Hydrogen bonding in nitroaniline analogues: Hydrogen-bonded sheets in 2-amino-4,6-dimethoxy-5-nitropyrimidine and π-stacked hydrogen-bonded sheets in 4-amino-2,6-dimethoxy-5-nitropyrimidine

Article abstract of DOI:10.1107/S0108270102020322

In 2-amino-4,6-dimethoxy-5-nitropyrimidine, C₆H₈N₄O₄, the molecules are linked by one N-H...N and one N-H...O hydrogen bond to form sheets built from alternating R₂²(8) and R₆⁶(32) rings. In isomeric 4-amino-2,6-dimethoxy-5-nitropyrimidine, C₆H₈N₄O₄, which crystallizes with Z′ = 2 in P1macr;, the two independent molecules are linked into a dimer by two independent N-H...N hydrogen bonds. These dimers are linked into sheets by a combination of two-centre C-H...O and three-centre C-H...(O)₂ hydrogen bonds, and the sheets are further linked by two independent aromatic π-π-stacking interactions to form a three-dimensional structure.

Full text of DOI:10.1107/S0108270102020322

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
(
Bernstein et al., 1995) motif (McWilliam et al., 2001; Glidewell
et al., 2002a). The dominant motif in simple 2-nitroanilines is a
combination of intramolecular S(6) rings and simple C(6) or
0
C (12) chains, depending upon whether Z is 1 or 2
2
2
ISSN 0108-2701
(
2
Dhaneshwar et al., 1978; Ellena et al., 1996; Cannon et al.,
001; Garden et al., 2002).
However, where other hydrogen-bond acceptors are
Hydrogen bonding in nitroaniline
analogues: hydrogen-bonded sheets in
present, whether as a heteroatom within the aromatic ring or
in a C-substituent, formation of NÐHÁ Á ÁN hydrogen bonds
can effectively compete with NÐHÁ Á ÁO hydrogen-bond
formation, leading to the production of very elegant sheets
containing alternating large and small rings (Glidewell et al.,
2
-amino-4,6-dimethoxy-5-nitropyrimi-
dine and p-stacked hydrogen-bonded
sheets in 4-amino-2,6-dimethoxy-
2
001; Glidewell et al., 2002b). Continuing our exploration of
5
-nitropyrimidine
this theme, we have now investigated two isomeric examples,
namely 2-amino-4,6-dimethoxy-5-nitropyrimidine, (I), and
4-amino-2,6-dimethoxy-5-nitropyrimidine, (II), containing
two additional types of potential hydrogen-bond acceptor,
namely a pair of N atoms within the ring and a pair of O atoms
in C-substituents.
a
b
Christopher Glidewell, * John N. Low, ² Manuel
c
d
Melguizo and Antonio Quesada ³
a
School of Chemistry, University of St Andrews, St Andrews, Fife KY16 9ST,
b
Scotland, Department of Chemistry, University of Aberdeen, Meston Walk,
c
Old Aberdeen AB24 3UE, Scotland, Departamento de Qu Âõ mica Inorg a nica y
d
Org a nica, Universidad de Ja e n, 23071 Ja e n, Spain, and School of Engineering,
University of Dundee, Dundee DD1 4HN, Scotland
Correspondence e-mail: cg@st-andrews.ac.uk
Received 5 November 2002
Accepted 7 November 2002
Online 10 December 2002
In 2-amino-4,6-dimethoxy-5-nitropyrimidine, C H N O , the
4
6
8
4
molecules are linked by one NÐHÁ Á ÁN and one NÐHÁ Á ÁO
2
2
hydrogen bond to form sheets built from alternating R (8) and
6
R (32) rings. In isomeric 4-amino-2,6-dimethoxy-5-nitro-
6
0
pyrimidine, C H N O , which crystallizes with Z = 2 in P1,
4
6
8
4
the two independent molecules are linked into a dimer by two
independent NÐHÁ Á ÁN hydrogen bonds. These dimers are
linked into sheets by a combination of two-centre CÐHÁ Á ÁO
and three-centre CÐHÁ Á Á(O) hydrogen bonds, and the sheets
2
are further linked by two independent aromatic ꢀ±ꢀ-stacking
interactions to form a three-dimensional structure.
