Eingekapselte Carbonsäure‐Dimere mit verkürzten Wasserstoffbrücken

Abstract

Molekularer Autoklav: In molekularen Kapseln eingeschlossene Carbonsäure‐Dimere unterliegen einem Binnendruck von 5–10 kbar, der zu einer Verkürzung der Wasserstoffbrücken führt (siehe Schema).

Analysis of Proton NMR in Hydrogen Bonds in Terms of Lone‐Pair and Bond Orbital Contributions

“…The negative sign is reminiscent of the negative signs often found for" throughs pace" couplings, [81] and therefore we interpret it as drivenb yn oncovalent interactions involvings ome overlap.T he K(H a ···X b )n uclear spin-spin coupling changes sign when going from traditional H-bonds with relatively large D XHX to more symmetric cases with small D XHX (the latter being represented by NMA and HSA molecules among our samples), with ap ositive sign signaling ac ovalentc haracter of the H a ···X b interaction and the on-seto fO c ÀH a ···O b 3-center s bonding. A "shielding" map derived for the NMA molecule from quantum chemicalc alculations confirms that the H a shielding is the lowest for the most symmetric arrangements, in agreement with the previously determined correlation of low D XHX with low H a shielding by Limbach et al [26,31,32] We remindt he reader that low shielding translates into alarge chemical shift. The mainc haracteristico ft he shielding of H a as well as the spin-spin couplings to the heteroatoms in the hydrogenb ond arises fromt wo blocks of LMOs: orbitals centeredf ullyo rp artially on X c ,a nd LMOs centered fully or partially on the X b atom.…”

“…The K (H a ⋅⋅⋅X b ) nuclear spin–spin coupling changes sign when going from traditional H‐bonds with relatively large D XHX to more symmetric cases with small D XHX (the latter being represented by NMA and HSA molecules among our samples), with a positive sign signaling a covalent character of the H a ⋅⋅⋅X b interaction and the on‐set of O c H a ⋅⋅⋅O b 3‐center σ bonding. A “shielding” map derived for the NMA molecule from quantum chemical calculations confirms that the H a shielding is the lowest for the most symmetric arrangements, in agreement with the previously determined correlation of low D XHX with low H a shielding by Limbach et al 26. 31, 32 We remind the reader that low shielding translates into a large chemical shift.…”

“…[25] For the former, as trong dependence of the protons hifts on the OÀHd istances has been noted. [26] Reference [26] and other studies by Limbach and co-workersn oted av ery good correlation between the distance (r)d ifference D OHO = (r O···H Àr OÀH )/2a nd the 1 HNMR shifts in OÀH···O hydrogen bond environments. [27,24] Furthermore, H/D isotope effects on other chemical shifts in the vicinity of the H-bond, [24,26,[28][29][30][31][32][33] such as 13 C, can be indicators of the average hydrogen positions.…”

“…Large proton shifts of 12 ppm and above are also found for intermolecular hydrogen bonds, such as for O⋅⋅⋅H in carboxylic acid dimers or N⋅⋅⋅H in DNA base pairs 25. For the former, a strong dependence of the proton shifts on the OH distances has been noted 26. Reference 26 and other studies by Limbach and co‐workers noted a very good correlation between the distance ( r ) difference D OHO =( r O⋅⋅⋅H − r OH )/2 and the 1 H NMR shifts in OH⋅⋅⋅O hydrogen bond environments 27.…”

“…Reference 26 and other studies by Limbach and co‐workers noted a very good correlation between the distance ( r ) difference D OHO =( r O⋅⋅⋅H − r OH )/2 and the 1 H NMR shifts in OH⋅⋅⋅O hydrogen bond environments 27. 24 Furthermore, H/D isotope effects on other chemical shifts in the vicinity of the H‐bond,24, 26, 28–33 such as 13 C, can be indicators of the average hydrogen positions. See also reference 34…”