Benzoxazine oligomers: Evidence for a helical structure from solid-state NMR spectroscopy and DFT-based dynamics and chemical shift calculations

Article abstract of DOI:10.1021/ja029059r

A combination of molecular modeling, DFT calculations, and advanced solid-state NMR experiments is used to elucidate the supramolecular structure of a series of benzoxazine oligomers. Intramolecular hydrogen bonds are characterized and identified as the d

Full text of DOI:10.1021/ja029059r

Published on Web 04/22/2003
Benzoxazine Oligomers: Evidence for a Helical Structure
from Solid-State NMR Spectroscopy and DFT-Based
Dynamics and Chemical Shift Calculations
Gillian R. Goward,†,§ Daniel Sebastiani,† Ingo Schnell,† Hans Wolfgang Spiess,*,†
Ho-Dong Kim,‡ and Hatsuo Ishida‡
Contribution from the Max-Planck-Institut fu¨r Polymerforschung, Postfach 3148,
D-55021 Mainz, Germany, and Department of Chemistry, Case Western ReserVe UniVersity,
10900 Euclid AVenue, CleVeland, Ohio 44106-7078
Received October 23, 2002; E-mail: spiess@mpip-mainz.mpg.de
Abstract: A combination of molecular modeling, DFT calculations, and advanced solid-state NMR
experiments is used to elucidate the supramolecular structure of a series of benzoxazine oligomers.
Intramolecular hydrogen bonds are characterized and identified as the driving forces for ring-shape and
helical conformations of trimeric and tetrameric units. In fast MAS 1H NMR spectra, the resonances of the
protons forming the hydrogen bonds can be assigned and used for validating and refining the structure by
means of DFT-based geometry optimizations and 1H chemical-shift calculations. Also supporting these
proposed structures are homonuclear 1H-1H double-quantum NMR spectra, which identify the local proton-
proton proximities in each material. Additionally, quantitative 15N-1H distance measurements obtained by
analysis of dipolar spinning sideband patterns confirm the optimized geometry of the tetramer. These results
clearly support the predicted helical geometry of the benzoxazine polymer. This geometry, in which the
N‚‚‚H‚‚‚O and O‚‚‚H‚‚‚O hydrogen bonds are protected on the inside of the helix, can account for many of
the exemplary chemical properties of the polybenzoxazine materials. The combination of advanced
experimental solid-state NMR spectroscopy with computational geometry optimizations, total energy, and
NMR spectra calculations is a powerful tool for structural analysis. Its results provide significantly more
confidence than the individual measurements or calculations alone, in particular, because the microscopic
structure of many disordered systems cannot be elucidated by means of conventional methods due to lack
of long-range order.
Introduction
building blocks of supramolecular interactions and show up
clearly in high-resolution solid-state 1H NMR studies.11-13 The
Supramolecular structures often suffer from a lack of long-
range crystallinity, due to the comparatively weak interactions
that determine their structures, for example, hydrogen bonding1-7
and π-stacking.8-10 Solid-state NMR, however, does not require
long-range ordering to provide structural details of these
fascinating systems. Hydrogen bonding and π-stacking are
powerful combination of quantum chemical calculations with
experimental NMR studies has been used previously to elucidate
structural details of other supramolecular systems.14-16 Here we
apply a combination of molecular modeling, density functional
theory (DFT) calculations, and experimental measurements to
obtain insight into the structure of a series of benzoxazine
oligomers and to show that hydrogen bonds have a strong
influence on the structural conformation that is adopted.
Supramolecular structures influenced by self-assembly and
hydrogen bonding are also frequently observed in polymeric
architectures.17-21 Polybenzoxazines are a classic example of
† Max-Planck-Institut fu¨r Polymerforschung.
‡ Case Western Reserve University.
§ Current address: Department of Chemistry, McMaster University,
Hamilton ON, Canada.
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Springer-Verlag: New York, 1991.
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10.1021/ja029059r CCC: $25.00 © 2003 American Chemical Society

Benzoxazine Oligomers
A R T I C L E S
such a structure. Polybenzoxazines exhibit a number of unusual
properties, including low volumetric shrinkage or expansion
upon curing,22 lack of water absorption and excellent resistance
to chemicals and UV light,23,24 as well as surprisingly high Tg
values given the low cross-linking density,25 all of which make
them attractive candidates for many commercial applications.
These properties have been attributed to the unique hydrogen-
bonding structure found in these materials, and therefore this
property has been the focus of recent investigation. In particular,
extensive crystallographic, NMR, and IR studies of the dimer
precursors have provided much insight.26,27 However, the
benzoxazine dimers organize themselves in a highly regular,
paired lattice; hence the model cannot be extended to account
for the unique properties of the parent polymer. Therefore,
synthetic efforts were made to procure other oligomers of the
benzoxazine material and thereby bridge the gap between the
fully determined dimer structure and the unknown polymer
structure.28 The objective here is to improve our understanding
of the polymer architecture, through studies of the hydrogen
bonding in the oligomers, and thereby understand the remarkable
physical characteristics of the polymers. In this paper, we focus
on the methyl-substituted benzoxazine trimer and tetramer,
which cannot be investigated by conventional X-ray diffraction
techniques, because suitable crystals could not be prepared.
