The title compound, C
10H
9NO, contains an acetyl group that is nearly coplanar with the indole ring system, with an angle between the planes of the heterocyclic ring and the acetyl group of 1.75 (17)°. The planes of the benzene and pyrrole rings in the indole system make a dihedral angle of 2.05 (11)°. Each molecule in the unit cell is linked through N-H
O hydrogen bonds to two other molecules, forming hydrogen-bonded chains in the [101] direction with graph set
C(6). The significance of this study lies in the analysis of the interactions occurring
via hydrogen bonds in this structure, as well as in the comparison drawn between the molecular structure of the title compound and those of several other indole derivatives possessing a 3-carbonyl group. The correlation between the IR spectrum of this compound and the structural data is also discussed.
Supporting information
CCDC reference: 697588
3-Acetylindole (98% pure), purchased from Sigma–Aldrich, was dissolved in a
mixture of acetone and water (1:1 v/v). After a few weeks, small
single crystals of (I), suitable for X-ray diffraction, were grown from the
solution by slow evaporation at 293 K. The IR spectrum of a polycrystalline
sample of (I) dispersed in KBr was measured at the temperature of liquid
nitrogen using an FT–IR Nicolet Magna 560 spectrometer operating at a
resolution of 2 cm-1. The IR spectrum was recorded in the range 1000–4000 cm-1 using an Ever-Glo source, a KBr beamsplitter and a DTGS detector.
Aromatic H atoms were treated as riding on their parent C atoms, with C—H =
0.96 Å, and with Uiso(H) = 1.2Ueq(C). Methyl H atoms were
also treated as riding on their parent C atoms, with C—H = 0.93 Å, and
Uiso(H) = 1.5Ueq(C). Atom H1, which takes part in hydrogen
bonding, was located in a difference Fourier map (ΔF) and refined
freely with isotropic displacement parameters. [Please check rephrasing to
reflect that only one atom was treated in this way]
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: ORTEP-3 (Farrugia, 1997) and Mercury (Macrae et al.,
2006); software used to prepare material for publication: publCIF (Westrip, 2008).
Crystal data top
C10H9NO | F(000) = 336 |
Mr = 159.18 | Dx = 1.309 Mg m−3 |
Monoclinic, P21/n | Melting point = 461–465 K |
Hall symbol: -P 2yn | Mo Kα radiation, λ = 0.71073 Å |
a = 9.5665 (19) Å | Cell parameters from 1709 reflections |
b = 7.6553 (15) Å | θ = 2.8–32.8° |
c = 11.031 (2) Å | µ = 0.09 mm−1 |
β = 90.77 (3)° | T = 110 K |
V = 807.8 (3) Å3 | Plate, colourless |
Z = 4 | 0.54 × 0.32 × 0.03 mm |
Data collection top
Oxford Diffraction KM-4-CCD Sapphire3 diffractometer | 974 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.069 |
Graphite monochromator | θmax = 25.0°, θmin = 2.8° |
ω scans | h = −11→11 |
4759 measured reflections | k = −4→9 |
1405 independent reflections | l = −13→13 |
