Theoretical and experimental studies of the charge density in urea

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293

199

Lattice parameters for urea have been measured at seven temperatures in the range 12 to 173 K and the crystal structure has been determined at 12, 60 and 123 K from neutron diffraction data. With 342 reflections (sin 0/A <0.77 A -I) measured for an octant of reciprocal space, full-matrix least-squares refinement gave Rw(F2) = 0.030, 0.029 and 0.029 at 12, 60 and 123 K. The atomic anisotropic thermal parameters are consistent with overall rigid-body motion together with intramolecular librations of the NH2 groups.

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

The crystal structure and molecular thermal motion of urea at 12, 60 and 123 K from neutron diffraction

Swaminathan¹,

Craven²,

McMullan³

1984

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293

199

“…version of an earlier code (STRUFAC) used in several previous analyses of model X-ray data generated from theory (see, for example, Spackman, 1979;Swaminathan, Craven, Spackman & Stewart, 1984;Ritchie, Cromer, Stewart, Wasserman & Ryan, 1985;Chandler & Phillips, 1986;Chandler, Figgis, Reynolds & Wolff, 1994). It incorporates an analytical Fourier transformation of Gaussian orbital products (Chandler & Spackman, 1978) and the option to include anisotropic thermal motion parameters for each atom.…”

Section: Compute Molecular Propertiesmentioning

confidence: 99%

Molecular electric moments and electric field gradients from X-ray diffraction data: model studies

Spackman¹,

Byrom²

1996

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Model X-ray data sets, with and without the inclusion of experimental thermal motion parameters, have been computed via Fourier transformation of ab initio molecular electron densities for 12 different molecular crystals. These datasets were then analysed with three different multipole models of varying sophistication and, from the multipole functions, molecular dipole and second moments, as well as electric field gradients (EFG's), at each nuclear site were computed and compared with results obtained from the original ab initio wavefunctions. The results provide valuable insight into the reliability of these properties, extracted in the same way from experimental X-ray data. Not all molecular systems display identical trends, but a general pattern is discernible. Specifically, dipole moments are typically underestimated by a small but significant amount (~ 10-15%), the trace of the second moment tensor is well determined but overestimated by a few per cent and electric field gradients at protons are confirmed to be well within reach of a careful charge density analysis of X-ray diffraction data.

“…These features were of interest to us because of recent studies of the charge-density distribution in related crystal structures, including parabanic acid at 298 K (Craven & McMullan, 1979) and at 123 K (He, Swaminathan, Craven & McMullan, 1986), urea (Swaminathan, Craven, Spackman & Stewart, 1984) and thiourea (Mullen, Heger & Treutmann, 1978;Mullen, 1982;Kutoglu, Scheringer, Meyer & Schweig, 1982). We have redetermined the crystal structure of TUPA with greater accuracy using X-ray (Mo Ka)…”

Section: Introductionmentioning

confidence: 99%

Structure and charge density of the 1:1 complex of thiourea with parabanic acid at 298 K

Weber¹,

Craven²

1987

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Even though the calculated values for the lattice energy could not be directly compared with the experimental ones because of the limitations of the approach used, it can be illustrative to compare the values obtained in the different runs when the hydrogen bonds are omitted. When the electrostatic term is not included, values of the lattice energy for rigid molecules (no subrotations) are in the range 70-100 kJ mol -~, whereas when molecular subrotations are relaxed lattice-energy values are in the range 100-160 kJ mo1-1. For runs including the electrostatic term, the lattice energy is in the range 100-170 kJ mol -~ when rigid molecules are considered and 130-200kJ mol -~ when molecular subrotations are relaxed. So, an unexpectedly large Coulombic contribution to the lattice energy, ranging from 30 to 60% of the total estimated lattice energy, is found. In our opinion, this high contribution is a qualitatively significant feature, in spite of the approach and the potential parameters used and the method of estimating effective atomic charges. Generally, it was tacitly assumed that the Coulombic contribution to the lattice energy is negligible but some previous results indicate that this assumption is not valid in all hydrocarbon crystals, reaching a value of 29% of the total lattice energy of benzene and up to 59% in other cases (Williams & Cox, 1984).If the Coulombic contribution to the lattice energy seems to be significant for these crystals, it is true, however, that the inclusion of the electrostatic term does not significantly affect the values of the optimized structural parameters. The overall fit between the theoretical configuration and experimental structure is, in general, slightly more satisfactory for runs including the electrostatic contribution, but conclusive general results on the effects of its inclusion on molecular rotation and translation would be difficult to establish.Within the limits of the approach and the potential functions used, we think that the reported results clearly indicate that the electrostatic term makes an important contribution to the lattice energy of these compounds but has little effect on the optimized structural parameters. A major limitation for accuracy in these calculations, in our opinion, is the description of the hydrogen bonds, but further improvements should be possible by defining a realistic potential function for these interactions.