The title compound, C
9H
7NO, has two symmetry-independent molecules in the asymmetric unit, which have different conformations of the hydroxy group with respect to the quinoline ring. One of the molecules adopts a
cis conformation, while the other shows a
trans conformation. Each type of independent molecule links into a separate infinite O-H
N hydrogen-bonded chain with the graph-set notation
C(7). These chains are perpendicular in the unit cell, one extended in the
a-axis direction and the other in the
b-axis direction. There is also a weak C-H
O hydrogen bond with graph-set notation
D(2), which runs in the
c-axis direction and joins the two separate O-H
N chains. The significance of this study lies in the comparison drawn between the experimental and calculated data of the crystal structure of the title compound and the data of several other derivatives possessing the hydroxy group or the quinoline ring. The correlation between the IR spectrum of this compound and the hydrogen-bond energy is also discussed.
Supporting information
CCDC reference: 724209
Powder of 6-HQ was purchased from Sigma–Aldrich and used without further
purification. It took nearly four years to obtain crystals suitable for X-ray
diffraction analysis. During this time, many crystal growth trials were
carried out from mixtures of various solvents, such as chloroform, petroleum
ether, ethyl acetate, ethanol and water. Suitable crystals were grown only by
very slow evaporation of an acetone mixture of 6-HQ by a very thin glass
capillary at 280.0 (1) K.
The aromatic H atoms were treated as riding on their parent atoms, with C—H =
0.95 Å and with Uiso(H) = 1.2Ueq(C). H atoms involved in
hydrogen bonding were located in a difference Fourier map and refined freely
with isotropic displacement parameters.
Data collection: CrysAlis CCD (Oxford Diffraction, 2006); cell refinement: CrysAlis RED (Oxford Diffraction, 2006); data reduction: CrysAlis RED (Oxford Diffraction, 2006); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2003) and Mercury (Macrae et al., 2006); software used to prepare material for publication: publCIF (Westrip, 2009).
Crystal data top
C9H7NO | Dx = 1.389 Mg m−3 |
Mr = 145.16 | Melting point: 466 K |
Orthorhombic, Pbca | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2ac 2ab | Cell parameters from 8457 reflections |
a = 14.2061 (2) Å | θ = 3.0–34.6° |
b = 13.4440 (2) Å | µ = 0.09 mm−1 |
c = 14.5341 (2) Å | T = 100 K |
V = 2775.82 (7) Å3 | Polyhedron, colourless |
Z = 16 | 0.41 × 0.18 × 0.1 mm |
F(000) = 1216 | |
Data collection top
Oxford Diffraction diffractometer with Sapphire3 CCD detector | 3361 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.044 |
