tert‐ Butylphosphonic Acid: From the Bulk to the Gas Phase

The structure of tert‐ butylphosphonic acid in the solid, in solution, and in the gas phase was studied by single‐crystal X‐ray diffraction, 1 H and 31 P NMR spectroscopic studies in solution, solid‐state 31 P NMR spectroscopy, and electrospray ionization mass spectrometry. In addition, density functional theory (DFT) calculations at the B3LYP/6‐31G*, B3LYP/6‐31+G*, and B3LYP/6‐311+G* level of theory for a large number of H‐bonded aggregates of the type ( t BuPO 3 H 2 ) n ( C n , P n ; n =1–7) support the experimental work. Crystallization of t BuPO 3 H 2 from polar solvents such as CH 3 CN or THF gives the H‐bonded one‐dimensional polymer 2 , whereas crystallization from the less polar solvent CDCl 3 favors the formation of the H‐bonded cluster ( t BuPO 3 H 2 ) 6 ⋅CDCl 3 ( 1 ). In CDCl 3 the hexamer ( t BuPO 3 H 2 ) 6 ( C 6 ) is replaced by smaller aggregates down to the monomer with decreasing concentration. DFT calculations and natural bond orbital (NBO) analyses for the clusters C 1 – C 7 and the linear arrays P 1 – P 7 reveal the hexamer C 6 to be the energetically favored structure resulting from cooperative strengthening of the hydrogen bonds in the H‐bonded framework. However, the average hydrogen bond strengths calculated for C 6 and P 2 do not differ significantly (42–43 kJ mol −1 ). The average distances r O⋅⋅⋅O , r OH , r PO , and r POH in C 1 – C 7 and P 1 – P 7 are closely related to the hydrogen bond strength. Electrospray ionization mass spectrometry shows the presence of different anionic species of the type [( t BuPO 3 H 2 ) n ‐H] − ( A 1 – A 7 , n= 1–7) depending on the instrumental conditions. DFT calculations at the B3LYP/6‐31G* level of theory were carried out for A 1 – A 6 . We suggest the dimer [( t BuPO 3 H 2 ) 2 ‐H] − ( A 2 ) and the trimer [( t BuPO 3 H 2 ) 3 ‐H] − ( A 3 ) are the energetically favored anionic structures. A hydrogen bond energy of approximately 83 kJ mol −1 was calculated for A 2 . Electrospray ionization mass spectrometry is not suitable to study the assembling process of neutral H‐bonded tert‐ butylphosphonic acid since the removal of a proton from the neutral aggregates has a large influence on the hydrogen bond strength and the cluster structure.