CH Stretch Vibrational Spectroscopy
and Tunneling Dynamics
in Vinyl Radical
JILA
F. Dong1, M. A. Roberts, R. S.
Walters and D. J. Nesbitt
JILA, Department of Chemistry
and Biochemistry,
University of Colorado
1Los
Gatos Research, Inc.
Mountain View, CA.





Prototype for unsaturated radical combustion chemistry
Large amplitude in-plane tunneling pathway
Mid-IR: H. Kanamori, Y. Endo, E. Hirota. J.Chem. Phys. 92,
197 (1990).
FTIR: L. Letendre, D.-K. Liu, C. D. Pibel, J. B. Halpern, H.-L.
Dai. J. Chem. Phys. 112, 9209 (2000).
mm: K. Tanaka, M. Toshimitsu, K. Harada, T. Tanaka. J.
Chem. Phys. 120, 3604 (2004).
Time-Resolved FTIR (Dai and coworkers)
Medium resolution IR emission from the photolysis of vinyl-Br at 193 nm.
Bands near 3235 cm-1 attributed to vinyl CH stretches.
Provides starting point for high resolution frequency searches.
Vinyl Expectations CH Stretches
DFT (B3LYP) Vinyl Radical
sym. CH2 asym. CH2 lone C-H
harmonic
3045.0
3141.4
3242.4
scaled
2934.5
3027.4
3124.7
- anharmonic DFT prediction vs. FTIR data…~ 200 cm-1 freq. search window!
High Resolution Slit-Jet Infrared Spectrometer
Sub-Doppler molecular linewidths (~ 30 MHz)
High resolution tunable IR radiation (1-2 MHz)
Stabilized Transfer Cavity (~ 10 MHz)
Shot-noise limited Sensitivity (~1.5 x 10-5 → 107 #/cc/qs)
Jet Cooled Hydrocarbon Radicals




localized
discharge
Clean synthesis of radicals
by dissociative attachment
(RX + e- → R + X-)
High radical densities at slit
orifice (1013-15 #/cm3)
Localized discharge (~ 105
cm/s, ~ 1 ms transit time) –
can avoid radical-radical
chemistry!
“Simple” high resolution
spectroscopy at 5-10 K
Vinyl Spectral Analysis




Search for vinyl bands beginning at
~3250 cm-1…signature bands found near
2900 cm-1.
3 A-type progressions observed.
Unambiguous confirmation of vinyl
assignment via high precision 2-line and
4-line combination differences.
Identical spectra obtained with multiple
vinyl precursors.
Constants
Set I (a)
Set II (a)
(cm–1)
Upper
Upper
Set III
Lower
Upper
A
7.9057 (9)
7.9239 (5)
7.78 (8)
7.78 (8)
B
1.08115 (10)
1.081305 (4)
1.06513 (7)
1.06297 (6)
C
0.94416 (8)
0.94606 (4)
0.94769 (5)
0.94470 (5)
N (104)
0.17 (2)
0.028 (11)
--
--
NK (102)
-0.151 (5)
0.028 (3)
--
--
K (102)
--
--
-0.32 (1)
--
2901.8603 (7)
2901.9319 (4)
--
2897.2264 (3)



0.0015
0.00082
0.00086
Bending/Tunneling Potentials
(CCSD(T) AVnZ, CBS)




High level CCSD(T)
AVnZ (n=2,3,4) surface
calculations
Extrapolated to
complete basis set CBS
limit (Dunning, Peterson
et al)
Corrected for ZPE in
3N-7 orthogonal
coordinates
Adiabatic PES for
ground state as well as
vibrationally excited
states
Reduced Mass (m)



Tunneling path (q) as psuedo 1D
coordinate embedded in full 3N-6
D space
Reduced mass as rigorous
function of full 1D tunneling and
3N-7 D orthogonal coordinates…
…calculated at each point q
(Rush-Wiberg)
q
Large Amplitude Tunneling Methods
(Rush-Wiberg/HBJ)
G (q)
1
 I xx
 I
  xy
  I xz

 X 11
 I xy
I yy
 I xz
 I yz
 I yz
X 21
I zz
X 31
X 11 
X 21 
X 31 

Y11 
X ik   m [ri  (
a 1
N
Yik   m [(
a 1
r
)]i
qk
r
r
)  (  )]
qi
qk
1.2
I ik   m ri rk (i  k )
a 1
I ii   m (r  r  ri )
2
a 1
m(q) (amu A2)
N
N
N
1.0
0.8
0.6
0.4
0.2
m (q)  1 / G44



0.0
-100
-50
0
50
100
CCH bend angle q (degrees)
Based on LAM rotational Hamiltonian methods by Hougen, Bunker and Johns
Matrix of inertial moments and displacements along 3N-6 tunneling path
Works for arbitrary choice (e.g. angles or distances) of LAM coordinate (q)
Wave Functions/Tunneling Splittings
Vinyl Radical

(CCH Large Amplitude Bending)

1000
-1
V(q) (cm )
500
0

-500
-1000
-1500
-2000
-100

1640 cm-1
-50
0
50
q (degrees)
100
Modified S.E.
Predicts tunneling
splitting of  = 0.513
cm-1
Near quantitative
agreement with mm
wave studies of Tanaka
et al ( = 0.543 cm-1)
Improved barrier height
prediction of 1640(50)
cm-1
Summary





First high resolution CH stretch spectra of vinyl
radical.
Three A-type bands observed: two tunneling (a,s)
bands out of ground state and one hot band.
Tunneling splittings consistent with previous studies.
Agreement with ab initio anharmonic predictions for
symmetric CH2 stretch.
Vibrationally adiabatic tunneling PES and barrier
heights.
Thanks
Feng Dong,
Melanie Roberts
Mike Deskivich
Funding:
Symmetry and Spin statistics (sym. CH2 : A-type)
for 180o rot. around the a-axis
b
   el vib tunneling rot ns  asym.
Ground State
a
c
A-type: N=0, ±1; Ka = 0; Kc = ±1
Elec.
Vib.
tunneling
Rot.
Ns.
-
+
+
+
+
-
+
+
-
-
-
+
-
+
-
-
+
-
-
+
Excited State
GROUND state:
Lower tunneling level (+1) :
Ka=even (3);
Ka=odd (1)
Upper tunneling level (-1) :
Ka=even (1);
Ka=odd (3)
Elec.
Vib.
tunneling
Rot.
Ns.
-
+
+
+
+
-
+
+
-
-
-
+
-
+
-
-
+
-
-
+
High E Discharge
all Hydrogens interchange…
…quickly quenched in expansion
Sym. CH2