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Electric field induced dipolar couplings in the proton NMR spectra of nitrobenzene and 2,4-dimethylnitrobenzene
Volume 7; number 6
CHEMICAL
.ELECTRIC NMR
FIELD
SPECTRA
OF
INDUCED
DIGOLAR
NITROBENZENE
Chemical taborato>y
1.5December
PHYSICS LETTERS
COUPLINGS
AND
IN
THE
L970
PROTON
2.4-DIMETHYLNITROBENZENE
C. W. HILBERS and C. MACLEAN of lhe Free Vnircvsily. At~slcrdam,
Tlw Nctkerlat~ls
Received 2 October 1070
Revised lnanuscript received 23 October 197~ By the application of an external electric field to a polar liquid the molecules are slightly nlignecl. The dipole-dipole interaction between the nuclear spins is then not averaged out and the NhlR spectrum
is modified. Results are presented for nitrobenoene and 2.4-dimethylnitrot,enzene.
1. INTRODUCTION
In the last decade several attempts have been made to detect, by application of an external electric field, the direct spin-spin dipolar couplings in NMR spectra. The first experiment was performed by Buckingham and McLauchlan [l] on liquid para-nitrotoluene; the sign of ortho coupling constant was reported. Subsequent experiments by Sears and Hahn [2] did not confirm their results. Attempts to observe effects of external electric fields on the proton NMR spectra of nitromethane and other compounds failed [2,3j. In 1968 the first positive quadrupolar electric field effects were reported [4]. From experiments where a quadrupolar splitting of the I4N NMR signal of nitrobenzene was detected, it could be deduced that the dipolar electric field effect should amount to a few Hz at 50 kVicm, a value experimentally accessible. In this note we report results on nitrobenzene and 2.4-dimethyl nitrobenzene for which successful dipolar electiic field experiments have been performed.
2. EXPERIMENTAL In an investigation of electric field effects in NMFt it is a pre-requisite that the sample is free from ionic impurities, because electric conduction can give rise to disturbing effects. Two of
these have been discussed before [5], viz. the production of an inhomogeneous electric field over the sample and a narrowing of proton NMR lines. These difficulties can be solved by apply.1% tjle technique of electrodialysis [5,6]. During the course of our experiments a new phenomenon
was encountered. which. if not recognized. may completely invalidate the results. II the magnet is controlled by a flux stabilizer the magnetic fieid may change dramatically as soon as an electric current flows through the sample. This effect can be minimized by carefully deionizing the sample. However. it can only be controlled completely if use is made of a luck system. Therefore the mea: urements have been performed with a Varian DA 60 NMR spectrom@er? equipped with a V 4353 external reference proton stabilizer. The application of electrodialysis brings about a variety of materials in the design of the sample cell. each having its own diamagnetic susceptibility. Because of this the magnetic field homogeneity over the sample will soon be insufficient for proton NMR. For this reason we have modified the sample cell, formerly described [51 in the following way: 1. Distance between the electrodes is 4 mm instead of 3 mm. 2. Copper electrodes have been replaced by gold coated
glass
plates.
3. A shim coil has been attached to the sample cell. With this design we obtained spectra with linewidths somewhat better than 2 Hz. In our experiments the electric and magnetic fields were parallel.
