The crystal packing and interaction energy of benzyl carbamate, C
8H
9NO
2, have been analysed in detail by the
PIXEL method. Benzyl carbamate forms layers of hydrogen-bonded molecules, with the layers connected by weaker C-H
interactions. According to the
PIXEL analysis, combinations of C-H
X (
X = O, N or
) interactions are comparable in energy with hydrogen bonding. These interactions are necessary for explaining the geometry and the assembly of the layers.
Supporting information
CCDC reference: 873891
Benzyl carbamate in methanol [Quantities?] was heated under reflux until
totally dissolved. The solution was filtered and hexane was added dropwise
until the solution was slightly cloudy. Slow evaporation of the solvent at
room temperature in a covered flask spiked by a hollow needle gave colourless
crystals of (I) suitable for single-crystal X-ray diffraction.
The size of the specimen crystal was small (100 × 50 × 50 µm), so
the diffraction intensities were not strong. The space group was initially set
to Pbcm according to the extinction rules, then relaxed to
Pbc21, hence the nonstandard setting. H atoms were situated at
calculated positions and treated as riding atoms, with Csp3—H =
0.97 Å, Csp2—H = 0.93 Å and N—H = 0.86 Å, and with
Uiso(H) = 1.2Ueq(C,N). In the absence of significant
anomalous scattering effects, Friedel pairs were not measured.
Data collection: CAD-4 EXPRESS (Enraf–Nonius, 1994); cell refinement: CAD-4 EXPRESS (Enraf–Nonius, 1994); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).
Crystal data top
C8H9NO2 | F(000) = 640 |
Mr = 151.16 | Dx = 1.269 Mg m−3 |
Orthorhombic, Pca21 | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: P 2c -2ac | Cell parameters from 25 reflections |
a = 10.037 (2) Å | θ = 16.0–29.8° |
b = 5.330 (1) Å | µ = 0.09 mm−1 |
c = 29.570 (4) Å | T = 293 K |
V = 1581.9 (5) Å3 | Prism, colourless |
Z = 8 | 0.1 × 0.05 × 0.05 mm |
Data collection top
Enraf–Nonius CAD-4 diffractometer | 913 reflections with I > 2σ(I) |
Radiation source: Enraf Nonius FR590 | Rint = 0.040 |
Graphite monochromator | θmax = 25.0°, θmin = 1.4° |
non–profiled ω scans | h = 0→11 |
Absorption correction: gaussian a grid of 8 x 8 x 8 = 512 sampling points was used | k = 0→6 |
Tmin = 0.992, Tmax = 0.996 | l = −35→0 |
1472 measured reflections | 2 standard reflections every 60 min |
1422 independent reflections | intensity decay: none |
Refinement top
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)] = 0.076 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.233 | H-atom parameters constrained |
S = 1.08 | w = 1/[σ2(Fo2) + (0.138P)2 + 0.3889P] where P = (Fo2 + 2Fc2)/3 |
1422 reflections | (Δ/σ)max < 0.001 |
199 parameters | Δρmax = 0.27 e Å−3 |
1 restraint | Δρmin = −0.47 e Å−3 |
Crystal data top
C8H9NO2 | V = 1581.9 (5) Å3 |
Mr = 151.16 | Z = 8 |
Orthorhombic, Pca21 | Mo Kα radiation |
a = 10.037 (2) Å | µ = 0.09 mm−1 |