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)] = 0.048 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.117 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.00 | w = 1/[σ2(Fo2) + (0.0626P)2] where P = (Fo2 + 2Fc2)/3 |
1405 reflections | (Δ/σ)max < 0.001 |
114 parameters | Δρmax = 0.23 e Å−3 |
0 restraints | Δρmin = −0.32 e Å−3 |
Crystal data top
C10H9NO | V = 807.8 (3) Å3 |
Mr = 159.18 | Z = 4 |
Monoclinic, P21/n | Mo Kα radiation |
a = 9.5665 (19) Å | µ = 0.09 mm−1 |
b = 7.6553 (15) Å | T = 110 K |
c = 11.031 (2) Å | 0.54 × 0.32 × 0.03 mm |
β = 90.77 (3)° | |
Data collection top
Oxford Diffraction KM-4-CCD Sapphire3 diffractometer | 974 reflections with I > 2σ(I) |
4759 measured reflections | Rint = 0.069 |
1405 independent reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.048 | 0 restraints |
wR(F2) = 0.117 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.00 | Δρmax = 0.23 e Å−3 |
1405 reflections | Δρmin = −0.32 e Å−3 |
114 parameters | |
Special details top
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 | 0.27957 (16) | 0.17171 (18) | 0.27207 (13) | 0.0328 (4) | |
C1 | 0.6625 (2) | 0.1790 (2) | 0.47915 (17) | 0.0212 (5) | |
C2 | 0.7988 (2) | 0.1203 (2) | 0.46763 (19) | 0.0284 (5) | |
H2 | 0.8672 | 0.1404 | 0.5293 | 0.034* | |
C3 | 0.8308 (2) | 0.0312 (2) | 0.3624 (2) | 0.0309 (5) | |
H3 | 0.9232 | −0.0110 | 0.3516 | 0.037* | |
C4 | 0.7306 (2) | 0.0020 (2) | 0.27215 (19) | 0.0288 (5) | |
H4 | 0.7553 | −0.0628 | 0.2022 | 0.035* | |
C5 | 0.5958 (2) | 0.0653 (2) | 0.28195 (18) | 0.0255 (5) | |
H5 | 0.5290 | 0.0481 | 0.2185 | 0.031* | |
C6 | 0.5597 (2) | 0.1557 (2) | 0.38817 (17) | 0.0201 (5) | |
C7 | 0.4321 (2) | 0.2334 (2) | 0.43329 (16) | 0.0215 (5) | |
C8 | 0.4657 (2) | 0.2961 (2) | 0.54712 (18) | 0.0228 (5) | |
H8 | 0.4018 | 0.3538 | 0.5988 | 0.027* | |
C9 | 0.2987 (2) | 0.2394 (2) | 0.37279 (17) | 0.0233 (5) | |
C10 | 0.1788 (2) | 0.3329 (3) | 0.43273 (19) | 0.0288 (5) | |
H10A | 0.1839 | 0.4580 | 0.4145 | 0.043* | |
H10B | 0.1850 | 0.3156 | 0.5207 | 0.043* | |
H10C | 0.0900 | 0.2858 | 0.4019 | 0.043* | |
N1 | 0.60021 (19) | 0.2651 (2) | 0.57561 (15) | 0.0244 (4) | |
H1 | 0.654 (2) | 0.288 (3) | 0.649 (2) | 0.036 (6)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O1 | 0.0364 (10) | 0.0411 (8) | 0.0206 (9) | 0.0021 (7) | −0.0071 (7) | −0.0027 (6) |
C1 | 0.0244 (12) | 0.0209 (9) | 0.0183 (11) | −0.0026 (8) | 0.0035 (8) | 0.0045 (7) |
C2 | 0.0273 (14) | 0.0311 (11) | 0.0266 (13) | −0.0050 (9) | −0.0024 (10) | 0.0076 (9) |
C3 | 0.0275 (13) | 0.0323 (11) | 0.0329 (13) | 0.0027 (9) | 0.0078 (10) | 0.0078 (9) |
C4 | 0.0347 (14) | 0.0271 (10) | 0.0249 (13) | 0.0016 (10) | 0.0115 (10) | 0.0020 (9) |
C5 | 0.0308 (14) | 0.0256 (10) | 0.0202 (11) | −0.0032 (9) | 0.0023 (9) | 0.0027 (8) |
C6 | 0.0239 (12) | 0.0198 (9) | 0.0167 (11) | −0.0027 (8) | 0.0026 (9) | 0.0036 (7) |
C7 | 0.0288 (13) | 0.0201 (9) | 0.0155 (10) | −0.0025 (8) | −0.0005 (8) | 0.0033 (8) |
C8 | 0.0256 (13) | 0.0227 (10) | 0.0200 (11) | −0.0004 (8) | 0.0002 (9) | 0.0029 (8) |