3. RESULTS AND DISCUSSION With the experimental
arrangement
described
above we have measured the proton spectrum of nitrobenzene exposed to electric fields up to 50 kV/cm. At the highest voltages electric field 587
effects were cle& ‘&sible. In fig. lc the ‘spectrmu of nitr&enz&ie.is giyen:witboukand iti .fig. Id with an applied electric,field (42:s Wkcrn-); The low field absorptions are:due:to the ortho protons 2 and 6, while the othei multipi& repreSei& the -’ meta and par? protons .3, 4.+d 5.. Con?parison_of these two specirit‘lc~an~ Id shows that; especially in the low Field m$tip?et, the lines shove to., @&I-.. W&S%a e0Ii.g ek&ric %..&I is appk?d. In fig-.-la the theoretical spectruni-of t cos5pt tra@c Ziqbd is .given, 2 ~_s~~n a modSed LAUC0ON LIIprogram. The s$&.ztral parameters J and (r used in this simulation tie
m&i with the ~i&&i’&?~. in :a-paper"by &n andCastelia.no’[?]r :.ln:the:‘calcuIaLLbn:of sp&t&im la ‘a ltiewidtli of .2;_I-Jswas $t!itrodu&d. .- - -:.-. Tlie th&retickl .$&rum .&jr th‘e“al&ned liquid -, iS gi+&itifig~:lb (li&%idtk:2;51 Hi); .-it-‘has been .. ,@%il&e.~‘~addi~& to, the’ hqr+ltdntan the&pole. ‘&&’ c&$-i;gs -j$> &_m&,&j i; &,$-&. These &&uplidgs corrftkpc@d to an.av&age orieafa_ - SF
-
’
collected in table 1. They have been’ obtained from the interpretation of a detailed nitrobenzene spectrum, measured with an A 56-60 Varian NMR specfromebzr. 33s J-coq2ing-5 Z?F=ifi gcKFderE+
5z!+S&Gked
Qz <&xs?~
;
ZjE = 3~3 x IQ--_
‘,The magnitude. of. the aI$+en2 +ay ~EMZ compared wjfks Us val& 3.3 X_3 0’ &kxkx? from the g~a&-~_
fie!deffect ‘detected previously [5]. between the results may be explained bi_ a slight uncerttity in the comparison of the experimental and theorktickl spectra. Because.of some line-broadening (= 0.5 Hz) at high &>k?#+ a ~&k+~&&%? is >nirD&ZZS& z1 I!??? polar-electric
-The difference
a
C
._ Fig. 1. *II NMR
588
_ :
spectrum of nitro6enzene without (c) and-with (d) an appRed efectrfc field (4’G.S‘kVfimj. The corresponding theoretical spectra aregivqn in .(a) and (b): ._ .: . .
: ‘.
iry@njy 7,.,..n$%r _.
--,,
.
,:. -
.a,.._
’
..I,’ .., . . .-
.
December
Table 1 _. Spectral parameters used in the simulation of the nitrobenzene spectra. .Vahes are gNen in Hz. D-coupjings correspond with an orientation (3~0~20 - 1) -_____________. _ _______5~.L!??_ ., Chemical shift --__
J - couplings ---------_ J (2,3) = J(6.5) = 8.42
J$Z, = wty) 7 0
J (2,4) = J(6.4) = 1.13
73)
inferrid-value conside?
15
CHEMICAL PHYSICS LETTERS
6
____.___
D - couplings
--- L_-___.__--.--_--__D (2,3) = D(6.5) : -2.12 .
D&4)
= D&,4)
d c,zsg = J@i.,3-j = G3,I
agL,5): = q6.3)
J(3.4)
D(3.4)
= J(5.4) = 7.56
= -0.25 -:
%a3
= D(5.4) = 0.26
J(3. 5) = 1.5% Dt3. .5)----= 0.20 ---._..._ __._. . _ _ __._. _._- ..---.____--.- --._.---.-----. -. --.--^---.---of the alignment. At present we The spectrum obtained for an aligned liquid
the results
to agree within experimental
accuracy.
E@xiS=z?Z-SSx? S%?#?~&Szz+inp rnZC&R>Srn p&wvkk&~ the .??IEx?k+C IMY? t-k?Sigrl OI
(3~0~~8 - l)E is unambiguously
known. As a
ZCXS.I~~ the same applies to the direct d&&e-
dipole.couplings
1970
between the nuclear spins.
depends,
-among other parameters, on the sign of relative to the D-couplings. The C_S&YKE*&Ex?ff tk? G&?i-rrru~ 2rrws ZlI #Se 2cwJ field muIfipl’ei is given by &3 f 44 + J25 in which J23 = J5S is dominant. Under the influence of the e&t& Eield the separation between the peaks bgcomes smaller. Therefore the sign of
the J-couplings
cv
b
-HO Fig.
2.
lH NiUR spectrum of 2.4-dimethylnitrobenzene without (a) and with @) an applied electric field (49.2 W/cm). . _
,_-_--,+ _..x_ t-
589
Volume-T:
nilmber ..
023 = D56!
6
which coupling
k much greater
than.
the other dinole-diooie intekac&ns, is opposite to the sign of Jig. *The coI$ldte &$ is.&ien.in table 1. -The signs_of the J-couplings in nitrobenzene a&ee with the results obtained for other aromatic molecules using a liquid crystal as an orienting medium 181. In our experiments, however: no assumptions have to be made about the si&m of the D-couplings_ The results obtained for nitrobenzene are
corrpborated nitrobenzene.
by investigations
590
I,
._
-
:
,__
_.
..,I
_,
‘.:..:‘._,..-
:.
