An Ab Initio Investigation of the Ground and Excited Electronic State Properties of a Series of Bromine- and Iodine-Containing Singlet Carbenes

Abstract

Ab initio calculations have been performed to determine the structure and energies of the ground and first excited electronic states of bromine-and iodine-containing singlet carbenes. Effective core potential basis sets augmented with polarization functions were utilized at the CASSCF, CASPT2, and CISD levels of theory.Validation of the effective core potential basis sets for the ground and excited states of the singlet carbenes was carried out by comparison with previous results from all-electron basis set calculations. As was the case in previous studies of chlorine-and fluorine-containing halocarbenes, the bromine-and iodine-containing singlet carbenes are characterized by small bond angles in their ground states, ranging from 100°to 112°, and dramatically larger bond angles in their first excited states, ranging from 125°to 132°. This increase is due to the promotion of an electron from a carbon lone pair orbital coplanar with the carbon-halogen bonds to a carbon p-type orbital perpendicular to the bonds. Adiabatic transition energies for transitions from the ground to first excited state for the singlet carbenes determined at the CASPT2(18,12) and CISD levels range from 21 277 to 10 870 cm -1 and are in excellent agreement with experimental measurements where comparisons are available.

C-H Functionalisation Through Singlet Chlorocarbenes Insertions – MP2 and DFT Investigations

“…So the barrier heights increase dramatically from 4.87 to 20.57 kcal/mol at B3LYP and 7.05 to 18.83 kcal/mol at MP2 calculations respectively for methane. Both in the σ and the π orientations the enhanced nucleophilicity of ethane seems to be the reason for the barrier height lowering through ~ 3.3 to 3.7 kcal/mol for 1 CHCl insertion. Similarly, the barrier height for 1 CCl 2 into ethane does show the same trend (a reduction through ~4.5 to 5.9 kcal/mol).…”

“…2 kcal/mol lower than those of the corresponding B3LYP/6-31g (d, p) values. Replacement of hydrogen by chlorine in 1 CHCl decreases its electrophilicity [32]. So the barrier heights increase dramatically from 4.87 to 20.57 kcal/mol at B3LYP and 7.05 to 18.83 kcal/mol at MP2 calculations respectively for methane.…”

“…The σ approaches of 1 CHCl and 1 CCl 2 towards the C-H of methane are associated with the activation energies of 4.87 and 20.57 kcal/mol respectively at B3LYP/6-31g (d, p) ( Table 1). The MP2/6-31g (d, p) value for 1 CHCl insertion is ca. 2 kcal/mol higher and that for 1 CCl 2 insertion is ca.…”

“…This aspect is clear from the fact that the reactivity decreases as the chlorination increases in the carbene. An approximately two to five fold increase in activation barrier (at B3LYP and MP2 levels) for inverted σ/p approach of 1 CHCl/ 1 CCl 2 may be attributed to the fact that the migrating hydrogen approaches the smaller lobe of the σ orbital of carbene carbon having the lone pair of electrons. Moreover the predominant π-donation over σ-attraction by chlorine atom could be the reason for this enormous increase in the activation barrier with the degree of chlorination.…”

“…The refrigerants hydrochloroflurocarbons (HCFCs) and hydroflurocarbons (HFCs) are not harmful to ozone layer as they are oxidized / photodissociated to halocarbenes [1]. The halocarbenes and carbenes are important transient intermediates in organic synthesis [2], in organometallic chemistry [3] and in gas phase combustion [4,5].…”