The complete valence shell photoelectron spectra of cytosine, thymine and adenine have been investigated experimentally and theoretically. Vertical ionization energies and spectral intensities have been evaluated using the many-body Green's function method, thereby enabling theoretical photoelectron spectra to be derived. In cytosine, the influence of tautomers and rotational conformers has been investigated. The calculated spectra display a satisfactory agreement with the experimental data and this has allowed most of the photoelectron bands to be assigned. Photoelectron asymmetry parameters have been determined from angle resolved spectra recorded with synchrotron radiation. The experimental data show that the electronic configuration of the five outer orbitals in cytosine, thymine and adenine is π, σ, π, σ, π. Vertical ionization energies have been measured for all the outer-valence orbitals even though some of the associated bands overlap significantly.

The mechanisms of fundamental base‐promoted acetylene reactions, namely, nucleophilic addition to the triple C ≡ C bond (vinylation) and nucleophilic addition of acetylenic carbanion to a carbonyl group (ethynylation), are addressed using three models of different complexity—pentasolvate, monosolvate, and anionic—which describe the catalytic superbasic systems MOH(OBut)/DMSO (suspensions of alkali hydroxides or tert‐butoxides in dimethyl sulfoxide). The above acetylene reactions and sequential transformations of reagents arranged by the superbasic center are modeled within the framework of the most complete pentasolvate model, in which the superbase is represented by the KOH·5DMSO (KOBut·5DMSO) complexes. We have developed approaches to the construction of simplified models (monosolvate and anionic) to describe transformations in complex systems. The mechanisms of cascade assemblies of 6,8‐dioxabicyclo[3.2.1]octanes, cyclopentenones, and furan cycles from ketones and acetylenes in the superbasic environment are investigated using a uniform B2PLYP/6‐311+G**//B3LYP/6‐31+G* approach, and the energy profiles of these different carbo‐ and heterocycles are analyzed.

A rapidly developing approach adding new dimensions to acetylene chemistry relying on employment of high basicity media such as alkali metal hydroxide suspensions in dimethyl sulfoxide (DMSO) has been, for the first time, investigated theoretically using ab initio models. Extending our recently introduced model of superbase catalysis with a nondissociated KOH (or NaOH) participation, we present here a model for a superbasic reaction center with the first solvation shell explicitly included. The alkali metal hydroxides in a DMSO solution were found to form KOH·5DMSO and NaOH·4DMSO complexes that are stabilized due to the interligand interaction. Our present MP2/6‐311++G**//B3LYP/6‐31+G* computations show that 1 and 2 water molecules can build themselves into the MOH close surrounding without substantially perturbing the DMSO ligands and easily travel between different insertion positions. Our results predict that the activation energies in the series of reactions of nucleophilic addition to a triple bond with water, methanol, methanethiol, sodium hydrosulfide, and acetone in the presence of dihydrated complexes should be larger than those obtained with the participation of monohydrated ones, which is in fair agreement with the experimental findings. The present model also explains an increase in the ethynylation reaction yield in the presence of water by suppression of the competitive enolization reaction.

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A CBS-Q//B3 based study has been carried out to elucidate the mechanism of the KOH/DMSO superbase catalyzed ketones nucleophilic addition to alkyl propargyl and alkyl allenyl ethers yielding, along with (Z)-monoadducts, up to 26% of unexpected (E)-diadducts. The impact of different substrates (alkynes versus allenes) on the reaction mechanism has been discussed in detail. Along with the model reaction of acetone addition to propyne and allene, the addition of acetone and acetophenone to methyl propargyl and methyl allenyl ethers is considered. The limiting reaction stage of the starting ketone carbanion addition to propargyl and allenyl systems occurs with activation energies typical for vinylation of ketones. In contrast, the addition of intermediate α-carbanions to the terminal position of methyl allenyl ether is associated with unusually low activation barriers. The results obtained explain the composition of the reaction products and indicate the participation of mainly the allene form in the reaction.

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ABSTRACT:The reaction mechanism of methanol vinylation with acetylene involving a nondissociated KOH molecule was studied using the MP2/6-311þþG**// HF/6-31þG* approach with the DMSO molecule explicitly included into reaction system. The whole conversion cycle of methanol vinylation including active nucleophile generation, bonding of the latter to the acetylene moiety yielding an intermediate carbanion, final methyl vinyl ether formation, and potassium hydroxide regeneration could be realized in the coordination shell of nondissociated KOH.

ABSTRACT:The mechanism of base-catalyzed nucleophilic addition of methanol to acetylene triple bond (vinylation) in dimethyl sulfoxide (DMSO) and methanol solution was studied using the MP2/6-311ϩϩG**//B3LYP/6-31G* calculations with solvent effects included via continuum model. The proton abstraction from methanol by nondissociated alkali in DMSO surrounding media to form alkali metal methoxides CH 3 OM (M ϭ Li, Na, K) was found to occur with a negligible activation barrier. The reasons for facilitation of base-catalyzed alcohol vinylation in the DMSO medium are discussed in the light of both poor solvation of methoxide ion and a specific coordination of reactants by nondissociated alkali in the MOH/DMSO mixtures.