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THE AMINO PROTON SHIFTS O F SOME SUBSTITUTED ANILINES IN
CYCLOHEXANE
W. F. H.
T. YONE~~OTO,' REYNOLDS,~R'I. HUTTON, T . SCH~IEFER~
AND
Department of Cltenzistry, University of Manitoba, Winnipeg, Ma?titoba
Received M a y 13, 1965
ABSTRACT
The amino proton shifts of 41 ortho, meta, and para substituted anilines a t low concen-
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trations in cyclohexane are reported. T h e shifts for meta and para substituents correlate with
Hammett u constants and it is shown t h a t they are probably proportional to the r-electron
density on the nitrogen atom. These shifts also correlate with the pK, values of the corre-
sponding anilinium ions in aqueous acid. The amino proton shifts for the ortho substituents,
apart from methyl groups, follow a correlation line which lies about 0.5 p.p.m. t o low field
from that for the meta a n d para substituents. This negative deviation is discussed in terms of a
weak hydrogen bonding interaction with the ortho substituents and the I-I-N-H bond angles
a s derived from infrared data.
INTRODUCTION
In recent years, correlations have been noted between Hammett a constants and the
chemical shift of the ring protons in p-substituted benzenes (I), formyl protons in nz- and
p-substituted benzaldehydes (2), hydroxyl protons in m- and $-substituted phenols (3),
methoxy protons of substituted anisoles (4), and amino protons in m- and $-substituted
For personal use only.
anilines (5, 6). T h e anilines were measured in carbon tetrachloride (5, 6) and acetonitrile
(G), where hydrogen bonding between solvent and amino protons is probable (see below).
The ring proton shifts a t the para position of substituted benzenes are proportional to
the calculated a-electron densities a t the carbon atom (7). We report 11ere the amino
proton shifts of 41 o-, m-,and p-substituted anilines in cyclohexane solution, where
interactions with the solvent are a t a minimunl, and discuss the results in terms of
Hanlnlett a constants of the substituents, the a-electron density a t the nitrogen atom, the
PI<, values of the corresponding aniliniuln ions, and the H-N-H angles derived from
high resolution infrared measurements. Low solubility in cyclohexane precluded measure-
ments on a number of compounds, among them the nitroanilines, aminophenols, and o-
and 9-phenylenediamines.
EXPERIMENTAL
1. Sarrzple Preparation
Solid anilines were sublimed before use, except for o-bromoaniline and p-iodoaniline which were recrystal-
lized from cyclohexane. T h e liquid haloanilines were distilled in vacuum over fresh potassium hydroxide.
Spectroscopy-grade cyclohexane was used a s a solvent without further purification. As long a s solubility
allowed, samples of 5,2, and 1 mole % concentration were prepared by weighing.
2. Spectra
T h e proton resonance spectrum of the amino group was recorded on a DP60 spectrometer. T h e solvent
resonance peak provided a convenient reference signal, sidebands of which were used t o find the shift of the
amino proton resonance. T h e line width of the latter, a broad band due to exchange and relaxation phenomena
involving the ",U quadrupole, was u s ~ ~ a labout 3-5 c/s a t 5 mole % a n d tended t o increase with dilution. In
ly
some cases the linewidth approached 25 c/s. A dozen recordings were averaged for each solution. T h e
reproducibility of the shifts was about 1 c/s in most cases but for very broad or weak signals it decreased t o
3 c/s. T h e shifts obtained a t the three concentrations were extrapolated to infinite dilution. Most of the
solutions showed no significant dilution shift in this concentration range. Low solubility in some cases
liVationa1 Research Coz~ncil Postdoctorate Fellow, 1962-1964.
2Natio?zalResearch Coz~ncil! Bolder. 1962-1965.
Stz~de?ttsl~ii3
r
3iVational Research Corrncil ~ e n i oReharclt Fellow, 1964-1966.
Canadian Journal of Chemistry. Volume 43 (1DG6)
2668
YOhTEMOTO ET AL.: PROTON SHIFTS 2669
demanded the use of the shift of the 1 mole % solution only. These are given the superscript a in Table I. In
only allowed the measurement of 2 or 5 mole % solutions.
other cases large line widths a t lower conce~ltrations
These are given the superscript b in Table I.
