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Anion‐Dependent Tendency of Di‐Long‐Chain Quaternary Ammonium Salts to Form Ion Quadruples and Higher Aggregates in Benzene
Author(s) -
Rocchigiani Luca,
Bellachioma Gianfranco,
Ciancaleoni Gianluca,
Crocchianti Stefano,
Laganà Antonio,
Zuccaccia Cristiano,
Zuccaccia Daniele,
Macchioni Alceo
Publication year - 2010
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201000530
Subject(s) - chemistry , benzene , counterion , ion , ammonium , crystallography , analytical chemistry (journal) , diffusion , bar (unit) , volume (thermodynamics) , chromatography , organic chemistry , thermodynamics , physics , meteorology
The self‐aggregation tendency of [N(CH 3 ) 2 (C 18 H 37 ) 2 ]X [ 1 X; X − =BF 4 − , PF 6 − , OTf − , NTf 2 − , BPh 4 − , BTol 4 − , BAr F− , and B(C 6 F 5 ) 4 − ] salts to form ion quadruples (IQs) and higher aggregates (HAggs) in [D 6 ]benzene is investigated by means of diffusion NMR spectroscopy. The experimental results indicate that salts containing small anions ( 1 BF 4 , 1 PF 6 , and 1 OTf) are present in solution as IQs even at the lowest investigated concentration of C =5×10 −5 M and show a limited tendency to further self‐aggregate, reaching a maximum average aggregation number (N= V H / ${V_{\rm{H}}^{{\rm{0IP}}} }$ , where V H =measured hydrodynamic volume and ${V_{\rm{H}}^{{\rm{0IP}}} }$ =hydrodynamic volume of the ion pair) of about 6–8 ( C =0.050–0.100 M ). Salts with larger counterions [ 1 BPh 4 , 1 BTol 4 , 1 BAr F , and 1 B(C 6 F 5 ) 4 ] form instead ion pairs at low concentration but steadily self‐aggregate (especially the non‐fluorinated ones) on increasing their concentration up to N values exceeding 50 ( C =0.030–0.050 M ). 1 NTf 2 behaves in an intermediate fashion. The self‐aggregation tendency of salts is quantified by formulating the dependence of V H on C by means of the equations of indefinitive aggregation models. The following rankings for the formation of IQs and HAggs are obtained: IQs: 1 BF 4 ≈ 1 PF 6 ≈ 1 OTf> 1 NTf 2 > 1 B(C 6 F 5 ) 4 ≥ 1 BPh 4 ≥ 1 BTol 4 ≥ 1 BAr F ; HAggs: 1 BTol 4 > 1 BPh 4 > 1 NTf 2 > 1 B(C 6 F 5 ) 4 > 1 BAr F > 1 BF 4 ≈ 1 PF 6 ≈ 1 OTf. Interionic NOE NMR studies and DFT calculations were conducted in order to determine the relative anion–cation orientation in the self‐aggregating units.