b = 5.330 (1) Å | T = 293 K |
c = 29.570 (4) Å | 0.1 × 0.05 × 0.05 mm |
Data collection top
Enraf–Nonius CAD-4 diffractometer | 913 reflections with I > 2σ(I) |
Absorption correction: gaussian a grid of 8 x 8 x 8 = 512 sampling points was used | Rint = 0.040 |
Tmin = 0.992, Tmax = 0.996 | 2 standard reflections every 60 min |
1472 measured reflections | intensity decay: none |
1422 independent reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.076 | 1 restraint |
wR(F2) = 0.233 | H-atom parameters constrained |
S = 1.08 | Δρmax = 0.27 e Å−3 |
1422 reflections | Δρmin = −0.47 e Å−3 |
199 parameters | |
Special details top
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes)
are estimated using the full covariance matrix. The cell esds are taken
into account individually in the estimation of esds in distances, angles
and torsion angles; correlations between esds in cell parameters are only
used when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell esds is used for estimating esds involving l.s. planes. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
C1A | 0.1646 (7) | 0.0357 (16) | 0.7035 (3) | 0.043 (2) | |
C3A | 0.1084 (9) | 0.3910 (18) | 0.6598 (4) | 0.064 (3) | |
H3A1 | 0.1505 | 0.5491 | 0.652 | 0.076* | |
H3A2 | 0.0444 | 0.4244 | 0.6837 | 0.076* | |
C4A | 0.0353 (8) | 0.2951 (15) | 0.6193 (3) | 0.052 (2) | |
C5A | 0.0809 (11) | 0.094 (2) | 0.5942 (4) | 0.071 (3) | |
H5A | 0.1592 | 0.013 | 0.6027 | 0.085* | |
C6A | 0.0112 (14) | 0.010 (2) | 0.5568 (4) | 0.088 (3) | |
H6A | 0.0379 | −0.1326 | 0.5412 | 0.106* | |
C7A | −0.0984 (13) | 0.145 (3) | 0.5432 (4) | 0.091 (3) | |
H7A | −0.1426 | 0.0964 | 0.517 | 0.109* | |
C8A | −0.1448 (11) | 0.346 (3) | 0.5668 (4) | 0.088 (3) | |
H8A | −0.2212 | 0.4299 | 0.5576 | 0.106* | |
C9A | −0.0760 (9) | 0.420 (2) | 0.6043 (4) | 0.070 (3) | |
H9A | −0.1055 | 0.5598 | 0.6202 | 0.084* | |
N1A | 0.2641 (6) | −0.1082 (12) | 0.7174 (3) | 0.0587 (19) | |
H1A1 | 0.2485 | −0.2337 | 0.7349 | 0.07* | |
H1A2 | 0.3443 | −0.0761 | 0.709 | 0.07* | |
O1A | 0.0481 (4) | 0.0079 (11) | 0.7133 (2) | 0.0574 (17) | |
O2A | 0.2085 (5) | 0.2237 (11) | 0.6771 (2) | 0.0568 (15) | |
C1B | 0.0542 (7) | 0.4570 (18) | 0.7901 (3) | 0.049 (2) | |
C3B | 0.1095 (9) | 0.097 (2) | 0.8333 (3) | 0.062 (2) | |
H3B1 | 0.1769 | 0.0712 | 0.8103 | 0.074* | |
H3B2 | 0.0675 | −0.0639 | 0.8391 | 0.074* | |
C4B | 0.1759 (8) | 0.1840 (16) | 0.8756 (3) | 0.053 (2) | |
C5B | 0.1265 (13) | 0.372 (2) | 0.9013 (4) | 0.084 (3) | |
H5B | 0.0482 | 0.4526 | 0.8929 | 0.101* | |
C6B | 0.1940 (16) | 0.446 (3) | 0.9410 (5) | 0.108 (4) | |
H6B | 0.1629 | 0.5826 | 0.9574 | 0.129* | |
C7B | 0.3026 (14) | 0.322 (3) | 0.9553 (4) | 0.097 (4) | |
H7B | 0.344 | 0.3662 | 0.9822 | 0.116* | |
C8B | 0.3502 (11) | 0.133 (3) | 0.9303 (4) | 0.097 (4) | |
H8B | 0.4268 | 0.0496 | 0.9396 | 0.116* | |
C9B | 0.2871 (10) | 0.060 (2) | 0.8908 (4) | 0.078 (3) | |
H9B | 0.3205 | −0.0743 | 0.8743 | 0.094* | |