Graphite monochromator | θmax = 34.6°, θmin = 3.0° |
ω scans | h = −22→13 |
31753 measured reflections | k = −21→20 |
5637 independent reflections | l = −23→22 |
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)] = 0.044 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.102 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.00 | w = 1/[σ2(Fo2) + (0.0458P)2] where P = (Fo2 + 2Fc2)/3 |
5637 reflections | (Δ/σ)max < 0.001 |
208 parameters | Δρmax = 0.45 e Å−3 |
0 restraints | Δρmin = −0.23 e Å−3 |
Crystal data top
C9H7NO | V = 2775.82 (7) Å3 |
Mr = 145.16 | Z = 16 |
Orthorhombic, Pbca | Mo Kα radiation |
a = 14.2061 (2) Å | µ = 0.09 mm−1 |
b = 13.4440 (2) Å | T = 100 K |
c = 14.5341 (2) Å | 0.41 × 0.18 × 0.1 mm |
Data collection top
Oxford Diffraction diffractometer with Sapphire3 CCD detector | 3361 reflections with I > 2σ(I) |
31753 measured reflections | Rint = 0.044 |
5637 independent reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.044 | 0 restraints |
wR(F2) = 0.102 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.00 | Δρmax = 0.45 e Å−3 |
5637 reflections | Δρmin = −0.23 e Å−3 |
208 parameters | |
Special details top
Experimental. The IR spectrum of a polycrystalline sample of 6-HQ was measured with the
transmission method at room temperature using the KBr pellet technique on a
Nicolet Magna 560?FT–IR spectrometer and with 4?cm-1 resolution. |
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. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
O1 | 0.15147 (5) | 0.00582 (6) | 0.30239 (6) | 0.02329 (18) | |
N1 | 0.50579 (6) | 0.13624 (7) | 0.19933 (6) | 0.01853 (18) | |
C2 | 0.51744 (7) | 0.22768 (8) | 0.16756 (7) | 0.0209 (2) | |
H2 | 0.5789 | 0.2475 | 0.1495 | 0.025* | |
C3 | 0.44410 (7) | 0.29694 (8) | 0.15904 (7) | 0.0205 (2) | |
H3 | 0.4561 | 0.3615 | 0.1352 | 0.025* | |
C4 | 0.35519 (7) | 0.27062 (8) | 0.18537 (7) | 0.0186 (2) | |
H4 | 0.3049 | 0.3170 | 0.1805 | 0.022* | |
C5 | 0.24796 (7) | 0.13988 (7) | 0.24618 (7) | 0.01720 (19) | |
H5 | 0.1955 | 0.1836 | 0.2428 | 0.021* | |
C6 | 0.23569 (7) | 0.04410 (8) | 0.27652 (7) | 0.0171 (2) | |
C7 | 0.31387 (7) | −0.02134 (8) | 0.28168 (7) | 0.0183 (2) | |
H7 | 0.3049 | −0.0877 | 0.3023 | 0.022* | |
C8 | 0.40209 (7) | 0.01017 (7) | 0.25724 (7) | 0.0180 (2) | |
H8 | 0.4539 | −0.0342 | 0.2619 | 0.022* | |
C9 | 0.41673 (6) | 0.10792 (7) | 0.22516 (7) | 0.01538 (19) | |
C10 | 0.33861 (7) | 0.17398 (7) | 0.21987 (7) | 0.01497 (19) | |
H1O | 0.1101 (9) | 0.0505 (9) | 0.2992 (9) | 0.022* | |
O2 | 0.16710 (5) | −0.05798 (5) | 0.01944 (6) | 0.02581 (19) | |
N2 | 0.15881 (6) | 0.35388 (6) | 0.02634 (6) | 0.01920 (18) | |
