Estimation of Charge Density on Nitrogen in Amides
by Measurement of One-Bond Carbon-Hydrogen Nuclear Coupling
Constants in N - C H 3 Group* **
Paul H a a k e
Department of Molecular Biology and Biochemistry, Wesleyan University, Middletown,
Connecticut 06459, USA
Donald A. Tyssee
Department of Chemistry, UCLA, Los Angeles, CA 90024, USA
Z. Naturforsch. 48a, 5 8 - 6 2 (1993); received November 26, 1991
One-bond, 1 3 C - X H coupling constants, J j ( C - H ) , in amines, ammonium ions, and carboxylic
amides correlate with structure and support the concept that the value of J ^ C - H ) is related to the
charge density on the nitrogen atom; for example, amine oxides have nearly the same charge density
at nitrogen as does the tetramethylammonium ion. The J j ( C - H ) values for methyls bonded to
nitrogen in various amides then give an experimental estimate of the charge density at the nitrogen
atom that enables an estimate of the bond order in the C - N amide-bond; the data suggest that
carboxylic amides have a C - N bond order of about 1.35, that sulfonamides have an S - N bond
order of about 1.45, and that phosphinamides, R 2 P ( 0 ) N ( C H 3 ) 2 , have a P - N bond order of about
1.3. In contrast, aminephosphines have a P - N single bond. The value for carboxylic amides is in
reasonable agreement with bond distances in amides.
Key words: Amides; Amine oxides; Charge density at nitrogen; Sulfonamides; Phosphinamides.
Introduction
O u r research with carbon-hydrogen coupling constants through one bond, J ^ C - H ) [1], is based upon
the pioneering work by Lauterbur [2], Muller and
Pritchard [3,4] and Shoolery [5], In particular, Muller
and Pritchard noted interesting effects on the magnitude of these couplings for methyl groups bonded to
various heteroatoms [3, 4], dominance of the Fermi
contact term in J ^ C - H ) , and experimental data in
support of a direct relationship between J j ( C - H ) and
percent s-character in the carbon atomic orbital [3-6].
Although there have been some doubts expressed
a b o u t this simple view of C - H couplings [7, 8] the
pattern that has emerged [9] supports the early conclusions [ 3 - 5 ] . Support for this view comes from such
applications as the correlation of J x ( C - H ) with struc-
* Presented at the Sagamore X Conference on Charge, Spin
and Momentum Densities, Konstanz, Fed. Rep. of Germany,
September 1 - 7 , 1991.
** Research supported in part by the National Science Foundation.
Reprint requests to Prof. Dr. Paul Haake, Department of
Molecular Biology and Biochemistry, Wesleyan University,
Middletown, CT 06459, USA.
ture in strained systems [10,11] and with the acidity of
C - H bonds [12, 13],
The research reported here is based on experimental results that indicate an empirical relationship between J t ( C - H ) in methyl groups b o n d e d to nitrogen
atoms in a variety of structures. We find that Jl ( C - H )
increases as the a m o u n t of formal positive charge on
nitrogen increases. This enables insight into charge
density and bonding at the nitrogen atom. The empirical relationship we have found is supported by concepts of bonding [8] and by the direct relationship [4, 5]
between J j ( C - H ) and percent s-character in the carbon AO of the C - H bond, because change in structure and charge on nitrogen must cause a change in
C - N bonding which, in turn, must cause a change in
C - H bonding. F r o m a simple viewpoint, a more positive N atom demands more p-character in the bonding of carbon to N ; this, in turn, causes m o r e s-character in the bonding of carbon to the H atoms, so the
C - H coupling increases [ 3 - 5 , 8, 9]. Although bonding is not localized from an L C A O viewpoint, the
results fit with the valence-bond framework of concepts of structure and bonding. O u r method is an
intramolecular probe, via the methyl group, of the
electropositive nature of the nitrogen atom.
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P. Haake and D. A. Tyssee • Charge Density at Nitrogen and Bond Orders in Amides
Table 1. Coupling constants for pyramidal N c o m p o u n d s a .
