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Andreas Orthaber combined calculation and experiment to examine the hydrogen chloride adduct of trimethylphosphine oxide. The electronic-structure analysis describes a strongly polar phosphorus–oxygen sigma bond and oxygen lone-pair negative hyperconjugation, illustrating how bonding descriptions depend on the chosen orbital analysis.^[ref]

Andreas Orthaber, Ferdinand Belaj and Rudolf Pietschnig, Comptes Rendus Chimie 13 (2010), 923–928. Section 3.2 analyses the strongly polar P–O sigma bond and oxygen lone-pair negative hyperconjugation; this is not a claim that every orbital-localization scheme assigns the same bond order.