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Nuclear magnetic resonance studies of iodine-amine complexes
AdvancesinMolecularRelaxationandIntemctionRocesses, 15(1979)135-150 OElsevier Scientific PublisbingCompaay,Amsterdam -Printed inTheNetherlands
135
NUCLEAR MAGNETIC RESONANCE STUDIES OF IODINE-AMINE COMPLEXES
M. CIUREANU and M. CONTINEANU Department of Physical Chemistry, National Institute of Chemistry, Bucharest, Bd. Republicii 13 (Romania) (Received 14 June 1978; in final form 23 October 1978) ABSTRACT The charge transfer equilibria in aromatic amine-iodine systems are investigated by chemical shift measurements. The formation constants K, determined by a new method, are compared with the results obtained from optical data. The pure complex shifts A0 are also determined and compared with those obtained by two classical n.m.r. methods. The A0 values of the amino protons are found to vary linearly with the Bammett o constant of the ring substituent in the donor molecule. Various contributions to the shift of the pure complex are discussed in relation to the structure of the complex and the determining role of the intermolecular charge transfer is stressed. The change of electron density on the nitrogen atom is used to evaluate the change in diamagnetic shielding as a result of intermolecular charge transfer. The A0 values for the aromatic protons are found to be smaller than for the amino protons.and are assigned mainly to the delocalization of the nitrogen lone pair electrons on the aromatic ring.
INTRODUCTION Charge transfer complexes of iodine with aromatic amines have been extensively investigated by visible, ultraviolet [l-3] and infrared spectroscopy [4-61. There are very few reports on their n.m.r. spectra, though chemical shift data are expected to provide valuable information concerning the structure of the molecular complexes. Fratiello [7] measured the chemical shifts of aniline containing a high concentration (1 M) of iodine and noted a large downfield shift of the amino proton signal and some small shifts of the ring proton signals. He concluded that the donor-acceptor interaction was localized at the nitrogen atom, a conclusion drawn earlier by Tsubomura [2] and Mukerjee [3] from optical data.
136
No attempts were made until now to determine the equilibrium constants of these complexes by n.m.r. spectrometry. The main reason for this lack of information is the relatively low solubility of iodine in most suitable organic solvents, which prevents the use of classical procedures [8-131. The usual treatments of charge transfer equilibria in solution would require measurements of the shifts of the donor nuclei in the presence of a large excess of the acceptor: [D] << [A]
0.
0
In our systems it was not possible to make the iodine concen-
tratia large compared with the donor, so that a different treatment became necessary. Fratiello's experiments have pointed out that for the amino proton absorption it is possible to obtain large values of the experimental association shifts even if only a small fraction of the amine is complexed, that is, even if
[Al o << iDlo.
Under these circumstances, it was possible to develop a new pro-
cedure of determining equilibrium constants and association shifts; this procedure was used to investigate the association equilibria in aromatic amine-iodine systems.
GENERAL METHOD This method is based upon the determination of the amino proton chemical shifts for several solutions containing a fixed concentrationof iodine (a) and various concentrationsof the aromatic amine (c); the latter exceeds the iodine concentration by a factor ranging from 5
to
100.
If the donor is not self-associated,the observed chemical shift of the amino protons will be given by the general equation for fast equilibria in solution:
6
= 5 6c + (1
where 6 is the observed shift of the amino protons in complexing media; 61
0) and
6c are the shifts of the NH2 protons in uncomplexed form and in the pure complex, respectively; y is the complex concentration in molar units. Defining A = 6 - 61 and A0 = hc - 61, eq. (1) becomes:
A
=
:A0
(2)
For an ideal solution the equilibrium constant is given by:
K
=
Y Cc - Y) (a - Y)
(3)
137 If one assumes that y and a are negligibly small with respect to c (y << c, a
d
1
1
iy+zy
=
(4)
If the acceptor concentration cannot be neglected with respect to c, one gets instead of (4): a 1 x=iz-o
+ +
o
(c + a)
(4')
A plot of a/A against (c + a) should be linear. The gradient of the line yields l/Ao, while the intercept yields l/KAo. Thus, both K and A0 can be evaluated. Eq. (4') has to be modified if the aromatic amine is self-associated. Infrared measurements made by Whetsel and Lady [14] have shown that the aromatic amines are partially dimerized in solution. These observations have been substantiated by n.m.r. measurements of the amino proton shifts as a function of concentration 114-181. If the self-associationof the donor is taken into account, the observed NH2 chemical shift in complexing media will be the weighted average of 61, 6c and 82:
6
=
(1 _ C+)&l
d +;62+
f6
c
(5)
where 62 is the chemical shift of the amino proton in pure dimer and 61 is the NH2 chemical shift at infinite dilution. The chemical shift of the NH2 protons in a solution containing the same amine concentration with no iodine added is:
6’
=
(1 - $)6l
d' 62 + 7
(6)
In (5) and (6) d and d' are the dimer concentration corresponding to the same donor concentration,with and without iodine added. They differ by a small quantity, hereafter denoted by a:
d
=
d'-a
(7)
It may easily be shown that CLis given by:
a
=*
(‘3)
138
From eq.
(8), the following
equation,
which
is a modified
form of eq.
(l), is
obtained:
(9) If y <<
c and d' < c, a is negligibly
in (9) can be omitted,
small,so
that d Q d' and the last term
so that:
and
(11)
Here A is the observed #a reference constant
K
=
where
solution
K will
be given
resonance
relative
the same donor concentration
c.
to that of
The equilibrium
by:
Y (cx - ~)(a - Y)
(12)
x is the fraction
associated:
x
shift of the amino proton
containing
of the initial donor
concentration
which
is not self-
l
=- c - 2d'
(13)
C
(10) - (131, one gets a modified
From eqs. ax d=
1 q+Ao
L
6' = 6 1, then eq.
investigate
(14) and
to provide
the above method
(14)
hydrogen
to study phenol-amide
of amine
bonded
from a plot of y
(4') become
evidence
we have applied
The first method
measurements
(4')
(cx + a)
Thus, K and A0 can be evaluated
In order
form of eq.
against
(cx + a).
When x = 1,
identical.
for the correctness two classical
of the values
procedures,
obtained
previously
by
used to
systems.
was similar
to the technique
interactions;
for two series of solutions,
(cl and ~2) and various
used by Takahashy
this procedure
involves
each containing
concentrations
and Li [19]
chemical
shift
a fixed concentration
of the acceptor.
A plot of 6 vs.
139
a yields two straight lines with different slopes, from which
A0 and K can be
obtained:
6 =
q+
6
CT;+
Kxl UC - 61) 1 + Kclxl Kx2Qc
=
a
(15)
- 61)
1 + Kc2x2
a
(16)
The second procedure was the method proposed by Creswell and Allred [20]. The ho values were obtained according to eq. (ll), from a plot of A against y/c. Unlike the original method, the y/c ratios were calculated from the equilibrium constants K, determined from spectrophotometricmeasurements.
