The unusual solid state structure of heroin hydrochloride monohydrate and its selective detection using NQR spectroscopy

Abstract

Heroin hydrochloride monohydrate, the normal crystalline form of this material, has been studied by X-ray crystallography, nuclear quadrupole resonance spectroscopy, and theoretical calculations. The X-ray data at 120 K could be refined in either of the two space groups P4₁ or P4₁2₁2 with the latter higher symmetry form being the more likely, but for the NQR spectra at 4.2 K the doublet structure suggests that a slight symmetry breaking has ocurrred, which lowers the symmetry to P4₁. The crystals have the unusual unit cell dimensions of a = b = 7.5335(2) Å, c = 71.976(4) Å, with Z = 8 (and Z′ = 2 in P4₁); the molecules are arranged on a four-fold helix of pitch 72.0 Å with its axis parallel to the c axis, requiring a total of eight molecules in order to complete one turn. As in the free base the nitrogen ring system of the N-protonated heroin molecule has a chair conformation; in P4₁ the N–H hydrogen is bonded to a Cl⁻ with N–H⋯Cl distances of 3.056(15) and 3.095(14) Å and each Cl⁻ ion is also hydrogen bonded to two water molecules. ¹⁴N and ³⁵Cl quadrupole resonance frequencies have been observed in both the pure material and material from a drugs seizure by two double resonance techniques; in cross-relaxation spectra, a broad ^35,37Cl resonance is found near 1.95 MHz at room temperature and an analysis of the line shape predicts an average quadrupole coupling constant for the two ions of 3.817 MHz and an asymmetry parameter of 0.4. In double resonance by level crossing experiments, ¹⁴N doublets are observed near 0.965 and 1.035 MHz at 4.2 K, predicting a mean quadrupole coupling constant of 1.333 MHz and an asymmetry parameter of 0.158. A doublet ³⁵Cl signal is also observed at 1.965 MHz. The doublet patterns observed are consistent with a P4₁ space group at 4.2 K. The similarity of the NQR data from the pure sample and the seizure confirms that the illicit sample is of the helical monohydrate, suggesting that these methods can be used to detect the narcotic in real situations. Theoretical calculations with Gaussian at the HF/6-31+G* level for two molecules at the configurations they adopt in the solid state are in reasonable agreement with these conclusions for ¹⁴N and one of the ³⁵Cl ions.