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Nuclear magnetic resonance studies of tetraalkyl compounds of group III and group V elements—III
Spectrochimica Acta, 1967,vol. 23.4, pp. 1235to 1242. Pergamon PressLtd. Printedin Northern Ireebnd
Nuclear magnetic resonance studies of tetraalkyl compounds of group III and group V elements-III E. W. RANDALL
and D. S~llw
Queen Mary College, Mile End Road, London E.l (Received 18 July 1900) Abstract-The proton NMR spectra of the ions NMe,EtA, have been studied and the coupling constants and shifts measured. The effects of the counter-ion, concentration and solvent have been investigated for the tetraethyl-ammonium ion. Solvent and concentration studies for tetraethyl-arsoniumiodide are also discussed.
1. INTRODTJCTI~N have previously studied [l-3] the proton magnetic resonance spectra of tetraalkyl derivatives of elements in Group V of the periodic table employing a variety of solvents and, in some cases, of gegen ions. Except for phosphorus, and one or two examples for As and Sb, the discussion was limited to the highly symmetrical cases of MEt,+ and MMe,+ in order that broadening of the spectra by electric quadrupole effects of M should be minimised. We have now investigated the mixed systems NMe,Et$_ in two solvents. Previous studies of the effect of gegen ion and solvent upon the internal chemical shift, A, were restricted to the cases of AsEt,+ [l-3] and ethylpyridinium derivatives [3, 41. Similar studies have now been made for NEt,+. The interactions governing the rates of quadrupolar relaxation in ionic solutions have recently been a subject of some interest, not to say controversy. In Parts I and II of this series we have adopted the view that the formation of ion pairs occurred in some of our systems and was an important factor not only for line widths but also for chemical shifts [l,21. It was, however, recognised [2] that on the microscopic level ion-ion and ion-solvent effects are not independent and that no unique model could be presented. The theoretical model initially used by VALIEV [5] was based on the assumption that solvent effects were the dominant interactions governing line widths. RICHARDS and YORKE showed that most of the cases they studied did fit the model, and the few exceptions were attributed to ion-ion contribution to the line widths [6]. HERTZ had developed a more general theory, which allowed contributions to the line width parameter from ion-ion interactions [7] and a similar approach was adopted by VALIEV [8]. The subsequent experimental studies of WE
[l] A. G. MASSEY,E. W. RANDALL and D. SHAW, Spectrochim. acta 20,379 (1964). [2] A. G. USEY, E. W. RANDALL and D. Smw, Spectrochim. Acta 21,263 (1965). [3]R. J. CHUNK,A. G. MASSEY, E. W. RANDALL and D. S-W, Nuclear Magnetic. Resonance in Chemistry (Edited by B. PESCE), p. 189,Academic Press (1906). [4] R. J. CWCK and E. W. RANDALL, Spectrochim. Acta 22, 221 (1966). [5] K. A. VALIEV, Soviet. Phys. JETP. 883 (1960). [S] R. E. RICHARDSand B. A. YORXE, Mol. Phya. 6, 287 (1963). [7] H. G. HERTZ, 2. Elektrochem. 66, 20 (1960). [S]K. A. VALIEV, Zhur. Stmkt. Khim. 5, 477 (1964). 1235
1236
E. W. RANDALLand D. SHAW
DEVERELL, FROST and RICHARDS on bromine and iodine resonances in solutions of salts were used to test the generalized model [9]. It was concluded that line widths were dominated by ion-solvent interactions for strongly hydrated cations (alkaline earths, Li+ and Na+) and by ion-ion effects for weakly solvated cations [9]. Other authors have discussed the importance of ion-ion interactions for the hexafluoroarsenate ion [lo]. We report here concentration studies for tetraalkyl-arsonium iodide in water and chloroform, together with an extended range of solvents including nitro-benzene. 2. EXPERIMENTALAND RESULTS The NMR spectra were obtained at 60 MC/S using a Varian Associates A-60 spectrometer at 35°C. They were calibrated by the standard audio side-band technique [ll].Tetramethylsilane was used as internal standard for non-aqueous solutions. In aqueous solutions sodium 3-(trimethylsilyl)-l-propanesulphonate was employed. Shifts are given in parts per million (ppm) downfield from the standard. 2.1 Tetraethylammonium
ion
Salts of the following anions were prepared by neutralization of solutions of Analar tetraethylammonium hydroxide with Analar acids: F-, Cl-, Br-, I-, NO,-, SOd2-, ClO,- and MeCOO-. In each of these cases the spectrum as previously reported [l,12-141 consists of a normal ethyl spectrum with each of the lines of the triplet split into three lines of approximately equal height due to coupling with the central Ni4 nucleus of spin 1. The coupling constants are independent of anion and solvent [12]. The internal chemical shift, A, between the methylene and methyl protons has now been found to depend on solvent, and in the case of chloroform solutions on anion. Only the bromide, hydroxide, acetate and nitrate were soluble in pure chloroform to the extent of 10 per cent. If, however, about 1 per cent methanol was added each of the other salts dissolved to give 10 per cent solutions except the fluoride and perchlorate which gave only 2 per cent solutions. The spectrum of the bromide was independent of concentration in the ranges studied. The results are summarized in Tables 1 and 2.