N1B | −0.0445 (6) | 0.5982 (14) | 0.7761 (3) | 0.0598 (19) | |
H1B1 | −0.0289 | 0.7237 | 0.7586 | 0.072* | |
H1B2 | −0.1247 | 0.5654 | 0.7844 | 0.072* | |
O1B | 0.1717 (5) | 0.4871 (11) | 0.7804 (2) | 0.0588 (17) | |
O2B | 0.0103 (6) | 0.2681 (11) | 0.8161 (2) | 0.0582 (15) | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
C1A | 0.034 (4) | 0.055 (5) | 0.042 (5) | −0.003 (4) | 0.002 (3) | −0.006 (4) |
C3A | 0.076 (6) | 0.042 (5) | 0.073 (6) | −0.003 (4) | 0.005 (5) | 0.009 (5) |
C4A | 0.057 (5) | 0.047 (5) | 0.052 (5) | −0.011 (4) | 0.007 (4) | 0.009 (4) |
C5A | 0.092 (7) | 0.063 (6) | 0.057 (6) | 0.012 (5) | 0.003 (5) | 0.004 (5) |
C6A | 0.126 (9) | 0.081 (7) | 0.058 (7) | −0.008 (7) | 0.001 (7) | −0.005 (6) |
C7A | 0.112 (9) | 0.104 (9) | 0.056 (6) | −0.027 (8) | −0.007 (7) | 0.004 (7) |
C8A | 0.086 (8) | 0.104 (9) | 0.075 (7) | −0.009 (6) | −0.010 (6) | 0.013 (7) |
C9A | 0.065 (5) | 0.077 (6) | 0.068 (7) | 0.003 (5) | −0.004 (5) | 0.010 (6) |
N1A | 0.033 (3) | 0.075 (4) | 0.068 (5) | 0.001 (3) | 0.002 (4) | 0.001 (5) |
O1A | 0.034 (3) | 0.074 (4) | 0.064 (4) | 0.003 (3) | 0.006 (2) | 0.012 (3) |
O2A | 0.052 (3) | 0.058 (3) | 0.060 (4) | −0.002 (3) | 0.006 (3) | 0.003 (3) |
C1B | 0.035 (5) | 0.061 (5) | 0.050 (5) | −0.003 (4) | −0.001 (3) | −0.014 (5) |
C3B | 0.075 (6) | 0.058 (6) | 0.051 (5) | −0.006 (5) | 0.005 (4) | −0.011 (5) |
C4B | 0.062 (5) | 0.050 (5) | 0.048 (5) | −0.004 (4) | 0.010 (4) | 0.000 (5) |
C5B | 0.120 (9) | 0.067 (6) | 0.066 (7) | 0.022 (6) | −0.012 (6) | −0.022 (6) |
C6B | 0.156 (12) | 0.105 (9) | 0.061 (7) | 0.023 (9) | −0.013 (8) | −0.033 (7) |
C7B | 0.112 (10) | 0.116 (10) | 0.064 (7) | −0.005 (9) | −0.014 (7) | 0.006 (8) |
C8B | 0.080 (8) | 0.126 (11) | 0.083 (9) | 0.010 (7) | −0.011 (7) | 0.016 (9) |
C9B | 0.072 (6) | 0.096 (7) | 0.067 (7) | 0.008 (6) | 0.003 (5) | −0.016 (6) |
N1B | 0.036 (4) | 0.076 (5) | 0.068 (5) | −0.008 (3) | 0.002 (3) | 0.001 (5) |
O1B | 0.036 (3) | 0.072 (4) | 0.068 (4) | −0.002 (3) | 0.003 (3) | 0.003 (4) |
O2B | 0.054 (3) | 0.063 (3) | 0.058 (4) | −0.008 (3) | 0.010 (3) | 0.011 (3) |
Geometric parameters (Å, º) top
C1A—O1A | 1.214 (8) | C1B—O1B | 1.224 (9) |
C1A—N1A | 1.325 (10) | C1B—N1B | 1.311 (10) |
C1A—O2A | 1.345 (10) | C1B—O2B | 1.340 (10) |
C3A—O2A | 1.436 (11) | C3B—O2B | 1.444 (11) |
C3A—C4A | 1.496 (13) | C3B—C4B | 1.491 (13) |
C3A—H3A1 | 0.97 | C3B—H3B1 | 0.97 |
C3A—H3A2 | 0.97 | C3B—H3B2 | 0.97 |
C4A—C9A | 1.374 (12) | C4B—C5B | 1.352 (14) |
C4A—C5A | 1.383 (14) | C4B—C9B | 1.371 (13) |
C5A—C6A | 1.382 (16) | C5B—C6B | 1.410 (17) |
C5A—H5A | 0.93 | C5B—H5B | 0.93 |
C6A—C7A | 1.373 (17) | C6B—C7B | 1.344 (17) |
C6A—H6A | 0.93 | C6B—H6B | 0.93 |
C7A—C8A | 1.359 (18) | C7B—C8B | 1.335 (19) |
C7A—H7A | 0.93 | C7B—H7B | 0.93 |
C8A—C9A | 1.365 (15) | C8B—C9B | 1.386 (16) |
C8A—H8A | 0.93 | C8B—H8B | 0.93 |
C9A—H9A | 0.93 | C9B—H9B | 0.93 |
N1A—H1A1 | 0.86 | N1B—H1B1 | 0.86 |
