Far-infrared spectra, conformational stability, barriers to internal rotation, ab initio calculations, r0 structural parameters, and vibrational assignment of ethyl methyl ether

The far-infrared spectra (350-35 cm(-1)) of gaseous ethyl methyl ether-do and ethyl methyl-d(3)-ether have been recorded at a resolution of 0.10 cm(-1). For the do species, the fundamental asymmetric torsion of the more stable trans conformer (two methyl moieties are trans to one another) has been observed at 115.40 cm(-1) with four upper state transitions falling to lower frequency, whereas, for the gauche form, it has been observed at 93.56 cm(-1) with two excited states falling to lower frequency. the corresponding series for the d(3) species start from 106.00 and 87.10 cm(-1), respectively. From these data, the asymmetric torsional potential coefficients for the do species have been determined to be: V-1 = 572 +/- 30; V-2 = 85 +/- 8; V-3 = 619 +/- 30; V-4 = 175 +/- 18, and V-6 = -28 +/- 3 cm(-1). The trans to gauche and gauche to gauche barriers were calculated to be 958 cm(-1) (11.5 kJ/mol) and 631 cm(-1) (7.55 kJ/mol), respectively, with an energy difference of 550 +/- 6 cm(-1) (6.58 +/- 0.07 kJ/mol). Utilizing three conformer pairs, variable temperature studies (-105 to -150degreesC) of the infrared spectra of the do sample dissolved in liquid krypton gave an enthalpy difference of 547 +/- 28 cm(-1) (6.54 +/- 0.33 kJ/mol) with the trans conformer the more stable rotamer. It is estimated that there is only 4% of the gauche conformer present at ambient temperatures. The structural parameters, conformational stabilities, barriers to internal rotation, and fundamental vibrational frequencies, which have been determined experimentally, are compared to those obtained from ab initio gradient predictions from RHF/6-31G* and with full electron correlation at the MP2 level with three different basis sets. The adjusted r(0) structural parameters have been obtained for the trans conformer from combined ab initio MP2/6-311+G** predictions and previously reported microwave rotational constants. The reported distances should be accurate to 0.003 Angstrom and the angles to 0.5degrees. These results are compared to the corresponding quantities obtained for some similar molecules.