EXPERIMENTAL Materials Carbon tetrachloride,used as a solvent in our experiments was dried over molecular sieves and distilled twice over K2CO3. Iodine was sublimed twice and stored in a dark bottle. Solid anilines were purified by recrystallization liquid anilines were distilled in vacuum over fresh potassium hydroxide. Sample solutions were prepared by adding a concentrated iodine solution in carbon tetrachloride to a determined quantity of amine.
N.M.R. Measurements The n.m.r. spectra were recorded with a Tesla BS 487C spectrometer at
a0 Mc.p.s. Chemical shift measurements were made relative to external tetramethylsilane. The bulk susceptibility corrections were made in the usual manner. In a series of measurements, the temperature was kept constant within *lo. The reported shifts at 25' are accurate to 0.5 c.p.s. The spectra of the samples were recorded immediately after preparation, in order to avoid the effect of ion-pair formation or other chemical changes of the solutions upon the measurements. The n.m.r. spectra of the complex solutions were compared with those of the reference samples. The chemical shift of the NH2 protons in the uncomplexed amines at infinite dilution in CC14 were in a very good agreement with those reported by other authors [15-la]. The reference samples showed a marked increase of the line width of the NH2 resonance signal when the donor was purified. For this reason, the error in
140 measurement was larger for the purified solutions and repeated recordings were made for each sample. The same effect of purification on the n.m.r. spectra of anilines was noted by Yonemoto et al [15]. At the same donor concentration, the NH2 resonance signal was narrower for the solutions containing iodine than for the reference samples; this effect is probably due to an increase of the hydrogen exchange rate, similar to that observed by other authors for related charge transfer complexes [21], or for protonated anilines [22]. In a series of measurements we used carbon tetrachloride solutions containing a fixed iodine concentration (about 0.03-0.06 M) and various amine concentrations ranging from 3.7 to 0.17 M.
In several cases, this range had to be severely
reduced for solubility reasons. Five to nine different samples were prepared in each case. A typical example of association shifts is given in Table I for m-toluidine. It can be seen that the molecular complexing causes a large downfield shift of the resonance signal with respect to the NH2 resonance of the reference sample; this shift increases when the donor concentration is lowered. In our experiments the maximum values of this shift were of about 80 c.p.s.
TABLE 1 NH2 chemical shifts of m-toluidine in CClt,in the absence and in the presence of a fixed iodine concentration (a = 4.05 x 10B2 M) c(M)
-6 (C.P.S.)
-6' (C.P.S.)
-A (c.P.s.)
3.010 2.240 1.495 0.707 0.380
268.0 272.0 278.5 294.0 315.0
260.5 262.0 264.0 266.5 268.0
7.5 10.0 14.5 27.5 47.0
RESULTS Typical plots of ax/A vs. (cx + a) are shown in figures l-3.' It can be seen that good straight lines are obtained in all cases. The x values were determined from dilution shift measurements by a method described elsewhere 1231. In the concentration range used in our experiments little improvement was gained by using eq. (14) instead of (4'). The experimental values of the equilibrium constants and the A0 values are given in Table 2 for a series of eight molecular complexes.
141
TABLE 2 N H 2 pure complex shifts and e q u i l i b r i u m aromatic amines K uv
constants
-A
with
Donor
o
PKa [24-26]
o (i mole -1 ) (I mole -l ) (c,p.s.)
-A' -A" o o (c.p.s.) (c.p.s.)
diphenylamine m-bromoaniline m-chloroaniline p-bromoaniline m-anisidine aniline m-toluidine p-toluidine o-toluidine 2,5-xylidine 2,6-xylidine N-methylaniline
-
0.84 3.44 3.57 3.91 4.23 4.58 4.68 5.08 4.44 4.57 3.89 4.86
1.48 2.O1 1.84 3.69 5.79 8.76 9.17 13.8 12.4 8.85 4.35 20.4
1142 636 626 639 631 715 571 880
0.39 0.37 0.23 O. 12 O.O -0.07 -O.17 -O.17 -0.24 -0.34 -
K nmr
for iodine complexes
-
1127 596 607 622 645 698 591 885
5,29 8.62 9.67 13.8 11.6 8.54 3.84 22.4
103 340 360 400 IiiO 550 580 610 635 680 600 890
0.0
-1.0
- 2.0 (,1
o qr--
x
{~
-3.(3
-
-4.0-
I
-5.C
0.0
1.0
I 2.0
|O
CX + a ( m o l e I "1 )
Fig.
i. Plots of ax/A vs.
(cx + a).
x 2,6-xylidine-12;
o m - t o l u i d i n e - I 2.
142
0
-1.
-2 m 9
X $4I
-3
-4
-5
>
1.0
2.0
3.0
cx + a (mole I-’ )
Fig. 2.
Plots of ax/O E
Ccx + a). o p-toluidine-12;x 2,5-xylidine-12;
A m-anisidine-12.
Table 2 presents also a compar> [A] . The latter results 0
showed also a strong wavelength dependence, whereas the values presented in table 2 were found to be wavelength independent. Similar observations, noted by several groups of workers [27] for other types of charge transfer complexes have been subject to extensive investigation,but there is no satiofacotry explanation of this phenomena.
143
I
I
1
1.0
I
2.0
3.c
cx + a (mole 1-l )
Fig. 3. Plots of ax/A vs. (cx + a). N-methylaniline-IZ.
l
aniline-12; o o-toluidine-12;Ij
The determination of K and A0 values by the method of Takahashi and Li 1191 required chemical shift measurements for two series of solutions, each containing a fixed concentration of amine (within the range 0.3-1.0 M) and various concentrations of iodine, ranging from 0.01 to 0.06 M. (17) - (18) are shown in Figure 4.
Typical plots of eqs.
The equilibrium constants K determined by
this method are not very accurate (as also noted by Takahashi), but the A
0
values, hereafter denoted by A'.are in a good agreement with those determined by our method (table 2). The determination of A0 values by the method of Creswell and Allred [ZO] was made using typical plots such as presented in Figure 5.
The straight lines
thus obtained are an evidence for the correctness of the Kuv values determined
144 - 260 -270
\
-3201 0.0
I 1.0
I I 3.0 4.0 a - lo2 (mole I-’) I 2.0
I 5.0
I 6.0
Fig. 4. Typical plot of NH2 chemical shift vs. a for m-toluidine complexed with iodine (cl = 0.369 M, cp = 0.698 M).
from optical data; the A0 values, hereafter denoted by AZ, are in a good agreement with those determined by our method (table 2). This procedure can be used to investigate systems with smaller A, values, for which our method yields rather large errors; therefore, we have used it to evaluate the A, values for the amino protons in the complexes formed by iodine with diphenylamine and halogenoanilines, as well as the pure complex shifts for the aromatic hydrogen atoms (table 3). The experiment& error in the latter determinations is rather high, owing to the smallness of the observed A shifts.
