Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536802006050/cf6160sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S1600536802006050/cf6160Isup2.hkl |
CCDC reference: 185782
A solution of 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine (0.118 g, 0.5 mmol) in methanol (15 ml) was mixed with aqueous nitric acid (2 mol dm-3; 0.5 ml, 1 mmol). Yellow rectangular blocks of the title compound were formed in one month. Analysis found (calculated): C 41.05 (40.90), H 3.45 (3.40), N 23.40 (23.85)%. IR cm-1 (all bands due to cation unless stated otherwise): 1697 (m), 1637 (s), 1612 (s), 1570 (m), 1500 (s), 1384 (w, NO3-), 819 (s), 744 (s).
H atoms bonded to N atoms were refined with geometrical restraints, while those bonded to C atoms were constrained with a riding model.
Data collection: SMART (Bruker, 1997); cell refinement: SAINT (Bruker, 2000); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: SHELXL97 and PLATON (Spek, 2001).
C12H12N4O2+·2NO3− | Dx = 1.614 Mg m−3 |
Mr = 352.28 | Melting point: unknown K |
Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
a = 7.9639 (9) Å | Cell parameters from 3001 reflections |
b = 14.017 (2) Å | θ = 2.9–27.6° |
c = 13.0563 (15) Å | µ = 0.14 mm−1 |
β = 95.995 (2)° | T = 150 K |
V = 1449.5 (3) Å3 | Rectangular block, yellow |
Z = 4 | 0.26 × 0.13 × 0.08 mm |
F(000) = 728 |
Bruker SMART1000 CCD area-detector diffractometer | 2275 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.040 |
Graphite monochromator | θmax = 28.9°, θmin = 2.1° |
ω scans | h = −10→10 |
10487 measured reflections | k = −17→18 |
3595 independent reflections | l = −12→16 |
Refinement on F2 | Primary atom site location: direct method |
Least-squares matrix: full | Secondary atom site location: difference Fourer method |
R[F2 > 2σ(F2)] = 0.042 | Hydrogen site location: difference Fourier map |
wR(F2) = 0.116 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.00 | w = 1/[σ2(Fo2) + (0.0615P)2] where P = (Fo2 + 2Fc2)/3 |
3449 reflections | (Δ/σ)max < 0.001 |
235 parameters | Δρmax = 0.32 e Å−3 |
3 restraints | Δρmin = −0.20 e Å−3 |
C12H12N4O2+·2NO3− | V = 1449.5 (3) Å3 |
Mr = 352.28 | Z = 4 |
Monoclinic, P21/n | Mo Kα radiation |
a = 7.9639 (9) Å | µ = 0.14 mm−1 |
b = 14.017 (2) Å | T = 150 K |
c = 13.0563 (15) Å | 0.26 × 0.13 × 0.08 mm |
β = 95.995 (2)° |
Bruker SMART1000 CCD area-detector diffractometer | 2275 reflections with I > 2σ(I) |
10487 measured reflections | Rint = 0.040 |
3595 independent reflections |
R[F2 > 2σ(F2)] = 0.042 | 3 restraints |
