Statistical fluctuations in counting rates etc., as well as defects in the structural model, can introduce bias in the estimation of parameters by least-squares refinements. Of the residuaIs in common use, only unweighted R2 = Σ(Io - Ic)2 is free from statistical bias. Order-of-magnitude estimates of the bias can be derived, but it seems better to avoid it by adjusting the weights. To the second order, refinement of R2 is unbiased if the intensity used in calculating the usual weights is not Io but ⅓ (Io + 2Ic). There seems to be no simple method of avoiding bias in R1.

Statistical bias in least-squares refinement

Abstract A family of fully bridged triphenylamines with embedded 5‐ and 7‐membered rings is presented. The compounds are potent electron donors capable to undergo donor/acceptor interactions with strong cyano‐based acceptors both in the solid state and solution. These interactions were evaluated by IR and UV/vis spectroscopy as well as X‐ray crystallography. The vinylene‐bridged compound was oxidized to the corresponding 1,2‐diketone which readily underwent acid‐catalyzed condensation with selected 1,2‐phenylenediamines. The resulting π‐extended quinoxaline derivatives represent valuable building blocks for the development of functional chromophores upon appropriate functionalization.

/pdf/fully-bridged-triphenylamines-comprising-five-and-seven-11q1ur10.pdf

Fully Bridged Triphenylamines Comprising Five‐ and Seven‐Membered Rings

Hydrogen-bonded supramolecular assemblies of citric acid, CA, with some heterocyclic compounds (N-donor and N-oxide)—acridine (acr), phenazine (phenz), 1,10-phenanthroline (110phen), 1,7-phenanthroline (17phen), 4,7-phenanthroline (47phen), 1,4-diazabicyclo[2.2.2]octane (dabco), and 4,4′-bipyridyl-N,N′-dioxide (bpydo)—have been reported. Among these, only the N-donors phenz and N-oxide (bpydo) form neutral co-crystals, while the others form salts owing to the deprotonation of −COOH. Thus, depending on the nature of the aggregate (salt/co-crystal), recognition between the co-formers is established through O–H···N/N+–H···O/N+H···O–heteromeric hydrogen bonding. Additionally, CA molecules establish homomeric interactions mediated by O–H···O hydrogen bonds. Moreover, CA forms a cyclic network with the co-formers or on its own, with a noteworthy feature of formation of host–guest networks in the assemblies with acr and phenz (solvated). In the assembly of acr, the CA molecules form a host network and captivate acr molecules as guest species, while in the case of phenz assembly, both the co-formers together encapsulate the solvent in the channels. However, the observed cyclic networks in the other structures form three-dimensional topologies in the form of ladders, a sandwich, lamellar layers, and interpenetrated networks. The structural features of the ensembles are evaluated unequivocally by the single-crystal X-ray diffraction method, while the homogeneity and phase purity are evaluated by using the powder X-ray diffraction method and differential scanning calorimetry. Further, conformational analysis of CA molecules reveals three types of conformations—T-shape (type I), syn–anti (type II), and syn (type III) as also observed in the literature for other CA co-crystals. In addition, the strength of the intermolecular interactions is quantified by performing Hirshfeld analysis.

https://pubs.acs.org/doi/pdf/10.1021/acsomega.3c03446

Systematic Exploration of Structural Topologies in Hydrogen-Bonded Supramolecular Assemblies of Citric Acid with Different Heterocyclic Compounds

During the course of research into the structure of 2,3,5,6-tetra­fluoro-7,7,8,8-tetra­cyano­quinodi­methane (F4TCNQ), C12F4N4, an important com­pound in charge-transfer and organic semiconductor research, a previously unreported polymorph of F4TCNQ was grown concomitantly with the known polymorph from a saturated solution of di­chloro­methane. The structure was elucidated using single-crystal X-ray diffraction and it was found that the new polymorph packs with mol­ecules in parallel layers, in a similar manner to the layered structure of F2TCNQ. The structure was analysed using Hirshfeld surface analysis, fingerprint plots and pairwise inter­action energies, and com­pared to existing data. The structure of a toluene solvate of F4TCNQ is also reported.

/pdf/structural-investigations-into-a-new-polymorph-of-f4tcnq-1p1dwv6qvi.pdf

Structural investigations into a new polymorph of F4TCNQ: towards enhanced semiconductor properties

/pdf/csd-communications-of-the-cambridge-structural-database-1exs4ccm.pdf

CSD Communications of the Cambridge Structural Database

The rigorous analysis of crystallographic models, refined through the use of least-squares minimization, is founded on the expectation that the data provided have a normal distribution of residuals. Processed single-crystal diffraction data rarely exhibit this feature without a weighting scheme being applied. These schemes are designed to reflect the precision and accuracy of the measurement of observed reflection intensities. While many programs have the ability to calculate optimal parameters for applied weighting schemes, there are still programs that do not contain this functionality, particularly when moving beyond the spherical atom model. For this purpose, CAPOW (calculation and plotting of optimal weights), a new program for the calculation of optimal weighting parameters for a SHELXL weighting scheme, is presented and an example of its application in a multipole refinement is given.

/pdf/capow-a-standalone-program-for-the-calculation-of-optimal-qnu2yas4ob.pdf

CAPOW: a standalone program for the calculation of optimal weighting parameters for least-squares crystallographic refinements

/pdf/evolving-the-cambridge-structural-database-234usucx.pdf

Natalie T. Johnson

Papers

Structural investigations into a new polymorph of F4TCNQ: towards enhanced semiconductor properties

CSD Communications of the Cambridge Structural Database

CAPOW: a standalone program for the calculation of optimal weighting parameters for least-squares crystallographic refinements

Evolving the Cambridge Structural Database