Charge density studies on small organic molecules around 20 K: oxalic acid dihydrate at 15 K and acetamide at 23 K

Zöbel, Dieter; Luger, Peter; Dreissig, W.; Koritsánszky, T.

doi:10.1107/s0108768192005433

Cited by 45 publications

(39 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…A calculation is also reported where the electron density of the crystal is given as a superposition of the densities calculated for the oxalic acid and water molecules separately. Finally, it should be mentioned that model (a) was used by Zobel et al (1992) (although it would have made no difference in his promolecule density model) and model (b) was used by in their calculations. Model (d) is displayed in Fig.…”

Section: Unconstrained Wavefunction Calculations For the Electron Denmentioning

confidence: 99%

“…The atomic positions and thermal parameters used in our calculations are the high-angle set re®ned by Zobel and co-workers at 15 K (Zobel et al, 1992), using the atomic form-factor method. The set of unique experimental structure factors was supplied by Zobel but are different to those deposited with the IUCr with their paper (Zobel et al, 1992;Zobel, 1996). Following their method, structure factors F with F`2', where ' is the estimated experimental error, were rejected from the data set.…”

Section: Data For the Oxalic Acid Dihydrate Crystalmentioning

confidence: 99%

“…The original atomic parameters were re®ned (Zobel et al, 1992) using atomic form factors on the high-angle data only, creating a bias toward atomic densities. This bias is not seen in the 1 2 for the low-angle data.…”

Section: Acta Cryst (2001) A57 87±100mentioning

confidence: 99%

See 2 more Smart Citations

Wavefunctions derived from experiment. II. A wavefunction for oxalic acid dihydrate

Grimwood

Jayatilaka

2001

Acta Crystallogr A Found Crystallogr

View full text Add to dashboard Cite

A wavefunction has been derived for the oxalic acid dihydrate molecule using accurate low-temperature X-ray electron-density structure-factor data. The electron density from this constrained theoretical wavefunction is compared to those of unconstrained theoretical wavefunctions. Fitted electron densities around hydrogen atoms show signi®cant deviation compared to Hartree±Fock calculations. In particular, hydrogen bonding appears enhanced in the crystal over theoretical predictions, while the density usually attributed to lone-pair electrons of the oxalic acid oxygen atoms is decreased. The constrained ®tting procedure improves the overall agreement of the calculated structure factors even for structure factors that were not used as input to the ®tting procedure. The pictures obtained from the constrained ®tting procedure are insensitive to random errors introduced into the data. Similarly, the ®tting procedure is able to reproduce features that arise from more accurate theoretical calculations. However, we are unable to ®t our wavefunction to within the experimentally quoted error bounds without allowing an unreasonably large change in the energy of the constrained wavefunction. Large Hartree±Fock and density functional theory (DFT) cluster calculations involving up to 86 atoms in size also do not show signi®cantly improved agreement with the experimentally observed structure factors. Derived properties from the constrained wavefunction fragments, such as the kinetic energy, electrostatic potential and the electron localization function, are also presented. In general, there are no dif®culties in extracting experimental wavefunctions and the associated derived properties from elastic X-ray scattering data for crystal fragments of the order of 20 atoms.

show abstract

Section: Unconstrained Wavefunction Calculations For the Electron Denmentioning

confidence: 99%

Section: Data For the Oxalic Acid Dihydrate Crystalmentioning

confidence: 99%

See 1 more Smart Citation

Wavefunctions derived from experiment. II. A wavefunction for oxalic acid dihydrate

Grimwood

Jayatilaka

2001

Acta Crystallogr A Found Crystallogr

View full text Add to dashboard Cite

show abstract

“…Oxalic acid (OXAC) dihydrate is an exemplary system displaying strong hydrogen bonding, examined repeatedly by X-ray and neutron diffraction [5][6][7][8][9] . Early vibrational spectroscopic 4 studies by Villepin and Novak [10][11][12] and single crystal far infrared study by Wyncke et al 13 attracted many other researchers to tackle the dynamics of this crystal using deuteron NMR 14 , ac conductivity measurements 15 , Raman 16,17 , high pressure X-ray 18 and neutron diffraction 19 , teraherz spectroscopy accompanied with density functional theory calculation 20 and Car-Parrinello molecular dynamics simulation 21 .…”

