Microwave Spectroscopy of
1,1-Difluorocyanomethyl
Radical, ĊF2CN
Lu Kang
Department of Natural Sciences, Union College, KY 40906
Stewart E. Novick
Department of Chemistry, Wesleyan University, CT 06459
Planar, D 3h
H
H
C
H
F
a = 0°
C
C
H
H
H
a <= 5°
Tetrahedral, T d
Pyramidal, C 3v
Pyramidal, C s
Quasiplanar, C s
C
F
F
F
F
a = 15.63°
F
a = 18.15°
Geometry: planar, quasiplanar, or pyramidal?
F
F
H
C
F
a
F
F
C
C
C
F
a = 19.47° (Umbrella angle: the angle between C-F bond and F-F-F plane)
Ab initio calculations
 UB3LYP/aug-cc-pVQZ (g01→ gdv → Gaussian 03)
Geometry optimization
Rotational constants
Centrifugal distortion constants
Dipole moments
Fine & hyperfine constants
 My DIYed computer: Heron (2001, Wesleyan, $1,300)
 Dual processors: 2 × 1.2 GHz AMD Athlon CPU
 1 GB memory, SCSI, RAID 0
 Benchmark test: Sun Graphics 123 s : DIYed Heron 125 s
 Swallowtail cluster (2008, computing center @ Wesleyan)

For each node: 8 × 2.66 GHz Intel CPU
 16 GB memory
Ab initio calculation: UB3LYP/aug-cc-pVQZ
The resonance structures of HCCCF2
1,1-Difluoropropargyl
F
1.201(1)
H 1.051(1) C
1.3
37
( 2)
HCCCF3, HCCCF2, and H2CCCF2
C 1.474(5) C
108.3°(2)
1.2177
H 1.0611 C
1.3
22
1
F
C 1.3534 C
H
C
C
C
F
F
F
F
H
C
C
F
C
111.72°
F
H
117.8°(2)
H
1.306(2)
C
)
6(3
1.08
1.3
23
(11
)
F
C1.302(12)C
F
C
C
C
F
110°(1)
H
3,3-Difluoropropadienyl
F
F
The geometry of ĊF2CN?
 ĊF2CCH:
 Kang & Novick: J. Chem. Phys. 125, 054309, (2006)
 Inertia defect, Δc = -0.085147(44) amuÅ2
 Planar or quasi-planar geometry.
 ĊF2CN:
 −C≡N is an isoelectronic analog of −C≡CH
 −C≡N is a pseudohalogen  pyramidal geometry.
 Planar or non-planar?
Density functional theory predictions
Opt. geometry: Cs
r(C ≡ N)
1.163 Å
r(C = C)
1.379 Å
r(C − F)
1.313 Å
(C−C≡N)
176.1º
(C−C−F)
122.1º
Ф(CCF–CCF)
162.7º
μa = 1.9 D, μc = 0.08 D
Δc = -0.30 amuÅ2
Experimental
Discharge stack
for insertion
into the mirror
thickness
cathode
3 mm
4 mm
3 mm
10 mm
ground
 Sample: 0.3% CF3CN/Ne  -900 V DC  ĊF2CN
 Helmholtz coils / Geomagnetic field
 FP-FTMW spectrometers: 6.5 – 40 GHz
hole dia
4 mm
4 mm
5 mm
5 mm
The paramagnetic transitions of ĊF2CN
Helmholtz coils on
Helmholtz coils off
Coupling schemes
 Spin:
S=½
IN = 1
IF1 = ½
IF2 = ½
 Coupling scheme 1: uncoupled scheme
J=N+S
F1 = F + IF1
F = J + IN
F2 = F1 + IF2
 Coupling scheme 2: coupled scheme
J=N+S
F = J + IN
IF = IF1 + IF2
F1 = F1 + IF
Hamiltonian
 Coupling scheme 1:
H = Hrot + Hsr + Hhfs(N) + Hhfs(F1) + Hhfs(F2)
 Coupling scheme 2:
H = Hrot + Hsr + Hhfs(N) + Hhfs(F)
 Hrot: Watson’s A-reduction Hamiltonian
 Hsr: electron spin – molecular over all rotation
 Hhfs(N): quadrupole coupling & Fermi contact
interactions
 Hhfs(F), Hhfs(F1), Hhfs(F2) : Fermi contact interactions
Nuclear spin statistics & the selection rules
IF = IF1 + IF2 = 0, 1 according to the coupled scheme
