1. Chong Tao, Calvin Mukarakate, Scott A. Reid Marquette University Richard H. Judge University of Wisconsin-Parkside 63 rd International Symposium on Molecular Spectroscopy June 19, 2008 RA06 A HIGH RESOLUTION VIEW OF INTERSYSTEM CROSSING IN AN ISOLATED SINGLET CARBENE: STIMULATED EMISSION PUMPING SPECTROSCOPY OF CH 35 Cl

2. Electronic structure of carbenes Carbenes contain a divalent carbon and feature low-lying singlet and triplet states Depending on the magnitude of the singlet-triplet gap, both states may play a role in carbene chemistry A central goal in carbene chemistry has been to determine the magnitude of the singlet-triplet gap (  E ST ) Singlet Carbene S0S0 T1T1 S1S1 C C S 0 T 1 S 1 C

3. Monohalocarbenes (CHX) Smallest carbenes with singlet ground states, show systematic variation in predicted  E ST CHF CHCl CHBr CHI S0S0 T1T1 S1S1 Singlet-triplet gap decreases Spin-orbit coupling increases 0 4000 8000 12000 16000 20000 Energy in cm -1 C C S 0 T 1 S 1 C

4. Background on CHCl/CDCl Sears and co-workers have reported a number of high resolution studies near the electronic origin Chang and co-workers first observed the triplet origin in CHCl and CDCl using emission spectroscopy Following a new survey of the visible spectroscopy of CHCl/CDCl, we recently obtained emission spectra which reveal many previously unassigned levels in both the singlet and triplet manifolds K a -sorted emission spectra confirmed the position of the triplet origin Our results were compared with the predictions of recent high level ab initio studies: 1 Yu, et al., Mol. Phys. 104, 47(2006); J. Chem. Phys. 115, 5433 (2001). 2 Tarczay, et al., Phys. Chem. Chem. Phys. 7, 2881 (2005). T 00 (ã-X) = 2172(2) cm -1 (exp), 2170 cm -1 (theory) 2

5. Triplet levels as observed in SVL emission spectra CHCl CDBr

6. Estimate of the triplet A constant for CHCl from SVL emission spectra From the observed K-type structure, (A-B) = 26(2) cm -1, in good agreement with the prediction of Sendt and Bacskay (25.1 cm -1 ) at CCSD(T)/cc-pVQZ level For comparison, nearby singlet levels have (A-B) constants of ~15 cm -1 The spectrum is consistent with the expected  K a selection rules for a A”-A” vibronic transition KaKa 0 1 1 2 0 KaKa

7. Stimulated Emission Pumping spectroscopy Experimental setup: Detector G HV Pump Dump time Intensity Gate 1 Gate 2 PumpDump

8. SEP spectroscopy of CH 35 Cl J”=4, K a ” = 1 J’=5, K a ’ = 0 J’=6 5 4 K a ’ = 1

9. SEP spectroscopy of CH 35 Cl J”=4, K a ” = 1 J’=5, K a ’ = 0 J’=6 5 4 K a ’ = 1

10. SEP spectroscopy of triplet CH 35 Cl (K a = 0→1 subband) Selection rules in case (B) limit:  J = 0,±1  N = 0,±1,±2  K a = 0,±1,…  K c = ±1,±3,…

11. The origin of spin-splittings In a state with S ≥ 1, two contributions are responsible for spin- splittings, (a) spin-spin and (b) spin-rotation interactions. Spin-spin interaction: Spin-rotation interaction: In fitting our spectra, we used a case b basis and the STROTA program of R.H. Judge [J. Kodet and R. H. Judge, Comp. Phys. Comm. 176, 601 (2007).]

12. Fit results and comparison with theory 1 Tarczay, et al., Phys. Chem. Chem. Phys. 2005, 7, 2881. 2 Yu, et al., Mol. Phys. 2006, 104, 47. 3 Sendt, et al., Int. J. Quant. Chem. 2000, 76, 297 Level Parameter (in cm -1 ) ã 3 A˝ (0,0,0) this work theory ã 3 A˝ (0,0,1) this work theory ã 3 A˝ (0,1,0) this work theory Term Energy2169.06(10) 2170 1) 2163.6 2) 2160.3 2) 3055.63(12) 3053.0 1) 3071.0 2) 3071.9 2) 3133.35(6) 3113.0 1) 3118.2 2) 3108.5 2) A25.47(10)25.64 3) 25.27(10)…25.66(4)… 0.5984(18)0.584 3) 0.5767(24)…0.5856(7)… 0.0071(9)…………… 00 -0.06(3)…-1.72(7)…2.43(2)… a ……0.09(4)…0.02(1) a0a0 ……0.07(4)…-0.04(1)  0.15…0.17…0.09… N36…27…50…

13. The origin of spin-splittings There are two contributions to the spin-spin interaction: (a) the pure dipolar spin-spin coupling, and (b) spin-orbit coupling with nearby electronic states. Michl and co-workers have shown [Coll. Czech. Chem. Comm 1998, 63, 1485] that the pure dipolar contribution is small and nearly constant across the series CHX (X = H, F, Cl, Br). The spin-orbit contribution reflects interactions with nearby singlet levels: When the level density is sparse, typically one term in this sum will dominate, and the sign of  0 will depend on the energy ordering of the states.  so 

14. Vibrational state dependence of spin-spin constant Singlet (calc) 1 Obs. Triplet (calc.) 2 0,0,0 0,1,1 0,2,0 0,0,3 0,0,1 0,1,0 0,0,4 0,2,1 0,1,2 1,0,0 a 3 A˝ Level    (cm -1 ) (0,1,0)2.43(2) (0,0,1)-1.72(7) (0,0,0)-0.06(3) 1 Predictions of Dunham expansion fit. 2 Tarczay, et al., Phys. Chem. Chem. Phys. 2005, 7, 2881.

15. The spin-spin constant provides detailed information on spin-orbit coupling 0,0,1 0,1,0 0,0,4 0,2,1 0,1,2 1,0,0 Interacting Levels  SO  (cm -1 )  E for unperturbed states (exp.)  E (calc.) 1,2 a 3 A˝(0,1,0)-X 1 A´(0,2,1)9.94858,54 a 3 A˝(0,0,1)-X 1 A´(0,1,2)22.9265263,275    ss  so  ss = 0.259 cm -1 To derive the spin-orbit contribution, we take the value of the spin-spin contribution as that of triplet methylene: 1 Tarczay, et al., Phys. Chem. Chem. Phys. 2005, 7, 2881. 2 Yu, et al., Mol. Phys. 2006, 104, 47.

16. Summary Rotationally resolved singlet-triplet spectra of CHCl were obtained using SEP The dominant triplet spin parameter is the spin-spin interaction, with a major contribution from spin-orbit coupling with nearby singlet state levels As a result, the spin-spin coupling constant displays a pronounced vibrational state dependence The derived singlet-triplet gap (T 00 = 2169.06(10) cm -1 or 6.2017(3) kcal mol -1 is in excellent agreement with theory

17. Acknowledgements People: Mihaela Deselnicu Yulia Mishchenko Danielle Brusse Zack Terranova Funding: National Science Foundation (CHE-0353596, CHE-0717960)