Transcript H2CS-laff-JM.ppt

RO-VIBRATIONAL ANALYSIS OF THE ν4 , ν6, AND ν3
BANDS OF THIOFORMALDEHYDE (H2CS):
EXAMPLE OF A MASSIVE A-TYPE CORIOLIS
RESONANCE
W. J. LAFFERTY
Optical Technology Division,
NIST, Gaithersburg, MD 20899,USA
J.-M. FLAUD, A. PERRIN,
LISA, CNRS/ Univ. Paris 12 et 7
61 Av. du Général de Gaulle
904910 Créteil, France
H. BECKERS, Y, S. KIM, H. WILLNER
Anorg. Chemistry, University of Wuppertal,
D-42119 Wuppertal, GERMANY
Introduction
• H2CS is one of the many molecular species found in interstellar space.
Not only is the normal isotopic species observed but also the –d1 and –d2
species as well.
• The molecule is of spectroscopic interest because the two lowest
vibrations are only about 0.8 cm-1 apart and are coupled by a massive
Coriolis resonance. The ro-vibration levels are close to being 50% mixed.
• The first spectroscopic works on H2CS were reported at NBS by
Johnson, Powell and Kirchoff in the MW region and by Johns and Olson
in the 3 μm region.
• The three low frequency fundamentals ν4 , ν6 and ν3 were studied using
laser-stark techniques by Bedwell and Duxbury.
• A very impressive study of H2CS and D2CS was reported in 1981 by
Turner, Halonen and Mills using FTS spectra ( resolution ~ 0.1 cm-1).
Unfortunately, the low resolution limited the results of this work.
• A generation later we have returned to study this molecule once
again with 0.005 cm-1 resolution, 32 m optical path and low pressure.
EXPERIMENTAL CONDITIONS
•Bruker 120 HR FTS with globar source, KBr beam splitter and MCT
detector
•Resolution – 0.005 cm-1
•Pressures - 0.045 and 0.113 Torr
•White-type multiple pass cell with 32 m optical path
•H2CS produced by low pressure pyrolysis of CH3SCl or C3H5SCH3 and
swept into the cell in an Ar stream.
CH2=CH-CH2--S-CH3 → CH2=CH-CH3 + S=CH2 (560o C)
Cl-S-CH3 → HCl + S=CH2 (1150o C)
Structure and symmetry properties
b (x)
C2v Point Group for H2CS
H
S
C
a (z)
H
E
C2z
σyz
σxz
A1
1
1
1
1
Tz , ν1, ν2,
ν3
A2
1
1
-1
-1
Rz
B1
1
-1
1
-1
Ty ,Rx ,ν4
B2
1
-1
-1
1
Tx , Ry , ν5,
ν6
Nuclear Statistical Weights for the Ground State Rotational Levels
Ka
Kc
Nuclear Spin Weight
e
e
1
e
o
1
o
o
3
o
e
3
Line assignment procedure
The 3 lowest fundamental frequencies are at relatively high wavenumbers→ Hot band
transitions are not a problem; however, the 34S species produces 4% of the total lines.
ν3 1059.2 cm-1 C=S stretching vibration: A-type selection rules.
ν4 990.18 cm-1 Out-of-plane H-wagging: C-type selection rules.
ν6 991.02 cm-1. In-plane C-H rock:
B-type selection rules.
The 31 levels interact with the two lower levels, but are far enough away so that the ν3
transitions can be easily established by pattern recognition and the assignments
confirmed with the aid of GSCD.
Assignment of the transitions of ν4 and ν6, is another matter! The states are
separated by only 0.84 cm-1 and the levels are linked by a massive z-type Coriolis
resonance → The rotational levels are mixed 50-50 and the familiar patterns are
nonexistent.
The constants from Bedwell and Duxbury were used to produce a first estimate of the
spectrum. Then line series were picked out and verified using GSCD. It was necessary
to make a number of line intensity measurements to really verify the line assignments
with confidence.
Q-branch features near the band centers of 4 and 6
 
"
"
Ka = 1, Kc = J - 1
J = 0
"
"
Ka = 0
J = 0
980
985
990
"
Ka = 1, Kc = J - 1
J = 0
J = 0
995
Wavenumber in cm
"
1000
-1
"
Ka = 1, Kc = J
Hamiltonian matrix used to calculate
the energy levels of the of the {41,61
and 31} interacting states of H2CS
41
61
31
41
HW
Herm conj
Herm conj
61
CA
HW
Herm conj
31
CB
CC
HW
v-diagonal operators:
HW = EV + [Av – ½ (B + C)] Jz2 + ½(Bv+Cv)J2 + ½(Bv-Cv) Jxy2 + . . . .
v-off-diagonal operators:
CA = h1A Jz + h2A J2Jz + h3A Jz2Jz +
CB = h1BJx + h2B [Jx ,Jz2] + . . . .
CC = h1C iJy + h2C J2 i Jy + . . . .
with [A,B] = AB + BA and Jxy2 = Jx2 – Jy2
. . . .
Vibrational Band Centers, Rotational and
Interaction Constants of H2CS in cm-1
00
Ev
41
61
31
990.18033(30)a
991.01859(40)
1059.20254(30)
A
9.727262(270)b
9.82307(220)
9.64719(220)
9.7163189
B
0.5903972(210)
0.5882697(190)
0.59100470(470)
0.5863150(180)
C
0.5554395(210)
0.55562267(240)
0.55284801(430)
0.55319658(150)
ΔK x 103
0.78434(1300)
0.645162(5700)
0.887535(6200)
0.796018(2600)
ΔJK x 104
0.173575(2800)
0.195237(6400)
0.192807(6800)
0.154089(1400)
ΔJ x 106
0.64429(2000)
0.641346(100)
0.640092(3400)
0.659471(1700)
δK x 104
0.12507(2000)
c
0.14410(1400)
c
δJ x 104
0.40047(3900)
c
c
c
HK x 106
0.2680(1800)
c
0.13133(2100)
0.47867(3400)
a Uncertainties are 1σ., b G S constants from MW data and IR GSCD, c Fixed at GS values.
Interaction Constants
Jz
<61 | 41>
= -10.01844(100)
<31| 61>
iJy = 0.22078(2900) x 10-1
+ Rotational corrections
<31| 41>
Jx = 0.203266(3300)
Relative line intensities
The equivalent widths were measured for 113 lines in
the P, Q, R branches with J values ranging from 2 to 35
and Ka values from 0 to 6.
The statistical analysis of the fit is as follows:
0< δ < 12% 52.2% of the lines
12< δ < 24% 26.6 % of the lines
24< δ < 48% 21.2 % of the lines
δ = │ECalc-EObs│/EObs
In relative units the band intensities are:
(004)-(000)
1
(006)-(000)
0.83
(003)-(000)
0.44
CONCLUSION
The IR spectrum of the unstable molecule, H2CS, has been obtained over
the range 800 to 3100 cm-1.
At this point, the three lowest fundamental bands have been assigned.
The two lowest bands, ν4 and ν6, are coupled by an intense A-type Coriolis
resonance in which the ro-vibrational levels are mixed 50-50. They are
both also coupled to ν3.
In order to verify some line assignments, a number of transition intensities
have been measured.
The transitions of all three states have been fit simultaneously and the
rotational constants and interaction constants derived.
Work is now proceeding on the assignments and fitting of the 3 μ bands.