C12 | 0.08103 (7) | 0.39882 (8) | 0.05397 (8) | 0.0218 (2) | |
H12 | 0.0801 | 0.4695 | 0.0542 | 0.026* | |
C13 | −0.00072 (7) | 0.34831 (8) | 0.08308 (8) | 0.0226 (2) | |
H13 | −0.0532 (8) | 0.3859 (9) | 0.1054 (9) | 0.027* | |
C14 | −0.00133 (7) | 0.24703 (8) | 0.08257 (7) | 0.0198 (2) | |
H14 | −0.0559 | 0.2118 | 0.1017 | 0.024* | |
C15 | 0.08434 (7) | 0.08970 (7) | 0.05082 (7) | 0.0182 (2) | |
H15 | 0.0313 | 0.0514 | 0.0690 | 0.022* | |
C16 | 0.16512 (7) | 0.04274 (7) | 0.02212 (7) | 0.0182 (2) | |
C17 | 0.24385 (7) | 0.09989 (8) | −0.00572 (7) | 0.0183 (2) | |
H17 | 0.2994 | 0.0671 | −0.0258 | 0.022* | |
C18 | 0.24119 (7) | 0.20138 (7) | −0.00418 (7) | 0.0178 (2) | |
H18 | 0.2946 | 0.2384 | −0.0234 | 0.021* | |
C19 | 0.15930 (7) | 0.25172 (7) | 0.02587 (7) | 0.01545 (19) | |
C20 | 0.07987 (7) | 0.19446 (7) | 0.05339 (7) | 0.01594 (19) | |
H2O | 0.2272 (9) | −0.0800 (9) | 0.0172 (8) | 0.024* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O1 | 0.0119 (3) | 0.0219 (4) | 0.0361 (5) | −0.0005 (3) | 0.0020 (3) | 0.0076 (3) |
N1 | 0.0127 (4) | 0.0244 (5) | 0.0185 (4) | −0.0009 (3) | 0.0010 (3) | −0.0004 (4) |
C2 | 0.0156 (5) | 0.0284 (6) | 0.0188 (5) | −0.0044 (4) | 0.0008 (4) | −0.0001 (4) |
C3 | 0.0213 (5) | 0.0200 (5) | 0.0202 (5) | −0.0051 (4) | −0.0027 (4) | 0.0023 (4) |
C4 | 0.0176 (5) | 0.0179 (5) | 0.0203 (5) | 0.0007 (4) | −0.0027 (4) | 0.0004 (4) |
C5 | 0.0113 (4) | 0.0187 (5) | 0.0216 (5) | 0.0027 (4) | −0.0005 (4) | 0.0011 (4) |
C6 | 0.0118 (4) | 0.0203 (5) | 0.0191 (5) | −0.0015 (4) | −0.0007 (4) | 0.0010 (4) |
C7 | 0.0176 (5) | 0.0160 (5) | 0.0212 (5) | 0.0010 (4) | −0.0004 (4) | 0.0023 (4) |
C8 | 0.0148 (4) | 0.0195 (5) | 0.0197 (5) | 0.0050 (4) | −0.0004 (4) | 0.0008 (4) |
C9 | 0.0125 (4) | 0.0189 (5) | 0.0148 (4) | 0.0006 (4) | −0.0005 (3) | −0.0018 (4) |
C10 | 0.0132 (4) | 0.0163 (4) | 0.0154 (4) | 0.0000 (4) | −0.0014 (4) | −0.0009 (4) |
O2 | 0.0212 (4) | 0.0133 (4) | 0.0429 (5) | 0.0002 (3) | 0.0062 (4) | 0.0018 (3) |
N2 | 0.0197 (4) | 0.0148 (4) | 0.0231 (4) | 0.0013 (3) | −0.0021 (3) | −0.0019 (3) |
C12 | 0.0247 (5) | 0.0158 (5) | 0.0249 (5) | 0.0056 (4) | −0.0031 (4) | −0.0033 (4) |
C13 | 0.0202 (5) | 0.0262 (6) | 0.0215 (5) | 0.0088 (5) | −0.0008 (4) | −0.0014 (5) |
C14 | 0.0156 (4) | 0.0254 (5) | 0.0184 (5) | 0.0024 (4) | 0.0015 (4) | 0.0022 (4) |
C15 | 0.0148 (4) | 0.0175 (5) | 0.0222 (5) | −0.0026 (4) | 0.0014 (4) | 0.0022 (4) |
C16 | 0.0189 (5) | 0.0132 (5) | 0.0226 (5) | −0.0006 (4) | 0.0016 (4) | 0.0000 (4) |
C17 | 0.0153 (4) | 0.0167 (5) | 0.0228 (5) | 0.0010 (4) | 0.0029 (4) | −0.0014 (4) |
C18 | 0.0148 (4) | 0.0170 (5) | 0.0216 (5) | −0.0025 (4) | 0.0018 (4) | −0.0009 (4) |