Fig. 5. Typical plot of the observed NH2 complex shifts Vs.1 calculated y/c ratios for 2,5-xylidine complexed with iodine.
145
TABLE 3 Complex shifts for the ring protons in iodine complexes with several aromatic amines (in p.p.m.) Donor
ortho
meta
para
aniline o-toluidine m-toluidine p-toluidine 2,5-xylidine 2,6-xylidine diphenylamine
-0.99 -0.94 -0.93
+0.24 0.0 +0.29 -0.25 -0.12 -0.32 +0.41
-0.52 -0.61 -1.11 -0.61 -0.60 -0.64 +0.62
-0.95 +0.62
A DISCUSSION OF THE A0 VALUES The amino protons A consideration of the pure complex shifts in table 2 shows first, that all NH2 complex shifts are to low field and second, that they become larger as the ccsnplexbecomes stronger. A plot of the A0 values vs. the Hammettoconstant of the ring substituents yields a straight line (Figure 6). This correlation proves the influence of the electronic structure of the donor on the NH2 shifts. The exception provided by the 2,6dimethylaniline-12 complex is obviously due to steric effects. One of the initial objectives of this work was to gain insight into the structure of molecular complexes from the value of A . 0
NH2
The results obtained for
pure complex shifts can be rationalized by a consideration of the possible
effects on the NH2 resonance signal due to molecular complexing:
O!
-2001
to-400d u' u -6OOa" -800
0
I
-0.4
I
-0.2
I
I
0.0
I
I
0.2
I
I
0.4
Fig. 61.NH2 pure complex shifts as a function of Hammett u constant of the ring substituent in the donor molecule.
146 (1) Change of the electronic charge distribution surrounding the nucleus being measured; (2) bulk-susceptibilitydifferences between the complex and the amine; (3) alteration of paramagnetic contribution to the amino proton shifts; (4) neighbour atom anisotropy effects; (5) electric field effects of acceptor on donor protons; (6) modification of ring currents in donor by interaction with accept&. Effect 1 would be primarily due to the decrease of the electronic density on the amino nitrogen atom, as a result of intermolecularcharge transfer. This decrease is expected to produce a downfield shift of the NH2 resonance signal, the magnitude of which can be evaluated using the relation:
*d =
Q,,*q,
(17)
where A is the change in the diamagnetic shielding of the NH2 protons due to d intermolecularcharge transfer; AqN is the change of the electron density on nitrogen; Q,, is a constant. Equation (17) is similar with that used to correlate the chemical shift changes of the C-H protons with the changes of the Pelectron density in aromatic systems; MO calculations [28,29] suggested that a change of one electron charge in the x-electron density at an aromatic carbon would produce a change of about lo-11 p.p.m. in the chemical shift of the proton bonded to it. This value of the constant seems to be too small to describe the dependence of the amino proton shift on the electronic charge on the neighbouring nitrogen; the QNH value would be expected to be larger, considering that the N-H bond is much more polarisable than the C-H bond. Since there are no quantitative studies on this subject, we have performed SCF MO calculations of the charge densities for a series of eleven substituted anilines [l]; the data thus obtained have shown that the n-electron density on nitrogen varies by 0.0239 electrons when the Hammett o constant of the ring substituent changes by a unit; experimental n.m.r. data in carbon tetrachloride solutions have shown that in this u range, the amino proton shift changes by 0.53 p.p.m.; thus, the QNH value was estimated to be 22.2 p.p.m./electron in the series of aromatic amines. This large value is supported by the results reported by Yonemoto [15], which have shown that the chemical shift of the amino protons in aromatic amines can be interpreted in terms of the change of the electron density on nitrogen, if one admits that the N-H bonds are about twice more polarisable than the C-H bonds. The change of the electron density on nitrogen by molecular complexing (Aq,) was calculated by one of us [30] using a double perturbation procedure. As shown in
147
Figure 7 for a typical example, AqN is very large compared with the changes of the electron densities on the ring atoms; therefore, the intermolecular charge transfer is strongly localized on the nitrogen atom. The calculated AqN values are presented in Table 4.
'TABLE 4 Calculated and experimental pure complex shifts for the amino hydrogen atoms (in p.p.m.) Donor
-MN
0
m-bromoaniline m-chloroaniline p-bromoaniline m-anisidine aniline m-toluidine p-toluidine o-toluidine 2,5_dimethylaniline 2,6_dimethylaniline N-methylaniline
0.39
0.37 0.23 0.12 0.0 -0.07 -0.17 -0.17
-0.24 -0.34
0.1117 0.1177 0.1239 0.2062 0.2274 0.2356 0.2418 0.2460 0.2464 0.2654 0.2942
_gexp 0
4.25 4.50 5.00 14.10 7.43 7.55 7.80 7.97 8.72 7.34 11.06
-Ab 0.79
0.85 0.74 0.88 0.92 0.88 0.91 0.89 0.82 0.82 0.89
talc
-A a
-Ad
-AO
1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4
2.48 2.61 2.75 4.57 5.04 5.22 5.36 5.45 5.46 5.88 6.52
4.67 4.86 4.89 6.85 7.36 7.50 7.67 7.74 7.68 8.10 8.81
In this way, an approximate estimate may be made of the expected values of changes in diamagnetic shielding (A,) due to intermolecular charge transfer. The values thus obtained are given in table 4, showing that the charge transfer is primarily responsible for the magnitude of pure complex shifts. The increase of the (-AqN) values in the series of complexes studied offers an explanation for the observed increase of A0 with the electron-releasingpower of the ring substituent. One should note that another possible source of change in the electron charge distribution surrounding the amino protons could be the change of the hybridization of the bonding orbitals from sp* to sp3; the decrease of the "s character" in the bonding orbitals of the nitrogen atom (demonstratedby infrared measureeents [5,6]) would result in an upfield shift of the NH2 resonance; the chemical shift data reported for uncomplexed amines [15] prove that the amino proton shifts are not very sensitive to hybridization state changes, so that this effect is expected to be unimportant. Effect 2 would result in a downfield shift of the NH2 resonance; this shift was calculated according to the relation [31]:
*b
=
2.6(x; - xv,)
CW
148 v
”
where xa and x are the volume susceptibilitiesof the amine and of the complex. C The Ab values thus obtained are presented in Table 4. Effect 3 is generally small for hydrogen atoms and was neglected in our calculations. Effect 4 would give a downfield shift of the NH2 resonance. A quantitative evaluation of this shift could be made using the point dipole approximation and McConnell's equation [32]:
'a
=
(Ax/3R3)(1- cos2e)
(1%
where Ax is the anisotropy in the magnetic susceptibilityof iodine, R and 6 define the positions of the amino hydrogens relative to the position of the anisotropic group. This approximation is valid only if R >> r, where r is the orbital radius of the anisotropic group; for the iodine atoms, this condition is not fulfilled. We have therefore estimated this term by an analogy with the shift due to neighbour anisotropy effect in methyl iodide [33,34]. Effect 5 was ruled out, since the acceptor has no dipole moment. Effect 6 would be also very small; this can be seen by carrying classical ring current calculations. We have therefore calculated the A0 value as a sum, according to the formula:
AO
=
A.d+Ab+A a
The A0 values thus obtained are compared in table 4 with the experimental data (evaluated as media of the values obtained by the three methods), showing a satisfactory agreement. The exceptions provided by the complexes of m-anisidine and N-methylaniline can be explained in terms of a subsequent reaction leading to the formation of an "inner complex"; a support for this idea is provided by our spectrophotometricdata [l]. In view of the approximationsused in our calculations and expermental errors, it seems reasonable to conclude that the amino proton shifts are mainly dependent on the electron density change at the N atom.
