31 July 1998 Chemical Physics Letters 292Ž. 1998 97±109 Decomposition modes of dioxirane, methyldioxirane and dimethyldioxirane Ð a CCSDž/ T , MR-AQCC and DFT investigation Dieter Cremer a,), Elfi Kraka a, Peter G. Szalay b a Department of Theoretical Chemistry, UniÕersity of Goteborg,ÈÊ Kemigarden 3, S-41296, Goteborg, È Sweden b Department of Theoretical Chemistry, EotÈÈÕos Lorand  ±UniÕersity, P.O. Box 32, H-1518, Budapest, Hungary Received 7 April 1998; accepted 8 June 1998 Abstract Formation and decomposition of dioxirane Ž.2a , methyldioxirane Ž.2b and dimethyldioxirane Ž.2c in the gas phase were investigated by carrying out CCSDŽ. T , MR-AQCC and B3LYP calculations with the 6-31G Ž d, p . , 6-311qG Ž 3df, 3pd . and cc-VTZ2Pqf, d basis sets. The inclusion of f functions in the basis set was essential to determine the heat of formation o y r D Hf Ž.298 of carbonyl oxide Ž.1a and 2a to be 27.0 and 0.3 kcal mol, respectively. With the latter value, we calculate the same ring strain energy for cyclopropane, oxirane and 2a. Molecule 2a decomposes at 298 K with an activation enthalpy of 18 kcalrmol to methylenebisŽ.Ž. oxy 3a , which is calculated to be 1.2 kcalrmol less stable than 2a, in contrast to previous investigations. Two methyl substituents increase the ring opening barrier to 23 kcalrmol and, thereby guarantee the kinetic stability of 2c. The biradicals 3 decompose with barriers smaller than 4 kcalrmol to esters and therefore will be difficult to intercept in dioxirane decomposition reactions. q 1998 Elsevier Science B.V. All rights reserved. 1. Introduction oxygen reactions, the reactions of singlet oxygen with diazo compounds, alkyl radical±ozone reac- Dioxiranes and their acyclic isomers carbonyl ox- tions, the matrix photooxidation of oxygen±alkene ides play an important role in many oxidation pro- mixtures and as intermediates in reactions of car- cesseswx 1±4 . The former are ;20 kcalrmol more bonyl compounds with atomic oxygenwx 1±4 . Curci stable than the latterwx 5 and, accordingly, it was et al.wx 10 postulated dioxiranes as oxidants in the possible to synthesize and investigate the parent caroate±ketone system and Murray and Jeyaraman dioxiranewx 6 and several substituted dioxiranes w 1± wx11 based their synthesis of dialkyldioxiranes on this 3,7±9x . Both isomers are probably generated as de- system. Apart from this, dioxiranes may play a role composition products of primary ozonides in the in the chemistry of the polluted atmospherewx 12,13 ozonolysis of alkeneswx 1±5 . Furthermore, they are and in enzymatic processeswx 14 . produced or supposed to be produced in carbene± Due to the importance of dioxiranes in many oxidation processes there is a large literature on ) Corresponding author. Fax: q46-31-7722933. these compounds covering different aspects of their 0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S0009-2614Ž. 98 00678-2 98 D. Cremer et al.rChemical Physics Letters 292() 1998 97±109 Scheme 1. chemistry viewed both from the point of experimen- be formed by direct decomposition of 3 Ž.Scheme 1 . tal and theoretical investigationswx 1±5 . In particular, Our investigation will be based on coupled cluster quantum chemical investigations added much to the Ž.CC , multi-reference averaged quadratic CC Ž MR- understanding of the properties of dioxiranes and AQCC.Ž. and density functional theory DFT calcula- their chemistrywx 5,9,15,16 . Nevertheless, it is still tions. The primary goal of our work will be to unclear why certain dioxiranes are stable, but others explain the increase in stability when substituting are not. Murray and Jeyaramanwx 11 succeeded in dioxiranes by methylŽ. alkyl groups and to predict synthesizing dimethyldioxirane, which can be kept which products can be expected when generating for several days at room temperature, while dioxi- dioxiranes. rane itself has only been observed as a labile species in the gas-phase ozonolysis of ethenewx 6 . Mass spectrometry investigations have found that dioxi- 2. Computational methods wx rane decomposes to CO22 , H , CO and H 2 O 4 . However, in matrix isolation experiments esters as For the parent systems 1a to 7a, CCSDŽ.Ž T CCSD dioxirane rearrangement products were observed with perturbative inclusion of tripleŽ. T excitations . rather than the decomposition productswx 1,7,9 . wx17 were carried out using Dunning's correlation We will investigate in this Letter the rearrange- corrected cc-VTZ2Pqf, d basis, which is composed ment and decomposition modes of dioxirane and its of aŽ. 10s 5p 2dwx 4s 3p 2d contraction augmented by a methyl substituted derivatives. For this purpose, we set of spherical f functions for the heavy atoms and a investigate the rearrangement of carbonyl oxide Ž.1a , set of spherical d-functions for H atoms ŽŽ.a C: syn- and anti-methylcarbonyl oxide Ž1b-syn and 1b- 1.097, 0.318Ž. d , 0.761 Ž. f ; a ŽO . : 2.314, 0.645 Ž. d , anti.