From the trimer, it might be possible to obtain crystallites which
can be studied by electron crystallography. Work along this line
is in progress.
A critical feature of these materials is their propensity to form
both intra- and intermolecular hydrogen bonds. The relative
number and strength of such interactions may influence the
supramolecular geometry adopted by the system. In particular,
we note the different structures observed among dimer pairs,
which differ only in the substitution at the nitrogen center. A
methyl group results in an exclusive dimer-dimer geometry,
whereas ethyl, propyl, and butyl substituents cause a twist about
the central nitrogen, resulting in an extended ladder structure.
These differences were first elucidated using double quantum
solid-state NMR27 and later confirmed using X-ray diffraction.29
An unanswered question is the effect of substituents on the
extended structures - both their packing geometries and their
physical properties. We begin to explore possible answers in
this paper, focusing on the methyl-substituted oligomers and
polymer. The influence of hydrogen bonding on the structural
and physical properties of the benzoxazine family can be
considered in the more general category of supramolecular
structures, and a particular challenge is to determine the
hydrogen-bonded structure in systems whose heavy-atom struc-
ture is not known. Here we combine advanced solid-state NMR
with advanced computational strategies to meet this challenge.
Applying fast magic-angle spinning (MAS), we have suc-
cessfully demonstrated that high-resolution solid-state NMR is
able to provide detailed structural information about the
hydrogen-bonding arrangements in benzoxazines. Both qualita-
tive descriptions of packing geometries, based on 2D 1H double
quantum (DQ) spectra,27 and quantitative analysis of the N-H
distance in the methyl benzoxazine dimer30 derived from
heteronuclear 1H-15N dipolar sideband patterns, have been
presented. Here we demonstrate a powerful combination of
solid-state NMR and DFT-based calculations, which enables
us to characterize essential structural features of the methyl
benzoxazine oligomers and to propose molecular structures that
are based on energy minimization, hydrogen-bond measurement,
and chemical-shift evaluation.
Homonuclear DQ NMR methods provide an excellent, facile
route to qualitative structural evaluation, based on the proton-
proton proximities evidenced in these spectra. The method relies
on the generation of double quantum coherences between
1
1
proximal H spins, covering a H-1H distance range of up to
approximately 3.5 Å, thereby probing the spatial arrangement
of protons based on the strength of the through-space dipolar
coupling.31-34 Heteronuclear 1H-15N recoupled polarization
transfer (REPT) is a complementary method for both correlations
and quantitative distance measurements, which uses rotor-
encoding of dipolar couplings via sideband patterns recorded
in the indirect dimensions.35 Other methods for heteronuclear
recoupling of the dipolar coupling have been demonstrated and
used for N-H distance measurements;36,37 however, we selected
REPT here for its robust performance under fast MAS condi-
tions. An enhancement of the signal intensity provided by this
method can be achieved both through the use of inverse
detection30,38 and, moreover, through the addition of spoil-
gradients (Gz) at appropriate positions in the pulse sequence,
as described recently by Saalwa¨chter and Schnell.39 This method
has been successfully used to precisely measure N-H bond
lengths (on the order of 110 pm) with 15N in natural abundance.
However, for the longer distances relevant in the benzoxazines,
we were unable to excite a strong enough signal under natural
abundance conditions. Therefore, the 15N-1H distance measure-
ments described here were performed on a 15N-enriched (>95%)
methyl benzoxazine tetramer and are compared with the
optimized structure.
We have performed geometry optimizations and NMR
chemical shift calculations in the framework of density func-
tional theory (DFT).40 In recent years, it has become possible
to calculate NMR chemical shifts not only for isolated mol-
ecules, but also for extended systems such as amorphous or
crystalline solids and liquids. Here, we use a recently developed
(30) Goward, G. R.; Schnell, I.; Brown, S. P.; Spiess, H. W.; Kim, H. D.; Ishida,
H. Magn. Reson. Chem. 2001, 39, S5-S17.
(20) Monkman, A. P.; Palsson, L. O.; Higgins, R. W.; Wang, C.; Bryce, M. R.;
Batsanov, A. S.; Howard, J. A. J. Am. Chem. Soc. 2002, 124, 6049-6055.
(21) Ezuhara, T.; Endo, K.; Aoyama, Y. J. Am. Chem. Soc. 1999, 121, 3279-
3283.
(22) Ishida, H.; Low, H. Y. Macromolecules 1997, 30, 1099-1106.
(23) Kim, H. D.; Ishida, H. J. Appl. Polym. Sci. 2001, 79, 1207-1219.
(24) Macko, J. A.; Ishida, H. J. Polym. Sci., Part B: Polym. Phys. 2000, 38,
2687-2701.
(25) Ishida, H.; Rodriguez, Y. Polymer 1995, 36, 3151-3158.