The ring protons The pure crmrplexshifts of the ring protons are much smaller than those exhibited by the amino hydrogen atoms; this observation can be interpreted in terms of a localization of the donor-acceptor interaction at the nitrogen atom - a conclusion drawn earlier on the basis of infrared and ultraviolet spectral data [l-6].
149 A consideration shifts
of the values
are, in general,
The factors
contributing
tons, though
their relative
tropy effect
should
important
complex).
shifts are the same as for the amino proThus,
is different.
because
of the larger
the neighbour
11-H distance.
This effect
should be larger.
in the case of diphenylamine-Izcomplex;
this case can be rationalized current
to these
effect
3 shows that the ortho and para-proton
(except for the diphenyl-amine-12
importance
be reduced
the ring current
trary,
in Table
to low field
On the con-
is particularly
the observed
in terms of the determining
aniso-
upfield
shifts
in
role of the ring-
effect.
The effect
of the intermolecular
the NH
shifts
because
example
presented
in Figure
so that the changes
charge
of the smaller
7, the charge
of the electron
I
I I I
would be smaller
besides,
transfer
density
I
‘I
transfer
CC, value;
is localized
of the carbon
than for
as seen from the at the N atom,
atoms are very
small.
0.1245
0.1395
I
; -0.0176<
>~k,
-0.0007
Fig.
-0.0171
7. Calculated
An important
charge displacements
role is expected
tion of the lone pair electrons mation.
This effect
we believe
this case.
in aniline-12.
to be played
by the decrease
on the aromatic
would reduce
in view of the observed
positions; > meta),
-0.2274
the shielding
ring as a result difference
order of the ring proton
that the contribution
of the delocalizaof complex
between shifts
of this effect might
for-
the ring (ortho > para
be decisive
in
REFERENCES 1 This paper is based on portions of a thesis submitted by Mariana Ciureanu to the Chemistry Faculty of Bucharest in fulfilment of the requirements for the degree of Doctor in Chemistry. 2 H. Tsubomura, J. Am. Chem. Sot., 82 (1960) 41. 3 A.K. Chandra and D.C. Mukerjee, Trans. Faraday Sot., 60 (1964) 62. 4 J. Lauransan and .I. Corset, Ann. Chim., 4 (1969) 475. 5 G. Lichtfus and 2. Zeegers Huyskens, Bull. Chem. Sot. Belge, 82 (1973) 123. 6 M. Ciureanu, Adv. Mol. Relaxation Interact. Processes, 15 (1979) 25. 7 A. Fratiello, J. Chem. Phys., 41 (1963) 2204. 8 P.J. Trotter and M.W. Hanna, J. Amer. Chem. Sot., 88 (1966) 3724. 9 T.C. Nehman and A.I. Popov, J. Phys. Chem., 70 (1966) 3688. 10 R. Foster and C.A. Fyfe, J. Chem. Sot. B, (1966) 926. 11 R. Foster and C.A. Fyfe, Nature, 213 (1967) 591. 12 R. Mathur, E.D. Becker, R.B. Bradley and N.C. Li, J. Phys. Chem., 67 (1963) 2190. 13 E.N. Gurianova, I.P. Golstein and I.P. Romm, "Donorno Actzeptornaia Sviazi", Isd Himia, Moscow, 1973. 14 K.S. Whetsel and J.H. Lady, J. Phys. Chem., 69 (1963) 1596. 15 T. Yonemoto, W.F. Reynolds, T. Schaefer and M. Hutton, Canad. J. Chem., 43 (1965) 2668. 16 W.B. Smith, J. Org. Chem., 27 (1963) 4641. 17 C. Giesner-Pretre, Comet. Rend., 252 (1961) 3238. 18 G.V. Sandul, V.C. Kutz and B.D. Pohodenko, Theor. i exp. Himia, 7 (1975) 659. 19 T. Takahashy and N.C. Li, J. Phys. Chem., 69 (1965) 1623. 20 C.J. Creswell and A.L. Allred, J. Phys. Chem., 66 (1962) 1469. 21 F.M. Mengerand and G. Saito, J. Org. Chem., 40 (1975) 2003. ,22 G. Fraenkel, J. Chem. Phys., 39 (1963) 1614. 23 M. Contineanu, M. Ciureanu and V.E. Sahini, Rev. Roumaine Chim., (in press). 24 A. de Courville, D. Pellier, Bull. Sot. Chim. France, (1967) 2165. 25 A. de Courville, Compt. Rend., 262 (1966) 1196. 26 R.N. Beale, J. Chem. Sot., (1954) 4494. 27 R. Foster and C.A. Fyfe, in "Progress in Nuclear magnetic Resonance Spectroscopy", Pergamon Press, Oxford, 1969, chap. 1. 28 G. Fraenkel, R.E. Carter, A. McLaghlan and J.H. Richards, J. Am. Chem. Sot., 82 (1960) 5846. 29 J.R. Leto, F.A. Cotton and J.S. Waugh, Nature, 180 (1957) 978. 30 M. Ciureanu, Rev. Roumaine Chim., 24 (1979) 31. 31 A.A. Bothner-By and R.E. Glick, J. Chem. Phys., 26 (1957) 1647. 32 H. MC. Connell, J. Chem. Phys., 27 (1957) 226. 33 H. Spieske and W.G. Schneider, J. Chem. Phys., 35 (1961) 722. 34 If the geometry of the complex is considered to be tetrahedral, as shown By infrared data [4-61 the II-H distance is about 2.78 A, compared with 2.6 A in methyl iodide.
