Solid-State NMR Characterization of 13C- and 15N-Labeled Phthalimides as Model Compounds for Studying Polyimides

Article abstract of DOI:10.1006/jmra.1995.0004

15N CSA spectra were acquired for four isotopically labeled bisphthalimide compounds.The 15N CSA tensor elements (δ₁₁, δ₂₂, and δ₃₃) for the labeled derivatives were determined from CSA lineshape analysis.In addition, the CSA spectrum for the 15N, 13C-labeled N-methylphthalimide was obtained.From CSA lineshape analysis, it was determined that δ₃₃ lies approximately along the N-C bond axis, with δ₂₂ assumed to be approximately perpendicular to the imide plane.The angles α and β were found to be 90 deg and 7 deg, respectively.The dipolar coupling constant for the 13C-15N bond was 883 Hz, corresponding to a bond length of 1.51 Angstroem.

Full text of DOI:10.1006/jmra.1995.0004

JOURNAL OF MAGNETIC RESONANCE, Series A 116, 156–160 (1995)
13
15
Solid-State NMR Characterization of C- and N-Labeled
Phthalimides as Model Compounds for Studying Polyimides
W
ILLIAM L. JARRETT, C. GREG JOHNSON, AND LON J. MATHIAS*
University of Southern Mississippi, Department of Polymer Science, Hattiesburg, Mississippi 39406-0076
Received November 14, 1994; revised April 3, 1995
15N CSA spectra were acquired for four isotopically labeled
bisphthalimide compounds. The 15N CSA tensor elements (d11
d22 , and d33 ) for the labeled derivatives were determined from CSA
lineshape analysis. In addition, the CSA spectrum for the 15N, 13C-
labeled N-methylphthalimide was obtained. From CSA lineshape
analysis, it was determined that d33 lies approximately along the
N –C bond axis, with d22 assumed to be approximately perpendicu-
Tensor evaluations of biological polymers are usually
based on labeled-model-compound studies; thus, a series of
labeled model imide compounds based on the phthalimide
,
moiety (C6H4 (CO)2N*–R with R Å 0H,
state 15N NMR spectroscopy. In addition, N-[13C]methyl-
[15N]phthalimide was synthesized and its solid-state 15N
NMR spectrum acquired in order to obtain information on
0
CH3 ,
0
C16H33 ,
0
CH2C6H5 ) have been synthesized and analyzed by solid-
lar to the imide plane. The angles
a and b were found to be 90Њ
and 7Њ, respectively. The dipolar coupling constant for the 13C –
15
the orientation of the d tensor with respect to the local molec-
˚
N bond was 883 Hz, corresponding to a bond length of 1.51 A.
ular geometry, with the eventual purpose of this study to
᭧ 1995 Academic Press, Inc.
apply this information to 15N-labeled polyimides.
INTRODUCTION
EXPERIMENTAL
High-resolution solution and solid-state NMR techniques
have been used for a number of years to characterize biologi-
cal and polymeric systems. Until the past 10–15 years, how-
ever, techniques which only observe the isotropic chemical-
shift (diso ) value were utilized, thereby ignoring the valuable
NMR Measurements
CP/MAS spectra. Solid-state CP/MAS NMR spectra
were obtained on a Bruker MSL-200 equipped with a Bruker
MAS probe operating at 50.32 and 20.287 MHz for 13C and
15N, respectively. Powdered samples were placed in fused
zirconia rotors fitted with Kel-F caps and spun with dry air,
with rotation rates of 4 to 5 kHz for carbon MAS and 2 to
3 kHz for nitrogen MAS. The 13C spectra were referenced
to external adamantane (29.5 ppm), while 15N spectra were
referenced downfield from external glycine (0 ppm). The
information that may be obtained from the
d tensor, which
is sensitive to the structural and chemical environment of the
observed site. Recently, with the advent of isotopic labeling
techniques, researchers have been able to determine the prin-
cipal components of the
d tensor (d11 , d22 , and d33 ) for a
variety of 13C- and 15N-labeled organic compounds (1). In
addition, the use of dipole-coupled solid-state NMR spec-
troscopy to determine the orientation of the d33 tensor ele-
ment with respect to the dipole coupling axis has been used
on peptides and polypeptides (2–6), nitrosobenzenes (7),
oximes (8), and acetanilides (9). A two-dimensional varia-
tion of this technique utilizing the 15N– 1H dipolar interaction
has been used to study the chain conformation and dynamics
for oriented proteins (10). Although this particular technique
has been used for biological systems, its application to syn-
thetic polymers has been rare. In addition, little has appeared
1H 90 s for 13C acquisition and 6.0
Њ pulse was 4.5 m ms for
15N acquisition, while a mixing pulse of 2 ms and a acquisi-
tion period of 50 ms were used in acquiring spectra for each
nuclei. A recycle delay of 210 s, corresponding to
T1 values was used for observation of both nuclei.
Powder spectra. Solid-state 15N chemical-shift anisot-
ropy (CSA) spectra were acquired with the same probe used
for CP/MAS work except that the spinning rate was set to
zero. For 15N powder spectra, a spin-echo sequence with a
Ç
3–4 1H
tau delay of 200
pulse bleedthrough. In addition, a 90
implemented at the end of acquisition in order to circumvent
ms was used to minimize artifacts due to
regarding the magnitude and orientation of the
d tensor for
Њ
flip-back pulse was
imides, a key component in the polyimide family of semi-
crystalline, heterocyclic polymers possessing excellent phys-
ical properties and thermal stability.
1
1
the long H T1 values. The H 90
Њ
pulse width was
Ç
3.98–
4.05 s, the acquisition time was 50 ms, and the recycle
m
delay was 30 s. The cross-polarization contact time was 5
ms. The number of scans acquired varied from 2560 to 3200.
* To whom correspondence should be addressed.
156
1064-1858/95 $12.00
Copyright
᭧ 1995 by Academic Press, Inc.
All rights of reproduction in any form reserved.

