Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270111038005/bi3021sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270111038005/bi3021Isup2.hkl | |
Chemical Markup Language (CML) file https://doi.org/10.1107/S0108270111038005/bi3021Isup3.cml |
CCDC reference: 851749
For related literature, see: Acheson et al. (1980); Allen et al. (1987); Bernstein et al. (1995); Bocelli & Grenier-Loustalot (1982); Boys & Bernardi (1970); Crumeyrolle-Arias, Tournaire, Cane, Launay, Barritault & Medvedev (2004); Desiraju & Steiner (1999); England et al. (2007); Etter et al. (1990); Fatima et al. (2007); Flakus & Hachuła (2012); Flakus et al. (2011); Foster & Weinhold (1980); Frisch (2009); Galliford & Scheidt (2007); Glendening et al. (1993); Hachuła et al. (2008, 2012); Jeffrey (1997); Karle et al. (1964); Lipkowski et al. (1995); Midoh et al. (2010); Motherwell et al. (1999); Nerskov-Lauritsen, Burgi, Hofmann & Schmidt (1985); Ng (2007); Parr & Yang (1989); Peddibhotla (2009); Pedireddi et al. (1994); Schuster & Mikenda (1999); Schuster et al. (1976); Smith et al. (2003); Steiner (2002); Wei et al. (2004); Zukerman-Schpector, Pinto, da DaC, Silva & Barcellos (1993).
6-Chloro-2-oxindole (Sigma–Aldrich, 97% pure) was dissolved in acetone and left to evaporate under ambient conditions. Plate-shaped crystals of (I) appeared after a few days. Differential scanning calorimetry of the bulk showed only one sharp endothermic peak at 466.43 K, which corresponds to the melting process. Spectroscopic analysis: IR (KBr; ν, cm-1): 1700 (s) (calculated 1800) νC═O; 1622–1002 (m) (calculated 1670–1092) νC═C and νC—C; 1333 (m) (calculated 1332) νC—N; 702 (m) (calculated 708) νC—Cl.
H atoms bound to C atoms were placed in idealized positions and refined as riding, with C—H = 0.95 Å and Uiso(H) = 1.2Ueq(C). Atom H1 of the NH group was located in a difference Fourier map and refined freely with an isotropic displacement parameter.
DFT calculations were carried out at the B3LYP/6-31 G(d,p) level using GAUSSIAN09 (Parr & Yang, 1989; Frisch et al., 2009), starting from the X-ray geometry. Natural bond orbital calculations (Foster & Weinhold, 1980) were carried out using the program NBO (Glendening et al., 1993) included in the GAUSSIAN09 package.
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., 2008); software used to prepare material for publication: publCIF (Westrip, 2010).
C8H6ClNO | F(000) = 344 |
Mr = 167.59 | Dx = 1.510 Mg m−3 |
Monoclinic, P21/c | Melting point: 466 K |
Hall symbol: -P 2ybc | Mo Kα radiation, λ = 0.71073 Å |
a = 4.2475 (1) Å | Cell parameters from 6067 reflections |
