Theoretical investigation of electronic structure and nuclear quadrupole interactions in cocaine free base

The nuclear quadrupole interaction (NQI) parameters for the ¹⁴N nucleus, namely, the quadrupole coupling constant (e²qQ) and asymmetry parameter (η) have been obtained through first-principles Hartree-Fock investigations on cocaine free base (C₁₇H₂₁NO₄). The calculated value -5.038 MHz of e²qQ is in very good agreement with the experimentally observed magnitude of 5.0229 for e²qQ, and the small value 0.067 for η is in keeping with the value of 0.0395 from nuclear quadrupole resonance measurements. These results provide strong support for the nature of the electron distribution in the cocaine molecule, including its anisotropy around the ¹⁴N nucleus, obtained by the first-principles Hartree-Fock method, in common with the conclusion from recent investigations on the energetic molecules RDX (1,3,5-trinitro-5-triazine, C₃H₆N₆O₆) and β-HMX (octahydro-1,3,5,7-terranitro-1,3,5,7-tetrazocine, C₄H₈N₈O₈). Additional support for this conclusion is provided by the fact that the positions of three hydrogen atoms in cocaine obtained from structural measurements were reported erroneously in the literature and were redetermined in the present work through total energy minimization by the Hartree-Fock procedure, the calculated results for e²qQ and η for ¹⁴N being based on these corrected H positions. Possible sources that could resolve the small but significant difference for η between theory and experiment are discussed. The calculated NQI parameters for ¹⁷O and ²H (deuteron) are presented for cocaine with the hope that they will be measured in the future to allow comparison with theory and thus provide a more complete picture of the electron distribution in this molecule.