Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270106003106/dn3001sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270106003106/dn3001Isup2.hkl |
CCDC reference: 603196
Palmitic acid, C16H32O2, was purchased from Fluka with a purity higher than 99.5% as determined by GC–MS. Tests with several solvents were carried out at different temperatures but it was not possible to obtain single crystals of suitable quality. Although with some indications of being slightly twinned, eventually a thin plate grown for 15 d at 298 K from a saturated solution of toluene was selected.
All C-bound H atoms were included in calculated positions and refined with free isotropic displacement parameters using a riding model, with a common variable C—H bond distance [Range of final C–H distances?]. The H atom of the carboxyl function was included in the refinement from an observed position and refined as a free isotropic atom.
Data collection: SMART (Bruker, 2001); cell refinement: SAINT-Plus (Bruker, 2001); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997a); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997a); molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: PARST (Nardelli, 1995).
C16H32O2 | F(000) = 576 |
Mr = 256.42 | Dx = 1.027 Mg m−3 |
Monoclinic, P21/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2ybc | Cell parameters from 2055 reflections |
a = 35.620 (11) Å | θ = 2.3–23.1° |
b = 4.9487 (16) Å | µ = 0.07 mm−1 |
c = 9.406 (3) Å | T = 298 K |
β = 90.447 (5)° | Plate, colourless |
V = 1658.0 (9) Å3 | 0.48 × 0.46 × 0.09 mm |
Z = 4 |
Bruker SMART APEX CCD area-detector diffractometer | 2912 independent reflections |
Radiation source: fine-focus sealed tube | 1918 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.064 |
Detector resolution: 8.26 pixels mm-1 | θmax = 25.1°, θmin = 1.7° |
ω rotations with narrow frames scans | h = −42→42 |
Absorption correction: multi-scan (SADABS in SAINT-Plus; Bruker, 2001) | k = −5→5 |
Tmin = 0.956, Tmax = 0.991 | l = −11→11 |
14859 measured reflections |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.092 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.263 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.13 | w = 1/[σ2(Fo2) + (0.1015P)2 + 1.0414P] where P = (Fo2 + 2Fc2)/3 |
2910 reflections | (Δ/σ)max = 0.006 |
215 parameters | Δρmax = 0.32 e Å−3 |
0 restraints | Δρmin = −0.21 e Å−3 |
C16H32O2 | V = 1658.0 (9) Å3 |