135
NUCLEAR MAGNETIC RESONANCE STUDIES OF IODINE-AMINE COMPLEXES
M. CIUREANU and M. CONTINEANU Department of Physical Chemistry, National Institute of Chemistry, Bucharest, Bd. Republicii 13 (Romania) (Received 14 June 1978; in final form 23 October 1978) ABSTRACT The charge transfer equilibria in aromatic amine-iodine systems are investigated by chemical shift measurements. The formation constants K, determined by a new method, are compared with the results obtained from optical data. The pure complex shifts A0 are also determined and compared with those obtained by two classical n.m.r. methods. The A0 values of the amino protons are found to vary linearly with the Bammett o constant of the ring substituent in the donor molecule. Various contributions to the shift of the pure complex are discussed in relation to the structure of the complex and the determining role of the intermolecular charge transfer is stressed. The change of electron density on the nitrogen atom is used to evaluate the change in diamagnetic shielding as a result of intermolecular charge transfer. The A0 values for the aromatic protons are found to be smaller than for the amino protons.and are assigned mainly to the delocalization of the nitrogen lone pair electrons on the aromatic ring.
INTRODUCTION Charge transfer complexes of iodine with aromatic amines have been extensively investigated by visible, ultraviolet [l-3] and infrared spectroscopy [4-61. There are very few reports on their n.m.r. spectra, though chemical shift data are expected to provide valuable information concerning the structure of the molecular complexes. Fratiello [7] measured the chemical shifts of aniline containing a high concentration (1 M) of iodine and noted a large downfield shift of the amino proton signal and some small shifts of the ring proton signals. He concluded that the donor-acceptor interaction was localized at the nitrogen atom, a conclusion drawn earlier by Tsubomura [2] and Mukerjee [3] from optical data.
136
No attempts were made until now to determine the equilibrium constants of these complexes by n.m.r. spectrometry. The main reason for this lack of information is the relatively low solubility of iodine in most suitable organic solvents, which prevents the use of classical procedures [8-131. The usual treatments of charge transfer equilibria in solution would require measurements of the shifts of the donor nuclei in the presence of a large excess of the acceptor: [D] << [A]
0.
0
In our systems it was not possible to make the iodine concen-
tratia large compared with the donor, so that a different treatment became necessary. Fratiello's experiments have pointed out that for the amino proton absorption it is possible to obtain large values of the experimental association shifts even if only a small fraction of the amine is complexed, that is, even if
[Al o << iDlo.
Under these circumstances, it was possible to develop a new pro-
cedure of determining equilibrium constants and association shifts; this procedure was used to investigate the association equilibria in aromatic amine-iodine systems.
GENERAL METHOD This method is based upon the determination of the amino proton chemical shifts for several solutions containing a fixed concentrationof iodine (a) and various concentrationsof the aromatic amine (c); the latter exceeds the iodine concentration by a factor ranging from 5
to
100.
If the donor is not self-associated,the observed chemical shift of the amino protons will be given by the general equation for fast equilibria in solution:
6
= 5 6c + (1
where 6 is the observed shift of the amino protons in complexing media; 61
0) and
6c are the shifts of the NH2 protons in uncomplexed form and in the pure complex, respectively; y is the complex concentration in molar units. Defining A = 6 - 61 and A0 = hc - 61, eq. (1) becomes:
A
=
:A0
(2)
For an ideal solution the equilibrium constant is given by:
K
=
Y Cc - Y) (a - Y)
(3)
137 If one assumes that y and a are negligibly small with respect to c (y << c, a
d
1
1
iy+zy
=
(4)
If the acceptor concentration cannot be neglected with respect to c, one gets instead of (4): a 1 x=iz-o
+ +
o
(c + a)
(4')
A plot of a/A against (c + a) should be linear. The gradient of the line yields l/Ao, while the intercept yields l/KAo. Thus, both K and A0 can be evaluated. Eq. (4') has to be modified if the aromatic amine is self-associated. Infrared measurements made by Whetsel and Lady [14] have shown that the aromatic amines are partially dimerized in solution. These observations have been substantiated by n.m.r. measurements of the amino proton shifts as a function of concentration 114-181. If the self-associationof the donor is taken into account, the observed NH2 chemical shift in complexing media will be the weighted average of 61, 6c and 82:
6
=
(1 _ C+)&l
d +;62+
f6
c
(5)
where 62 is the chemical shift of the amino proton in pure dimer and 61 is the NH2 chemical shift at infinite dilution. The chemical shift of the NH2 protons in a solution containing the same amine concentration with no iodine added is:
6’
=
(1 - $)6l
d' 62 + 7
(6)
In (5) and (6) d and d' are the dimer concentration corresponding to the same donor concentration,with and without iodine added. They differ by a small quantity, hereafter denoted by a:
d
=
d'-a
(7)
It may easily be shown that CLis given by:
a
=*
(‘3)
138
From eq.
(8), the following
equation,
which
is a modified
form of eq.
(l), is
obtained:
(9) If y <<
c and d' < c, a is negligibly
in (9) can be omitted,
small,so
that d Q d' and the last term
so that:
and
(11)
Here A is the observed #a reference constant
K
=
where
solution
K will
be given
resonance
relative
the same donor concentration
c.
to that of
The equilibrium
by:
Y (cx - ~)(a - Y)
(12)
x is the fraction
associated:
x
shift of the amino proton
containing
of the initial donor
concentration
which
is not self-
l
=- c - 2d'
(13)
C
(10) - (131, one gets a modified
From eqs. ax d=
1 q+Ao
L
6' = 6 1, then eq.
investigate
(14) and
to provide
the above method
(14)
hydrogen
to study phenol-amide
of amine
bonded
from a plot of y
(4') become
evidence
we have applied
The first method
measurements
(4')
(cx + a)
Thus, K and A0 can be evaluated
In order
form of eq.
against
(cx + a).
When x = 1,
identical.
for the correctness two classical
of the values
procedures,
obtained
previously
by
used to
systems.
was similar
to the technique
interactions;
for two series of solutions,
(cl and ~2) and various
used by Takahashy
this procedure
involves
each containing
concentrations
and Li [19]
chemical
shift
a fixed concentration
of the acceptor.
A plot of 6 vs.
139
a yields two straight lines with different slopes, from which
A0 and K can be
obtained:
6 =
q+
6
CT;+
Kxl UC - 61) 1 + Kclxl Kx2Qc
=
a
(15)
- 61)
1 + Kc2x2
a
(16)
The second procedure was the method proposed by Creswell and Allred [20]. The ho values were obtained according to eq. (ll), from a plot of A against y/c. Unlike the original method, the y/c ratios were calculated from the equilibrium constants K, determined from spectrophotometricmeasurements.
