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WEAK CHWEAK CH……F BRIDGES AND INTERNAL DYNAMICSF BRIDGES AND INTERNAL DYNAMICS
IN THE CHIN THE CH33F-CHFF-CHF33
MOLECULAR COMPLEX MOLECULAR COMPLEX
Walther Caminati, Università di Bologna, ItalyJuan C. Lopez, Jose L. Alonso, Universidad de Valladolid, Spain
Jens-Uwe Grabow, Universität Hannover, Germany
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Hydrogen Bond Interactions
A) Normal Hydrogen Bond (Strong, ≈ 25 kJ/mol)B) Improper Hydrogen Bond (Weak, << 25 kJ/mol)
C) Anti Hydrogen Bond (Weak, << 25 kJ/mol)
The weak hydrogen bond in structural chemistry and biology, IUCr Monographs on crystallography, Vol. IX (G.R. Desiraju, T. Steiner eds.), Oxford University Press (2001).
E. Kryachko, S. Scheiner, J. Phys. Chem. A 108 (2004) 2527.
S. N. Delanoye, W.A. Herrebout, B.J. Van der Veken, J. Am. Chem. Soc. 124 (2002) 11854.
data are available from X-ray diffraction, theoretical calculations, IR absorption in rare gas solutions, and rotationally resolved spectroscopy.
T. Steiner, Angew. Chem. 114 (2002) 50; Angew. Chem. Int. Ed. 41 (2002) 48.
C. G. Cole, A. C. Legon, Chem. Phys. Lett. 396 (2003) 31.
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•Supersonic Jet ExpansionMolecular ClustersHydrogen BondingConformational
Equilibria •Molecular Dynamics
Large Amplitude MotionsInternal Rotation
FT-MW spectroscopy of species withweak hydrogen bonds (WHB)
Standard ab-initio and DFT calculations: Gaussian or Gamess.
Economy calculations: Distributed polarizability model.
Conformations and potential energy surfaces of molecular adducts.
Effective Hamiltonian fits: Watson, coupled, …
Analysis withobs.-calc. deviations down to a few kHz (< 10-7 cm-1).
Effective potential function fits: flexible model
Inclusion of structural relaxations.
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Bologna supersonic jet FT-MW Spectrometer:
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Coaxial oriented Beam-Resonator Arrangement (COBRA)
Fabry-Perot resonator
resonatortuning
FT
FID
Impulse
polarization pulse:
coherence between
rotating molecular dipoles
oscillating macroscopic
dipole moment:
electromagnetic field at frequencies
of molecular transitions
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Plausible Conformations of CH3F…CHF3
MP2/6-311++G(2df,2p):
E/cm-1 I: 713.8 II: 0.0 III: 29.7 IV: 113.9 V: 119.7
I II III IV V
I + II: minima.Three non-linearweek hydrogen-bonds (WHB)different dipole-dipole
interaction.III + V: saddle points.
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Permutation inversion group theory
CH3 group & CF3 group: C3(1)C3(2) = G9
Cs point group symmetry: E,
Molecular symmetry group: G18
H1
Cl
H3H2
C3 axis
C3 E C3 C3²
E (123) (123)² = (132)A1 1 1 1
E 1 *
1 *
H. C. Longuet-Higgins, Mol. Phys. 6 (1963), 445.J. T. Hougen, J. Chem. Phys. 37 (1962), 1433; J. Chem. Phys. 39 (1963), 358.
C3 group
X1
X2X3
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Tunneling pathways and Tunneling Hamiltonian Matrix
symmetric matrix, 5 eigenvalues:
W1= W(A1)=H11+2HH+2HF+2HA+2HG
W2=W3=W(E1)=H11+2HH-HF-HA-HG
W4=W5=W(E2)=H11-HH+2HF-HA-HG
W6=W7=W(E3)=H11-HH-HF+2HA-HG
W8=W9=W(E4)=H11-HH-HF-HA+2HG
HH: rotation of CH3-groupHF: rotation of CF3-groupHA: anti-geared rotation of CX3-groupsHG: geared rotation of CX3-groups
H11
H11
H11
H11
H11
H11
H11
H11
H11
HH HH
HH
HH
HH
HH
HHHH
HH
HF
HF
HF
HF
HF
HF
HF
HF
HF HA
HA
HA
HA
HA
HA
HA
HA
HA HG
HG
HG
HG
HG
HG
HG
HG
HG
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Energy level splitting diagram for K = 0
HH >> HF > HA ≈ -HG Γ(G18)
J0J
-CH3 -CF3
E
EE
EA
AE
A
AA
E4
E2
E1
A1
Δ'AE
ΔAE
E3
2HH
2HF
2HA 2HG
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Internal Rotation Fine Structure
J,Ka,Kc ← J,Ka,Kc = 31,3←21,2
CH3 top & CF3 top
Only one splitting observed
Effective moment of inertia Iα = 86.0(3) uÅ2 for A1-E1 splitting of CH3F-CHF3
only slightly smaller thanIα = 89.23(2) uÅ2 for isolated CHF3
E1
A1 E1
A1
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CH3F-CHF3 CH3F-13CHF3 13CH3F-CHF3
A/MHz 6451.696(2) 6456(5) 6440(2)
B/MHz 1459.976(6) 1456.23(2) 1430.48(1)
C/MHz 1413.440(6) 1410.10(2) 1386.47(1)
DJ/kHz 2.740 (3) 2.720(7) 2.668(3)
DJK/kHz 53.44(2) 52.2(5) 51.2(2)
DK/kHz 16.3(3) [16.3] [a] [16.3] [a]
dJ/kHz 0.222(2) 0.219(5) 0.204(2)
dK/kHz 1.4(90) [1.4] [a] [1.4] [a]
I / u Å2 86.0(3) 89(3) 86(2)
(a, i) / 51.4(1) 53(2) 50.6(8)
V3 / kJ mol-1 0.840(5) 0.80(3) 0.88(2)
Pbb/uÅ2 44.865(1) [b] 44.82(4) 44.85(2)
CH3F-CHF3 Spectrocopic Constants and CF3-Top Internal Rotation
[a]: Parameter fixed to value of normal species.
