title: Matrix isolation studies of carbonic acid--the vapor phase above the β-polymorph.
abstract: Twenty years ago two different polymorphs of carbonic acid, α- and β-H2CO3, were isolated as thin, crystalline films. They were characterized by infrared and, of late, by Raman spectroscopy. Determination of the crystal structure of these two polymorphs, using cryopowder and thin film X-ray diffraction techniques, has failed so far. Recently, we succeeded in sublimating α-H2CO3 and trapping the vapor phase in a noble gas matrix, which was analyzed by infrared spectroscopy. In the same way we have now investigated the β-polymorph. Unlike α-H2CO3, β-H2CO3 was regarded to decompose upon sublimation. Still, we have succeeded in isolation of undecomposed carbonic acid in the matrix and recondensation after removal of the matrix here. This possibility of sublimation and recondensation cycles of β-H2CO3 adds a new aspect to the chemistry of carbonic acid in astrophysical environments, especially because there is a direct way of β-H2CO3 formation in space, but none for α-H2CO3. Assignments of the FTIR spectra of the isolated molecules unambiguously reveal two different carbonic acid monomer conformers (C(2v) and C(s)). In contrast to the earlier study on α-H2CO3, we do not find evidence for centrosymmetric (C(2h)) carbonic acid dimers here. This suggests that two monomers are entropically favored at the sublimation temperature of 250 K for β-H2CO3, whereas they are not at the sublimation temperature of 210 K for α-H2CO3.
authors: Institute of Physical Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria.
authors: Jürgen
authors: J
authors: Bernard
authors: Roland G
authors: RG
authors: Huber
authors: Klaus R
authors: KR
authors: Liedl
authors: Hinrich
authors: H
authors: Grothe
authors: Thomas
authors: T
authors: Loerting
mesh_terms: D002255:Carbonic Acid / Q000302:isolation & purification*; D011108:Polymers / Q000737:chemistry*; D014835:Volatilization
chemical_list: D011108:Polymers; D002255:Carbonic Acid
references: J Chem Phys. 2009 May 28;130(20):204308
references: 19485450
references: Angew Chem Int Ed Engl. 2009;48(15):2690-4
references: 19212997
references: J Phys Chem A. 2011 Nov 3;115(43):12011-6
references: 21978172
references: J Chem Phys. 2007 May 28;126(20):204315
references: 17552770
references: J Chem Phys. 2011 Jan 28;134(4):044319
references: 21280737
references: Science. 2009 Dec 18;326(5960):1690-4
references: 19965381
references: J Phys Chem A. 2012 Feb 16;116(6):1638-47
references: 22239118
references: J Am Chem Soc. 2007 Nov 14;129(45):13863-71
references: 17944463
references: Science. 2009 Jul 3;325(5936):61-4
references: 19574384
references: Angew Chem Int Ed Engl. 2000 Mar;39(5):891-894
references: 10760883
references: Chemphyschem. 2010 Aug 2;11(11):2305-9
references: 20397242
references: Inorg Chem. 2006 Jul 24;45(15):5961-70
references: 16842001
references: Science. 2009 Jul 3;325(5936):64-7
references: 19574385
references: Chemphyschem. 2012 Sep 17;13(13):3087-91
references: 22707186
references: J Phys Chem A. 2006 May 25;110(20):6472-81
references: 16706404
references: Science. 1998 Feb 27;279(5355):1332-5
references: 9478889
references: Angew Chem Int Ed Engl. 2011 Feb 18;50(8):1939-43
references: 21328675