2.2 Tetraulkylammonium
iodides, NMe,Et+_,I-
(4 > n > 1)
These quaternary salts were prepared by reacting stoichiometric amounts of ethereal solutions of the methyl iodide with the corresponding trialkylamine, NMe,_,Et,_,. The spectra are typified by the case of triethyl-methyl-iodide (n = 1) which is shown in Fig. 1. The long range N14-H coupling is observed in each case and is approximately the same as the value in tetra-ethylammonium ion. The methyl lines show broadening due to unresolved coupling with N14 which appears to be less than O-2c/sin each case. The results are reported in Table 3. [9] C. DEVERELL,D.J.FROST and R.E.RIcHARDs,MoZ. Phys. 9,565(1965). [lo] M.ST.J. ARNOLD and K.J. PACKER, Mol. Phys. 10,141(1966). [ll] J.T.ARNoLD andM.E.P~cxm~, J.C?wm. Phy.v.19, 1608 (1951). [12] E.B~LocK,D.G.TucK~~~E.J.WOODHOUSE, J.Chem.Phya.38,2318 (1963). [13] J. M.ANDERSON,J.D.BALDESC~ELER,D.C.DIT~ER~~~D.PHILLIPS, J.Chem.Phya. 38, 1260 (1963). 1141 D. G. GASSW and P. C. HECKERT,J. Org. Chem. 30,3859 (1965).
1237
Nuclear magnetic resonance studies of tetraalkyl compounds Table 1. Effect
of gegen ion on internal chemical shift of NEta
Aqueous soln. Compound
(ppm f062)
Chloroform soln. (ppm f 0.02)
W&),SC,
(ppm
* *
2.00 2.01 2.01 2.01 2.02 2.01 2.02 2.01 2.01
NEt,F NEt*c1 NEt,Br NEt,I NEt,OH NEt,NO, NEt,c10, NEt,Ac
Chloroform + 1y. methanol * 0.02) 2.09 2.08 2.12 2.07 2.10 2.06 2.04 2.10 2.10
2.13 * 2.13 2.06 * 2.10 *
* Insoluble.
Table 2. Effect of concentration of proton magnetic resonance shifts of NEt*Br in aqueous solution Concentration (gmole/litre)
@pm -
7.27 0.93 0.43 0.18
Table 3. NMR Compound
J(N’4-C-C-H)~c/s
* t $ $ A
CH, (ppm +O*Ol)
1.25 1.25 1.24 1.24
- 3.29 -3.26 - 3,24 -3.26
c/s c/s
A(CHs-CH,) (ppm &@Ol) 2.04 2.01 2.00 2.02
data for quaternary ammonium
NEt,1
*CHs (Ha0 soln.) *CHa (l&O soln.) *CH,$ (H,O soln.) A(H,O soln.) ?CH, (CHCl, soln.) -tCH, (CHCl, soln.) tCH,$ (CHCl, soln.) A(CHC1, soln.)
J(Nr4-C-H) J(H-H)
CH, +041)
- 1.29 -3.25 2.00 - 1.20 -3.41 2.13 1.80 * 0.05 7.3
NEtsMe -1.22 -3.30 -2.91 2.08 - 1.44 -3.65 - 3.24 2.21
I 2.10 f 0.05 <0.2 7.5
NEtaMes
salts
NEtMe,
-1.30 -3.33 -3.00 2.03
-1.34 -3.38 -3.07 2.04
NMe,T - 3.20 -
Insoluble
2.10 f 0.05 <0.2 7.7
2.20 + 0.05 to.3 7.8
0.55 -
Shifts in ppm w.r.t. sodium 3-(trimethylsilyl)-l-propanesulphonate. Shifts in ppm w.r.t. tetramethylsilane. CHs in methyl group. These results are in agreement with GASSMAN and HECKERT [la]. Internal chemical shift difference.
E. W.
1238
RANDALL and D. SHAW
Proton-nitrogen decoupling experiments on these ions and on the mixed phosphonium ions were carried out using the apparatus already described [15]. Substituent effects on the shifts of the central atom have been evaluated as well as for arsenic [ 161. These results and ion or solvent effects on the central shifts will be published elsewhere [ 171. 2.3 Tetramethyl and ethylarsonium iodide These compounds were prepared as before [l]. The proton spectra have been studied as a function of concentration and solvent. The results are presented in Tables 4-7. EH,
C”3
J H-H
J.H-H
NKH, CH,$H:
I
I
I -3 23
-2
III J Ni4H
J N!H
91
-I 22 ppm
Fig. 1. 60 MC/~ PMR of triethylmethylammonium
iodide in aqueous solution.
