Transcript cyanamide.ppt

63rd OSU International Symposium on Molecular Spectroscopy
WK08
Detailed analysis of the 0+-0- inversion
doublet in D2NCN and HDNCN cyanamide
Zbigniew Kisiel, Adam Krasnicki
Institute of Physics, Polish Academy of Sciences
Brenda P. Winnewisser, Manfred Winnewisser
Department of Physics, The Ohio State University
The cyanamide molecule ( H2NCN ) :
5 atoms, ma = 4.319(4) D
NH2 inversion motion with a low
barrier double-minimum potential:
H2NCN: 0+-0- = 49 cm-1
cmw:
to 120 GHz:
srb analysis:
to 500 GHz:
14N splitting:
FT far ir:
ir to 980 cm-1:
Millen,Topping,Lide, J.Mol.Spectrosc. 8, 153 (1962)
Tyler,Sheridan,Costain, J.Mol.Spectrosc. 43, 248 (1972)
Johnson,Suenram,Lafferty, Astrophys. J. 208, 245 (1976)
Brown,Godfrey,Kleibomer, J.Mol.Spectrosc. 114, 257 (1985)
Read,Cohen,Pickett, J.Mol.Spectrosc. 115, 316 (1986)
Brown et al., J.Mol.Spectrosc. 130, 213 (1988)
Birk,Winnewisser, J.Mol.Spectrosc. 159, 69 (1993)
Moruzzi,Jabs,2Winnewisser, J.Mol.Spectrosc. 190, 353 (1998)
astrophysical:
Turner et al., Astrophys. J. 201, L149 (1975)
The spectra :
MMW spectrum (Istok BWO based
spectrometers in Giessen and Koln):
118 – 179 GHz
202 – 221 GHz
570 – 650 GHz
(all three species)
Far-infrared spectrum (IFS 120HR
in Giessen, res. 0.00167 cm-1):
8 – 350 cm-1
(only D2NCN)
Transitions:
a-type rotational:
c-type interstate:
0+  0+
0-  00-  0+
0+  0-
Wolfgang Jabs, Giessen 1998
The J = 9  8 rotational transition of HDNCN:
white = 0+
green = 0-
Transition in fit
The J = 9  8 rotational transition of D2NCN:
5, 05, 0+
white = 0+
yellow = 0-
Ka = 2, 0+
2, 0-
The 0-  0+ cQ-branch in D2NCN for Ka = 11  10 :
The 0+  0- cQ-branch in D2NCN for Ka = 7  6 :
LW plot for the 0-  0+ cR-branch in D2NCN for Ka = 8  7 :
strip width = 0.5 cm-1
central frequencies
are from 92.7 cm-1
to 119.8 cm-1
LW plot for successive Ka = 7 aR-branch transitions D2NCN :
Comparison of MMW and FTIR spectra of D2NCN :
The Hamiltonian :
This will be in block form: Each state will have a diagonal block for the
rotational Hamiltonian and its vibrational energy or energy separation DE.
H = Hrot(0+)
Hint
Hint
Hrot(0-) + DE
For an inversion doublet the off-diagonal blocks can be set up in the reduced
axis system of Pickett. In D2NCN the inverting NH2 group pivots around the baxis (in the ac plane) so that:
Hint = (Fca + FcaJ P 2 + FcaK Pz 2 + …) (Pc Pa + Pa Pc )
For HDNCN the inversion plane is slightly off the ac plane so that:
Hint = (Fca + FcaJ P 2 + FcaK Pz 2 + …) (Pc Pa + Pa Pc ) +
(Fbc + FbcJ P 2 + FbcK Pz 2 + …) (Pb Pc + Pc Pb )
Comparison of fitted constants for HDNCN and D2NCN :
(Rotational and centrifugal distortion constants)
Comparison of constants for H2NCN, HDNCN and D2NCN :
(Interaction constants)
Data distribution plots for the 0+ - 0- system in D2NCN:
symbols  (o-c)i / si
red for o-c > 3 s
fitted lines: 3039
rms:
1.16
Specific 0+  0- perturbations in D2NCN at low Ka :
B+C
Similar Ka but without
J-selective perturbation
The aR-branch in D2NCN aligned on a perturbed sequence:
cm-1
Conclusions :
 The rotational and vibration-rotation transitions in D2NCN 0+-0- inversion
doublet have been assigned and fitted up to J = 84, Ka = 15, and 173 cm-1
 The transitions in the 0+-0- inversion doublet of HDNCN have been fitted up
to J = 33, Ka = 10, and 646 GHz.
 There is a delicate balance between the lengths of centrifugal distortion
expansions in Hrot and Hint
 The three accurate 0+-0- energy level separations allow useful tests of isotope
effects in double-minimum potentials (in progress).
We are indebted to Wolfgang Jabs (Giessen) who recorded all of the spectra
used in this work.