Comment
Supramolecular aggregation in simple C-substituted nitro-
anilines is dominated by NÐHÁ Á ÁO hydrogen bonds. In
Compounds (I) and (II) crystallize in space groups P2
1
/c
0
and P1 with Z = 1 and 2, respectively. In (I) (Fig. 1), both
methoxy groups have their methyl C atoms essentially in the
plane of the pyrimidine ring, such that both methyl groups are
directed away from the nitro substituent. This conformation
matches that of the analogous 2-amino-4,6-dimethoxy-5-
nitrosopyrimidine (Glidewell et al., 2002). Although there is
no crystallographic symmetry relating the two halves of the
molecule across the N2Á Á ÁN5 line, the corresponding dimen-
sions for the two halves (Table 1) are very similar. Both of the
exocyclic CÐN distances are very short for their types (Allen
et al., 1987), while the NÐO distances are long. The N1ÐC6
and N3ÐC4 distances are signi®cantly shorter than the N1Ð
C2 and C2ÐN3 distances, and these observations, taken
4
-nitroanilines containing no other hydrogen-bonding substi-
tuents, there are two dominant patterns of supramolecular
aggregation. Either each molecule is linked to four other
molecules by means of such hydrogen bonds, giving rise to
two-dimensional (Tonogaki et al., 1993; Glidewell et al., 2002a)
or three-dimensional (Ferguson et al., 2001) hydrogen-bonded
structures, or else the molecules are linked into chains via
2
paired NÐHÁ Á ÁO hydrogen bonds which form an R (6)
2
²
Postal address: School of Engineering, University of Dundee, Dundee
DD1 4HN, Scotland.
y
On leave from: Departamento de Qu Âõ mica Inorg a nica Org a nica,
³
Universidad de Ja e n, 23071 Ja e n, Spain.
o14 # 2003 International Union of Crystallography
DOI: 10.1107/S0108270102020322
Acta Cryst. (2003). C59, o14±o18

organic compounds
6
together, point to the charge-separated form (Ia) as an
important contributor to the overall molecular±electronic
structure.
different R (32) ring, such that four methoxy groups are
6
directed into each of these rings, effectively occupying the
whole interior of these rings. Thus, it may well be that these
methoxy groups act as templates for the formation of the
The two independent molecules in (II) (Fig. 2) have iden-
tical conformations and in each the CÐO bonds of the two
methoxy groups are almost parallel. The intramolecular
distances in the two molecules (Table 3) are very similar. As
observed in (I), the exocyclic CÐN bonds are short for their
types but, in contrast with (I), in each molecule of (II) there is
a signi®cant difference between the two NÐO bond lengths,
with that involved in the characteristic intramolecular S(6)
hydrogen-bonded motif being the longer in each molecule.
These observations, together with the pattern of CÐN
distances within the rings, point to (IIa) and (IIb) as important
contributors to the overall molecular±electronic structure. The
nitro groups in (II) are almost coplanar with the adjacent
pyrimidine rings, presumably in¯uenced by the intramolecular
hydrogen bonds. In (I), the nitro group is twisted out of the
6
6
R (32) rings, just as in 3-tri¯uoromethyl-4-nitroaniline there
4
are pairs of CF groups lying within hydrogen-bonded R (32)
3
4
rings (Glidewell et al., 2002a). There are neither CÐ
HÁ Á Áꢀ(arene) hydrogen bonds nor aromatic ꢀ±ꢀ-stacking
interactions in the structure of (I), which thus depends solely
on hard hydrogen bonds as the only direction-speci®c inter-
actions.
Within the asymmetric unit of (II) (Fig. 2), there are two
independent NÐHÁ Á ÁO hydrogen bonds (Table 4), each
generating an S(6) motif, and there are two independent NÐ
HÁ Á ÁN hydrogen bonds, which together generate a pseudo-
2
2
centrosymmetric R (8) ring. However, a search of possible
additional symmetry revealed none. These dimeric units are
then linked into sheets via a series of two CÐHÁ Á ÁO hydrogen
bonds (Table 4). Methoxy atom C121 in the type 1 molecule
ꢀ
ring plane by ca 30 (Tables 1 and 3).
