Single Vibronic Level (SVL) emission spectroscopy of CHBr: Vibrational structure of the X1A and a3A  states

Simple carbenes S0 T1 S1 S0 T1 S1 S0 T1 S1 Carbenes have a divalent carbon which leads to energetically similar singlet and triplet states C S0 T1 S1 Typical molecule S0 T1 S1 Carbene S0 T1 S1 Or reversed

Monohalocarbenes (CHX) Smallest carbenes with singlet ground states, show systematic variation in predicted DEST C C C S0 T1 S1 20000 S1 16000 12000 Energy in cm-1 8000 T1 4000 S0 CHF CHCl CHBr CHI Singlet-triplet gap decreases Spin-orbit coupling increases

Methodology CHBr/CDBr were generated by pulsed electrical discharge through a 1% mixture of CH(D)Br3 in He. The laser system is an etalon narrowed dye laser (Lambda-Physik Scanmate 2E) pumped by a Nd:YAG laser (Continuum NY-61 or Powerlite 7010). SVL emission spectra were acquired using a 0.3 m spectrograph with 600 or 1800 l/mm grating and gated intensified CCD detector Spectra were acquired in photon counting mode, averaged over typically 7500 to 10000 laser shots with 100 mm slit width Calibration spectra were obtained using a Fe:Ne hollow cathode lamp. Background spectra were obtained with the laser blocked to check for emission lines from species in the discharge

Example: SVL emission spectra of CHF By pumping different transitions (different vibronic states) we can “light up” different modes in the ground state Pump bending state, “light up” bending transitions Pump C-F stretching state, “light up” states containing C-F stretch Pump C-H stretch, “light up” states containing C-H stretch

Previous studies of CHBr and CDBr Previous studies using emission1 and high resolution hot-band spectroscopy2 show perturbations due to triplet state, particularly in bending manifold Some controversy concerning C-H stretching frequency exists We recorded excitation spectra in the 450-750 nm range, rotationally analyzing 30 cold bands, and observed bands in the C-H stretching progression for the first time 1Chang and co-workers., J. Mol. Spectrosc. 202, 131 (2000). 2Yu, et al., J. Chem. Phys. 115, 5433 (2001). Yu, et al., J. Chem. Phys. 115, 5433 (2001).

SVL emission spectra of CHBr Emission spectra from C-H stretching states allows firm assignment of CH stretch.

SVL emission spectra of CDBr

Dunham expansion fit results The term energies were fit to a standard anharmonic model (Dunham expansion) and compared with ab initio (MRCI) calculations which incorporated spin-orbit coupling (Yu, et al., J. Chem. Phys. 115, 5433 (2001). CHBr Dunham expansion poorly reproduces term energies for levels that are strongly perturbed by spin-orbit interactions, but better represents C-Br stretching states. Fit standard deviation is ~16 cm-1 for 40 levels, ~ 116 cm-1 for 27 levels from MRCI calculations CDBr All levels below 3000 cm-1 with exception of (0,3,0) are fit by Dunham expansion to within 1 cm-1. Overall fit standard deviation is ~46 cm-1 for 40 levels, ~ 76 cm-1 for 33 levels from MRCI calculations. MRCI calculations well reproduced perturbations in bending levels.

Energy level diagrams for CHBr 0,2,1 0,1,0 1,0,0 0,0,1 0,0,4 0,1,2 0,2,0 0,0,0 0,0,3 0,1,1 Singlet (DE) Obs. Triplet (calc.) 1 Yu, et al., J. Chem. Phys. 115, 5433 (2001).

Energy level diagrams for CDBr 0,0,5 0,4,0 0,3,1 0,2,2 1,1,0 0,0,1 0,1,3 0,0,4 0,1,0 0,3,0 0,2,1 0,1,2 1,0,0 0,0,0 Singlet (DE) Obs. Triplet (calc.) 0,0,3 1 Yu, et al., J. Chem. Phys. 115, 5433 (2001).

Examples of triplet levels in SVL emission spectra CDBr (15 triplet levels obs.) CHBr (8 triplet levels obs.)

CHBr: Experiment vs. Theory DFT calculations: B3LYP/ aug-cc-pVTZ

CDBr: Experiment vs. Theory DFT calculations: B3LYP/ aug-cc-pVTZ

Conclusion SVL emission spectra were recorded following excitation of the pure bending levels and the combination states in the A1A”-X1A’ system of CHBr and CDBr. This reveal rich detail concerning the vibrational structure of the X state up to ~ 9 000 cm-1. The spectra show significant perturbations due to spin-orbit interaction with the low-lying a3A” state. An effective spectroscopic Hamiltonian (Dunham expansion) works well for levels with pure C-Br stretch but poorly reproduces the experimental term energies of levels in the bending manifold. A comparison is made with previous experimental and theoretical studies. The results lead to revised assignments, including the X1A’ C-H stretching fundamental. Density functional calculations of the ground state vibrational frequencies were performed; the results are in excellent agreement with the experimentally derived vibrational parameters.

Acknowledgements GROUP MEMBERS FUNDING NSF