Stimulated Emission Pumping Spectroscopyof the [X ˜ ] 1 A′ State of CHF Calvin Mukarakate, † Chong Tao, † Christopher D. Jordan, ‡ William F. Polik,* ,‡ and Scott A. Reid* ,† Department of Chemistry, Marquette UniVersity, Milwaukee, Wisconsin 53201-1881, and Department of Chemistry, Hope College, Holland, Michigan 49422-9000 ReceiVed: September 4, 2007; In Final Form: October 24, 2007 We have recorded stimulated emission pumping (SEP) spectra of the A ˜ 1 A′′ f X ˜ 1 A′ system of CHF, which reveal rich detail concerning the rovibronic structure of the X ˜ 1 A′ state up to ∼7000 cm -1 above the vibrationless level. Using several intermediate A ˜ 1 A′′ state levels, we obtained rotationally resolved spectra for 16 of the 33 levels observed in our previous single vibronic level (SVL) emission study (Fan et al., J. Chem. Phys. 2005, 123, 014314), in addition to one new level. An anharmonic effective Hamiltonian model poorly reproduces the term energies even with the improved set of data because of the extensive interactions among levels in a given polyad (p) having combinations of ν 1 , ν 2 , ν 3 , which satisfy the relationship p ) 2ν 1 + ν 2 + ν 3 . However, the precise A rotational constants determined from the SEP data were invaluable in clarifying the assignments for these strongly perturbed levels, and the data are well reproduced using a multiresonance effective Hamiltonian model. The derived vibrational parameters are in good agreement with high level ab initio calculations. The experimental frequencies were combined with those of CDF to derive a harmonic force field and average (r z , r e z ) structures for the ground state. Introduction Carbenes are important reactive intermediates that are found in organic synthesis, combustion, thermal decomposition of small organic molecules, and atmospheric and interstellar chemistry. 1-13 The chemistry of carbenes is fascinating because the divalent carbon gives rise to low-lying singlet and triplet states with similar energies but distinct chemical reactivity. A central issue in carbene chemistry, and the focus of many theoretical and some experimental studies over the years, has been the precise determination of the singlet-triplet energy gap, ∆E TS . 14-40 The monohalocarbene CHF is the smallest carbene with a singlet ground state, and is thus a prototype for exploring the spectroscopy, dynamics, Renner-Teller effect in and electronic structure of singlet carbenes. 14-34,41-59 Using laser spectroscopy, recent progress has been made on the experimental side in probing the vibrational structure of the X ˜ 1 A′ state and spin- orbit coupling for the set of monohalocarbenes HCX (X ) F, Cl, and Br). 34,36-40,60-65 For example, SVL emission spectra of CHCl, CDCl, CHBr, and CDBr show transitions to triplet levels, which borrow intensity from nearby levels of the X ˜ 1 A′ state. 36-40,60-65 Experimental estimates for the magnitude of ∆E TS derived from the emission studies are in very good agreement with theoretical predictions. 36-40,61-65 We previously reported single vibronic level (SVL) emission spectra of CHF and CDF which mapped the vibrational levels up to ∼10,000 cm -1 above the vibrationless level of the X ˜ 1 A′ state, encompassing the region where the triplet origin is predicted. (Theoretical values for ∆E ST range from 4617 to 5537 cm -1 , 14-22,24-27,29-33 while an experimental value of 5210 ( 140 cm -1 was derived from photoelectron spectra of CHF 28 .) An anharmonic effective Hamiltonian model well reproduced the experimental term energies of CDF. However, the spectra of CHF were complicated by Fermi resonances among levels in a given polyad (p) having combinations of ν 1 , ν 2 , and ν 3 , which satisfied the relationship p ) 2ν 1 + ν 2 + ν 3 . 60 Because of the extensive anharmonic interactions, the assignments for many levels of CHF were considered tentative. In this study, we have applied rotationally resolved stimulated emission pumping (SEP) spectroscopy to probe the vibrational mixing in the X ˜ 1 A′ state and identify spin-orbit interactions with the low lying a ˜ 3 A′′ state, which have previously been observed in the A ˜ 1 A′′ state using quantum beat spectroscopy. 56 Suzuki and Hirota reported emission and high-resolution SEP spectroscopy from the 0 0 level of CHF, 50 identifying a resonance between 1 1 and 2 1 3 1 and an unusually small anharmonic C-H stretching frequency [2643.0393(26) cm -1 ]. However, their study investigated only a few levels up to 3000 cm -1 above the vibrationless level of the X ˜ 1 A′ state. Building upon this work and our previous studies using fluorescence excitation 56 and SVL emission 60 spectroscopy, we have extended the SEP studies up to ∼7000 cm -1 above the vibrationless level of the X ˜ 1 A′ state. The precise A rotational constants determined from the SEP data have clarified the assignments for many levels, and we have modeled the data using a two-resonance effective Hamil- tonian model. The derived vibrational parameters are in excellent agreement with high level ab initio calculations, and the experimental harmonic frequencies are combined with those of CDF to derive a harmonic force field and average (r z ) structure for the ground state. Experimental Procedures The apparatus, pulsed discharge nozzle, and data acquisition procedures have been described in previous studies. 55-66 CHF was generated by a pulsed electrical discharge through a 1% to * Corresponding author. E-mail: scott.reid@mu.edu (S.A.R.); polik@ hope.edu (W.F.P.). † Marquette University. ‡ Hope College. 466 J. Phys. Chem. A 2008, 112, 466-471 10.1021/jp077108m CCC: $40.75 © 2008 American Chemical Society Published on Web 01/03/2008