Fluorescence Excitation and Emission Spectroscopy of the X̃¹A′ → ùA′′ System of CHI and CDI

Abstract

We report on the first detailed studies of the spectroscopy of an iodocarbene, measuring fluorescence excitation and emission spectra of the X1A' --> A1A'' system of :CHI and the deuterated isotopomer :CDI. Due to similar bending and C-I stretching frequencies in the upper state, fluorescence excitation spectra of :CHI show polyads composed of members of the 2(0)(n-x)3(0)x progressions with x = 0-3. For :CDI, only progressions with x = 0, 1 are observed. Extrapolation of the 20n term energies for both isotopomers to a common origin places the electronic origin of the X1A' --> A1A'' system near 10500 cm-1, in good agreement with theoretical predictions. Rotational analysis of the 16 observed bands for CHI and 13 observed bands for :CDI yields rotational constants for the upper and lower states that are also in good agreement with theory. To investigate the controversial issue of the ground state multiplicity of :CHI, we measured single vibronic level emission spectra from many A1A'' levels. These spectra show conclusively that the ground state is a singlet, as for both isotopomers the ã3A'' origin is observed, lying well above the origin of the X1A' state. At energies above the ã3A'' origin, the spin-orbit mixing is so severe that few vibrational assignments can be made. Analysis of the emission spectra provides a lower limit on the singlet-triplet gap of 4.1 kcal mol-1, in excellent agreement with theoretical predictions.

Spectroscopy and heats of formation of CXI (X = Br, Cl, F) iodocarbenes: quantum chemical characterisation of the X̃(1 A′), ã(3 A″) and Ã(1 A″) states

“…All other iodocarbenes, including CHI, have been predicted to have singlet ground states. The most recent computed value of the singlet-triplet splitting in CHI is 4.4 kcal mol −1 [18], in good agreement with the experimental lower bound of 4.1 kcal mol −1 , as determined by Tao et al [22]. Similar values were obtained by Hajgató et al [23] as well as in several earlier computations [28][29][30][31].…”

“…As noted above, the spectroscopic work of Tao and co-workers on CHI [21,22] has confirmed that its ground state is indeed singlet.…”

“…The only spectroscopic data on iodocarbenes come from the negative ion photoelectron work of the Lineberger group [19,20] and more recently the fluorescence excitation and emission spectroscopic studies of CHI and CDI by Tao and co-workers [21,22]. In particular, a value of −1 ± 3 kcal mol −1 was deduced for the singlet-triplet gap in CI 2 from the photoelectron experiments [20], the negative value implying a triplet ground state.…”

Quantum Chemical Characterization of theX̃(1A′),ã(3A′′) andÃ(1A′′) States of CHBr and CHI and Computed Heats of Formation for CHI and CI†

“…[53][54][55] The theoretical work of Hajgató, et al, 51 however, contradicted this, by predicting that in CHI, for example, the 3 A′′ state will be as much as 4.5 kcal mol -1 higher in energy than the 1 A′ state. The recent spectroscopic work of Reid et al 21,22 has confirmed that the ground state is indeed singlet and the above lower bounds they deduced are consistent with the theoretical prediction of this work as well as of the earlier study of Hajgató et al 51 The à r X excitation energies (T 00 ) are predicted to be 12343 and 10852 cm -1 for CHBr and CHI respectively, which are 371 and 352 cm -1 , i.e., ∼1 kcal mol -l higher than the experimental values. A similar level of agreement between theory and experiment was achieved for CHCl 11 and CClBr 12 at comparable levels of theory, as well as for the C 2 molecule.…”

“…On the other hand, CHI has only recently been detected spectroscopically for the first time, 21,22 which makes the current theoretical study very topical.…”