Mr = 256.42 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 35.620 (11) Å | µ = 0.07 mm−1 |
b = 4.9487 (16) Å | T = 298 K |
c = 9.406 (3) Å | 0.48 × 0.46 × 0.09 mm |
β = 90.447 (5)° |
Bruker SMART APEX CCD area-detector diffractometer | 2912 independent reflections |
Absorption correction: multi-scan (SADABS in SAINT-Plus; Bruker, 2001) | 1918 reflections with I > 2σ(I) |
Tmin = 0.956, Tmax = 0.991 | Rint = 0.064 |
14859 measured reflections |
R[F2 > 2σ(F2)] = 0.092 | 0 restraints |
wR(F2) = 0.263 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.13 | Δρmax = 0.32 e Å−3 |
2910 reflections | Δρmin = −0.21 e Å−3 |
215 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. |
x | y | z | Uiso*/Ueq | ||
O1 | 0.49465 (7) | 0.2631 (5) | −0.3717 (2) | 0.0577 (7) | |
H1 | 0.5159 (16) | 0.323 (14) | −0.453 (7) | 0.19 (3)* | |
O2 | 0.45853 (6) | 0.6125 (5) | −0.4249 (2) | 0.0552 (7) | |
C1 | 0.46616 (8) | 0.4111 (6) | −0.3504 (3) | 0.0428 (8) | |
C2 | 0.44219 (9) | 0.3338 (6) | −0.2281 (3) | 0.0471 (8) | |
H2A | 0.4573 (4) | 0.3431 (6) | −0.143 (2) | 0.093 (14)* | |
H2B | 0.4347 (2) | 0.148 (5) | −0.2405 (4) | 0.060 (10)* | |
C3 | 0.40732 (9) | 0.4999 (7) | −0.2052 (4) | 0.0505 (9) | |
H3A | 0.3914 (4) | 0.4857 (7) | −0.287 (2) | 0.072 (11)* | |
H3B | 0.41416 (18) | 0.685 (5) | −0.1940 (4) | 0.062 (10)* | |
C4 | 0.38612 (9) | 0.4074 (8) | −0.0753 (4) | 0.0556 (9) | |
H4A | 0.38142 (16) | 0.208 (6) | −0.0840 (4) | 0.106 (16)* | |
H4B | 0.4025 (5) | 0.4361 (10) | 0.011 (3) | 0.095 (14)* | |
C5 | 0.34925 (9) | 0.5460 (7) | −0.0504 (4) | 0.0566 (9) | |
H5A | 0.3339 (4) | 0.5213 (9) | −0.130 (2) | 0.071 (12)* | |
H5B | 0.35342 (14) | 0.730 (5) | −0.0401 (4) | 0.064 (11)* | |
C6 | 0.32914 (10) | 0.4419 (8) | 0.0799 (4) | 0.0594 (10) | |
H6A | 0.32613 (12) | 0.234 (6) | 0.0703 (4) | 0.096 (14)* | |
H6B | 0.3460 (5) | 0.4782 (12) | 0.168 (3) | 0.104 (15)* | |
C7 | 0.29103 (9) | 0.5629 (8) | 0.1063 (4) | 0.0591 (10) | |
H7A | 0.2754 (5) | 0.5328 (10) | 0.024 (2) | 0.079 (13)* | |
H7B | 0.29374 (11) | 0.755 (6) | 0.1182 (5) | 0.076 (12)* | |
C8 | 0.27149 (10) | 0.4491 (8) | 0.2348 (4) | 0.0606 (10) | |
H8A | 0.26916 (11) | 0.248 (7) | 0.2229 (5) | 0.090 (14)* | |
H8B | 0.2877 (5) | 0.4828 (12) | 0.321 (3) | 0.108 (16)* | |
C9 | 0.23303 (10) | 0.5634 (8) | 0.2624 (4) | 0.0616 (10) | |
H9A | 0.2170 (5) | 0.5321 (11) | 0.177 (3) | 0.084 (13)* | |
H9B | 0.23533 (11) | 0.762 (6) | 0.2756 (5) | 0.088 (13)* | |
