Thermodynamic Properties of the Methylmethoxy Radical with Intricate Treatment of Two-Dimensional Hindered Internal Rotations

Abstract

Thermodynamic properties of the methylmethoxy (CH 3 OCH 2 ) radical relevant to the pyrolysis and combustion of dimethyl ether are presented from quantum chemical calculations. The potential energy surface with torsional coordinates of the methyl and methylene groups of CH 3 OCH 2 is obtained at the CCSD(T)/aug-cc-pVTZ// B2PLYP/TZVP level. Internal rotations in the CH 3 OCH 2 geometry are found to generate two structures which are nonsuperposable mirror images, and a "double-well" feature is observed on the one-dimensional potential of the methylene rotation. Using the resulting torsional potentials, multiple structure and torsional anharmonicitities of CH 3 OCH 2 are evaluated by the multistructural method to obtain conformationally averaged partition functions which then serve as a basis for calculations of thermochemical parameters. The thermodynamic properties C p o , S o , and H T − H 0 at 298 K for CH 3 OCH 2 are predicted to be 64.54 J K −1 mol −1 , 283.73 J K −1 mol −1 , and 14.34 kJ mol −1 , respectively. The computational method CCSD(T)/cc-pV(5,6)Z//B2PLYP/TZVP with isodesmic reactions determine Δ f H o 298 (CH 3 OCH 2 ) to be on average 1.97 ± 0.64 kJ mol −1 with anharmonic torsion included, and this value is predicted to be 1.72 ± 0.62 kJ mol −1 by the G4 method. Results from atomization energy calculations using the CCSD(T)/cc-pV(5,6)Z//B2PLYP/TZVP method yields an enthalpy of formation value of 4.14 kJ mol −1 . The isodesmic reaction scheme is found to give an enthalpy of formation more accurate than atomization energy approach.

Computational Study of the Reaction of Dimethyl Ether with Nitric Oxide. Mechanism and Kinetic Modeling

“…From Table , the hydrogen-abstraction reaction from DME by NO is calculated to be an endothermic process, which is consistent with the structural analyzes based on Hammond’s postulate. The BMK/MG3S level provides a reaction enthalpy by 203.51 kJ mol –1 with ZPE correction included at 298 K, showing best agreement with the experimentally based determination of 201.82 kJ mol –1 by utilizing the following experimentally based enthalpies of formation of the involved species: Δ H ° f,298 (CH 3 OCH 3 ) = −184.01 kJ mol –1 , Δ H ° f,298 (NO) = 91.121 kJ mol –1 , and Δ H ° f,298 (HNO) = 106.96 kJ mol –1 taken from ref and Δ H ° f,298 (CH 3 OCH 2 ) = 1.97 kJ mol –1 in ref . At the B2PLYP/TZVP and mPW2PLYP/TZVP levels, this value is calculated to be 208.49 and 209.37 kJ mol –1 , respectively, which is about 7 and 8 kJ mol –1 , respectively, more than the experimentally based value.…”

Identification of Torsional Modes in Complex Molecules Using Redundant Internal Coordinates: The Multistructural Method with Torsional Anharmonicity with a Coupled Torsional Potential and Delocalized Torsions

“…Many methods to properly consider torsional anharmonicity have been developed based on Pitzer and Gwinn’s seminal work in the 1940s, − including separable one-dimensional hindered rotor (1D-HR), , n -dimensional hindered rotor ( n D-HR), extended hindered rotor (EHR), extended two-dimensional torsion method (E2DT), and the multistructural and torsional method (MS-T). , Among these methods, the MS-T method has the advantage of balancing the computational costs, ease of use, and accuracy, and it has been used for thermochemistry and kinetic studies of various systems. − Specifically, by using a nonredundant-internal-coordinate representation and the full Kilpatrick–Pitzer treatment, the MS-T method enables one to describe torsions including kinetic and potential energy coupling of torsions and Coriolis coupling to rotation without calculating the fully coupled many-dimensional torsional potential energy surface. The nonredundant-internal-coordinate representation plays an important role in the MS-T scheme because internal coordinates are used to separate the torsional modes from the overall vibrational Hessian.…”