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Covalency and Ionicity Do Not Oppose Each Other—Relationship Between Si−O Bond Character and Basicity of Siloxanes
Author(s) -
Fugel Malte,
Hesse Maxie F.,
Pal Rumpa,
Beckmann Jens,
Jayatilaka Dylan,
Turner Michael J.,
Karton Amir,
Bultinck Patrick,
Chandler Graham S.,
Grabowsky Simon
Publication year - 2018
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.201802197
Subject(s) - siloxane , chemistry , hydrogen bond , ionic bonding , crystallography , covalent bond , disiloxane , molecule , intermolecular force , molecular geometry , computational chemistry , organic chemistry , catalysis , ion , polymer
Covalency and ionicity are orthogonal rather than antipodal concepts. We demonstrate for the case of siloxane systems [R 3 Si−(O−SiR 2 ) n −O−SiR 3 ] that both covalency and ionicity of the Si−O bonds impact on the basicity of the Si‐O‐Si linkage. The relationship between the siloxane basicity and the Si−O bond character has been under debate since previous studies have presented conflicting explanations. It has been shown with natural bond orbital methods that increased hyperconjugative interactions of LP(O)→σ*(Si‐R) type, that is, increased orbital overlap and hence covalency, are responsible for the low siloxane basicity at large Si‐O‐Si angles. On the other hand, increased ionicity towards larger Si‐O‐Si angles has been revealed with real‐space bonding indicators. To resolve this ostensible contradiction, we perform a complementary bonding analysis, which combines orbital‐space, real‐space, and bond‐index considerations. We analyze the isolated disiloxane molecule H 3 SiOSiH 3 with varying Si‐O‐Si angles, and n ‐membered cyclic siloxane systems Si 2 H 4 O(CH 2 ) n −3 . All methods from quite different realms show that both covalent and ionic interactions increase simultaneously towards larger Si‐O‐Si angles. In addition, we present highly accurate absolute hydrogen‐bond interaction energies of the investigated siloxane molecules with water and silanol as donors. It is found that intermolecular hydrogen bonding is significant at small Si‐O‐Si angles and weakens as the Si‐O‐Si angle increases until no stable hydrogen‐bond complexes are obtained beyond φ SiOSi =168°, angles typically displayed by minerals or polymers. The maximum hydrogen‐bond interaction energy, which is obtained at an angle of 105°, is 11.05 kJ mol −1 for the siloxane–water complex and 18.40 kJ mol −1 for the siloxane–silanol complex.