Crystal Structures of n-Alkanes with Branches of Different Size in the Middle

Wax Formation in Linear and Branched Alkanes with Dissipative Particle Dynamics

We present a dissipative particle dynamics (DPD) model for wax formation (i.e., the freezing transition) in linear and branched alkanes at room temperature (298 K) and atmospheric pressure. We parametrize the model using pure liquid phase densities and the onset of wax formation as a function of alkyl chain length. Significant emphasis is placed on building an accurate representation of the underlying molecular architecture by careful consideration of bond lengths and angles, aided by distributions obtained from molecular dynamics simulation. Using the derived model, we observe wax formation in n-alkanes when the alkyl chain length is greater than 18 (n-octadecane), in excellent agreement with experimental observations. Further, we reproduce the behavior of branched alkanes and mixtures including solubilities of heavy alkanes in light alkane solvents.

Raman Spectroscopy Study of Structurally Uniform Hydrogenated Oligomers of α-Olefins

The expansion of the range of physico-chemical methods in the study of industrially significant α-olefin oligomers and polymers is of particular interest. In our article, we present a comparative Raman study of structurally uniform hydrogenated dimers, trimers, tetramers, and pentamers of 1-hexene and 1-octene, that are attractive as bases for freeze-resistant engine oils and lubricants. We found out that the joint monitoring of the disorder longitudinal acoustic mode (D-LAM) and symmetric C–C stretching modes allows the quantitative characterization of the number and length of alkyl chains (i.e., two structural characteristics), upon which the pour point and viscosity of the hydrocarbons depend, and to distinguish these compounds from both each other and linear alkanes. We demonstrated that the ratio of the contents of CH₂ and CH₃ groups in these hydrocarbons can be determined by using the intensities of the bands in the spectra, related to the asymmetric stretching vibrations of these groups. The density functional theory (DFT) calculations were applied to reveal the relations between the wavenumber and bandshape of the symmetric C–C stretching mode and a conformation arrangement of the 1-hexene and 1-octene dimers. We found that the branched double-chain conformation results in the splitting of the C–C mode into two components with the wavenumbers, which can be used as a measure of the length of branches. This conformation is preferable to the extended-chain conformation for hydrogenated 1-hexene and 1-octene dimers.

Crystal structures of eight mono-methyl alkanes (C26-C32) via single-crystal and powder diffraction and DFT-D optimization

The crystal structures of eight mono-methyl alkanes have been determined from single-crystal or high-resolution powder X-ray diffraction using synchrotron radiation. Mono-methyl alkanes can be found on the cuticles of insects and are believed to act as recognition pheromones in some social species, e.g. ants, wasps etc. The molecules were synthesized as pure S enantiomers and are (S)-9-methylpentacosane, C26H54; (S)-9-methylheptacosane and (S)-11-methylheptacosane, C28H58; (S)-7-methylnonacosane, (S)-9-methylnonacosane, (S)-11-methylnonacosane and (S)-13-methylnonacosane, C30H62; and (S)-9-methylhentriacontane, C32H66. All crystallize in space group P21. Depending on the position of the methyl group on the carbon chain, two packing schemes are observed, in which the molecules pack together hexagonally as linear rods with terminal and side methyl groups clustering to form distinct motifs. Carbon-chain torsion angles deviate by less than 10° from the fully extended conformation, but with one packing form showing greater curvature than the other near the position of the methyl side group. The crystal structures are optimized by dispersion-corrected DFT calculations, because of the difficulties in refining accurate structural parameters from powder diffraction data from relatively poorly crystalline materials.

Crystal structures of n-alkane with three functional groups in the middle and at both ends

We synthesized a linear alkane, K35DA, with a main-chain carbon number n = 33 and three functional groups, a carbonyl group in the middle and carboxyl groups at both ends, and studied influences of the functional groups as well as chain length on morphologies of samples prepared by solution-grown and bulk crystallization methods (SG-K35DA and BK-K35DA) from differential scanning calorimetry (DSC), X-ray diffraction, and IR absorption measurements. Data analyses reveal that at room temperature an orthorhombic crystal of type P2(1)2(1)2(1), together with a considerable amount of amorphous fraction, is predominantly realized in BK-K35DA due to the van der Waals force between neighboring long methylene sequences, whereas a monoclinic type of crystal belonging to the same space group (P2(1)/c) as reported for linear dicarboxylic acid crystals with odd carbon numbers is coexistent for SG-K35DA. The crystalline structures appear to be distorted with increasing temperature, as the dipole-dipole interaction between the carbonyl groups tends to be weakened, and both orthorhombic and monoclinic crystals undergo the solid-solid phase transition to the hexagonal crystalline structure at a temperature about 10 K below their respective T(m)s, which can be regarded as a new example of the Brill transition.