The many-body expansion approach to ab initio calculation of electric field gradients in molecular crystals.

Accurate calculation of electric field gradients (EFGs) in molecular crystals, despite big advances in ab initio techniques, is still a challenge. Here, we present a new approach to calculate the EFGs in molecular crystals by employing the many-body expansion (MBE) technique with electrostatic embedding. This allows for (i) a reduction in the computational cost or an alternative increase in the level of theory (we use the MP2/6-311++G) and (ii) the ability to monitor EFG convergence by progressively adding more surrounding molecules and/or adding higher many-body interactions.

A rapid determination of nuclear quadrupole resonance frequencies using field-cycling magnetic resonance and frequency modulated RF excitations.

A modification of Slusher-Hahn's double resonance technique is described and experimentally tested. It is based on application of multiple frequency sweeps and can be used for a rapid location of nuclear quadrupole resonance (NQR) frequencies. The resolution of the present technique is relatively low but, when the NQR frequencies are located, it is easy to use either the Slusher-Hahn's technique or pulse NQR to determine the NQR frequencies with a higher precision.

N NQR spectroscopy reveals the proton position in N-HN bonds: a case study with proton sponges.

The position of protons in hydrogen bonds is often uncertain to some degree, as the technique most often used for structure determination, X-ray diffraction, is sensitive to electron density, which is not particularly abundant around protons. In hydrogen bonds, protons introduce an additional problem: the potential for proton motion is inherently anharmonic and thus requires the consideration of nuclear quantum effects (NQEs). Here, we demonstrate that 14N NQR spectroscopy is able to rather accurately determine proton positions in N-HN bonds, in certain cases with an accuracy comparable to that of X-ray and neutron diffraction at room temperature.

N NQR lineshape in nanocrystals: An ab initio investigation of urea.

N nuclear quadrupole resonance (NQR) lineshapes mostly contain information of low interest, although in nanocrystals they may display some unexpected behaviour. In this work, we present an ab initio computational study of the N NQR lineshapes in urea nanocrystals as a function of the nanocrystal size and geometry, focusing on the surface induced broadening of the lineshapes. The lineshapes were obtained through a calculation of the electric field gradient for each nitrogen site in the nanocrystal separately, taking into account the individual crystal field by embedding the molecule of interest in a suitable lattice of point multipoles representing other urea molecules in the nanocrystal.

(1)H-(14)N cross-relaxation spectrum analysis in sildenafil and sildenafil citrate.

Here we describe a method for the extraction of (14)N quadrupole parameters from a (1)H-(14)N cross-relaxation spectrum by fitting the lineshapes of the (14)N quadrupole transitions. The procedures used typically to fit quadrupole lineshapes are not directly applicable to fit the (1)H-(14)N cross-relaxation spectrum, because the presence of proton homonuclear dipolar interaction broadens the lineshapes considerably and prevents a reliable determination of Cq and η from a single lineshape. Instead, one must fit two or even three lineshapes originating from the same nitrogen site simultaneously.