C9 | 0.0299 (14) | 0.0235 (10) | 0.0164 (11) | −0.0034 (9) | −0.0022 (9) | 0.0053 (8) |
C10 | 0.0280 (13) | 0.0314 (11) | 0.0271 (13) | 0.0007 (9) | −0.0004 (9) | 0.0041 (9) |
N1 | 0.0278 (12) | 0.0287 (9) | 0.0165 (9) | −0.0034 (8) | −0.0025 (8) | 0.0020 (7) |
Geometric parameters (Å, º) top
O1—C9 | 1.238 (2) | C5—H5 | 0.9500 |
C1—C2 | 1.387 (3) | C6—C7 | 1.453 (3) |
C1—N1 | 1.392 (3) | C7—C8 | 1.378 (3) |
C1—C6 | 1.407 (3) | C7—C9 | 1.433 (3) |
C2—C3 | 1.384 (3) | C8—N1 | 1.342 (3) |
C2—H2 | 0.9500 | C8—H8 | 0.9500 |
C3—C4 | 1.391 (3) | C9—C10 | 1.512 (3) |
C3—H3 | 0.9500 | C10—H10A | 0.9800 |
C4—C5 | 1.383 (3) | C10—H10B | 0.9800 |
C4—H4 | 0.9500 | C10—H10C | 0.9800 |
C5—C6 | 1.408 (3) | N1—H1 | 0.97 (2) |
| | | |
C2—C1—N1 | 129.50 (19) | C8—C7—C9 | 127.51 (19) |
C2—C1—C6 | 122.84 (19) | C8—C7—C6 | 105.53 (18) |
N1—C1—C6 | 107.66 (18) | C9—C7—C6 | 126.95 (17) |
C3—C2—C1 | 117.11 (19) | N1—C8—C7 | 111.31 (19) |
C3—C2—H2 | 121.4 | N1—C8—H8 | 124.3 |
C1—C2—H2 | 121.4 | C7—C8—H8 | 124.3 |
C2—C3—C4 | 121.4 (2) | O1—C9—C7 | 121.66 (19) |
C2—C3—H3 | 119.3 | O1—C9—C10 | 119.16 (18) |
C4—C3—H3 | 119.3 | C7—C9—C10 | 119.17 (17) |
C5—C4—C3 | 121.5 (2) | C9—C10—H10A | 109.5 |
C5—C4—H4 | 119.2 | C9—C10—H10B | 109.5 |
C3—C4—H4 | 119.2 | H10A—C10—H10B | 109.5 |
C4—C5—C6 | 118.43 (19) | C9—C10—H10C | 109.5 |
C4—C5—H5 | 120.8 | H10A—C10—H10C | 109.5 |
C6—C5—H5 | 120.8 | H10B—C10—H10C | 109.5 |
C5—C6—C1 | 118.68 (19) | C8—N1—C1 | 108.88 (17) |
C5—C6—C7 | 134.68 (19) | C8—N1—H1 | 131.1 (14) |
C1—C6—C7 | 106.62 (17) | C1—N1—H1 | 119.9 (14) |
| | | |
N1—C1—C2—C3 | 177.25 (18) | C1—C6—C7—C8 | −0.5 (2) |
C6—C1—C2—C3 | −1.8 (3) | C5—C6—C7—C9 | −1.2 (3) |
C1—C2—C3—C4 | 0.1 (3) | C1—C6—C7—C9 | −179.14 (17) |
C2—C3—C4—C5 | 1.9 (3) | C9—C7—C8—N1 | 178.82 (17) |
C3—C4—C5—C6 | −2.2 (3) | C6—C7—C8—N1 | 0.2 (2) |
C4—C5—C6—C1 | 0.5 (3) | C8—C7—C9—O1 | −176.80 (18) |
C4—C5—C6—C7 | −177.26 (18) | C6—C7—C9—O1 | 1.5 (3) |
C2—C1—C6—C5 | 1.5 (3) | C8—C7—C9—C10 | 4.1 (3) |
N1—C1—C6—C5 | −177.72 (16) | C6—C7—C9—C10 | −177.63 (17) |
C2—C1—C6—C7 | 179.86 (16) | C7—C8—N1—C1 | 0.2 (2) |
N1—C1—C6—C7 | 0.63 (19) | C2—C1—N1—C8 | −179.68 (19) |
C5—C6—C7—C8 | 177.5 (2) | C6—C1—N1—C8 | −0.5 (2) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O1i | 0.97 (2) | 1.83 (2) | 2.788 (2) | 171 (2) |
Symmetry code: (i) x+1/2, −y+1/2, z+1/2. |
Experimental details
Crystal data |
Chemical formula | C10H9NO |
Mr | 159.18 |
Crystal system, space group | Monoclinic, P21/n |
Temperature (K) | 110 |
a, b, c (Å) | 9.5665 (19), 7.6553 (15), 11.031 (2) |
β (°) | 90.77 (3) |
V (Å3) | 807.8 (3) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.09 |
Crystal size (mm) | 0.54 × 0.32 × 0.03 |
|
Data collection |
Diffractometer | Oxford Diffraction KM-4-CCD Sapphire3 diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 4759, 1405, 974 |
Rint | 0.069 |
(sin θ/λ)max (Å−1) | 0.595 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.048, 0.117, 1.00 |