The supramolecular structure of (I) is generated by just two
hydrogen bonds, one each of the NÐHÁ Á ÁN and NÐHÁ Á ÁO
types (Table 2). The amino atom N2 in the molecule at (x, y, z)
acts as a hydrogen-bond donor, via atom H2A, to ring atom
N1 in the molecule at (1 � x, � y, 1 � z), so generating a
2
2
1
2
1
2
centrosymmetric R (8) ring centred at ( ,0, ) (Fig. 3). The same
atom N2 also acts as a donor, but this time via atom H2B, to
1
1
nitro atom O51 in the molecule at (� x, y � , � z), and
2
2
propagation of this hydrogen bond produces a C(8) chain
running parallel to the [010] direction and generated by the 2
1
1
4
2
screw axis along (0,y, ). The combination of R (8) rings and
C(8) chains generates a (102) sheet, built from alternating
2
2
6
centrosymmetric R (8) and R (32) rings and incorporating the
1
2
6
3
C(8) chains along (0,y, ) and (1,� y, ) (Fig. 3).
4
4
Neither of the methoxy substituents participates directly in
the hydrogen-bonding arrangement but, in each molecule, the
two methoxy groups are directed into the interior of a
Figure 2
A view of the two independent molecules of (II), showing the atom-
labelling scheme and the NÐHÁ Á ÁN hydrogen bonds within the
asymmetric unit. Displacement ellipsoids are drawn at the 30%
probability level and H atoms are shown as small spheres of arbitrary
radii.
Figure 1
A view of the molecule of (I), showing the atom-labelling scheme.
Displacement ellipsoids are drawn at the 30% probability level and H
atoms are shown as small spheres of arbitrary radii.
Figure 3
A stereoview of part of the crystal structure of (I), showing the formation
2
6
of a hydrogen-bonded (102) sheet built from R (8) and R (32) rings.
2
6
ꢁ
Acta Cryst. (2003). C59, o14±o18
Christopher Glidewell et al.
6 8 4
Two isomers of C H N O
4
o15

organic compounds
(
containing atoms N11 etc.) at (x, y, z) acts as a hydrogen-bond
corresponding rings at both (� x, � y, 2 � z) and (1 � x, � y,
Ê
donor, via atom H12B, to nitro atom O151 in the type 1
molecule at (x, 1 + y, z), while atom C221 in the type 2 mol-
ecule (containing atoms N21 etc.) at (x, y, z) similarly acts as a
donor, via atom H22B, to nitro atom O251 at (x, y � 1, z). In
2 � z). The centroid separations are 3.462 (2) and 3.414 (2) A,
Ê
the interplanar spacings are 3.269 (2) and 3.241 (2) A, and the
Ê
centroid offsets are 1.140 (2) and 1.073 (2) A, respectively.
The combined effect of these two interactions is the genera-
tion of a ꢀ-stacked chain running parallel to the [100] direction
(Fig. 6), linking the hydrogen-bonded (100) sheets.
2
6
6
this manner, a chain of edge-fused R (8) and R (22) rings
2
running parallel to [010] is generated by translation (Fig. 4).
Similarly, methoxy atom C161 in the type 1 molecule at (x,
y, z) forms, via atom H16B, a planar three-centre CÐHÁ Á Á(O)
2
hydrogen bond, in which the two acceptors are methoxy atom
O26 and nitro atom O252, both in the type 2 molecule at (x,
y � 1, 1 + z). Methoxy atom C261 at (x, y, z) forms, via atom
H26B, another such three-centre system, where the acceptors
are methoxy atom O16 and nitro atom O152 at (x, 1 + y, z � 1).
The combination of these two three-centre interactions
1
2
2
1
2
generates a tricyclic R (6)R (6)R (6) motif containing the
2
unusual pseudo-anthracene synthon A (see Scheme), and
propagation of this motif by translation generates a chain
running parallel to the [011] direction (Fig. 5).
The combination of the [010] (Fig. 4) and [011] chains
Fig. 5) generates a sheet parallel to (100), and adjacent sheets
(
are linked to form a three-dimensional structure by two
independent aromatic ꢀ±ꢀ-stacking interactions involving
type 1 molecules only. The pyrimidine ring of the type 1
molecule at (x, y, z) forms stacking interactions with the
Figure 6
Part of the crystal structure of (II), showing the formation of a ꢀ-stacked
chain of type 1 molecules along [100]. Atoms marked with an asterisk (*)
or hash (#) are at the symmetry positions (1 � x, � y, 2 � z) and (� x, � y,
2
� z), respectively. For the sake of clarity, the unit-cell box and H atoms
bonded to C atoms have been omitted.