C10 | 0.21355 (10) | 0.4461 (8) | 0.3900 (4) | 0.0633 (10) | |
H10A | 0.2302 (5) | 0.4786 (11) | 0.478 (3) | 0.098 (15)* | |
H10B | 0.21120 (11) | 0.240 (6) | 0.3762 (5) | 0.093 (14)* | |
C11 | 0.17498 (10) | 0.5601 (8) | 0.4183 (4) | 0.0659 (11) | |
H11A | 0.17731 (12) | 0.762 (7) | 0.4322 (6) | 0.107 (16)* | |
H11B | 0.1587 (5) | 0.5290 (12) | 0.331 (3) | 0.105 (16)* | |
C12 | 0.15543 (10) | 0.4424 (8) | 0.5454 (5) | 0.0685 (11) | |
H12A | 0.1728 (5) | 0.4767 (12) | 0.639 (3) | 0.086 (13)* | |
H12B | 0.15302 (11) | 0.226 (6) | 0.5309 (6) | 0.117 (17)* | |
C13 | 0.11678 (11) | 0.5566 (9) | 0.5726 (5) | 0.0733 (12) | |
H13A | 0.11902 (12) | 0.753 (7) | 0.5851 (6) | 0.14 (2)* | |
H13B | 0.1011 (6) | 0.5250 (13) | 0.488 (3) | 0.103 (17)* | |
C14 | 0.09692 (11) | 0.4418 (9) | 0.6993 (5) | 0.0745 (12) | |
H14A | 0.1143 (5) | 0.4824 (13) | 0.796 (3) | 0.092 (14)* | |
H14B | 0.09483 (12) | 0.218 (6) | 0.6865 (5) | 0.122 (17)* | |
C15 | 0.05804 (13) | 0.5541 (11) | 0.7226 (6) | 0.0937 (16) | |
H15A | 0.05982 (14) | 0.760 (9) | 0.7311 (6) | 0.16 (2)* | |
H15B | 0.0418 (7) | 0.5112 (19) | 0.635 (4) | 0.18 (3)* | |
C16 | 0.03875 (14) | 0.4452 (13) | 0.8519 (6) | 0.112 (2) | |
H16A | 0.0379 (7) | 0.221 (6) | 0.8466 (15) | 0.16 (3)* | |
H16B | 0.0096 (8) | 0.525 (5) | 0.8564 (18) | 0.089 (13)* | |
H16C | 0.0545 (6) | 0.509 (5) | 0.949 (2) | 0.18 (3)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0654 (15) | 0.0519 (15) | 0.0560 (15) | 0.0123 (12) | 0.0208 (11) | 0.0121 (11) |
O2 | 0.0641 (15) | 0.0521 (14) | 0.0496 (13) | 0.0064 (11) | 0.0137 (10) | 0.0189 (11) |
C1 | 0.0511 (18) | 0.0367 (16) | 0.0406 (17) | −0.0057 (14) | 0.0058 (13) | −0.0008 (14) |
C2 | 0.060 (2) | 0.0390 (18) | 0.0420 (17) | −0.0056 (14) | 0.0097 (15) | 0.0072 (14) |
C3 | 0.058 (2) | 0.0446 (19) | 0.049 (2) | −0.0013 (15) | 0.0119 (16) | 0.0012 (15) |
C4 | 0.056 (2) | 0.062 (2) | 0.049 (2) | −0.0007 (17) | 0.0128 (16) | 0.0057 (17) |
C5 | 0.061 (2) | 0.056 (2) | 0.053 (2) | −0.0036 (16) | 0.0109 (17) | 0.0036 (17) |
C6 | 0.059 (2) | 0.066 (3) | 0.053 (2) | −0.0007 (17) | 0.0154 (17) | 0.0053 (18) |
C7 | 0.060 (2) | 0.059 (2) | 0.058 (2) | 0.0000 (17) | 0.0127 (18) | 0.0042 (18) |
C8 | 0.059 (2) | 0.063 (3) | 0.060 (2) | −0.0015 (17) | 0.0162 (18) | 0.0020 (18) |
C9 | 0.060 (2) | 0.060 (3) | 0.065 (2) | 0.0007 (17) | 0.0155 (18) | 0.0031 (19) |
C10 | 0.060 (2) | 0.063 (3) | 0.067 (2) | −0.0007 (17) | 0.0186 (19) | 0.0013 (19) |
