Microwave, infrared, and Raman spectra, vibrational assignment, normal coordinate analysis, and barrier to internal rotation of dimethylphosphine-<i>d</i>3

Abstract

Microwave spectra of dimethylphosphine-d3 and -d6 have been obtained and assigned in the range from 18 to 40 GHz. The six independent rotational constants along with the nine previously determined ones are used to obtain a least-squares fit of the eight structural parameters as follows: r(CP)=1.853±0.006 Å, r(CHα)=1.090±0.020 Å, r(CHβ)=1.086±0.010 Å, r(PH)=1.418±0.020 Å, uHαCP=109.1±1.8 °, uHβCP=110.7±1.0 °, uCPC=99.76 °±0.10 °, and uHPC=96.5 °±1.2 °. The barrier to internal rotation of the methyl group has been determined to be 823.6 cm−1 (2.35 kcal/mole) and 816.7 cm−1 (2.34 kcal/mole) by using two different methods of analyzing the excited state splitting data from the rotational transitions. The infrared spectra (80–3200 cm−1) and Raman spectra (10–3200 cm−1) of gaseous, liquid, and solid dimethylphosphine-d3 have been obtained and the spectra interpreted in detail. Similar information was also obtained for the -d4 species and an assignment of the P–H bending modes is given. The torsional data obtained from the low frequency Raman and far infrared spectra of gaseous dimethylphosphine-d3 have been used to determine the potential function based on the semirigid model and the effective barrier to internal rotation was found to be 832.3 cm−1 (2.38 kcal/mole). V60 and V06 were found to be −28.5 cm−1 (−0.081 kcal/mole) and −37.2 cm−1 (−0.11 kcal/mole), respectively. both the sine–sine (V′33) and cosine–cosine (V33) coupling terms were found to be zero. Spectra of the -d0 species were also recorded and these torsional data were also analyzed on the basis of the semirigid model. The barrier to internal rotation was found to be 810.1 cm−1 (2.32 kcal/mole) with both the cosine–cosine and sine–sine coupling terms being significant. A normal coordinate analysis has been carried out using seven diagonal force constants and 14 interaction constants in order to fit 61 fundamental frequencies. Revised assignments are provided for the PH bending and CH3 rocking motions. Extensive coupling was found for the normal modes in the 700–1000 cm−1 spectral region.