EXPERIMENTAL Materials Carbon tetrachloride,used as a solvent in our experiments was dried over molecular sieves and distilled twice over K2CO3. Iodine was sublimed twice and stored in a dark bottle. Solid anilines were purified by recrystallization liquid anilines were distilled in vacuum over fresh potassium hydroxide. Sample solutions were prepared by adding a concentrated iodine solution in carbon tetrachloride to a determined quantity of amine.
N.M.R. Measurements The n.m.r. spectra were recorded with a Tesla BS 487C spectrometer at
a0 Mc.p.s. Chemical shift measurements were made relative to external tetramethylsilane. The bulk susceptibility corrections were made in the usual manner. In a series of measurements, the temperature was kept constant within *lo. The reported shifts at 25' are accurate to 0.5 c.p.s. The spectra of the samples were recorded immediately after preparation, in order to avoid the effect of ion-pair formation or other chemical changes of the solutions upon the measurements. The n.m.r. spectra of the complex solutions were compared with those of the reference samples. The chemical shift of the NH2 protons in the uncomplexed amines at infinite dilution in CC14 were in a very good agreement with those reported by other authors [15-la]. The reference samples showed a marked increase of the line width of the NH2 resonance signal when the donor was purified. For this reason, the error in
140 measurement was larger for the purified solutions and repeated recordings were made for each sample. The same effect of purification on the n.m.r. spectra of anilines was noted by Yonemoto et al [15]. At the same donor concentration, the NH2 resonance signal was narrower for the solutions containing iodine than for the reference samples; this effect is probably due to an increase of the hydrogen exchange rate, similar to that observed by other authors for related charge transfer complexes [21], or for protonated anilines [22]. In a series of measurements we used carbon tetrachloride solutions containing a fixed iodine concentration (about 0.03-0.06 M) and various amine concentrations ranging from 3.7 to 0.17 M.
In several cases, this range had to be severely
reduced for solubility reasons. Five to nine different samples were prepared in each case. A typical example of association shifts is given in Table I for m-toluidine. It can be seen that the molecular complexing causes a large downfield shift of the resonance signal with respect to the NH2 resonance of the reference sample; this shift increases when the donor concentration is lowered. In our experiments the maximum values of this shift were of about 80 c.p.s.
TABLE 1 NH2 chemical shifts of m-toluidine in CClt,in the absence and in the presence of a fixed iodine concentration (a = 4.05 x 10B2 M) c(M)
-6 (C.P.S.)
-6' (C.P.S.)
-A (c.P.s.)
3.010 2.240 1.495 0.707 0.380
268.0 272.0 278.5 294.0 315.0
260.5 262.0 264.0 266.5 268.0
7.5 10.0 14.5 27.5 47.0
RESULTS Typical plots of ax/A vs. (cx + a) are shown in figures l-3.' It can be seen that good straight lines are obtained in all cases. The x values were determined from dilution shift measurements by a method described elsewhere 1231. In the concentration range used in our experiments little improvement was gained by using eq. (14) instead of (4'). The experimental values of the equilibrium constants and the A0 values are given in Table 2 for a series of eight molecular complexes.
141
TABLE 2 N H 2 pure complex shifts and e q u i l i b r i u m aromatic amines K uv
constants
-A
with
Donor
o
PKa [24-26]
o (i mole -1 ) (I mole -l ) (c,p.s.)
-A' -A" o o (c.p.s.) (c.p.s.)
diphenylamine m-bromoaniline m-chloroaniline p-bromoaniline m-anisidine aniline m-toluidine p-toluidine o-toluidine 2,5-xylidine 2,6-xylidine N-methylaniline
-
0.84 3.44 3.57 3.91 4.23 4.58 4.68 5.08 4.44 4.57 3.89 4.86
1.48 2.O1 1.84 3.69 5.79 8.76 9.17 13.8 12.4 8.85 4.35 20.4
1142 636 626 639 631 715 571 880
0.39 0.37 0.23 O. 12 O.O -0.07 -O.17 -O.17 -0.24 -0.34 -
K nmr
for iodine complexes
-
1127 596 607 622 645 698 591 885
5,29 8.62 9.67 13.8 11.6 8.54 3.84 22.4
103 340 360 400 IiiO 550 580 610 635 680 600 890
0.0
-1.0
- 2.0 (,1
o qr--
x
{~
-3.(3
-
-4.0-
I
-5.C
0.0
1.0
I 2.0
|O
CX + a ( m o l e I "1 )
Fig.
i. Plots of ax/A vs.
(cx + a).
x 2,6-xylidine-12;
o m - t o l u i d i n e - I 2.
142
0
-1.
-2 m 9
X $4I
-3
-4
-5
>
1.0
2.0
3.0
cx + a (mole I-’ )
Fig. 2.
Plots of ax/O E
Ccx + a). o p-toluidine-12;x 2,5-xylidine-12;
A m-anisidine-12.
Table 2 presents also a compar
showed also a strong wavelength dependence, whereas the values presented in table 2 were found to be wavelength independent. Similar observations, noted by several groups of workers [27] for other types of charge transfer complexes have been subject to extensive investigation,but there is no satiofacotry explanation of this phenomena.
143
I
I
1
1.0
I
2.0
3.c
cx + a (mole 1-l )
Fig. 3. Plots of ax/A vs. (cx + a). N-methylaniline-IZ.
l
aniline-12; o o-toluidine-12;Ij
The determination of K and A0 values by the method of Takahashi and Li 1191 required chemical shift measurements for two series of solutions, each containing a fixed concentration of amine (within the range 0.3-1.0 M) and various concentrations of iodine, ranging from 0.01 to 0.06 M. (17) - (18) are shown in Figure 4.
Typical plots of eqs.
The equilibrium constants K determined by
this method are not very accurate (as also noted by Takahashi), but the A
0
values, hereafter denoted by A'.are in a good agreement with those determined by our method (table 2). The determination of A0 values by the method of Creswell and Allred [ZO] was made using typical plots such as presented in Figure 5.
The straight lines
thus obtained are an evidence for the correctness of the Kuv values determined
144 - 260 -270
\
-3201 0.0
I 1.0
I I 3.0 4.0 a - lo2 (mole I-’) I 2.0
I 5.0
I 6.0
Fig. 4. Typical plot of NH2 chemical shift vs. a for m-toluidine complexed with iodine (cl = 0.369 M, cp = 0.698 M).
from optical data; the A0 values, hereafter denoted by AZ, are in a good agreement with those determined by our method (table 2). This procedure can be used to investigate systems with smaller A, values, for which our method yields rather large errors; therefore, we have used it to evaluate the A, values for the amino protons in the complexes formed by iodine with diphenylamine and halogenoanilines, as well as the pure complex shifts for the aromatic hydrogen atoms (table 3). The experiment& error in the latter determinations is rather high, owing to the smallness of the observed A shifts.