SOLID-STATE SPECTRA OF LABELED PHTHALIMIDES
157
All chemical-shift values are referenced to crystalline gly- where d33 corresponds to the most shielded component of
cine (0 ppm).
the chemical-shift tensor, will be used. If the nucleus being
studied is dipolar-coupled to another spin-12 system,
is given by
v(u, f)
CSA simulations. CSA spectra were simulated using the
POWFIT program developed by Dr. T. G. Oas (11). Fits
were done by performing analyses on either the zeroth or
first derivative of selected regions of the experimental spec-
trum. In addition, the theoretical spectrum was convoluted
with Lorentzian and Gaussian line broadening to reproduce
the natural line broadening present. All simulations were
done on a MicroVax 3100.
Dv(u, f)
v
(
u,
f
)
Å
v
(
u
,
f
)
{
,
{
2
where the splitting due to the dipole coupling interaction
Dv
(
u
,
f
) is
Synthesis
Dv
(
u
,
f
)
All reagents were obtained from Aldrich Chemical Co.
and used without purification. N,N-dimethylformamide
(DMF) was reagent grade and used as received.
A typical procedure for the preparation of an N-substituted
phthalimide is as follows: A clean, dry flask was charged
with equimolar amounts of [15N] potassium phthalimide and
methyl iodide (for the N-methylphthalimide) followed by
enough DMF to give a mixture that was 15–20% solids by
weight. The flask was submerged in a preheated oil bath at
Å
D{1
0
3[sin
b
sin
u
cos(
f
0
a
)
/
cos b cos u
]2 }.
Here D is the dipolar coupling constant (g1g2h/r12 ) and
b
and are the polar and azimuthal angles describing the
a
orientation of the dipolar vector with respect to the principal-
axis system of the chemical-shielding tensor. In making ini-
tial guesses for the values of
a and b, the procedure outlined
by Teng and Cross was utilized (6a).
approximately 60ЊC for 2 to 3 h. The cooled reaction mixture
RESULTS AND DISCUSSION
was poured into rapidly stirring water (10 times reaction
volume). The precipitate was collected and recrystallized
from aqueous ethanol (70 to 85% yield). For the doubly
labeled N-[13C]-methyl-[15N]-phthalimide compound, the
above procedure was followed except that 13C-labeled
methyl iodide was used.
15N-Labeled Model Imides CSA Spectra
Figure 1 shows the CSA spectra for the four model imides
along with the spectral simulations used to determine the
values of d11 , d22 , and d33 ; these values are summarized in
Table 1. The calculated spectra fit well to the experimental
data, with some variation probably due to anisotropic T2
and cross-polarization effects (11). All the N-substituted
THEORETICAL BACKGROUND
phthalimides have approximately the same width (d11
of 120 ppm. However, there are substantial changes in the
isotropic shift diso , mainly due to the d22 tensor element. This
0
d33 )
The theory of NMR powder spectra has been given
elsewhere (12) and will be only outlined here. The powder
spectrum for a nucleus undergoing a particular transition is
given by (11)
Ç
is also reflected in the rather large differences in
systems.
h for these
p
2p
Si (
v
)
Å
Pi gi,u,f
(
v
,
vi
(
u
,
f
)) sin
u
Ì
u f,
Ì
͐ ͐
Doubly Labeled N-methylphthalimide
uÅ0 fÅ0
Figure 2 shows the experimental and simulated CSA spec-
tra for 15N– 13C labeled N-methylphthalimide. The CSA
spectrum is modulated by both the magnitude of the 15N–
13C dipolar coupling and its orientation with respect to the
where Pi is the probability of the transition, gi is the lines-
hape function, and and are the polar and azimuthal
angles describing the orientation of the magnetic field with
respect to some molecular frame of reference. Pi and gi
are usually assumed to be orientation independent. For the
u
f
d
axis system. In the simulation, the values of d11
, d22 , and
d33 were initially held fixed and D, , and were allowed
a
b
chemical-shift interaction,
itly:
v(u, f) can be expressed explic-
to vary. After these parameters were optimized, the chemi-
cal-shift-tensor elements were then permitted to vary in order
to get the best fit to the spectrum. Table 2 gives the best fit
v
(
u,
f
)
Å
s11sin2
u
cos2
f
/
s22sin2
u
sin2
f
/
s33cos2
u.
values used in Fig. 2. From the values of
orientation of the N–CH3 bond vector with respect to the
s22 , and s33 are the principal components of the axis system was determined; this is shown in Fig. 3. Al-
chemical-shielding tensor (s33 s22 s11 ). Experimen- though the orientation of d33 with respect to the molecular
tally, the chemical-shift-tensor elements d11 d22 , and d33 are structure cannot be determined strictly from the data pre-
measured, and in this paper the convention d11 d22 d33
sented here, from previous work performed on sp2 ring nitro-
a and b, the
d
Here s11
,
§
§
,
§
§
,