(26) Dunkers, J.; Zarate, E. A.; Ishida, H. J. Phys. Chem. 1996, 100, 13514-
13520.
(27) Schnell, I.; Brown, S. P.; Low, H. Y.; Ishida, H.; Spiess, H. W. J. Am.
Chem. Soc. 1998, 120, 11784-11795.
(28) Ishida, H.; Moran, C. Macromolecules 1998, 31, 2409.
(29) Kim, H.-D.; Ishida, H. J. Macromol. Chem. Macromol. Symp. 2003,
accepted.
(31) Schnell, I.; Spiess, H. W. J. Magn. Reson. 2001, 151, 153-227.
(32) Lee, Y. K.; Kurur, N. D.; Helmle, M.; Johannessen, O. G.; Nielsen, N. C.;
Levitt, M. H. Chem. Phys. Lett. 1995, 242, 304-309.
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8554.
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Chem. Phys. 1998, 108, 2686-2694.
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418.
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(37) Zhao, X.; Sudmeier, J. L.; Bachovchin, W. W.; Levitt, M. H. J. Am. Chem.
Soc. 2001, 123, 11097-11098.
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Sijbesma, R. P.; Spiess, H. W. J. Magn. Reson. 2001, 150, 57-70.
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9
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A R T I C L E S
Goward et al.
Scheme 1. Synthesis of Model Trimer
Scheme 2. Synthesis of Model Tetramer
pared by the procedure described by Dunkers and Ishida.42 To obtain
the intermediate dimer II, equimolar portions of the monomer I and
p-cresol were heated at 80 °C for 12 h, and the resulting yellow product
was recrystallized from n-hexane. This intermediate dimer II was
reacted again with an equimolar portion of the monomer I without
reaction solvent at 80 °C for 48 h. The resulting yellow product was
cooled and was subsequently purified by column chromatography with
silica gel using hexane/acetone (20:1) as the eluent. The product was
fine white crystalline powder. 1H NMR (200 MHz, CDCl3, 298 K) δ:
2.20, 2.21 (15H, Ar-CH3), 2.22 (6H, N-CH3), 3.68 (8H, Ar-CH2-
N), and 6.70, 6.84, 6.86 (6H, Ar-H). 13C NMR (50.1 MHz, CDCl3,
298 K) δ: 15.81, 20.27 (5C, Ar-C), 40.95 (2C, N-CH3), 58.78, 59.59
(4C, Ar-C-N), and 122.14-154.05 (18C, Ar). Anal. Calcd for
C29H38N2O3: C, 75.29; H, 8.28; N, 6.06. Found: C, 75.09; H, 8.30; N,
6.11. Mass spectrometry (MS-FD) exp., 462.4; calcd. for C29H38N2O3,
462.3.
method for extended systems41 for the benzoxazine dimer in
its crystal structure, and we use a supercell technique for the
benzoxazine trimer and tetramer in vacuo.
The isolated structures of both trimer and tetramer have strong
intramolecular OH‚‚‚N and weak OH‚‚‚O hydrogen bonds that
keep them in a closed-ring like geometry. In the condensed
phase, it would be further stabilized by packing effects such as
van der Waals interactions between phenyl rings of neighboring
molecules. However, because the material does not form large
crystals, these effects are assumed to be of moderate strength.
So far, there is little evidence for intermolecular hydrogen bonds,
but we are currently investigating the possibility of a competing
arrangement which would form ladderlike microstructures.
Experimental Section
Synthesis of N,N-Bis{2-hydroxyl-5-methyl-3-[(N-3,5-dimethyl-2-
hydroxyl-benzyl)-N-methylaminomethyl]}-methylamine (VI, Scheme
2, Methyl-tetramer). The model tetramer for polybenzoxazine was
synthesized according to the following procedure using 2,4-dimeth-
Synthesis of 2,6-Bis[N-(3,5-dimethyl-2-hydroxybenzyl)-N-methyl-
amino-methyl]-p-cresol (III, Scheme 1, Methyl-trimer). The starting
monomer, 3,4-dihydro-3,6,8-trimethyl-2H-1,3-benzoxazine, I, was pre-
(41) Sebastiani, D.; Parrinello, M. J. Phys. Chem. A 2001, 105, 1951-1958.
(42) Dunkers, J.; Ishida, H. Spectrochim. Acta, Part A 1995, 51, 855-867.