No. of reflections | 1405 |
No. of parameters | 114 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.23, −0.32 |
Selected geometric parameters (Å, º) topO1—C9 | 1.238 (2) | C7—C8 | 1.378 (3) |
C1—N1 | 1.392 (3) | C8—N1 | 1.342 (3) |
| | | |
C9—C7—C6 | 126.95 (17) | O1—C9—C10 | 119.16 (18) |
O1—C9—C7 | 121.66 (19) | C7—C9—C10 | 119.17 (17) |
| | | |
C8—C7—C9—O1 | −176.80 (18) | C6—C7—C9—C10 | −177.63 (17) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O1i | 0.97 (2) | 1.83 (2) | 2.788 (2) | 171 (2) |
Symmetry code: (i) x+1/2, −y+1/2, z+1/2. |
The indole ring system is the structural element of many natural and/or synthetic organic compounds exhibiting biological and pharmacological activities, such as anti-allergic (Shigenaga et al., 1993), antimicrobial (Oh et al., 2006), antifungal and antibacterial properties (Quetin-Leclercq et al., 1995; Singh et al.., 2000; Ablordeppey et al., 2002), as well as anti-cancer properties (Dashwood et al., 1994; Andreani et al., 2001; Gul & Hamann, 2005; Gupta et al., 2007). The most well known indole compounds are the tryptophan-derived tryptamine alkaloids, i.e. the neurotransmitter serotonin (Simonenkov & Fedorov, 2002; Nichols & Nichols, 2008), the hormone melatonin (Cardinali et al., 2004; Karasek, 2004), the hallucinogens psilocybin (Passie et al., 2002) and N,N-dimethyltryptamine (DMT) (Strassman, 1996), 2-(5-methoxy-1H-indol-3-yl)-N,N-dimethylethanamine (5-MeO-DMT) (Gouzoulis-Mayfrank et al., 2005) or the ergolines such as lysergic acid diethylamide (LSD) (Appel et al., 2004). The indolic derivatives, including the plant hormone auxin (indolyl-3-acetic acid, IAA) (Chandrasekhar & Raghunathan, 1982; Nigović et al., 2000), the anti-inflammatory drug indomethacin (Chen et al., 2002; Cox & Manson, 2003) and the betablocker pindolol (Nair et al., 2004), should also be mentioned. Over the last few years, the synthesis of indole derivatives, especially the triptans, has been a subject of fundamental interest to organic and medicinal chemists. Triptans are selective serotonin (5-HT1B/D) agonists developed for the relief of migraine symptoms by targeting the disease pathology (Perry & Markham, 1998; de Vries et al., 1999; Ferrari et al., 2002; Pascual et al., 2007). So far, seven triptans have become available in the USA, viz. sumatriptan (Ravikumar et al., 2006), zolmitriptan (Ravikumar et al., 2007a), naratriptan (Massiou, 2001), rizatriptan (Ravikumar et al., 2007b), frovatriptan (Balbisi, 2006), eletriptan (Goadsby et al., 2000) and almotriptan (Ravikumar et al., 2008). Indole derivatives are also of great importance for the synthesis of pyrroloindoles and carbazoles via Diels–Adler cycloaddition or Suzuki–Miyaura reactions (Wenkert et al., 1988; Gribble, 2003; Pathak et al., 2006) or for nucleophilic addition reactions (Agbalyan et al., 1974; Ottoni et al., 1998; Pelkey et al., 1999).