C11 | 0.063 (2) | 0.062 (3) | 0.073 (3) | 0.0011 (18) | 0.020 (2) | 0.003 (2) |
C12 | 0.064 (2) | 0.067 (3) | 0.074 (3) | −0.0011 (19) | 0.022 (2) | 0.000 (2) |
C13 | 0.065 (2) | 0.070 (3) | 0.086 (3) | 0.0034 (19) | 0.023 (2) | 0.000 (2) |
C14 | 0.070 (3) | 0.075 (3) | 0.078 (3) | −0.005 (2) | 0.025 (2) | 0.002 (2) |
C15 | 0.075 (3) | 0.101 (4) | 0.106 (4) | 0.009 (3) | 0.037 (3) | 0.001 (3) |
C16 | 0.089 (4) | 0.142 (6) | 0.105 (4) | −0.008 (3) | 0.047 (3) | −0.002 (4) |
O1—C1 | 1.269 (4) | C9—C10 | 1.507 (5) |
O1—H1 | 1.12 (7) | C9—H9A | 0.9924 |
O2—C1 | 1.247 (4) | C9—H9B | 0.9924 |
C1—C2 | 1.488 (4) | C10—C11 | 1.511 (5) |
C2—C3 | 1.506 (4) | C10—H10A | 1.0310 |
C2—H2A | 0.9646 | C10—H10B | 1.0310 |
C2—H2B | 0.9646 | C11—C12 | 1.505 (5) |
C3—C4 | 1.512 (4) | C11—H11A | 1.0104 |
C3—H3A | 0.9544 | C11—H11B | 1.0104 |
C3—H3B | 0.9543 | C12—C13 | 1.512 (5) |
C4—C5 | 1.501 (5) | C12—H12A | 1.0828 |
C4—H4A | 1.0037 | C12—H12B | 1.0827 |
C4—H4B | 1.0037 | C13—C14 | 1.503 (5) |
C5—C6 | 1.515 (5) | C13—H13A | 0.9809 |
C5—H5A | 0.9296 | C13—H13B | 0.9809 |
C5—H5B | 0.9296 | C14—C15 | 1.510 (6) |
C6—C7 | 1.506 (5) | C14—H14A | 1.1183 |
C6—H6A | 1.0376 | C14—H14B | 1.1183 |
C6—H6B | 1.0376 | C15—C16 | 1.502 (6) |
C7—C8 | 1.509 (5) | C15—H15A | 1.0250 |
C7—H7A | 0.9632 | C15—H15B | 1.0250 |
C7—H7B | 0.9632 | C16—H16A | 1.1120 |
C8—C9 | 1.507 (5) | C16—H16B | 1.1120 |
C8—H8A | 1.0060 | C16—H16C | 1.1120 |
C8—H8B | 1.0060 | ||
C1—O1—H1 | 120 (3) | C8—C9—H9A | 108.6 |
O2—C1—O1 | 122.9 (3) | C10—C9—H9A | 108.5 |
O2—C1—C2 | 121.1 (3) | C8—C9—H9B | 108.6 |
O1—C1—C2 | 116.0 (3) | C10—C9—H9B | 108.6 |
C1—C2—C3 | 116.7 (3) | H9A—C9—H9B | 107.6 |
C1—C2—H2A | 108.2 | C9—C10—C11 | 115.0 (3) |
C3—C2—H2A | 108.1 | C9—C10—H10A | 108.5 |
C1—C2—H2B | 108.2 | C11—C10—H10A | 108.5 |
C3—C2—H2B | 108.1 | C9—C10—H10B | 108.6 |
H2A—C2—H2B | 107.3 | C11—C10—H10B | 108.6 |
C2—C3—C4 | 111.6 (3) | H10A—C10—H10B | 107.5 |
C2—C3—H3A | 109.3 | C10—C11—C12 | 115.0 (4) |
C4—C3—H3A | 109.3 | C12—C11—H11A | 108.5 |
C2—C3—H3B | 109.3 | C10—C11—H11A | 108.5 |
C4—C3—H3B | 109.3 | C12—C11—H11B | 108.5 |
H3A—C3—H3B | 108.0 | C10—C11—H11B | 108.5 |
C3—C4—C5 | 115.5 (3) | H11A—C11—H11B | 107.5 |
C5—C4—H4A | 108.4 | C11—C12—C13 | 114.7 (4) |
C3—C4—H4A | 108.4 | C11—C12—H12A | 108.6 |
C5—C4—H4B | 108.4 | C13—C12—H12A | 108.6 |
C3—C4—H4B | 108.4 | C11—C12—H12B | 108.6 |
H4A—C4—H4B | 107.5 | C13—C12—H12B | 108.6 |
