Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270102007266/na1570sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270102007266/na1570Isup2.hkl |
CCDC reference: 188619
Melamine was dissolved in a 20% solution of glutaric acid and the resulting solution was slowly evaporated. After several days, colourless crystals of (I) appeared.
All H atoms were refined except those of the CH2 groups, which were constrained as riding, with C—H = 0.97 Å.
Data collection: KM-4 CCD Software (Kuma Diffraction, 2000); cell refinement: KM-4 CCD Software; data reduction: KM-4 CCD Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Sheldrick, 1990); software used to prepare material for publication: SHELXL97.
C3H7N6+·C5H7O4−·H2O | F(000) = 584 |
Mr = 276.27 | Dx = 1.449 Mg m−3 Dm = 1.444 Mg m−3 Dm measured by flotation |
Monoclinic, P21/c | Melting point: dehydrated K |
Hall symbol: -P 2ybc | Mo Kα radiation, λ = 0.71073 Å |
a = 4.539 (1) Å | Cell parameters from 1600 reflections |
b = 28.544 (6) Å | θ = 3–28° |
c = 9.863 (2) Å | µ = 0.12 mm−1 |
β = 97.57 (3)° | T = 293 K |
V = 1266.7 (5) Å3 | Parallelepiped, colourless |
Z = 4 | 0.20 × 0.18 × 0.12 mm |
Kuma KM-4 with CCD area-detector diffractometer | 2920 independent reflections |
Radiation source: fine-focus sealed tube | 1605 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.019 |
Detector resolution: 1024 x 1024 with blocks 2 x 2 pixels mm-1 | θmax = 28.0°, θmin = 2.9° |
ω scan | h = −5→5 |
Absorption correction: analytical face-indexed (SHELXTL; Sheldrick, 1990) | k = −37→37 |
Tmin = 0.976, Tmax = 0.985 | l = −12→13 |
11010 measured reflections |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.029 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.070 | w = 1/[σ2(Fo2) + (0.0274P)2] where P = (Fo2 + 2Fc2)/3 |
S = 1.01 | (Δ/σ)max = 0.004 |
2920 reflections | Δρmax = 0.14 e Å−3 |
203 parameters | Δρmin = −0.12 e Å−3 |
0 restraints | Extinction correction: SHELXL97 (Sheldrick, 1997), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.0084 (10) |
C3H7N6+·C5H7O4−·H2O | V = 1266.7 (5) Å3 |
Mr = 276.27 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 4.539 (1) Å | µ = 0.12 mm−1 |
b = 28.544 (6) Å | T = 293 K |
c = 9.863 (2) Å | 0.20 × 0.18 × 0.12 mm |
β = 97.57 (3)° |
Kuma KM-4 with CCD area-detector diffractometer | 2920 independent reflections |
Absorption correction: analytical face-indexed (SHELXTL; Sheldrick, 1990) | 1605 reflections with I > 2σ(I) |
Tmin = 0.976, Tmax = 0.985 | Rint = 0.019 |