wR(F2) = 0.116 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.00 | Δρmax = 0.32 e Å−3 |
3449 reflections | Δρmin = −0.20 e Å−3 |
235 parameters |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. Pyridinium H atoms were located from /DF syntheses and refined with the N—H distance restrained to be 0.88 (1) A and with Uiso(H) = 1.5Ueq(N). Aromatic H atoms, after location from /DF syntheses were placed geometrically and refined with a riding model for which the C—H distance was constrained to be 0.96 A and Uiso(H) = 1.2Ueq(C). |
x | y | z | Uiso*/Ueq | ||
O3R | 0.96466 (17) | −0.07286 (9) | 0.76440 (10) | 0.0307 (3) | |
O1R | 1.02768 (17) | −0.07953 (10) | 0.60743 (10) | 0.0354 (4) | |
O2R | 1.14799 (17) | −0.17627 (10) | 0.72197 (11) | 0.0394 (4) | |
N1R | 1.04898 (19) | −0.10988 (11) | 0.69672 (13) | 0.0277 (4) | |
O1N | −0.14682 (17) | 0.86438 (10) | 0.26074 (11) | 0.0388 (4) | |
N1N | −0.24467 (19) | 0.89916 (11) | 0.32060 (13) | 0.0282 (4) | |
O3N | −0.23428 (17) | 0.86905 (10) | 0.41123 (11) | 0.0355 (3) | |
O2N | −0.34792 (18) | 0.96135 (10) | 0.29023 (12) | 0.0439 (4) | |
N1 | −0.02143 (19) | 0.70651 (11) | 0.38730 (14) | 0.0333 (4) | |
H1 | −0.079 (2) | 0.7550 (11) | 0.3583 (15) | 0.040* | |
C6 | 0.0371 (2) | 0.71637 (13) | 0.48571 (16) | 0.0322 (5) | |
H6 | 0.0103 | 0.7718 | 0.5227 | 0.039* | |
C5 | 0.1372 (2) | 0.64558 (13) | 0.53424 (15) | 0.0282 (4) | |
H5 | 0.1808 | 0.6521 | 0.6044 | 0.034* | |
C4 | 0.1732 (2) | 0.56442 (12) | 0.47855 (14) | 0.0222 (4) | |
C3 | 0.1057 (2) | 0.55751 (13) | 0.37634 (14) | 0.0269 (4) | |
H3 | 0.1267 | 0.5024 | 0.3373 | 0.032* | |
C2 | 0.0095 (2) | 0.62969 (14) | 0.33207 (16) | 0.0335 (5) | |
H2 | −0.0355 | 0.6253 | 0.2619 | 0.040* | |
C7 | 0.2803 (2) | 0.48987 (12) | 0.53032 (15) | 0.0253 (4) | |
H7 | 0.3254 | 0.4971 | 0.6002 | 0.030* | |
N8 | 0.31189 (18) | 0.41513 (10) | 0.47982 (12) | 0.0247 (3) | |
N9 | 0.41337 (18) | 0.34818 (11) | 0.53276 (12) | 0.0245 (3) | |
H9 | 0.451 (2) | 0.3590 (14) | 0.5970 (8) | 0.029* | |
C10 | 0.4481 (2) | 0.26557 (13) | 0.48578 (15) | 0.0274 (4) | |
O11 | 0.39318 (19) | 0.24550 (10) | 0.39825 (11) | 0.0424 (4) | |
C12 | 0.5661 (2) | 0.19818 (13) | 0.54911 (14) | 0.0251 (4) | |
C13 | 0.6137 (2) | 0.11657 (13) | 0.49977 (15) | 0.0297 (4) | |
H13 | 0.5722 | 0.1050 | 0.4300 | 0.036* | |
C14 | 0.7217 (2) | 0.05217 (13) | 0.55246 (15) | 0.0313 (5) | |
H14 | 0.7550 | −0.0040 | 0.5192 | 0.038* | |
N15 | 0.77876 (19) | 0.06924 (11) | 0.65007 (13) | 0.0281 (4) | |
H15 | 0.844 (2) | 0.0264 (11) | 0.6845 (14) | 0.034* | |
C16 | 0.7354 (2) | 0.14668 (13) | 0.70104 (15) | 0.0298 (4) | |