3-Acetylindole, (I), and indole-3-carboxaldehyde have been the subject of a study by us of the generation mechanism of the IR spectra of hydrogen-bonded molecular crystals (Flakus & Hachuła, 2008; Hachuła et al., 2008). Measurement of the IR spectra of polycrystalline and monocrystalline samples of the indole derivatives and theoretical analysis of the results was mainly focused on spectroscopic effects corresponding to the intensity distribution, the influence of temperature, linear dichroism, and the isotopic substitution of deuterium in the above-mentioned molecules measured in the frequency range of the proton and the deuterium stretching vibration bands, νN–H and νN–D, respectively. These spectroscopic studies were preceded by an analysis of the X-ray crystal structures of the compounds. The crystal structure of indole-3-carboxaldehyde has been described previously (Ng, 2007). The molecules of this compound are linked into linear [helical in orginal article?] chains along the crystallographic c axis by intermolecular N—H···O hydrogen bonds. The structure of (I) has not yet been reported.
Compound (I) crystallizes with one molecule in the asymmetric unit (Fig. 1). The five- and six-membered rings are essentially planar, with r.m.s. deviations from the mean plane of 0.0178 Å. The largest deviations from the least-squares indole plane are observed for atoms C4 [-0.0304 (15) Å] and C6 [0.0225 (16) Å]. Atom N1 is out of the indole plane by -0.0185 (13) Å. The dihedral angle between the planes of the pyrrole and benzene rings is 2.05 (11)°. By comparison, this angle is 4.22° in indole-3-carboxaldehyde (Ng, 2007) and 0.29° in indole-3-carboxylic acid (Smith et al., 2003). The N1—C1 and N1—C8 bond lengths [1.392 (3) and 1.342 (3) Å] differ from the corresponding mean values of 1.372 (7) and 1.370 (12) Å, respectively, reported for the indole ring system by Allen et al. (1987). This significant asymmetry of the two endocyclic N—C bonds of the heterocyclic ring of (I) may be a result of the electron-withdrawing character of the acetyl group and the preferential conjugation of the C9═O1 bond with C7═C8. Moreover, the N1—C8 bond in (I) is shortened, whereas C7═C8 is elongated (Table 1). The C—C bond opposite to the heteroatom is considerably longer than the others. An almost identical shortening of N1—C8 and lengthening of the C7═C8 double bond are found in similar structures, e.g. indole-3-carboxaldehyde (Ng, 2007), 3-acetyl-1-methoxyindole (Acheson et al., 1980), indole-3-carboxylic acid (Smith et al., 2003) and indole-3-acetic acid (Karle et al., 1964), or in other indole derivatives possessing a 3-carbonyl group but which are unsubstituted at the ring N atom (Damak & Riche, 1977; Hu et al., 2005). This fact is consistent with electron delocalization from the N atom into the acetyl group. Thus, the substitution of an electronegative O atom on the pyrrole ring makes little difference to the π-donor ability of N (Morrin Acheson et al., 1980). The sum of the angles around the indole N atom is 359.88°, which indicates that the geometry around N is normal sp2 coordination, as expected for π-conjugation of the indole ring (Huang et al., 2004). The endocyclic C—C bond distances and the bond angles in (I) are in the normal ranges and are comparable with those of other indole derivatives (Allen et al., 1987).