11010 measured reflections |
R[F2 > 2σ(F2)] = 0.029 | 0 restraints |
wR(F2) = 0.070 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.01 | Δρmax = 0.14 e Å−3 |
2920 reflections | Δρmin = −0.12 e Å−3 |
203 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. |
x | y | z | Uiso*/Ueq | ||
N1 | 0.3304 (2) | 0.48729 (3) | 0.14798 (10) | 0.0362 (3) | |
N2 | 0.7210 (2) | 0.48827 (3) | 0.33912 (10) | 0.0372 (3) | |
N3 | 0.4139 (2) | 0.55352 (4) | 0.28920 (11) | 0.0394 (3) | |
H3 | 0.364 (3) | 0.5831 (5) | 0.3114 (13) | 0.047* | |
C1 | 0.5606 (3) | 0.46749 (4) | 0.22968 (12) | 0.0359 (3) | |
C2 | 0.6430 (3) | 0.53172 (4) | 0.36590 (12) | 0.0355 (3) | |
C3 | 0.2611 (3) | 0.53069 (4) | 0.17996 (12) | 0.0356 (3) | |
N4 | 0.6369 (3) | 0.42445 (4) | 0.20079 (12) | 0.0464 (3) | |
H41 | 0.538 (3) | 0.4089 (4) | 0.1233 (13) | 0.056* | |
H42 | 0.793 (3) | 0.4105 (5) | 0.2586 (13) | 0.056* | |
N5 | 0.7921 (3) | 0.55529 (4) | 0.46793 (12) | 0.0477 (3) | |
H51 | 0.930 (3) | 0.5404 (5) | 0.5198 (14) | 0.057* | |
H52 | 0.743 (3) | 0.5834 (5) | 0.4846 (14) | 0.057* | |
N6 | 0.0379 (3) | 0.55329 (4) | 0.10906 (12) | 0.0448 (3) | |
H61 | −0.052 (3) | 0.5403 (4) | 0.0303 (14) | 0.054* | |
H62 | 0.012 (3) | 0.5823 (5) | 0.1306 (13) | 0.054* | |
O1 | 1.0579 (2) | 0.84827 (3) | 0.68347 (10) | 0.0539 (3) | |
O2 | 1.3274 (2) | 0.85847 (3) | 0.88434 (10) | 0.0564 (3) | |
O3 | 0.8772 (2) | 0.63348 (3) | 0.64853 (10) | 0.0611 (3) | |
O4 | 0.5275 (2) | 0.68672 (3) | 0.60591 (11) | 0.0635 (3) | |
H4 | 0.443 (4) | 0.6680 (6) | 0.5261 (18) | 0.095* | |
C4 | 1.1598 (3) | 0.83364 (4) | 0.80016 (14) | 0.0411 (3) | |
C5 | 1.0881 (3) | 0.78512 (4) | 0.84331 (13) | 0.0487 (4) | |
H5A | 0.9546 | 0.7875 | 0.9121 | 0.058* | |
H5B | 1.2702 | 0.7707 | 0.8864 | 0.058* | |
C6 | 0.9484 (3) | 0.75291 (4) | 0.73177 (13) | 0.0447 (3) | |
H6A | 1.0730 | 0.7518 | 0.6591 | 0.054* | |
H6B | 0.7564 | 0.7654 | 0.6937 | 0.054* | |
C7 | 0.9081 (3) | 0.70353 (4) | 0.78398 (13) | 0.0445 (3) | |
H7A | 1.1000 | 0.6914 | 0.8234 | 0.053* | |
H7B | 0.7817 | 0.7047 | 0.8559 | 0.053* | |
C8 | 0.7741 (3) | 0.67098 (4) | 0.67485 (13) | 0.0410 (3) | |
O5W | 0.4736 (3) | 0.35602 (4) | 0.98748 (11) | 0.0658 (3) | |
H5W1 | 0.314 (4) | 0.3486 (6) | 0.9284 (19) | 0.099* | |
H5W2 | 0.633 (4) | 0.3536 (6) | 0.9389 (19) | 0.099* |
U11 | U22 | U33 | U12 | U13 | U23 | |
N1 | 0.0397 (6) | 0.0300 (5) | 0.0364 (6) | 0.0043 (4) | −0.0041 (5) | −0.0040 (4) |
N2 | 0.0401 (6) | 0.0310 (5) | 0.0375 (6) | 0.0021 (4) | −0.0063 (5) | −0.0038 (4) |