3. Effect of Pz~rificatiorton Spectra
An investigation of the effect of impurity on the shift was made with aniline solutions. I n Fig. 1, line 1 was
obtained for aniline distilled in vacuulu over potassium hydroxide. Line 2 shows the results for the distilled
aniline if the solutions were prepared 1 d after opening the distillation vessel, while line 3 was obtained for
undistilled deeply colored aniline. T h e resonance peaks obtained for the solutions prepared from unpurified
aniline were narrower than for purified aniline, probably because of faster proton exchange catalyzed b y
water or other impurities. For this reason, the error in measurement was larger for the purified aniline solu-
tions. Consequently, although the solutions of unpurified aniline appear to show both a smaller dilution shift
and a high-field shift conlpared t o the solutions of purified aniline, the experimental error is such that little
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was gained by purification.
109-
-
110-
For personal use only.
0 1 2 3 4 5
MOLE OI. in C H
6 12
FIG.I. Line I (open circles) shows the chemical shift in c/s t o low field from cyclohexane of the amino
protons of vacuum-distilled aniline a t three concentrations in cyclohexane. Line 2 (half-filled circles) is for
aniline which was used one day after vacuum distillation. Line 3 (filled circles) is for unpurified, deeply colored
aniline.
RESULTS AND DISCUSSION
In Table I the amino proton shifts, Av, of 41 o-, m-, and p-substituted anilines are given
relative to aniline itself. Prior to the discussion of these shifts in terms of lnolecular para-
meters, the state of the aniline molecules in solution must be considered. If our Av values
represented the true shift a t infinite dilution, they would naturally be those of aniline
nlolecules isolated from each other by solvent molecules. Because of signal-to-noise
problems, however, the extrapolation to infinite dilution is probably solnewhat unreliable.*
There may be a substantial, a t present undetectable, change in shift between 1 and 0 mole
yo, especially if self-association through hydrogen bonding occurs.
The intensity of the first overtone of the N-1-1 stretching band of aniline in cyclohexane
has beell interpreted in terms of hydrogen-bonding association ( 8 ) . A dilnerization model
is consistent with the data up to 0.2 M. The equilibrium constant for dimer fornlation is
about 0.4 a t room temperature ( 8 ) . For a 0.2 M solution, which happens to be about 2
mole %, only about 5% of the aniline nlolecules are therefore in the form of dimers. On an
n.1n.r. time scale the dilners will break up and reform rapidly, so that the observed
shift represents the weighted average of the nlonolner and dinler shifts. Unfortunately,
the difference in shift between dimer and monomer is not known. However, if we assume
* T h e viable, i f expensive, procedure to follow Itere woz~ldbe the zrse of l5X sszlbstituted co~rapoz~nds eliminate
to
qz~adrz~pole or2
relnsatiofr broadeni~tgand a conapz~terof average transieizts to allow ~neasz~reiiaents very dilzlte
solz~tions.
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TABLE I
Chemical shifts, Au, in p.p.m. of the amino-protons of some substituted anilines in cyclohexane solutions
For personal use only.
No. Substituent AuC No. Substituent Auc ud ApKac
3,4-diCI -0.02 0.60 - u
3,5-diCla -0.12 0.74 - 2
2,4-diCI -0.45 0.43 - 2:
2,5-diCI -0.53 0.57 - u
2,6-diCI
2,6-diBr
-0.97
-1.10
0.40
0.42
-
-
2
0
2:
2,4-diCH3 0.20 -0.34 - >
2,3-diCH3 0.13 -0.24 -0.01 r
2,6-diCH3 0.10 -0.34 - o
T
2,5-diCH3 0.13 -0.24 -
0
3,s-diCHaa 0.20 -0.14 - rC
3.4-diCH2 0.22 -0.24 0.67 F?
a b~eeesperimental procedure.
=Relativeto aniline.
d~-Iamrnettn-constants. See discussion for choice of values.
CTakenfrom refs. 35 and 36.
,As recornniended in ref. 14.
YONEMOTO ET AL.: PROTON SHIFTS 267 1
that the substituted anilines have very similar association constants to aniline then the
discussions below should still be valid. Certainly, the absortivity-concentration relation-
ships of 7n-methylaniline and m-cl~loroanilineare qualitatively similar to the ones for
aniline, although they are solnewhat different for o-chloroaniline (9). A reasonable expecta-
tion is that o-substituents would hinder self-association and that self-association through
hydrogen bonding leads to low-field shifts. Therefore the observed large low-field shifts for
o-substituents can also reasonably be discussed in terms of the properties of the isolated
molecules, as is done below.
i. Correlation of Av with a Constants
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In Fig. 2, AV is plotted versus the Hainmett a constants. The effect of more than one
substituent is taken as additive (10) and the recent a values froin the analysis of McDaniel
and Brown (11) were used when available. For o-substituents the a values derived by
T a f t (12) were used since these are on the same scale as the Haminett a values for m- and
p-substituents.