C4—C5—C6 | 113.0 (3) | H12A—C12—H12B | 107.6 |
C4—C5—H5A | 108.9 | C12—C13—C14 | 115.3 (4) |
C6—C5—H5A | 109.0 | C14—C13—H13A | 108.5 |
C4—C5—H5B | 109.0 | C12—C13—H13A | 108.5 |
C6—C5—H5B | 109.0 | C14—C13—H13B | 108.4 |
H5A—C5—H5B | 107.8 | C12—C13—H13B | 108.4 |
C5—C6—C7 | 115.5 (3) | H13A—C13—H13B | 107.5 |
C7—C6—H6A | 108.4 | C13—C14—C15 | 114.5 (4) |
C5—C6—H6A | 108.4 | C13—C14—H14A | 108.6 |
C7—C6—H6B | 108.4 | C15—C14—H14A | 108.6 |
C5—C6—H6B | 108.4 | C13—C14—H14B | 108.7 |
H6A—C6—H6B | 107.4 | C15—C14—H14B | 108.7 |
C6—C7—C8 | 113.9 (3) | H14A—C14—H14B | 107.6 |
C6—C7—H7A | 108.8 | C14—C15—C16 | 114.3 (5) |
C8—C7—H7A | 108.7 | C16—C15—H15A | 108.8 |
C6—C7—H7B | 108.8 | C14—C15—H15A | 108.8 |
C8—C7—H7B | 108.8 | C16—C15—H15B | 108.5 |
H7A—C7—H7B | 107.7 | C14—C15—H15B | 108.6 |
C7—C8—C9 | 115.1 (3) | H15A—C15—H15B | 107.6 |
C9—C8—H8A | 108.5 | C15—C16—H16A | 109.5 |
C7—C8—H8A | 108.6 | C15—C16—H16B | 109.6 |
C9—C8—H8B | 108.5 | H16A—C16—H16B | 109.5 |
C7—C8—H8B | 108.5 | C15—C16—H16C | 109.3 |
H8A—C8—H8B | 107.5 | H16A—C16—H16C | 109.5 |
C8—C9—C10 | 114.7 (3) | H16B—C16—H16C | 109.5 |
O1—C1—C2—C3 | −177.4 (3) | C2—C3—C4—C5 | −175.2 (3) |
O2—C1—C2—C3 | 3.6 (4) | C3—C4—C5—C6 | 179.2 (3) |
C1—C2—C3—C4 | −178.1 (3) | C4—C5—C6—C7 | −177.0 (3) |
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O2i | 1.12 (6) | 1.51 (6) | 2.621 (3) | 173 (5) |
Symmetry code: (i) −x+1, −y+1, −z−1. |
Experimental details
Crystal data | |
Chemical formula | C16H32O2 |
Mr | 256.42 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 298 |
a, b, c (Å) | 35.620 (11), 4.9487 (16), 9.406 (3) |
β (°) | 90.447 (5) |
V (Å3) | 1658.0 (9) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.07 |
Crystal size (mm) | 0.48 × 0.46 × 0.09 |
Data collection | |
Diffractometer | Bruker SMART APEX CCD area-detector diffractometer |
Absorption correction | Multi-scan (SADABS in SAINT-Plus; Bruker, 2001) |
Tmin, Tmax | 0.956, 0.991 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 14859, 2912, 1918 |
Rint | 0.064 |
(sin θ/λ)max (Å−1) | 0.596 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.092, 0.263, 1.13 |
No. of reflections | 2910 |
No. of parameters | 215 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.32, −0.21 |
Computer programs: SMART (Bruker, 2001), SAINT-Plus (Bruker, 2001), SAINT-Plus, SHELXS97 (Sheldrick, 1997a), SHELXL97 (Sheldrick, 1997a), PLATON (Spek, 2003), PARST (Nardelli, 1995).
