Tetranitromethane: A Nightmare of Molecular Flexibility in the Gaseous and Solid States

Abstract

After numerous attempts over the last seven decades to obtain a structure for the simple, highly symmetric molecule tetranitromethane (C(NO ) , TNM) that is consistent with results from diffraction experiments and spectroscopic analysis, the structure has now been determined in the gas phase and the solid state. For the gas phase, a new approach based on a four-dimensional dynamic model for describing the correlated torsional dynamics of the four C-NO units was necessary to describe the experimental gas-phase electron diffraction intensities. A model describing a highly disordered high-temperature crystalline phase was also established, and the structure of an ordered low-temperature phase was determined by X-ray diffraction. TNM is a prime example of molecular flexibility, bringing structural methods to the limits of their applicability.

Tetranitromethan – ein Albtraum molekularer Flexibilität in Gasphase und Festkörper

“…[29] An dieser Stelle bleibt lediglich zu subsumieren, dass die Untersuchungen an der kristallinen Hochtemperaturphase eine überaus große Flexibilitätd er TNM-Moleküle belegen. Dazu züchteten wir einen makroskopisch gut ausgeprägten TNM-Kristall fürR B-Experimente.M it einem Satz an bei 200 K gewonnenen Beugungsdaten war es mçglich, die Struktur dieser scheinbar kubischen Hochtemperaturphase in verschiedener Weise zu lçsen und zu verfeinern.…”

“…[30] Die drei unabhängigen Moleküle A, B, Ch aben erstaunlich ähnliche Strukturen (Überblendung siehe Abbildung 2). Über deren Auftreten wurde bereits früher berichtet, doch ihre Struktur wurde bisher nicht gelçst.…”

“…Statt des Abtastens der PES wurden viele einzelne Punkte auf ihr im Monte-Carlo-Verfahren erprobt. Dazu fand der klassische Metropolis-Algorithmus [27,28] [30] Die drei unabhängigen Moleküle A, B, Ch aben erstaunlich ähnliche Strukturen (Überblendung siehe Abbildung 2). [20,21] und übersteigen den im festen Tr initromethan von 127.2(1)8 8.…”

Structure Determination Techniques Flex Their Muscles

“…[2,4] Structure analysis by single-crystal X-ray diffraction (SCXRD) has an even longer pedigree.E ver since the first crystal structure determination was published in 1913, the technique has been at the forefront of many of the most important scientific developments of the past century [5] and is as tandard characterization technique for new materials and for investigating structural variability.T he Cambridge Structural Database of small-molecule crystal structures [6] alone now contains around 900 000 entries.T he method does have its limitations,the most obvious being the requirement forideally-relatively large single crystals composed of unit cells with ac onsistent repetition of atomic positions.H owever, given suitable hardware,s oftware,s kill, and time,c onsiderable deviations from this ideal can be accommodated. [7] Another feature of structures determined by SCXRD is that molecules are in close proximity to each other, and the effects of this proximity can range from insignificant to extreme.The field of crystal engineering, [8] which involves the design, construction, and analysis of crystal structures of organic and metal-organic compounds,i sb ased on the existence of significant intermolecular interactions in the solid state.So,g iven the power and maturity of these techniques for structure determination, why is the successful determination of the structure of as imple molecule such as tetranitromethane (Figure 2a)inthe gas phase and the solid state such ab ig deal?Part of the appeal of the Communication by Mitzel and co-workers [9] lies in the resolution of many decades of uncertainty over the true structure of tetranitromethane. Figure 1.…”

“…Part of the appeal of the Communication by Mitzel and co-workers [9] lies in the resolution of many decades of uncertainty over the true structure of tetranitromethane. Figure 1.…”