COSTAIN, J. Chem. Phys. 29, 864 (1958).
P&s.
(1968).
43, 3442 (1965).