Melting Point Alternation Research Articles

On the Question of the Relationship between the Rotational Motion of Molecules and the Alternation of Properties in Homological Series

In the paper the emphasis is placed on attempt to describe the effect of alternation of melting points in homologous series using the example of melting points of alkanes of normal structure from the frame of reference of the concept of rotational motion of molecules in a liquid. In the existing literature, the presence of separate sequences for the melting points of even and odd homologues is usually explained by packing effects, in which, due to different symmetry, even and odd representatives crystallize with different densities of the crystal lattice. However, this behavior can also be explained by the difference in the entropies of the rotational motion of molecules in the liquid phase, which is observed due to the different positions of the axis of rotation for even and odd homologues. The theoretically predicted difference in the moments of inertia for molecules with an even and an odd number of carbon atoms in the chain was confirmed by calculating the moments of inertia of n-alkane molecules, for which the atomic coordinates were optimized using density functional theory calculations

Shedding Light on the Interactions of Hydrocarbon Ester Substituents upon Formation of Dimeric Titanium(IV) Triscatecholates in DMSO Solution.

The dissociation of hierarchically formed dimeric triple lithium bridged triscatecholate titanium(IV) helicates with hydrocarbyl esters as side groups is systematically investigated in DMSO. Primary alkyl, alkenyl, alkynyl as well as benzyl esters are studied in order to minimize steric effects close to the helicate core. The 1H NMR dimerization constants for the monomer–dimer equilibrium show some solvent dependent influence of the side chains on the dimer stability. In the dimer, the ability of the hydrocarbyl ester groups to aggregate minimizes their contacts with the solvent molecules. Due to this, most solvophobic alkyl groups show the highest dimerization tendency followed by alkenyls, alkynyls and finally benzyls. Furthermore, trends within the different groups of compounds can be observed. For example, the dimer is destabilized by internal double or triple bonds due to π–π repulsion. A strong indication for solvent supported London dispersion interaction between the ester side groups is found by observation of an even/odd alternation of dimerization constants within the series of n‐alkyls, n‐Ω‐alkenyls or n‐Ω‐alkynyls. This corresponds to the interaction of the parent hydrocarbons, as documented by an even/odd melting point alternation.

From Molecules to Interactions to Crystal Engineering: Mechanical Properties of Organic Solids.

Mechanical properties of organic molecular crystals have been noted and studied over the years but the complexity of the subject and its relationship with diverse fields such as mechanochemistry, phase transformations, polymorphism, and chemical, mechanical, and materials engineering have slowed understanding. Any such understanding also needs conceptual advances-sophisticated instrumentation, computational modeling, and chemical insight-lack of such synergy has surely hindered progress in this important field. This Account describes our efforts at focusing down into this interesting subject from the viewpoint of crystal engineering, which is the synthesis and design of functional molecular solids. Mechanical properties of soft molecular crystals imply molecular movement within the solid; the type of property depends on the likelihood of such movement in relation to the applied stress, including the ability of molecules to restore themselves to their original positions when the stress is removed. Therefore, one is interested in properties such as elasticity, plasticity, and brittleness, which are linked to structural anisotropy and the degree to which a structure veers toward isotropic character. However, these matters are still by no means settled and are system dependent. While elasticity and brittleness are probably displayed by all molecular solids, the window of plasticity is perhaps the one that is most amenable to crystal engineering strategies and methods. In all this, one needs to note that mechanical properties have a kinetic component: a crystal that is elastic under slow stress application may become plastic or brittle if the same stress is applied quickly. In this context, nanoindentation studies have shown themselves to be of invaluable importance in understanding structural anisotropy. Several problems in solid state chemistry, including classical ones, such as the melting point alternation in aliphatic straight chain dicarboxylic acids and hardness modulation in solid solutions, have been understood more clearly with this technique. The way may even be open to picoindentation studies and the observation of molecular level movements. As in all types of crystal engineering, an understanding of the intermolecular interactions can lead to property oriented crystal design, and we present examples where complex properties may be deliberately turned on or off in organic crystals: one essentially fine-tunes the degree of isotropy/anisotropy by modulating interactions such as hydrogen bonding, halogen bonding, π···π interactions, and C-H···π interactions. The field is now wide open as is attested by the activities of several research groups working in the area. It is set to take off into the domains of smart materials, soft crystals, and superelasticity and a full understanding of solid state reactivity.

