Multipodal coordination of a tetracarboxylic crown ether with ${\rm NH}_4^+$ NH 4+: A vibrational spectroscopy and computational study

Abstract

The elucidation of the structural requirements for molecular recognition by the crown ether (18-crown-6)-2,3,11,12-tetracarboxylic acid (18c6H(4)) and its cationic complexes constitutes a topic of current fundamental and practical interest in catalysis and analytical sciences. The flexibility of the central ether ring and its four carboxyl side arms poses important challenges to experimental and theoretical approaches. In this study, infrared action vibrational spectroscopy and quantum mechanical computations are employed to characterize the conformational structure of the isolated gas phase complex formed by the 18c6H(4) host with NH(4)(+) as guest. The results show that the most stable gas-phase structure is a barrel-like conformation sustained by tetrapodal H-bonding of the ammonia cation with two C=O side groups and with four oxygen atoms of the ether ring in a bifurcated arrangement. Interestingly, a similar structure had been proposed in previous crystallographic studies. The experiment also provides evidence for a significant contribution of a higher energy bowl-like conformer with features resembling those adopted by 18c6H(4) in the analogous complexes with secondary amines. Such a conformation displays H-bonding between confronted side carboxyl groups and tetrapodal binding of the NH(4)(+) with the ether ring and with one C=O group. Structures involving even more extensive intramolecular H-bonding in the 18c6H(4) substrate are found to lie higher in energy and are ruled out by the experiment.

Chiral Recognition of Amino Acid Enantiomers by a Crown Ether: Chiroptical IR-VCD Response and Computational Study

“…Crown ethers have played a leading role in the development of Supramolecular Chemistry during the past decades. − The importance of crown ethers can be largely attributed to their ability to form inclusion complexes with cationic species, in particular protonated primary and secondary amines. − This property has motivated a large body of work devoted to the design of chiral substituted crown ethers, mainly for applications in asymmetric separation with electrophoresis , and high performance liquid chromatography. − …”

Microwave spectroscopy and quantum chemical investigation of nine low energy conformers of the 15-crown-5 ether

“…In order to face this problem from first principles, different groups have probed the most stable conformers of isolated crown ethers and their cationic complexes by means of gas-phase spectroscopic techniques. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Gas phase studies have the fundamental advantage of leading to benchmark structural information that can be put into direct relation with quantum chemical computations. 18 This approach provides valuable insights into the interplay between the intra-and intermolecular interactions, the intrinsic flexibility of the ether ring and potential solvent effects.…”

Conformation of Benzo-12-Crown-4 Complexes with Ammonium Ions Investigated by Cold Gas-Phase Spectroscopy

“…2,28−32 For CE complexes with molecular ion guests, IR spectroscopy in the gas phase was performed for ammonium ions and protonated amino acids. 15,19,33 Recently, we studied dibenzo-18-crown-6 (DB18C6) complexes with primary alkylammonium ions (RNH 3 + ) by cold UV spectroscopy in the gas phase. 34 The DB18C6 complexes with RNH 3 + ions are formed through three N−H•••O hydrogen bonds, which induce the formation of a novel encapsulation structure absent in the metal-ion complexes [e.g., K + (DB18C6)].…”

“…Recently, gas-phase IR spectroscopy was performed to reveal the conformation of the CE complexes with metal ions. − For benzo-CE complexes, UV spectroscopy under cold gas-phase conditions provided information on the conformation. − These complexes showed sharp vibronic structures in their UV spectra, allowing the application of UV–UV and IR–UV double-resonance spectroscopy to examine the number of conformers and their structures. For molecular ion guests, ammonium ions containing the NH 3 + group also form stable complexes with CEs. ,− For CE complexes with molecular ion guests, IR spectroscopy in the gas phase was performed for ammonium ions and protonated amino acids. ,, Recently, we studied dibenzo-18-crown-6 (DB18C6) complexes with primary alkylammonium ions (RNH 3 + ) by cold UV spectroscopy in the gas phase . The DB18C6 complexes with RNH 3 + ions are formed through three N–H···O hydrogen bonds, which induce the formation of a novel encapsulation structure absent in the metal-ion complexes [e.g., K + (DB18C6)] …”