Structures, intramolecular rotation barriers, and thermochemical properties of methyl ethyl, methyl isopropyl, and methyl tert-butyl ethers and the corresponding radicals

Structures and thermochernical properties, Delta(f298)degrees, Sdegrees(T), and C-p(T) (50 less than or equal to T/K less than or equal to 5000) of three ethers and the corresponding radicals were determined by ab initio and density functional calculations. Molecular structures and vibration frequencies were determined at the B3LYP/6-31G(d,p) and MP2/6-31G(d,p) levels, with single point calculations for the energy at the B3LYP/6-311+G(3df,2p) and MP2/6-311+G(2df,2p) levels, respectively, and with composite methods CBSQ and G3(MP2) with B3LYP/6-31G(d,p) and MP2/6-31G(d,p) optimized geometries. Enthalpies of formation (DeltaH(f298)degrees,) were determined at each calculation level using the group balance isodesmic reactions. Standard entropy, Sdegrees(T), and heat capacity, C-p(T), from vibrational, translational, and external rotational contributions were calculated using the rigid-rotor-harmonic-oscillator approximation based on the vibration frequencies and structures obtained from the density functional study. Potential barriers for internal rotation were calculated at the B3LYP/6-31G(d) level, and hindered internal rotational contributions to entropy and heat capacity were calculated by summation over the energy levels obtained by direct diagonalization of the Hamiltonian matrix of hindered internal rotations. Evaluations of data from the isodesmic reactions at each calculation level results in the enthalpy of formation being -52.22 +/- 0.84, -60.13 +/- 0.94, and -67.78 +/- 1.44 kcal/mol for methyl ethyl ether (CCOC), methyl isopropyl ether (C2COC), and methyl tert-butyl ether (OCOC), respectively. Standard enthalpies are -1.67 +/- 0.98, -9.31 +/- 2.18, and -7.93 +/- 1.81 kcal/mol for methyl ethyl ether radicals, C.H2CH2OCH3 (C.COC), CH3C.HOCH3 (CC.OC), and CH3CH2OC.H-2 (CCOC.), respectively. Standard enthalpies are -10.06 +/- 0.85, -17.33 +/- 2.38, and -16.75 +/- 1.71 kcal/mol for methyl isopropyl ether radicals, C.H2CH(CH3)OCH3 (C2.COC), (CH3)(2)C.OCH3 (C2C.OC, and (CH3)(2)CHOC.H-2 (C2COC.), respectively. Methyl tert-butyl ether radicals have enthalpies of - 17.74 +/- 1.13 and -24.54 +/- 1.97 kcal/mol for C.H-2(CH3)(2)COCH3 (C3.COC) and (CH3)(3)COC.H-2 (C3COC.), respectively. Bond strengths on the ethers are in order C-H bond > C-C bond > central C-O bond > terminal C-O bond, with one exception of C-C bond energy for C2COC being slightly lower than that of the C-O bond (C2C-OC) by 0.5 kcal/mol. Thermodynamic properties of the O/C2 group. and the ether gauche interaction term were determined for group additivity application. The hydrogen bond increment group values for ROCJ, RJCOC, and RCJOC were also derived.