The structural role of the methoxy substituents thus appears
to be entirely different between (I) and (II). In (I), these
substituents appear to act as templates for the formation of
large hydrogen-bonded rings, while in (II), they link dimeric
units into sheets.
Figure 4
Part of the crystal structure of (II), showing the formation of an edge-
2
6
fused chain of R (8) and R (22) rings along [010]. Atoms marked with an
asterisk (*) or hash (#) are at the symmetry positions (x, 1 + y, z) and (x,
y � 1, z), respectively.
2
6
Experimental
Samples of (I) and (II) were prepared by oxidation of the corre-
sponding nitroso compounds (Marchal et al., 2002) with 3-chloro-
peroxobenzoic acid (1.1 molar equivalents) in acetonitrile solution.
After recrystallization from ethyl acetate, the products had melting
points of 462 [for (I)] and 452 K [for (II)]. Spectroscopic analysis for
1
(
I), H NMR (ꢁ, DMSO-d
6
, p.p.m.): 3.91 (s, 6H, O±CH
3
), 7.65 (bs, 2H,
, p.p.m.): 54.6,
13
NH
12.2, 160.7, 163.5. Spectroscopic analysis for (II), H NMR (ꢁ,
DMSO-d , p.p.m.): 3.88 (s, 3H, O±CH ), 3.93 (s, 3H, O±CH ), 8.33
, p.p.m.):
4.8, 54.9, 111.6, 160.1, 163.3, 165.9. Crystals suitable for single-crystal
2
, exchanges with D
2
O); C NMR (ꢁ, DMSO-d
6
1
1
6
3
3
Figure 5
13
2 2 6
(bs, 2H, NH , exchanges with D O); C NMR (ꢁ, DMSO-d
Part of the crystal structure of (II), showing the formation of a chain of
rings along [011]. For the sake of clarity, the unit-cell box has been
omitted. Atoms marked with an asterisk (*) or hash (#) are at the
symmetry positions (x, 1 + y, z � 1) and (x, y � 1, 1 + z), respectively.
5
X-ray diffraction were grown by slow evaporation of solutions in
ethyl acetate [for (I)] or diethyl ether [for (II)].
ꢁ
o16 Christopher Glidewell et al.
6 8 4
Two isomers of C H N O
4
Acta Cryst. (2003). C59, o14±o18

organic compounds
Compound (I)
Data collection
Nonius KappaCCD area-detector
diffractometer
' scans, and ! scans with ꢆ offsets
Absorption correction: multi-scan
(DENZO±SMN; Otwinowski &
Minor, 1997)
3699 independent re¯ections
2510 re¯ections with I > 2ꢇ(I)
Crystal data
�
3
C
6
H N
8 4
O
4
D
x
= 1.646 Mg m
Rint = 0.065
ꢀ
M
r
= 200.16
Mo Kꢃ radiation
Cell parameters from 1835
re¯ections
ꢄmax = 27.5
Monoclinic, P2 =c
h = � 8 ! 8
1
Ê
a = 7.3443 (2) A
k = � 12 ! 12
l = � 17 ! 16
Ê
b = 7.8437 (2) A
ꢀ
ꢄ = 3.0±27.5
ꢅ = 0.14 mm
T = 120 (2) K
Tmin = 0.946, Tmax = 0.978
12 543 measured re¯ections
Ê
c = 14.9129 (4) A
� 1
ꢀ
3
ꢂ = 109.9470 (13)
Ê
V = 807.54 (4) A
Z = 4
Re®nement
Block, yellow
0.50 Â 0.40 Â 0.15 mm
2
Re®nement on F
2
H-atom parameters constrained
2
2
2
o
2
R[F > 2ꢇ(F )] = 0.050
wR(F ) = 0.135
w = 1/[ꢇ (F
where P = (F
(Á/ꢇ)max = 0.001
) + (0.0779P) ]
Data collection
2
2 2
o c
+ 2F )/3
Nonius KappaCCD area-detector
diffractometer
1835 independent re¯ections
1569 re¯ections with I > 2ꢇ(I)
S = 1.02
3699 re¯ections
257 parameters
Ê
� 3
Áꢈmax = 0.50 e A
Áꢈmin = � 0.39 e AÊ
� 3
'
scans, and ! scans with ꢆ offsets
Absorption correction: multi-scan
DENZO±SMN; Otwinowski &
Minor, 1997)
min = 0.936, Tmax = 0.985
065 measured re¯ections
Rint = 0.062
ꢀ
ꢄ
max = 27.5
(
h = � 8 ! 9
k = � 9 ! 10
l = � 19 ! 17
T
Table 3
Selected geometric parameters (A, ) for (II).