N1A—H1A2 | 0.86 | N1B—H1B2 | 0.86 |
| | | |
O1A—C1A—N1A | 125.5 (8) | O1B—C1B—N1B | 125.3 (9) |
O1A—C1A—O2A | 123.1 (7) | O1B—C1B—O2B | 123.3 (8) |
N1A—C1A—O2A | 111.4 (6) | N1B—C1B—O2B | 111.4 (6) |
O2A—C3A—C4A | 114.5 (7) | O2B—C3B—C4B | 114.0 (8) |
O2A—C3A—H3A1 | 108.6 | O2B—C3B—H3B1 | 108.7 |
C4A—C3A—H3A1 | 108.6 | C4B—C3B—H3B1 | 108.7 |
O2A—C3A—H3A2 | 108.6 | O2B—C3B—H3B2 | 108.7 |
C4A—C3A—H3A2 | 108.6 | C4B—C3B—H3B2 | 108.7 |
H3A1—C3A—H3A2 | 107.6 | H3B1—C3B—H3B2 | 107.6 |
C9A—C4A—C5A | 118.1 (9) | C5B—C4B—C9B | 118.1 (10) |
C9A—C4A—C3A | 119.5 (8) | C5B—C4B—C3B | 122.6 (9) |
C5A—C4A—C3A | 122.2 (8) | C9B—C4B—C3B | 119.2 (9) |
C6A—C5A—C4A | 120.7 (10) | C4B—C5B—C6B | 119.9 (11) |
C6A—C5A—H5A | 119.6 | C4B—C5B—H5B | 120 |
C4A—C5A—H5A | 119.6 | C6B—C5B—H5B | 120 |
C7A—C6A—C5A | 118.2 (11) | C7B—C6B—C5B | 120.9 (12) |
C7A—C6A—H6A | 120.9 | C7B—C6B—H6B | 119.5 |
C5A—C6A—H6A | 120.9 | C5B—C6B—H6B | 119.5 |
C8A—C7A—C6A | 122.4 (11) | C8B—C7B—C6B | 119.1 (12) |
C8A—C7A—H7A | 118.8 | C8B—C7B—H7B | 120.4 |
C6A—C7A—H7A | 118.8 | C6B—C7B—H7B | 120.4 |
C7A—C8A—C9A | 118.1 (11) | C7B—C8B—C9B | 120.9 (12) |
C7A—C8A—H8A | 120.9 | C7B—C8B—H8B | 119.5 |
C9A—C8A—H8A | 120.9 | C9B—C8B—H8B | 119.5 |
C8A—C9A—C4A | 122.2 (11) | C4B—C9B—C8B | 120.9 (11) |
C8A—C9A—H9A | 118.9 | C4B—C9B—H9B | 119.6 |
C4A—C9A—H9A | 118.9 | C8B—C9B—H9B | 119.6 |
C1A—N1A—H1A1 | 120 | C1B—N1B—H1B1 | 120 |
C1A—N1A—H1A2 | 120 | C1B—N1B—H1B2 | 120 |
H1A1—N1A—H1A2 | 120 | H1B1—N1B—H1B2 | 120 |
C1A—O2A—C3A | 116.1 (6) | C1B—O2B—C3B | 116.8 (7) |
| | | |
O2A—C3A—C4A—C9A | 169.8 (9) | O2B—C3B—C4B—C5B | 15.3 (13) |
O2A—C3A—C4A—C5A | −14.7 (11) | O2B—C3B—C4B—C9B | −168.7 (9) |
C9A—C4A—C5A—C6A | −4.1 (14) | C9B—C4B—C5B—C6B | 4.0 (17) |
C3A—C4A—C5A—C6A | −179.6 (10) | C3B—C4B—C5B—C6B | 180.0 (12) |
C4A—C5A—C6A—C7A | 4.8 (16) | C4B—C5B—C6B—C7B | −4 (2) |
C5A—C6A—C7A—C8A | −4.0 (18) | C5B—C6B—C7B—C8B | 3 (2) |
C6A—C7A—C8A—C9A | 2.4 (18) | C6B—C7B—C8B—C9B | −2 (2) |
C7A—C8A—C9A—C4A | −1.7 (16) | C5B—C4B—C9B—C8B | −2.7 (16) |
C5A—C4A—C9A—C8A | 2.5 (15) | C3B—C4B—C9B—C8B | −178.9 (10) |
C3A—C4A—C9A—C8A | 178.2 (9) | C7B—C8B—C9B—C4B | 1.7 (18) |
O1A—C1A—O2A—C3A | −1.2 (11) | O1B—C1B—O2B—C3B | 0.9 (12) |
N1A—C1A—O2A—C3A | 179.7 (8) | N1B—C1B—O2B—C3B | 179.4 (7) |
C4A—C3A—O2A—C1A | −80.8 (9) | C4B—C3B—O2B—C1B | 84.0 (10) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N1A—H1A1···O1Bi | 0.86 | 2.15 | 2.996 (10) | 169 |
N1A—H1A2···O1Aii | 0.86 | 2.08 | 2.903 (8) | 159 |
N1B—H1B1···O1Aiii | 0.86 | 2.16 | 3.012 (10) | 169 |
N1B—H1B2···O1Biv | 0.86 | 2.07 | 2.888 (8) | 160 |
C9A—H9A···O2Aiv | 0.93 | 2.77 | 3.595 (12) | 149 |
C9B—H9B···O2Bii | 0.93 | 2.77 | 3.601 (13) | 150 |
C3A—H3A1···N1Aiii | 0.97 | 2.89 | 3.530 (12) | 124 |
C3B—H3B2···N1Bi | 0.97 | 2.83 | 3.509 (12) | 128 |
C3A—H3A2···N1B | 0.97 | 3.02 | 3.923 (13) | 155 |
C3B—H3B1···N1A | 0.97 | 3.04 | 3.917 (12) | 152 |