O1—C1 | 1.269 (4) | C2—C3 | 1.506 (4) |
O2—C1 | 1.247 (4) | C14—C15 | 1.510 (6) |
C1—C2 | 1.488 (4) | C15—C16 | 1.502 (6) |
O2—C1—O1 | 122.9 (3) | C1—C2—C3 | 116.7 (3) |
O2—C1—C2 | 121.1 (3) | C2—C3—C4 | 111.6 (3) |
O1—C1—C2 | 116.0 (3) | C3—C4—C5 | 115.5 (3) |
O1—C1—C2—C3 | −177.4 (3) | C2—C3—C4—C5 | −175.2 (3) |
O2—C1—C2—C3 | 3.6 (4) | C3—C4—C5—C6 | 179.2 (3) |
C1—C2—C3—C4 | −178.1 (3) | C4—C5—C6—C7 | −177.0 (3) |
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O2i | 1.12 (6) | 1.51 (6) | 2.621 (3) | 173 (5) |
Symmetry code: (i) −x+1, −y+1, −z−1. |
Carboxylic or fatty acids represent a very common type of natural compound used in different industrial fields (pharmaceutical, food and cosmetic industries). A new application for unbranched saturated carboxylic acids CnH2nO2 (abbreviated Cn) lies in the field of energy storage and thermal protection, due to their high melting enthalpy (162–232 kJ g−1 for n in the range 10–20) and other related properties, such as their stability during thermal cycling. Against this background, our group, integrated in REALM (Réseau Européen sur les Alliages Moléculaires, or European Network on Molecular Alloys), has been studying the polymorphism and solid-state miscibility of this family of substances.
Normal saturated acids show a complex polymorphic behaviour, only partially characterized to date, due to the difficulty of obtaining suitable single crystals for X-ray analysis: problems arise involving crystallization processes, several crystalline habits and forms, and crystallization conditions. In general, different forms, or a mixture of them, are usually observed after a crystallization process. Their occurrence depends on several variables, such as the number of C atoms in the chain, the temperature, the solvent and precipitant used, the grinding (after crystallization) of the sample or the crystallization rate (Kaneko et al., 1998; Sato et al., 1991; Moreno et al., 2006). Typically, the forms are named A, B, C, D and E for fatty acids with an even number of C atoms, and A', B', C', D' and E' for those with an odd number. Some of these forms have been characterized for specific members of the family.
In general, the structure of these forms consists of molecules packed in bilayers containing two molecules, with the saturated carbon chains in the all-trans conformation (except for B and some triclinic forms, where the molecules adopt a gauche conformation), and with their carboxyl groups forming dimers through a typical R22(8) hydrogen-bond system (Bernstein et al., 1995). These dimers are arranged so as to exhibit monolayers of terminal methyl and carboxyl groups, except for some triclinic forms, Asuper and A1, where both terminal groups coexist in the same monolayer face.
The A' (von Sydow, 1954a, 1955b; Goto & Asada, 1980) and B' forms (von Sydow, 1954b; Goto & Asada, 1984) are triclinic, P1, with Z = 2 and Z = 4, respectively. Different triclinic A forms (also P1) have been observed for C12: an Asuper form (Goto & Asada, 1978a) and an A1 form (Lomer, 1963), with Z = 6 and Z = 2, respectively. A different triclinic A2 form was found for C16 (Kobayashi et al., 1984). Two different polytypes have been distinguished for the B and E forms, one monoclinic, P21/a, with Z = 4 (Goto & Asada, 1978b; Kaneko et al., 1990), and the other orthorhombic, Pbca, with Z = 8 (Kaneko et al., 1994a,b). On raising the temperature, all these forms transform to a monoclinic C form for even acids (Vand et al., 1951; Malta et al., 1971) and to C' for odd acids (von Sydow, 1955a). In the case of even acids, the transition is irreversible and the C form is always obtained from quenching the melt.
