Transcript bform.ppt

63rd OSU International Symposium on Molecular Spectroscopy
WF09
Assignment and analysis of the rotational
spectrum of bromoform enabled by
broadband FTMW spectroscopy
Z.Kisiel, A.Krasnicki, L.Pszczolkowski
Institute of Physics, Polish Academy of Sciences
S.T. Shipman, L. Alvarez-Valtierra, B.H. Pate
Department of Chemistry, University of Virginia
The bromoform molecule ( HCBr3 ) :
5 atoms
9 normal modes
Oblate symmetric top
mc  1 D
Problems:
Four species of comparable isotopic population due to 79Br:81Br  1:1
Complex and extensive hyperfine structure in low-J transitions
Early studies:
Kojima et al., J.Chem.Phys. 20,804(1952)
J=10 transition at 2.5 GHz
Williams,Cox,Gordy, J.Chem.Phys. 20,1524(1952)
J=11  10 to 15 14 transitions at 27-37 GHz
The spectra recorded for bromoform:
 Supersonic expansion chirped-pulse FTMW spectra of HCBr3 and
DCBr3 (averaged over 200000 gas pulses) for:
partial J = 1  0 transition at 2.5 GHz
complete J = 2  1 transition at 5 GHz
complete J = 3  2 transition at 7.4 GHz
 Some Balle-Flygare type supersonic expansion measurements for the
J = 3  2 to J = 5  4 transitions of HCBr3
 Room temperature millimetre wave spectra of HCBr3 at 166-318
GHz.
The analysis was made with SPFIT/SPCAT and the AABS package.
Initial predictions were made using ab initio calculations of
quadrupole splitting constants and of quartic centrifugal distortion
constants.
The J = 2  1 rotational transition of bromoform at 5 GHz:
Hyperfine splitting dominates over
the isotopic and asymmetry splitting
in asymmetric species
(A-B  20 MHz so that Kc = 1
splitting is  40 MHz)
Line in dataset for
HC79Br281Br
HC81Br3
HC79Br81Br2
HC79Br3
HC79Br281Br
… and the same J transition zoomed in:
HC79Br281Br
The J = 3  2 rotational transition of bromoform at 7.4 GHz:
Isotopic and asymmetry splitting
begin to win over the hyperfine
splitting
HC79Br281Br
… as for the lower J transition all lines have been recorded at excellent S/N and
can be measured to better than 10 kHz:
How to assign and fit such a spectrum:
 Use the techniques for dealing with symmetric top three-quadrupolar
hyperfine structure developed previously for CH3CCl3 and CHCl3
(J.Mol.Spectrosc. 189,228(1998); 238,72(2006)). Only three
adjustable quadrupolar parameters are required:
 Use SPFIT/SPCAT for fitting/prediction
 Use carefully scaled ab initio predictions of hyperfine constants on
the basis of known experimental data for CH3Br and CH2Br2
 Carry out assignment in graphical mode and set up the datafiles with
the AABS package. Use the AABS package also to keep track of
what is in the dataset and of lines from the other isotopic species
The J = 90  89 rotational transition of HC81Br3 at 218 GHz:
(low K region)
K=0
10
20
… and the same J transition at higher K where hyperfine splitting appears.
K = 45
50
Evolution of line profiles for MMW transitions of bromoform:
HC81Br3
J = 90  89
218 GHz
HC79Br281Br
J = 88  87
217 GHz
K from
6 to 55
Kc from
17 to 65
Global fits for bromoform – rotational part of the Hamiltonian:
Chirped-pulse FTMW:
Cavity FTMW:
MMW:
308 lines
59 lines
348 lines
s = 7.17 kHz
2.16 kHz
42.6 kHz
Global fits for bromoform – quadrupole Hamiltonian:
The number of the
bromine nucleus
(Br1 is always on a
symmetry plane)
Similar results have been obtained
for the four DCBr3 isotopic species
Components of the inertial and principal c tensors:
Comparison of B (MHz) with previous work:
Good agreement +
interesting systematic
difference of 0.1 MHz
The r0 geometry of bromoform:
From fit to 18 moments of inertia for 8 isotopic species to an average
deviation of 0.0117 uÅ2
The very small cBr = 0.0336 Å causes some problems.
Conclusions:
 The assignment and analysis of the rotational spectrum of bromoform was only
possible because of the availability of the broadband, chirped-pulse FTMW
spectrum.
 Assignment of the FTMW spectrum and carefully scaled ab initio calculations
made it possible to find corresponding ground state transitions in the MMW
region.
 Global fits of all of the available data resulted in very precise rotationl, c.d., and
hyperfine splitting constants for eight isotopic species of bromoform.
 Many cross-checks on the derived constants confirm the validity of the fitted
model.
 Implications of the derived values on the molecular properties of bromoform
and investigation of the evolution of such properties in the series CH3Br,
CH2Br2, CHBr3 is in progress.