In the title compound, C(16)H(21)N(3)O(3), the piperazine ring adopts a chair conformation, with its N-C bonds in pseudo-equatorial orientations. In the crystal, mol-ecules are linked by O-H⋯N hydrogen bonds, generating C(5) chains propagating in [101]. Weak aromatic π-π stacking inter-actions also occur [centroid-centroid separation = 3.899 (1) Å].

The title compound, C(16)H(19)NO(5), which was synthesized from p-meth-oxy-cinnamic acid, has intra-molecular O-H⋯O and N-H⋯O hydrogen-bonding inter-actions. In the crystal, mol-ecules are linked by weak C-H⋯O hydrogen bonds and aromatic π-π stacking inter-actions [minimum ring centroid-centroid separation = 3.790 (1) Å].

The application of the weak-beam method of electron microscopy to the measurement of the separation of partial dislocations 2 2.0 nrn is discussed. The accuracy of the experimental results is analysed, and approximations in obtaining values of the stacking-fault energy, y, are outlined.

In the mol-ecule of the title compound, [Ni(C(5)H(4)NO(3)S)(2)(H(2)O)(4)], the Ni(II) cation is located on an inversion center and is coordinated by four water mol-ecules and two pyridine-3-sulfonate anions with an NiN(2)O(4) distorted octa-hedral geometry. The face-to-face separation of 3.561 (5) Å between parallel pyridine rings indicates the existence of weak π-π stacking between the pyridin...

In the title compound, C18H15NO2, the dihedral angle between the mean planes of the quinoline ring system and the phenyl ring is 78.8 (1)°. The mean plane of the carboxyl-ate group is twisted from the mean planes of the quinoline ring system and phenyl ring by 1.5 (9) and 77.6 (4)°, respectively. In the crystal, mol-ecules are linked by weak C-H⋯O inter-actions, generating C(8) chains along [00...

In the racemic title compound, C21H18O4, the chromone ring is essentially planar [maximum deviation from the least-squares plane = 0.026 (3) Å], with a dihedral angle of 78.18 (12)° between the benzene rings of the chromanone and chromenone moieties. In the crystal, there are weak π-π stacking inter-actions [minimum ring centroid separation = 3.9286 (17) Å].

In the crystal structure of the title compound, C(8)H(5)NO(6), both carboxyl groups are involved in inter-molecular centrosymmetric cyclic O-H⋯O hydrogen-bonding associations, which give a zigzag chain structure extending along (2[Formula: see text]1). Weak π-π stacking inter-actions are also present [minimum ring centroid separation = 3.893 (4) Å].

The title compound, C20H10Cl2N2, has crystallographic twofold rotational symmetry [maximum deviation from the least-squares plane = 0.038 (1) Å]. In the crystal, weak π-π ring stacking inter-actions occur down the a-axis direction [minimum centroid-centroid separation = 3.7163 (8) Å].

In the title chalcone derivative, C(15)H(8)Cl(4)O, the C=C double bond exists in an E configuration and the dihedral angle between the two benzene rings is 48.13 (11)°. In the crystal, mol-ecules are arranged into columns and stacked down the a axis featuring possible weak aromatic π-π stacking inter-actions [centroid-centroid separation = 3.888 (2) Å].

Several features of subfitness are analyzed. Isbell’s Spatiality Theorem leads to the concept of T1-spatiality which is compared with the TD-spatiality. Subfitness is put into relation with other weak separation axioms (and with the existence of nearness), both in spaces and locales, and seeming discrepancies are explained. A new characteristic of subfitness by means of codensity is presented.