Odd-Even Effect in the Elastic Modulii of α,ω-Alkanedicarboxylic Acids

Nanoindentation is a probe used to quantitatively assess mechanical behavior of small volume materials. In this technique, load applied vs. the depth of penetration of the indenter into the specimen are measured simultaneously and with high precision and resolution. By analyzing the data, one can obtain the elastic modulus and hardness of crystals. Though this technique has been extensively used to characterize inorganic and engineering materials, we have recently extended its utility to study weak interactions and to establish structure-property relationships in molecular crystals. Being able to assess the relative strength of weak interactions such a technique has become relevant to the subject of crystal engineering which is concerned with the design of molecular solids with desired properties and functions. In our recent studies through nanoindentation on the alkanedicarboxylic acids reveals that the elastic modulus shows similar alternation property as the melting point alternation. Our results are endorsing the strained molecular conformations hypothesis for the melting point alternation of diacids, proposed by Thalladi et al. These results support the development of crystal engineering because nanoindentation may be used as a direct measure of molecular and crystal energies of molecular crystals.

Melting-point alternation in the primary amines

Melting-point alternation in the primary amines

The crystal structures and melting point properties of isonicotinamide cocrystals with alkanediacids HO2C(CH2)n−2CO2H n = 7–9

The tunability of physiochemical properties of crystalline materials through controlled cocrystal formulation is an aim for both the pharmaceutical industry and crystal engineers. In this paper we report that the melting point alternation behaviour of odd and even alkanedicarboxylic acids is mimicked in a set of cocrystals in which isonicotinamide is combined in a 1 : 1 stoichiometry with pimelic acid, suberic acid and azelaic acid. All three structures contain hydrogen bonded chains of alternating acid and amide molecules between which acid–pyridine and acid–amide synthons are formed. Both isonicotinamide : pimelic acid and isonicotinamide : azelaic acid form structures in which the acid moiety adopts a twisted alkyl backbone conformation similar to that observed in the pure odd alkanediacid materials. The isonicotinamide : suberic acid cocrystal differs from the others reported here retaining both the elevated melting point and the planar acid conformation displayed by even alkanediacid materials in this series.

Remarkable reversal of melting point alternation by co-crystallization

The pronounced melting point alternation of α,ω-alkanedicarboxylic acids (HOOC–(CH2)n−2–COOH, n = 4–10) is maintained in co-crystals of the same acids with 1,2-bis(4-pyridyl)ethane (BPA), which contains an even number of carbon atoms in the chain, and is reversed in co-crystals with 1,2-bis(4-pyridyl)propane (BPP), which contains an odd number of carbon atoms in the aliphatic chain.

Inversion of the melting point alternation in n-alkyl carboxylic acids by co-crystallization with pyrazine

Download options Please wait... Supplementary files Crystal structure data CIF (43K) Details of the r.m.s. deviations between the pyrazine positions in the structures of C 7–C 13, and details of the calculation of the van der Waals contribution to the lattice energy for C 7–C 13 PDF (165K) Crystal structure data TXT (43K) Additions and corrections Correction HTM (2K) Article information DOI https://doi.org/10.1039/B517513E Article type Paper Submitted 12 Dec 2005 Accepted 17 Mar 2006 First published 27 Mar 2006 Download Citation CrystEngComm, 2006,8, 333-337 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Inversion of the melting point alternation in n-alkyl carboxylic acids by co-crystallization with pyrazine A. D. Bond, CrystEngComm, 2006, 8, 333 DOI: 10.1039/B517513E To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Andrew D. Bond

On the crystal structures and melting point alternation of the n-alkyl carboxylic acids

The crystal structures of the complete series of n-alkyl carboxylic acids from hexanoic to pentadecanoic acid have been determined following in situ crystallisation. The structures reveal that the melting point alternation across the series is correlated with alternating crystal density. All even acids crystallise from the liquid as the C modification, while the odd acids exhibit a transition from the C′ to the C″ modification between undecanoic and tridecanoic acid. In each structure, molecules form hydrogen-bonded dimers arranged into bilayers, with a rectangular packing arrangement in the plane perpendicular to the dimer long axes. The packing density within bilayers is comparable in each case and the alternating crystal density can be attributed solely to alternating packing density between bilayers. The carboxyl groups are identically disposed in all structures, but adjacent dimers are offset to differing degrees parallel to their long axes to reduce in-plane O⋯O repulsions, giving rise to angles between n-alkyl chains. The extent of the lateral offset diminishes in general across the series as the n-alkyl chains exert an increasing driving force towards parallel alignment. The systematic alternation in the packing density between bilayers arises as a result of different orientations adopted by the terminal C–C bonds with respect to the methyl group interface. Several of the key structural features may be rationalised using a simple two-dimensional model that describes the packing of modified parallelograms and trapezoids, representing the even and odd acids, respectively. Extension of the model into three dimensions permits complete rationalisation of the structures.

Mechanochemical preparation of hydrogen-bonded adducts between the diamine 1,4-diazabicyclo[2.2.2]octane and dicarboxylic acids of variable chain length: an X-ray diffraction and solid-state NMR study.