Compound
(CH 3 ) 3 N
Solvent
neat
(CH 3 ) 3 NH + CL-
CC14
CF 3 CO 2 H
DOD
(CH 3 ) 4 N + CR
CF3CO2H
DOD
(CH 3 ) 2 NH 2 C1~
CF3CO2H
DOD
CH 3 NH+CP
(CH 3 ) 3 N + -0~
sulfuric acid
sulfuric acid
sulfuric acid
DOD
DOD
CF,CO 2 H
sulfuric acid
+
(CH 3 ) 3 N -OCH 3 CD 3 NO 2
State of compound
uncharged
uncharged
+
(CH 3 ) 3 NH
(CH 3 ) 3 NH +
(CH 3 ) 3 NH +
(CH 3 ) 3 NH +
(CH 3 ) 4 N +
(CH 3 ) 4 N +
(CH 3 ) 4 N +
(CH 3 ) 2 NH 2
(CH 3 ) 2 NH 2
CH 3 NH+
(CH 3 ) 3 N + -O(CH 3 ) 3 N + -OH
(CH 3 ) 3 N + -OH
(CH 3 ) 3 N + -OCH 3
•A ( C - H )
(Hz)
131.7
131.9
144.1
143.6
144.6
144.2
144.2
144.5
144.5
143.3
144.8
143.6
143.6
146.3
146.1
146.1
Table 2. Coupling constants for trigonal N c o m p o u n d s including carboxylic amides.
Compound
Solvent
State of
compound
7, ( C - H ) Bond
(Hz)
order a
C 6 H 5 CH = NCH 3
CH 3 NO 2
neat
neat
CF 3 CO 2 H
neat
DOD
sulfuric acid
uncharged
neutral
neutral
neutral
H-bonded
protonated
133.4
146.7
146.7
138.1
139.0
144.1
1.38
1.45
1.86
CH3C(0)N(CH3)2
neat
CF 3 CO 2 H
neutral
137.5
141.1
1.33
1.62
C6H5C(0)N(CH3)2
neutral
CC14
sulfuric acid protonated
phosphoryPOCl 3
lated
137.6
144.3
146.3
1.34
1.87
20.3
HC(0)N(CH3)2
(dimethylformamide)
a
The bond order refers to the C - N amide b o n d and is
determined by the equation
Bond order = 1 -(- [ ( ^ ( C - H ) - 133.4 H z ) / 1 2 . 5 H z ] .
a
In some cases, the N a t o m is exactly tetrahedral, and in all
these c o m p o u n d s the N atom has bond angles within a few
degrees of tetrahedral.
The value of 133.4 Hz comes from the top entry in this table,
and the value of 12.5 Hz is the difference between trimethylamine and tetramethylammonium ion (Table 1); see the discussion in the text.
Results and Discussion
in converting an attached c a r b o n f r o m sp 3 to sp 2 , as
we find for nitrogen by comparing ( C H 3 ) 3 N and
C6H5CH=NCH3.
Effect of Charge on Nitrogen. The Jx ( C - H ) value
of a methyl g r o u p on a positive nitrogen in the sp 3 state is 144.2 Hz for ( C H 3 ) 4 N + in D 2 0 (Table 1); that
is, there is a 12.5 Hz increase in coupling constant on
quaternizing trimethylamine. Other d a t a in Table 1
support this value: note the values for trimethylamine
in trifluoroacetic acid, t r i m e t h y l a m m o n i u m ion, and
tetramethylammonium ion. There is significant deviation f r o m this value only when there are strong solvation effects on trimethylammonium ion in D 2 0 or
sulfuric acid and, moreover, those solvation effects
are in the expected direction and they are n o t large.
The values for trimethylamine oxide also s u p p o r t this
number. Neutral trimethylamine oxide with a coupling
of 143.6 H z is only marginally lower t h a n 144.2 Hz
for the tetramethylammonium ion; as expected, when
trimethylamine oxide is p r o t o n a t e d , there is a significant increase in coupling.
The d a t a in Tables 1 and 2 indicate that ^ ( C - H )
correlates with charge at the nitrogen a t o m . In
N ( C H 3 ) 3 , the ^ ( C - H ) coupling is 131.8 Hz (Table 1).
When nitrogen is quaternized, the coupling increases
to a b o u t 144 Hz a n d remains very close to this value
regardless of concentration, the n a t u r e of the counterion, or other perturbations. O n the other h a n d , it is
possible to p e r t u r b the coupling; for example, trimethyl a m m o n i u m ion in D 2 0 has a coupling a b o u t
1 Hz less than in sulfuric acid, presumably owing to
hydrogen bonding of D 2 0 to the N H hydrogen.