H16 | 0.7778 | 0.1559 | 0.7711 | 0.036* | |
C17 | 0.6291 (2) | 0.21258 (13) | 0.65112 (15) | 0.0294 (4) | |
H17 | 0.5985 | 0.2682 | 0.6864 | 0.035* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O3R | 0.0361 (7) | 0.0301 (7) | 0.0256 (7) | 0.0046 (6) | 0.0026 (6) | −0.0009 (6) |
O1R | 0.0419 (8) | 0.0371 (8) | 0.0266 (8) | 0.0087 (6) | 0.0004 (6) | 0.0032 (6) |
O2R | 0.0326 (7) | 0.0334 (8) | 0.0512 (10) | 0.0106 (6) | −0.0009 (7) | 0.0088 (7) |
N1R | 0.0257 (8) | 0.0230 (8) | 0.0334 (10) | 0.0015 (6) | −0.0017 (7) | 0.0002 (7) |
O1N | 0.0387 (8) | 0.0422 (8) | 0.0346 (8) | 0.0072 (7) | 0.0001 (7) | −0.0048 (7) |
N1N | 0.0268 (8) | 0.0209 (8) | 0.0348 (10) | 0.0008 (6) | −0.0067 (7) | 0.0009 (7) |
O3N | 0.0373 (7) | 0.0342 (8) | 0.0345 (9) | 0.0054 (6) | 0.0012 (6) | 0.0061 (6) |
O2N | 0.0415 (8) | 0.0318 (8) | 0.0551 (10) | 0.0136 (6) | −0.0110 (7) | 0.0060 (7) |
N1 | 0.0264 (8) | 0.0214 (8) | 0.0491 (11) | 0.0042 (7) | −0.0097 (8) | 0.0046 (8) |
C6 | 0.0276 (9) | 0.0218 (10) | 0.0472 (13) | 0.0007 (8) | 0.0041 (9) | −0.0074 (9) |
C5 | 0.0277 (9) | 0.0301 (10) | 0.0262 (10) | −0.0031 (8) | 0.0003 (8) | −0.0027 (8) |
C4 | 0.0189 (8) | 0.0199 (8) | 0.0279 (10) | −0.0006 (7) | 0.0027 (7) | 0.0035 (7) |
C3 | 0.0284 (9) | 0.0245 (9) | 0.0272 (10) | 0.0014 (7) | 0.0004 (8) | −0.0033 (8) |
C2 | 0.0354 (10) | 0.0318 (11) | 0.0310 (12) | −0.0005 (8) | −0.0077 (9) | 0.0009 (8) |
C7 | 0.0243 (9) | 0.0240 (9) | 0.0268 (10) | 0.0005 (7) | −0.0002 (8) | 0.0031 (8) |
N8 | 0.0225 (7) | 0.0217 (8) | 0.0299 (9) | 0.0027 (6) | 0.0020 (6) | 0.0052 (7) |
N9 | 0.0250 (7) | 0.0234 (8) | 0.0240 (9) | 0.0055 (6) | −0.0031 (6) | 0.0028 (7) |
C10 | 0.0303 (10) | 0.0263 (10) | 0.0253 (11) | 0.0020 (8) | 0.0005 (8) | 0.0029 (8) |
O11 | 0.0597 (10) | 0.0366 (8) | 0.0282 (8) | 0.0143 (7) | −0.0082 (7) | −0.0001 (6) |
C12 | 0.0244 (9) | 0.0228 (9) | 0.0279 (10) | 0.0015 (7) | 0.0024 (8) | 0.0062 (8) |
C13 | 0.0347 (10) | 0.0310 (10) | 0.0229 (10) | 0.0011 (8) | 0.0010 (8) | 0.0002 (8) |
C14 | 0.0359 (10) | 0.0256 (10) | 0.0331 (12) | 0.0054 (8) | 0.0076 (9) | −0.0041 (8) |
N15 | 0.0255 (8) | 0.0241 (8) | 0.0335 (9) | 0.0056 (6) | −0.0019 (7) | 0.0037 (7) |
C16 | 0.0318 (9) | 0.0296 (10) | 0.0267 (11) | −0.0013 (8) | −0.0025 (8) | −0.0027 (8) |
C17 | 0.0305 (9) | 0.0229 (9) | 0.0342 (11) | 0.0037 (8) | 0.0004 (8) | −0.0042 (8) |
O3R—N1R | 1.275 (2) | C7—N8 | 1.277 (2) |
O1R—N1R | 1.236 (2) | C7—H7 | 0.9500 |
O2R—N1R | 1.2418 (19) | N8—N9 | 1.376 (2) |
O1N—N1N | 1.258 (2) | N9—C10 | 1.353 (2) |