6
Ê
ꢀ
Re®nement
2
2
2
2
N11ÐC12
C12ÐN13
N13ÐC14
C14ÐC15
C15ÐC16
C16ÐN11
C14ÐN14
C15ÐN15
N15ÐO151
N15ÐO152
C12ÐO12
C16ÐO16
1.3333 (19)
1.3201 (19)
1.3497 (19)
1.429 (2)
N21ÐC22
C22ÐN23
N23ÐC24
C24ÐC25
C25ÐC26
C26ÐN21
C24ÐN24
C25ÐN25
N25ÐO251
N25ÐO252
C22ÐO22
C26ÐO26
1.3348 (19)
Re®nement on F
2
o
w = 1/[ꢇ (F ) + (0.0617P)
1.3212 (19)
1.3522 (19)
1.433 (2)
1.420 (2)
1.323 (2)
1.3266 (19)
1.4254 (19)
1.2456 (16)
1.2255 (16)
1.3304 (18)
1.3215 (17)
2
R[F > 2ꢇ(F )] = 0.040
wR(F ) = 0.110
S = 1.00
+ 0.2772P]
where P = (F
(Á/ꢇ)max < 0.001
Áꢈmax = 0.26 e A
Áꢈmin = � 0.32 e AÊ
2
2
2
c
o
+ 2F
)/3
1.416 (2)
Ê
� 3
1
1
835 re¯ections
29 parameters
1.3341 (19)
1.3292 (18)
1.4362 (19)
1.2411 (16)
1.2153 (17)
1.3259 (18)
1.3199 (17)
� 3
H-atom parameters constrained
Table 1
Selected geometric parameters (A, ) for (I).
Ê
ꢀ
N1ÐC2
C2ÐN3
N3ÐC4
C4ÐC5
C5ÐC6
C6ÐN1
1.3515 (17)
1.3515 (16)
1.3167 (16)
1.4076 (18)
1.4151 (17)
1.3244 (16)
C2ÐN2
C5ÐN5
N5ÐO51
N5ÐO52
C4ÐO4
C6ÐO6
1.3299 (16)
C14ÐC15ÐN15ÐO151
C16ÐC15ÐN15ÐO151 179.95 (13)
� 0.4 (2)
C24ÐC25ÐN25ÐO251
C26ÐC25ÐN25ÐO251 175.77 (13)
� 4.9 (2)
1.4367 (16)
1.2368 (14)
1.2287 (14)
1.3265 (15)
1.3273 (15)
N11ÐC12ÐO12ÐC121
N11ÐC16ÐO16ÐC161
� 3.7 (2)
N21ÐC22ÐO22ÐC221
N21ÐC26ÐO26ÐC261
0.2 (2)
� 2.44 (19)
� 1.36 (19)
C4ÐC5ÐN5ÐO51
C6ÐC5ÐN5ÐO51
� 29.94 (17)
N1ÐC6ÐO6ÐC61
N3ÐC4ÐO4ÐC41
3.10 (17)
2.14 (16)
149.95 (12)
Table 4
Hydrogen-bonding geometry (A, ) for (II).
Ê
ꢀ
DÐHÁ Á ÁA
DÐH
HÁ Á ÁA
DÁ Á ÁA
DÐHÁ Á ÁA
Table 2
Hydrogen-bonding geometry (A, ) for (I).
