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Moderate and Advanced Intramolecular Proton Transfer in Urea–Anion Hydrogen‐Bonded Complexes
Author(s) -
Baggi Giorgio,
Boiocchi Massimo,
Fabbrizzi Luigi,
Mosca Lorenzo
Publication year - 2011
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201100490
Subject(s) - chemistry , hydrogen bond , intramolecular force , proton , molecule , urea , ion , acceptor , crystallography , cationic polymerization , hydrogen , medicinal chemistry , stereochemistry , polymer chemistry , organic chemistry , physics , quantum mechanics , condensed matter physics
The study of the interactions of the three urea‐based receptors AH , BH + and CH 2+ with a variety of anions, in MeCN, has made it possible to verify the current view that hydrogen bonding is frozen proton transfer from the donor (the urea NH fragment in this case) to the acceptor (the anion X − ). The poorly acidic, neutral receptor AH establishes two equivalent hydrogen bonds NH⋅⋅⋅X − , with all anions, including CH 3 COO − and F − , in which moderate proton transfer from NH to the anion takes place. The strongly acidic, dicationic receptor CH 2+ forms, with most anions, complexes in which two inequivalent hydrogen bonds are present: one involving moderate proton transfer (NH⋅⋅⋅X − ) and one in which advanced proton transfer has taken place, described as N − ⋅⋅⋅HX. The degree of proton advancement is directly related to the basic tendencies of the anion. The cationic receptor BH + of intermediate acidic properties only forms complexes with two inequivalent hydrogen bonds (moderate+advanced proton transfer) with CH 3 COO − and F − , and complexes with two equivalent hydrogen bonds (moderate proton transfer) with all the other anions. Moreover, [ B ⋅⋅⋅HF] and [ C ⋅⋅⋅HF] + , on addition of a second F − ion, lose the bound HF molecule to give HF 2 − . Release of CH 3 COOH, with the formation of [CH 3 COOH⋅⋅⋅CH 3 COO] − , also takes place with the [ B ⋅⋅⋅CH 3 COOH] complex in the presence of a large excess of anion.