Transcript dimet.ppt
The microwave spectrum of partially deuterated species of dimethyl ether a b D. Lauvergnat, L. Margulès, R. A. Motyenko, c d J.-C. Guillemin, and L. H. Coudert a LCP, CNRS/Université Paris-Sud, Orsay, France b PhLAM, CNRS/Université des Sciences et Technologies de Lille 1 Villeneuve d'Ascq, France c Sciences Chimiques de Rennes, Rennes, France d LISA, CNRS/Universités Paris Est et Denis Diderot, Créteil, France b Why are we interested in deuterated species? A large number of unidentified lines in the ISM may be due to partially deuterated species. Measuring partially deuterated species provides astronomers with a tool to measure the [D]/[H] ratio. In this talk the microwave spectrum of the partially deuterated species of dimethyl ether CH2DOCH3 will be investigated theoretically. Outline • Torsional energy levels of the normal species • Torsional energy levels of the deuterated species • Torsional Hamiltonian • Potential energy surface • Torsional functions • Nature of the torsional energy levels • Microwave spectrum of the deuterated species Torsional coordinates The torsional angles a1 and a2 are used PES of the normal species with: 1. Durig, Li, and Groner, JMS 62 (1976) 159. 2. MP2 with cc-PVTZ basis set. Torsional energy levels: normal species 4 tunneling sublevels G36 Torsional function: normal species (a1,a2) A1 sublevel What happens when the molecule is deuterated? The kinetic energy part of the torsional Hamiltonian is modified because of the mass change. The effective potential energy function for the torsion is changed due to zero-point energy effects. Kinetic energy change a1 Potential energy function change -1 V10 = 13.3 cm D-out of plane 10 cm-1 D-in plane Lauvergnat et al., JMS 256 (2009) 204 and Margulès et al., JMS 254 (2009) 55. Torsional energy level calculation Torsional energy levels: deuterated species 9 tunneling sublevels 3 nondegenerate 3 doubly degenerate Torsional function of the 1st A-type sublevel D-in plane Torsional function of the 2nd A-type sublevel D-out of plane Torsional function of the 3rd A-type sublevel D-out of plane Torsional energy levels D-out of plane Tunneling between a1 = 60 and a1 = 300 Internal rotation of the other methyl group D-in plane Torsional function centered around a1 = 180 Internal rotation of the other methyl group The microwave spectrum Two sets of transitions. Set I Set II The next steps Overall rotation will be taken into account. Rotational dependence of the various tunneling splittings will 1 be determined using the water dimer formalism. The energy difference D between the two sublevel sets should be calculated accurately. We can begin analyzing the microwave spectrum. 1. Hougen, JMS 114 (1985) 395 and Coudert and Hougen, JMS 139 (1990) 259.