Microwave spectra, molecular structures, and theoretical calculations of (E)-sp and (E)-ac isomers of 2-methylpropanal oxime

The microwave spectra of two conformers of (E)-2-methylpropanal oxime ((CH3)2CHCHNOH) and its deuterated species ((CH3)2CHCHNOD) were observed in the frequency range from 8 to 40 GHz. The absorption lines in the ground and excited vibrational states were assigned. The rotational constants of normal species in the ground vibrational state were determined to be A = 7343.5(25), B = 1847.20(2), and C = 1607.29(2) MHz for (E)-sp isomer and A = 7073.15(84), B = 2006.01(1), and C = 1686.62(1) MHz for (E)-ac isomer. The planar moments (Pbb = (Ic + Ia − Ib)/2) of the normal and deuterated species were determined to be 54.829(15) and 54.829(8) uÅ2 for the (E)-sp, and 59.578(6) and 59.768(7) uÅ2 for the (E)-ac, respectively. The Pbb values of the (E)-sp of normal and deuterated species suggest that the (E)-sp has Cs symmetry. The rs coordinates of the hydrogen atoms of the hydroxyl group were determined for the (E)-sp and the (E)-ac. They indicated that their OH bonds took trans positions with respect to the CN double bond. The four and six structural parameters were adjusted by fitting to the rotational constants of the (E)-sp and (E)-ac, respectively. The observed rotational constants (B and C) of the (E)-sp and the (E)-ac isomers were in better agreement with those calculated using the MP2/6-31++G(d, p) level than with those calculated using the MP2/6-31G(d, p) level.