9
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Benzoxazine Oligomers
A R T I C L E S
ylphenol, p-cresol, formaldehyde, and methylamine. The starting
monomer, 3,4-dihydro-3,6-dimethyl-2H-1,3-benzoxazine, IV, was pre-
pared by the procedure described by Dunkers and Ishida.42 The
intermediate dimer V was synthesized by heating equimolar portions
of the monomer IV and p-cresol at 80 °C for 12 h, and the resulting
yellow product was recrystallized from n-hexane. The mixture of
intermediate dimer V and monomer I (1:2 mole ratio) was refluxed in
chloroform for 48 h. After the reaction mixture was cooled to room
temperature, chloroform was removed with a rotary evaporator, and
the resulting yellow solid was subsequently purified by column
chromatography with silica gel using hexane/acetone (20:1) as the
eluent. 15N-enriched methyl-tetramer was prepared by the same method
as the methyl-tetramer with the exception of methylamine-15N hydro-
large unit cell to simulate isolated molecules. The simulation boxes
were chosen such as to leave a distance of approximately 5 Å of empty
space between the molecules to exclude interactions between its images
in neighboring cells. Possibilities for packing effects are the focus of
ongoing simulations and will be presented in a further work. The use
of simulated annealing with molecular dynamics can be more efficient
than a standard geometry optimization. Further, an MD simulation
shows less tendency to get stuck in a local minimum or a saddle point
of the potential energy surface, thus avoiding the necessity of
performing a calculation of the second derivatives.
A recently developed method for calculating NMR chemical
shielding tensors in DFT under periodic boundary conditions41 that is
implemented in CPMD was used to obtain the NMR resonance
frequencies. It bears some resemblance to the IGLO approach,48 but is
actually a variant of the CSGT formulation49 extended to periodic
systems in a pseudopotential plane wave formulation.44 The method
has been shown to yield a very good agreement with experiment, in
particular, concerning the effects of hydrogen bonding.50-52 To compare
to experimental results, the shieldings were referenced to tetrameth-
ylsilane (TMS) according to δ(H) ) σ(TMS) - σ(H) with σ(TMS)
being the shielding calculated for an isolated TMS molecule at the same
level of theory. A Gaussian broadening with a half-width of 0.15 ppm
was applied to the individual proton shift values, yielding the spectra
shown below.
1
chloride. The product was a fine white powder. H NMR (200 MHz,
CDCl3, 298 K) δ: 2.19, 2.21 (18H, Ar-CH3), 2.23 (9H, N-CH3),
3.63, 3.66 (12H, Ar-CH2-N), and 6.66, 6.84, 6.86 (8H, Ar-H). 13C
NMR (50.1 MHz, CDCl3, 298 K) δ: 15.81, 20.27 (6C, Ar-C), 40.95
(3C, N-CH3), 57.57, 58.38, 59.59 (6C, Ar-C-N), and 121.84-153.65
(24C, Ar). Anal. Calcd for C39H51N3O4: C, 74.85; H, 8.21; N, 6.71.
Found: C, 74.66; H, 7.99; N, 6.84. Mass spectrometry (MS-FD) exp.,
627.6; calcd. for C39H51N3O4 (with 100% 15N isotope), 628.4.
NMR Methods. All experiments were carried out on a Bruker
Avance 700 solid-state NMR spectrometer running at Larmor frequen-
cies of 700.13 and 70.93 MHz for 1H and 15N, respectively. The
measurements were performed using a 2.5 mm MAS probe with typical
rotor frequencies of 30 kHz, π/2 pulse lengths of 2-2.5 µs for both
15N and 1H pulses, and recycle delays of 2 s. 1H double-quantum
experiments were performed using the compensated BaBa pulse
sequence.31 The heteronuclear recoupling sequence used to probe dipolar
couplings was 15N-1H indirectly detected REPT-HDOR, with selection
We estimate the error in the DFT calculation of the chemical
shieldings to be in the range of about (0.5 ppm. It has two independent
origins. First, the description by DFT as such can give rise to a
maximum deviation of (0.5 ppm in the NMR parameters, when
compared to quasi-exact methods such as coupled-cluster approaches.
In most situations, however, relative shifts and trends are much better
reproduced. The second origin is the geometry obtained from the DFT
energy minimization, which does not include finite temperature effects
and can thus be intrinsically wrong by about 0.05 Å. With a typical
1
of the H-15N coupling supported by spoil gradients (Gz) of 100 µs
duration and 100-200 G/cm strength (pulse sequence given in ref 39);
rotor-encoded sideband patterns were acquired applying dipolar recou-
pling for 20 rotor periods (τr) during both excitation and reconversion.
The t1 dimension was incremented in steps of 1.67 µs (corresponding
to τr/20), and 40 t1 slices were collected, which were subsequently
catenated over 40 rotor periods prior to Fourier transformation (as
described in ref 43).
1
slope of 5 ppm/Å for H shifts as a function of its location within a
hydrogen bond, the geometric error could lead to a shift deviation of
another (0.3 ppm.
For all calculations, we used a self-built Beowulf-cluster of 16
Pentium-4 machines with a MyriNet interconnect, running under the
Linux operating system. The computational time was typically 1000-
2000 processor-hours per system for the geometry optimization and
the NMR calculations. This corresponds to a big total of roughly 1016
floating point operations.
Computational Details. To calculate the minimum energy geom-
etries for all systems, we used the Car-Parrinello Molecular Dynamics
simulation package, a density functional theory code based on a plane
wave representation of the electronic structure.44 Because of its known
ability to reproduce reliably the character and strength of hydrogen
bonds, the Becke-Lee-Yang-Parr (BLYP)45 exchange-correlation
functional was used, together with Goedecker-type pseudopotentials.46
A plane wave cutoff of 70 Ry was chosen for all calculations, and the
sampling of the Brillouin zone was restricted to the Γ-point. In all
systems, the atomic positions were subsequently optimized until the
maximum component of the energy gradient dropped below 5 × 10-4
au.
