1. 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

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

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

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

5. 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

6. 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

7. 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

8. 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.

9. 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

10. 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

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

12. 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

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

14. 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)

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

16. Rotational Constants 0+ 0- A” 0.792502 (21) 0.792524 (15) B”
(23)
(15) C”
(22)
(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’
(56)
(41)
(54) B’
(54)
(42)
(58) C’
(22)
(15)
(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)
(32)
-7.3 (22)
2.25 (72)
1+←0+
1-(a)←0-
1-(b)←0- n0
(21)
(20)
(23)
Units : cm-1;