¹³C Chemical‐shift tensors in an analogous series of heterosubstituted polycyclic aromatic compounds

On Librational and Rotational Motions of Aromatic Rings in Layered Sn(IV) and Zr(IV) Phosphonate Materials: A Variable-Temperature 13C, 31P Solid-State NMR Study

“…As we believe, these results are not in a conflict if motions in materials 1−3 affecting their 31 P T 1 relaxation and the 13 C CSA are not simple rotational diffusion but are composite, consisting of 180°flips around C(1)−C(4) axis (Figure 3B) and librations around the same axis (Figure 3C). The amplitude of the librations, added to the flips, grows with the temperature, shifting these composite motions toward simple rotational diffusion at high temperatures.…”

“…As seen in Table , the 13 C CSA values, measured in 1 – 3 at room temperature, are remarkably smaller than those in flexible biphenyl and ibuprofen molecules. , However, due to the well-known influence of substituents on the 13 C CSA in the aromatic rings, − motionless molecules 4 and 5 are preferable for such a comparison: they clearly show that the 13 C CSA of 1 – 3 is actually reduced in the fast-moving aromatic rotors. These motions at low energy barriers of 1.4–3.0 kcal/mol are fast enough to reduce completely the 13 C CSA values in Table , if the mechanism of the motions is rotational diffusion.…”

“…In the present work, dynamics of aromatic rotors in 1−3 was characterized by 31 P T 1 time measurements and, in addition, by determinations of the carbon chemical shift anisotropy, 13 C CSA, which is sensitive to internal molecular motions. 23−28 Here and below, 13 C CSA = δ 11 − (δ 22 + δ 33 )/2, and δ 11 > δ 22 > δ 33 .…”

“…22 This situation is similar to the fast-rotating phenylene groups of organic frameworks X(C 6 H 4 ) 4 (X = C, Si) also showing short 13 C T 1 times of aromatic CH carbons. 21 The variable-temperature 31 P T 1 times measured in 3 are even shorter (Table S2 and Figure S4) to give, via the expression written for dipolar proton− phosphorus interactions 22 (see comments in SI), a very low activation energy of 1.4 kcal/mol characterizing the aromatic rotor in 3. Thus, the aromatic rings in 1−3 are free enough to experience very fast motions at frequencies of >10 8 Hz even at low temperatures requiring low activation energies of 1.4−3.0 kcal/mol.…”

Carbon-13 Chemical-Shift Tensors in Polycyclic Aromatic Compounds: Fluoranthene and Decacyclene

“…The measurement of 13 C chemical-shift tensors (CSTs) of polycyclic aromatic hydrocarbons (PAHs) has received considerable attention in recent years. − Challenges to measuring CSTs in aromatic microcrystalline powders include spectral complexity due to the spinning sideband patterns, the relatively close isotropic chemical shifts of the various carbons in the molecule (typically 120−140 ppm), and coincidental overlap of equivalent molecular positions in crystallographically inequivalent sites. Spectral complexity is reduced in this work by application of the FIREMAT experiment, a two-dimensional (2D) magic angle turning (MAT) experiment that isolates individual sideband patterns associated with different isotropic chemical shifts .…”