C19 | 0.0157 (4) | 0.0136 (4) | 0.0171 (5) | 0.0004 (4) | −0.0021 (4) | −0.0013 (4) |
C20 | 0.0139 (4) | 0.0185 (5) | 0.0154 (4) | 0.0012 (4) | −0.0001 (4) | 0.0014 (4) |
Geometric parameters (Å, º) top
O1—C6 | 1.3557 (11) | O2—C16 | 1.3550 (12) |
O1—H1O | 0.842 (13) | O2—H2O | 0.904 (12) |
N1—C2 | 1.3236 (13) | N2—C12 | 1.3218 (13) |
N1—C9 | 1.3735 (12) | N2—C19 | 1.3735 (12) |
C2—C3 | 1.4027 (15) | C12—C13 | 1.4102 (15) |
C2—H2 | 0.9500 | C12—H12 | 0.9500 |
C3—C4 | 1.3665 (13) | C13—C14 | 1.3617 (15) |
C3—H3 | 0.9500 | C13—H13 | 0.958 (12) |
C4—C10 | 1.4124 (14) | C14—C20 | 1.4178 (13) |
C4—H4 | 0.9500 | C14—H14 | 0.9500 |
C5—C6 | 1.3722 (14) | C15—C16 | 1.3746 (13) |
C5—C10 | 1.4194 (13) | C15—C20 | 1.4104 (13) |
C5—H5 | 0.9500 | C15—H15 | 0.9500 |
C6—C7 | 1.4188 (14) | C16—C17 | 1.4159 (14) |
C7—C8 | 1.3697 (13) | C17—C18 | 1.3650 (14) |
C7—H7 | 0.9500 | C17—H17 | 0.9500 |
C8—C9 | 1.4099 (14) | C18—C19 | 1.4150 (13) |
C8—H8 | 0.9500 | C18—H18 | 0.9500 |
C9—C10 | 1.4235 (13) | C19—C20 | 1.4233 (13) |
| | | |
C6—O1—H1O | 109.3 (8) | C16—O2—H2O | 110.4 (8) |
C2—N1—C9 | 117.90 (8) | C12—N2—C19 | 117.58 (9) |
N1—C2—C3 | 123.67 (9) | N2—C12—C13 | 124.02 (10) |
N1—C2—H2 | 118.2 | N2—C12—H12 | 118.0 |
C3—C2—H2 | 118.2 | C13—C12—H12 | 118.0 |
C4—C3—C2 | 119.34 (10) | C14—C13—C12 | 119.02 (10) |
C4—C3—H3 | 120.3 | C14—C13—H13 | 121.7 (7) |
C2—C3—H3 | 120.3 | C12—C13—H13 | 119.2 (7) |
C3—C4—C10 | 119.46 (9) | C13—C14—C20 | 119.66 (10) |
C3—C4—H4 | 120.3 | C13—C14—H14 | 120.2 |
C10—C4—H4 | 120.3 | C20—C14—H14 | 120.2 |
C6—C5—C10 | 120.32 (9) | C16—C15—C20 | 120.28 (9) |
C6—C5—H5 | 119.8 | C16—C15—H15 | 119.9 |
C10—C5—H5 | 119.8 | C20—C15—H15 | 119.9 |
O1—C6—C5 | 123.88 (9) | O2—C16—C15 | 119.01 (9) |
O1—C6—C7 | 116.16 (9) | O2—C16—C17 | 121.18 (9) |
C5—C6—C7 | 119.96 (9) | C15—C16—C17 | 119.80 (9) |
C8—C7—C6 | 120.72 (9) | C18—C17—C16 | 121.06 (9) |
C8—C7—H7 | 119.6 | C18—C17—H17 | 119.5 |
C6—C7—H7 | 119.6 | C16—C17—H17 | 119.5 |
C7—C8—C9 | 120.60 (9) | C17—C18—C19 | 120.39 (9) |
C7—C8—H8 | 119.7 | C17—C18—H18 | 119.8 |
C9—C8—H8 | 119.7 | C19—C18—H18 | 119.8 |
N1—C9—C8 | 118.99 (9) | N2—C19—C18 | 118.95 (9) |
N1—C9—C10 | 122.04 (9) | N2—C19—C20 | 122.36 (9) |
C8—C9—C10 | 118.97 (9) | C18—C19—C20 | 118.68 (9) |
C4—C10—C5 | 122.96 (9) | C15—C20—C14 | 122.84 (9) |
C4—C10—C9 | 117.58 (9) | C15—C20—C19 | 119.80 (9) |
C5—C10—C9 | 119.43 (9) | C14—C20—C19 | 117.36 (9) |
| | | |
C9—N1—C2—C3 | 0.11 (15) | C12—C13—C14—C20 | 0.31 (16) |
N1—C2—C3—C4 | 0.65 (16) | C20—C15—C16—O2 | −179.32 (10) |
C2—C3—C4—C10 | −0.57 (15) | C20—C15—C16—C17 | −0.56 (16) |
C10—C5—C6—O1 | −179.64 (9) | O2—C16—C17—C18 | 179.06 (10) |
C10—C5—C6—C7 | 0.00 (15) | C15—C16—C17—C18 | 0.33 (16) |
