60th International Symposium on Molecular Spectroscopy Slit Discharge IR Spectroscopy of Jet-Cooled Cyclopropyl Radical : Structure, Tunneling and IVR Dynamics JILA, CU-Boulder Feng Dong Scott Davis David J. Nesbitt 60th International Symposium on Molecular Spectroscopy June 21, 2005 TJ08

Motivation Prototypical alkyl ring radical Tunneling dynamics? Prospects for chiral synthesis? Unimolecular ring-opening? NO high-resolution spectra reported! DH0 = -22 kcal/mol

Experimental Setup ∆n ≈ 0.0003 cm-1 sensitivity ~ 5 x 10-7/Hz1/2 T ~20 K

Predicted Transitions for In-phase Antisymmetric CH2 Stretch aa ss a s Nuclear spin statistics in the ground state (Ka + Kc) Symmetric level  even :odd = 6 : 10 Asymmetric level  even :odd = 10 : 6

Sample Spectra of Q-branch Fully assigned according to four-line and two-line combination difference Local perturbations in some high J levels in the excited state

Symmetric to Symmetric Tunneling Only nuclear spin statistics different for the simulations Experiment agrees well with the nuclear spin ratio 6:10 Transitions from the symmetric tunneling level Experiment Simulation Simulation

Asymmetric to Asymmetric Tunneling Two similar bands fully assigned Two line combination differences in the ground state agree within ~ 15 MHz Same nuclear spin statistics in the ground state Asymmetric to asymmetric tunneling transition IVR in the excited state Simulation Experiment Experiment Simulation

Effective Molecule Structure Expt. Equilibrium Transition State r-CC (Å) 1.470(1) 1.4925 1.4779 r-CC (Å) 1.532(1) 1.5513 1.5621 -CH 56.6(2) 57.2 57.1 Fixed :ra-CH=1.09 Å, qa-CH=37º First precise structural information Ab initio overestimate of the ring strain at CCSD(T)/AVDZ.

Boltzmann Analysis of Tunneling Splitting ESR measurement – indistinguishable for the hyperfine splittings of syn and anti b-H DEtun” = 3.2(3) cm-1 Theoretical prediction DEtun”~ 1.1 cm-1 DEtun’ = 4.9(3) cm-1 Fast inversion rate k0  2.0×1011 s-1  impossible to freeze the inversion even at low temperature

Tunneling Barrier V0= 780 ± 40 cm-1 Reported predicted barrier height V0 = 870 cm-1 High level ab initio 1D PES along the a-CH bending coordinate Numerov-Cooley method to solve 1D Schrodinger equation Scale the PES vertically to map out the relationship between the DEtun” vs V0 Refined barrier height V0 = 780(40) cm-1 V0= 780 ± 40 cm-1

IVR Anharmonic Coupling 50/50 intensity distribution  two states very close to each other Deperturbation d12 = 0.037(6) cm-1 b12 = 0.2196(2) cm-1 Vibrational density of states r< 1 state/cm-1 Lack of fundamental frequencies and anharmonicity ‘Bright State’ ‘Dark State’

Summary First high-resolution detection of cyclopropyl, in-phase anti-symmetric CH stretch (n7) Tunneling dynamics of a-CH across the CCC plane (3.2 cm-1 and 4.9 cm-1) IVR in excited vibrational state due to the anharmonic coupling Precision structural information Fast inversion rate (k0  2.0×1011 s-1) Refined barrier height 780(40) cm-1 from 1D quantum modeling

Acknowledgements David J. Nesbitt Scott Davis Chandra Savage Erin S. Whitney NSF AFOSR

Background Experimentally EPR (Schuler 1963): nuclear hyperfine parameters Photoelectron spectro. (Dyke 1985): na-CH~1000±70 cm-1 IR in Ar-matrix (Hotzhauer, 1990): nCH2(asym)=3049/3042cm-1 Conversion between cyclopropyl and allyl radicals observed in both directions (Greig 1966; Holtzhauer 1990) Theoretically Follow-up on the explanation of hyperfine parameters from EPR Tunneling splitting predicted (Barone) Barrier: ~ 1280cm-1 (1993), 870cm-1 (1996) Splitting in ground state: ~1cm-1 (1993)

Cyclopropyl : Theoretical Predictions Freq.* (cm-1) Inten. (Km/mol) Assignment 3099.13 6 a-CH (n1) 3042.12 20 ip. asym. CH2 (n7) 3029.89 0.3 oop. asym. CH2 (n11) 2969.30 12 oop. sym. CH2 (n14) 2969.29 19 ip. sym. CH2 (n2) * Scaled by 0.9628

Rotational Constants 0+ 0- A” 0.792502 (21) 0.792524 (15) B” 0.689617 (23) 0.689155 (15) C” 0.43945 (22) 0.43911 (14) DN” (10-6) 1.96 (54) 1.13 (31) DNK” (10-5) 1.81 (16) -1.83 (11) 1+ 1- (a) 1- (b) A’ 0.791812 (56) 0.793114 (41) 0.792362 (54) B’ 0.683635 (54) 0.686741 (42) 0.686567 (58) C’ 0.43771 (22) 0.43413 (15) 0.43643 (15) DN’ (10-5) -2.42 (18) 1.48 (13) 1.31 (27) DNK’ (10-5) 32.37 (45) -4.80 (29) -5.77 (91) DK’ (10-5) -25.69 (32) -7.3 (22) 2.25 (72) 1+←0+ 1-(a)←0- 1-(b)←0- n0 3040.66907 (21) 3042.56059 (20) 3042.11813 (23) Units : cm-1;