The effect of temperature was studied for the cases of aqueous solutions of tetraethyl-arsonium and tetramethyl-arsonium : a slight broadening of lines occurred on heating (Fig. 2). [15] D. G. GILLIES and E. W. RANDALL, J. Sci. In&r. 43, 466 (1966). [16] D. G. GILLIES and D. SHAW, Ph.D. Theses, University of London, [17] D. G. GILLIES, E. W. RANDALL and D. SHAW, to be published.
1965.
Nuclear magnetic resonance studies of tetraalkyl compounds Table 4. Effects of concentration
Concentration (gmole/litre)
of the proton shifts of AsEt, solution
CHs (ppm f0.01)
in chloroform
A(CH,-CH,)
‘332
@pm
(ppm kO@l)
3041)
0.84
- 1.42
-2.74
1.32
061
-1.41
-2.75
1.34
0.44
-1.42
-2.75
1.33
0.28
-1.41
-2.75
1.34
0.24
- 1.42
-2.75
1.33
0.18
- 1.42
-275
1.33
0.16
-1.41
-2.75
1.34
0.06
-1.40
-2.75
1.35
0.03
-1.14
-275
1.34
0.01
-1.40
-2.75
1.34
Table 5. Effect of concentration
on NMR
1239
spectrum of aqueous AsEt,
Concentration (gmole/litre)
CHs shift (ppm f0.01)
3.78
-1.30
18.0
-2.45
3.5
1.15
2.10
-1.29
20.0
-2.40
4.0
1.11
1.26
-1.30
21.0
-2.40
6.0
1.10
0.68
-1.29
21.0
-2.38
6.0
1.09
0.33
-1.29
21.0
-2.38
6.0
I.09
0.05
-1.31
22.0
-2.39
6.0
1.08
Line width (c/s fO.6)
CH2
shift (ppm &O*Ol)
Line width (c/sf0.5)
A(CH,-CH,) (ppm %@Ol)
Table 6. Effect of solvent dielectric constant on the NMR spectrum of AsEt, Dielectric constant 20”
Solvent
A(CHs-CH,) Line width
ppm
Water
80.4
1.08 f
0.01
large
Formic acid
58.0
I.10 f
0.05
large
40.2
1.04 f
0.05
intermediate
Nitrobenzene
34.8
1.35 f
0.02
small
Liq. sulphur dioxide
14.0
1.05 f
0.06
large
Arsenic trichloride
139
1.05 f
0.06
large
Triiluoroacetic
acid*
Acetic acid
6.2
1.27 f
0.05
SmSll
Chloroform
4.0
I.34
0.01
small
* Solution decomposes
f
over a period of half an hour, iodine being liberated.
E. W. RANDAU and D. SHAW
1240
Table 7. Effect of concentrationon NMR spectrum of AsMe
in aqueous solution
Concentration (gmole/litre)
Chemicsl shift (ppm i 0.01)
Line width (c/s f 0.5)
2.40 1.19 0.48 0.20 0.11 0.40
1.95 1.92 1.89 1.89 1.87 1.87
9.0 8.0 11.0 11.0 11.0 12.0
-_-
--
------
Fig. 2. 60 MC/SPMR of satureted aqueous solution of tetraethylarsoniumiodide: (a) at 90°C; (b) at 35°C. 3. DISCUSSION
3.1 Substituent effects in alkylammonium
ions
It has been shown previously [l-3, 12, 141 that there is no solvent effect on the line width parameter x for NEt,+ and NMe,+ whereas pronounced effects occur for The general conclusion was that the larger ions AsEt, + , AsMe,+ and SbEt,+ [l-3]. the quadrupole relaxation rates for As and Sb are particularly susceptible to environmental factors (e.g. substituent and solvent) compared with N because of the larger quadrupole moments and Sternheimer anti-shielding parameters. The observation that substitution of a methyl group for an ethyl group in the tetraethyl X ions affects the line width when X = As or Sb, but not when X is N is consistent with this idea. Other authors have recently studied a wide range of substituents on nitrogen. Thus LEHN and FRANCK-NEUMANN [18] report that resolvable H1-N14 coupling in [18] J. M. LEHN and M. FRANCK-NEUMANN, J. Chem. Phys. 43, 1421 (1965).
1241
Nuclear magnetic resonance studies of tetraalkyl compounds
ethyl groups in the bromide salts of Et,N+(CH,),N+Et, occurs if n > 6. Progressively narrower (decoupled) lines are obtained as n is decreased until n = 0, when, as we GASSMAN and HECKERT [la] similarly have seen, coupling is once more resolved. report a number of acyclic quaternary ammonium iodides as well as the mixed methyl, ethyl, ammonium ions considered here. coupling constant increases as n increases in The value of the J(N~4-C-C-H) the series NMe,Et,_,. This behaviour in a sequence of this type has been observed before and has been rationalized assuming that the Fermi contact term dominates the coupling mechanism [2, and references therein]. Table 8. Comparison of the effect of solvent on CH, and CH, shifts CH, (ppm) CHCl, soln.
