Spectra and structure of organophosphorus compounds. XXV—Raman and infrared spectra and conformational stability of ethyldimethylphosphine

Abstract

The Raman (10-3500 cm-') and infrared (50-3500 cm-') spectra have been recorded for gaseous and solid ethyldimethylphosphine, CH3CH2P(CH& Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. From the fact that several distinct Raman lines disappear on going from the fluid phases to the solid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers in the fluid phase with the gauche conformer being more stable and the only one present in the spectrum of the solid. From a temperature study of the Raman spectrum of the liquid, the enthalpy difference between the gauche and trans conformers was determined to be 134f32 cm-' (383f92 cal mol-I). Relying on group frequencies and relative intensities of the infrared and Raman bands, and in some cases infrared band contours, a complete vibratibnal assignment is proposed for the gauche conformer. The assignment is supported by a normal coordinate calculation which was carried out utilizing a modified valence force field to obtain the frequencies of the normal modes and the potential energy distribution. The CH3-P torsions have been observed at 208 and 185 cm-' in the gas phase and from these frequencies the periodic barriers to internal rotation have been calculated to be 905f95 cm-' (2.56 kcal mol-I). The CH,-C torsion was also observed in the gas phase at 217 cm-' from which a periodic barrier of 1134 cm-' (3.22 kcal mol-') was calculated. The asymmetric torsional mode has been observed for the gauche conformer in both the infrared and Raman spectra of the gas at 91 cm-' with evidence of 'hot bands' at lower frequencies. All of these results are compared with corresponding quantities for several other organophosphines.

Molecular mechanics (MM4) force field development for phosphine and its alkyl derivatives

“…Comparisons among experimental 20 and MM4 calculated vibrational frequencies gave root mean squared deviations of 44 and 39 cm )1 for trans and gauche conformers (Tables S8 and S9). Since the assignment of the vibrational modes was not done by isotopic substitution, we used a smaller weighting factor for these two sets of vibrational frequencies in our force field development.…”

“…Experimental structural data were found in the literature for phosphine, 6,7 methylphosphine, 8,10,11 dimethylphosphine, 8,12 trimethylphosphine, 9,13À15 ethylphosphine, 16,17 isopropylphosphine, 18,19 ethyldimethylphosphine, 20,21 and tert-butylphosphine. 22,23 The molecular structures were determined from either electron diffraction or microwave spectroscopy, while almost all of their vibrational frequencies are available from infrared, Raman, or microwave spectroscopy.…”

Vibrational spectra, force constants and conformations of molecules of trivalent tricoordinate phosphorus compounds

“…Alkylphosphines and their derivatives with the general formula AlkPX 2 (Alk = C 2 H 5 , n-C 3 H 7 , iso-C 3 H 7 ; X = H, F, Cl, CH 3 ), which are liquids or gases at room temperature, are characterised by the existence of an equilibrium between the G and T conformers formed owing to internal rotation about the P7C bond (see Figs 1, 2). The T conformers are more energetically favourable than the G conformers, 15 ± 21, 23, 56 ± 61 except for the molecule of ethyl(dimethyl)phosphine; 46,154,155 the energy `gain' is nearly 1.6 kJ mol 71 in the liquid phase and 0.8 kJ mol 71 in the gas phase. It should be noted that the enthalpy differences (DH ) between the T and G conformers are small: they vary from * 0.4 to * 4.2 kJ mol 71 and decrease on going from the liquid to the gas phase.…”

Raman and infrared spectra, conformational stability, ab initio calculations and vibrational assignment of fluoromethyl phosphonothioic dichloride

“…There are a large number of organophosphorus molecules with formula (Z \ lone pair, O, CH 3 CH 2 P(Z)X 2 S or group and X \ H, F, Cl or methyl group) BH 3 which exist in two conformations in the Ñuid states at ambient temperature.1h10 For most of these molecules the more stable conformer is the anti form where the methyl group is trans to Z, with the ethyldimethylphosphine molecule being one of the few exceptions. 11 When the group is replaced by the CH 2 isoelectronic oxygen atom for the and CH 3 OPF 2 molecules, only the anti conformer is CH 3 OPCl 2 present in the Ñuid states at ambient temperature,12,13 although some earlier conformational studies suggested14,15 the presence of more than one conformer for the Ñuoride. Ab initio calculations support the conclusion of the presence of one conformer at ambient temperature with the gauche form calculated to be more than 1600 cm~1 (19.1 kJ mol~1) higher in energy than the anti conformer at the MP2/6È31G* level.…”