Stretching and folding of 2-nanometer hydrocarbon rods

Abstract

Linear alkanes CnH2n+2 in vacuum isolation are finite models for an infinite polyethylene chain. Using spontaneous Raman scattering in supersonic jet expansions for n = 13-21 in different spectral ranges, we determine the minimal chain length nh for the cohesion-driven folding of the preferred extended all-trans conformation into a hairpin structure. We treat fully stretched all-trans alkanes as molecular "nanorods" and derive Young's modulus E for the stretching of an isolated single-strand polyethylene fibre by extrapolating the longitudinal acoustic mode to infinite chain length. Two key quality parameters for accurate intra- and intermolecular force fields of hydrocarbons (nh = 18 ± 1, E = 305 ± 5 GPa) are thus derived with high accuracy from experimental spectroscopy.

Polymer Segments at the Folding Limit: Raman Scattering for the Diglyme Benchmark

“…Although the full vibrational Raman spectrum including the complex CH stretching manifold was recorded (Figure ), the focus will be on the conformation‐sensitive and low‐anharmonicity backbone stretching or longitudinal acoustic modes . Figure shows a helium expansion spectrum of diglyme in the LAM region.…”

“…The n ‐alkane folding case worked within certain error bars, although the barriers are at the upper limit compatible with this method. Chains up to 17±1 carbon atoms were shown to prefer the stretched state over a hairpin fold . If one of the terminal hydrogens is replaced by a phenyl ring, the folding is already observed for chains as short as 8 carbon atoms …”

Benchmarking Quantum Chemical Methods: Are We Heading in the Right Direction?

“…Different variants of CCSD(T) calculations also agree with experiment86, 87, 88 but now for much better reasons. It is actually quite possible that these calculations are more accurate than the experiment, which retains an estimated error bar of ±1 chain segments 89. This remains a possibility despite multiple spectroscopic evidence and the ZPVE bottleneck in the calculations.…”

Quantenchemische Methoden im Leistungsvergleich: Stimmt die Richtung noch?

“…[85] Ein relativ neues Beispiel dafüri st der Wechsel von gestreckten linearen Alkanen zu dispersionsgetriebenen Haarnadelkonformationen als Funktion der Kettenlänge.D ie MM2-Kraftfeld-Vorhersage liegt innerhalb des experimentellen Fehlerbalkens, [86] aber nur weil dieses Kraftfeld die konformative Faltungsenergie und die Dispersionswechselwirkung der Kettenenden gleichermaßen überschätzt. [89] -Jeder kann einmal das Glück haben, richtig vorherzusagen, dass es an Weihnachten schneit. -M ehrere Va rianten von CCSD(T)-Rechnungen stimmen bei der Alkanfaltung ebenfalls mit dem Experiment über-ein, [86][87][88] aber jetzt aus sehr viel besseren Gründen.…”

“…Es ist sogar denkbar,d ass diese Rechnungen genauer als das Experiment sind, das nach wie vor einen geschätzten Fehlerbalken von AE 1K ettensegmenten aufweist. [89] …”