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TheC–F group, which is found in a large number of smallorganic molecules and drugs available in the market, has still notbeen fully understood in terms of the strength and directionalityof the interactions offered by this group in guiding the formationof crystal lattices. In this manuscript, we have tried to understandthe role played by the C–F group, using a model system of N-benzylideneanilines, on which we have previously donea systematic study with fluorine as a substituent on both rings. Theeffect on the packing of these molecules by replacing one of the fluorineatoms by either Cl or Br has been comprehended in this manuscript.It was observed that the features of the difluorinated analogues remainedintact when the noninteracting fluorine atom was replaced by Cl orBr, while with the replacement of the interacting fluorine by Cl orBr, completely different packing characteristics were found to bedeveloped. To quantify the strength of the interactions offered by“organic fluorine”, stabilization energies of the dimers(which has been found to interact through the C–H···Fhydrogen bond) have been calculated by Gaussian 09 at the MP2 levelusing a 6-31+G* basis set. These values were found to be between −0.3and −6.0 kcal/mol. To study the topological properties of theinteracting molecular pair, AIM calculations have also been done usingAIM2000. In the studied dimers, the existence of bond critical points (BCPs)at the C–H···F hydrogen bond have always beenseen and Laplacian at those BCPs has also been found to be positive,which is clearly an indication of a closed shell type of interactionbetween the C–H and F–C groups. [ABSTRACT FROM AUTHOR]

The azobenzenes, known for theirvarious importance in the industry,have been chosen as model compounds to understand the role of weakinteractions involving the C–X (X = F, Cl, and Br) bond usingsingle-crystal X-ray diffraction technique, especially in the absenceof other stronger intermolecular forces such as hydrogen bonds. Thefluorinated compounds have been found to pack in the lattice by utilizingC–H···F hydrogen bonds, whereas the chlorinatedand brominated analogues have been found to prefer C–X···X–C,C–X···π, and π···πinteractions while packing in the lattice. The stabilization energyoffered by the C–H···F hydrogen bonds and theC–X···X–C interactions have been estimatedby computational methods using Gaussian 09, and the topological propertieshave been determined by using the AIM2000 package. The lattice energydecomposition has been done using semiclassical density sums (SCDS)PIXEL method. Our studies indicate that the stabilization energy offeredby each C–H···F hydrogen bond lies in the rangefrom −0.8 to −1.0 kcal/mol, while that for the C–X···X–Cinteraction has been found to be −0.35 kcal/mol for the X =Cl interaction and −0.73 kcal/mol for the X = Br interaction.Further, the analysis of these interaction by atoms in molecules (AIM)theory indicates that the electron densities (ρc)at the bond critical points (BCP) for C–H···Fand C–X···X–C (X = Cl and Br), calculatedusing the AIM2000 package, are small (<0.007 eÅ–3), and the values of Laplacian (∇2ρc) are positive. This indicates that these interactions are of thehydrogen bond type. A detailed study of these interactions by experimentaland computational methods has been described in the manuscript. [ABSTRACT FROM AUTHOR]

Thecrystal structures of anhydrous enrofloxacin (EFC) (1), enrofloxacin hexahydrate (EFC·6H2O) (2), enrofloxacin maleate (EFC-M) (3), enrofloxacin hemifumarate(EFC-F) (4), enrofloxacin hemisuccinate (EFC-S) (5), enrofloxacin hemioxalate (EFC-O) (6), enrofloxacinacetate (EFC-A) (7) and ammonium salt of enrofloxacin(Am-EFC) (8) have been determined by using single crystalX-ray diffraction (SCXRD). The crystals of 1were grownfrom a dilute solution of EFC in dilute ammonium hydroxide. The crystalsof 2were grown from a solution of EFC with nicotinicacid. Solvent drop grinding experiments on enrofloxacin with variousdicarboxylic acids resulted in four new salts (3–6). When a solution of EFC in glacial acetic acid evaporatedto dryness over a period of time, crystals of 7weregrown. Crystals of 8were found to grow when EFC wasdissolved in liquid ammonia and solution was slowly evaporated atroom temperature (RT). All these products (1–8) were characterized by 1H and 13CNMR spectroscopy, Fourier transform infrared (FTIR) spectroscopy,differential scanning calorimetry (DSC) and powder X-ray diffraction(PXRD). Single crystal X-ray diffraction study has indicated thatthe products 3–6had a cation ofenrofloxacin and an anion of the acid in common. Four of these eightproducts have been found to contain water of crystallization. Threeof these salts were found to have one neutral acid molecule. The protontransfer and hydrate stoichiometry were confirmed from single crystalX-ray diffraction. Solubility of 1and 3–6was determined in water (pH = 6.8) using UV–visspectroscopy at RT. The alteration between the neutral, zwitterionic,anionic and cationic forms EFC has led to very interesting structuresof these salt/cocrystals and also the pure forms (1and 2). [ABSTRACT FROM AUTHOR]