Difference
-3.25
-3.41
+0.16
-3.30
-3.47
10.17
+0*04
-2.21
-2.47
+0.26
-1.31
+0.13
-2.13
-2.51
+o-28
-1.21
-1.31
+0.10
-2.18
-2.51
-to.33
AsEt
-1.29
-1.41
+0.12
-2.38
-2.75
-to.37
AsEtsMeI
-1.25
-1.40
1-0.15
-2.37
-2.78
+o-41
SbEt,I
-1.30
-1.32
t-o.02
-2.27
-2.48
+0.61
H,O soln.
CH, (ppm) CHCI, soln.
Difference
H,O soln.
NEt4Br
-1.25
-1.28
+0.03
NEtsMe
- 1.22
-1.25
+0.03
PEt,I
-1.21
-1.25
PEtsMe
-1.18
PEtzMezI
Compound
3.2
Solution effects for XEt,+
ions (X = N;
As)
We have previously pointed out in discussing solution chemistry the problem of defining precisely the terms “ion-ion effect” and “ion-solvent effect” [2, 31. It is important to recognize that the two forms of interaction are not necessarily independent. Thus ion pair formation could modify the ion solvent contribution to the observed spectral parameter even if the ion-ion contribution is small. We would refer to this case as an ion-ion effect. 3.2.1 Proton chemical shifts. The results presented above in Table 1 for NEt;+ in chloroform solutions show that A is dependent upon gegen ion whereas no such dependence was found for aqueous solutions. This behaviour is the same as noted previously for AsEt,+ and for l-ethylpyridinium ions [2,3,4], and may be rationalized in the same way in terms of ion pair formation in solvents of low dielectric constant. For each of the elements in Group V, A is larger in chloroform than in aqueous solution [2, 31. Moreover the variation is due mainly to a change in the methylene rather than the methyl shifts (see Table 8). This sensitivity of the methylene resonances extends to cases where the anion is changed. Similar effects were reported in the case of 1-ethylpyridinium ions [4] and tetrabutylammonium ions [19]. The sitiplest rationalization of these results is that the ion-pair formation polarizes the ionic centre (X) and the X-C bond rather than C-C or C-H bonds. For the case of AsEt,& A has been found to be the same for water, arsenic trichloride, liquid sulphur dioxide [2, 31 and now formic acid. These solvents are [19] R. C. BUCKSON
and G. S. SMITE,
J. Chem. Phy8. 68, 1857 (1964).
1242
E. W. RANDALLand D. SHAW
characterized by high dielectric constants in which ion pair formation is not anticipated. The ion-solvent effects on the proton shift must thus be either small or comparable in each case. The solvents chloroform and acetic acid on the other hand have low dielectric constants, promote ion-pair formation and give rise to different values for A, presumably due to ion-ion effects. Nitro-benzene (and possibly trifluoroacetic acid) gives a clear exception to the above simple views in that it has a high dielectric constant but gives a A value for AsEt,+ which is indicative of ion-pair formation. Two explanations may be offered: (i) nitro-benzene gives a large solvent effect; (ii) a more complicated complex The second possibility was expressed involving solvent and ion-pair is produced. by HYNE in consideration of electrochemical studies of ammonium salts in nitrobenzene solution, he postulated that nitro-benzene molecules act as ‘templates’ for ion pairs and so promote ion pair association [20]. The use of high concentrations to bring about ion pairing even in aqueous solutions produces the large values of A characteristic of the ion pairing situation for the cases of NEt,I, AsEt, (Tables 2, 5). 3.2.2 Line widths for AsEt,I. The broadening of proton lines for tetraethyl-arsonium ions in aqueous solution has been attributed to spin-spin interactions between the protons and the arsenic nucleus. The alternative of exchange broadening was eliminated by proton-arsenic double resonance experiments [3, 161. Similar results have been obtained for the tetramethyl-arsonium ion. The anticipated effects on line widths of variation of solvent and of concentration in aqueous solution using the ion-pairing hypothesis are found (Tables 5 and 6). Thus large concentrations in aqueous solution or the use of solvents of low dielectric constant produce narrow lines due to rapid quadrupole relaxation of the arsenic. The anomalous cases are nitro-benzene which although it has a high dielectric constant gives narrow lines, and possibly trifluoroacetic acid which gives lines of intermediate width. These solvents conceivably, as discussed above, produce a large ion-solvent effect or promote the formation of solvent-ion pair aggregates. me grateful to the U.S. Department of the Army through its European Research Office for support of this research. Aclwwwledgemm&--We
[20] J. B. HYNE, J. Am. Chem. Sot. 85, 304 (1963).
