Experimental, Hartree-Fock, and density functional theory investigations of the charge density, dipole moment, electrostatic potential, and electric field gradients in L-asparagine monohydrate

We have investigated the charge density, ρ(r), its curvature, δ²ρ/δr(ij), the dipole moment, μ, and the electrostatic potential, Φ(r), in L-asparagine monohydrate by using high-resolution single-crystal X- ray crystallography and quantum chemistry. In addition, we have compared electric field gradient, ΔE, results obtained from crystallography and quantum chemistry with those obtained from single-crystal ¹⁴N nuclear magnetic resonance spectroscopy. A multipole model of the X-ray ρ(r) is compared to Hartree-Fock and density functional theory predictions, using two different large basis sets. The quality of the calculated charge densities is evaluated from a simultaneous comparison of eight Hessian-of-ρ(r) tensors at bond critical points between non-hydrogen atoms. These tensors are expressed in an icosahedral representation, which includes information on both tensor magnitude and orientation. The best theory-versus-experiment correlation is found at the B3LYP/6-311++G(2d,2p) level, which yields a slope of 1.09 and an R² value of 0.96. Both DFT and HF results give molecular dipole moments in good accord with the value extracted from the X-ray diffraction data, 14.3(3) D, and both sets of calculations are found to correctly reproduce the experimental molecular electrostatic potential, Φ(r). The intermolecular hydrogen bond ρ(r) is also subjected to a detailed theoretical and experimental topological analysis, and again good agreement is found between theory and experiment. For the comparison of the ΔE tensors, the icosahedral representation is again used. There is found to be moderate accord between theory and experiment when using results obtained from diffraction data, but much better accord when using results obtained from NMR data (slope = 1.14, R² = 0.94, for the 12 icosahedral tensor elements for N1 and N2). Overall, these results strongly support the idea that both HF and DFT methods give excellent representations of the electrostatic properties ρ(r), δ²ρ/δr(ij), μ, Φ(r), and ΔE, for crystalline L-asparagine monohydrate, encouraging their future use in situations where experimental results are lacking, such as in peptides and in enzyme active sites.