Seven Conformations of the Macrocycle Cyclododecanone Unveiled by Microwave Spectroscopy

Preliminary assessment of new single and blended volatile binding media for temporary consolidation of cultural heritage

Volatile Binding Media (VBM) are waxy solids that can be used for temporary consolidation of heritage artifacts and architectural surfaces thanks to their spontaneous sublimation at room temperature. They are used to temporary shelter, consolidate or protect materials during high-risk operation, such as excavation, transportation, water-based treatments, etc. Although they are employed since the 1990s, research focused almost entirely on one of them, cyclododecane (CDD), which is by far the most used in onsite applications. However, CDD exhibits some drawbacks, including a fixed sublimation speed that hardly fits into all the possible applications and climates, hence the development of new VBM is strongly needed. In recent years, a certain attention was addressed to menthol as a possible alternative, but the research on other possible substitutes is still lacking. In this paper, a range of different VBM for temporary consolidation of cultural heritage materials was prepared and investigated, including five pure compounds (CDD, cyclododecanol, cyclododecanone, menthol and camphene) and fifteen mixes. These new materials are expected to provide tunable properties in terms of melting temperatures and sublimation rates, allowing their use in a variety of climatic contexts and applications, and to exhibit better properties for onsite applications compared to CDD, such as lower flash point, lower hazard for conservators' health and/or higher availability.

Revealing Internal Rotation and 14N Nuclear Quadrupole Coupling in the Atmospheric Pollutant 4-Methyl-2-nitrophenol: Interplay of Microwave Spectroscopy and Quantum Chemical Calculations

The structure and interactions of oxygenated aromatic molecules are of atmospheric interest due to their toxicity and as precursors of aerosols. Here, we present the analysis of 4-methyl-2-nitrophenol (4MNP) using chirped pulse and Fabry-Pérot Fourier transform microwave spectroscopy in combination with quantum chemical calculations. The rotational, centrifugal distortion, and ¹⁴N nuclear quadrupole coupling constants of the lowest-energy conformer of 4MNP were determined as well as the barrier to methyl internal rotation. The latter has a value of 106.4456(8) cm^-1, significantly larger than those from related molecules with only one hydroxyl or nitro substituent in the same para or meta positions, respectively, as 4MNP. Our results serve as a basis to understand the interactions of 4MNP with atmospheric molecules and the influence of the electronic environment on methyl internal rotation barrier heights.

Conformational Panorama of Cycloundecanone: A Rotational Spectroscopy Study

The conformational landscape of the medium-size cyclic ketone cycloundecanone has been investigated using chirped-pulse Fourier transform microwave spectroscopy and computational calculations. Nine conformations were observed in the rotational spectrum and identified from the comparison of experimental and theoretical rotational constants as well as the observed and predicted types of rotational transitions. All singly substituted ¹³C isotopologues were observed for the most abundant conformer, which allowed the determination of partial substitution and effective structures. The most abundant conformer dominates the rotational spectrum and is almost 40 times more abundant than the least abundant conformer. Conformational preferences are governed by the combination of transannular H···H and eclipsed HCCH interactions.