The rotational spectrum of bromoacetyl chloride I. Merke a, * , N. Vaeck b , D. Petitprez c , G. Wlodarczak c a Institut fu ¨r Physikalische Chemie, RWTH Aachen, D-52056 Aachen, Germany b Laboratoire de Chimie Quantique et Photophysique, CP 160/09 Universite ´ Libre de Bruxelles, 50 Av. F. Roosevelt, B-1050, Bruxelles, Belgium c Laboratoire de Physique des lasers, Atomes et Mole ´cules and CERLA, UMR CNRS 8523, Universite ´ de Lille 1, F-59655 Villeneuve d’Ascq, France Received 18 May 2005; revised 30 June 2005; accepted 30 June 2005 Available online 26 August 2005 Dedicated to Dr Jean Demaison for his major contribution to spectroscopy and molecular structure determinations Abstract The rotational spectrum of bromoacetyl chloride, BrCH 2 COCl, has been assigned using a pulsed molecular beam Fourier transform microwave spectrometer. It has been possible to determine the rotational and quartic centrifugal distortion constants of the energetically favoured conformer (anti-periplanar) as well as the complete bromine and chlorine quadrupole coupling tensors including their off diagonal elements for the following isotopomers: 79 BrCH 2 CO 35 Cl, 81 BrCH 2 CO 35 Cl, 79 BrCH 2 CO 37 Cl, and 81 BrCH 2 CO 37 Cl. Experimental results are supported by quantum chemical calculations. q 2005 Elsevier B.V. All rights reserved. Keywords: Bromoacetyl chloride; Microwave spectroscopy; Quadrupole coupling 1. Introduction Bromoacetyl chloride is often considered as a textbook example to emphasize the importance of non-adiabatic interaction in chemistry [1]. Following previous experimen- tal work [2,3] the quantum dynamics of its photodissociation process has been recently investigated by ab initio methods [4,5]. During the course of these last theoretical calculations, an optimized geometry was derived for the ground electronic state and compared to experimental electron diffraction data [6] showing a ‘good agreement’. As no previous spectro- scopic studies were undertaken before, we decided to check experimentally the geometry of the most stable conformer of bromoacetyl chloride in the ground electronic state by the analysis of its rotational spectrum. 2. Experimental details and spectrum analysis Bromoacetyl chloride was purchased from Aldrich and used without further purification. Fourier transform microwave measurements were performed in the frequency region from 5 to 20 GHz using a supersonic molecular beam [7]. The sample was introduced as a mixture of about 1% bromoacetyl chloride in neon at a stagnation pressure of approximately 150 kPa. Even under supersonic beam conditions the spectrum appeared to be extremely crowded because all heavy atoms are close to the inertial a axis, and therefore, causing overlapping hyperfine patterns of the bromine and chlorine isotopomers for all observed rotational transitions. When starting the assignment we performed a first guess of the spectrum using structural data taken from ab initio calculations [4]. Quadrupole coupling constants of bromine and chlorine for this prediction were calculated by rotating the coupling tensors of bromine and chlorine determined in molecules containing such nuclei in similar environment into the principal inertial axes system of bromoacetyl chloride. Using these values as starting point it was possible to assign the spectra of the following isotopomers of the anti-periplanar conformer: 79 BrCH 2 CO 35 Cl, 81 BrCH 2- CO 35 Cl, 79 BrCH 2 CO 37 Cl, and 81 BrCH 2 CO 37 Cl. The spectra have been analysed according to Watson’s A-reduction, I r -representation using Pickett’s program [8]. The isotopomer 79 BrCH 2 CO 35 Cl was studied in great detail. We assigned more than 400 hyperfine com- ponents belonging to 61 different rotational transitions. Journal of Molecular Structure 780–781 (2006) 306–309 www.elsevier.com/locate/molstruc 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.06.040 * Corresponding author. E-mail address: ilona.merke@rwth-aachen.de (I. Merke).