Some of the new unique features of the MOLCAS quantum chemistry package version 7 are presented in this report. In particular, the Cholesky decomposition method applied to some quantum chemical methods is described. This approach is used both in the context of a straight forward approximation of the two-electron integrals and in the generation of so-called auxiliary basis sets. The article describes how the method is implemented for most known wave functions models: self-consistent field, density functional theory, 2nd order perturbation theory, complete-active space self-consistent field multiconfigurational reference 2nd order perturbation theory, and coupled-cluster methods. The report further elaborates on the implementation of a restricted-active space self-consistent field reference function in conjunction with 2nd order perturbation theory. The average atomic natural orbital basis for relativistic calculations, covering the whole periodic table, are described and associated unique properties are demonstrated. Furthermore, the use of the arbitrary order Douglas-Kroll-Hess transformation for one-component relativistic calculations and its implementation are discussed. This section especially focuses on the implementation of the so-called picture-change-free atomic orbital property integrals. Moreover, the ElectroStatic Potential Fitted scheme, a version of a quantum mechanics/molecular mechanics hybrid method implemented in MOLCAS, is described and discussed. Finally, the report discusses the use of the MOLCAS package for advanced studies of photo chemical phenomena and the usefulness of the algorithms for constrained geometry optimization in MOLCAS in association with such studies.

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The main purpose of this study is to assess the relative importance of diradical or peroxirane (perepoxide) intermediates in the singlet oxygen cycloaddition reactions with alkenes that lead to dioxetanes. The relevant nonconcerted pathways are explored for ethene, methyl vinyl ether, and s-trans-butadiene by CAS-MCSCF optimizations followed by multireference perturbative CAS-PT2 energy calculations and by DFT(B3LYP) optimizations. The two different theoretical approaches gave similar results (reported below). These results show that methoxy or vinyl substitution does not affect qualitatively the reaction features evidenced by the unsubstituted system. Peroxirane turns out to be attainable only by passing through the diradical, due to the nature of the critical points involved. The energy barriers for the transformation of the diradical to peroxirane in the case of ethene (ΔE ⧧ = 13−15 kcal mol-1) and methyl vinyl ether (ΔE ⧧ = 12−13 kcal mol-1) are higher than those for the diradical closure to dioxetane (ΔE ⧧ = 8−9 kcal mol-1, for ethene, and 9 kcal mol-1, for methyl vinyl ether). In all three systems, the peroxirane pathway to dioxetane is prevented by the high energy barrier for the second step, leading from peroxirane to dioxetane (ΔE ⧧ = 26−27, 27−31 and 22 kcal mol-1, for ethene, methyl vinyl ether, and butadiene, respectively). By contrast, peroxirane can very easily back-transform to the diradical (with a ΔE ⧧ estimate of 3 kcal mol-1, for ethene and methyl vinyl ether, and close to zero, for butadiene). These results indicate that, although a peroxirane intermediate might form in some cases, it corresponds to a dead-end pathway which cannot lead to dioxetane.

In the tropospheric oxidation of benzene and methylated benzenes, unsaturated dicarbonyls are commonly detected products. Aldehydes are known to contribute on their own to some aspects of air pollution, and hexa-2,4-dien-1,6-dial (muconaldehyde) in particular is interesting because of its multiform toxicity. This study investigates the likelyhood of some benzene oxidation steps and is especially focused on ring opening and generation of muconaldehyde. With sufficiently high NO x concentration, O abstraction by NO from the cis peroxyl group in the 2-hydroxy-cyclohexadienyl peroxyl radical III can play a role. In fact, it is shown to open a facile cascade of oxidation steps by first forming the 2-hydroxy-cyclohexadienyl oxyl radical VI. This intermediate is prone to ring opening via β-fragmentation and generates the open-chain delocalized 6-hydroxy-hexa-2,4-dienalyl radical VII, in which one terminus is the first carbonyl group of the final dialdehyde. The second one can form either by simple H abstraction operated by O2 or by O2 addition followed by HOO• elimination. The overall free-energy drop with respect to III is estimated to be 48 kcal mol-1. Exploration of other pathways, possibly playing a major role in yielding aldehydes in the case of low NO x concentration, indicates that only ring closure of the 2-hydroxy-cyclohexadienyl peroxyl radical III to the [3.2.1] bicyclic endo-peroxy allyl radical intermediate XIII is promising. In this case, however, the outcome of a subsequent ring opening can ultimately be the production of 1,2 and 1,4 dialdehydes (as direct oxidation of muconaldehyde itself can actually do).