Trigonal Nitrogen (see Table 2). The J1 ( C - H ) values for N - C H 3 groups change f r o m 131.7 H z f o r
N ( C H 3 ) 3 to 133.4 H z f o r C 6 H 5 C H = N C H 3 . T h e
higher coupling constant for the sp 2 -state of N is
explained by considering the increase in the electronegativity of the nitrogen, as the a m o u n t of s-character in the bonding atomic orbital of nitrogen to the
methyl is increased; that is, the sp 2 -orbital of planar,
trigonal N is m o r e electron-withdrawing t h a n the sp 3 orbital of pyramidal N , because the electron density in
the sp 2 -orbital is closer to the N a t o m [8]. O u r findings are consistent with the increase of 2 H z in the
7 1 ( C - H ) of the methyl groups of ( C H 3 ) 2 C = C H 2
compared to neopentane [4]; that is, the effect on
^ ( C - H ) in the attached methyl groups is the same
The J ! ( C - H ) value of a methyl g r o u p on a positive
nitrogen in the sp 2 -state can be estimated f r o m the
value for neutral nitrogen, 133.4 Hz for C 6 H 5 C H =
= N C H 3 , and the increase in coupling when N is
quaternized, 12.5 Hz, as discussed in the previous
p a r a g r a p h . The result, 145.9 Hz, is supported by the
coupling for nitromethane: 146.3 Hz. Because of the
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P. Haake and D. A. Tyssee • Charge Density at Nitrogen and Bond Orders in Amides 72
special b o n d i n g in n i t r o m e t h a n e , we prefer to use
12.5 H z , the value f r o m p y r a m i d a l N , for the effect of
conversion of a n e u t r a l N c o m p o u n d to the cation.
Carboxylic Amides. T h e N a t o m is trigonal in carboxylic amides [13, 14]. T h e r e f o r e we use a base value
of Jl ( C - H ) = 133.4 H z for neutral, trigonal nitrogen.
Table 2 d e m o n s t r a t e s t h a t neutral amides have couplings considerably greater t h a n this value. This suggests t h a t J1 ( C - H ) values might be used to estimate
the a m o u n t of positive charge on N due to electron
d e r e a l i z a t i o n . F o r a full positive charge on N, the
coupling should increase by a b o u t 12.5 H z (previous
p a r a g r a p h ) . This enables an estimate of the a m o u n t of
positive charge o n nitrogen in the neutral carboxylic
amides in Table 2 using the e q u a t i o n
% N + contributor
(1)
= [ ^ ( C - H ) observed —Jx ( C - H ) for neutral N]/12.5 Hz,
which is based o n the discussion in the previous p a r a graphs.
T h e f o r m a m i d e coupling of 138.4 H z leads to a n
estimate of % N + : (138.1 - 133.4)/12.5 = 3 8 % N + .
Similarly, the couplings for dimethylacetamide a n d
d i m e t h y l b e n z a m i d e give 3 3 % a n d 3 4 % N + . The average value for the three c o m p o u n d s is 3 5 % , leading to
a C - N b o n d o r d e r of 1.35.
T h e structural research on carboxylic amides [14,
15] d e m o n s t r a t e s t h a t the N a t o m is trigonal p l a n a r
(sp 2 ), a n d the b o n d lengths indicate contributions to
structure of a b o u t 4 0 % f r o m the c o n t r i b u t o r with a
C = N d o u b l e b o n d a n d a positive charge on nitrogen.
O u r results are in reasonable agreement with these
structural studies. In fact, since the s t a n d a r d b o n d
lengths for single a n d d o u b l e C - N b o n d s in the sp 2 state are difficult to establish, a n d since the dipolar
c h a r a c t e r of amides m a y lead to b o n d contraction
beyond the a m o u n t induced by 7r-bonding, the ^ ( C - H )
values m a y be a better guide to b o n d order t h a n the
observed b o n d lengths. Rotational barriers in amides
also correlate with the b o n d order of a b o u t 3 5 % calculated f r o m J ^ ( C - H ) [16].
T h e addition of D 2 0 to an amide increases ^ ( C - H ) .