Mechanical mixing of solid dicarboxylic acids of variable chain length HOOC(CH(2))(n)COOH (n = 1-7) with solid 1,4-diazabicyclo[2.2.2]octane generates the corresponding salts or co-crystals of the formula [N(CH(2)CH(2))(3)N]-H-[OOC(CH(2))(n)COOH] (n=1-7). Preparation of the same systems from solution has been instrumental for a full characterization of the mechanochemical products by means of single-crystal and powder-diffraction X-ray analyses, as well as by solid-state NMR. The acid-base adducts, whether involving proton transfer from the COOH group to the N-acceptor, that is having ((-))O...H-N((+)) interactions, or the formation of neutral O-H...N hydrogen bonds, show a melting point alternation phenomenon analogous to that shown by the neutral carboxylic acids. The carbon chemical shift tensors of the COOH group obtained from the sideband intensity of low speed spinning NMR spectra provide a reliable criterion for assigning the protonation state of the adducts.

Molecular Complexes of Homologous Alkanedicarboxylic Acids with Isonicotinamide: X-ray Crystal Structures, Hydrogen Bond Synthons, and Melting Point Alternation

Crystallization of α,ω-alkanedicarboxylic acids (HOOC−(CH2)n-2−COOH, n = 2−6) with isonicotinamide (IN) is carried out in 1:2 and 1:1 stoichiometry. Five cocrystals of (diacid)·(IN)2 composition (diacid = oxalic acid, malonic acid, succinic acid, glutaric acid, and adipic acid) are characterized by X-ray diffraction at 153(2) K. Tapes of acid−pyridine O−H···N and amide−amide N−H···O hydrogen bond synthons stabilize these five crystal structures as predicted by the hierarchic model: the best donor (COOH) and best acceptor group (pyridine N) hydrogen bond as acid−pyridine and the second best donor−acceptor group (CONH2) aggregates as an amide dimer. Glutaric acid and adipic acid cocrystallize in 1:1 stoichiometry also, (diacid)·(IN), with acid−pyridine and acid−amide hydrogen bonds. Synthon energy calculations (ΔEsynthon, RHF/6-31G**) explain the observed hydrogen bond preferences in 1:2 (five examples) and 1:1 (two examples) cocrystals. The acid−pyridine hydrogen bond is favored over the acid−amide dimer ...

Alternation of Melting Points in Odd- and Even-Numbered Monoacid Triacylglycerols

Alternation of melting points in the series of odd- and even-numbered monoacid triacylglycerols has been known for a long time, but hitherto little experimental evidence has been available to support existing theories. In this paper the melting point alternation of β-CnCnCn-type triacylglycerols is discussed, based on the crystal structures of β-CnCnCn (n = number of C atoms per hydrocarbon chain = 10, 12, 14, 16, 18 and n = 13, described in this paper). The crystal structure of β-1,2,3-tris(tridecanoyl)glycerol (β-C13C13C13) has been determined from high-resolution synchrotron X-ray powder diffraction data and is presented here. Grid search and Rietveld refinement have been used to determine and refine the structure, respectively. Like the even-numbered monoacid triacylglycerols, β-C13C13C13 is crystallized in an asymmetric tuning-fork conformation and its acyl chains are laterally packed, resulting in a layered structure. Within a molecular layer, the odd- and even-numbered monoacid triacylglycerols are identically packed, but the packing of adjacent molecular layers is different, clarifying the alternation of melting points.

The Melting Point Alternation in alpha,omega-Alkanediols and alpha,omega-Alkanediamines: Interplay between Hydrogen Bonding and Hydrophobic Interactions The Melting Point Alternation in n-Alkanes and Derivatives, Part 2. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. V.R.T. thanks the Alexander von Humboldt Foundation for a postdoctoral fellowship. Part 1: ref. 6.

Efficient hydrophobic packing, driven by molecular offset within the hydrogen-bonded layers of α,ω-alkanediols and diamines, is allowed by the geometry of the even-numbered members (left in the picture). In contrast, the geometry of the odd-numbered members (right in the picture) forbids such packing and leads to the observed density and melting point alternation.

The Melting Point Alternation in α,ω-Alkanedithiols

α,ω-Alkanedithiols show an alternation in their melting points with even members exhibiting systematically higher melting points than odd members. In this study, single crystals of the first nine members of α,ω-alkanedithiols (HS−(CH2)n−SH, n = 2−10) have been grown in situ using a miniature zone melting procedure, and their X-ray analyses have been performed. The structural similarities and differences between even and odd members have been analyzed based on the packing arrangements of hydrocarbon chains and SH functionalities. Whereas the hydrocarbon chains maintain an intergrooving pattern, the SH groups associate in an antiparallel fashion in both the members. The SH groups do not form hydrogen bonds. In the even members the molecules are offset along their length, whereas such an offset is not observed in the odd members. The densities of even members are systematically higher than those of odd members and show an alternating trend. Using a simple geometrical model, describing even and odd members as modified parallelograms and trapezoids, respectively, the density features of the packing patterns in both the members are illustrated. The melting point alternation in dithiols is attributed to the geometry allowed or forbidden molecular offset toward a higher packing density.