For solubility reasons, the range of a is only 1.2 units as compared to 1.6 units available
in acetonitrile solution (6). Disregarding the o-substituents, except for methyl groups, the
range of AV is very small and we feel there is little to be gained f r o ~ n least squares treat-
a
ment. Instead, the dashed line has been drawn as shown in Fig. 2. As expected, the
electron-withdrawing substituents (positive a) lead to a low-field shift. The average
deviation for 7n- and p-substituents (but including o-methyl) is just over 0.03 p.p.m. which
For personal use only.
is probably within experimental error.
ppm. -0.4
I
o para
m meta
-0.8 -
-1.0 -
.
A ortho
di-ortho
..
26-CI2 A 2.CN
2.6-Br2
L I I I I
-0.50 -0.25 0 0.25 0.50 0.75
w
FIG.2. T h e chemical shift in p.p.m. relative to aniline of t h e amino protons in substituted anilines is
plotted versus the Hamlnett u-constants of the substituents. The unidentified ortho points clustered about
the top lineare those with methyl groups in the ortho position.
Per unit change in a, Av changes by 0.35 p.p.m. which compares with 0.53 p.p.m. in
carbon tetrachloride and 0.97 p.p.m. in acetonitrile solutions (6). The larger values in the
latter solutions can be understood in terms of hydrogen bonding to the solvent. As u
increases algebraically the N-H bond increases in acidity, and hydrogen bonding with an
acceptor solvent is favored, causing larger shifts. That carbon tetrachloride should hydrogen
bond to N-1-1 is perhaps unexpected, yet there is evidence that it does so even t o C11 --
bonds. As an example, proton shifts of many haloalkanes (13) in cyclohexane and carbon
2672 CANADIAN JOURAT.4L O F O.
CHEMISTRY. V L 43. 196.5
tetrachloride solution indicate that the latter offers chlorine atoms for hydrogen bonding
to the C-H bonds of the solute. Further, it is known that some substituted anilines react
rapidly with carbon tetrachloride (14, 15). For the discussioil in the next subsection,
measurement of ainino proton shifts in solvents like cyclohexane is therefore essential.
2. AVand T-Electron Density on the Nitrogen Atom
In p-substituted benzenes, the p-proton shift changes by 1 p.p.111. when the I-Iammett u
constant changes by 1.4 units (1). For this u range, the calculated a-electron density on the
p-carbon atoll1 varies by 0.1 electron (7), indicating a proton shift of about 10 p.p.in./
electron. The electron density on the nitrogen atoin of substituted anilines is expected to
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be much less sensitive than the p-carbon atom t o the influence of a substituent on the
ring. Thus, the calculated T-density on the nitrogen atom in N,N-dimethylaniline is only
0.007 electrons less than in N,N-dimethyl-p-anisidine (16). This compares with 0.031
electron more a t the 9-carbon atom in anisole than in benzene (7) (these two calculations
used the same bond and core parameters for the oxygen atom in the nlethoxy group).
In this way an approximate estimate may be made of the expected change in AV of the
amino protons for a change of one unit in u in the substituted anilines. I t is Av = 0.007/1.4
X 0.031 = 0.16 p.p.rn., about half the observed value. However, the N-1-1 bond is much
more polarizable than the C-H bond. From chemical shift data, Musher (17) deduces
that it is roughly 5/3 illore polarizable, leading to a calculated Av of 0.27 p.p.m.
The electrostatic field of the excess charge on the nitrogen atoin produces a potential
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differencebetween the two ends of the N-I3 bond which drives electrons into the hydrogen
1s orbital. However, the hybridization of the nitrogen atom changes when the ring is
substituted (see below) and this will change the excitation energies of the orbitals involved
and therefore change any assumed proportionality between a-density and proton shielding.
In view of the approximations and experimental errors involved, it seems reasonable to
conclude that the ainino proton shift in m- and p-substituted anilines is primarily dependent
on the a-electron density a t the nitrogen atom. In this connection, it is interesting to note
a shift of the zero point of about 0.05 p.p.m. to high field in Fig. 2. Such a shift also occurs
for the proton shifts in p-substituted benzenes and is evidence for an additional factor in
determining the u constants. The latter depend on the electronic structure of a reaction
intermediate related to the substituted benzoic acids.
