63 rd OSU International Symposiumon Molecular Spectroscopy WF

63 rd OSU International Symposiumon Molecular Spectroscopy WF 09 Assignment and analysis of the rotational spectrum ofbromoformenabled 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 bromoformmolecule ( HCBr 3 ) : 5 atoms 9 normal modes Oblate symmetric top mc 1 D Problems: Four species of comparable isotopic population due to 79 Br: 81 Br 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 forbromoform: ð Supersonic expansion chirped-pulse FTMW spectra of HCBr 3 and DCBr 3 (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 ð Some Balle-Flygare type supersonic expansion measurements for the J = 3 2 to J = 5 4 transitions of HCBr 3 ð Room temperature millimetre wave spectra of HCBr 3 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 ofbromoformat 5 GHz: Hyperfine splitting dominates over the isotopic and asymmetry splitting in asymmetric species (A -B 20 MHz so that K c = 1 splitting is 40 MHz) Line in dataset for HC 79 Br 281 Br HC 81 Br 3 HC 79 Br 81 Br 2 HC 79 Br 3 HC 79 Br 281 Br

… and the same J transition zoomed in: HC 79 Br 281 Br

The J = 3 2 rotational transition ofbromoformat 7. 4 GHz: Isotopic and asymmetry splitting begin to win over the hyperfine splitting HC 79 Br 281 Br

… as for the lower J transition all lines have been recorded at excellent S/N and can be measured to better than 10 k. Hz:

How to assign and fit such a spectrum: ð Use the techniques for dealing with symmetric top three-quadrupolar hyperfine structure developed previously for CH 3 CCl 3 and CHCl 3 (J. Mol. Spectrosc. 189, 228(1998); 238, 72(2006)). Only three adjustable quadrupolar parameters are required: ð Use SPFIT/SPCAT for fitting/prediction ð Use carefully scaled ab initio predictions of hyperfine constants on the basis of known experimental data for CH 3 Br and CH 2 Br 2 ð 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

81 Br at 218 GHz: The J = 90 89 rotational transition of HC 3 (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: HC 81 Br 3 J = 90 89 218 GHz HC 79 Br 281 Br J = 88 87 217 GHz K from 6 to 55 Kc from 17 to 65

Global fits forbromoform– rotational part of the Hamiltonian: Chirped-pulse FTMW: Cavity FTMW: MMW: 308 lines 59 lines 348 lines s = 7. 17 k. Hz 2. 16 k. Hz 42. 6 k. Hz

Global fits forbromoform– quadrupole Hamiltonian: The number of the bromine nucleus (Br 1 is always on a symmetry plane) Similar results have been obtained for the four DCBr 3 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 r 0 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 c. Br = 0. 0336 Å causes some problems. 106. 96(3) 1. 105(3) Å Å ) 3 ( 89 1. 91 111. 86

Conclusions: ð The assignment and analysis of the rotational spectrum of bromoform was only possible because of the availability of the broadband, chirped-pulse FTMW spectrum. ð Assignment of the FTMW spectrum and carefully scaled ab initio calculations made it possible to find corresponding ground state transitions in the MMW region. ð 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. ð Many cross-checks on the derived constants confirm the validity of the fitted model. ð Implications of the derived values on the molecular properties of bromoform and investigation of the evolution of such properties in the series CH 3 Br, CH 2 Br 2, CHBr 3 is in progress.