N3 | 0.0431 (6) | 0.0299 (5) | 0.0418 (6) | 0.0044 (5) | −0.0069 (5) | −0.0067 (5) |
C1 | 0.0370 (7) | 0.0309 (6) | 0.0381 (7) | −0.0002 (5) | −0.0017 (6) | −0.0032 (5) |
C2 | 0.0370 (7) | 0.0301 (5) | 0.0378 (7) | −0.0016 (5) | −0.0017 (6) | −0.0020 (6) |
C3 | 0.0358 (7) | 0.0324 (6) | 0.0370 (6) | −0.0021 (5) | −0.0013 (6) | 0.0007 (5) |
N4 | 0.0505 (7) | 0.0350 (6) | 0.0489 (7) | 0.0088 (5) | −0.0111 (6) | −0.0086 (5) |
N5 | 0.0530 (7) | 0.0346 (5) | 0.0501 (7) | 0.0057 (5) | −0.0132 (6) | −0.0115 (6) |
N6 | 0.0485 (7) | 0.0333 (5) | 0.0476 (7) | 0.0063 (5) | −0.0120 (6) | −0.0048 (5) |
O1 | 0.0609 (6) | 0.0449 (5) | 0.0502 (6) | −0.0108 (4) | −0.0143 (5) | 0.0129 (4) |
O2 | 0.0675 (7) | 0.0389 (5) | 0.0556 (6) | −0.0158 (5) | −0.0189 (5) | 0.0083 (5) |
O3 | 0.0632 (7) | 0.0411 (5) | 0.0721 (7) | 0.0099 (5) | −0.0172 (6) | −0.0140 (5) |
O4 | 0.0646 (7) | 0.0500 (6) | 0.0664 (7) | 0.0159 (5) | −0.0270 (6) | −0.0190 (5) |
C4 | 0.0419 (7) | 0.0368 (6) | 0.0427 (7) | −0.0039 (6) | −0.0011 (7) | 0.0058 (6) |
C5 | 0.0672 (9) | 0.0360 (7) | 0.0398 (7) | −0.0118 (6) | −0.0044 (7) | 0.0005 (6) |
C6 | 0.0593 (9) | 0.0348 (6) | 0.0384 (7) | −0.0055 (6) | 0.0003 (7) | 0.0001 (6) |
C7 | 0.0523 (8) | 0.0350 (6) | 0.0423 (7) | −0.0039 (6) | −0.0080 (7) | −0.0013 (6) |
C8 | 0.0439 (8) | 0.0332 (6) | 0.0434 (8) | −0.0003 (5) | −0.0032 (7) | 0.0010 (6) |
O5W | 0.0577 (7) | 0.0820 (7) | 0.0554 (7) | −0.0094 (6) | −0.0009 (5) | −0.0191 (6) |
N1—C3 | 1.327 (2) | O2—C4 | 1.266 (2) |
N1—C1 | 1.356 (2) | O3—C8 | 1.210 (2) |
N2—C2 | 1.326 (2) | O4—C8 | 1.310 (2) |
N2—C1 | 1.356 (2) | O4—H4 | 0.986 (18) |
N3—C2 | 1.354 (2) | C4—C5 | 1.497 (2) |
N3—C3 | 1.367 (2) | C5—C6 | 1.508 (2) |
N3—H3 | 0.907 (13) | C5—H5A | 0.9700 |
C1—N4 | 1.318 (2) | C5—H5B | 0.9700 |
C2—N5 | 1.321 (2) | C6—C7 | 1.520 (2) |
C3—N6 | 1.321 (2) | C6—H6A | 0.9700 |
N4—H41 | 0.945 (13) | C6—H6B | 0.9700 |
N4—H42 | 0.938 (14) | C7—C8 | 1.490 (2) |
N5—H51 | 0.865 (14) | C7—H7A | 0.9700 |
N5—H52 | 0.853 (13) | C7—H7B | 0.9700 |
N6—H61 | 0.908 (14) | O5W—H5W1 | 0.89 (2) |
N6—H62 | 0.867 (13) | O5W—H5W2 | 0.92 (2) |
O1—C4 | 1.254 (2) | ||
C3—N1—C1 | 115.9 (1) | O1—C4—C5 | 120.1 (1) |
C2—N2—C1 | 116.0 (1) | O2—C4—C5 | 117.8 (1) |
C2—N3—C3 | 119.6 (1) | C4—C5—C6 | 116.3 (1) |
C2—N3—H3 | 119.3 (8) | C4—C5—H5A | 108.2 |
C3—N3—H3 | 121.1 (8) | C6—C5—H5A | 108.2 |
N4—C1—N1 | 117.6 (1) | C4—C5—H5B | 108.2 |
N4—C1—N2 | 116.8 (1) | C6—C5—H5B | 108.2 |
N1—C1—N2 | 125.6 (1) | H5A—C5—H5B | 107.4 |
N5—C2—N2 | 120.3 (1) | C5—C6—C7 | 112.1 (1) |