3. Correlation o AVwith PI<, Valzies
f
In Fig. 3, AV is plotted versus ApK, of the protonated anilines in aqueous acid. The
average deviation from the straight line is less than 0.03 p.p.in. However, because of the
slight slope of the line a measurement of the anlino proton shift in cyclohexane allows the
prediction of the PI<, a substituted aniline with an accuracy of only about 1t0.25 units.
of
For this purpose, it would be better to use a polar solvent like acetonitrile for which the
correlation line has a much steeper slope. As in Fig. 2, there is a zero point shift of the
correlation line. Also, the point for m-aminoaniline shows a rather large negative deviation,
a possible result of increased hydrogen bonding for this compound in cyclohexane solution.
The points for o-substituted anilines (apart froin o-fluoro and o-methyl anilines) show
large negative deviations but they appear to parallel the correlation line for the m- and
p-substituents. The reason for this will become clearer from the discussion below.
4.Ortho-S,zibstitutedA nilines
In Fig. 2, the o-substituted anilines fall, on an average, about 0.50 p.p.111. below the
correlation line for the m- and p-substituted anilines. Although the scatter is considerable,
YONEMOTO ET AL.: PROTON SHIFTS 2673
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F^"FIG.3. The chemical shift in p.p.m. relative to aniline of the amino protons in substituted anilines is
plotted versus the available ApK, values of the corresponding anilinium ions in aqueous acid; ApK, is relative
t o t h e pK, value of aniliniu~n
ion.
we consider that two parallel lines can be drawn, one for m- and 9-substituents and one for
the o-substituents (with the exceptions of o-methyl and o-fluoro which are discussed later).
A number of possible reasons for this low-field shift suggest themselves. These are a
steric hindrance to n-overlap of the amino group with the ring, the magnetic anisotropy of
For personal use only.
the substituent (IS, 19), the electric field of the substituent dipole (20), and an intra-
molecular van der Waals or dispersion interaction (21). These will now be discussed in
order.
(a) Steric Hindrance to n-overlap
Electronic absorption spectra (22) and 13Cchemical shifts (23) of substituted anilines
agree that even bulkier substituents than those under consideration do not cause steric
hindrance to n-overlap of the amino group with the ring. In any event, steric hindrance of
this nature would lead to high-field shifts of the amino protons since less charge would be
delocalized from the amino group into the aromatic nucleus.
(b) DIagnetic Anisotropy and Electric Field Efects
We have carried out calculations for both these effects, using the point dipole approxi-
mation. The geometry of the systenl was assumed to be planar with H-N-H angles of
120°, and the mean of the calculated shifts for the two protons was examined. Because of
the unfavorable angle between the internuclear vector and the C-X bond, reasonable
values of the magnetic anisotropy of the C-X bond lead to quite small shifts (but see the
discussion of the cyano group below). In the dipole approximation, the electric field effect
of the bond dipole moments (24) dominates. Using the recommended coefficients for the
N-H bond (17) in Buckingham's equation (20), low-field shifts are calculated which
decrease in the order F > C1 > Br > I, opposite to the observed order of shifts. This is
so whether the dipole is placed midway along the C-X bond or on the at0111 X . We do
not reproduce a table of the calculations here since they do not account for the observed
shifts.
I t is perhaps possible that a judicious choice of anisotropy and bond dipole moments,
together with a variation in the location of the dipoles, ~vould lead to agreement with the
observed shifts for some of the substituents (see cyano group below). The significance of
such an agreement is doubtful. Martin and Dailey (25) point out similar difficulties in
accounting for the o-proton shifts in substituted benzenes. A further relevant point these
3674 CANADIAN JOURNAL O F O.
CHEMISTRY. V L 43, 1965
authors make is that the electric dipole field of the substituent does not contribute
significantly to m- or @-protonchemical shifts in these molecules.