Results and Discussion
Figure 1 shows the DFT/BLYP-optimized structures of the
methyl benzoxazine trimer and tetramer together with the paired-
dimer structure.26 While the dimer forms pairs of molecules
through intermolecular hydrogen bonding, the trimer and
tetramer exhibit intramolecular hydrogen bonding, such that a
single oligomer is self-contained in its geometry. Supporting
data from solution state IR dilution studies show that, in both
the trimer and the tetramer, the hydrogen bonds persist
unchanged even in extremely dilute solution and are thus
attributed to intramolecular hydrogen bonds.29 The question then
arises whether these structures are also present in the solid,
where packing effects can lead to the formation of ladder-type
arrangements, based on both intra- and intermolecular H-
bonds.27,29
As the starting point for the benzoxazine dimer, we used the known
crystal structure.26 The geometry optimization was performed under
periodic boundary conditions, taking into account the monoclinic crystal
lattice. For the isolated benzoxazine trimer and tetramer molecules,
we started from geometries generated by simulated annealing calcula-
tions with a standard all-atom force-field using point charges fitted by
the restricted electrostatic potential method (RESP).47 All atoms were
subsequently relaxed at the DFT/BLYP level of theory in a sufficiently
(43) Saalwa¨chter, K.; Spiess, H. W. J. Chem. Phys. 2001, 114, 5707-5728.
(44) Hutter, J.; Ballone, P.; Bernasconi, M.; Focher, P.; Fois, E.; Goedecher,
S.; Parrinello, M.; Tucherman, M. CPMD Computer Code, 1995, MPI fu¨r
Festko¨rperforschung, Stuttgart, Germany, and IBM Zu¨rich Research Lab.
(45) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (b) Lee, C.; Yang, W.; Parr,
R. G. Phys. ReV. B 1988, 37, 785.
(46) Goedecker, S.; Teter, M.; Hutter, J. Phys. ReV. B 1996, 54, 1703-1710.
(47) Bayly, C. I.; Cieplak, P.; Cornell, W. D.; Kollman, P. A. J. Phys. Chem.
1993, 97, 10269-10280.
(48) Kutzelnigg, W. Isr. J. Chem. 1980, 19, 193-200.
(49) Keith, T. A.; Bader, R. F. W. Chem. Phys. Lett. 1993, 210, 223-231.
(50) Sebastiani, D.; Goward, G.; Schnell, I.; Parrinello, M. Comput. Phys.
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9
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A R T I C L E S
Goward et al.
Figure 1. Methyl benzoxazine dimer, trimer, and tetramer structures in their ab initio optimized geometries. The dimer (a) forms pairs of molecules via
double intermolecular N-H‚‚‚O‚‚‚H-O hydrogen bonds. The trimer structure (b) comprises a nearly planar ring, while in the tetramer, (c), the fourth
1
monomer unit overlaps the start of the ring geometry, depicting the beginning of a helix. The experimental H chemical shifts are assigned to the protons
forming hydrogen bonds.
1
A comparison of the experimental H MAS NMR spectra
is the increasing number of resolved N-H resonances with
increasing oligomer length, from one to two, to three. The
chemical shifts are listed in Table 1. The N-H resonances begin
at 11.7 ppm in the dimer and spread around this central
frequency, as more resonances are added in the oligomers.
Considering the O-H resonances, we see in this case a trend
toward higher resonance frequency with increasing oligomer
length.
The agreement between the experimental and calculated
spectra is striking. Most satisfying is the correlation between
the number and position of the resonances in the N-H region
(10-15 ppm). In the dimer, only one N-H resonance is
determined at 11.7 and 12.2 ppm by experiment and calculation,
respectively. In addition, the dimer’s O-H resonance is found
at 6.9 ppm experimentally and at 7.0 ppm by calculation. In
the trimer, two N-H resonances are found, at 10.7 and 12.3
1
and the calculated H NMR spectra is given in Figure 2. The
latter were obtained from DFT-based calculations of a periodic
structure built up from the models shown in Figure 1, while
the MAS spectra were obtained from 10 mg of each sample,
packed in a 2.5 mm rotor which was spun at 30 kHz. First, we
1
consider the experimental H MAS NMR spectra of the series
of oligomers, the top traces in Figure 2. The spectra are
interpreted according to an alphabetical assignment, beginning
with the H-bonded resonances. N-H’s at the highest resonance
frequency are labeled “A” and differentiated with primes for
increasing number of resolved N-H resonances. O-H’s are
labeled “B”, aromatic resonances are labeled “C,D”, and
aliphatic resonances are labeled “E,F”. Qualitatively, it is evident
that the hydrogen-bonding structures are different for the dimer,
trimer, and tetramer. The clearest difference among the spectra
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5796 J. AM. CHEM. SOC. VOL. 125, NO. 19, 2003

Benzoxazine Oligomers
A R T I C L E S
Table 1. Calculated and Experimental 1H Chemical Shifts of
Benxoxazine Dimer, Trimer, and Tetramera
N−H resonances O−H aromatic aromatic aliphatic aliphatic
A′′ A′
chemical shifts
in ppm
A
B
C
D
E
F
dimer, expt.
dimer, calc.
trimer, expt.
trimer, calc.