_ :.--
: --. __
<
-’ --we’ thank Mi. .T;G&& @k. skilful etieriinental’?he$. Tde &nip_&&idnal a_ssi&ce bf J:_Gerritsen (MT SC.) tid MS. G: K&pm&Is gratefully. acknov;iledged. -,One of the -authors ‘C.W.H.. thanks Z.W;O.‘.(Ned,erlandse organisatic, voor.,Zuiyer Wetensdhappelijlr, Onderzoek) for financial support.
on 2.4-dimethyl-
The low-field part of the spectrum (fig. 2) is caused by the ring protons: the peaks at the low field side belong- to proton 6 while the other multiplet is due to protons 3 and 5. The doublet at high field originates from the methylgroups which have a different chemical shift. The absorption of the paramcthyl group is found at the high field side. Again the apparent coupling between protons 5 and 6 becomes smaller in the presence of a strong electric field (49.2 kV:cm) (compare figs. 2a and 2b). Most convincing, however. is the change of the signal originating from the para methylgroup. This line is on the point of splitting into a triplet. while no change is observed in the neighbouring peak of the ortho methylgroup. This is to be expected: the ratio
between the splittings para-CH3 is -1,;8.
,- ~&&&&DGEME;N+ ;.
of the ortho-CH3
and the
REFEF$ENCES Buckingham and K. A. l&Lauchlan, Proc. Chcm. sot. (1963) 144. [Zj R: Ei J-Sears and E.L.Hahn; J. Chem. Phys. 45 (1966) 2753;. 47 (1967) 348. [3] J.D. Macomber. N. S. Ham and J. S. Waugh, J. Chem. Phys. 46 (1967) 2855. [4j C. %‘. HiIb& Ad C. Maclean, Chem. Phys. Letters 2 (1968) 445. [5] C. W. Hilbers ‘and C. MacLean. Mol. Phys. 1G (1969) 275. 161 G. Briere and F. Gaspard. Chem. Phys. Letters 1 (1969) 706. [‘I] S. Castellano and C. Sun. J_ Am. Chem. Sot. 83 (1966) 4742. [8] A. SauPe. Z. Naturforsch. 20a (19ti5) 572; L.C. Snyder, .J. Chem. I%ys. 43 (1965) 4041; P. Diehl and C. L. Khetrapal, in: Basic principles and progress in NiHR. Vol. 1. eds. P. Diehl, E. Fluck and R. Kosfeld (Springer. Berlin, 1969). [l J A. p.
CHEMICAL
.ELECTRIC NMR
FIELD
SPECTRA
OF
INDUCED
DIGOLAR
NITROBENZENE
Chemical taborato>y
1.5December
PHYSICS LETTERS
COUPLINGS
AND
IN
THE
L970
PROTON
2.4-DIMETHYLNITROBENZENE
C. W. HILBERS and C. MACLEAN of lhe Free Vnircvsily. At~slcrdam,
Tlw Nctkerlat~ls
Received 2 October 1070
Revised lnanuscript received 23 October 197~ By the application of an external electric field to a polar liquid the molecules are slightly nlignecl. The dipole-dipole interaction between the nuclear spins is then not averaged out and the NhlR spectrum
is modified. Results are presented for nitrobenoene and 2.4-dimethylnitrot,enzene.
1. INTRODUCTION
In the last decade several attempts have been made to detect, by application of an external electric field, the direct spin-spin dipolar couplings in NMR spectra. The first experiment was performed by Buckingham and McLauchlan [l] on liquid para-nitrotoluene; the sign of ortho coupling constant was reported. Subsequent experiments by Sears and Hahn [2] did not confirm their results. Attempts to observe effects of external electric fields on the proton NMR spectra of nitromethane and other compounds failed [2,3j. In 1968 the first positive quadrupolar electric field effects were reported [4]. From experiments where a quadrupolar splitting of the I4N NMR signal of nitrobenzene was detected, it could be deduced that the dipolar electric field effect should amount to a few Hz at 50 kVicm, a value experimentally accessible. In this note we report results on nitrobenzene and 2.4-dimethyl nitrobenzene for which successful dipolar electiic field experiments have been performed.
