Ab Initio Studies of Halogenated Methyl and Methylene Radicals:  Molecular Structure, Vibrational Frequencies, and Enthalpies of Formation

Abstract

Fluorine- and chlorine-substituted methyl and methylene radicals have been studied by ab initio quantum chemical methods in order to determine the molecular structure and vibrational frequencies as well as the enthalpies of formation. The equilibrium geometries of radicals have been optimized at various levels, and the vibrational frequencies have been calculated at the optimized geometries. The calculated results, particularly at the MP2/ 6-311G(d) level, are in good agreement with the experimental data reported previously, suggesting that the present levels of theory can yield reasonably accurate estimates for the molecular properties of halogenated radicals when the experimental data are lacking. The standard enthalpies of formation at 298 K, ΔH f°(298 K), of these radicals have been obtained, utilizing the electronic energies calculated by the G2 or the CBS method. Enthalpies of formation for methyl radicals calculated with the original G2 method are close to the experimental values although the claimed accuracy of 2 kcal/mol is not quite achieved, but the errors in enthalpies of formation from other methods are rather high. For methylene radicals whose enthalpies of formation show large discrepancies among the values reported, some assessments have been made based on the calculated enthalpies of formation.

An ab Initio Investigation of Halocarbenes

“…There is no accurate experimental enthalpy of formation of CH 3 F. Therefore, values of ∆ f H°for fluorine-containing carbenes were computed from reactions of the type Enthalpies of formation for halocarbenes, computed from both G2 and QCISD(T) energies, are contained in Table 4, together with available experimental data 7-9,39 and earlier reported computed values of ∆ f H°. 27,39,[49][50][51][52] We note that the computed values of ∆ f H°for the five chlorinated carbenes were determined using eq 1 with CH 3 Cl as the reference halomethane. Since there is also an accurate experimental enthalpy of formation for CH 2 T)] reveals that enthalpies of formation calculated by the two methods agree to within 3 kJ mol -1 for all of the halocarbenes, with the sole exception of CHBr, for which the deviation is approximately 9 kJ mol -1 .…”

Geometries, vibrational frequencies, and excitation energies of a series of fluorine-substituted carbenes, FCX (X = H, F, Cl, Br, and I): A high-level multireference configuration interaction study

“…For the triplet states, as well as the ground singlet‐lowest triplet energy gap (S‐T gap), experimental studies are sparse, mostly by negative ion photoelectron spectroscopy. On the other hand, a number of ab initio theoretical studies were performed on the fluorine‐substituted carbenes with different calculation methods . For example, Gaussian‐2 and Quadratic Configuration Interaction (QCI) theory with basis sets up to 6–311+G(3df,2p) were employed to obtain geometries and vibrational frequencies of the singlet and triplet states, as well as the S‐T gaps of all halocarbenes .…”

Photodissociation dynamics of dichlorocarbene at 248 nm

“…There has also been a comparable level of effort in theoretical studies of the halocarbenes. 6,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] It should be noted that the X ˜1A 1 and A ˜1B 1 states coalesce to a degenerate electronic state at linear geometry, and hence these two states are coupled by the Renner-Teller effect.…”