Venkat R. Thalladi

The melting point alternation in a,w-alkanediols and a,w-alkanediamines: interplay between hydrogen bonding and hydrophobic interactions.

The structural details of a,w-alkanediols and a,w-alkanediamines which are characterized by H bonding and hydrophobic interactions. These systems represent a balance between structural simplicity and interaction complexity, which makes the study of interference between interactions a feasible exercise. The anal. of the crystal structures of diols and diamines revealed the interplay between 2 important intermol. interactions that are possible in these amphiphilic compds., namely, H bonding and hydrophobic interactions. While H bonding and hydrophobic interactions operate in consonance in even members, and therefore culminate in dense packing, they run into geometrical conflicts in odd members, leading to looser packing. Accordingly, the odd members of the diol and diamine series have relatively lower densities and m.ps. than the even members.

Octupolar versus dipolar crystalline structures for nonlinear optics: a dual crystal and propagative engineering approach

A new type of crystalline structure for nonlinear optics whereby octupolar symmetry features are displayed at both molecular and crystalline levels is exemplified by the prototype 2,4,6-triaryloxy-1,3,5-triazine (TPOT) crystal and analyzed in terms of both individual molecular responses and crystal packing features. Polarized harmonic light scattering permits the full determination of the molecular β hyperpolarizability tensor and confirms the octupolar trigonal symmetry of the TPOT molecule. An oriented gas model is used to infer therefrom an estimate of the crystalline nonlinear d tensor which is predicted to be of the same order as that of the reference dipolar N-4-nitrophenyl-(L)-prolinol crystal. The concept of optimal packing toward quadratic nonlinear optics, which had been initially introduced in the realm of quasi-one-dimensional structures, is revisited and enlarged to encompass more isotropic uniaxial structures potentially amenable, in the case of octupoles, to larger optimal values than in th...

C–H group acidity and the nature of C–H···N interactions: crystal structural analysis of pyrazine and methyl substituted pyrazines

Crystal structures of methyl-, 2,3-, 2,5- and 2,6-dimethyl-, and trimethylpyrazine (2–6) have been determined by X-ray diffraction analysis. Four of these compounds (2–4 and 6) are liquids and their single crystals have been grown in situ on the diffractometer using a miniature zone melting procedure. These structures are analysed together with those of pyrazine (1) and tetramethylpyrazine (7) in the context of C–H···N interactions. Compounds 1–7 were chosen because they contain only C, H and N atoms, and also C–H groups of variable acidity. This facilitates the comparison of C–H···N geometries with respect to the C–H group acidity. Compounds 1 and 2 are dominated by sp2 C–H groups in their molecules and their supramolecular structures are generated by sp2 C–H···N. On the other hand the domination of sp3 C–H groups at the molecular level in 6 and 7 results in crystal structures that are governed by sp3 C–H···N. A balance of sp2 and sp3 C–H groups in the dimethyl derivatives 3–5 leads to a situation where both kinds of C–H groups play a structure directing role. While the sp2 C–H groups form C–H···N, the sp3 C–H groups are involved in C–H···π interactions. Thus the C–H group interactions follow a property unique to hydrogen bonds—stronger donors interact with stronger acceptors while weaker donors approaching weaker acceptors. Inter-structural comparison revealed that H···N distances decrease with increasing C–H acidity and therefore C–H···N interactions could be considered as weak hydrogen bonds.

Role of weak hydrogen bonds in the crystal structures of phenazine, 5,10-dihydrophenazine and their 1:1 and 3:1 molecular complexes.

Phenazine (1) and 5,10-dihydrophenazine (2) form not only a 1:1 complex (3), as expected from N–H···N hydrogen bond requirements, but surprisingly also a 3:1 complex (4). Single crystal X-ray analyses of compound 2 and complexes 3 and 4 have been carried out. While the overall packing in complex 3 is as efficient as in pure 1 and 2, acidic C–H groups, which contribute actively to the structural assembly in pure 2, are not involved in hydrogen bonding in 3. It is shown that the quest for hydrogen bonding by these C–H groups steers the formation of complex 4 with an unprecedented N–H···N and C–H···N hydrogen bond mediated tape structure.

A comparative study of the crystal structures of tetrahalogenated hydroquinones and γ-hydroquinone

gamma-Hydroquinone (1) and its tetrafluoro, tetrachloro and tetrabromo derivatives (2), (3) and (4) adopt crystal structures that have an almost invariant system of O-H.O hydrogen bonds. However, within this O-H.O framework, the four structures display variations that are characteristic of the C-H, C-F, C-Cl and C-Br groups. In the parent compound (1) aromatic rings are packed with a herringbone geometry, whilst in the halogenated derivatives (3) and (4), polarization-type halogen.halogen contacts are optimized. The fluoro derivative (2) is exceptional in that neither of the above possibilities is adopted, even though the O-H.O scaffolding does not per se prohibit either of them geometrically.

Why is the melting point of propane the lowest among n-alkanes?

Ethane, propane and n-butane form layer structures in the solid state. By describing their molecular geometries as a rectangle, an irregular pentagon and an irregular hexagon, respectively, it is shown here that propane cannot close-pack like its congeners. The melting point of propane is therefore lower than expected in the series.

Hexagonal supramolecular networks in the crystal structure of the 1:1 molecular complex trimethylisocyanurate–1,3,5-trinitrobenzene

Trimethylisocyanurate 2 and 1,3,5-trinitrobenzene 3 form a hexagonal C–H ⋯ O mediated 1:1 complex wherein distinct molecular layers are formed, the molecular symmetry of the components being retained in the crystal.

Supramolecular synthons in crystal engineering. Structure simplification, synthon robustness and supramolecular retrosynthesis

The analogy between crystal engineering and traditional organic synthesis is outlined with reference to a family of crystal structures which incorporate iodo⋯nitro and carboxyl dimmer supramolecular synthons.

Crystal Structures and Packing of 2,4,6-tris(4-Fluorophenoxy)-1,3,5-triazine and 2,4,6-tris(3,4-Dimethylphenoxy)-1,3,5-triazine. New Materials for Octupolar Nonlinear Optics

The crystal structures and packing of 2,4,6-tris(4-fluorophenoxy)-1,3,5-triazine and 2,4,6-tris(3,4-dimethylphenoxy)-1,3,5-triazine are discussed. These structures have been determined as a continuation of a series of octupolar NLO materials we have been investigating. The crystal structures are characterized by C–H...F and C–H...π hydrogen bonds, respectively. A characteristic of these triazine structures is the presence of dimeric Piedfort Units (PU) that are extended into more elaborate two-dimensional (2-D) networks. The structure of the fluoro derivative is compared with that of the corresponding unsubstituted and chloro/bromo-substituted derivatives. The structure of the dimethyl triazine is compared with that of the corresponding 4-methyl derivative. The noncentrosymmetric nature of the dimethyl derivative was confirmed by a powder SHG signal at 1.064 μm of the order of ∼0.5 × KDP. Interestingly, the dimethyl derivative studied here is isostructural with the corresponding 4-methyl triazine. This H/Me isostructurality is shown to be an uncommon phenomenon by an analysis with the CSD.

Steering non-centrosymmetry into the third dimension: crystal engineering of an octupolar nonlinear optical crystal

The ability of CH3 groups to form helical chains of C–H‥π interactions with phenyl rings leads to polar stacking of trigonal octupolar networks in a substituted triazine, and therefore to three-dimensional non-centrosymmetry.