O1—C6—C7—C8 | −179.88 (9) | C16—C17—C18—C19 | 0.28 (16) |
C5—C6—C7—C8 | 0.45 (15) | C12—N2—C19—C18 | 179.39 (9) |
C6—C7—C8—C9 | −0.93 (15) | C12—N2—C19—C20 | 0.25 (15) |
C2—N1—C9—C8 | 178.56 (9) | C17—C18—C19—N2 | −179.82 (10) |
C2—N1—C9—C10 | −0.93 (14) | C17—C18—C19—C20 | −0.64 (15) |
C7—C8—C9—N1 | −178.55 (9) | C16—C15—C20—C14 | −179.81 (10) |
C7—C8—C9—C10 | 0.95 (15) | C16—C15—C20—C19 | 0.19 (15) |
C3—C4—C10—C5 | −178.13 (10) | C13—C14—C20—C15 | −179.85 (10) |
C3—C4—C10—C9 | −0.20 (15) | C13—C14—C20—C19 | 0.15 (15) |
C6—C5—C10—C4 | 177.93 (10) | N2—C19—C20—C15 | 179.55 (10) |
C6—C5—C10—C9 | 0.04 (15) | C18—C19—C20—C15 | 0.40 (14) |
N1—C9—C10—C4 | 0.98 (14) | N2—C19—C20—C14 | −0.44 (15) |
C8—C9—C10—C4 | −178.51 (9) | C18—C19—C20—C14 | −179.59 (9) |
N1—C9—C10—C5 | 178.99 (9) | C5—C6—O1—H1O | −2.8 (9) |
C8—C9—C10—C5 | −0.50 (14) | C7—C6—O1—H1O | 177.6 (9) |
C19—N2—C12—C13 | 0.25 (16) | C15—C16—O2—H2O | −162.3 (8) |
N2—C12—C13—C14 | −0.54 (17) | C17—C16—O2—H2O | 19.0 (9) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1O···N1i | 0.842 (13) | 1.877 (13) | 2.7125 (11) | 171.4 (12) |
O2—H2O···N2ii | 0.904 (12) | 1.852 (13) | 2.7442 (11) | 168.6 (12) |
C13—H13···O1iii | 0.958 (12) | 2.518 (12) | 3.4411 (13) | 161.8 (10) |
Symmetry codes: (i) x−1/2, y, −z+1/2; (ii) −x+1/2, y−1/2, z; (iii) −x, y+1/2, −z+1/2. |
Experimental details
Crystal data |
Chemical formula | C9H7NO |
Mr | 145.16 |
Crystal system, space group | Orthorhombic, Pbca |
Temperature (K) | 100 |
a, b, c (Å) | 14.2061 (2), 13.4440 (2), 14.5341 (2) |
V (Å3) | 2775.82 (7) |
Z | 16 |
Radiation type | Mo Kα |
µ (mm−1) | 0.09 |
Crystal size (mm) | 0.41 × 0.18 × 0.1 |
|
Data collection |
Diffractometer | Oxford Diffraction diffractometer with Sapphire3 CCD detector |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 31753, 5637, 3361 |
Rint | 0.044 |
(sin θ/λ)max (Å−1) | 0.799 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.044, 0.102, 1.00 |
No. of reflections | 5637 |
No. of parameters | 208 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.45, −0.23 |
Selected bond lengths (Å) topO1—C6 | 1.3557 (11) | O2—C16 | 1.3550 (12) |
C5—C6 | 1.3722 (14) | C15—C16 | 1.3746 (13) |
C6—C7 | 1.4188 (14) | C16—C17 | 1.4159 (14) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1O···N1i | 0.842 (13) | 1.877 (13) | 2.7125 (11) | 171.4 (12) |
O2—H2O···N2ii | 0.904 (12) | 1.852 (13) | 2.7442 (11) | 168.6 (12) |
C13—H13···O1iii | 0.958 (12) | 2.518 (12) | 3.4411 (13) | 161.8 (10) |
Symmetry codes: (i) x−1/2, y, −z+1/2; (ii) −x+1/2, y−1/2, z; (iii) −x, y+1/2, −z+1/2. |
