The use of structural information in phase determination

Klop, Enno A.; Krabbendam, H.; Kroon, Juan A. Valiente

doi:10.1107/s0108767390001829

Cited by 7 publications

(1 citation statement)

References 7 publications

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“…It is promising that this high-reliability triplet estimation can be achieved even for very low DR or very 'light' heavy atoms (Table 10). Fan, Han, Qian & Yao (1984) and Klop, Krabbendam & Kroon (1990) proposed expressions to solve this sign problem. However, the expressions developed in these papers depend on N -1/2 and consequently the reliability of these formulae is low for large structures.…”

Section: Test Results and Discussionmentioning

confidence: 99%

On direct-methods phase information from differences between isomorphous structure factors

Kyriakidis¹,

Peschar²,

Schenk³

1993

Acta Cryst Sect A

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An efficient procedure is presented for the derivation of joint probability distributions of isomorphous data sets. The new technique is based on the use of the differences of isomorphous structure factors as random variables. It will be shown that the usual probabilistic techniques, applied to these random variables, finally result in the joint probability distribution of three single differences of isomorphous structure factors comprising three doublet and eight triplet phase sums. An advantage of the new technique is that the inherent correlation between the isomorphous data sets is removed if a probabilistic procedure is set up for the small difference itself. In this way, an enormous mathematical simplification is obtained while the final results are much better than those obtainable by previous probabilistic expressions. The final triplet distribution seems to be of sufficient quality to be used in a normal directmethods procedure. In contrast to usual approaches, the heavy-atom substructure need not be solved first. The probabilistic expression will be explained in detail for one and three single differences. Applications for the cases of single anomalous scattering, two different wavelengths and single isomorphous replacement (excluding anomalous-scattering effects)

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Section: Test Results and Discussionmentioning

confidence: 99%

On direct-methods phase information from differences between isomorphous structure factors

Kyriakidis¹,

Peschar²,

Schenk³

1993

Acta Cryst Sect A

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2′‐Methyl‐3′‐nitroflavone

Kendi

Özbey

Tunçbìlek

et al. 1996

Cryst. Res. Technol.

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The crystal and molccular structure of the title compound has been determined by direct methods and rcfincd to a final R of 0.044. The compound crystallizes in space group P21/n with cell dimcnsions a = 16.681(2), 6 = 3.838(1), c = 20.550(4) A, and / 3 = 104.21(2)". The phenyl and benzene rings of the title molecule are quite planar. The y-pyrone ring exhibits maximum deviation 0.032(4) A from planarity and makes a dihedral angle of 139.8(2)" with the 2-phenyl substituent.

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A combined nuclear magnetic resonance and computational study of monohydroxyflavones applied to product ion mass spectra

Burns¹,

Ellis²,

Li³

et al. 2007

Rapid Comm Mass Spectrometry

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A method is presented for the estimation of 13C-chemical shifts for carbon atoms in protonated and deprotonated molecules; in principle, this method can be applied to ions in general. Experimental 13C-chemical shifts were found to vary linearly with computed atomic charges using the PM3 method. Pseudo-13C-chemical shifts for atoms in protonated and deprotonated molecules can be estimated from computed atomic charges for such atoms using the above linear relationship. The pseudo-13C-chemical shifts obtained were applied to the rationalization of product ion mass spectra of protonated and deprotonated molecules of flavone and 3-, 5-, 6-, 7-, 2'-, 3'-, and 4'-hydroxyflavones, where product ion formation is due to either cross-ring cleavage of the C-ring (retro-Diels-Alder reaction) or to cleavage of a C-ring bond followed by loss of either a small neutral molecule or a radical. The total product ion abundance ratio of C-ring cross cleavage to C-ring bond cleavage, gamma, varied by a factor of 660 for deprotonated monohydroxyflavones, i.e., from 0.014:1 to 9.27:1. The magnitude of gamma, which is dependent on the relative bond orders within the C-ring of the protonated and deprotonated molecules of monohydroxyflavones, can be rationalized on the basis of the magnitudes of the 13C- and 1H-chemical shifts as determined by nuclear magnetic resonance spectroscopy.

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The use of structural information in phase determination

Cited by 7 publications

References 7 publications

On direct-methods phase information from differences between isomorphous structure factors

On direct-methods phase information from differences between isomorphous structure factors

2′‐Methyl‐3′‐nitroflavone

A combined nuclear magnetic resonance and computational study of monohydroxyflavones applied to product ion mass spectra

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