Three different attacks of 3 Σ g O 2 on the hydroxycyclohexadienyl radical intermediate I (generated from the reaction of OH • with benzene) have been studied by Density Functional Theory. Both abstraction by O 2 of the hydrogen gem to OH in I (affording phenol) and O 2 addition to the π-delocalized system of I (producing a hydroxycyclohexadienyl peroxyl radical intermediate) appear to be very viable, with ∆H q ) 3-4 kcal mol -1 . The former reaction is exothermic by 27 kcal mol -1 , the latter only by 1 kcal mol -1 . In contrast, a recently repropounded pathway, which would lead to benzene oxide/oxepin, via hydrogen abstraction from the hydroxyl in I operated by O 2 , appears not to be competitive, showing a significantly higher barrier (∆H q ) 32 kcal mol -1 ). Benzene oxide and oxepin are estimated to lie 21 and 19 kcal mol -1 above I, respectively.

Alkyl peroxyls form in the atmospheric oxidation of hydrocarbons and in their combustion. When NO concentration is low, they can appreciably react with themselves. This reaction has both propagation and termination channels. Multireference second-order perturbative energy calculations CAS(16,12)-PT2/6-311G(2df,p) have been carried out on the CAS(8,8)-MCSCF/6-311G(d,p) geometries pertaining to the reaction pathways explored. The tetroxide intermediate put forward first by Russell in 1957 is found as a stable energy minimum, but the calculations indicate that, as the system moves from atmospheric to combustion temperatures, its formation becomes problematic. A concerted synchronous transition structure, apt to connect it with the termination products, formaldehyde, methanol, and dioxygen, is not found. The concerted dissociation of the two external O–O bonds in the tetroxide leads to the (CH23O•)23⋯3O2 complex, with overall singlet spin multiplicity. Both termination via H transfer, to give H2CO, CH3OH, and O2, or dissociation to 2 CH3O•+O2 (possible propagation) are feasible. The former could occur in principle with production of either excited O21 or excited H23CO. However, if a sufficiently easy intersystem crossing (ISC) could take place in the complex, the process would end up with all ground-state molecules. The (possible) propagation channels are favored by higher temperatures, while lower temperatures favor the ISC mediated termination channel. A fairly good qualitative agreement with experimental T dependence of the relevant branching ratio is found. From the tetroxide over again, dissociation of a single external O–O bond leads to CH3O• and CH3O3•, or possibly to a (CH3O•⋯CH3O3•)1 complex, but further transformations along this line are not competitive.

The [1,2] and [2,3] migration steps in the Stevens and Sommelet-Hauser rearrangements which occur in the ylides of quaternary ammonium salts have been studied at M05-2x levels. The Stevens migration has been found to take place through a diradical pathway in several cases (tetramethylammonium, benzyltrimethylammonium, benzylphenacyldimethylammonium ylides). By contrast, in the phenyltrimethylammonium ylide this reaction takes place through a concerted process. The Sommelet-Hauser rearrangement takes place through a concerted transition structure. The most important factor determining the extent of competition with the Stevens rearrangement is the difference in the reaction energies as the formation of the Sommelet-Hauser intermediate is significantly less endoergic.

The mechanisms of the fragmentation and isomerization pathways of o-benzyne were studied at the multi-configurational second-order perturbative level [CAS(12,12)-PT2]. The direct fragmentation of o-benzyne to C2H2 + C4H2 follows two mechanisms: a concerted mechanism and a stepwise mechanism. Although the concerted mechanism is characterized by a single closed-shell transition structure, the stepwise pathway is more complex and structures with a strong diradical character are seen. A third diradicaloid fragmentation pathway of o-benzyne yields C6H2 as the final product. As an alternative to fragmentation, o-benzyne can also undergo rearrangement to its meta and para isomers and to the open chain cis and trans isomers of hexa-3-en-1,6-diyne (HED). These easily fragment to C2H2 + C4H2 or C6H2. Kinetic modelling at several different temperatures between 800 and 3000 K predicted that the thermal decomposition of o-benzyne should yield C2H2, C4H2 and C6H2 as the main products. Small amounts of the HED isomers accumulated at temperatures <1200 K, but they rapidly decompose at higher temperatures. Between 1000 and 1400 K, C2H2 + C4H2 are formed exclusively from the decomposition of trans-HED. At temperatures >1400 K, C2H2 + C4H2 also form from the direct fragmentation of o-benzyne. The formation of C2H2 + C4H2 prevails up to 1600 K but above this temperature the formation of C6H2 prevails. At temperatures >2400 K, the direct fragmentation of o-benzyne again leads to the formation of C2H2 + C4H2. The formation of hydrogen atoms is also explained by our proposed mechanisms.