Nuclear magnetic resonance studies of tetraalkyl compounds of group III and group V elements-III E. W. RANDALL
and D. S~llw
Queen Mary College, Mile End Road, London E.l (Received 18 July 1900) Abstract-The proton NMR spectra of the ions NMe,EtA, have been studied and the coupling constants and shifts measured. The effects of the counter-ion, concentration and solvent have been investigated for the tetraethyl-ammonium ion. Solvent and concentration studies for tetraethyl-arsoniumiodide are also discussed.
1. INTRODTJCTI~N have previously studied [l-3] the proton magnetic resonance spectra of tetraalkyl derivatives of elements in Group V of the periodic table employing a variety of solvents and, in some cases, of gegen ions. Except for phosphorus, and one or two examples for As and Sb, the discussion was limited to the highly symmetrical cases of MEt,+ and MMe,+ in order that broadening of the spectra by electric quadrupole effects of M should be minimised. We have now investigated the mixed systems NMe,Et$_ in two solvents. Previous studies of the effect of gegen ion and solvent upon the internal chemical shift, A, were restricted to the cases of AsEt,+ [l-3] and ethylpyridinium derivatives [3, 41. Similar studies have now been made for NEt,+. The interactions governing the rates of quadrupolar relaxation in ionic solutions have recently been a subject of some interest, not to say controversy. In Parts I and II of this series we have adopted the view that the formation of ion pairs occurred in some of our systems and was an important factor not only for line widths but also for chemical shifts [l,21. It was, however, recognised [2] that on the microscopic level ion-ion and ion-solvent effects are not independent and that no unique model could be presented. The theoretical model initially used by VALIEV [5] was based on the assumption that solvent effects were the dominant interactions governing line widths. RICHARDS and YORKE showed that most of the cases they studied did fit the model, and the few exceptions were attributed to ion-ion contribution to the line widths [6]. HERTZ had developed a more general theory, which allowed contributions to the line width parameter from ion-ion interactions [7] and a similar approach was adopted by VALIEV [8]. The subsequent experimental studies of WE
[l] A. G. MASSEY,E. W. RANDALL and D. SHAW, Spectrochim. acta 20,379 (1964). [2] A. G. USEY, E. W. RANDALL and D. Smw, Spectrochim. Acta 21,263 (1965). [3]R. J. CHUNK,A. G. MASSEY, E. W. RANDALL and D. S-W, Nuclear Magnetic. Resonance in Chemistry (Edited by B. PESCE), p. 189,Academic Press (1906). [4] R. J. CWCK and E. W. RANDALL, Spectrochim. Acta 22, 221 (1966). [5] K. A. VALIEV, Soviet. Phys. JETP. 883 (1960). [S] R. E. RICHARDSand B. A. YORXE, Mol. Phya. 6, 287 (1963). [7] H. G. HERTZ, 2. Elektrochem. 66, 20 (1960). [S]K. A. VALIEV, Zhur. Stmkt. Khim. 5, 477 (1964). 1235
1236
E. W. RANDALLand D. SHAW
DEVERELL, FROST and RICHARDS on bromine and iodine resonances in solutions of salts were used to test the generalized model [9]. It was concluded that line widths were dominated by ion-solvent interactions for strongly hydrated cations (alkaline earths, Li+ and Na+) and by ion-ion effects for weakly solvated cations [9]. Other authors have discussed the importance of ion-ion interactions for the hexafluoroarsenate ion [lo]. We report here concentration studies for tetraalkyl-arsonium iodide in water and chloroform, together with an extended range of solvents including nitro-benzene. 2. EXPERIMENTALAND RESULTS The NMR spectra were obtained at 60 MC/S using a Varian Associates A-60 spectrometer at 35°C. They were calibrated by the standard audio side-band technique [ll].Tetramethylsilane was used as internal standard for non-aqueous solutions. In aqueous solutions sodium 3-(trimethylsilyl)-l-propanesulphonate was employed. Shifts are given in parts per million (ppm) downfield from the standard. 2.1 Tetraethylammonium
ion
Salts of the following anions were prepared by neutralization of solutions of Analar tetraethylammonium hydroxide with Analar acids: F-, Cl-, Br-, I-, NO,-, SOd2-, ClO,- and MeCOO-. In each of these cases the spectrum as previously reported [l,12-141 consists of a normal ethyl spectrum with each of the lines of the triplet split into three lines of approximately equal height due to coupling with the central Ni4 nucleus of spin 1. The coupling constants are independent of anion and solvent [12]. The internal chemical shift, A, between the methylene and methyl protons has now been found to depend on solvent, and in the case of chloroform solutions on anion. Only the bromide, hydroxide, acetate and nitrate were soluble in pure chloroform to the extent of 10 per cent. If, however, about 1 per cent methanol was added each of the other salts dissolved to give 10 per cent solutions except the fluoride and perchlorate which gave only 2 per cent solutions. The spectrum of the bromide was independent of concentration in the ranges studied. The results are summarized in Tables 1 and 2.
