First and Second Proton Affinities of Carbon Bases
- 10 July 2008
- journal article
- research article
- Published by Wiley in Chemphyschem
- Vol. 9 (10), 1474-1481
- https://doi.org/10.1002/cphc.200800208
Abstract
Quantum chemical calculations at the MP2/TZVPP//BP86/SVP level are reported for the first and second proton affinities (PAs) of divalent carbon-donor molecules. The molecules investigated are imidazol-2-ylidenes (“normal” NHCs) and the tautomeric imidazol-4/5-ylidenes (“abnormal” NHCs). PAs are also calculated for acyclic and cyclic carbodiphosphoranes, carbophosphoranesulfide, unsaturated and saturated carbodicarbenes, tetraaminoallenes and carbon suboxide. The results are discussed in terms of divalent carbon(II) compounds (carbenes) CR2, which have one lone electron pair at carbon, and carbon(0) compounds CL2, which have two lone pairs at carbon and two C←L donor–acceptor bonds. Divalent C(0) compounds (carbones) not only have very high first PAs, but the second PA is also large and strong enough to isolate doubly protonated C(0) species as salts in a condensed phase. The first PA of divalent carbon(II) compounds (carbenes) are also large. However, they have much smaller second PAs than the divalent carbon(0) compounds. The divalent C(0) character of a compound is not always obvious when the bonding situation in the equilibrium geometry is considered. This is the case, for example, for tetraaminoallenes (TAAs). Protonation of TAAs changes the bonding situation of the central moiety from doubly bonded (R2N)2CCC(NR2)2 to a donor–acceptor description (R2N)2C→C(H+)n←C(NR2) [n=1, 2]. The atomic partial charge at the carbon donor atom does not correlate with the PA and the trend of the second PA may be quite different from the trend of the first. The trends of the first and second PA correlates quite well with the eigenvalues of the highest-lying carbon lone-pair orbitals.Keywords
This publication has 85 references indexed in Scilit:
- Synthesis of a Mixed Phosphonium–Sulfonium Bisylide R3PCSR2Angewandte Chemie, 2007
- Valence, Oxidation Number, and Formal Charge: Three Related but Fundamentally Different ConceptsJournal of Chemical Education, 2006
- 1,8-Bis(hexamethyltriaminophosphazenyl)naphthalene, HMPN: A Superbasic Bisphosphazene “Proton Sponge”Journal of the American Chemical Society, 2005
- Counter-ion effects switch ligand binding from C-2 to C-5 in kinetic carbenes formed from an imidazolium salt and IrH5(PPh3)2Chemical Communications, 2002
- Evaluated Gas Phase Basicities and Proton Affinities of Molecules: An UpdateJournal of Physical and Chemical Reference Data, 1998
- Experimental and theoretical study of carbon suboxide C3O2, protonated carbon suboxide C3HO2+ and C3HO2· radical in the gas phaseInternational Journal of Mass Spectrometry and Ion Processes, 1995
- Electronic structure of a stable nucleophilic carbeneThe Journal of Physical Chemistry, 1991
- Structure and triboluminescence of polymorphs of hexaphenylcarbodiphosphoraneJournal of the American Chemical Society, 1978
- Allenetetramine and DialkoxydiaminoalleneAngewandte Chemie-International Edition, 1973
- HEXAPHENYLCARBODIPHOSPHORANE, (C6H5)3PCP(C6H5)3Journal of the American Chemical Society, 1961