b = 12.3838 (2) Å | θ = 3.3–34.4° |
c = 14.0372 (2) Å | µ = 0.45 mm−1 |
β = 92.898 (2)° | T = 100 K |
V = 737.42 (2) Å3 | Plate, colourless |
Z = 4 | 0.60 × 0.46 × 0.16 mm |
Oxford Sapphire3 CCD area-detector diffractometer | 1283 independent reflections |
Radiation source: fine-focus sealed tube | 1203 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.012 |
Detector resolution: 16.0328 pixels mm-1 | θmax = 25.1°, θmin = 3.3° |
ω scans | h = −5→2 |
Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2006) | k = −14→14 |
Tmin = 0.775, Tmax = 0.932 | l = −16→16 |
4675 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)] = 0.025 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.068 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.06 | w = 1/[σ2(Fo2) + (0.038P)2 + 0.345P] where P = (Fo2 + 2Fc2)/3 |
1283 reflections | (Δ/σ)max = 0.001 |
104 parameters | Δρmax = 0.21 e Å−3 |
0 restraints | Δρmin = −0.25 e Å−3 |
C8H6ClNO | V = 737.42 (2) Å3 |
Mr = 167.59 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 4.2475 (1) Å | µ = 0.45 mm−1 |
b = 12.3838 (2) Å | T = 100 K |
c = 14.0372 (2) Å | 0.60 × 0.46 × 0.16 mm |
β = 92.898 (2)° |
Oxford Sapphire3 CCD area-detector diffractometer | 1283 independent reflections |
Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2006) | 1203 reflections with I > 2σ(I) |
Tmin = 0.775, Tmax = 0.932 | Rint = 0.012 |
4675 measured reflections |
R[F2 > 2σ(F2)] = 0.025 | 0 restraints |
wR(F2) = 0.068 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.06 | Δρmax = 0.21 e Å−3 |
1283 reflections | Δρmin = −0.25 e Å−3 |
104 parameters |
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. Least-squares planes (x,y,z in crystal coordinates) and deviations from them (* indicates atom used to define plane) 3.3241 (0.0007) x + 4.4223 (0.0044) y + 6.5930 (0.0018) z = 5.6248 (0.0013) * -0.0097 (0.0007) Cl1 * 0.0006 (0.0010) N1 * -0.0264 (0.0009) O1 * 0.0112 (0.0011) C1 * 0.0225 (0.0010) C2 * -0.0025 (0.0011) C3 * -0.0181 (0.0011) C4 * -0.0041 (0.0010) C5 * 0.0068 (0.0011) C6 * 0.0247 (0.0011) C7 * -0.0050 (0.0011) C8 Rms deviation of fitted atoms = 0.0149 3.3538 (0.0014) x + 4.3195 (0.0062) y + 6.5174 (0.0067) z = 5.5896 (0.0018) * -0.0071 (0.0009) C1 * 0.0074 (0.0009) C2 * -0.0015 (0.0009) C3 * -0.0048 (0.0009) C4 * 0.0055 (0.0009) C5 * 0.0005 (0.0009) C6 Rms deviation of fitted atoms = 0.0052 3.3075 (0.0017) x + 4.5066 (0.0073) y + 6.6126 (0.0080) z = 5.6636 (0.0031) Angle to previous plane (with approximate esd) = 1.13 ( 0.09 ) * 0.0051 (0.0008) C1 * -0.0094 (0.0008) C6 * 0.0099 (0.0008) C7 * -0.0077 (0.0008) C8 * 0.0022 (0.0008) N1 Rms deviation of fitted atoms = 0.0075 |