2. EXPERIMENTAL In an investigation of electric field effects in NMFt it is a pre-requisite that the sample is free from ionic impurities, because electric conduction can give rise to disturbing effects. Two of
these have been discussed before [5], viz. the production of an inhomogeneous electric field over the sample and a narrowing of proton NMR lines. These difficulties can be solved by apply.1% tjle technique of electrodialysis [5,6]. During the course of our experiments a new phenomenon
was encountered. which. if not recognized. may completely invalidate the results. II the magnet is controlled by a flux stabilizer the magnetic fieid may change dramatically as soon as an electric current flows through the sample. This effect can be minimized by carefully deionizing the sample. However. it can only be controlled completely if use is made of a luck system. Therefore the mea: urements have been performed with a Varian DA 60 NMR spectrom@er? equipped with a V 4353 external reference proton stabilizer. The application of electrodialysis brings about a variety of materials in the design of the sample cell. each having its own diamagnetic susceptibility. Because of this the magnetic field homogeneity over the sample will soon be insufficient for proton NMR. For this reason we have modified the sample cell, formerly described [51 in the following way: 1. Distance between the electrodes is 4 mm instead of 3 mm. 2. Copper electrodes have been replaced by gold coated
glass
plates.
3. A shim coil has been attached to the sample cell. With this design we obtained spectra with linewidths somewhat better than 2 Hz. In our experiments the electric and magnetic fields were parallel.
3. RESULTS AND DISCUSSION With the experimental
arrangement
described
above we have measured the proton spectrum of nitrobenzene exposed to electric fields up to 50 kV/cm. At the highest voltages electric field 587
effects were cle& ‘&sible. In fig. lc the ‘spectrmu of nitr&enz&ie.is giyen:witboukand iti .fig. Id with an applied electric,field (42:s Wkcrn-); The low field absorptions are:due:to the ortho protons 2 and 6, while the othei multipi& repreSei& the -’ meta and par? protons .3, 4.+d 5.. Con?parison_of these two specirit‘lc~an~ Id shows that; especially in the low Field m$tip?et, the lines shove to., @&I-.. W&S%a e0Ii.g ek&ric %..&I is appk?d. In fig-.-la the theoretical spectruni-of t cos5pt tra@c Ziqbd is .given, 2 ~_s~~n a modSed LAUC0ON LIIprogram. The s$&.ztral parameters J and (r used in this simulation tie
m&i with the ~i&&i’&?~. in :a-paper"by &n andCastelia.no’[?]r :.ln:the:‘calcuIaLLbn:of sp&t&im la ‘a ltiewidtli of .2;_I-Jswas $t!itrodu&d. .- - -:.-. Tlie th&retickl .$&rum .&jr th‘e“al&ned liquid -, iS gi+&itifig~:lb (li&%idtk:2;51 Hi); .-it-‘has been .. ,@%il&e.~‘~addi~& to, the’ hqr+ltdntan the&pole. ‘&&’ c&$-i;gs -j$> &_m&,&j i; &,$-&. These &&uplidgs corrftkpc@d to an.av&age orieafa_ - SF
-
’
collected in table 1. They have been’ obtained from the interpretation of a detailed nitrobenzene spectrum, measured with an A 56-60 Varian NMR specfromebzr. 33s J-coq2ing-5 Z?F=ifi gcKFderE+
5z!+S&Gked
Qz <&xs?~
;
ZjE = 3~3 x IQ--_
‘,The magnitude. of. the aI$+en2 +ay ~EMZ compared wjfks Us val& 3.3 X_3 0’ &kxkx? from the g~a&-~_
fie!deffect ‘detected previously [5]. between the results may be explained bi_ a slight uncerttity in the comparison of the experimental and theorktickl spectra. Because.of some line-broadening (= 0.5 Hz) at high &>k?#+ a ~&k+~&&%? is >nirD&ZZS& z1 I!??? polar-electric
-The difference
a
C
._ Fig. 1. *II NMR
588
_ :
spectrum of nitro6enzene without (c) and-with (d) an appRed efectrfc field (4’G.S‘kVfimj. The corresponding theoretical spectra aregivqn in .(a) and (b): ._ .: . .
: ‘.
iry@njy 7,.,..n$%r _.
--,,
.
,:. -
.a,.._
’
..I,’ .., . . .-
.
December
Table 1 _. Spectral parameters used in the simulation of the nitrobenzene spectra. .Vahes are gNen in Hz. D-coupjings correspond with an orientation (3~0~20 - 1) -_____________. _ _______5~.L!??_ ., Chemical shift --__
J - couplings ---------_ J (2,3) = J(6.5) = 8.42
J$Z, = wty) 7 0
J (2,4) = J(6.4) = 1.13
73)
inferrid-value conside?
15
CHEMICAL PHYSICS LETTERS
6
____.___
D - couplings
--- L_-___.__--.--_--__D (2,3) = D(6.5) : -2.12 .
