Does hydrohalic acid HX (X = F, Cl) form true N-protonated twisted amide salts? Effects of anions on the ion-pair interactions and on the amide moiety in N-protonated tricyclic twisted amide salts

摘要

The geometries of N-protonated tricyclic twisted amide salts (I-VI) and amide cations I+ have been optimized at the DFT-D3(BJ) level using density functionals PBE and PBE0. The optimized structure of I and II at the DFT/PBE-D3(BJ) level of theory is in excellent agreement with the experimental structure of Stoltz's 2-quinuclidone tetrafluoroborate salt (Nature 2006, 441, 731) and Aube's medium-bridged tricyclic N-protonated amide salt (J. Am. Chem. Soc., 2010, 132, 8836). In the studied N-protonated amide salts (I-IV), the [BF4](-) and [RSO3](-) anions interact with the cationic fragment [I](+) through N-H center dot center dot center dot F and N-H center dot center dot center dot O hydrogen bonds, respectively. The hydrohalic acids HX (X = F, Cl) interact with non-protonated amides with very weak N center dot center dot center dot H-X and X-H center dot center dot center dot O(CO) hydrogen bonds. The ion pair dissociation energies of amides [I+][X-] (I, X = BF4-, II, X = OTs-, III, X = MeSO3-, IV, X = HSO3-) are in the range 8.2-12.7 kcal mol(-1) and in the range 6.2-10.6 kcal mol(-1) in water solvents. Charge analysis is consistent with the formulation of the salts of twisted amides (I-VI) as [I](q+)[X](q-) with q = 0.81 in I, 0.72 in II, 0.66 in III, 0.70 in IV, 0.39 in V, 0.45 in VI. Only 0.10e and 0.23e charge flows from the non-protonated tricyclic amide to HF and HCl, respectively. The nature of N-H center dot center dot center dot F, N-H center dot center dot center dot O, N center dot center dot center dot H-F and N center dot center dot center dot H-Cl bonds has also been investigated using spectral studies (infrared frequencies, H-1 NMR chemical shifts, N-14 nuclear quadrupole coupling constants) to gain theoretical insights.