158
JARRETT, JOHNSON, AND MATHIAS
FIG. 2. Experimental and simulation 15N CSA spectra of doubly labeled
N-methylphthalimide. The experimental data are represented by open cir-
cles, the spectral simulation by a solid line. The raw data were processed
in the same manner as the spectra in Fig. 1.
FIG. 1. 15N CSA spectra of (a) phthalimide, (b) N-methylphthalimide,
(c) N-stearylimide, and (d) N-benzylphthalimide. All spectra were apodized
using a Kaiser digital filter ( 4, N 3–5 ms) and zero-filled to 8092
data points before Fourier transformation. Experimental data are represented
by open circles, spectral simulation by a solid line.
a
Å
Å
stant, the length of the 15N– 13C bond is calculated to be
˚
1.51 A. The X-ray structure for N-methylphthalimide is not
gens, the d33 tensor was found to be perpendicular to the
ring (13, 14). Accordingly, we are tentatively placing d33
perpendicular to the imide plane. Figure 4 shows the orienta-
tion of the principal components of the chemical-shift tensor.
From the magnitude of the 15N– 13C dipolar coupling con-
known, although the calculated bond length at the AM1
level is 1.45 A (15). The N–C bond distance for a related
compound, bis(N-hexylphthalimide) was determined via
˚
˚
single-crystal X-ray methods to be 1.44
{
0.01 A (16).
Since NMR spectroscopy and X-ray diffraction measure dif-
ferent vibrational averages ( ), the exper-
»
r03 01/3 versus
…
»r…
imental result is in good agreement with what is expected
for this system.
TABLE 1
15N Chemical-Shift Data for 15N-Labeled Model Imidesa
(exptal)b
iso
(calc)c
iso
d
d
d
Compounda
d
d
d11
d22
d33
he
TABLE 2
15N Chemical-Shift Parameters for N-[13C]-methyl[15N]-
phthalimidea
Phthalimide
121.5
119.8
128.9
134.2
121
119
128
135
165
177
172
179
136
125
157
165
62
55
55
60
0.492
0.813
0.274
0.189
N-methylphthalimide
N-stearylphthalimide
N-benzylphthalimide
d11
d22
d33
a
b
D
a All data reported in parts per million and referenced to glycine (diso
179
125
55
7
90
883
Å
0.0 ppm).
b Based on 15N CP/MAS spectral analysis.
a All values for the chemical-shift tensor are parts per million. The values
and are referenced with respect to the N CH3 bond axis. The value
of the dipolar coupling constant D is in hertz. Uncertainties are 3.0 ppm
for for , and for , respectively.
1
c Determined by
(
is
d11
/
d22
3.0 ppm.
diso).
/
d33).
of
a
b
{
3
d Uncertainty in
d
{
{
e h
Å
(
d22
0
d11)/(d33
0
d
,
{
5Њ
a
{
2Њ
b

