Methylene: A study of the X̃ 3B1 and ã 1A1 states by photoelectron spectroscopy of CH−2 and CD−2

Abstract

Photoelectron spectra are reported for the CH2(X̃ 3B1)+e−←CH−2 (X̃ 2B1) and CH2(ã 1A1)+e−←CH−2 (X̃ 2B1) transitions of the methylene and perdeuterated methylene anions, using a new flowing afterglow photoelectron spectrometer with improved energy resolution (11 meV). Rotational relaxation of the ions to ∼300 K and partial vibrational relaxation to <1000 K in the flowing afterglow negative ion source reveal richly structured photoelectron spectra. Detailed rotational band contour analyses yield an electron affinity of 0.652±0.006 eV and a singlet–triplet splitting of 9.00±0.09 kcal/mol for CH2. (See also the following paper by Bunker and Sears.) For CD2, results give an electron affinity of 0.645±0.006 eV and a singlet–triplet splitting of 8.98±0.09 kcal/mol. Deuterium shifts suggest a zero point vibrational contribution of 0.27±0.40 kcal/mol to the observed singlet–triplet splitting, implying a Te value of 8.7±0.5 kcal/mol. Vibrational and partially resolved rotational structure is observed up to ∼9000 cm−1 above the zero point vibrational level of the 3B1 states, revealing a previously unexplored region of the quasilinear potential surface of triplet methylene. Approximately 20 new vibration-rotation energy levels for CH2 and CD2 are measured to a precision of ∼30 cm−1 in the v2=2–7 region (bent molecule numbering). Bending vibrational frequencies in the methylene anions are determined to be 1230±30 cm−1 for CH− and 940±30 cm−1 for CD−2, and the ion equilibrium geometries are bracketed. The measured electron affinity also provides values for the bond strength and heat of formation of CH−2, and the gas phase acidity of CH3. A detailed description of the new flowing afterglow photoelectron spectrometer is given.

Once upon Anion: A Tale of Photodetachment

“…It showed unequivocally that serious hot-band contamination was present in the earlier photoelectron spectra (31). The new singlet-triplet splitting measurement of 9.00 ± 0.09 kcal mol −1 was in much better agreement with other new experiments and with high-level quantum calculations (39).…”

Bonding and fluxionality in group-4 metal complexes with pyrrolyl ligands

“…Fig. 6, using the calculated DH R and the following experimental values (kcal mol -1 ): D(M-Cl) = 83 (Ti), 112 (Zr), 114 (Hf) [30], EA(Cl) = 83 [39], EA(pyr) = 55 [40], and EA(dmp) ≈ EA(pyr).…”

“…The M-pyr′ bond dissociation enthalpy, in each complex, can be estimated using equation (1) in Fig. 6, with the experimental values for EA(Cl) [39], EA(pyr′) [40] and D(M-Cl) 1 , and the calculated enthalpy variation, DH R , for the exchange ligand reaction between [M(pyr)Cl 3 ] and MCl 4 . The electron affinity of dmp was assumed equal to the one published for pyr.…”

Theoretical study on interactions of N-heterocyclic carbene with the bare first-row transition metals

“…The low-lying states of 1 A 1 (σ 2 π 0 ) and 1 B 1 (σ 1 π 1 ) are 10.4 and 33.1 kcal/mol [45][46][47][48][49] above the ground state, respectively (Schemes 2, 3).…”