2.2 Tetraulkylammonium
iodides, NMe,Et+_,I-
(4 > n > 1)
These quaternary salts were prepared by reacting stoichiometric amounts of ethereal solutions of the methyl iodide with the corresponding trialkylamine, NMe,_,Et,_,. The spectra are typified by the case of triethyl-methyl-iodide (n = 1) which is shown in Fig. 1. The long range N14-H coupling is observed in each case and is approximately the same as the value in tetra-ethylammonium ion. The methyl lines show broadening due to unresolved coupling with N14 which appears to be less than O-2c/sin each case. The results are reported in Table 3. [9] C. DEVERELL,D.J.FROST and R.E.RIcHARDs,MoZ. Phys. 9,565(1965). [lo] M.ST.J. ARNOLD and K.J. PACKER, Mol. Phys. 10,141(1966). [ll] J.T.ARNoLD andM.E.P~cxm~, J.C?wm. Phy.v.19, 1608 (1951). [12] E.B~LocK,D.G.TucK~~~E.J.WOODHOUSE, J.Chem.Phya.38,2318 (1963). [13] J. M.ANDERSON,J.D.BALDESC~ELER,D.C.DIT~ER~~~D.PHILLIPS, J.Chem.Phya. 38, 1260 (1963). 1141 D. G. GASSW and P. C. HECKERT,J. Org. Chem. 30,3859 (1965).
1237
Nuclear magnetic resonance studies of tetraalkyl compounds Table 1. Effect
of gegen ion on internal chemical shift of NEta
Aqueous soln. Compound
(ppm f062)
Chloroform soln. (ppm f 0.02)
W&),SC,
(ppm
* *
2.00 2.01 2.01 2.01 2.02 2.01 2.02 2.01 2.01
NEt,F NEt*c1 NEt,Br NEt,I NEt,OH NEt,NO, NEt,c10, NEt,Ac
Chloroform + 1y. methanol * 0.02) 2.09 2.08 2.12 2.07 2.10 2.06 2.04 2.10 2.10
2.13 * 2.13 2.06 * 2.10 *
* Insoluble.
Table 2. Effect of concentration of proton magnetic resonance shifts of NEt*Br in aqueous solution Concentration (gmole/litre)
@pm -
7.27 0.93 0.43 0.18
Table 3. NMR Compound
J(N’4-C-C-H)~c/s
* t $ $ A
CH, (ppm +O*Ol)
1.25 1.25 1.24 1.24
- 3.29 -3.26 - 3,24 -3.26
c/s c/s
A(CHs-CH,) (ppm &@Ol) 2.04 2.01 2.00 2.02
data for quaternary ammonium
NEt,1
*CHs (Ha0 soln.) *CHa (l&O soln.) *CH,$ (H,O soln.) A(H,O soln.) ?CH, (CHCl, soln.) -tCH, (CHCl, soln.) tCH,$ (CHCl, soln.) A(CHC1, soln.)
J(Nr4-C-H) J(H-H)
CH, +041)
- 1.29 -3.25 2.00 - 1.20 -3.41 2.13 1.80 * 0.05 7.3
NEtsMe -1.22 -3.30 -2.91 2.08 - 1.44 -3.65 - 3.24 2.21
I 2.10 f 0.05 <0.2 7.5
NEtaMes
salts
NEtMe,
-1.30 -3.33 -3.00 2.03
-1.34 -3.38 -3.07 2.04
NMe,T - 3.20 -
Insoluble
2.10 f 0.05 <0.2 7.7
2.20 + 0.05 to.3 7.8
0.55 -
Shifts in ppm w.r.t. sodium 3-(trimethylsilyl)-l-propanesulphonate. Shifts in ppm w.r.t. tetramethylsilane. CHs in methyl group. These results are in agreement with GASSMAN and HECKERT [la]. Internal chemical shift difference.
E. W.
1238
RANDALL and D. SHAW
Proton-nitrogen decoupling experiments on these ions and on the mixed phosphonium ions were carried out using the apparatus already described [15]. Substituent effects on the shifts of the central atom have been evaluated as well as for arsenic [ 161. These results and ion or solvent effects on the central shifts will be published elsewhere [ 171. 2.3 Tetramethyl and ethylarsonium iodide These compounds were prepared as before [l]. The proton spectra have been studied as a function of concentration and solvent. The results are presented in Tables 4-7. EH,
C”3
J H-H
J.H-H
NKH, CH,$H:
I
I
I -3 23
-2
III J Ni4H
J N!H
91
-I 22 ppm
Fig. 1. 60 MC/~ PMR of triethylmethylammonium
iodide in aqueous solution.