N5—C2—N3 | 118.1 (1) | C5—C6—H6A | 109.2 |
N2—C2—N3 | 121.6 (1) | C7—C6—H6A | 109.2 |
N6—C3—N1 | 121.3 (1) | C5—C6—H6B | 109.2 |
N6—C3—N3 | 117.3 (1) | C7—C6—H6B | 109.2 |
N1—C3—N3 | 121.4 (1) | H6A—C6—H6B | 107.9 |
C1—N4—H41 | 120.2 (8) | C8—C7—C6 | 112.9 (1) |
C1—N4—H42 | 117.6 (8) | C8—C7—H7A | 109.0 |
H41—N4—H42 | 122.2 (11) | C6—C7—H7A | 109.0 |
C2—N5—H51 | 117.3 (9) | C8—C7—H7B | 109.0 |
C2—N5—H52 | 120.4 (9) | C6—C7—H7B | 109.0 |
H51—N5—H52 | 122.2 (13) | H7A—C7—H7B | 107.8 |
C3—N6—H61 | 118.6 (8) | O3—C8—O4 | 121.3 (1) |
C3—N6—H62 | 117.1 (9) | O3—C8—C7 | 125.1 (1) |
H61—N6—H62 | 122.8 (12) | O4—C8—C7 | 113.6 (1) |
C8—O4—H4 | 116.3 (10) | H5W1—O5W—H5W2 | 105.5 (16) |
O1—C4—O2 | 122.2 (1) |
D—H···A | D—H | H···A | D···A | D—H···A |
N3—H3···O2i | 0.91 (2) | 1.83 (2) | 2.728 (2) | 168 (1) |
N4—H41···O5Wii | 0.95 (2) | 2.01 (2) | 2.895 (2) | 155 (1) |
N4—H42···O3iii | 0.94 (2) | 2.07 (2) | 2.991 (2) | 166 (1) |
N5—H51···N2iii | 0.87 (2) | 2.13 (2) | 2.991 (2) | 173 (1) |
N5—H52···O3 | 0.85 (2) | 2.18 (2) | 2.850 (2) | 135 (1) |
N6—H61···N1iv | 0.91 (2) | 2.17 (2) | 3.074 (2) | 171 (1) |
N6—H62···O1i | 0.87 (2) | 2.05 (2) | 2.903 (2) | 167 (1) |
O4—H4···O2i | 0.99 (2) | 1.62 (2) | 2.597 (2) | 173 (2) |
O5W—H5W1···O1v | 0.89 (2) | 1.89 (2) | 2.764 (2) | 165 (2) |
O5W—H5W2···O1vi | 0.92 (2) | 1.97 (2) | 2.891 (2) | 174 (2) |
Symmetry codes: (i) x−1, −y+3/2, z−1/2; (ii) x, y, z−1; (iii) −x+2, −y+1, −z+1; (iv) −x, −y+1, −z; (v) −x+1, y−1/2, −z+3/2; (vi) −x+2, y−1/2, −z+3/2. |
Experimental details
Crystal data | |
Chemical formula | C3H7N6+·C5H7O4−·H2O |
Mr | 276.27 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 293 |
a, b, c (Å) | 4.539 (1), 28.544 (6), 9.863 (2) |
β (°) | 97.57 (3) |
V (Å3) | 1266.7 (5) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.12 |
Crystal size (mm) | 0.20 × 0.18 × 0.12 |
Data collection | |
Diffractometer | Kuma KM-4 with CCD area-detector diffractometer |
Absorption correction | Analytical face-indexed (SHELXTL; Sheldrick, 1990) |
Tmin, Tmax | 0.976, 0.985 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 11010, 2920, 1605 |
Rint | 0.019 |
(sin θ/λ)max (Å−1) | 0.660 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.029, 0.070, 1.01 |
No. of reflections | 2920 |
No. of parameters | 203 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.14, −0.12 |
Computer programs: KM-4 CCD Software (Kuma Diffraction, 2000), KM-4 CCD Software, SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), SHELXTL (Sheldrick, 1990), SHELXL97.