N14ÐH14BÁ Á ÁO151
N24ÐH24BÁ Á ÁO251
N14ÐH14AÁ Á ÁN23
N24ÐH24AÁ Á ÁN13
C121ÐH12BÁ Á ÁO152
C221ÐH22BÁ Á ÁO252
0.88
0.88
0.88
0.88
0.98
0.98
0.98
0.98
0.98
0.98
2.00
2.01
2.16
2.14
2.42
2.44
2.54
2.50
2.54
2.48
2.624 (2)
2.630 (2)
3.027 (2)
3.010 (2)
3.378 (2)
3.394 (2)
3.438 (2)
3.356 (2)
3.441 (2)
3.338 (2)
127
126
169
170
165
165
153
146
152
146
Ê
ꢀ
DÐHÁ Á ÁA
DÐH
HÁ Á ÁA
DÁ Á ÁA
DÐHÁ Á ÁA
i
i
N2ÐH2AÁ Á ÁN1
0.88
0.88
2.26
2.13
3.1210 (16)
2.9951 (15)
165
166
ii
ii
N2ÐH2BÁ Á ÁO51
iii
C161ÐH16BÁ Á ÁO26
iii
1
1
2
C161ÐH16BÁ Á ÁO252
Symmetry codes: (i) 1 � x; � y; 1 � z; (ii) � x; y � 2
;
� z.
iv
C261ÐH26BÁ Á ÁO16
iv
C261ÐH26BÁ Á ÁO152
Symmetry codes: (i) x; 1 ‡ y; z; (ii) x; y � 1; z; (iii) x; y � 1; 1 ‡ z; (iv) x; 1 ‡ y; z � 1.
Compound (II)
Crystal data
1
For compound (I), space group P2 /c was uniquely assigned from
C
6
H N
8 4
O
4
Z = 4
= 1.601 Mg m
Mo Kꢃ radiation
the systematic absences. Compound (II) is triclinic, and space group
P1 was selected and con®rmed by the subsequent analysis. H atoms
Ê
were treated as riding atoms, with CÐH distances of 0.98 A and NÐ
�
3
M
r
= 200.16
Triclinic, P1
D
x
Ê
a = 6.8427 (2) A
Cell parameters from 3699
re¯ections
Ê
b = 9.9742 (5) A
Ê
H distances of 0.88 A.
Ê
c = 13.2752 (5) A
ꢀ
ꢄ = 3.2±27.5
For both compounds, data collection: KappaCCD Server Software
(Nonius, 1997); cell re®nement: DENZO±SMN (Otwinowski &
Minor, 1997); data reduction: DENZO±SMN; program(s) used to
solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to
ꢀ
ꢀ
ꢀ
� 1
ꢃ = 79.007 (2)
ꢂ = 75.577 (3)
ꢉ = 72.547 (2)
V = 830.46 (6) A
ꢅ = 0.14 mm
T = 120 (1) K
Block, colourless
0.40 Â 0.25 Â 0.20 mm
Ê
3
ꢁ
Acta Cryst. (2003). C59, o14±o18
Christopher Glidewell et al.
6 8 4
Two isomers of C H N O
4
o17

organic compounds
re®ne structure: SHELXL97 (Sheldrick, 1997); molecular graphics:
PLATON (Spek, 2002); software used to prepare material for
publication: SHELXL97 and PRPKAPPA (Ferguson, 1999).
Dhaneshwar, N. N., Tavale, S. S. & Pant, L. M. (1978). Acta Cryst. B34, 2507±
509.
Ellena, J., Punte, G. & Rivero, B. E. (1996). Acta Cryst. C52, 2074±2076.
Ferguson, G. (1999). PRPKAPPA. University of Guelph, Canada.
Ferguson, G., Glidewell, C., Low, J. N., Skakle, J. M. S. & Wardell, J. L. (2001).
Acta Cryst. C57, 315±316.
Garden, S. J., Fontes, S. P., Wardell, J. L., Skakle, J. M. S., Low, J. N. &
Glidewell, C. (2002). Acta Cryst. B58, 701±709.
Glidewell, C., Low, J. N., McWilliam, S. A., Skakle, J. M. S. & Wardell, J. L.
(2001). Acta Cryst. C57, 1209±1211.
Glidewell, C., Low, J. N., McWilliam, S. A., Skakle, J. M. S. & Wardell, J. L.
2
The authors thank Dr A. Marchal for the nitroso
compounds used as precursors in the synthesis of (I) and (II).
The X-ray data were collected at the EPSRC X-ray Crystal-
lographic Service, University of Southampton, England; the
authors thank the staff for all their help and advice. JNL
thanks NCR Self-Service, Dundee, for grants which have
provided computing facilities for this work.
(
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Two isomers of C H N O
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Acta Cryst. (2003). C59, o14±o18