Strukturverfeinerung von Harnstoff mit Neutronenbeugungsdaten bei 60, 123 und 293 Κ und X-N- und X-X(1s²)-Synthesen bei etwa 100 Κ

Abstract

Neutron diffraction data were collected on urea at 60, 123 and 293 Κ, and the nuclear parameters were determined [/? (F) = 0.034,0.033 and 0.043 respectively]. The parameters at 293 Κ are compared with the neutron parameters of Pryor and Sanger by means of half normal plots and χ 2 tests, and the low-temperature parameters with the ls 2 -core parameters which were determined by Mullen and Hellner with X-ray data. Significant differences of0.0030 Â, on average, were found between the t/ 33 components from our low-temperature determination and from the Is 2 -core refinement. X-Ν and X-X(1 s 2 ) maps are calculated for about 100 Κ with two different scale factors and, after an assessment of all parameters, a "best" experimental deformation density is calculated. The effect of large librations of the Ν atom about the C-O axis (4.2 rms degrees) on the density distribution, and the effect of using not fully correct phases are discussed. The errors in the density distribution which are caused by errors in the X-ray data, in the scale factor and in the neutron parameters, are assessed to be 0.07 e -3 at the bond peaks, and 0.45 eA" 3 at the nuclear positions. The "best" experimental deformation density has higher bond peaks (except for the C-O bond) than a (published) theoretical difference density (4-31G basis), but lower peaks in the lone-pair regions of the oxygen atom and no polarization minimum at the oxygen atom.

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

“…According to the a ,2 test, these values and the above neutron diffraction values can be accepted as having a common probability distribution at the 93% level of confidence. There are highly significant differences from the neutron diffraction values reported by Guth et al (1980), such as /333 = 0-0119 (5) .~z for the O atom at 123 K. In a similar X 2 test involving the two sets of neutron diffraction values, the confidence level is less than 1%. the molecules and the H-bonds confined to planes parallel to (110), (1T0) which intersect and leave tunnels having a square cross section 3.94 x 3.94 A.…”

“…Corrections to the urea bond lengths of Guth et al (1980), which were proposed by Scheringer (1980), involve the rigid-body motion of urea together with important intramolecular motion of the H atoms. The latter was determined from the infrared and Raman spectra of crystalline urea, assuming that the molecules vibrate independently (Yamaguchi, Miyazawa, Shimanouchi & Mizushima, 1957;Saito, Machida & Uno, 1971;Ishii & Scheringer, 1979).…”

“…The present neutron diffraction studies were undertaken when large discrepancies were found between the anisotropic thermal parameters at 123 K reported by Guth et al (1980) and the corresponding values obtained from an X-ray analysis at the same temperature (Swaminathan, Craven, Spackman & Stewart, 1984). It seemed desirable to investigate these discrepancies as part of a detailed extensive study of the molecular structure and thermal motion of urea over a range of temperature.…”

2-Pyridone, C5H5NO, crystal structure refinements at 295 Κ and 120 Κ, experimental and theoretical deformation density studies

“…The density maxima of about 0.3eÄ~3 may be compared to the density maxima of the lone pairs in urea (Guth et al, 1980). …”

Ultra-wideline 14N NMR spectroscopy as a probe of molecular dynamics

“…1a. The majority of the spectrum corresponds to a single nitrogen site (the two sites in the molecule are equivalent in the crystal structure) 12 with C Q = 3.47 MHz and asymmetry parameter Z Q = 0.31. These parameters are in reasonably good agreement with values predicted using the CASTEP software 13 (C Q = À3.86 MHz and Z Q = 0.28, details of these simulations can also be found elsewhere).…”