N1—C3 | 1.327 (2) | O1—C4 | 1.254 (2) |
N1—C1 | 1.356 (2) | O2—C4 | 1.266 (2) |
N2—C2 | 1.326 (2) | O3—C8 | 1.210 (2) |
N2—C1 | 1.356 (2) | O4—C8 | 1.310 (2) |
N3—C2 | 1.354 (2) | C4—C5 | 1.497 (2) |
N3—C3 | 1.367 (2) | C5—C6 | 1.508 (2) |
C1—N4 | 1.318 (2) | C6—C7 | 1.520 (2) |
C2—N5 | 1.321 (2) | C7—C8 | 1.490 (2) |
C3—N6 | 1.321 (2) | ||
C3—N1—C1 | 115.9 (1) | O2—C4—C5 | 117.8 (1) |
C2—N2—C1 | 116.0 (1) | C4—C5—C6 | 116.3 (1) |
C2—N3—C3 | 119.6 (1) | C5—C6—C7 | 112.1 (1) |
N1—C1—N2 | 125.6 (1) | C8—C7—C6 | 112.9 (1) |
N2—C2—N3 | 121.6 (1) | O3—C8—O4 | 121.3 (1) |
N1—C3—N3 | 121.4 (1) | O3—C8—C7 | 125.1 (1) |
O1—C4—O2 | 122.2 (1) | O4—C8—C7 | 113.6 (1) |
O1—C4—C5 | 120.1 (1) |
D—H···A | D—H | H···A | D···A | D—H···A |
N3—H3···O2i | 0.91 (2) | 1.83 (2) | 2.728 (2) | 168 (1) |
N4—H41···O5Wii | 0.95 (2) | 2.01 (2) | 2.895 (2) | 155 (1) |
N4—H42···O3iii | 0.94 (2) | 2.07 (2) | 2.991 (2) | 166 (1) |
N5—H51···N2iii | 0.87 (2) | 2.13 (2) | 2.991 (2) | 173 (1) |
N5—H52···O3 | 0.85 (2) | 2.18 (2) | 2.850 (2) | 135 (1) |
N6—H61···N1iv | 0.91 (2) | 2.17 (2) | 3.074 (2) | 171 (1) |
N6—H62···O1i | 0.87 (2) | 2.05 (2) | 2.903 (2) | 167 (1) |
O4—H4···O2i | 0.99 (2) | 1.62 (2) | 2.597 (2) | 173 (2) |
O5W—H5W1···O1v | 0.89 (2) | 1.89 (2) | 2.764 (2) | 165 (2) |
O5W—H5W2···O1vi | 0.92 (2) | 1.97 (2) | 2.891 (2) | 174 (2) |
Symmetry codes: (i) x−1, −y+3/2, z−1/2; (ii) x, y, z−1; (iii) −x+2, −y+1, −z+1; (iv) −x, −y+1, −z; (v) −x+1, y−1/2, −z+3/2; (vi) −x+2, y−1/2, −z+3/2. |
This study is a continuation of our investigation into the characterization of the hydrogen bonds formed by melamine in the solid state (Janczak & Perpétuo, 2001a,b,c,d; Perpétuo & Janczak, 2002). Melamine, as well as its organic and inorganic complexes or salts, can develop supramolecular structures via multiple hydrogen bonds by self-assembly of components which contain complementary arrays of hydrogen-bonding sites (MacDonald & Whitesides, 1994; Row, 1999; Krische & Lehn, 2000; Sherrington & Taskinen, 2001). To expand the understanding of the solid state physical-organic chemistry of compounds containing multiple and other hydrogen-bonding systems, herein we present the solid state structure of singly protonated melaminium glutarate monohydrate, (I). Additionally, the geometries of both oppositely charged parts, i.e. the singly protonated melaminium cation and the glutarate anion, are compared with the ab initio fully optimized parameters calculated at the HF/6–31 G(d,p) level (Frisch et al., 1995). The ab initio molecular orbital calculation was carried out on the isolated ions, and the results are illustrated in Fig. 1. \sch
The asymmetric unit of (I) consists of one melaminium residue protonated at one of the three ring N atoms, one glutatrate anion and one water molecule (Fig. 2). The six-membered aromatic ring of the singly protonated melaminium residue exhibits significant distortions from the ideal hexagonal form. The internal C—N—C angle at the protonated N atom is greater than the other two C—N—C angles within the ring. This is a result of the steric effect of a lone-pair electron, predicted by the valence-shell electron-pair repulsion theory (VSEPR; Gillespie, 1963, 1992). As a result of the protonation of the melamine ring at one N atom, the internal N—C—N angle involving only non-protonated N atoms is significantly greater than the N—C—N angles involving both protonated and non-protonated N atoms. The correlation between the internal C—N—C angles within the melaminium ring is similar to those reported for the crystals of barbituric acid with melamine (Zerkowski et al., 1994), melaminium phthalate (Janczak & Perpétuo, 2001a), melaminium chloride hemihydrate (Janczak & Perpétuo, 2001c), bis(melaminium) sulfate dihydrate (Janczak & Perpétuo, 2001 d) and melaminium acetate monohydrate acetic acid solvate (Perpétuo & Janczak, 2002), i.e., those single protonated melaminium salts that have been previously structurally characterized. The ab initio optimized geometry calculated for the singly protonated melaminium residue in (I) (see Fig. 1a) shows a similar correlation between the C—N—C and N—C—N angles within the ring as that seen in the crystal. Thus, the ring distortion of the singly protonated melaminium residue mainly results from the protonation and, to a lesser degree, from the hydrogen-bonding system and crystal packing. The C—N bond lengths within the ring in the optimized melaminium residue are slightly shorter than those in the crystal of (I), while the C—N bond lengths which join the amine groups are quite similar in both treatments. The lengthening of the C—N bonds in the melaminium ring in the crystal is likely to be due to interaction of the π-π clouds between the rings in the stack.