Symmetry codes: (i) x, y−1, z; (ii) x, −y+1/2, z+1/2; (iii) x, y+1, z; (iv) x−1, −y+3/2, z+1/2. |
Experimental details
Crystal data |
Chemical formula | C8H9NO2 |
Mr | 151.16 |
Crystal system, space group | Orthorhombic, Pca21 |
Temperature (K) | 293 |
a, b, c (Å) | 10.037 (2), 5.330 (1), 29.570 (4) |
V (Å3) | 1581.9 (5) |
Z | 8 |
Radiation type | Mo Kα |
µ (mm−1) | 0.09 |
Crystal size (mm) | 0.1 × 0.05 × 0.05 |
|
Data collection |
Diffractometer | Enraf–Nonius CAD-4 diffractometer |
Absorption correction | Gaussian a grid of 8 x 8 x 8 = 512 sampling points was used |
Tmin, Tmax | 0.992, 0.996 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 1472, 1422, 913 |
Rint | 0.040 |
(sin θ/λ)max (Å−1) | 0.594 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.076, 0.233, 1.08 |
No. of reflections | 1422 |
No. of parameters | 199 |
No. of restraints | 1 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.27, −0.47 |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N1A—H1A1···O1Bi | 0.86 | 2.15 | 2.996 (10) | 168.6 |
N1A—H1A2···O1Aii | 0.86 | 2.08 | 2.903 (8) | 159.4 |
N1B—H1B1···O1Aiii | 0.86 | 2.16 | 3.012 (10) | 168.9 |
N1B—H1B2···O1Biv | 0.86 | 2.07 | 2.888 (8) | 159.6 |
C9A—H9A···O2Aiv | 0.93 | 2.77 | 3.595 (12) | 149 |
C9B—H9B···O2Bii | 0.93 | 2.77 | 3.601 (13) | 149.6 |
C3A—H3A1···N1Aiii | 0.97 | 2.89 | 3.530 (12) | 124.1 |
C3B—H3B2···N1Bi | 0.97 | 2.83 | 3.509 (12) | 128.2 |
C3A—H3A2···N1B | 0.97 | 3.02 | 3.923 (13) | 155.4 |
C3B—H3B1···N1A | 0.97 | 3.04 | 3.917 (12) | 151.8 |
Symmetry codes: (i) x, y−1, z; (ii) x, −y+1/2, z+1/2; (iii) x, y+1, z; (iv) x−1, −y+3/2, z+1/2. |
Pairs of molecules, interaction energies (Ei) and Ei
components from PIXEL analysis (kJ mol-1) top | | Eia | Coulombicb | Polarizationb | Dispersionb | Repulsionb | Intermolecular interactionc |
1 | A···Bi | -60.0/-58.1 | -64.7 | -18.4 | -15.2 | 38.3 | N1A···O1B, N1B···O1A |
2 | B···Bii | -33.5/-32.1 | -31.5 | -11.6 | -19.7 | 29.3 | N1B···O1B, C5B···πB |
3 | A···Aiii | -32.9/-31.9 | -30.7 | -11.4 | -22.4 | 31.6 | N1A···O1A, C5A···πA |
4 | A···Aiv | -17.0/-16.2 | -5.5 | -2.4 | -19.4 | 10.3 | C3A···N1A |
5 | B···Bi | -15.9/-15.2 | -5.1 | -2.4 | -17.7 | 9.4 | C3B···N1B |
6 | B···Biii | -13.2/-12.7 | -4.3 | -1.3 | -14.8 | 7.1 | C9B···O2B, C8B···πB |
7 | A···Aii | -12.7/-12.1 | -4.4 | -1.4 | -14.3 | 7.4 | C9A···O2A, C8A···πA |
8 | A···B | -11.1/-10.6 | -1.2 | -3.0 | -15.4 | 8.6 | C3A···N1B, C3B···N1A |
9 | B···Av | -7.2/-7.1 | -1.4 | -0.4 | -7.7 | 2.4 | C7B···πA |
10 | B···Avi | -5.6/-4.9 | -1.3 | -1.4 | -10.4 | 7.6 | C6B···πA |
11 | A···Bvii | -5.3/-5.2 | -1.2 | -0.3 | -5.0 | 1.2 | C7A···πB |
Notes: (a) The first number is the interaction energy for the pair of molecules
in the crystal structure and the second is for an isolated pair of molecules
with crystal geometry. (b) The components refer to Ei in the crystal
structure; (c) πA and πB refer to π orbitals in the phenyl ring of A and B,
respectively.
