The rovibrational absorption spectrum of the weakly bound cluster CO 2 -N 2 O has been observed in the region of the ν 3 CO 2 asymmetric stretch (∼2350 cm -1 ). Clusters were formed by supersonic expansion of a mixture of N 2 O:CO 2 in a 4:1 ratio using He carrier gas. Approximately 340 lines have been assigned. Both parallel and perpendicular transitions were observed. The ground state rotational constants A, B, and C are 0.294 924-(12), 0.058 004(6), and 0.048 400(6) cm -1 , respectively. The quartic centrifugal distortion constants are D j ) 5.03(17) × 10 -7 cm -1 , D jk ) -3.92(9) × 10 -6 cm -1 , D k ) 1.26(2) × 10 -5 cm -1 , δ j ) 1.61(92) × 10 -8 cm -1 , and δ k ) 1.77(80) × 10 -6 cm -1 . Ab initio calculations for several CO 2 -N 2 O equilibrium structures have been done on the Hartree-Fock self-consistent field level using a 6-311g* basis set. Møller-Plesset MP2 and MP3 calculations were carried out for the two possible slipped parallel configurations of the dimer. The planar slipped parallel geometry with the oxygen atom of N 2 O nearest the CO 2 monomer was found to be the most stable structure. The spectral constants and stabilization energies were compared to those of the CO 2 and N 2 O homodimers. A planar slipped parallel geometry is observed with the O of N 2 O nearly over the C of CO 2 . R cm and θ are 3.4701 Å and 60.1°, respectively, assuming the CO 2 and N 2 O are parallel to each other.
The absorption spectrum of the weakly bound complex CO2−Br2 has been observed by probing the asymmetric stretch of the CO2 moiety near 2349 cm-1. The complex was formed by the supersonic expansion of a mixture of CO2 and bromine vapor with He as a carrier gas. CO2−Br2 was found to have a linear structure with one Br atom close to the center of mass of the system. The isotopic mixture of the second Br provided the splitting of the observed peaks into two. The measured rotational and distortion constants for 79Br and 81Br are B‘ = 0.0148044(26), D‘ = 6.43(17) × 10-8, B‘‘ = 0.0146591(26), D‘‘ = 6.01(16) × 10-8 and B‘ = 0.0146763(27), D‘ = 6.42(18) × 10-8, B‘‘ = 0.0145321(28), D‘‘ = 6.04(19) × 10-8, respectively (in cm-1). The distances from the Br2 bond center to the C atom are 5.116 and 5.083 Å for the ground and excited states, respectively, and the force constant of the van der Waals stretching mode is estimated to be 0.004 mdyn/Å in both states. The experimental values are compared with the results of ab initio calculations performed at the Hartree−Fock and MP2 levels. The origin of van der Waals binding between CO2 and Br2 and the floppiness of the complex are discussed.
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