11.7 6.8 6.5-5.5 N/A
12.2 7.0 6.0-5.0 N/A
12.3 10.7 7.4 7.0-5.0 6.0-5.5 3.0-1.5 1.5-1.0
12.2 10.7 7.4 6.2-5.5 4.8-4.2 3.0-1.5 1.5-1.0
3-0 ppm N/A
3-0 ppm N/A
tetramer, expt. 12.9 11.7 10.9 7.9 7.2-5.5 N/A
tetramer, calc. 13.1 11.8 11.1 7.0 6.5-5.8 N/A
3.5-1.0 2.0-0.0
4.2-2.0 2.0-0.5
a We note that the spectral resolution within the aromatic and aliphatic
regions is poor, due to significant overlap, and thus ranges of resonance
frequencies are given.
dimensional structural conformations proposed. Further support
for these molecular geometries is found in the 2D homonuclear
DQ NMR spectra of the materials, discussed below.
To investigate the relative strength of the intramolecular
hydrogen bonds in more detail, we have performed further
calculations on the benzoxazine trimer and tetramer. First, the
OH‚‚‚O hydrogen bond that is responsible for the ring closure
was opened by imposing an O‚‚‚O distance of about 3 Å, while
allowing the rest of the molecule to relax completely. This
resulted in an increase in total energy of 10-12 kJ/mol for both
molecules. Furthermore, a variety of alternative geometries were
generated with the combined force-field and DFT geometry
optimization approach. In all alternative geometries, the
OH‚‚‚N intramolecular hydrogen bonds remained intact, thus
always imposing an internal curvature on the molecule. The
outer OH‚‚‚O bond was conserved in the majority of the
structures obtained, yielding approximately the same overall
geometry and a similar NMR pattern. In the cases where the
ring-closing did not take place, other OH‚‚‚O bonds were
formed, but the total energies of the systems were always more
than 25 kJ/mol higher than that in the helical arrangement. In
addition, the calculated NMR spectra showed no resemblance
to the experimentally observed spectrum. Thus, there is con-
clusive evidence that the trimer and tetramer form ring-shaped
and helical structures, respectively, which are held together by
a sufficiently strong intramolecular OH‚‚‚O hydrogen bond.
With respect to the dimer, we need to add that the result of
the geometry optimization of the periodic structure resulted in
a N-H distance of 1.70 Å, which is significantly shorter than
the previously determined value of (1.96 ( 0.05) Å.30 Remark-
ably, the chemical shift of the NH proton occurs at 11.7 ppm
in the nonlabeled sample (spectrum shown in Figure 2), which
differs from that observed at 11.2 ppm in both the labeled
material on which the distance measurement was made and a
previously prepared sample.27,30 We also note that the sample
examined in ref 27 was not uniform, but also contained a
fraction of the ladder-type packing arrangement, as was observed
Figure 2. Comparison of calculated (blue) and experimental 1H MAS NMR
spectra for the dimer (a), trimer (b), and tetramer (c). 1H DQ MAS spectrum
of cross-linked polybenzoxazine based on methylamine (d). Resolved
resonances are labeled according to the following pattern; A, N-H protons;
B, O-H protons; C,D, aromatic protons; E,F, aliphatic protons. A, A′, and
A′′ indicate the increasing number of N-H resonances observed with
increasing oligomer length.
1
in the 2D H DQ spectrum of that material. Therefore, we
attribute the difference not to an inaccuracy in the measurement
or the geometry optimization, but rather to an imperfect crystal
packing, which results in the longer-than-optimal N-H distance.
This conclusion is based primarily on the extreme sensitivity
of the proton chemical shift to the NH distance. To test this
hypothesis, a NH distance was fixed, and the geometry
optimization was performed under the constraint of 1.9 Å. The
energy difference per dimer pair (i.e., 4 H-bonds) was small,
approximately 3 kJ/mol, and the resulting 1H chemical shift for
the NH resonance was less than 11 ppm, in agreement with the
ppm in the experimental spectrum and at 10.7 and 12.2 ppm in
the calculated spectrum. In contrast, the tetramer shows three
N-H resonances in both experimental and calculated spectra,
in the range from 10.0 to 13.5 ppm. The reduced resolution in
the experimental spectrum of the tetramer demonstrates that the
material is not as highly ordered as the trimer. Nevertheless,
on the basis of the similarity between our experimental and
calculated data, we are confident in the validity of the three-
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J. AM. CHEM. SOC. VOL. 125, NO. 19, 2003 5797

A R T I C L E S
Goward et al.