Symmetry codes: (i) x, y-1, z; (ii) x, y+1, z; (iii) x+1/2, -y, z;
(iv) x-1/2, -y+1, z; (v) -x, -y+1, z+1/2; (vi) -x, -y, z-1/2;
(vii) -x+1/2, y, z+1/2. |
Geometry of intermolecular contacts (Å, °) in pairs of molecules in Table 1 topD—H···A | D—H | H···A | D···A | D—H···A |
N1A—H1A1···O1Bi | 0.86 | 2.15 | 2.996 (10) | 168.6 |
N1B—H1B1···O1Aii | 0.86 | 2.16 | 3.012 (10) | 168.9 |
N1A—H1A2···O1Aiii | 0.86 | 2.08 | 2.903 (8) | 159.4 |
N1B—H1B2···O1Biv | 0.86 | 2.07 | 2.888 (8) | 159.6 |
C9A—H9A···O2Aiv | 0.93 | 2.77 | 3.595 (12) | 149.0 |
C9B—H9B···O2Biii | 0.93 | 2.77 | 3.601 (13) | 149.6 |
C3A—H3A1···N1Aii | 0.97 | 2.89 | 3.530 (12) | 124.1 |
C3B—H3B2···N1Bi | 0.97 | 2.82 | 3.509 (12) | 128.2 |
C3A—H3A2···N1B | 0.97 | 3.02 | 3.923 (13) | 155.4 |
C3B—H1B1···N1A | 0.97 | 3.04 | 3.917 (12) | 151.8 |
C5A—H5A···CgAiii | 0.93 | 3.40 | 4.246 | 152.4 |
C5B—H5B···CgBiv | 0.93 | 3.54 | 4.391 | 153.5 |
C8A—H8A···CgAiv | 0.93 | 3.70 | 4.546 | 152.3 |
C8B—H8B···CgBiii | 0.93 | 3.60 | 4.436 | 151.3 |
C6B—H6B···CgAv | 0.93 | 4.02 | 4.788 | 142.2 |
C7A—H7A···CgBvi | 0.93 | 3.66 | 4.551 | 162.0 |
C7B—H7B···CgAvii | 0.93 | 3.56 | 4.401 | 152.0 |
CgA and CgB refer to the centroids of the phenyl rings of
molecules A and B, respectively.
Symmetry codes: (i) x, y-1, z; (ii) x, y+1, z; (iii) x+1/2, -y, z;
(iv) x-1/2, -y+1, z; (v) -x, -y+1, z+1/2; (vi) -x, -y, z-1/2;
(vii) -x+1/2, y, z+1/2. |
The carbamate group is known in biochemistry for its role in biological processes. For example, it tunes haemoglobin affinity for O2 during physiological respiration (O'Donnell et al., 1979). Carbamate derivatives present significant pharmacological activity, in some cases exhibiting potential as anticancer drugs (Bubert et al., 2007). In the solid state, the carbamate group acts as both donor and acceptor in hydrogen bonding, favouring the formation of highly stable synthons. Thus, the carbamate group has been proposed in crystal engineering as a building block for hydrogen-bonded solids (Gosh et al., 2006).