IF = (±½) – (±½) = 0 (25%) nuclear spin singlet state
IF = (±½) + (±½) = 1 (75%) nuclear spin triplet state
Fermions: Ψtot must be anti-symmetric (Ө)
Ψtot = Ψelec  Ψvib  Ψrot  Ψnucl = Ө → Ψrot  Ψnucl = 
because Ψelec = Ө (2B1 state), Ψvib =  (v = 0)
[1] Ψrot = Ө & Ψnucl = Ө: IF = ½ - ½ = 0 → Ka = 1, 3, 5, ···
Ψnucl = Ө: 2-½[α(1)β(2) - β(1)α(2)]
[2] Ψrot =  & Ψnucl = : IF = ½ + ½ = 1 → Ka = 0, 2, 4, ···
Ψnucl = : α(1)α(2), 2-½[α(1)β(2) + β(1)α(2)], β(1)β(2)
Spectroscopic constants of ĊF2CN
Spectroscopic
constants /MHz
UB3LYP /
aug-cc-pVQZ
ĊF2CN
( IF = 0, 1 )
ĊF2CN
( IF = 1 )
ĊF2CN
( IF = 0 )
A0
11010.2
11011.040(36)
11010.702[b]
11010.702(93)
B0
4080.5
4081.7276(4)
4081.6989(43)
4081.7257(7)
C0
2982.5
2989.9342(3)
2989.9666(46)
2989.9361(8)
ΔN  103
0.566
0.624(9)
1.01(8)
0.63(5)
ΔNK  103
16.3
17.93(5)
16.5(5)
18.7(2)
ΔK  103
-1.27
-11(7)
[a]
[a]
δN  103
0.160
0.178(4)
[b]
0.124(11)
δK  103
9.70
[a]
[a]
[a]
N  103
N/A
N/A
[b]
2.2(8)
aa
N/A
-46.565(5)
-46.661(19)
-46.546(6)
bb
N/A
-23.685(2)
-23.741(10)
-23.669(4)
cc
N/A
-0.1248(9)
-0.092(8)
-0.121(2)
χaa(N)
-4.97
-4.379(1)
-4.396(6)
-4.386(5)
(χbb-χcc)(N)
2.05
1.248(6)
1.320(49)
1.249(8)
χac(N)
-0.18
[a]
[a]
[a]
aF(N)
4.49
7.907(2)
7.901(3)
7.913(3)
Taa(N)
-17.9
-12.154(3)
-12.170(6)
-12.165(4)
(Tbb-Tcc)(N)
-45.7
-32.618(6)
-32.600(50)
-32.646(3)
Tab(N)
N/A
2.9(4)
11.2(24)
12.8(13)
aF(F)
95.9
189.16(3)
189.22(3)
N/A
Taa(F)
-189
-178.664(4)
-178.672(4)
N/A
(Tbb-Tcc)(F)
-600
-565.19(3)
-565.27(3)
N/A
3.9 kHz / 156
7.1 kHz / 78
6.2 kHz / 64
σ / # of lines
[a]
: Fixed at the DFT (UB3LYP/aug-cc-pvqz) calculation predicted value.
[b]
: Fixed as the value obtained from the nuclear spin singlet state ĊF2CN (IF = 0).
Molecular geometry & Fermi contact terms
 Structural analysis:
 Inertia defect of ĊF2CN: Δc= -0.68566(15) amuÅ2
 Inertia defect of ĊF2CCH: Δc= -0.085147(44) amuÅ2
 Fermi contact coupling constant, aF
 Pred. aF(N) = 4.5 MHz
Meas. aF(N) = 7.9 MHz
 Pred. aF(F) = 96 MHz
Meas. aF(F) = 189 MHz
 Why there are so large discrepancies?
aF = -(8π/3)gSgIβSβI |ψ(0)|2

How to chose the basis sets for your ab initio calculations?
Discussions - unexpected doublet splittings
 Hougen’s comment
 My explanations
 Vibration-rotation interaction
 :CF2 inversion → 0+ & 0 Similar to the δ-potential
 Very narrow barrier: ~ 0.2Å
 The barrier could be high
 not all authors agree
Future work
 c-type transitions
 Fit vibration-rotation coupling constants
 Calculate the barrier height:
UCCSD(T)/aug-cc-pVQZ // B3LYP/aug-cc-pVQZ
 ĊF2CCD, ĊF2CCF
Acknowledgements
 Dr. James R. Cheeseman, Gaussian Inc., CT
 Dr. Michael J. Frisch, Gaussian Inc., CT
 Prof. Patrick Thaddeus, CFA, Harvard, MA
 Dr. Michael C. McCarthy, CFA, Harvard, MA
 Prof. Wei Lin, University of Saint Mary, KS