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 > 2sigma(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 | ||
Cl1 | 0.86752 (9) | 0.45746 (3) | 0.10744 (2) | 0.02315 (14) | |
O1 | 0.0776 (2) | 0.37168 (8) | 0.56074 (7) | 0.0206 (2) | |
N1 | 0.2761 (3) | 0.43530 (10) | 0.42204 (8) | 0.0152 (3) | |
H1 | 0.185 (4) | 0.4949 (16) | 0.4220 (12) | 0.025 (4)* | |
C1 | 0.4698 (3) | 0.39741 (11) | 0.35143 (9) | 0.0143 (3) | |
C2 | 0.5605 (3) | 0.45152 (11) | 0.27111 (9) | 0.0158 (3) | |
H2 | 0.4979 | 0.5238 | 0.2577 | 0.019* | |
C3 | 0.7493 (3) | 0.39346 (11) | 0.21108 (9) | 0.0168 (3) | |
C4 | 0.8478 (3) | 0.28858 (11) | 0.22936 (10) | 0.0185 (3) | |
H4 | 0.9761 | 0.2519 | 0.1863 | 0.022* | |
C5 | 0.7555 (3) | 0.23749 (11) | 0.31230 (10) | 0.0177 (3) | |
H5 | 0.8231 | 0.1659 | 0.3266 | 0.021* | |
C6 | 0.5653 (3) | 0.29183 (11) | 0.37342 (9) | 0.0153 (3) | |
C7 | 0.4280 (3) | 0.26059 (11) | 0.46629 (10) | 0.0170 (3) | |
H7A | 0.5960 | 0.2458 | 0.5160 | 0.020* | |
H7B | 0.2910 | 0.1962 | 0.4584 | 0.020* | |
C8 | 0.2381 (3) | 0.36009 (11) | 0.49083 (9) | 0.0161 (3) |
U11 | U22 | U33 | U12 | U13 | U23 | |
Cl1 | 0.0314 (2) | 0.0229 (2) | 0.0156 (2) | −0.00561 (14) | 0.00590 (15) | −0.00066 (13) |
O1 | 0.0260 (5) | 0.0181 (5) | 0.0182 (5) | −0.0005 (4) | 0.0065 (4) | 0.0020 (4) |
N1 | 0.0174 (6) | 0.0116 (6) | 0.0168 (6) | 0.0011 (5) | 0.0021 (5) | 0.0006 (5) |
C1 | 0.0129 (6) | 0.0143 (7) | 0.0153 (7) | −0.0019 (5) | −0.0019 (5) | −0.0025 (5) |
C2 | 0.0171 (7) | 0.0138 (7) | 0.0163 (7) | −0.0017 (5) | −0.0019 (5) | 0.0000 (5) |
C3 | 0.0175 (7) | 0.0196 (7) | 0.0131 (6) | −0.0053 (5) | −0.0002 (5) | −0.0015 (5) |
C4 | 0.0179 (7) | 0.0188 (7) | 0.0189 (7) | −0.0016 (5) | 0.0005 (5) | −0.0064 (5) |
C5 | 0.0181 (7) | 0.0132 (7) | 0.0213 (7) | 0.0004 (5) | −0.0030 (5) | −0.0032 (5) |
C6 | 0.0142 (6) | 0.0152 (7) | 0.0159 (6) | −0.0032 (5) | −0.0033 (5) | −0.0003 (5) |
C7 | 0.0190 (7) | 0.0139 (7) | 0.0179 (7) | −0.0009 (5) | −0.0008 (5) | 0.0022 (5) |
C8 | 0.0166 (6) | 0.0152 (7) | 0.0162 (7) | −0.0037 (5) | −0.0014 (5) | −0.0004 (5) |
Cl1—C3 | 1.7523 (14) | C3—C4 | 1.385 (2) |
Cl1—Cl1i | 3.4378 (6) | C4—C5 | 1.398 (2) |
O1—C8 | 1.2312 (17) | C4—H4 | 0.950 |
N1—C8 | 1.3572 (18) | C5—C6 | 1.3827 (19) |
N1—C1 | 1.4006 (17) | C5—H5 | 0.950 |
N1—H1 | 0.833 (19) | C6—C7 | 1.5053 (18) |
C1—C2 | 1.3829 (19) | C7—C8 | 1.5219 (19) |
C1—C6 | 1.3989 (19) | C7—H7A | 0.990 |
C2—C3 | 1.3921 (19) | C7—H7B | 0.990 |
C2—H2 | 0.950 | ||
C3—Cl1—Cl1i | 170.87 (5) | C6—C5—C4 | 119.58 (13) |