F o r example, the coupling c o n s t a n t s of the N-methyl
g r o u p s of N , N - d i m e t h y l f o r m a m i d e a n d N , N - d i m e t h y l m e t h a n e s u l f o n a m i d e increase by 0.9 H z a n d
0.8 H z when D 2 0 is used as solvent. This observation
is consistent with h y d r o g e n b o n d i n g of D 2 0 to the
oxygen a t o m a n d , as a result, increased charge on
nitrogen.
W h e n the amides are p r o t o n a t e d or p h o s p h o r y lated ( k n o w n to occur at the O a t o m ) , the resulting
b o n d o r d e r s are in r e a s o n a b l e agreement with the
expectation t h a t p r o t o n a t i o n or p h o s p h o r y l a t i o n
occurs on the O a t o m , leading to d o m i n a n c e of the
> C = N + ( C H 3 ) 2 c o n t r i b u t o r to the structure. F o r example, d i m e t h y l f o r m a m i d e in sulfuric acid gives a
coupling only slightly smaller t h a n for the tetramethyla m m o n i u m ion.
Application of J1 (C-H)
to the Structure of Phosphorus and Sulfur Amides. In view of the results o n
carboxylic a m i d e structure in u n c h a r g e d a n d p r o t o nated f o r m s f r o m / j ( C - H ) values, we have applied
o u r m e t h o d to amides where the structure is n o t well
u n d e r s t o o d . T h e d a t a a n d results are in Table 3.
In agreement with expectations, there a p p e a r s to be
a higher b o n d o r d e r in s u l f o n a m i d e s t h a n in sulflnamides. T h e s u l f o n a m i d e s d o n o t a p p e a r to be p r o t o nated in trifluoroacetic acid, based o n the similarity to
the couplings in D 2 0 . In sulfuric acid, the sulfonamides are clearly p r o t o n a t e d a n d the coupling is even
larger t h a n t h a t expected f o r a n a m m o n i u m ion. This
suggests t h a t the N a t o m bears m u c h of the positive
charge in a p r o t o n a t e d s u l f o n a m i d e .
In the p h o s p o r u s amides there is a large difference
between ( C H 3 ) 2 N P ( C 6 H 5 ) 2 a n d ( C H 3 ) 2 N P ( 0 ) ( C 6 H 5 ) 2 .
A bond order of 1.15 for N,N-dimethylaminodiphenylp h o s p h i n e indicates a m i n o r a m o u n t of ^-interaction
of the lone-pair electrons o n nitrogen with the
d-orbitals of p h o s p h o r u s . T h i s is consistent with the
observed nucleophilic b e h a v i o r of these c o m p o u n d s .
It has been s h o w n t h a t the p h o s p h o r u s a t o m is m o r e
nucleophilic t o w a r d v a r i o u s electrophiles t h a n is the
nitrogen a t o m [ 1 7 - 2 0 ] .
Sufficient structural d a t a exist to allow a r o u g h
estimate to be m a d e of the P - N b o n d order in N,N-dimethyldiphenylphosphinamide, ( C H 3 ) 2 N P ( 0 ) ( C 6 H 5 ) 2 ;
the structure of this a m i d e h a s been r e p o r t e d [21] with
a P - N b o n d length of 0.167 n m . T h e length of the
P - N b o n d in the p h o s p h o r a m i d a t e ion, ( H 3 N + - P 0 3 ) ~ ,
is reported to be 0 . 1 7 7 - 0 . 1 7 9 n m [22], whereas in a
p h o s p h o n i t r i l e t e t r a m e r the length of the exocyclic
P - N b o n d s is r e p o r t e d to be 0 . 1 7 5 - 0 . 1 8 4 n m [23].
Taking the value of a single P - N b o n d to be
0.179 n m , t h a t of a P - N b o n d of b o n d o r d e r 0.15 to
be 0.16 n m [24] a n d a s s u m i n g t h a t the relation between b o n d length a n d b o n d o r d e r is linear over this
range, results in a structure-based [21] b o n d order for
the P - N b o n d in N , N - d i m e t h y l d i p h e n y l p h o s p h i n amide of 1.32. T h e b o n d o r d e r of N , N - d i m e t h y l -
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P. Haake and D. A. Tyssee • Charge Density at Nitrogen and Bond Orders in Amides
Amide
Solvent
State of
amide
7i ( C - H )
(Hz)
Bond order a
CH3so2N(CH3)2
CDC1 3
DOD
H2SO4
neutral
H-bonded
protonated
139.3
141.1
147.8
1.47
C6H5S02N(CH3)2
CC14
CF 3 CO 2 H
H2SO4
neutral
protonated
138.8
141.9
148.5
1.43
1.60
«2
Table 3. Coupling constants for sulfur and phosphorus amides.