(c) Dispersion Interactions
There is considerable overlap of the van der Waals radii of the aillino proton and the
heavier halogen substituents. Such an overlap would be expected to lead to low-field
shifts of the amino protons because of the fluctuating electric fields arising froin the
instantaneous dipoles in the halogen atoms (effect proportional to the mean square of
these fields (26)). However, the methoxy group causes much the same effect as the heavier
halogens and, furthermore, the latter cause nearly the same shift. The similar shielding
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for broino and chloro substituents could possibly be rationalized as follows. The hydrogen-
iodine distance is so small (about 2.5 A compared to the suill of the van der FVaals radii of
1.2 + 2.15 = 3.35 A) that a compensating high-field shift arises from the magnetic
anisotropy of the iodine, just as the proton in hydrogen iodide is more shielded than in
hydrogen bromide (27). This argument not only serves to show the unreliability of the
point dipole approxiination in calculating inagnetic anisotropy effects a t short distances,
but also illustrates the tenuousness of any explanation of such low-field shifts in terins of
long-range models of electric field and magnetic anisotropy; or dispersion effects. A recent
paper from this laboratory (28) concerning the o-proton and o-fluorine shifts in ethylenes,
propylenes, benzenes, and perfluorobenzenes throws soine doubt on their explanation in
terms of a single one of these effects.
For personal use only.
(d) Weak Hydrogen Bonding
I t is well known that hydrogen bonding causes a shift to low field of the proton involved.
As originally suggested (27) it is probably a combination of the electric field and inagnetic
anisotropy effects of the proton acceptor. A recent calculation of the shift of the chloro-
form proton caused by weak hydrogen bonds to nitrogen bases illustrates the difficulties
involved in the calculation (27a). I t turns out that the nitrogen lone pair is responsible
for a t least 95% of the electric field a t the proton in a linear intermolecular hydrogen bond.
The point dipole approxi~nation therefore useless. The calculation also illustrates that
is
the magnetic anisotropy effect is much less than the electric field effect, even for the cyano
group.
In our compounds, we consider it most sensible to discuss the ortho shifts qualitatively
in terins of an interaction of the N-H bond with the substituent lone pairs, which we
prefer to call weak hydrogen bonding as opposed to the strong hydrogen bond found in
2-amino-5-chloro-benzophenone (see below). The plausibility of this approach is supported
by the fact that the low-field shift for the o-substituents in Fig. 2 increases as a becomes
more positive, i.e., as the N-H bond becomes more acidic its interaction with the
substituent increases; as would be expected for hydrogen bonding.
Further support comes froin the H-N-H bond angles calculated from the frequencies
of the symmetric and antisymmetric stretching bonds of these coinpounds (14). In Fig. 4,
Av is plotted versus the H-N-H angle. With the exception of the o-trifluoromethyl and
o-cyano groups (discussed below) there is a roughly parallel change in the expected
direction of Av with the H-N-H angle. The enhanced H-N-H angle on ortho sub-
stitution has been attributed to intramolecular hydrogen bonding (14).
Finally, the splitting of the first overtone of the N-H symmetric stretching band in
o-substituted anilines has been interpreted in terms of a double-minimum potential for the
hydrogen bonded proton (29). This is further support for the above interpretation of the
low-field shifts.
YONEMOTO ET AL.: PROTON SHIFTS 2675
A\A
\\
\\
'
\ A A
o para
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A
o meta or meta,para A ,-,5 \
A ortho \
-10
. l
109"
1
10
1'
I
Ill0 12
1' 13
1"
I
14
1'
b2-CN
115'
I
1'
16
H-N-H ANGLE
FIG.4. The chemical shift in p.p.m. relative to aniline of the amino protons in substituted anilines is
plotted versus the H-N-H angle derived from the symmetric and antisymmetric stretching frequencies
in ref. 14.
From the viewpoint of T-electron density on the nitrogen atom it is important to realize
that the range in A V of about 1 p.p.m. in Fig. 4 should not be taken as representing a
For personal use only.
proportionate change in the former. The electron density changes, as discussed above,
\vould be reasonably expected to account only for about 0.3 p.p.m. in Av. I t is true that as
the H-N-H angle increases towards 120" the T-overlap between the amino group and
the ring will increase but not to the extent that a naive interpretation of the Av values
u~ould indicate. The excess low-field shift for the o-substituted anilines is probably caused
by the perturbation of the electron density near the proton by the substituent lone pairs.
and
(e) The o-Fl~roro o-Methyl Szlbstitztents
In Fig. 2, the amino proton shift of o-fluoroaniline does not deviate far from the straight
line. This is to be expected if, as indicated by infrared data (14), the small size of the
fluorine atom prevents the close approach of the hydrogen atom to its lone pair orbitals.
The amino proton shifts of the compounds containing an o-methyl group do not indicate a
large anisotropy of the C-C bond (30), unless the anisotropy of the C-H bonds leads
to a fortuitous cancellation of the C-C bond anisotropy.
