Katjonska-pi interakcija

Katjonska–π interakcija između benzena i natrijumskog katjona.

Katjonska–π interakcija je nekovalentna molekulska interakcija između lica jednog elektronski bogatog π sistema (e.g. benzena, etilena, acetilena) i susednog katjona (e.g. Li+, Na+). Ova interakcija je primer nekovalentnog vezivanja između monopola (katjona) i kvadrupola (π sistema). Energije vezivanja su znatne, sa vrednostima u rastvoru u opsegu reda veličina sličnom vodoničnim vezama i sonim mostovima. Slično drugim nekovalentnim vezama, katjonske–π interakcije imaju važnu ulogu u prirodi, a posebno u proteinskim strukturama, molekulskom prepoznavanju i enzimskoj katalizi. Ovaj efekat nalazi primenu u sintetičkim sistemima.[1][2]

π sistem iznad i ispod benzenovog prstena dovodi do kvadrupolne distribucije naelektrisanja.

Poreklo efekta

Benzen, model π sistema, nema permanentni dipolni moment, jer se doprinosi slabih veza između ugljenika i vodonika poništavaju usled molekulske simetrije. Međutim, elektronski bogati π sistem iznad i ispod benzenovog prstena ima parcijalno negativno naelektrisanje. Protivtežno pozitivno naelektrisanje se javlja na ravani benzenovih atoma, te se formira električni kvadrupol (par dipola, poravnatih kao paralelogrami tako da nema neto molekulskog dipolnog momenta). Negativno naelektrisani region kvadrupola može da formira interakcije sa pozitivno naelektrisanim jonima; pri čemu se posebno jak efekat javlja u prisustvu katjona visoke gustine naelektrisanja.[2]

Priroda katjonske-π interakcije

Većina izučavanih katjonskih-π interakcija obuhvata vezivanje između aromatičnog π sistema i alkalnih metala ili azotnih katjona. Optimalna interakciona geometrija ima katjon u van der Valsovom kontaktu sa atomskim aromatičnim prstenom, koji je centriran iznad π lica duž centralne normalne ose.[3] Istraživanja su pokazala da elektrostatički doprinos dominira interakcije kod jednostavnih sistema, i relativne energije vezivanja su u dobroj korelaciji sa elektrostatičkom potencijalnom energijom.[4][5]

Reference

  1. ^ Eric V. Anslyn; Dennis A. Dougherty (2004). Modern Physical Organic Chemistry. University Science Books. ISBN 978-1-891389-31-3. 
  2. ^ а б Dougherty, D. A.; J.C. Ma (1997). „The Cation-π Interaction”. Chemical Reviews. 97 (5): 1303—1324. PMID 11851453. doi:10.1021/cr9603744. 
  3. ^ Tsuzuki, Seiji; Yoshida, Masaru; Uchimaru, Tadafumi; Mikami, Masuhiro (2001). „The Origin of the Cation/π Interaction: The Significant Importance of the Induction in Li+and Na+Complexes”. The Journal of Physical Chemistry A. 105 (4): 769. doi:10.1021/jp003287v. 
  4. ^ S. Mecozzi; A. P. West & D. A. Dougherty (1996). „Cation-π Interactions in Simple Aromatics: Electrostatics Provide a Predictive Tool”. Journal of the American Chemical Society. 118 (9): 2307. doi:10.1021/ja9539608. 
  5. ^ S. E. Wheeler & K. N. Houk (2009). „Substituent Effects in Cation/π Interactions and Electrostatic Potentials above the Center of Substituted Benzenes Are Due Primarily to through-Space Effects of the Substituents”. J. Am. Chem. Soc. 131 (9): 3126—7. PMC 2787874 Слободан приступ. PMID 19219986. doi:10.1021/ja809097r. 

Literatura

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