Desipramine hydrochloride: a non-merohedrally twinned structure

The title compound, C18H23N2 +·Cl−, is a non-merohedrally twinned salt [domains 0.9288 (3) and 0.0712 (3)] which crystallizes with four independent cation–anion pairs in the asymmetric unit. The seven-membered ring in each of the cations adopts a boat conformation, thus creating a butterfly effect within the ring system. The average value of the dihedral angle between the two aromatic rings in the four cations is 57.1 (1)°. The crystal packing is stabilized only slightly by a collection of intermediate N—H⋯Cl hydrogen-bonding interactions, which produce a weak, but cooperative, infinite, one-dimensional, intermolecular hydrogen-bond network along the a axis. A MOPAC PM3 computational calculation gives support to these observations.

QNMHA thanks the University of Mysore for use of its research facilities. RJB acknowledges the NSF MRI program (grant No. CHE-0619278) for funds to purchase an X-ray diffractometer.
Desipramine hydrochloride works by increasing the amount of serotonin and norepinephrine in the brain so that the feeling of depression is prevented or relieved. By a separate mechanism, desipramine hydrochloride may also reduce pain related to peripheral neuropathy. The effect of desipramine hydrochloride on peripheral sympathetic nerve activity is reported (Cohen et al., 1990). A sensitive spectrophotometric method has been developed for the determination of desipramine hydrochloride (Nagaraja et al., 2000). Desipramine hydrochloride is proposed as a new reagent for detection of microamounts of blood in urine (Ahmed et al., 2002).
The title compound, (I), is a non-merohedrally twinned salt and crystallizes with four independent cation-anion pairs (A,B,C,D) in the asymmetric unit cell. Cation-anion pair A is shown in Fig. 1. These cation-anion pairs form two sets of a pair of enantiomers connected by a salt linkage between the proton that has been transfered from the hydrogen chloride to the desipramine (dp) group resulting in the protonation of the secondary-amine N atom of the side chain yielding an -NH 2 + -grouping and a chloride anion (Fig. 2). The dihedral angle between the C1-C6 and C9-C14 ring planes in the cation is 57.5 (5)° (A), 57.8 (3)° (B), 55.8 (3)° (C) and 57.1 (9)° (D), respectively. The average value, 57.1 (1)°, is significantly higher than that observed in the related structures of desipraminum picrate, 52.79 (6)°,  and desipraminum picrate monohydrate, 53.92 (8)°, (Harrison et al., 2007).  (Table   1) which produces a weak, but cooperative, infinite, one-dimensional, intermolecular hydrogen bond network along the a axis of the unit cell (Fig. 3).
From the results of a MOPAC PM3 computational calculation (Schmidt & Polik, 2007), the dihedral angle between the C1-C6 and C9-C14 ring planes in the cation is found to be 57.7 (3)°, just slightly higher than the 57.1 (1)° average value of the four molecules in the asymmetric unit. The least squares plane of the C1/C14/C15/N1 group makes dihedral angles of 40.09° with the C1-C6 ring plane and 55.60° with the C9-C14 ring plane, which is slightly higher than the C1-C6 ring plane average value in the crystal (39.3 (1)°) and slightly lower than the C9-C14 ring plane average value (56.9 (7)°) in the crystal. From these observations it is clear that the collective effect of the intermediate hydrogen bond interactions in (I) only slightly influences crystal packing stability, in sharp contrast to the traditional hydrogen bonding effects observed in desipraminium picrate  and desipraminium picrate monohydrate (Harrison et al., 2007), respectively, which resulted in much greater deviations in these values.

Experimental
The title compound was obtained as a gift sample from R.L. Fine Chem, Bangalore, India. The compound was used without further purification. X-ray quality crystals (m.p. 458-461 K) were obtained by slow evaporation from an aqueous solution.

Refinement
The crystal is a non-merohedral twin with domains of 0.9288 (3) and 0.0712 (3). The structure was solved using the nonoverlapping reflections and refined using all data in a SHELXTL HKLF5 format. In this format reflections cannot be merged hence the data appears to be more than 100% complete.
All of the H atoms were placed in their calculated positions and then refined using the riding model with N-H = 0.92, C-H = 0.95-0.99 Å, and with U iso (H) = 1.18-1.51U eq (C,N).  .Clshowing one of two sets of a pair of enantiomers connected by a salt linkage between the proton that has been transfered from the hydrogen chloride to the desipramine (dp) group resulting in the protonation of the secondaryamine N atom of the side chain yielding an -NH 2 + -grouping and a chloride anion.

Special details
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.