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 (tBuPO(3)H(2))(n) (C(n), P(n); n=1-7) support the experimental work. Crystallization of tBuPO(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 (tBuPO(3)H(2))(6).CDCl(3) (1). In CDCl(3) the hexamer (tBuPO(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(Obond;H), r(Pdbond;O), and r(Pbond;OH) 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 [(tBuPO(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 [(tBuPO(3)H(2))(2)-H](-) (A(2)) and the trimer [(tBuPO(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.