C8—N1—C1 | 111.65 (12) | C6—C5—H5 | 120.2 |
C8—N1—H1 | 122.4 (12) | C4—C5—H5 | 120.2 |
C1—N1—H1 | 125.8 (12) | C5—C6—C1 | 119.34 (12) |
C2—C1—C6 | 122.89 (12) | C5—C6—C7 | 132.56 (12) |
C2—C1—N1 | 127.77 (12) | C1—C6—C7 | 108.10 (11) |
C6—C1—N1 | 109.34 (12) | C6—C7—C8 | 102.77 (11) |
C1—C2—C3 | 115.85 (12) | C6—C7—H7A | 111.2 |
C1—C2—H2 | 122.1 | C8—C7—H7A | 111.2 |
C3—C2—H2 | 122.1 | C6—C7—H7B | 111.2 |
C4—C3—C2 | 123.38 (12) | C8—C7—H7B | 111.2 |
C4—C3—Cl1 | 118.78 (10) | H7A—C7—H7B | 109.1 |
C2—C3—Cl1 | 117.84 (11) | O1—C8—N1 | 125.29 (13) |
C3—C4—C5 | 118.94 (12) | O1—C8—C7 | 126.59 (12) |
C3—C4—H4 | 120.5 | N1—C8—C7 | 108.12 (11) |
C5—C4—H4 | 120.5 | ||
C8—N1—C1—C2 | −179.82 (13) | C2—C1—C6—C5 | −0.9 (2) |
C8—N1—C1—C6 | 0.27 (15) | N1—C1—C6—C5 | 179.06 (11) |
C6—C1—C2—C3 | 1.46 (19) | C2—C1—C6—C7 | 178.78 (12) |
N1—C1—C2—C3 | −178.43 (12) | N1—C1—C6—C7 | −1.30 (14) |
C1—C2—C3—C4 | −0.9 (2) | C5—C6—C7—C8 | −178.71 (14) |
C1—C2—C3—Cl1 | 179.26 (9) | C1—C6—C7—C8 | 1.71 (14) |
C2—C3—C4—C5 | −0.2 (2) | C1—N1—C8—O1 | −179.32 (12) |
Cl1—C3—C4—C5 | 179.61 (10) | C1—N1—C8—C7 | 0.87 (15) |
C3—C4—C5—C6 | 0.86 (19) | C6—C7—C8—O1 | 178.64 (13) |
C4—C5—C6—C1 | −0.36 (19) | C6—C7—C8—N1 | −1.56 (14) |
C4—C5—C6—C7 | −179.90 (13) |
Symmetry code: (i) −x+2, −y+1, −z. |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O1ii | 0.833 (19) | 2.01 (2) | 2.8399 (16) | 170.4 (17) |
C4—H4···O1iii | 0.95 | 2.39 | 3.2753 (16) | 155 |
Symmetry codes: (ii) −x, −y+1, −z+1; (iii) x+1, −y+1/2, z−1/2. |
Experimental details
Crystal data | |
Chemical formula | C8H6ClNO |
Mr | 167.59 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 100 |
a, b, c (Å) | 4.2475 (1), 12.3838 (2), 14.0372 (2) |
β (°) | 92.898 (2) |
V (Å3) | 737.42 (2) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.45 |
Crystal size (mm) | 0.60 × 0.46 × 0.16 |
Data collection | |
Diffractometer | Oxford Sapphire3 CCD area-detector diffractometer |
Absorption correction | Multi-scan (CrysAlis RED; Oxford Diffraction, 2006) |
Tmin, Tmax | 0.775, 0.932 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 4675, 1283, 1203 |
Rint | 0.012 |
(sin θ/λ)max (Å−1) | 0.596 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.025, 0.068, 1.06 |
No. of reflections | 1283 |
No. of parameters | 104 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.21, −0.25 |
Computer programs: CrysAlis CCD (Oxford Diffraction, 2006), CrysAlis RED (Oxford Diffraction, 2006), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), ORTEP-3 (Farrugia, 1997) and Mercury (Macrae et al., 2008), publCIF (Westrip, 2010).
