Infrared spectra and ab initio calculations of the matrix isolated (CO 2 )´(H 2 SO 4 ) and (CO 2 )´(SO 3 ) complexes Aharon Givan a , Aharon Loewenschuss a, * , Claus J. Nielsen b a Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel b Department of Chemistry, University of Oslo, Blindern, N-0315 Oslo, Norway Received 18 April 2001; accepted 8 May 2001 Abstract In Ar/CO 2 /H 2 SO 4 layers deposited at 15±18 K, infrared bands originating in the (CO 2 )´(H 2 SO 4 ) and (CO 2 )´(SO 3 ) complexes were identi®ed. The complexes were found to be produced predominantly during the matrix deposition process. Ab initio calculations on these complexes are presented and compared with the experimental results. The (CO 2 )´(H 2 SO 4 ) complex is formed by an end-on OyCyO´´´HO interaction. The (CO 2 )´(SO 3 ) interaction occurs mainly via an interaction between the O atom of CO 2 and the S atom of SO 3 and a secondary interaction between the C atom of CO 2 and the most adjacent O atom of SO 3 . q 2002 Elsevier Science B.V. All rights reserved. Keywords: Sulfuric acid complexes; Matrix isolation spectroscopy; Infrared spectroscopy; Ab initio calculations 1. Introduction Sulfuric acid is an important atmospheric species found in signi®cant abundance in polar stratospheric clouds and in low temperature sub-micron aerosol drops [1]. In several recent studies we investigated the infrared spectra of pure H 2 SO 4 [2] and D 2 SO 4 [3] vapors (containing SO 3 and H 2 O/D 2 O in equili- brium concentrations) trapped in solid argon matrices along with the relevant ab initio calculations of both the structure and vibrational frequencies. We also reported the infrared spectra of molecular complexes of H 2 SO 4 Ð its dimer [2] and its adducts with atmo- spheric species like H 2 O[2],CO[4],N 2 ,NOandN 2 O 2 [5] and HCl [6] isolated in solid argon. Several vibra- tional bands of the mixed (L)´(H 2 O)´(H 2 SO 4 ) complexes [4,5] were also identi®ed, leading to an understanding of their bonding scheme. The inevi- table presence, due to chemical equilibrium, of SO 3 in the sulfuric-acid/argon matrix samples, also facili- tated the assignment of infrared features of the (SO 3 ) 2 [2], (OC)´(SO 3 ) [4] and the, much weaker, (N 2 )´(SO 3 ) [5] species. However, no evidence was found for the stabilization of the parallel complexes of SO 3 with H 2 O, NO and with N 2 O 2 in solid argon, although other studies reported the existence of stable (H 2 O) 1±2 ´(SO 3 ) molecular complexes in nitrogen [7] and oxygen [8] solids. The spectral results indicated that attachment of CO, N 2 , NO, N 2 O 2 and HCl to the H 2 SO 4 moiety occurs through hydrogen bonding to the O±H moiety of H 2 SO 4 (HCl has a secondary link to the SyO 2 oxygen [6]), whereas the OC´´´SO 3 interaction occurs via the respective C and S atoms. The molecular species stabilized in solid argon samples were shown to essentially originate in the Journal of Molecular Structure 604 (2002) 147±157 0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S0022-2860(01)00644-5 www.elsevier.com/locate/molstruc * Corresponding author. Tel.: 1972-2-658-5313; fax: 1972-2-658-5319. E-mail address: loewena@chem.ch.huji.ac.il (A. Loewenschuss).