(fl o-Cyanoaniline
In Figs. 2 and 4, the Av value deviates strongly to low field from all regular trends. In
this particular case, the magnetic anisotropy of the C-X bond is rather large. If an
experimental (31) and theoretical value (32) for the triple bond of about 20 X
cin3/mole is used, one calculates a low-field shift of about 0.33 p.p.m. for the planar
geoilletry assumed above. This agrees roughly with the excess low-field shifts in Figs. 2
and 4, i.e., from the lines drawn for o-substituents in Fig. 2 and the line in Fig. 4.
(g) 0-Trifiz~oronzelhylaniline
The Av value falls roughly on the lower line in Fig. 2. This is not unexpected since the
lone pairs of the fluorine atoms are now considerably closer to the hydrogen atom than in
o-fluoroaniline. In Fig. 4, however, the Av value deviates strongly from the expected value.
The deviation is easily rationalized in terins of the geometry of the molecule. The inter-
action of the hydrogen atom and the fluorine atom in the trifluoromethyl group, although
stronger than in o-fluoroaniline, need not, because of their relative positions lead to an
2676 CANADIAN JOURNAL O F O.
CHEMISTRY. V L 43, 1965
enhancement of the H-N-H angle; but the interaction can lead t o a considerably
increased low-field shift because the lone pairs are now closer to the hydrogen atom.
(h) 2,6-Dichloro- and Dibromoanilines
The low-field shift in these two coinpounds is roughly twice that of the mono-substituted
compounds.This is reasonable in that both amino protons now interact with the substituent,
whereas proton exchange leads to an averaging of hydrogen bonded and unbonded shifts
for the monosubstituted compounds.
5. Ring Current Shifts
As the H-N-H angle increases the possibility arises of a change in ring current con-
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tributions to the amino proton shift (33). T o estimate this change we have calculated the
ring current contributions to the N-H proton shift for two extreme orientations of the
amino group with respect to the plane of the ring: planar, and N-H bonds perpendicular
to the ring. Using the procedure recommended by Dailey (34) we find that, a t most, the
protons in the perpendicular form would be shifted -0.1 p.p.in. relative to protons in the
planar form (the adverse angle is more than compensated for by the decrease in ring-
proton distance). However, since the H-N-H angle appears to vary between 109" and
115" in our compounds the changes in ring current shifts will amount to less than 0.05
p.p.in., which can be disregarded in our discussion.
6. Strong Hydrogen Bonding
The one compound in Table I expected to form a strong intramolecular hydrogen bond
For personal use only.
is 2-amino-5-chlorobenzophenone. Here the hydrogen bond to the carbonyl group forins
part of a six-membered ring and would be expected to be strong. In agreement, we find a
Av of -2.65 p.p.m. The H-N-H angle in this compound has not been calculated froin
the vibrational bands (14), but the splitting of the overtone of the syminetric stretching
band has been measured for 2-aminobenzophenone (29). From a plot of this splitting versus
H-N-H angle in other coinpounds we deduce an approximate value of 127" for 2-amino-
5-chlorobenzophenone.* Froin Fig. 4 a predicted shift of 2.50 f 0.33 p.p.n~.follows,
assuming the linearity of the plot. I t is, however, to be noted t h a t the carbonyl bond and
the other ring may contribute t o this shift to an unknown extent.
CONCLUSIONS
The small range of the chemical shift of the amino protons of m- and p-substituted
anilines in cyclohexane can be interpreted in terms of small changes in the T-electron
density on the nitrogen atom. The much larger shifts in the o-substituted anilines are a t
present best interpreted in terms of weak hydrogen bonding of the amino protons to the
substituent. The shifts can be used to predict pR, values of the anilinium ions but for
this purpose it is best to use a polar solvent.
We are very grateful to the National Research Council for financial support.
REFERENCES
1. H. SPIESECI~E W. G. SCHNEIDER.T. Chem. Phvs. 35.731 (1961).
and
2. R. E. ICLINCK J. B. STOTHERS.can. J. Chern. 40,1071 (1962). '
and
3. R. J. OUELLETTE.Can. J. Chem. 43,707 (1965).
4. C. HEATHCOTE.Can. J. Chern. 40,1565 (1962).
*2-Aminoacetopheno?ze has a n H-N-H i
angle of 133" (14) and the splitting of itsfirst overto~ze s greater than
for 2-aminobenzopltenone (19).
YONEMOTO ET AL.: PKOTON SHIFTS
5. W. I;. REYNOLDS.Ph.D. thesis, University of Manitoba. 1963.
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