Linear Relationship between ¹³C NMR Chemical Shifts and the Bending of sp‐Carbon Chains

Abstract

Polyynes show as trictly linear relationship between the energy impact and the bending of the polyyne chain. The energy,w hich is necessary to bend the acetylenic chain, decreases with the increasing number of acetylene units. Ad eviation from linearity in polyynes can be realizedi ns olution by violation of the mutual-exclusion principle betweenI Ra nd Raman spectra. However,t here is still no possibility to measure the extent of the nonlinearity in solution. Herein, we show that the 13 CNMR spectroscopy represents an appropriate tool for this as we found an almost perfect linear relationship between the bendingo ft he alkyne chain and the change of the chemical shift of the outer acetylenic carbon atoms. By using molecular bows in which the alkyne chain can be bent by switching the azobenzene unit, this correlation can be proved experimentally.I nt he future, this correlation should enablet he determination of the extent of the bendinga nd the strain energyi np olyynes. Consequently, polyynes could be employed as probesf or measuring further molecular forces.Polyynes consisting of alternate single and triple bonds can be found in interstellar space, in plants,m ushrooms,a nd in marine organisms. [1] They are also used intensively as building blocks for materials and in supramolecular chemistry. [2] Investigationso ft he stabilityo fp olyynes revealt hat the reactivity regardingt heir dimerization [3] and decomposition increases with growingc hain length. [4] Therefore, it is necessary for the synthesis of higherp olyynest oa ttach large, sterically demanding end-capping groups in order to preventd ecomposition of the polyyne. [5] Using this strategy the synthesis and isolationo fa polyyne with 22 conjugatedt riple bonds wasachieved. [6] With increasing number of acetylene units, the energy of the bending vibration along the molecular axis decreases and, therefore, ad eviation from the linearityo ft he molecular struc-

Measurement of Nanometer Distances in Solution via 13C NMR Spectroscopy—Shrinking of Macrocycles with Increasing Temperature

“…Bending of the alkyne chain was detected both in the solid phase [26] and in solution [27, 28] . The bending also results in a shift of the 13 C NMR signals of the sp carbon atoms [29, 30] . Using the molecular spring 1 as probe we could recently show that the bending β [for definition, see Eq.…”

Bicyclic Phenyl–Ethynyl Architectures: Synthesis of a 1,4‐Bis(phenylbuta‐1,3‐diyn‐1‐yl) Benzene Banister

“…Thus the dissolved arrangement sketched as 1 b (Figure 2 c) resembles the banister of a staircase with an inserted floor (Figure 1 f) and not the intended helical staircase. 13 C NMR chemical shifts depend on the bending of the acetylenic chains and analysis according to the work of Kreuzahler et al [34] . reveal that the lower macrocyclic ring A‐C‐D‐B is considerably more strained than the upper one (A‐C‐F‐E) which is reflected in higher shift differences for the acetylenic carbons (12.3 and 9.0 ppm vs. 7.8 and 4.7 ppm).…”

“…Altogether,t he recorded NOEs mildly favor as lightly bent terphenyl backbone with not entirelyo rthogonal phenylr ings A, B, and Ea longside the substantially bent longer1 ,4-bis(phenylbuta-1,3-diyn-1-yl) benzene oligomer.T hus the dissolved arrangements ketched as 1b (Figure2c) resembles the banister of as taircase with an inserted floor (Figure 1f)a nd not the intendedh elical staircase. 13 CNMR chemical shifts depend on the bending of the acetylenic chains and analysis according to the work of Kreuzahler et al [34] revealt hat the lower macrocyclic ring A-C-D-B is considerably more strained than the upper one (A-C-F-E)w hich is reflected in higher shiftd ifferences for the acetylenic carbons( 12.3 and 9.0 ppm vs.7 .8 and 4.7 ppm). It also corroborates that the acetylenic moietiesa tt he ends of compound 1 are more strained (12.3 and 7.8 ppm) than the ones adjacent to the central Cp henyl ring (9.0 and 4.7 ppm).…”

Messung von Nanometerabständen in Lösung mittels 13C NMR‐Spektroskopie – Schrumpfen von Makrocyclen mit zunehmender Temperatur

“…Eine Biegung der Alkinkette konnte sowohl im Feststoff [26] als auch in Lösung [27, 28] nachgewiesen werden. Zudem führt die Biegung zu einer Verschiebung der 13 C NMR‐Signale der sp‐Kohlenstoffatome [29, 30] . So konnten wir kürzlich unter Verwendung der molekularen Feder 1 zeigen, dass die Biegung β [für die Definition siehe Gl.…”