Figure 4. Regions of the 1H 2D DQ NMR spectra (shown in Figure 2)
relevant for the hydrogen bonds of the dimer (a), trimer (b), and the tetramer
(c). On the right, experimental spectra are compared to calculated 1H shifts
(blue).
correlations of like methyl (E), methylene (F), and phenyl
protons (C and D) in both oligomers. As well, various contacts
are observed between the resonances C, D, E, and F, all of
which, however, are of no significance with respect to the
hydrogen-bonding scheme, but merely reflect proton-proton
proximities that are obvious from the molecular structure.
More significant in terms of probing the structures are the
correlations among the hydrogen-bonded protons, whose reso-
nances occur in the lower left quadrant of the DQ spectrum.
An expansion of this region is given in Figure 4. The OH
resonances, labeled B, are evident in all three spectra, at 6.9
ppm in the dimer, 7.4 ppm in the trimer, and 8.0 ppm in the
tetramer. The N-H resonances, labeled A in the dimer, A and
A′ in the trimer, and A, A′, and A′′ in the tetramer, distinguish
the oligomers both molecularly, in the number of N-H
resonances present, and structurally, in the nature of the
correlations present. In the methyl benzoxazine dimer, trimer,
and tetramer, one, two, and three N-H resonances, respectively,
are observed. However, none of these N-H protons shows an
“auto” correlation of the type A-A, A′-A′, or A′′-A′′, which
means that the N-H protons are spatially separated from each
other. For the trimer, however, a correlation between A and A′
is found at 23.8 ppm in the DQ dimension, which indicates
that these two NH protons are in close proximity to each other
Figure 3. 1H 2D DQ NMR spectra for the dimer (a), trimer (b), and tetramer
(c), obtained at ambient temperature, under 30 kHz MAS with one rotor
period of dipolar recoupling. π/2 pulse lengths of 2 µs and recycle delays
of 2 s were implemented. Sixty-four transients were averaged for each of
32 slices acquired in the indirect dimension. Labels correspond to those
used in Figure 2.
trend we describe. Apparently, the position of the N-H can be
adjusted to cope with packing requirements. At this point, we
return to considering the trimer and tetramer structures, which
are the primary focus of this paper.
1
The 2D H-1H DQ NMR spectra obtained for the dimer,
trimer, and tetramer are shown in Figure 3. The reader is referred
to previous publications for analogous spectra obtained for the
methyl dimer.27,30 As in the case of the dimer, strong resonances
in the alphatic and aromatic regions are observed as expected
along the diagonal of the 2D spectrum and arise from self-
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5798 J. AM. CHEM. SOC. VOL. 125, NO. 19, 2003

Benzoxazine Oligomers
A R T I C L E S
Figure 5. (a) 15N-1H inverse CP-REPT-HDOR spectra for the benzoxazine tetramer with 15N enrichment, collected using τexc ) 20 τr under MAS at 30
kHz. The experimental 1D patterns shown in (b, c, d) were taken from the 2D spectrum shown in (a) at the resonance positions of the NH protons A′′, A′,
and A, respectively. The colored patterns in (b, c, d) are calculated using the N-H distances given on the right. The red and blue patterns represent the upper
and lower limits of the error margin, while the green pattern fits best to the experimental data.
in the ring structure. In contrast, in the spectrum of the tetramer,
no such A-A′ or A-A′′ contacts are observed, which is
consistent with an increase in the spatial separation between
the nitrogens in the larger oligomer. The A-B correlations
present in all spectra are consistent with intramolecular contacts
between the protons in the two types of hydrogen-bonded (N-H
and O-H) protons. Similarly, expected contacts between the
NH resonances and the methyl protons of the methyl at the
amine position are always present.
These data are in good agreement with the molecular
conformations illustrated in Figure 1. In particular, the quasi-
planar ring-shaped geometry of the trimer brings the NH protons
into spatial proximity with each other and is consistent with
the observed A-A′ contact. The respective proton-proton
distance in the molecular DFT model is 2.74 Å. Furthermore,
the quasi-helical tetramer structure occupies comparatively more
free volume, such that the N-H protons are separated from
each other by greater than 4.0 Å and therefore fall outside the
range of H-H distances observable via DQ NMR under these
conditions.27,31
conformation of the polymer. In such a geometry, the hydrogen
bonds are found on the inner side of the helix, where they are
protected from chemical attack. Such a geometry is consistent
with the properties such as lack of water absorption and lack
of contraction or expansion upon curing, the industrially
favorable properties of these polymers.