N1N—O2N | 1.2349 (19) | N9—H9 | 0.874 (9) |
N1N—O3N | 1.251 (2) | C10—O11 | 1.213 (2) |
N1—C6 | 1.327 (3) | C10—C12 | 1.515 (2) |
N1—C2 | 1.333 (3) | C12—C13 | 1.385 (3) |
N1—H1 | 0.882 (9) | C12—C17 | 1.388 (3) |
C6—C5 | 1.384 (3) | C13—C14 | 1.380 (3) |
C6—H6 | 0.9500 | C13—H13 | 0.9500 |
C5—C4 | 1.396 (3) | C14—N15 | 1.329 (2) |
C5—H5 | 0.9500 | C14—H14 | 0.9500 |
C4—C3 | 1.389 (2) | N15—C16 | 1.337 (2) |
C4—C7 | 1.469 (2) | N15—H15 | 0.887 (9) |
C3—C2 | 1.361 (3) | C16—C17 | 1.371 (3) |
C3—H3 | 0.9500 | C16—H16 | 0.9500 |
C2—H2 | 0.9500 | C17—H17 | 0.9500 |
O1R—N1R—O2R | 121.94 (17) | C4—C7—H7 | 120.6 |
O1R—N1R—O3R | 118.91 (14) | C7—N8—N9 | 115.92 (15) |
O2R—N1R—O3R | 119.14 (16) | C10—N9—N8 | 119.55 (15) |
O2N—N1N—O3N | 121.19 (17) | C10—N9—H9 | 121.2 (13) |
O2N—N1N—O1N | 120.59 (17) | N8—N9—H9 | 119.2 (13) |
O3N—N1N—O1N | 118.22 (15) | O11—C10—N9 | 123.70 (17) |
C6—N1—C2 | 122.80 (16) | O11—C10—C12 | 120.76 (17) |
C6—N1—H1 | 116.7 (14) | N9—C10—C12 | 115.52 (16) |
C2—N1—H1 | 120.5 (14) | C13—C12—C17 | 118.52 (16) |
N1—C6—C5 | 119.70 (18) | C13—C12—C10 | 116.44 (16) |
N1—C6—H6 | 120.2 | C17—C12—C10 | 125.04 (17) |
C5—C6—H6 | 120.2 | C14—C13—C12 | 119.67 (18) |
C6—C5—C4 | 119.05 (18) | C14—C13—H13 | 120.2 |
C6—C5—H5 | 120.5 | C12—C13—H13 | 120.2 |
C4—C5—H5 | 120.5 | N15—C14—C13 | 119.55 (17) |
C3—C4—C5 | 118.52 (16) | N15—C14—H14 | 120.2 |
C3—C4—C7 | 122.71 (17) | C13—C14—H14 | 120.2 |
C5—C4—C7 | 118.77 (17) | C14—N15—C16 | 122.88 (16) |
C2—C3—C4 | 120.02 (18) | C14—N15—H15 | 119.2 (13) |
C2—C3—H3 | 120.0 | C16—N15—H15 | 117.8 (13) |
C4—C3—H3 | 120.0 | N15—C16—C17 | 119.28 (18) |
N1—C2—C3 | 119.89 (18) | N15—C16—H16 | 120.4 |
N1—C2—H2 | 120.1 | C17—C16—H16 | 120.4 |
C3—C2—H2 | 120.1 | C16—C17—C12 | 120.10 (17) |
N8—C7—C4 | 118.85 (17) | C16—C17—H17 | 120.0 |
N8—C7—H7 | 120.6 | C12—C17—H17 | 120.0 |
C2—N1—C6—C5 | 1.2 (3) | N8—N9—C10—C12 | 177.82 (15) |
N1—C6—C5—C4 | −0.7 (3) | O11—C10—C12—C13 | 3.5 (3) |
C6—C5—C4—C3 | −0.5 (3) | N9—C10—C12—C13 | −175.19 (17) |
C6—C5—C4—C7 | 179.62 (17) | O11—C10—C12—C17 | −176.79 (19) |
C5—C4—C3—C2 | 1.2 (3) | N9—C10—C12—C17 | 4.5 (3) |
C7—C4—C3—C2 | −178.89 (17) | C17—C12—C13—C14 | −0.1 (3) |
C6—N1—C2—C3 | −0.4 (3) | C10—C12—C13—C14 | 179.64 (18) |
C4—C3—C2—N1 | −0.8 (3) | C12—C13—C14—N15 | 0.0 (3) |
C3—C4—C7—N8 | −0.6 (3) | C13—C14—N15—C16 | 0.5 (3) |
C5—C4—C7—N8 | 179.26 (17) | C14—N15—C16—C17 | −0.8 (3) |
C4—C7—N8—N9 | 179.86 (15) | N15—C16—C17—C12 | 0.7 (3) |
C7—N8—N9—C10 | 177.81 (17) | C13—C12—C17—C16 | −0.2 (3) |
N8—N9—C10—O11 | −0.8 (3) | C10—C12—C17—C16 | −179.97 (18) |