Section: Introductionmentioning

confidence: 99%

Evidence of Polaron Excitations in Low Temperature Raman Spectra of Oxalic Acid Dihydrate

2016

View full text Add to dashboard Cite

Low temperature Raman spectra of oxalic acid dihydrate (8K -300 K) both for polycrystalline and single crystal phase show strong variation with temperature in the interval from 1200 to 2000 cm -1 . Previous low temperature diffraction studies all confirmed the stability of the crystal P21/n phase with no indications of any phase transition, reporting the existence of a strong hydrogen bond between the oxalic acid and a water molecule. A new group of Raman bands in the 1200 -1300 cm -1 interval below 90 K is observed, caused by possible loss of the centre of inversion.This in turn could originate either due to disorder in hydroxyl proton positions, or due to proton transfer from carboxylic group to water molecule. The hypothesis of proton transfer is further supported by the emergence of new bands centered at 1600 cm -1 and 1813 cm -1 , which can be explained with vibrations of H3O + ions. The broad band at 1600 cm -1 looses intensity, while the band at 1813 cm -1 gains intensity on cooling. The agreement between quantum calculations of vibrational spectra and experimentally observed Raman bands of hydronium ions in oxalic acid sesquihydrate crystal corroborate this hypothesis.

show abstract

“…The advantages of lowering the temperature further in this way are mainly extensions of the points above (Larsen, 1995). The Fddd diffractometer described here has been developed to exploit the extended temperature range, to study low-temperature phase transitions and charge-density distributions at temperatures down to 9 K. Previously reported experimental arrangements for laboratory-based helium-temperature single-crystal X-ray diffraction (Samson, Goldish & Dick, 1980;Henriksen, Larsen & Rasmussen, 1986;Zobel, Luger, Dreissig & Koritsanszky, 1992) have used sealed tubes to generate X-rays. The rotating-anode generator used with the system reported here develops higher fluxes and consequently allows the use of smaller samples and/or a reduction in the measurement time.…”

Section: Introductionmentioning

confidence: 99%

TheFdddfour-circle diffractometer for single-crystal X-ray studies at temperatures down to 9 K

Copley¹,

Goeta²,

Lehmann³

et al. 1997

J Appl Cryst

View full text Add to dashboard Cite

The Fddd four-circle diffractometer system has been developed to perform X-ray diffraction experiments at temperatures as low as 9 K. The diffractometer consists of: (i) a Siemens rotatinganode generator; (ii) a Huber goniometer with offset X circle; (iii) a Siemens Fast Scintillation Detector; and (iv) an APD '202' Displex cryogenic refrigerator. The belt-driven rotating anode gives an X-ray flux that is far in excess of that obtained from a sealed X-ray tube and leads to faster data collections and the opportunity to study smaller samples. X-ray beam alignment requires precise movements of the 300 kg goniometer and this is achieved using compressed air pads. Steel-braided gas lines that allow the transfer of helium gas between the Displex and the compressor are supported by a counterbalance system. The stress on these lines has been reduced by attaching them to the Displex via high-pressure rotating joints and by passing them through a metal ring that is attached to the Z circle. A compact vacuum gauge has been mounted through one of the four ports on the top of the cryostat and gives valuable information on the pressure within the Displex sample chamber during an experiment. Crystals are mounted on sharpened 0.3 mm graphite pencil leads that are held in a newly designed sample mount. The temperature reported from the second stage of the Displex has been assessed by consideration of temperature standards that include the solid-state phase transitions of benzil (1,2-diphenylethanedione) and terbium vanadate.

show abstract

Charge density studies on small organic molecules around 20 K: oxalic acid dihydrate at 15 K and acetamide at 23 K

Cited by 45 publications

References 0 publications

Wavefunctions derived from experiment. II. A wavefunction for oxalic acid dihydrate

Wavefunctions derived from experiment. II. A wavefunction for oxalic acid dihydrate

Evidence of Polaron Excitations in Low Temperature Raman Spectra of Oxalic Acid Dihydrate

TheFdddfour-circle diffractometer for single-crystal X-ray studies at temperatures down to 9 K

Contact Info

Product

Resources

About