Three hydrohalogenides of organic nitrogen bases

The relation between crystal structure and the occurrence of quantum-rotor-induced polarization

Among hyperpolarization techniques, quantum-rotor-induced polarization (QRIP), also known as the Haupt effect, is a peculiar one. It is, on the one hand, rather simple to apply by cooling and heating a sample. On the other hand, only the methyl groups of a few substances seem to allow for the effect, which strongly limits the applicability of QRIP. While it is known that a high tunnel frequency is required, the structural conditions for the effect to occur have not been exhaustively studied yet. Here we report on our efforts to heuristically recognize structural motifs in molecular crystals able to allow to produce QRIP.

Weak hydrogen bonding and fluorous interactions in the chloride and bromide salts of 4-[(2,2,3,3-tetrafluoropropoxy)methyl]pyridinium

Neutralization of 4-[(2,2,3,3-tetrafluoropropoxy)methyl]pyridine with hydrohalo acids HX (X = Cl and Br) yielded the pyridinium salts 4-[(2,2,3,3-tetrafluoropropoxy)methyl]pyridinium chloride, C₉H₁₀F₄NO⁺·Cl^-, (1), and 4-[(2,2,3,3-tetrafluoropropoxy)methyl]pyridinium bromide, C₉H₁₀F₄NO⁺·Br^-, (2), both carrying a fluorous side chain at the para position of the pyridinium ring. Single-crystal X-ray diffraction techniques revealed that (1) and (2) are isomorphous. The halide anions accept four hydrogen bonds from N-H, ortho-C-H and CF₂-H groups. Two cations and two anions form a centrosymmetric dimeric building block, utilizing complimentary N-H...X...H-Csp³ connections. These dimers are further crosslinked, utilizing another complimentary Csp²-H...X...H-Csp² connection. The pyridinium rings are π-stacked, forming columns running parallel to the a axis that make angles of ca 44-45° with the normal to the pyridinium plane. There are also supramolecular C-H...F-C interactions, namely bifurcated C-H...F and bifurcated C-F...H interactions; additionally, one type II C-F...F-C halogen bond has been observed.

4-tert-Butylpyridinium chloride–4,4′-(propane-2,2-diyl)bis(2,6-dimethylphenol)–toluene (2/2/1)

In the title solvated salt, C₉H₁₄N⁺·Cl⁻·C₁₉H₂₄O₂·0.5C₇H₇, two mol­ecules of 4,4′-(propane-2,2-di­yl)bis­(2,6-di­methyl­phenol) are linked via O—H⋯Cl hydrogen bonds to two chloride ions, each of which is also engaged in N—H⋯Cl hydrogen bonding to a 4-tert-butyl­pyridinium cation, giving a cyclic hydrogen-bonded entity centred at 1/2, 1/2, 1/2. The toluene solvent mol­ecule resides in the lattice and resides on an inversion centre; the disorder of the methyl group requires it to have a site-occupancy factor of 0.5. No crystal packing channels are observed.

[CuCl3(H2O)](-) complexes aggregated to form hydrate columns in methyl-substituted pyridinium or piperidinium salts

1,2,3-Trimethylpyridinium aquatrichloridocuprate(II), (C(8)H(12)N)[CuCl(3)(H(2)O)], (I), 3,4-dimethylpyridinium aquatrichloridocuprate(II), (C(7)H(10)N)[CuCl(3)(H(2)O)], (II), and 2,3-dimethylpyridinium aquatrichloridocuprate(II), (C(7)H(10)N)[CuCl(3)(H(2)O)], (III), exhibit the same fundamental structure, with (I) and (II) isomorphous and with the unit-cell constants of (III) similar to the reduced unit-cell constants of (I) and (II). The distorted square-planar [CuCl(3)(H(2)O)](-) complex [mirror symmetric in (I) and (II)] forms two semicoordinate Cu···Cl bonds to a neighboring complex to produce a dimer with 2/m symmetry [only inversion symmetry in (III)]. The semicoordinate Cu...Cl bond length of the dimer shows significant elongation at 295 K compared with that at 100 K, while the coordinate Cu-Cl bond lengths are slightly contracted at 295 K compared with those at 100 K. The inorganic dimers are linked by eight hydrogen bonds to four neighboring dimers to establish a checkerboard network layer in the ab plane, with voids between the dimers that accommodate, on both sides, inversion-related organic cation pairs. The organic cations are required by mirror-plane symmetry to be disordered in (I) and (II). The organic cations and [CuCl(3)(H(2)O)](-) complexes are nearly coplanar and tilted out of the layer plane to establish a hybrid organic-inorganic layer structure parallel to (202) [(11-2) in (III)], with hydrate columns (defined by water molecules) and hydrophobic columns (defined by methyl groups) parallel to each other [and along the 2(1) axes in (I) and (II)]. In 1,1-dimethylpiperidinium aquatrichloridocuprate(II), (C(7)H(16)N)[CuCl(3)(H(2)O)], (IV), the bulkier organic cation prevents semicoordinate bonding between complexes, which are hydrogen bonded side-to-side in zigzag chains that place water molecules in columns along half of the 2(1) axes.

The nature of the complex formed between pyridine and hydrogen bromide in the gas phase: An experimental approach using rotational spectroscopy

The rotational spectra of two isotopomers, C(5)H(5)N...H(79)Br and C(5)H(5)N...H(81)Br, of a complex formed by pyridine and hydrogen bromide were observed by using a pulsed-jet, Fourier-transform microwave spectrometer that incorporated a mixing nozzle. Rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants for the (14)N and Br nuclei were determined. The rotational constants indicate that the observed complex has a planar geometry of C(2v) symmetry, with the HBr subunit lying along the C(2) axis of pyridine. These conclusions are in good agreement with those obtained earlier from infrared spectroscopy in cryogenic matrices and ab initio calculations. The distance r(N...H)=1.137(2) A was obtained by fitting the rotational constants under the assumption of an unchanged pyridine geometry and with r(H-Br) fixed at the value obtained through an ab initio calculation. The nuclear quadrupole coupling constants were interpreted, with the aid of ab initio calculations for the complex, as well as hydrogen-bonded and ion-pair limiting models for the interaction of pyridine and HBr, to establish that the complex may be visualized in terms of a substantial contribution (>60%) of the ionic structure C(5)H(5)NH(+)...Br(-) to its valence-bond description. The intermolecular force constant k(sigma)=57.8(16) N m(-1) determined from the centrifugal distortion constants Delta(J) of the two isotopomers is also consistent with a strongly bound complex involving partial proton transfer.