Ž. and dimethyl carbonyl oxide 1c to the corre- 1.428Ž. f ; a ŽH . : 1.407, 0.388 Ž p . , 1.057 Ž d ..wx 18 and sponding dioxiranes 2a, 2b and 2c, respectively; ring has proven to lead to reliable results in the case of opening of the latter to the methylenebisŽ. oxy biradi- dioxiraneswx 15,16 . The ring opening of 2 to form 3 cals 3a, 3b and 3c; rearrangement of the biradicals 3 represents a multi-reference problem involving two to formic acid or esters Ž.4a, 4b and 4c ; rotation of 4 configurations of the same symmetry. As a result, to the less stable conformation 5; decomposition of MR-AQCCwx 19 calculations were performed em- q s q wx either 4 or 5 to CO RO222Ž.7; for 7a,R O HO ploying a 6-311 GŽ. 3df, 3pd basis set 20 . MR- q s or R 22CO Ž.6; for 6a,R 22H where 6 may also AQCC is essentially a modified MR-CI procedure, D. Cremer et al.rChemical Physics Letters 292() 1998 97±109 99 which contrary to MR-CI leads to nearly size-exten- Since the extension of the CCSDŽ. T and MR- sive resultswx 21 . According to test calculations wx 22 , AQCC calculations to the methyl substituted systems potential energy surfaces obtained by MR-AQCC are 2b and 2c was not feasible because of computational parallel to those of full CI calculations and, there- limitations, we checked various methods as to fore, MR-AQCC provides reliable energy differ- whether they can provide a reasonable, but computa- ences. tionally much less expensive description of the reac- The reference space for the MR-AQCC of 2, tions of Scheme 1. It turned out that density func- TS2-3 and 3 was carefully chosen to give a balanced tional theoryŽ. DFT with Becke's three-parameter description at all three geometries. The highest occu- functional B3LYPwx 23 provided the best description. pied MOs and the LUMO of 2 are DFT covers significant correlation effects in an un- specified way and has proven to be useful in the s p 6a11 ± Ž.OO ; 2b ± ŽOO bond . ; description of typical single-determinant problems 1a ±pŽ.Ž.OO antibond, CO nonbond ; 4b ±s) OO . but also some multi-reference problems, in which 22one configuration is still dominating the wavefunc- The ground stateŽ. GS of 2 is defined by the tion. For biradicals such as 3, unrestricted DFT 2 2 2 electron configuration . .Ž. 6a112Ž. 2bŽ. 1a Ž.UDFT can provide reasonable descriptions, in par- 1 Ž A,41 p.. Ring opening to 3 formally involves a ticular if the symmetry of the molecule is broken rotation of the two 2p p electron pairs at the O Ž.broken symmetry UDFT as was shown in the case wx atoms into the ring plane so that both the 6a1 ±sŽ.OO of metal bonding for transition metal complexes 24 . and the 4b2 ±s)Ž.OO become doubly occupied and, Broken symmetry UDFT calculations of singlet bi- consequently, s-bonding between the O atoms is no radicals lead to a mixing of singlet and tripletŽ and longer possible. At the equilibrium OO distance of 3 higher.Ž. state s . Ž.close to 2 AÊ the two p MOs 1a21 and 2b are of Mixing might be assessed by calculating the ex- comparable energy and, therefore, both the pectation value of the operator S 2 for the Kohn± 2 2 2 ...Ž. 6a122Ž. 1aŽ. 4b and the Sham single determinant wavefunction. However, 2 2 2 2 . .Ž. 6a112Ž. 2bŽ. 4b electron configuration con- ²S : is of somewhat dubious value in DFT since the 1 tribute to the A,21 p GS of 3. The analysis of the Kohn±Sham wavefunction corresponds to the un- 1 ring opening process reveals that the A,21 p of 3 physical situation of non-interacting electrons. Calcu- corresponds to a low-lying excited state of dioxirane lation of ²S 2 : requires knowledge of the pair den- ™ ™ obtained by 2b124b or 1a 224b double excita- sity, which is not known. Therefore, we refrained 1 1 tion and that the A,411p and A,2p state undergo from improving the UDFT results by simple summa- an avoided crossing situation close to TS2-3. Hence, tion rules or spin-projection techniques applied for a consistent description of 2,TS2-3 and 3 requires ab initio wavefunctions and instead based the direct the inclusion of orbitals 6a112 , 2b , 1a and 4b 2 into use of the UDFT results on the fact that the electron the reference space, which in the present work was density is less sensitive to spin contamination effects extended by adding the unoccupied orbitals 7a1 and than the wavefunction.