Most carbamate compounds of interest are phenyl derivatives. In the known polymorphs of one such compound, phenyl carbamate, the molecular environment is very similar around the carbamate group but very different around the phenyl ring (Wishkerman & Bernstein, 2008). In this case, polymorphism apparently arises from a different assembly of the same supramolecular synthons, suggesting that weaker interactions, such as those involving the phenyl ring, can play an important role in directing this assembly. Thus, the final molecular packing comes from the interplay of a few strong hydrogen bonds around the carbamate group with a number of weaker interactions, most of them involving the phenyl ring. In order to obtain insight into the interplay of the carbamate group and the phenyl ring in the molecular packing, the related compound, benzyl carbamate, (I), has been crystallized, and the crystal structure analysed in detail using the PIXEL method (Gavezzotti, 2011).
Benzyl carbamate crystallizes in the noncentrosymmetric group Pbc21, presenting two independent molecules in the asymmetric unit, A and B, related by local symmetry centres (Fig. 1). The geometric parameters for molecules A and B are equal within three times the s.u. values. Two independent local symmetry centres are observed, situated at (0.240, 0.106, 0.252) and (0.740, 0.106, 0.252), as calculated from the average of non-H-atom pairs. The formation of a local symmetry centre close to x = 1/4, y = 1/8 is a common feature in this space group (Marsh et al., 1998). Crystal packing in (I) takes the form of layers perpendicular to the c axis, with the molecules inside each layer connected by strong N—H···O hydrogen bonds and neighbouring layers connected by weak C—H···π contacts. Each layer consists of two sublayers formed exclusively of molecules A or molecules B. Interlayer contacts involve one A and one B sublayer (Fig. 2).
The PIXEL method allows the calculation of the intermolecular interaction energy (Ei) inside the crystal structure from the electron density of isolated molecules with the crystal geometry, which is previously calculated from ab initio methods. PIXEL provides interaction energies (Ei) for pairs of molecules inside the crystal structure that take into account the polarization energy induced in the molecules by the crystal environment.
According to the calculated values of Ei, the eight lowest interaction energies correspond to pairs of molecules in the same layer, while the next three correspond to molecules in neighbouring layers. The rest of the calculated Ei values range from -2.5 to 2.0 kJ mol-1 and are for pairs of molecules involving two molecules that are not first neighbours in the crystal structure. As the 11 lowest Ei values account for 93% of the lattice energy, only the corresponding pairs of molecules will be considered in the following analysis. In Table 1, pairs of molecules are labelled in order of increasing Ei and identified by the most prominent intermolecular interaction, which does not mean that Ei is the energy of this particular interaction. Thus, when Ei is associated with, for example, a hydrogen bond, it should be understood as the interaction energy of the molecules connected by this hydrogen bond, and not the energy of the hydrogen bond alone. The geometry of the intermolecular interactions in Table 1 is given in Table 2.
In order to obtain an estimate of the effect of the crystal environment on the intermolecular interactions, Ei was also calculated for 11 isolated pairs of molecules with the same geometry as in the crystal structure. Ei is systematically 2–7% lower for pairs of molecules in the crystal structure, i.e. the crystal environment strengthens the intermolecular interactions, although its influence is small. According to this result, the crystal stability of benzyl carbamate arises from the interaction of each molecule with its primary neighbours, with little influence from the surrounding molecules.
The three lowest Ei values are for molecules connected by N—H···O hydrogen bonds, while the rest are associated with C—H···X (X = O, N or π). The qualitatively different character of these two kinds of interaction is revealed by the energy decomposition analysis provided by PIXEL. Thus, in strong hydrogen bonds, the most important contribution to Ei is the electrostatic energy, which is a good estimator of Ei because the sum of the other contributions (dispersion, polarization and repulsion) approximately cancels, as observed in other energy decomposition schemes based on the electron density (Abramov et al. 2000). In the remaining pairs of molecules, dispersion is the main contribution to Ei, while the sum of the electrostatic and polarization energies only partially compensates for the repulsion.
As shown in Fig. 2, molecule pair 1 corresponds to an R22(8) motif (Bernstein et al., 1995) formed by two N—H···O hydrogen bonds. This supramolecular motif connects molecules A and B and is built around one of the local symmetry centres. Given the large value of Ei compared with the rest of the interaction energies, the dimer defined by R22(8) can be considered as the building block of the crystal structure. The next two pairs correspond to N—H···O hydrogen bonds defining two identical C(4) motifs that run along [010]. Consecutive dimers along these chains are tilted, the planes defined by their R22(8) motifs forming an angle of 74°, calculated from the planes defined by atoms O2A/C1A/O1A/N1A and O2Bi/C1Bi/O1Bi/N1Bi [symmetry code: (i) x - 1, y, z] and that defined by the same atoms after applying the symmetry operator (-x, y + 1/2, z). Two parallel dimers along the same chain are bridged by two carbamate groups in tilted dimers, forming an R66(16) motif (Fig. 3). The same hydrogen-bond pattern is observed in metastable form I of phenyl carbamate (Wishkerman & Bernstein, 2008).