The acetyl group is almost coplanar with the heterocyclic ring [torsion angles C8—C7—C9—O1 = -176.80 (18)° and C6—C7—C9—C10 = -177.63 (17)°]. The dihedral angle between the plane of the indole ring system and the plane of the acetyl group, O1—C9—C10, is 1.75 (17)°. The C═O bond distance and O—C—C bond angle are similar to those in indole-3-carboxaldehyde and 3-acetyl-1-methoxyindole [C9—O1 = 1.228 (2) Å and O1—C9—C7 = 124.71 (18)°, and C9—O1 = 1.224 (2) Å and O1—C9—C7 = 120.9°, respectively]. The geometry of the acetyl group is governed by the repulsive interaction between the O1 carbonyl group and the heterocyclic ring, leading to an enlargement of the O1—C9—C7 and C9—C7—C6 angles and a diminution of O1—C9—C10 and C7—C9—C10 (Table 1). A similar repulsive interaction between the lone pairs on atom O1 and the neighbouring atoms of the indole ring system is observed in indole-3-carboxaldehyde (Ng, 2007). The crystal structure similarity between indole-3-carboxaldehyde and (I) leads to the conclusion that the replacement of the H atom in the aldehyde group of indole-3-carboxaldehyde by a methyl group does not significantly influence the crystal lattice parametrs.
There are four molecules in the unit cell at the symmetry positions (x, y, z), (3/2 - x, 1/2 + y, 1/2 - z), (1 - x, 1 - y, 1 - z) and (x - 1/2, 1/2 - y, 1/2 + z). Each of these molecules is linked by a single N—H···O hydrogen bond to the other two, forming zigzag chains. Atom N1 of the pyrrole N—H group in the molecule at (x, y, z) acts as a hydrogen-bond donor via atom H1 to carbonyl atom O1 belonging to the molecule at (1/2 + x, 1/2 - y, 1/2 + z) (Fig. 2). The result of this interaction is the formation of hydrogen-bonded chain with a graph-set motif of C(6) (Etter et al., 1990; Bernstein et al., 1995) running along the [101] direction. In the crystal structure of (I), two such chains, related to one another by an inversion centre symmetry operation, pass through each unit cell (Fig. 3). The same graph-set motif of C(6) is observed in indole-3-carboxaldehyde (Ng, 2007) and methyl indole-3-carboxylate (Hu et al., 2005). Similar molecular packing can also be found in 3-acetoxyindole (Chakraborty et al., 1991), in which the molecules are connected through N—H···O hydrogen bonds to form C(7) chains. Judging from the bond distances, the N—H···O hydrogen bond between two 3-acetylindole molecules appears to be slightly stronger [N1—H1···O1 = 2.788 (2) Å] than that involving two indole-3-carboxaldehyde molecules [N1—H1···O1 = 2.826 (2) Å]. The details of the hydrogen-bonding interactions are shown in Figs. 2 and 3 and given in Table 2.
The polycrystalline spectrum of (I) is shown in Fig. 4. The values of the H—N and N···O distances, as well as the N—H···O angle (Table 2), characterize this bond as a medium-strength hydrogen bond (Desiraju & Steiner, 1999; Steiner, 2002). The strength of the hydrogen bond in (I) is supported by spectroscopic measurements. The νN—H proton stretching vibration band of (I) extends over a frequency range of 3300–2400 cm-1. The polycrystalline N—H band is shifted towards the lower frequencies by ca 280 cm-1 compared with the unperturbed value of 3400 cm-1. This shift in the N—H stretching frequency proves that this N—H··· O hydrogen bond is of medium strength. A familiar correlation between the hydrogen-bond energy and the frequency shift of the proton (or deuteron) stretching vibration band is used to justify this statement (Schuster et al., 1976; Schuster & Mikenda, 1999). The N—H···O bond length [N1···O1 = 2.788 (2) Å] in (I) appears to be slightly shorter than the those in other compounds, e.g. the N-methylamide derivatives [mean N···O distance 2.85 Å; Leiserowitz & Tuval, 1978]. Consequently, the stronger N—H···O hydrogen bonds correspond to a larger frequency shift.