Electronic spectroscopy of CHBr and CDBr Chong Tao, Calvin Mukarakate, Mihaela Deselnicu and Scott A. Reid Department of Chemistry, Marquette University Wisconsin 53233

Simple carbenes Carbenes have a divalent carbon which leads to energetically similar singlet and triplet states Typical molecule S0S0 T1T1 S1S1 Carbene S0S0 T1T1 S1S1 C C S 0 T 1 S 1 C Or reversed

Motivations The two lowest singlet states are components of a Renner-Teller pair, and thus simple carbenes are model systems for study of the Renner-Teller effect Mapping the singlet- singlet spectra is important for subsequent studies of the triplet state properties Energy  X1AX1A Ã1AÃ1A a3Aa3A 180 o

Methodology Laser G HV CHBr was produced from pulsed discharge of CHBr 3 precursors seeded in He, with a backing P=40psi CDBr was produced from CDBr 3 precursor Spectrum was measured under jet- cooled condition

Spectra show polyad structure due to similar frequencies of 2 and 3. Example survey spectra: CHBr

At high energies, we also observe levels in the C-H stretching progression. Example survey spectra: CHBr

Example high resolution spectra: CHBr

Dixon plots for CHBr Dashed lines are polynomial fits, solid lines are calculations of Yu, et al… The derived barrier heights are: 2 n 1680 cm -1 2 n cm -1 2 n cm -1

Isotope shifts: CHBr The isotope shifts with increasing quanta of C-Br stretch (upper panel) are similar for all bands and consistent with a simple harmonic prediction (solid line) A small dependence of isotope shift on bending quantum number is observed (lower panel) 2n3m2n3m 79 Br, 81 Br

Excited state C-H stretching frequency We observe bands in the CH stretching progression (1 1 2 n ) for the first time A linear extrapolation to n = 0 of the term energy difference between these bands and the 2 n bands returns: E(1 1 2 n )- E(2 n ) = 1 o +n x ´ = 3100 cm -1 x 12 0 = -41 cm -1

Picture of bands observed for CDBr

Dashed lines are polynomial fits (third order) The derived barrier heights are: 2 n 2110 cm -1 2 n cm -1 2 n cm -1 Dixon plots for CDBr

CDBr isotope splitting

Excited state C-D stretching frequency Extrapolated excited state C-D stretching frequency 1 ´ = 2351 cm -1 x 12 0 = -25 cm -1

 LIF spectra of CHBr/CDBr in the nm region were recorded and rotationally analyzed.  CH/CD stretching excitation spectra were observed for the first time. Their frequencies were determined to be 3100, and 2351 cm -1, respectively.  The minima in Dixon plots indicate the barrier heights to linearity for CHBr and CDBr are 1680 cm -1 and 2110 cm -1, respectively, above vibrationless level. Summary

Members from Dr. Scott Reid’s group Funding from NSF and ACS/PRF Acknowledgements