D&4)
= D&,4)
d c,zsg = J@i.,3-j = G3,I
agL,5): = q6.3)
J(3.4)
D(3.4)
= J(5.4) = 7.56
= -0.25 -:
%a3
= D(5.4) = 0.26
J(3. 5) = 1.5% Dt3. .5)----= 0.20 ---._..._ __._. . _ _ __._. _._- ..---.____--.- --._.---.-----. -. --.--^---.---of the alignment. At present we The spectrum obtained for an aligned liquid
the results
to agree within experimental
accuracy.
E@xiS=z?Z-SSx? S%?#?~&Szz+inp rnZC&R>Srn p&wvkk&~ the .??IEx?k+C IMY? t-k?Sigrl OI
(3~0~~8 - l)E is unambiguously
known. As a
ZCXS.I~~ the same applies to the direct d&&e-
dipole.couplings
1970
between the nuclear spins.
depends,
-among other parameters, on the sign of relative to the D-couplings. The C_S&YKE*&Ex?ff tk? G&?i-rrru~ 2rrws ZlI #Se 2cwJ field muIfipl’ei is given by &3 f 44 + J25 in which J23 = J5S is dominant. Under the influence of the e&t& Eield the separation between the peaks bgcomes smaller. Therefore the sign of
the J-couplings
cv
b
-HO Fig.
2.
lH NiUR spectrum of 2.4-dimethylnitrobenzene without (a) and with @) an applied electric field (49.2 W/cm). . _
,_-_--,+ _..x_ t-
589
Volume-T:
nilmber ..
023 = D56!
6
which coupling
k much greater
than.
the other dinole-diooie intekac&ns, is opposite to the sign of Jig. *The coI$ldte &$ is.&ien.in table 1. -The signs_of the J-couplings in nitrobenzene a&ee with the results obtained for other aromatic molecules using a liquid crystal as an orienting medium 181. In our experiments, however: no assumptions have to be made about the si&m of the D-couplings_ The results obtained for nitrobenzene are
corrpborated nitrobenzene.
by investigations
590
I,
._
-
:
,__
_.
..,I
_,
‘.:..:‘._,..-
:.
_ :.--
: --. __
<
-’ --we’ thank Mi. .T;G&& @k. skilful etieriinental’?he$. Tde &nip_&&idnal a_ssi&ce bf J:_Gerritsen (MT SC.) tid MS. G: K&pm&Is gratefully. acknov;iledged. -,One of the -authors ‘C.W.H.. thanks Z.W;O.‘.(Ned,erlandse organisatic, voor.,Zuiyer Wetensdhappelijlr, Onderzoek) for financial support.
on 2.4-dimethyl-
The low-field part of the spectrum (fig. 2) is caused by the ring protons: the peaks at the low field side belong- to proton 6 while the other multiplet is due to protons 3 and 5. The doublet at high field originates from the methylgroups which have a different chemical shift. The absorption of the paramcthyl group is found at the high field side. Again the apparent coupling between protons 5 and 6 becomes smaller in the presence of a strong electric field (49.2 kV:cm) (compare figs. 2a and 2b). Most convincing, however. is the change of the signal originating from the para methylgroup. This line is on the point of splitting into a triplet. while no change is observed in the neighbouring peak of the ortho methylgroup. This is to be expected: the ratio
between the splittings para-CH3 is -1,;8.
,- ~&&&&DGEME;N+ ;.
of the ortho-CH3
and the
REFEF$ENCES Buckingham and K. A. l&Lauchlan, Proc. Chcm. sot. (1963) 144. [Zj R: Ei J-Sears and E.L.Hahn; J. Chem. Phys. 45 (1966) 2753;. 47 (1967) 348. [3] J.D. Macomber. N. S. Ham and J. S. Waugh, J. Chem. Phys. 46 (1967) 2855. [4j C. %‘. HiIb& Ad C. Maclean, Chem. Phys. Letters 2 (1968) 445. [5] C. W. Hilbers ‘and C. MacLean. Mol. Phys. 1G (1969) 275. 161 G. Briere and F. Gaspard. Chem. Phys. Letters 1 (1969) 706. [‘I] S. Castellano and C. Sun. J_ Am. Chem. Sot. 83 (1966) 4742. [8] A. SauPe. Z. Naturforsch. 20a (19ti5) 572; L.C. Snyder, .J. Chem. I%ys. 43 (1965) 4041; P. Diehl and C. L. Khetrapal, in: Basic principles and progress in NiHR. Vol. 1. eds. P. Diehl, E. Fluck and R. Kosfeld (Springer. Berlin, 1969). [l J A. p.