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

Abstract

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, C 26 H 54 ; (S)-9-methylheptacosane and (S)-11-methylheptacosane, C 28 H 58 ; (S)-7-methylnonacosane, (S)-9-methylnonacosane, (S)-11-methylnonacosane and (S)-13-methylnonacosane, C 30 H 62 ; and (S)-9-methylhentriacontane, C 32 H 66. All crystallize in space group P2 1 . 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.

Coping with the climate: Cuticular hydrocarbon acclimation of ants under constant and fluctuating conditions

“…Hydrocarbons with strong van der Waals bonds (e.g. n-alkanes, monomethyl alkanes) can even crystallize (Brooks et al, 2015). Thus, n-alkanes should provide the best waterproofing to the insect, especially if they have high chain lengths.…”

How do cuticular hydrocarbons evolve? Physiological constraints and climatic and biotic selection pressures act on a complex functional trait

“…At low temperatures, hydrocarbons with strong van der Waals bonds (e.g. n-alkanes and monomethyl alkanes) can crystallize [31,32]. This enhances waterproofing, but also increases the viscosity of the CHC layer, which is undesirable for substance diffusion and inter-individual CHC exchange and might therefore limit their function as a communication signal.…”

Cuticular Hydrocarbon Trails Released by Host Larvae Lose their Kairomonal Activity for Parasitoids by Solidification

“…Since H. sylvanidis females follow CHC trails of T. confusum host larvae by zig-zag movements along the trail, they most probably can directly contact the trail components; however, perception over a short distance cannot be excluded. Due to their melting behavior, some mono-and dimethylbranched CHCs are expected to be liquid in freshly laid larval trails at ambient temperatures, whereas other monomethyl-branched alkanes might begin to solidify right after trail deposition (Brooks et al 2015;Gibbs and Pomonis 1995). The liquid aggregate state of methyl-branched alkanes might be relevant for eliciting trail-following behavior in H. sylvanidis.…”