Quinoline derivatives are well known for their antifungal and antibacterial activities (Bambury, 1979). In the other hand they also have toxic, mutagenic and carcinogenic activities (Reinhardt & Britteli, 1981). Quinoline was metabolized by a Pseudomonas sp. and several intermediate products were obtained, such as 2-hydroxyquinoline and 8-hydroxycoumarin (Shukla, 1986). Hydroxyquinolines are good substrates for glucoronidation by UDP-glucoronosyltransferase (Kanou et al., 2002). Thus the biological function of 6-hydroxyquinoline (6-HQ) has attracted considerable interest in recent years. Quinoline and 6-HQ were degraded into quinolones by Brevundimonas diminuta, Pseudomonas diminuta and Bacillus circulans (Han & Andrade, 2005; Bott & Lingens, 1991). A series of 6-HQ derivatives, i.e. ethyl 6-hydroxyqiunoline-3-carboxylates, have important anti-hepatitis B virus activities in vitro (Liu et al., 2008). The major studies on 6-HQ were focused on the excited state proton transfers in aqueous solutions and on the photophysical properties of this compound (Yu et al., 1997; Kim et al., 1997, 2001; Mehata et al., 2002, 2003). The vibrational frequencies of the title compound have been calculated by using Hartree–Fock and density functional methods (Arici & Köksal, 2008).
The objects of our research are the IR spectra of the crystals with open hydrogen-bonded chains in the unit cell, measured in a frequency range of the proton and deuteron stretching vibrations of the hydrogen bridge (Flakus & Michta, 2003, 2004, 2005). Characteristic isotopic and spectroscopic effects, called the H/D isotopic self-organization effects, were observed in this vibration frequency range (Flakus, 1989, 2003; Flakus & Bańczyk, 1999). Measurements of polarized IR spectra of diverse spatially oriented hydrogen-bond systems present in the lattices of molecular crystals allow us to estimate the polarization properties of transitions found in the excited states of the proton vibrations in the crystals. These transitions contribute to the νX—H band generation mechanisms in the crystalline spectra. Thus for the reliable interpretations of the self-organization mechanism, the crystal structure of the hydrogen bond system must be known. In the case of 6-HQ, a crystallographic study has not yet been reported. A search of the Cambridge Structural Database [Version 5.28 (Allen, 2002); CONQUEST, Version 1.9 (Bruno et al., 2002)] for 6-HQ derivatives yielded only 11 complex structures with different and large substituents on the quinoline ring. From other hydroxyquinolines only the structure of 8-hydroxyquinoline with the centrosymmetric dimers of the hydrogen bonds in the unit is well known (Roychowdhury et al., 1978; Banerjee & Saha, 1986; Zhang & Wu, 2005).