Crystal Packing of Odd-Chain Saturated Triglycerides

Single crystal electron diffraction data from an odd-chain triglyceride, glycerol triheptadecanoate, are compared with the known structure of the even-chain homo-acid triglycerides. Both class­es of triglycerides are found to resemble one another in chain packing: they both have the same chain tilt (61 °) for the β-polymorph and, further, the orientation of the T∥ subcell in the crystal structure is identical. The existence of the T∥ subcell is quantitatively verified for the first time with the diffraction intensities from suitably tilted crystals. Given these similarities in β-poly­morph structure, the major difference between triglycerides with even or odd chains (indicated by published long spacing and melting point alternation) must be due to the methyl end group pack­ing - i.e., the end group packing volume for the odd-chain glycerides must be larger than found for the even-chain materials.

Analysis of Local Free Volume in Lamellar Crystals: An Aid for Understanding Radical Mobility in Solids

Abstract In crystals with two short axes it is easy to partition free volume into local components by plotting a profile of the free areas of planes parallel to the plane defined by the short axes. Local free volume is used to analyze radical motion in bis-undecanoyl peroxide. It is relevant to melting-point alternation in long-chain compounds and to packing in a lipid bilayer.

Alternating melting points, heats and entropies of fusion of linear aliphatic polyesters

Heats of fusion of polyethylene-adipate, pimelate, suberate and azelaate have been determined by two methods, viz. DTA and the pressure dependence of the melting point up to 6000 bar. Degrees of crystallization were measured dilatometrically. Entropies of fusion were divided into volume and conformational entropy terms, only the latter alternates. Alternation of melting points depends on enthalpy of fusion ΔH m , entropy of fusion ΔS m and volume change Δυ on melting; influence of the functions increases thus ΔV m < ΔH m < ΔS m and ΔS m is dominant. Entropy and heat of fusion alternation is explained by the conformational change on melting governed by the driving force of maximum H-bridge formation.

Lipid structures

AbstractMuch of the function of lipids, as in shortenings, confections, biological membranes, etc., depends on the structures they exhibit, i.e., their crystalline and mesomorphic states. Triglycerides may serve as a point of departure for considering the crystal state of lipids. The three principle cross‐sectional arrangements of long chains‐α, β′ and β‐in which there is kinship to hydrocarbons, offer a major basis for classification. Different crystal forms are rheologically different. Complications in the chain length direction lead to stepped configurations in the methyl planes and to double and triple chain length structures. A detailed consideration of the time‐honored melting point alternation of homologs sheds light on the lipid crystal picture. A startling feature is the catalysis of isomerization by the solid state of diglycerides, with a consequent shift in equilibrium far toward 100% 1,3‐diglyceride. Stepwise reversible transformations, as in hydrocarbons, are documented for high molecular weight 1‐monoglycerides. A common feature, the sharp drop with mixture of stable form melting points, to “expose” metastable forms as stable forms for intermediate compositions is strikingly exhibited by fatty alcohols. The two bases for mesomorphism, or liquid crystal formation, among lipids are discussed: (1) dichotomy between molecular moieties in degree of polarity, as in phospholipids; and (2) dichotomy in molecular geometry as in cholesteryl esters. The former type is associated with three main basic arrangements‐smectic or lamellar, middle or hexagonal (normal and reversed), and viscous isotropic or cubic (normal and reversed). The latter is associated with two main arrangements‐cholesteric (essentially limited to cholesteryl compounds) and smectic. The importance of mesomorphism in consideration of membranes and emulsions is emphasized. Developments in understanding the structures of interfacial lipid states (including monomolecular surface films) and the correspondence of interfacial with bulk states is a matter of great continuing interest.

Multimacrocyclic compounds. Part I. Novel triply bridged dibenzenoid cage compounds

Internal cyclotrimerisation of aromatic triynes on a Ziegler catalyst has given a series of compounds containing two aromatic rings joined by three polymethylene chains. Melting point alternation and spectral properties are similar to those of paracyclophanes.

Alternation of melting points in homologous series of long‐chain compounds

AbstractThe different possibilities of stacking of layers in long‐chain compounds have been analyzed with reference to structural differences between even and odd members of homologous series. It has been possible to correlate packing differences with alternation of melting points and explain the alternation behavior completely on this basis. The theory can also be applied to surface films and mesomorphous phases. Alternation of en‐antiotropic transition temperatures and other physical properties are also discussed.