Fig. 5. Typical plot of the observed NH2 complex shifts Vs.1 calculated y/c ratios for 2,5-xylidine complexed with iodine.
145
TABLE 3 Complex shifts for the ring protons in iodine complexes with several aromatic amines (in p.p.m.) Donor
ortho
meta
para
aniline o-toluidine m-toluidine p-toluidine 2,5-xylidine 2,6-xylidine diphenylamine
-0.99 -0.94 -0.93
+0.24 0.0 +0.29 -0.25 -0.12 -0.32 +0.41
-0.52 -0.61 -1.11 -0.61 -0.60 -0.64 +0.62
-0.95 +0.62
A DISCUSSION OF THE A0 VALUES The amino protons A consideration of the pure complex shifts in table 2 shows first, that all NH2 complex shifts are to low field and second, that they become larger as the ccsnplexbecomes stronger. A plot of the A0 values vs. the Hammettoconstant of the ring substituents yields a straight line (Figure 6). This correlation proves the influence of the electronic structure of the donor on the NH2 shifts. The exception provided by the 2,6dimethylaniline-12 complex is obviously due to steric effects. One of the initial objectives of this work was to gain insight into the structure of molecular complexes from the value of A . 0
NH2
The results obtained for
pure complex shifts can be rationalized by a consideration of the possible
effects on the NH2 resonance signal due to molecular complexing:
O!
-2001
to-400d u' u -6OOa" -800
0
I
-0.4
I
-0.2
I
I
0.0
I
I
0.2
I
I
0.4
Fig. 61.NH2 pure complex shifts as a function of Hammett u constant of the ring substituent in the donor molecule.
146 (1) Change of the electronic charge distribution surrounding the nucleus being measured; (2) bulk-susceptibilitydifferences between the complex and the amine; (3) alteration of paramagnetic contribution to the amino proton shifts; (4) neighbour atom anisotropy effects; (5) electric field effects of acceptor on donor protons; (6) modification of ring currents in donor by interaction with accept&. Effect 1 would be primarily due to the decrease of the electronic density on the amino nitrogen atom, as a result of intermolecularcharge transfer. This decrease is expected to produce a downfield shift of the NH2 resonance signal, the magnitude of which can be evaluated using the relation:
*d =
Q,,*q,
(17)
where A is the change in the diamagnetic shielding of the NH2 protons due to d intermolecularcharge transfer; AqN is the change of the electron density on nitrogen; Q,, is a constant. Equation (17) is similar with that used to correlate the chemical shift changes of the C-H protons with the changes of the Pelectron density in aromatic systems; MO calculations [28,29] suggested that a change of one electron charge in the x-electron density at an aromatic carbon would produce a change of about lo-11 p.p.m. in the chemical shift of the proton bonded to it. This value of the constant seems to be too small to describe the dependence of the amino proton shift on the electronic charge on the neighbouring nitrogen; the QNH value would be expected to be larger, considering that the N-H bond is much more polarisable than the C-H bond. Since there are no quantitative studies on this subject, we have performed SCF MO calculations of the charge densities for a series of eleven substituted anilines [l]; the data thus obtained have shown that the n-electron density on nitrogen varies by 0.0239 electrons when the Hammett o constant of the ring substituent changes by a unit; experimental n.m.r. data in carbon tetrachloride solutions have shown that in this u range, the amino proton shift changes by 0.53 p.p.m.; thus, the QNH value was estimated to be 22.2 p.p.m./electron in the series of aromatic amines. This large value is supported by the results reported by Yonemoto [15], which have shown that the chemical shift of the amino protons in aromatic amines can be interpreted in terms of the change of the electron density on nitrogen, if one admits that the N-H bonds are about twice more polarisable than the C-H bonds. The change of the electron density on nitrogen by molecular complexing (Aq,) was calculated by one of us [30] using a double perturbation procedure. As shown in
147
Figure 7 for a typical example, AqN is very large compared with the changes of the electron densities on the ring atoms; therefore, the intermolecular charge transfer is strongly localized on the nitrogen atom. The calculated AqN values are presented in Table 4.
'TABLE 4 Calculated and experimental pure complex shifts for the amino hydrogen atoms (in p.p.m.) Donor
-MN
0
m-bromoaniline m-chloroaniline p-bromoaniline m-anisidine aniline m-toluidine p-toluidine o-toluidine 2,5_dimethylaniline 2,6_dimethylaniline N-methylaniline
0.39
0.37 0.23 0.12 0.0 -0.07 -0.17 -0.17
-0.24 -0.34
0.1117 0.1177 0.1239 0.2062 0.2274 0.2356 0.2418 0.2460 0.2464 0.2654 0.2942
_gexp 0
4.25 4.50 5.00 14.10 7.43 7.55 7.80 7.97 8.72 7.34 11.06
-Ab 0.79
0.85 0.74 0.88 0.92 0.88 0.91 0.89 0.82 0.82 0.89
talc
-A a
-Ad
-AO
1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4
2.48 2.61 2.75 4.57 5.04 5.22 5.36 5.45 5.46 5.88 6.52
4.67 4.86 4.89 6.85 7.36 7.50 7.67 7.74 7.68 8.10 8.81
In this way, an approximate estimate may be made of the expected values of changes in diamagnetic shielding (A,) due to intermolecular charge transfer. The values thus obtained are given in table 4, showing that the charge transfer is primarily responsible for the magnitude of pure complex shifts. The increase of the (-AqN) values in the series of complexes studied offers an explanation for the observed increase of A0 with the electron-releasingpower of the ring substituent. One should note that another possible source of change in the electron charge distribution surrounding the amino protons could be the change of the hybridization of the bonding orbitals from sp* to sp3; the decrease of the "s character" in the bonding orbitals of the nitrogen atom (demonstratedby infrared measureeents [5,6]) would result in an upfield shift of the NH2 resonance; the chemical shift data reported for uncomplexed amines [15] prove that the amino proton shifts are not very sensitive to hybridization state changes, so that this effect is expected to be unimportant. Effect 2 would result in a downfield shift of the NH2 resonance; this shift was calculated according to the relation [31]:
*b
=
2.6(x; - xv,)
CW
148 v
”
where xa and x are the volume susceptibilitiesof the amine and of the complex. C The Ab values thus obtained are presented in Table 4. Effect 3 is generally small for hydrogen atoms and was neglected in our calculations. Effect 4 would give a downfield shift of the NH2 resonance. A quantitative evaluation of this shift could be made using the point dipole approximation and McConnell's equation [32]:
'a
=
(Ax/3R3)(1- cos2e)
(1%
where Ax is the anisotropy in the magnetic susceptibilityof iodine, R and 6 define the positions of the amino hydrogens relative to the position of the anisotropic group. This approximation is valid only if R >> r, where r is the orbital radius of the anisotropic group; for the iodine atoms, this condition is not fulfilled. We have therefore estimated this term by an analogy with the shift due to neighbour anisotropy effect in methyl iodide [33,34]. Effect 5 was ruled out, since the acceptor has no dipole moment. Effect 6 would be also very small; this can be seen by carrying classical ring current calculations. We have therefore calculated the A0 value as a sum, according to the formula:
AO
=
A.d+Ab+A a
The A0 values thus obtained are compared in table 4 with the experimental data (evaluated as media of the values obtained by the three methods), showing a satisfactory agreement. The exceptions provided by the complexes of m-anisidine and N-methylaniline can be explained in terms of a subsequent reaction leading to the formation of an "inner complex"; a support for this idea is provided by our spectrophotometricdata [l]. In view of the approximationsused in our calculations and expermental errors, it seems reasonable to conclude that the amino proton shifts are mainly dependent on the electron density change at the N atom.
