Rovibrational laser jet-cooled spectroscopy of SF6-rare gas complexes in the ν3 region of SF6.

High resolution infrared spectroscopy combining an external cavity quantum cascade laser with a pulsed pin hole supersonic jet is used to investigate small van der Waals (vdW) heteroclusters containing SF6 and rare gas (Rg) atoms in the ν3 region of SF6. In the first step of the analysis, the rovibrational band contours of parallel and perpendicular transitions of 1 : 1 SF6-Rg heterodimers (Rg = Ar, Kr, Xe) are simulated to derive ground and excited state parameters and hence ground state and equilibrium S-Rg distances with a precision better than 0.5 pm. These values are used to assess quantum chemistry calculations (DFT-D method) as well as semi-empirical predictions (combination rules). In the second step, the spectral signatures of the 1 : 1 heterodimers and of larger heteroclusters containing up to three Rg atoms have been identified by considering reduced vibrational red shifts, i.e., shifts normalized to the average 1 : 1 red shift. The reduced vibrational red shifts within the series of bands observed and assigned to 1 : 1 and 1 : 2 complexes are found to be independent of the Rg atom, which suggests similar 1 : 1 and 1 : 2 structures along the Rg series. In addition, the increasing number of bands when going from monomer to 1 : 2 complexes illustrates the increased lifting of vibrational degeneracy induced by Rg solvation. Finally, the vibrational shifts of the 1 : 1 SF6-Rg heterodimers are found to fit an intermolecular interaction model in which long-range attractive and short-range repulsive contributions to the vibrational shift are found to partially cancel out, the former being dominant. From the same model, well depths are obtained and are found to compare well with quantum chemistry calculations and semi-empirical combination rules.

Conformational landscape of the SF6 dimer as revealed by high resolution infrared spectroscopy and complexation with rare gas atoms.

Taking advantage of a versatile set-up, combining pulsed pin hole or slit nozzle supersonic expansion with an external cavity quantum cascade laser, the rovibrational absorption spectrum of the SF6 dimer in the ν3 mode region has been revisited at high resolution under various experimental conditions in SF6:He mixtures. Two new rotationally resolved spectral bands have been identified in the range of the parallel band of the dimer spectrum in addition to that previously reported. Among these three spectral features, two of them are assigned to conformations of the dimer (noted #1 and #2), clearly distinguished from their different S-S interatomic distances, i.e. 474 and 480 pm respectively. The third one is assigned to a (SF6)2-He complex, from comparison with additional experiments in which (SF6)2-Rg heterotrimers (Rg = Ne, Ar, Kr, Xe) are observed. A schematic picture of the potential energy landscape of the SF6 dimer in terms of a nearly flat surface is proposed to account for the conformational relaxation observed in the expansions and for the structure of the (SF6)2-Rg heterotrimers, which are exclusively formed from the conformer #2 dimer. Although modelling qualitatively supports this picture, much effort has still to be achieved from a theoretical point of view to reach a quantitative agreement with the present benchmark experimental data both in terms of structure and energetics.

Radical Ring-Opening Copolymerization of Cyclic Ketene Acetals and Maleimides Affords Homogeneous Incorporation of Degradable Units.

Radical copolymerization of donor-acceptor (D-A) monomer pairs has served as a versatile platform for the development of alternating copolymers. However, due to the use of conventional radical polymerization, the resulting copolymers have generally been limited to nondegradable vinyl polymers. By combining radical D-A copolymerization with radical ring-opening polymerization (rROP), we have synthesized an alternating copolymer with a high incorporation of degradable backbone units. Copolymerization of N-ethyl maleimide (NEtMI) with the cyclic ketene acetal (CKA) 2-methylene-4-phenyl-1,3-dioxolane (MPDL) was demonstrated to proceed in an alternating fashion, and controlled polymerization was achieved using reversible addition-fragmentation chain transfer (RAFT) polymerization. Spontaneous copolymerization, in the absence of an exogenous initiating source, occurred when the mixture of monomers was heated, presumably due to the large electron disparity between the comonomers. Chain-extension with styrene afforded well-defined P(MPDL-alt-NEtMI)-b-polystyrene copolymers, and degradation of the homopolymers and block copolymers showed complete breakdown of the alternating copolymer.