Singlet−Triplet Splittings in CX₂ (X = F, Cl, Br, I) Dihalocarbenes via Negative Ion Photoelectron Spectroscopy

Abstract

The 364 nm negative ion photoelectron spectra of CF2 -, CCl2 -, CBr2 -, and CI2 - exhibit transitions to two different electronic states, the 1A1 and 3B1. The CF2 - spectrum exhibits well-resolved transitions to both electronic states. In the cases of CCl2 -, CBr2 -, and CI2 -, the spectra exhibit extended, partially resolved vibrational progressions and the two states are overlapped, making a direct determination of the origin transition energy not possible. The overlapped spectra show that the singlet−triplet splittings in the heavier halocarbenes are much smaller than for CF2 -. The results of ab initio calculations have been used to generate Franck−Condon simulations of the spectra, which aid in the determination of the band origins. The 1A1 state is found to be the lower state for CF2, CCl2, and CBr2 and the electron affinities have been determined to be 0.180 ± 0.020, 1.59 ± 0.07, and 1.88 ± 0.07 eV, respectively. For CI2, the triplet state is apparently the lower lying state with an electron affinity of 2.09 ± 0.07 eV. The singlet−triplet splitting energy has been determined to be 54 ± 3, 3 ± 3, 2 ± 3, −1 ± 3 kcal/mol for CF2, CCl2, CBr2, and CI2, respectively. In addition, the bending and symmetric stretching vibrational frequencies have been determined for either one or both states.

Block Correlated Coupled Cluster Theory With A Complete Active-Space Self-Consistent-Field Reference Function: The General Formalism And Applications

“…For CF 2 , the S-T gaps from four methods seem to be fairly close to each other and in good agreement with the experimental data (54±3 kcal/mol [176]). For CCl 2 and CBr 2 , the S-T separations predicted from all four methods used here are quite different from the experimental data [177] (3±3 and −2±3 kcal/mol, respectively). So our results will be compared with theoretical estimates obtained at the CCSD(T)/ccpVQZ//CCSD(T)/cc-pVTZ level [174], which are 20.9 kcal/mol for CCl 2 , and 16.6 kcal/mol for CBr 2 .…”

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

“…73 When the triplet carbene geometry changes are substantially larger, as in the case of dichlorocarbene, the triplet origin is essentially unobservable and again led to an initially inaccurate assessment of E S-T . 74 At this time, a number of methods have been developed to allow the quantitative treatment of extended vibrational progressions with strong mode coupling. [75][76][77] These more elaborate analyses could show the inadequacy of the initial dichlorocarbene analysis, and indeed a better S-T splitting was determined, 78 although with considerably reduced accuracy compared with our normal measurements.…”

A redetermination of the heats of formation of chloro- and dichlorocarbene and the deprotonation of methyl cation, a spin forbidden process?

“…Chloro-and dichlorocarbene both are ground state singlets [17,18]; consequently, the deprotonation of their conjugate acids is a spin allowed transformation. For a ground state, triplet carbene this process is spin forbidden if it is formed in its lowest energy state.…”