SOLID-STATE SPECTRA OF LABELED PHTHALIMIDES
159
Discussion
The invariance of the d33 tensor indicates that the aroma-
ticity of the imide ring is fairly consistent for the substituents
given here. Instead, the largest changes appear in the d11 and
d22 tensor elements. For d11 , the changes in substituent would
correspond to differences in the N–R
s bond (d11 varies 14
ppm from phthalimide to N-benzylphthalimide). However,
it is not clear how the different substituents effect the d22
tensor element. Shoji and co-workers have shown that, in
the case of polypeptides, d22 was quite sensitive to molecular
composition and conformation, due to changes in the hydro-
gen bonding (17). Hiyama et al. have observed different
15N CSA spectra for two distinct crystal forms of Boc-glycyl-
glycylglycine benzyl ester, an effect attributed to a nonplanar
peptide bond for one of the crystal forms (5).
The orientation of the nitrogen chemical-shift-tensor ele-
ments with respect to the nitrogen–carbonyl plane for N-meth-
ylphthalimide is quite different compared to their orientation
for the amide linkage in polypeptides (2–6), including the
proline ring in Boc-l-alanyl-l-proline (4). In particular, it is
interesting to note how the d22 changes orientation from being
perpendicular to the amide plane for peptides to being in the
imide plane for the compound studied here as well as for l-
histidine (13). This change in tensor orientation probably
reflects the difference between the partial double-bond char-
acter for amides and the more delocalized bonding nature
for imides. Currently, computational and synthetic efforts
are in progress in order to further investigate the nature of
the bonding about the 15N site for imide systems.
FIG. 4. Orientation for the chemical-shift tensor with respect to the
imide plane. Here the terminology of Teng et al. (6a) has been used.
However, for imides, the tensor element perpendicular to the plane dcc
corresponds to d33 , while the tensor element along the N–C bond daa corre-
sponds to d11
.
as model compounds for studying polyimides. From solid-
state 15N NMR spectroscopic and computational methods,
the values of the
d shift tensor for the model imides were
determined. In addition, for N-[13C]-methyl-[15N]-phthali-
mide, the d11 tensor was found to lie approximately along
the N–CH3 bond axis, with the d33 tensor element perpendic-
ular to the imide plane. From the magnitude of the 15N– 13C
dipolar coupling, the N–CH3 bond length was determined
SUMMARY
˚
to be 1.51 A. Future work concerning the determination of
the tensor orientation with respect to the molecular frame
is in progress.
A procedure has been developed for incorporating 15N and
d
13C isotopic labeling into N-substituted phthalimides, for use
ACKNOWLEDGMENTS
The authors thank the Office of Naval Research for providing funds for
the purchase of the MSL-200 spectrometer and for a grant providing partial
support of this research and Dr. Terry Oas for providing the POWFIT
program.
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d axis

160
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