A feature not accounted for by hydrogen bonding is the stacked dimers along [100], giving rise to the second local symmetry centre between stacked dimers. The two lowest Ei values for non-hydrogen-bonded pairs of molecules (pairs 4 and 5) and the highest Ei value for an intralayer interaction (pair 8) are associated with this stacking. The short C3A—H3A1···N1A (pair 4) and C3B—H3B2···N1B (pair 5) distances and the conformation of the carbamate groups, with an interplanar distance of 3.174 Å, calculated from the planes defined by atoms C3A/O2A/C1A/O1A/N1A and C3Bi/O2Bi/C1Bi/O1Bi/N1Bi, and that defined by the same atoms after applying the symmetry operator (x + 1, y, z), and the oxo groups being almost superimposed in the direction perpendicular to the carbamate plane, suggests that these three pairs account for a complex stacking of the dimers that involves a combination of C—H···N and π–π interactions. According to PIXEL, this dimer stacking is similar in strength to an N—H···O hydrogen bond, playing an important role in the conformation of the layers. Thus, R22(8) motifs and dimer stacking define chains along [100] where each molecule in the chain is connected to its neighbours by local symmetry centres (Fig. 4). The assembly of these chains through hydrogen bonding results in layers where lines of local symmetry centres alternate with the b-glide plane of the crystal structure.
The only significant interaction between the carbamate and phenyl groups in the A sublayer is C9A—H9A···O2A (pair 7). The conformation of the molecule pair suggests that this interaction is reinforced by C8A—H8A···πA (pair 7). Each molecule A forms two C9A—H9A···O2A (pair 7) contacts and one C3A—H3A···N1A (pair 4) contact of similar Ei value with three consecutive members of the chain generated by the C(4) motif (Fig. 5). This is the strongest interaction bridging elements along C(4) and, together with the C5A—H5A···πA contact reinforcing N1A—H1A1···O1A (both in pair 3), stabilizes the tilted conformation of the molecules along the C(4) chains (pairs 2 and 3), to the detriment of the parallel conformation observed, for example, in the stable form II of phenyl carbamate (Wishkerman & Bernstein, 2008). The same pattern of contacts, involving pair 6, is observed in the B sublayer.
The molecule packing in a noncentrosymmetric space group arises from the assembly of layers through C—H···π contacts. These interactions can induce the formation of chiral helices arranged along the 21 axis, favouring crystallization in a noncentrosymmetric space group even in the case of achiral molecules (Tanaka et al., 2007). The only important differences in the molecular environments of A and B concern the interlayer C—H···π contacts (Fig. 6). Thus, the geometries of H7A···πB (pair 11) and H7B···πA (pair 9) are quite different, with the Ei value being significantly lower for the latter. Moreover, B is a donor in a third C—H···π contact (pair 10) with an unconventional conformation. In this pair, the phenyl rings are almost perpendicular but the donor presents a very large offset from the centroid of the acceptor. The result deviates from the T conformation usually associated with a C—H···π interaction, resembling an L conformation, with a large centroid-to-centroid distance CgB···CgAii = 6.008 Å [symmetry code: (ii) -x + 1, -y, z + 1/2] and a short separation from the donor centroid CgB to the plane defined by the phenyl acceptor πA of 3.257 Å. Because of this unusual conformation, this interaction can be easily missed, in spite of being comparable in strength with other C—H···π contacts.
In conclusion, the application of the PIXEL method to benzyl carbamate, (I), stresses features difficult to detect from a more conventional analysis of the structure based exclusively on geometry. Thus, the method reveals that a combination of C—H···N interactions induces a stacking that is comparable in energy with hydrogen bonding, that C—H···O and C—H···π interactions favour a tilted conformation of the molecules along hydrogen-bonded chains, and that relatively strong phenyl–phenyl interactions can take place with unusual geometries. Although hydrogen bonding is the strongest intermolecular force, there is no clear division between strong and weak interactions. Thus, even if strong hydrogen bonds are present, C—H···X (X = O, N or π) should be considered in explaining the crystal packing of benzyl carbamate.