Chong Tao, Calvin Mukarakate, Scott A. Reid Marquette University Richard H. Judge University of Wisconsin-Parkside 63 rd International Symposium on Molecular.

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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

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

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 Energy in cm -1 C C S 0 T 1 S 1 C

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

Triplet levels as observed in SVL emission spectra CHCl CDBr

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 KaKa

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

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

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

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,…

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).]

Fit results and comparison with theory 1 Tarczay, et al., Phys. Chem. Chem. Phys. 2005, 7, Yu, et al., Mol. Phys. 2006, 104, 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 Energy (10) ) ) ) (12) ) ) ) (6) ) ) ) A25.47(10) ) 25.27(10)…25.66(4)… (18) ) (24)…0.5856(7)… (9)…………… 0 (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…

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 

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.

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) ,54 a 3 A˝(0,0,1)-X 1 A´(0,1,2) ,275    ss  so  ss = 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, Yu, et al., Mol. Phys. 2006, 104, 47.

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 = (10) cm -1 or (3) kcal mol -1 is in excellent agreement with theory

Acknowledgements People: Mihaela Deselnicu Yulia Mishchenko Danielle Brusse Zack Terranova Funding: National Science Foundation (CHE , CHE )