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O1i | 0.833 (19) | 2.01 (2) | 2.8399 (16) | 170.4 (17) |
C4—H4···O1ii | 0.95 | 2.39 | 3.2753 (16) | 155.0 |
Symmetry codes: (i) −x, −y+1, −z+1; (ii) x+1, −y+1/2, z−1/2. |
WBO (A) | WBO (B) | WBO (A) | WBO (B) | ||
Cl1—C3 | 1.0522 | 1.0520 | C2—C3 | 1.3850 | 1.3888 |
O1—C8 | 1.7455 | 1.6204 | C3—C4 | 1.4179 | 1.4148 |
N1—C8 | 1.1050 | 1.1949 | C4—C5 | 1.3978 | 1.4011 |
N1—C1 | 1.0693 | 1.0641 | C5—C6 | 1.4452 | 1.4421 |
C1—C2 | 1.3855 | 1.3845 | C6—C7 | 1.0174 | 1.0170 |
C1—C6 | 1.3052 | 1.3106 | C7—C8 | 0.9667 | 0.9777 |
X-ray | DFT (A) | DFT (B) | |
Cl1—C3 | 1.7523 (14) | 1.7597 | 1.7599 |
O1—C8 | 1.2312 (17) | 1.2129 | 1.2303 |
N1—C8 | 1.3572 (18) | 1.3904 | 1.3684 |
N1—C1 | 1.4006 (17) | 1.3975 | 1.4002 |
C1—C2 | 1.3829 (19) | 1.3896 | 1.3896 |
C1—C6 | 1.3989 (19) | 1.4075 | 1.4065 |
C2—C3 | 1.3921 (19) | 1.3991 | 1.3986 |
C3—C4 | 1.385 (2) | 1.3933 | 1.3942 |
C4—C5 | 1.398 (2) | 1.4024 | 1.4020 |
C5—C6 | 1.3827 (19) | 1.3854 | 1.3856 |
C6—C7 | 1.5053 (18) | 1.5072 | 1.5070 |
C7—C8 | 1.5219 (19) | 1.5394 | 1.5341 |
C8—N1—C1 | 111.65 (12) | 112.638 | 111.793 |
C2—C1—N1 | 127.77 (12) | 128.503 | 128.007 |
C6—C1—N1 | 109.34 (12) | 109.208 | 109.632 |
C2—C3—Cl1 | 117.84 (11) | 118.361 | 118.436 |
C4—C3—Cl1 | 118.78 (10) | 119.154 | 119.034 |
C1—C6—C7 | 108.10 (11) | 108.314 | 107.932 |
C5—C6—C7 | 132.56 (12) | 132.242 | 132.494 |
C6—C7—C8 | 102.77 (11) | 103.711 | 103.079 |
O1—C8—N1 | 125.29 (13) | 125.579 | 126.044 |
O1—C8—C7 | 126.59 (12) | 128.293 | 126.392 |
N1—C8—C7 | 108.12 (11) | 106.128 | 107.564 |
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The structure determination of 6-chloro-2-oxindole, (I), is part of a series of structure determinations of indole derivatives (Hachuła et al., 2008, 2012). There is considerable chemical interest in the development of oxindole derivatives because the heterocyclic ring of oxindole (indolin-2-one) is the basic nucleus of a number of alkaloids (e.g. vincatine, mitraphylline, herbaline, crassanine and voachalotin) and of many natural and/or synthetic compounds exhibiting biological and pharmacological activity (Crumeyrolle-Arias et al., 2004; England et al., 2007; Fatima et al., 2007; Galliford & Scheidt, 2007; Peddibhotla, 2009; Midoh et al., 2010). Our interest in the compound comes from investigations of the IR spectra and hydrogen bonding in oxindole (Lipkowski et al., 1995; Hachuła et al., 2012) and in indole derivatives such as indole-3-carboxaldehyde and 3-acetylindole (Flakus et al., 2011; Flakus & Hachuła, 2012).
The molecule of (I) (Fig. 1) is essentially planar (r.m.s. deviation 0.0149 Å for all non-H atoms). The dihedral angle between the planes defined by the constituent pyrrolidine and benzene rings is 1.13 (9)°. 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 [1.4006 (17) and 1.3572 (18) Å, respectively] differ from the corresponding mean values of 1.419 and 1.331 Å reported for the γ-lactams (Allen et al., 1987). Comparable asymmetric bonding patterns are found in similar structures [e.g. 3-methylindolin-2-one (Lipkowski et al., 1995), 3,3-dimethylindolin-2-one (Lipkowski et al., 1995), 3,3-dichloroindolin-2-one (Zukerman-Schpector et al., 1993), 2,3-indolinedione (Bocelli & Grenier-Loustalot, 1982) and 5-hydroxyindolin-2-one (Wei et al., 2004)] and in other indole derivatives [e.g. indole-3-carboxaldehyde (Ng, 2007), 3-acetylindole (Hachuła et al., 2008), 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)]. This is consistent with electron delocalization from the N atom towards the carbonyl group. The C8═ O1 bond length [1.2312 (17) Å] is comparable with the mean value of 1.232 Å reported for the γ-lactams (Nerskov-Lauritsen et al., 1985). Natural bond orbital (NBO) analysis (Foster & Weinhold, 1980) of the electronic structure [Table 1; showing calculations for a single molecule (A) and for a hydrogen-bonded molecular pair (B)] confirms that the electrons of the lone pair on atom N1 contribute to the electronic density of the N1—C8 bond, lending it a partial double-bond character.
