Molecular, thermodynamic, and kinetic parameters of iodomethanes and iodomethyl radicals: ab initio calculations

Abstract

The wave functions and enthalpies of formation of the ground states of iodomethanes CH 4 3x I x and iodomethyl radicals CH 3 3x I x . (x = 133) were calculated ab initio with regard to electron correlation. The geometries of the molecules of these compounds were determined, as well as the normal mode frequencies and other parameters, which were used for calculating the thermodynamic functions in the 031500 K range.These functions were used for calculating the constants of the CH 4 3x I x 76 47 CH 4 3x I x 31 + I and CH 4 3x I x + I 76 47 CH 4 3x I x 31 + I 2 equilibria, which, in turn, were used for calculating the corresponding rate constants in the high concentration limit.Studies of gas-phase pyrolysis of alkyl halides have been successfully used in developing fundamentals of chemical kinetics for almost a century [1,2]. The key reactions for the alkyl halides being pyrolyzed are those between (halo)alkyl radical and atomic hydrogen: R . + H . 76 RH, halogen: R . + Hlg . 76 RHlg, or hydrogen halide molecules: R . + HHlg 76 RH + Hlg . , Hlg = Cl, Br, I.Hence, these pyrolytic experiments furnish kinetic and thermochemical information about free radicals [336]. For the last-named reactions (CH 3 . + HI, CH 3 . + HBr, C 2 H 5 . + HBr, CH 2 Br . + HI...), peculiar phenomenon of a negative activation energy was recently discovered [739], whose nature still remains to be elucidated [10313]. Accurate thermodynamic and kinetic parameters of chloro-, bromo-, and iodoalkanes and their corresponding radicals are also essential for solving less fundamental but practically vital problems, such as, e.g., simulation of the Earth ozone layer depletion or modeling of incineration of halogencontaining waste.

Hyperfine Resolved Fourier Transform Microwave and Millimeter-Wave Spectroscopy of the Iodomethyl Radical, CH2I (X̃2B1)

“…These bond lengths compare well with our initial prediction 71 or with the theoretical calculations made by Marshall et al (2.049 and 1.081 Å), 51 by Odelius et al (2.066 and 1.083 Å), 65 and by Liu et al (2.050 and 1.086 Å) 67 but not with the values by Dymov et al (2.103 and 1.070 Å). 63 Our result for the r(C-H) bond length is also very close to that obtained for CH 2 Br (r 0 (C-H) ) 1.084 Å) 73 A difference of this magnitude (0.090 Å) for the C-Br bond lengths also exists between the CH 2 Br and CH 3 Br species. 73,19 Measurements of the rotational spectrum of the dideuterated isotopologue, CD 2 I, are planned to be conducted in the near future; as a result, these structural parameters will likely evolve to become more accurate.…”

“…The present gas phase investigation has completed the series of studies by high-resolution rotational spectroscopy of the monohalogen-substituted methyl radicals CH 2 X, with X ) F, Cl, Br, and I. The rotational constants of CH 2 I in the ground vibronic state have been accurately determined, and they compare well with the ab initio calculations giving a planar equilibrium structure 43,51,65,71 except the ones by Dymov et al 63 The effective rotational constants have been transformed into the Watson's determinable parameters, as follows (constants in MHz, one standard deviation between parentheses): A 0 ) 276675.97(252), B 0 ) 8896.5037(15), and C 0 ) 8613.9648(15). The small positive value of the inertial defect obtained from the determinable rotational constants, ∆ 0 ) 0.03665(3) amu Å 2 , strongly suggests the planarity of the molecule.…”

“…The present gas phase investigation has completed the series of studies by high-resolution rotational spectroscopy of the monohalogen-substituted methyl radicals CH 2 X, with X = F, Cl, Br, and I. The rotational constants of CH 2 I in the ground vibronic state have been accurately determined, and they compare well with the ab initio calculations giving a planar equilibrium structure ,,, except the ones by Dymov et al…”

“…The oxidation mechanism of the CH 2 I radical was also investigated . Ab initio structural calculations of this radical at various levels of theory have been reported by a number of authors, resulting in nearly as many as geometric parameters showing planar ,,, and quasiplanar ,, equilibrium configurations with a dihedral angle comprised between 8.5° and 10.1°. Yet, the CH 2 I radical is expected to be planar in its equilibrium geometry by comparison with the chlorine and bromine analogues.…”

Suprachannel as a Radical Trap: Crystal Structure of Single Carbon Radicals

“…The two very strong modes at 113 and 169 cm –1 can be assigned by the stretching modes of I 2 with symmetries A g and B 3 g , respectively, , indicating the formation of iodine (I 2 ) via the photoreaction. The sharp peak at 195 cm –1 is a new signal, which can be explained by ·CH 2 I, while the other two peaks are close to the signals of the remaining “outside” CH 2 I 2 . The other peaks of the spectrum corresponding to the cyclodimeric skeleton remain virtually unchanged (Figure S6, Supporting Information).…”