Bond (2004) has recently determined, from single-crystal in situ crystallization, the high-temperature phases of the fatty acids with chain lengths from n = 6 to n = 15, i.e. the C form, P21/c, with Z = 4 for the even acids, and the C' form, P21/c, with Z = 4 for the odd acids C7, C9 and C11, and he has also pointed out the existence of a new monoclinic form C'', C2/c, with Z = 8 for C13 and C15 acids. By means of a geometrical approach based on structural data, Bond has shown the existence of a clear correlation between the alternating melting points for even and odd carboxylic acids and their crystal density: a different orientation adopted by the terminal C—C bonds with respect to the methyl-group interface explains the observed alternation in crystal density (the methyl groups approach each other less closely in the odd acids compared with the even acids). In a recent study, Gbabode (2005) has characterized the lower- and high-temperature forms for C11 to C23 odd acids from high-quality powder diffraction patterns, combining Rietveld refinements with force-field minimization methods.
This paper deals with the structure of the C form of hexadecanoic acid, (I). This structure determination has been accomplished in order to obtain a molecular model for longer even carboxylic acids that could be used in the structural Rietveld refinement from X-ray powder diffraction data.
A general view of the molecule is shown in Fig. 1. Selected bond distances and angles are presented in Table 1, and geometric details of the hydrogen-bonding scheme are given in Table 2. It is apparent from Table 2, and also from Fig. 2, which shows a representation of the structure in the ac plane, that molecules are linked into centrosymmetric dimers via O—H···O hydrogen bonds. These dimers are arranged into bilayers parallel to the crystallographic bc plane. The long axis of the molecules is tilted over this bc plane by an angle of 54.7 (1)°. It must be noted the intermolecular contact [3.050 (4) Å] between atom O1 and atom C1(−x + 1,+y − 1/2,-z − 1/2) of an adjacent molecule in the same layer indicates a stronger interchain interaction than that attributed only to van der Waals forces.
The average C—C bond length in the aliphatic carbon chain (from C3 to C16) is 1.508 (2) Å and the mean C—C—C bond angle (from C2 to C15) is 114.6 (1)°. All C—C bonds show an antiperiplanar (trans) conformation [mean absolute C—C—C—C torsion angle 178.6 (1)°]. The C2—C3—C4—C5 [−175.2 (3)°], C1—C2—C3—C4 [−178.1 (3)°] and O1—C1—C2—C3 [−177.4 (3)°] torsion angles deviate significantly from the ideal value of 180°. This could be explained by the steric hindrance produced by the hydrogen bonds. The carboxyl group is nearly coplanar with the skeletal plane of the hydrocarbon chain [dihedral angle 6.0 (2)°].
The C1—O1 and C1—O2 bond distances are very similar [1.269 (4) and 1.247 (4) Å, respectively], showing the involvement of both O atoms in the intermolecular hydrogen bonds that create the dimer aggregate. The O1···O2 hydrogen-bonded distance is 2.621 (3) Å and the O1—H1···O2 angle is 173 (5)°. This similarity of both C—O bond lengths within the carboxyl functional group was also observed by Bond (2004) for the C form of even carboxylic acids with 6 to 14 C atoms.
These results are also in agreement with the study of Hayashi et al. (1975) on the solid-state IR spectra of these C forms measured in the range from room to liquid-helium temperature. They observed, in the regions of the characteristic frequencies of the carboxyl group and in the region of the CH2 wagging modes, that many bands decrease in intensity with decreasing temperature and others near the latter ones increase in intensity. Their results suggest that the carboxyl group and the C2—C3 bond can adopt two relative configurations (cis and trans) around the C1—C2 bond at room temperature, concluding that the trans configuration is more stable at low temperatures for even acids. They pointed out that the transition between the two configurations involves a simple prototropic exchange between the two carboxylic acid groups forming the dimer. This cis–trans disordering was confirmed by Kanters et al. (1975) by X-ray crystallographic measurements for crystalline fluoromalonic acid, observing a splitting in the H-atom electron density of the carboxyl group at room temperature which disappears at liquid nitrogen temperature.
The similarity observed between the two C—O bond distances, in our case at room temperature, and the short O1···O2 distance, suitable for H-atom transfer, agree with the existence of this configurational disorder in hexadecanoic acid, as was previously observed for the C form of other related even carboxylic acids.