In the structure of (I), the molecules form dimers across inversion centres through two N—H···O hydrogen bonds (Fig. 2 and Table 2), forming an R22(8) graph-set motif (Etter et al., 1990; Bernstein et al., 1995; Motherwell et al., 1999). According to the hydrogen-bonding classification provided by Steiner (2002) and Desiraju & Steiner (1999), these are medium-strength electrostatic interactions. Neighbouring dimers interact through C4—H4···O1ii contacts [symmetry code: (ii) x + 1, -y + 1/2, z - 1/2], Cl···Cl interactions [C3—Cl1···Cl1i = 3.4378 (6) Å and 170.87 (5)°; symmetry code: (iii) -x + 2, -y + 1, -z] and stacking interactions [interplanar separation of 3.33 (1) Å]. According to the classification of Pedireddi et al. (1994), the Cl···Cl contacts are type I in nature, having equal C—Cl···Cl angles.
The experimental values of the bond lengths and angles are in good agreement with those obtained from density functional theory (DFT) calculations (Table 3). The DFT calculations were obtained for both a single molecule (A) and a hydrogen-bonded dimer (B), comprising two molecules of (I) linked through N1—H1···O1 hydrogen bonds. The largest differences between the calculated and experimental values for a single molecule are 0.0332 Å for the bond lengths and 2.0° for the bond angles, while the corresponding values for the hydrogen-bonded dimer are 0.0122 Å and 0.75°. The better agreement in the latter case could be considered as an indication that the intermolecular interactions slightly influence the molecular geometry of (I). The total energy for the geometry-optimized single molecule is -3.918 × 10 -15 J and the calculated dipole moment is 1.1904 D. The dipole vector lies in the molecular plane and is oriented approximately along the axis passing through atoms C1 and C4 (Fig. 1). In the structure, the molecular dipoles are arranged antiparallel to each other. The calculated total energy of the two intermolecular hydrogen bonds, corrected for basis set superposition error (BSSE) using a standard procedure (Boys & Bernardi, 1970), is -81.568 kJ mol-1. The calculated hydrogen-bond geometry [N—H = 1.031 Å, H···O = 1.823 Å, N···O = 2.847 Å and N—H···O = 171.98°] also corresponds well with the experimental data.
In the measured IR spectrum of (I), an intense and broad band in the region 3400–2400 cm-1 can be assigned to the νN—H stretching mode of the N—H group involved in hydrogen bonding. In the IR spectrum derived from the DFT calculations, this band is observed at 3295 cm-1. The shift in the N—H stretch to lower frequency (red shift) compared with the expected non-hydrogen-bonded value of 3451 cm-1 is associated with hydrogen bonding and a lengthening of the N—H bond (Schuster et al., 1976; Schuster & Mikenda, 1999; Jeffrey, 1997). The extent of the shift, Δνs, is related to important chemical and physical properties, such as hydrogen-bond energy and interatomic distances. For medium-strength hydrogen bonds, the X—H···A distance ranges from 2.5 to 3.2 Å [X—H ca 1.5–2.2 Å and X—H···A 130–180°], Δνs is expected to be 10–25% and the hydrogen-bond energy lies in the range 4–15 kcal mol-1 (ca 15–60 kJ mol-1; Jeffrey, 1997). Thus, the N—H···O hydrogen bonds in (I) are confirmed to be medium-strength. The geometric parameters of (I) are comparable with those of the N—H···O bond occurring in oxindole form II [N···O = 2.833 (2) and 2.857 (2) Å; Hachuła et al., 2012].