The effect of temperature was studied for the cases of aqueous solutions of tetraethyl-arsonium and tetramethyl-arsonium : a slight broadening of lines occurred on heating (Fig. 2). [15] D. G. GILLIES and E. W. RANDALL, J. Sci. In&r. 43, 466 (1966). [16] D. G. GILLIES and D. SHAW, Ph.D. Theses, University of London, [17] D. G. GILLIES, E. W. RANDALL and D. SHAW, to be published.
1965.
Nuclear magnetic resonance studies of tetraalkyl compounds Table 4. Effects of concentration
Concentration (gmole/litre)
of the proton shifts of AsEt, solution
CHs (ppm f0.01)
in chloroform
A(CH,-CH,)
‘332
@pm
(ppm kO@l)
3041)
0.84
- 1.42
-2.74
1.32
061
-1.41
-2.75
1.34
0.44
-1.42
-2.75
1.33
0.28
-1.41
-2.75
1.34
0.24
- 1.42
-2.75
1.33
0.18
- 1.42
-275
1.33
0.16
-1.41
-2.75
1.34
0.06
-1.40
-2.75
1.35
0.03
-1.14
-275
1.34
0.01
-1.40
-2.75
1.34
Table 5. Effect of concentration
on NMR
1239
spectrum of aqueous AsEt,
Concentration (gmole/litre)
CHs shift (ppm f0.01)
3.78
-1.30
18.0
-2.45
3.5
1.15
2.10
-1.29
20.0
-2.40
4.0
1.11
1.26
-1.30
21.0
-2.40
6.0
1.10
0.68
-1.29
21.0
-2.38
6.0
1.09
0.33
-1.29
21.0
-2.38
6.0
I.09
0.05
-1.31
22.0
-2.39
6.0
1.08
Line width (c/s fO.6)
CH2
shift (ppm &O*Ol)
Line width (c/sf0.5)
A(CH,-CH,) (ppm %@Ol)
Table 6. Effect of solvent dielectric constant on the NMR spectrum of AsEt, Dielectric constant 20”
Solvent
A(CHs-CH,) Line width
ppm
Water
80.4
1.08 f
0.01
large
Formic acid
58.0
I.10 f
0.05
large
40.2
1.04 f
0.05
intermediate
Nitrobenzene
34.8
1.35 f
0.02
small
Liq. sulphur dioxide
14.0
1.05 f
0.06
large
Arsenic trichloride
139
1.05 f
0.06
large
Triiluoroacetic
acid*
Acetic acid
6.2
1.27 f
0.05
SmSll
Chloroform
4.0
I.34
0.01
small
* Solution decomposes
f
over a period of half an hour, iodine being liberated.
E. W. RANDAU and D. SHAW
1240
Table 7. Effect of concentrationon NMR spectrum of AsMe
in aqueous solution
Concentration (gmole/litre)
Chemicsl shift (ppm i 0.01)
Line width (c/s f 0.5)
2.40 1.19 0.48 0.20 0.11 0.40
1.95 1.92 1.89 1.89 1.87 1.87
9.0 8.0 11.0 11.0 11.0 12.0
-_-
--
------
Fig. 2. 60 MC/SPMR of satureted aqueous solution of tetraethylarsoniumiodide: (a) at 90°C; (b) at 35°C. 3. DISCUSSION
3.1 Substituent effects in alkylammonium
ions
It has been shown previously [l-3, 12, 141 that there is no solvent effect on the line width parameter x for NEt,+ and NMe,+ whereas pronounced effects occur for The general conclusion was that the larger ions AsEt, + , AsMe,+ and SbEt,+ [l-3]. the quadrupole relaxation rates for As and Sb are particularly susceptible to environmental factors (e.g. substituent and solvent) compared with N because of the larger quadrupole moments and Sternheimer anti-shielding parameters. The observation that substitution of a methyl group for an ethyl group in the tetraethyl X ions affects the line width when X = As or Sb, but not when X is N is consistent with this idea. Other authors have recently studied a wide range of substituents on nitrogen. Thus LEHN and FRANCK-NEUMANN [18] report that resolvable H1-N14 coupling in [18] J. M. LEHN and M. FRANCK-NEUMANN, J. Chem. Phys. 43, 1421 (1965).
1241
Nuclear magnetic resonance studies of tetraalkyl compounds
ethyl groups in the bromide salts of Et,N+(CH,),N+Et, occurs if n > 6. Progressively narrower (decoupled) lines are obtained as n is decreased until n = 0, when, as we GASSMAN and HECKERT [la] similarly have seen, coupling is once more resolved. report a number of acyclic quaternary ammonium iodides as well as the mixed methyl, ethyl, ammonium ions considered here. coupling constant increases as n increases in The value of the J(N~4-C-C-H) the series NMe,Et,_,. This behaviour in a sequence of this type has been observed before and has been rationalized assuming that the Fermi contact term dominates the coupling mechanism [2, and references therein]. Table 8. Comparison of the effect of solvent on CH, and CH, shifts CH, (ppm) CHCl, soln.