a
The bond order refers to the S - N
or P - N amide bond and is determined by the equation
«2
Bond order
= 1+1(7, ( C - H )
- 1 3 3 . 4 Hz)/12.5 Hz],
C6H5SON(CH3)2
neat
neutral
137.4
1.32
(C6H5)2PN(CH3)2
neat
neutral
134.4
1.08
(C6H5)2P(0)N(CH3)2
CDC1 3
H2SO4
CF 3 CO 2 H
neutral
protonated
protonated
137.3
145.5
144.8
1.31
P-N+H(CH3)2
H2N+(CH3)2 b
[(CH 3 ) 2 N] 3 PO
neat
neutral
135.4
1.16
d i p h e n y l p h o s p h i n a m i d e determined by the / ^ C - H )
c o u p l i n g c o n s t a n t (1.36) is in g o o d a g r e e m e n t w i t h
this. O u r p r e v i o u s w o r k on p h o s p h o r u s a m i d e s def i n e d t h e p K a n d is c o n s i s t e n t w i t h p r o t o n a t i o n o n N
in b o t h s u l f u r i c a n d t r i f l u o r o a c e t i c a c i d s [25], b u t
a p p a r e n t l y t h e l a b i l i t y in t h e l a t t e r a c i d c a u s e d c l e a v age of the P - N b o n d before the spectrum was taken
( T a b l e 3).
Solvent
Effects.
T h e s o l v e n t e f f e c t s o b s e r v e d in T a -
bles 1 - 3 i n d i c a t e t h a t t h e s e c o u p l i n g s give u s e f u l inf o r m a t i o n even w h e n the effects a r e small. F o r e x a m ple, t h e t r i m e t h y l a m m o n i u m i o n ( T a b l e 1) h a s a l o w e r
c o u p l i n g in D
2
0 t h a n in t r i f l u o r o a c e t i c acid a n d t h e
v a l u e in t r i f l u o r o a c e t r i c acid is l o w e r t h a n in s u l f u r i c
acid, as w o u l d be expected by t h e relative ability of
t h e s e s o l v e n t s t o f u n c t i o n as h y d r o g e n b o n d a c c e p t o r s .
A s i m i l a r e f f e c t is s e e n f o r t h e
dimethylammonium
i o n . I n T a b l e s 2 a n d 3, a m i d e s s h o w t h e e x p e c t e d inc r e a s e in c o u p l i n g d u e t o h y d r o g e n b o n d d o n a t i o n
from D 2 0 .
Experimental
N-Benzylidenemethylamine
was prepared by adding
methylamine (0.26 moles) to benzaldehyde (0.26 moles) in
100 ml of methanol at room temperature. The mixture was
refluxed for 1 - 1 . 5 hours. Distillation gave a product boiling
between 179° and 182 °C (b.p. C 6 H 5 C H O 179 °C; b.p.
C 6 H 5 C H = N C H 3 180 °C. The 'H-nmr spectrum indicated
that approximately 15% of the benzaldehyde remained unreacted.
N,N-Dimethylformamide
was obtained from Matheson,
Coleman and Bell Chemical Company and used without
further purification.
b
Methyls are a doublet indicating
that the P - N bond is intact.
b
c
Methyls are a 1:2:1 triplet indicating that the P - N bond as been
cleaved, generating the dimethylammonium ion which is exchanging slowly.
N.N-Dimethylbenzamide
was prepared by adding an excess of dimethylamine directly to benzoyl chloride at 0 °C in
an open flask. The amide was taken up in acetone and the
amine hydrochloride filtered off. After removal of the acetone by evaporation, the amine was purified by vacuum
distillation: b.p. 115 °C at 0.5 Torr, m.p. 41 °C.