In this article, the results of our structural studies of the hydrogen bonds of 6-HQ are presented. 6-HQ crystallizes with two molecules in the asymmetric unit (Fig. 1), which exhibit comparable bond lengths and angles. Both six-membered rings of 6-HQ are essentially planar, with deviations from the mean plane in the range 0.0001–0.0231 Å. Larger r.m.s. deviations from the quinoline plane are observed for the N1-containing molecule than for the N2-containing one (0.0141 and 0.0027, respectively). The dihedral angles between the planes of the benzene and pyridine rings are 1.69 (4) and 0.20 (4)° for the N1- and N2-containing molecules, respectively. These angles compare well with those of related compounds, such as 8-hydroxyquinoline [1.37° (Roychowdhury et al., 1978) or 0.82° (Zhang & Wu, 2005)], α-naphtol (0.22°; Rozycka-Sokolowska et al., 2004), 7-bromoquinolin-8-ol [1.6 (3)°; Collis et al., 2003] or (+)-2-(1'-hydroxy-1'-methylethyl)-2,3-dihydrofuro(2,3-b)quinolin-6-ol (1.47°; Jurd & Wong, 1983). Atoms O1 and O2 of the hydroxy groups lie slightly out of the quinoline plane, by -0.0202 (7) and -0.0096 (7) Å, respectively. The C—O bond length is in the region of 1.355 Å and compares well with the calculated value, which is equal to 1.352 Å (Bach et al., 1998). The C—O distance is also similar to those of related compounds such as 8-hydroxyquinoline N-oxide (Desiderato et al., 1971) and ethyl 4-(4-chlorophenyl)-6-hydroxyquinoline-2-carboxylate (Wu et al., 2006). The hydroxy group exerts an influence on the distances of the C5—C6 (C15—C16) and C6—C7 (C16—C17) endocyclic bonds of the benzene ring. These bonds are elongated in comparison with the corresponding distances in quinoline (Davies & Bond, 2001). This behavior is probably a consequence of some degree of conjugation between the oxygen and quinoline system, which was observed also in other quinoline derivatives (Suszko-Purzycka et al., 1985).
The main difference between the two symmetry-independent molecules is the orientation of the hydroxy group with respect to the quinoline ring. In the N1-containing molecule the H atom from the hydroxy group is in a cis position relative to atom C5 and this is the cis-conformer of 6-HQ (6-HQA). In turn, the second molecule has a trans conformation of the C15—C16—O2—H2O group and this is the trans-conformer of 6-HQ (6-HQB). Thus the values of the C5—C6—O1—H1O and C7—C6 –O1—H1O torsion angles [-2.8 (9) and 177.6 (9)°, respectively] are different from those of the C15—C16—O2—H2O and C17—C16—O2—H2O angles [-162.3 (9) and 19.0 (9)°, respectively]. Similar calculated and experimental conformers with different orientation of the hydroxy group have been found in the structure of β-naphthol (Marciniak et al., 2003; Ahn et al., 2003). The values of the same dihedral angles were -1.39 and 178.66° for cis-β-naphthol and -159.42 and 22.13° for trans-β-naphthol, respectively (Marciniak et al., 2003). In the case of 6-HQ as well as β-naphtol the total energies of two rotamers in the ground state calculated by the ab initio self-consistent field method showed that the cis rotamer is more stable than the trans one (Bach et al., 1998).
In the crystal structure of 6-HQ, the two symmetry-independent molecules interact via O—H···N hydrogen bonds (Table 1), forming two separate extended zigzag chains with the graph-set notation C(7) (Fig. 2) (Bernstein et al., 1990; Grell et al., 1999). The chain formed by the cis conformers runs in the a-axis direction and the second chain formed by the trans conformers runs in the b-axis direction. Thus the two separate chains are perpendicular in the unit cell. There is also a weak C13—H13···O1 hydrogen bond with graph-set notation D(2) (Bernstein et al., 1990; Grell et al., 1999). This weak bond joins the two symmetry-independent chains (Fig. 3). Thus the second level graph-set notation give two possible sets of the hydrogen-bond motifs, i.e. D33(10) and D33(14) (Bernstein et al., 1990; Grell et al., 1999).
The values of the O—H···N hydrogen-bond distances are in the range 2.5-3.2 Å and therefore they can be regarded as strong hydrogen bonds (Desiraju & Steiner, 1999). The strength of the hydrogen bonds in this compound was also investigated with the help of IR spectroscopy. The band of the isolated O—H stretching vibration in alcohols, νO—H, is located at a frequency of about 3600 cm-1 (Günzler & Gremlich, 2002). In the case of 6-HQ, we observed the wide band of the O—H stretching vibration in the frequency range 3100–2200 cm-1 with a shift of about 900 cm-1 (Fig. 5). This relative shift is equal to 25% and this value lies on the border defined between a strong and a very strong hydrogen bond (Desiraju & Steiner, 1999).