Difference
-3.25
-3.41
+0.16
-3.30
-3.47
10.17
+0*04
-2.21
-2.47
+0.26
-1.31
+0.13
-2.13
-2.51
+o-28
-1.21
-1.31
+0.10
-2.18
-2.51
-to.33
AsEt
-1.29
-1.41
+0.12
-2.38
-2.75
-to.37
AsEtsMeI
-1.25
-1.40
1-0.15
-2.37
-2.78
+o-41
SbEt,I
-1.30
-1.32
t-o.02
-2.27
-2.48
+0.61
H,O soln.
CH, (ppm) CHCI, soln.
Difference
H,O soln.
NEt4Br
-1.25
-1.28
+0.03
NEtsMe
- 1.22
-1.25
+0.03
PEt,I
-1.21
-1.25
PEtsMe
-1.18
PEtzMezI
Compound
3.2
Solution effects for XEt,+
ions (X = N;
As)
We have previously pointed out in discussing solution chemistry the problem of defining precisely the terms “ion-ion effect” and “ion-solvent effect” [2, 31. It is important to recognize that the two forms of interaction are not necessarily independent. Thus ion pair formation could modify the ion solvent contribution to the observed spectral parameter even if the ion-ion contribution is small. We would refer to this case as an ion-ion effect. 3.2.1 Proton chemical shifts. The results presented above in Table 1 for NEt;+ in chloroform solutions show that A is dependent upon gegen ion whereas no such dependence was found for aqueous solutions. This behaviour is the same as noted previously for AsEt,+ and for l-ethylpyridinium ions [2,3,4], and may be rationalized in the same way in terms of ion pair formation in solvents of low dielectric constant. For each of the elements in Group V, A is larger in chloroform than in aqueous solution [2, 31. Moreover the variation is due mainly to a change in the methylene rather than the methyl shifts (see Table 8). This sensitivity of the methylene resonances extends to cases where the anion is changed. Similar effects were reported in the case of 1-ethylpyridinium ions [4] and tetrabutylammonium ions [19]. The sitiplest rationalization of these results is that the ion-pair formation polarizes the ionic centre (X) and the X-C bond rather than C-C or C-H bonds. For the case of AsEt,& A has been found to be the same for water, arsenic trichloride, liquid sulphur dioxide [2, 31 and now formic acid. These solvents are [19] R. C. BUCKSON
and G. S. SMITE,
J. Chem. Phy8. 68, 1857 (1964).
1242
E. W. RANDALLand D. SHAW
characterized by high dielectric constants in which ion pair formation is not anticipated. The ion-solvent effects on the proton shift must thus be either small or comparable in each case. The solvents chloroform and acetic acid on the other hand have low dielectric constants, promote ion-pair formation and give rise to different values for A, presumably due to ion-ion effects. Nitro-benzene (and possibly trifluoroacetic acid) gives a clear exception to the above simple views in that it has a high dielectric constant but gives a A value for AsEt,+ which is indicative of ion-pair formation. Two explanations may be offered: (i) nitro-benzene gives a large solvent effect; (ii) a more complicated complex The second possibility was expressed involving solvent and ion-pair is produced. by HYNE in consideration of electrochemical studies of ammonium salts in nitrobenzene solution, he postulated that nitro-benzene molecules act as ‘templates’ for ion pairs and so promote ion pair association [20]. The use of high concentrations to bring about ion pairing even in aqueous solutions produces the large values of A characteristic of the ion pairing situation for the cases of NEt,I, AsEt, (Tables 2, 5). 3.2.2 Line widths for AsEt,I. The broadening of proton lines for tetraethyl-arsonium ions in aqueous solution has been attributed to spin-spin interactions between the protons and the arsenic nucleus. The alternative of exchange broadening was eliminated by proton-arsenic double resonance experiments [3, 161. Similar results have been obtained for the tetramethyl-arsonium ion. The anticipated effects on line widths of variation of solvent and of concentration in aqueous solution using the ion-pairing hypothesis are found (Tables 5 and 6). Thus large concentrations in aqueous solution or the use of solvents of low dielectric constant produce narrow lines due to rapid quadrupole relaxation of the arsenic. The anomalous cases are nitro-benzene which although it has a high dielectric constant gives narrow lines, and possibly trifluoroacetic acid which gives lines of intermediate width. These solvents conceivably, as discussed above, produce a large ion-solvent effect or promote the formation of solvent-ion pair aggregates. me grateful to the U.S. Department of the Army through its European Research Office for support of this research. Aclwwwledgemm&--We
[20] J. B. HYNE, J. Am. Chem. Sot. 85, 304 (1963).