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, rho(r), its curvature, partial der ivative(2)rho/partial derivative r(ij), the dipole moment, mu, and the elec trostatic potential, Phi(r), in L-asparagine monohydrate by using high-reso lution single-crystal X-ray crystallography and quantum chemistry. In addit ion, we have compared electric field gradient, del E, results obtained from crystallography and quantum chemistry with those obtained from single-crys tal N-14 nuclear magnetic resonance spectroscopy. A multipole model of the X-ray rho(r) is compared to Hartree-Fock and density functional theory pred ictions, using two different large basis sets. The quality of the calculate d charge densities is evaluated from a simultaneous comparison of eight Hes sian-of-rho(r) tensors at bond critical points between non-hydrogen atoms. These tensors are expressed in an icosahedral representation, which include s information on both tensor magnitude and orientation. The best theory-ver sus-experiment correlation is found at the B3LYP/6-311++G(2d,2p) level, whi ch yields a slope of 1.09 and an R-2 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 f ound to correctly reproduce the experimental molecular electrostatic potent ial, Phi(r). The intermolecular hydrogen bond rho(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 th e del E tensors, the icosahedral representation is again used. There is fou nd to be moderate accord between theory and experiment when using results o btained from diffraction data, but much better accord when using results ob tained from NMR data (slope = 1.14, R-2 = 0.94, for the 12 icosahedral tens or elements for N1 and N2). Overall, these results strongly support the ide a that both HF and DFT methods give excellent representations of the electr ostatic properties rho(r), partial derivative 2 rho/partial derivative r(ij ), mu, Phi(r), and del E, for crystalline L-asparagine monohydrate, encoura ging their future use in situations where experimental results are lacking, such as in peptides and in enzyme active sites.