The melaminium residue in the crystal of (I) is involved in nine hydrogen bonds, in seven of them as donor H and in the remaining two as acceptor H. The pair of almost linear N—H···N bonds link the melaminium residue with two neighbouring melaminium residues to form a chain (Fig. 3) that is inclined by ~39, ~21 and ~44° to the a, b and c axes, respectively. The remaining five very bent N—H···O bonds link the chains of melaminium residues with the glutarate anions and the water molecule.
The glutarate anion is involved in seven hydrogen bonds, in six of them as acceptor and in one as donor. Atoms O1, O2 and O3 are involved in two hydrogen bonds as acceptors, while atom O4 is a donor. The C—O bond lengths in the ionized carboxyl group are intermediate between single Csp2—O and double Csp2═O (1.247–1.262 Å; Allen et al. 1987), indicating delocalization of the charge on both O atoms. The slight difference between the C4—O1 and C4—O2 bond lengths correlates well with the strength of the hydrogen bonds in which the O atoms are involved (Table 2). The C—O bond lengths in the protonated carboxyl group (COOH) are similar to the values found in other non-ionized carboxyl groups (Allen et al. 1987).
In the crystal of (I), the C4—C8 chain of the glutarate ion is almost planar; the average deviation of these C atoms from the weighted least-squares plane through them is 0.026 Å. The orientation of the carboxyl COO- and COOH groups with respect to the carbon chain is described by the O1—C4—C5—C6 [12.6 (2)°] and O3—C8—C7—C6 [127.3 (2)°] torsion angles. The optimized geometry of the glutarate anion is illustrated in Fig. 1 b. Both C—O bonds of the ionized carboxyl group are slightly shorter than in the crystal. In the COOH group, the double C═O bond is shorter and the single C—OH bond is longer in the optimized glutarate anion than in the crystal. This is likely to be due to the hydrogen-bond interactions present in the crystal. The ionized carboxyl group (COO-) in the optimized glutarate ion, in contrast with the crystal, is coplanar with the chain of the C atoms, with the O1—C4—C5—C6 torsion angle equal to 0.7°, while the O3—C8—C7—C6 torsion angle, describing the rotation of the COOH group, is equal to 107.4°. The differences between the orientation of the COO- and COOH groups in relation to the plane of the C atoms between the X-ray geometry and the optimized gas-phase calculations are due to the mutual arrangement of the oppositely charged units that form the hydrogen-bonding system (Fig. 3a), since the calculations were performed on the isolated glutarate ion.
The water molecule is involved in three hydrogen bonds, in two as donor and in one as acceptor. As donor, the water molecule forms O—H···O hydrogen bonds with two glutarate anions, whereas as acceptor it forms hydrogen bonds with the melaminium residue via the H atom of an amine group.
In the crystal of (I), the melaminium residues interconnected by N—H···N hydrogen bonds form complementary joined positively charged chains that form a stacking structure. Within one stack, the melaminium residues are separated by ~3.27 Å. This distance is shorter than that between π-aromatic ring systems (~3.4 Å; Pauling, 1960) and indicates a π-π interaction between the melaminium rings within the stack. The glutarate anions form hydrogen-bonded polymeric zigzag chains, [···HOOC(CH2)3COO···HOOC(CH2)3COO···]n, that interconnect neighbouring melaminium chains to form a layer (Fig. 3 b) located in the crystal along the [010] direction (Fig.3a) and which is inclined by ~46 and ~44° to the a and c axes, respectively. The water molecules join neighbouring layers via hydrogen bonds that stabilize the stacking structure.