Experimental details
Crystal data | |
Chemical formula | C12H12N4O2+·2NO3− |
Mr | 352.28 |
Crystal system, space group | Monoclinic, P21/n |
Temperature (K) | 150 |
a, b, c (Å) | 7.9639 (9), 14.017 (2), 13.0563 (15) |
β (°) | 95.995 (2) |
V (Å3) | 1449.5 (3) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.14 |
Crystal size (mm) | 0.26 × 0.13 × 0.08 |
Data collection | |
Diffractometer | Bruker SMART1000 CCD area-detector diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 10487, 3595, 2275 |
Rint | 0.040 |
(sin θ/λ)max (Å−1) | 0.679 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.042, 0.116, 1.00 |
No. of reflections | 3449 |
No. of parameters | 235 |
No. of restraints | 3 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.32, −0.20 |
Computer programs: SMART (Bruker, 1997), SAINT (Bruker, 2000), SAINT, SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), CAMERON (Watkin et al., 1996), SHELXL97 and PLATON (Spek, 2001).
Contact | X-H | H···O | X···O | X-H···O | H···O-N | torsa | |
N1-H1···O1N | 0.88 | 2.03 | 2.878 (3) | 161 | 93 | 13.4 | |
N9-H9···O3Rv | 0.87 | 2.09 | 2.940 (3) | 163 | 106 | 3.2 | |
N15-H15···O3R | 0.89 | 1.93 | 2.815 (3) | 174 | 101 | 2.8 | |
C16-H16···O11vi | 0.95 | 2.28 | 3.133 (3) | 149 | 129 | 17.9 | |
C5-H5···O1Nvii | 0.95 | 2.35 | 3.265 (3) | 162 | 104 | 40.4 | |
C6-H6···O1Rviii | 0.95 | 2.36 | 3.277 (3) | 163 | 98 | 44.6 | |
C14-H14···O3Niii | 0.95 | 2.28 | 3.201 (3) | 164 | 109 | 43.9 | |
C17-H17···O2Rv | 0.95 | 2.53 | 3.287 (3) | 137 | 82 | 44.4 | |
C17-H17···O3Rv | 0.95 | 2.39 | 3.315 (3) | 166 | 87 | 47.2 |
Contact | Minimum | Maximum | Mean |
N···O | 2.815 (3) | 2.940 (3) | 2.878 (3) |
H···O | 1.93 | 2.09 | 2.02 (8) |
N-H···O | 161 | 174 | 166 (7) |
H···O-N | 93 | 106 | 100 (6) |
H···O-N-O | 2.8 | 13.4 | 7(4) |
As part of our analysis of the roles of N—H···O and C—H···O hydrogen-bonding interactions in bipyridinium nitrates (Felloni et al., 2002), the synthesis and structural characterization of 3,6-bis(pyridin-4-io)-1,2,4,5-tetrazine dinitrate was targeted. The product of the reaction between 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine and aqueous nitric acid, however, was the hydrolysis product, 1-(4-pyridinio)-2'-[4-pyridinio-(E)-methylidene]hydrazide dinitrate, (I) (Fig. 1). Since the presence of the amido moiety in the section linking the two pyridinium rings radically alters the hydrogen-bonding potential of this cation compared with that of simple bipyridinium dinitrates, we thought it appropriate to consider its structure separately.