[b]: Smaller than sum of values of CH3F and CHF3.
CF3
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Oxirane-Difluoromethane: Planar Moment
Pii = 1/2 (-Iii + Ijj + Ikk) , i,j,k = a,b,c
S. Blanco, J.C. Lopez, A. Lesarri, W. Caminati, J.L. Alonso, ChemPhysChem 5 (2004) 1779.
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CH3-Top Internal Rotation
Pbb of the complex should amount to
Pbb(CHF3) + Pbb(CH3F) = 1.55 uÅ2 + 44.62 uÅ2 = 46.17 uÅ2.
Experimental Pbb of the complex isPbb(CHF3
…CH3F) = 44.69 uÅ2, i.e. 1.48 uÅ2 smaller.
Planar moment of inertia Pbb = (h/16π2)(-1/B + 1/A + 1/C)
is related to V3 barrier of internal rotor by:
A00 = Ar + W00(2) F ρa
2
B00 = Br
C00 = Cr+ W00(2) F ρc
2
Experimental Pbb is reproduced for
V3 (CH3) = 0.36 kJ/mol (= 30 cm-1).
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Barriers to Internal Rotation:CH3 and CF3 Tops
V3 (CH3) = 0.36 kJ/mol
V’3 (CF3) = 0.840(5) kJ/mol
MP2/6-311++G(2df,2p): EXPERIMENTAL:
from planar moments Pbb
from A1-E1 splittings
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Internal rotor CF3 CH3
I / u Å2 85.0 3.20
V3 / kJ mol-1 0.840 0.36
s = 4V3/9F 67.92 2.5
AE[a] / MHz 2.6·10-1 1.2·105
Nlevels[b] 12 2
CF3 and CH3 Internal Rotors:
Energy Spacing
Barrier & Inertia
Ratio 106
[a]: Energy spacing between A1 and E1 or E2 sublevels.
[b]: Number of torsional levels below the V3 barrier.
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CF3 and CH3 LAM Potential Surface
Weak CH···F Bridges and Internal Dynamics inthe CH3F·CHF3 Molecular Complex**
Walther Caminati,* Juan C. Lòpez, José L. Alonso, and Jens-Uwe Grabow
Angew. Chem. Int. Ed. 2005, 44, 3840-3844
Molecular Complexes VIP
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CH3F…CHF3 Complex:Force Constant and Dissociation Energy
Stabilized by three weak CHF hydrogen bonds (WHB)and electrostatic dipole-dipole interaction.
ks = 5.2 Nm-1
ED = 5.3 kJ·mol-1 1.8 kJ·mol-1/[CHF] (<< 25kJ·mol-1)
DJ = DJ(eff) – [-1/2 (ρb4 + ρc
4) W00(4) F] (small contribution)
ks = 16π4 (μ RCM)2 [4B4 + 4C4 - (B-C)2 (B+C)2] / (h DJ)ED(Lennard-Jones) = 1/72 ks RCM
2
V’3 (CF3) = 0.84 kJ·mol-1
V3 (CH3) = 0.36 kJ·mol-1
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Structural Parameters
r0 rs MP2/6-311 ++G(2df,2p)
r(FCH3F…HCHF3)/pm 242.7(10)[a] 240.5
α/° 124.1(3) [a] 122.3
β/° 115.5(3) [a] 109.9 r(FCHF3…HCH3F)/pm 305.2[b] 284.2 r(CCH3F…CCHF3)/pm 364.5[b] 360(2) [c] 352.2
[a] fitted parameter
[b] derived parameter[c] from rs coordinates
CCHF3: |a| = 84(6) pm
|b| = 0*
|c| = 43(4) pm
CCH3F: |a| = 261.9(7) pm
|b| = 0*
|c| = 57(3) pm
*set to zero because imaginary
rα
β
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AcknowledgementAcknowledgement
Deutsche Forschungsgemeinschaft (DFG)
Land Niedersachsen
Deutscher Akademischer
Austauschdienst (DAAD)
Directión General de Investigación –
Ministerio de Ciencia y Technología the
Junta de Castilla y León
Minister of Education, Spain
Università di Bologna
Minister of Education, Italy