N,N-Dimethylmethanesulfonamide
was prepared by adding
an excess of dimethylamine directly to methanesulfonyl chloride at 0 °C in an open flask. The amide was taken up in
acetone and the amine hydrochloride filtered off. The acetone was removed by evaporation and the amide recrystallized from water, m.p. 4 8 . 5 - 4 9 . 5 °C; reported m.p. 48 °C
[26].
N,N-Dimethylbenzenesulfonamide
was prepared by adding
an excess of dimethylamine directly to benzenesulfonyl
chloride at 0 °C in an open flask. The amide was taken up in
acetone and the amine hydrochloride filtered off. The acetone was removed by evaporation and the amide recrystallized from n-heptane: m.p. 48 °C.
N,N-Dimethylbenzenesulfinamide
was prepared by adding
dimethylamine in diethyl ether directly to benzenesulflnyl
chloride (prepared according to Douglas [27]) with stirring
and dry-ice cooling. The mixture was warmed to room temperature, the amine hydrochloride filtered off, and the ether
removed by vacuum. The amide was purified by vacuum
distillation, b.p. 130 °C at 20 Torr.
N,N-Dimethyldiphenylphosphinamide
was prepared by
adding dimethylamine directly to diphenylphosphinyl chloride
(prepared by oxidation of chlorodiphenylphosphine at icebath temperature). The residue was partially taken up in
aqueous sodium bicarbonate, extracted several times with
carbon tetrachloride, the carbon tetrachloride removed, and
the amide recrystallized from acetone-hexane: m.p. 104 °C
(lit. [28] 1 0 3 - 1 0 5 °C).
Measurement
of JX(C-H).
The C
constants were measured using an
1 3
-H
coupling
audio-oscillator
a n d f r e q u e n c y c o u n t e r a t a s w e e p w i d t h o f 50 o r
100 H z . D u p l i c a t e m e a s u r e m e n t s o n d i f f e r e n t d a y s
with different samples indicated that the data
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62
P. Haake and D. A. Tyssee • Charge Density at Nitrogen and Bond Orders in Amides 62
accurate to ± 0 . 2 H z ; generally, these duplicate determinations were accurate to ± 0 . 1 Hz. In some cases
the coupling was more difficult to determine because
of interference from other peaks or because of peak
broadening; in these cases the error in the reported
coupling may be as high as ± 0 . 3 Hz.
[1] Abbreviations: / j ( C - H ) = 1 3 C - * H nuclear coupling
constant through one bond; AO = atomic orbital.
[2] P. C. Lauterbur, J. Chem. Phys. 26, 217 (1957).
[3] N. Muller and D. E. Pritchard, J. Chem. Phys. 31, 768
(1959).
[4] N. Muller and D. E. Pritchard, J. Chem. Phys. 31, 1471
(1959).
[5] J. N. Shoolery, J. Chem. Phys. 31, 1427 (1959).
[6] G. M. Karabatsos and C. E. Orzech, J. Amer. Chem.
Soc. 86, 3574 (1964).
[7] D. M. Grant and W. Litchman, J. Amer. Chem. Soc. 87,
3994 (1965).
[8] H. A. Bent, Chem. Rev. 61, 275 (1961).
[9] J. H. Goldstein, V. S. Watts, and L. S. Rattet, 1 3 C - H
Satellite N M R Spectra, in: Progress in Nuclear Magnetic Resonance Spectroscopy 8 (2), 1 0 3 - 1 6 2 (1971).
[10] R. A. Alden, J. Kraut, and T. G. Traylor, J. Amer.
Chem. Soc. 90, 74 (1968).
[11] C. S. Foote, Tetrahedron Lett. 1963, 579.
[12] A. Streitwieser, Jr., and W. R. Young, J. Amer. Chem.
Soc. 91, 529 (1969).
[13] P. Haake, L. P. Bausher, and W. B. Miller J. Amer.
Chem. Soc. 91, 1113 (1969).
[14] L. Pauling, in: Symposium on Protein Structure (A.
Newberger, ed.), John Wiley & Sons, Inc., New York
1958.
[15] P. Chakrabarti and J. D. Dunitz, Helv. Chim. Acta 65,
1555 (1982). From the Cambridge Structural Database,
amide bond lengths are 0.1322 nm for primary amides,
0.1331 nm for secondary amides, and 0.1346 nm for
non-cyclic tertiary amides. The amides we have studied
are tertiary. There is also a study of esters: W. B.
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