¹⁹F chemical shifts, coupling constants and conformational preferences in monosubstituted perfluoroparacyclophanes

Abstract

In the process of studying the chemistry of perfluoro[2.2]paracyclophanes (PFPCs), a novel class of compounds, it became necessary to identify some disubstituted products. To achieve this goal, we characterize in this work some monosubstituted PFPCs, identifying their (19)F-(19) F coupling patterns, and establishing a methodology for the assignment of their (19)F chemical shifts. The pattern of coupling constants indicates a skewed geometry in which the upper deck moves towards or away from the substituent, depending on the substituent electron-donor character and size. Quantum chemical calculations, performed at the HF/6-311 + G(d,p)//B3LYP/EPR-III level of theory, confirmed the conformations inferred from coupling constants and reproduced well the values of the couplings. Transmission mechanisms for the FC term of four- and five-bond (19)F-(19) F couplings are discussed in detail. Understanding the conformational preferences of PFPCs and how they are reflected by the coupling constants facilitates the assignment of (19)F chemical shifts in monosubstituted PFPCs and the identification of the disubstituted products.

NMR spectral analysis of strongly second‐order 6‐, 8‐, 9‐ and 10‐ spin‐systems (1H─19F, 19F─19F, and 13C─19F) in perfluorotoluyl‐ and tetrafluoro‐pyridyl‐aromatics using the lineshape method ANATOLIA

“…It is prudent to consider a through‐space F─F coupling as a distance relationship in difluorocyclophanes show J FF couplings of up to 110 Hz at 2.419 Å, reducing to a few 10s of hertz at approximately 3.0 to 3.2 Å . For a series of structurally rigid 1,8‐difluoronaphthalene compounds the through‐space J FF coupling was between 85.2 to 36.6 Hz corresponding to distances of 2.452 to 2.718 Å, with the specific contributions to the measured experimental coupling being dissected, showing that the Fermi contact term of +56 Hz results from a combination of a through‐space coupling (+75 Hz) and σ through‐bond coupling (−19 Hz) . The contributions from the diamagnetic and paramagnetic parts of the 19 F nuclear magnetic shielding tensor, paramagnetic spin‐orbit term and Fermi contact contributions to indirect nuclear spin–spin coupling constants have been rationalised …”

Destination state screening of active spaces in spin dynamics simulations

“…With case-specific knowledge of individual pulse sequences, it should be possible to apply the screening at simulation stages that do not necessarily involve detection -if one has the knowledge of how the different subspaces feed into each other throughout the pulse sequence, it should be possible to set up screens for each evolution period, threading the (eventually) observable magnetization through several ''keyholes'' along the way. 13 C nuclei are assumed to be dilute.…”

“…3. The large J couplings in the 15-spin 19 F system of this perfluoroparacyclophane molecule [13] mean that the matrix dimension, even after state space restriction, symmetry factorization, and zero track elimination, is of the order of 3 Â 10 4 . Path tracing through the Liouvillian reveals that this space can be split into two independent subspaces of dimension $10 4 , and 15 subspaces of dimension 1.…”

“…The IK-2 basis set was used (for every spin the spin orders involving directly coupled spins are kept [8]). Chemical shifts and coupling constants are taken from [13]. The Liouville space dimension of $10 9 is reduced by existing state space restriction methods to $3 Â 10 4 .…”

Synthesis and modification of octafluoro[2.2]paracyclophane (parylene AF4)

“…The experimental data was found to be in good agreement with computational models. 44 The most recent report concerning 37 is the atropoisomerism of the acetoacetic ester produced by the reaction of 37 and ethyl acetoacetate. 45 Atropisomers are stereoisomers resulting from restricted rotation about a single bond where the barrier of rotation is sufficiently high that both isomers can be isolated.…”