The ring protons The pure crmrplexshifts of the ring protons are much smaller than those exhibited by the amino hydrogen atoms; this observation can be interpreted in terms of a localization of the donor-acceptor interaction at the nitrogen atom - a conclusion drawn earlier on the basis of infrared and ultraviolet spectral data [l-6].
149 A consideration shifts
of the values
are, in general,
The factors
contributing
tons, though
their relative
tropy effect
should
important
complex).
shifts are the same as for the amino proThus,
is different.
because
of the larger
the neighbour
11-H distance.
This effect
should be larger.
in the case of diphenylamine-Izcomplex;
this case can be rationalized current
to these
effect
3 shows that the ortho and para-proton
(except for the diphenyl-amine-12
importance
be reduced
the ring current
trary,
in Table
to low field
On the con-
is particularly
the observed
in terms of the determining
aniso-
upfield
shifts
in
role of the ring-
effect.
The effect
of the intermolecular
the NH
shifts
because
example
presented
in Figure
so that the changes
charge
of the smaller
7, the charge
of the electron
I
I I I
would be smaller
besides,
transfer
density
I
‘I
transfer
CC, value;
is localized
of the carbon
than for
as seen from the at the N atom,
atoms are very
small.
0.1245
0.1395
I
; -0.0176<
>~k,
-0.0007
Fig.
-0.0171
7. Calculated
An important
charge displacements
role is expected
tion of the lone pair electrons mation.
This effect
we believe
this case.
in aniline-12.
to be played
by the decrease
on the aromatic
would reduce
in view of the observed
positions; > meta),
-0.2274
the shielding
ring as a result difference
order of the ring proton
that the contribution
of the delocalizaof complex
between shifts
of this effect might
for-
the ring (ortho > para
be decisive
in
REFERENCES 1 This paper is based on portions of a thesis submitted by Mariana Ciureanu to the Chemistry Faculty of Bucharest in fulfilment of the requirements for the degree of Doctor in Chemistry. 2 H. Tsubomura, J. Am. Chem. Sot., 82 (1960) 41. 3 A.K. Chandra and D.C. Mukerjee, Trans. Faraday Sot., 60 (1964) 62. 4 J. Lauransan and .I. Corset, Ann. Chim., 4 (1969) 475. 5 G. Lichtfus and 2. Zeegers Huyskens, Bull. Chem. Sot. Belge, 82 (1973) 123. 6 M. Ciureanu, Adv. Mol. Relaxation Interact. Processes, 15 (1979) 25. 7 A. Fratiello, J. Chem. Phys., 41 (1963) 2204. 8 P.J. Trotter and M.W. Hanna, J. Amer. Chem. Sot., 88 (1966) 3724. 9 T.C. Nehman and A.I. Popov, J. Phys. Chem., 70 (1966) 3688. 10 R. Foster and C.A. Fyfe, J. Chem. Sot. B, (1966) 926. 11 R. Foster and C.A. Fyfe, Nature, 213 (1967) 591. 12 R. Mathur, E.D. Becker, R.B. Bradley and N.C. Li, J. Phys. Chem., 67 (1963) 2190. 13 E.N. Gurianova, I.P. Golstein and I.P. Romm, "Donorno Actzeptornaia Sviazi", Isd Himia, Moscow, 1973. 14 K.S. Whetsel and J.H. Lady, J. Phys. Chem., 69 (1963) 1596. 15 T. Yonemoto, W.F. Reynolds, T. Schaefer and M. Hutton, Canad. J. Chem., 43 (1965) 2668. 16 W.B. Smith, J. Org. Chem., 27 (1963) 4641. 17 C. Giesner-Pretre, Comet. Rend., 252 (1961) 3238. 18 G.V. Sandul, V.C. Kutz and B.D. Pohodenko, Theor. i exp. Himia, 7 (1975) 659. 19 T. Takahashy and N.C. Li, J. Phys. Chem., 69 (1965) 1623. 20 C.J. Creswell and A.L. Allred, J. Phys. Chem., 66 (1962) 1469. 21 F.M. Mengerand and G. Saito, J. Org. Chem., 40 (1975) 2003. ,22 G. Fraenkel, J. Chem. Phys., 39 (1963) 1614. 23 M. Contineanu, M. Ciureanu and V.E. Sahini, Rev. Roumaine Chim., (in press). 24 A. de Courville, D. Pellier, Bull. Sot. Chim. France, (1967) 2165. 25 A. de Courville, Compt. Rend., 262 (1966) 1196. 26 R.N. Beale, J. Chem. Sot., (1954) 4494. 27 R. Foster and C.A. Fyfe, in "Progress in Nuclear magnetic Resonance Spectroscopy", Pergamon Press, Oxford, 1969, chap. 1. 28 G. Fraenkel, R.E. Carter, A. McLaghlan and J.H. Richards, J. Am. Chem. Sot., 82 (1960) 5846. 29 J.R. Leto, F.A. Cotton and J.S. Waugh, Nature, 180 (1957) 978. 30 M. Ciureanu, Rev. Roumaine Chim., 24 (1979) 31. 31 A.A. Bothner-By and R.E. Glick, J. Chem. Phys., 26 (1957) 1647. 32 H. MC. Connell, J. Chem. Phys., 27 (1957) 226. 33 H. Spieske and W.G. Schneider, J. Chem. Phys., 35 (1961) 722. 34 If the geometry of the complex is considered to be tetrahedral, as shown By infrared data [4-61 the II-H distance is about 2.78 A, compared with 2.6 A in methyl iodide.