Combined matrix isolation IR spectroscopic and ab initio quantum chemical study of the molecular structure of aminomethylphosphinic acid

The molecular structure of 1-methylaminophosphinic acid (AMPA) was investig ated with the matrix isolation IR spectroscopy and sb initio calculations p erformed with RHF, MP2, MP3, MP4(DQ), MP4(SDQ) and MP4(SDTQ) methods. Three pseudopotential basis sets designed as CEP-31G were used in the calculatio ns: Basis Set I-CEP-31G with the d-functions on phosphorus. Basis Set II-CE P-31G with the d-functions on all heavy atoms; Basis Set III-CEP-31G with t he d-functions on all heavy atoms and p-functions on hydrogens. Four stable molecular and four stable zwitterion conformers of aminophosphinic acid we re found via ab initio calculations. According to the calculations, molecul ar conformers are always more stable than the zwitterion conformers, irresp ective of the basis set size and level of theory. This result is in good ag reement with matrix IR spectrum of the AMPA, The presence of the bands of O H stretching and NH2 bending vibrations and the absence of the bands of POO - and NH3+ vibrations are the evidence of molecular structure of AMPA in th e isolated state. An increased number of vibrational bands is found in the IR spectrum. It is explained by the high conformation lability of AMPA mole cules which is related to very low barrier of rotation about C-P bond. The IR spectrum is actually determined by multiple sites of AMPA molecule packe d in the Ar crystal, which considerably increases the number of bands in th e IR spectrum. (C) 1999 Elsevier Science B.V. All rights reserved.