Transcript vicndip.pptx

66th OSU International Symposium on Molecular Spectroscopy
MH06
Improved dipole moments for
acrylonitrile and propionitrile
Zbigniew Kisiel and Adam Kraśnicki
Institute of Physics, Polish Academy of Sciences,
Al. Lotników 32/46, 02-668 Warszawa, Poland
m
Suspiciously strong a-type lines at THz frequencies:
1075,102  1065,101
1085,103  1075,102
Kisiel, Pszczolkowski, Drouin, Brauer, Pearson, J.Mol.Spectrosc. 258, 26 (2009)
Dipole moments in acrylonitrile and propionitrile:
 | m |  4D determined by the CN group
 ma component considerably larger than mb (while it is mb that is
required for THz intensities)
 Complications from 14N nuclear quadrupole hyperfine structure
 
 +
m
Acrylonitrile (vinyl cyanide)
Propionitrile (ethyl cyanide)
Previous values (Debye):
Acrylonitrile
[1]
[2]
ma
mb
3.68
1.25
3.815(12)
0.894(68)
mtot
3.89(8) 3.918(19)
Propionitrile
[1]
[2]
ma
mb
3.78(4) 3.85(2)
1.38(4) 1.23(2)
mtot
4.02(4) 4.05(3)
[1] Wilcox et al., J.Chem.Phys. 22,516(1954)
[2] Stolze+Sutter, Z.Naturforsch. 40a,998(1985)
(0.894  1.25)2 = 0.51
[1] Laurie, J.Chem.Phys. 31,1500(1959)
[2] Heise et al., Z.Naturforsch. 29a,1345(1974)
(1.23  1.38)2 = 0.79
The IFPAN Stark electrodes for uniform field distribution
Electrode spacing is necessarily large
so that careful design is required for
high-quality measurements.
[1] Kisiel et al., Chem.Phys.Lett. 325,523(2000)
[2] Kisiel et al., J.Mol.Spectrosc. 208,113(2001)
Analysis with QSTARK = Quadrupolar STARK program:
 Prediction/fitting of Stark effect by matrix diagonalization for each
combination of E and M
 Any top symmetry with either 0 or 1 quadrupolar nuclei
 Cell calibration with both CH3CN and CH3I (OCS is less useful at low
fields)
Available on:
prospe.htm
Chem.Phys.Lett. 325,523(2000)
Acta.Phys.Pol.A 112,S95(2007)
Transitions available for Stark measurement:
A /MHz
B
C
k
Acrylonitrile
Propionitrile
49850.6969(7)
4971.21248(6)
4513.82848(6)
27663.6829(5)
4714.21316(2)
4235.05948(6)
-0.9798
-0.9591
Irel @ 1K
101←000
9485.
a
0.3
212←111
202←101
211←110
18513.
18966.
19427.
a
a
a
0.08
1.0
0.08
212←303
15982.
b
0.002
MF = 0,1,2
Stark measurements for acrylonitrile:
Visible mixed order Stark behaviour with sensitivity not only to the value of ma,
but also to mb.
101  000
202  101
Stark measurements for acrylonitrile:
Additional Stark components with higher sensitivity to the value of mb.
211  110
212  303
bP
Current dipole moment determination for acrylonitrile:
experimental
ma
mb
/D
/D
mtot /D
Nlines
sfit/kHz
calculated*
[1954] [1985]
this work
me
m0
3.68
1.25
3.815(12)
0.894(68)
3.821(4)
0.687(8)
3.844
0.702
3.802
0.696
3.89(8) 3.918(19)
3.882(4)
3.907
3.865
95
3.23
*Calculation with CFOUR at the CCSD(T)/aug-cc-pVDZ level
Relative intensities of pairs of a-type and b-type transitions chosen for:
 similar intensity,
 similar values of quantum numbers,
 frequency proximity.
newmb
old mb
2
 mb   0.687 
 old   
 m   0.894   0.59
 b 
new
2
Intensity of THz-region transitions of acrylonitrile:
old mb
new mb

b-types at 59% of
previous intensity
ma = 0.64 D
mb = 2.04 D
Stark measurements for propionitrile:
Nuclear quadrupole structure was remeasured in supersonic expansion and a new
global fit of the data made as an enabling step.
101  000
202  101
Current dipole moment determination for propionitrile:
experimental
calculated
[1959]
[1974]
this work
me*
me**
/D
/D
3.78(4)
1.38(4)
3.85(2)
1.23(2)
3.816(4)
1.236(1)
3.846
1.241
3.822
1.229
mtot /D
4.07(4)
4.05(3)
4.011(4)
4.041
4.015
ma
mb
Nlines
sfit/kHz
87
2.96
*Calculation at the CCSD/aug-cc-pVDZ level
**Calculation at the CCSD(T)/aug-cc-pVDZ level
SUMMARY:
 The dipole moments of acrylonitrile and propionitrile were measured in
supersonic expansion on Stark lobes of resolved components of the 14N nuclear
quadrupole hyperfine structure.
 For acrylonitrile the mb component was found to be significantly smaller than the
literature value.
 The new experimental value of mb is supported by relative intensity measurements
near 300 GHz and is in good agreement with precise ab initio calculations
inclusive of the ground state contribution.
 The new mb leads to significantly lower transition intensities of the b-type
rotational transitions of acrylonitrile that dominate its THz-region spectrum
= 59% of previous values.
 For propionitrile the new results offer increased precision but are in agreement
with previous values.