15N-1H Distance Measurements. In our previous investiga-
tion of the N-H‚‚‚O distance in the methyl benzoxazine dimer,
a distance of (1.96 ( 0.05) Å was measured in the 15N-labeled
sample prepared for that study. Here, we present distance
measurements for the N-H contacts in the 15N-labeled tetramer,
using an improved variant of the pulse sequence.39 The new
pulse sequence again makes use of 1H detection, with the added
advantage of pulsed field gradients (PFGs), or, alternatively,
1H R3 pulses, which dephase unwanted 1H magnetization. The
measurements of N-H distances in the tetramer are in good
agreement with the calculated structure. Figure 5 shows the
1
rotor-encoded H-15N dipolar sideband pattern (Figure 5a),
together with the 1D patterns taken from the respective regions
of the 2D spectrum and corresponding calculated patterns
A 1H 1D DQ MAS spectrum obtained for cross-linked, high
molecular weight polybenzoxazine based on methylamine is
shown in Figure 2d and clearly does not afford sufficient spectral
resolution for a full structure determination due to the overlap
among different types of hydrogen-bonded resonances, and
generally broadened spectral features. The trend toward an
increasingly complex hydrogen-bonding structure, with indi-
vidually resolved resonance lines observed for the small
(Figure 5b-d). We note that, while three H(N) resonances
1
1
could clearly be resolved in the H MAS and DQ spectra, the
1
resolution in the H dimension of the sideband experiment is
somewhat hampered due to the poorer signal-to-noise which
results from the extremely long recoupling time required to
generate the higher order sidebands necessary for accurate
measurements of these relatively long distances. Therefore, the
sideband patterns (in the F1 dimension) have been extracted from
1
1
oligomers, accounts for the broad H resonances observed in
the 2D spectrum at exactly those H resonance positions (in
the polymer, where a superposition of spectra for different
H-bonded structures is expected.
Turning to the chemical properties of the polybenzoxazine
polymers, we find supporting evidence for the structures
described here. The overlapped ring geometry of the tetramer
leads us to hypothesize a helical geometry as the most likely
the F2 dimension) which are known from the 1H MAS
experiment. A comparison with calculated sideband patterns
(colored patterns in Figure 5b-d) yields N-H distances of (1.84
( 0.05), (1.79 ( 0.05), and (1.70 ( 0.10) Å for the NH protons
A, A′, and A′′, respectively. In Figure 5b-d, the red and blue
patterns represent the upper and lower limits of the error margin,
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A R T I C L E S
Goward et al.
while the green pattern fits best to the experimental data. The
measured distances are in excellent agreement with the values
found in the optimized geometry of the tetramer, that is, 1.89,
1.78, and 1.72 Å, and also concur with the calculated and
experimental 1H chemical shifts of the respective protons. The
longest N-H distance is found for proton A, which has the
smallest chemical shift (at 10.7 ppm) and forms a hydrogen
bond to the central N-atom in the tetramer. The relative
weakness of the hydrogen bond indicates that the central part
of the tetramer experiences the most strain due to the helical
geometry. For the two other NH-protons, A′ and A′′, shorter
N-H distances as well as larger chemical shifts (of 11.7 and
12.9 ppm) are found. These observations correspond to stronger
hydrogen bonds, suggesting that the more peripheral parts of
the tetramer experience less conformational strain. These data
therefore confirm our proposed structure of the tetramer - the
first twist of a helix.
state NMR and FT-IR. Solid-state NMR is particularly suited
to detect fine-tuning of structures via the adjustment of the
NH‚‚‚O hydrogen bond.
Conclusions
In summary, we have presented a case study in which the
accord between DFT-based chemical shift calculations and high-
1
resolution solid-state H MAS NMR spectra provides ample
confirmation of a supramolecular hydrogen-bonding structure.
Moreover, we have established a structural motif for the methyl
benzoxazine trimer and tetramer, which, in contrast to the methyl
dimer pairs, are characterized exclusively by intramolecular
hydrogen bonds. Extrapolating from the trends found among
these oligomers, we conclude that the polybenzoxazines form
helices in the solid state: such a structural conformation
accounts for their favorable chemical properties.
In conclusion, this study demonstrates the remarkable success
of the combination of cutting-edge experimental and compu-
tational NMR methods in the investigation of structural driving
forces of supramolecular hydrogen-bonded systems.
The trend to shorter N-H distances with increasing proton
resonance frequency also supports our earlier discussion of the
N-H distances in the slightly differing structures of the
benzoxazine dimer, where the experimentally measured N-H
Acknowledgment. H.I. acknowledges support from the von
Humboldt Fellowship program. G.R.G. is grateful to NSERC
(Canada) for support in the form of a postdoctoral fellowship.
Financial support through the Deutsche Forschungsgemeinschaft
(SFB 625 in Mainz) is gratefully acknowledged. We thank M.
Hehn and H. Raich for their technical expertise and patience
required to modify the 2.5 mm probe to incorporate gradients.
1
distance of greater than 1.9 Å correlated to a H resonance
frequency of 11.2 ppm, whereas the distance of 1.7 Å
determined via geometry optimization correlated to the calcu-
lated resonance frequency of 12.4 ppm. These data show the
extreme sensitivity of the 1H chemical shift to its environment
- particularly in the presence of hydrogen bonding. Moreover,
the differences noted among the dimer samples in the solid state
were undetectable by other analysis methods including solution-
JA029059R
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5800 J. AM. CHEM. SOC. VOL. 125, NO. 19, 2003