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

Abstract

We synthesized four branched n-alkane samples C35-C1, C35-C4, C35-C6, and C35-C4Ph with the same number of carbons as the main chain, n = 35, to which the methyl, butyl, hexyl, and butyl phenyl groups were respectively attached at the middle, and also the corresponding linear homologue of C35, and studied their crystalline structures from DSC, IR, and Raman spectroscopy, X-ray diffraction measurement, and computer simulation. Solid-solid phase transitions characteristic of linear alkane C35 are not observed for any branched alkanes, and their melting temperatures Tm are lowered to 325.2, 318.5, 314.3, and 314.1K, respectively. Main chains of branched alkane molecules are not folded, irrespective of length and chemical structure of branches, but are extended to take the planar zigzag form in the solid state. The branches of C35-C4 and C35-C6 are also aligned inside the crystal in the extended form. Data analyses on solution-grown crystallized samples reveal that, with increasing the branch length, their crystal structures transform from polymorphic forms of the orthorhombic (P2(1)2(1)2(1)) and the triclinic (P) for C35-C1 and C35-C4 to the unique triclinic form for C35-C6 and C35-C4Ph, so as to minimize extra surface energy invoked by introduction of long branches.

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

“…The Raman-active vibrations, whose frequencies depend on a chain length of alkanes and related substances (fatty acids, alkenes, alkylammonium compounds, bromoalkanes, etc. ), belong to the longitudinal acoustic mode (LAM), disorder LAM (D-LAM), and symmetric C–C stretching mode [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ].…”

“…The LAM is well-known marker to evaluate a length of trans and quasi - trans segments in both linear and branched alkanes [ 8 , 9 , 10 , 11 ]. For example, Raman spectroscopic study in the LAM spectral region proved that molecules in the solid-state center-branched alkanes (C 17 H 35 ) 2 CHCH 3 and (C 19 H 39 ) 2 CHCH 3 are not folded and are crystallized as extended chains with 35 and 39 carbon atoms [ 9 , 10 ].…”

“…The LAM is well-known marker to evaluate a length of trans and quasi - trans segments in both linear and branched alkanes [ 8 , 9 , 10 , 11 ]. For example, Raman spectroscopic study in the LAM spectral region proved that molecules in the solid-state center-branched alkanes (C 17 H 35 ) 2 CHCH 3 and (C 19 H 39 ) 2 CHCH 3 are not folded and are crystallized as extended chains with 35 and 39 carbon atoms [ 9 , 10 ]. Experimental studies and ab initio calculations demonstrated that formulae for determining the chain length by analysis of the LAM in Raman spectra of branched and linear alkanes are the same, because of a minor effect of side alkyl chains on the LAM frequency [ 9 , 10 , 11 , 12 ].…”

“…For example, Raman spectroscopic study in the LAM spectral region proved that molecules in the solid-state center-branched alkanes (C 17 H 35 ) 2 CHCH 3 and (C 19 H 39 ) 2 CHCH 3 are not folded and are crystallized as extended chains with 35 and 39 carbon atoms [ 9 , 10 ]. Experimental studies and ab initio calculations demonstrated that formulae for determining the chain length by analysis of the LAM in Raman spectra of branched and linear alkanes are the same, because of a minor effect of side alkyl chains on the LAM frequency [ 9 , 10 , 11 , 12 ]. On the other hand, the LAM frequency in substituted alkanes like fatty acids, alkylamines, and bromoalkanes is affected by a mass of a terminal group in a hydrocarbon chain [ 13 , 14 ].…”

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

“…The crystal structures of methyl-branched alkanes have received less attention and knowledge of their crystal structures, molecular packing and preferred conformations is unclear. The symmetric molecules 20-methylnonatriacontane (C 19 H 39 ) 2 CH(CH 3 ) and 18-methylpentatriacontane (C 17 H 35 ) 2 -CH(CH 3 ), which have the methyl group attached to the central C atom of the main chain, were described with an extended chain conformation (Yamamoto et al, 2004;Ikedou et al, 2005), whereas melt-or solution-crystallization of the much longer C 96 H 193 CH(CH 3 )C 94 H 189 produced a once-folded conformation (Ungar & Zeng, 2001). The influence of the side-chain position on the packing and crystal structures of methyl alkanes has not been extensively studied.…”