The extended structure of (I) is complex. An extensive hydrogen-bonding network, primarily N—H···O interactions, but with supporting C—H···O contacts, leads to a three-dimensional matrix structure. The long-range structure can be considered in a number of different ways. Our preferred description is that of a cationic framework with bridging nitrate anions. The cations align along the [101] direction in the form of a one-dimensional zigzag chain (Fig. 2). A relatively short C—H···O interaction (C—H 0.95, H···O 2.28, C···O 3.13 Å and C—H···O 149°) involving α-pyridinium carbons and carbonyl O atoms links the cations in a C(6) motif (Bernstein et al., 1995). Adjacent chains are arranged in a centrosymmetric fashion in the (101) plane to generate a two-dimensional sheet structure (Fig. 2). The sheets stack in the [101] direction to give the three-dimensional matrix structure (Fig. 3).
Between the cations lie the nitrate anions which act as hydrogen-bond acceptors for both N—H···O and C—H···O interactions. Both crystallographically independent nitrate anions are surrounded by three cations; one acts as an acceptor to one N—H donor [N1—H1···O1N; Fig. 4(a)] and two C—H donors [C5—H5···O1N and C14—H14···O3N; Fig. 4(a)], the other as an acceptor to two N—H donors [N9—H9···O3R and N15—H15···O3R; Fig. 4(b)], one normal C—H donor [C6—H6···O1R; Fig 4(b)] and one bifurcated C—H donor [C17—H17···O2R,O3R; Fig. 4(b)]. The structural parameters for the N—H···O contacts (Fig. 5 and Table 1) fall into narrow ranges (Table 2), the means of which are typical of intermediate strength N—H···O hydrogen bonds (Jeffrey, 1997, 1995; Speakman, 1975). They are considerably weaker than those found in the simple bipyridinium nitrates, as illustrated by their longer N···O and H···O mean interatomic distances [2.88 and 2.02 Å (Table 3) cf. 2.74 and 1.89 Å (Felloni et al., 2002)].
The structural parameters of the C—H···O contacts also fall into a narrow range. Unlike the N—H···O contacts, however, they exhibit a lack of coplanarity between cation and anion which is manifest in H···O—N—O torsion angles in the range 40.4–47.2° (Table 1). They are typical of type-3 C—H···O contacts [Fig. 6(c); Felloni et al., 2002]. Type 1 contacts [Fig. 6(a)] are the conventional C—H···O interactions involving nitrate anions, in which the cation and anion are coplanar and the H···O—N angle is typical of sp2-hybridized oxygen (~120°). Type 2 contacts [Fig. 6(b)] differ from type 1 contacts solely in the direction of approach of the N—H donor which bisects two O atoms, giving a much larger H···O—N angle (~180°).
The structural parameters of the cation-anion C—H···O contacts in the title compound, when compared to those of the inter-cation C—H···O contact, for which not only is the torsion angle smaller but also the interatomic distances H···O and N···O are shorter (Table 1), suggest that the inter-cation C—H···O contact is the stronger interaction. As this observation conflicts with conventional arguments regarding electrostatics, it is clear that the C—H···O interactions must be considerably inferior to the N—H···O contacts.