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We report a density functional theory (DFT) investigation of the chemical reactivity of the S&dbd;C&dbd;S···Cl complex toward 2,3-dimethylbutane (DMB) and compare it to the related reaction of a Cl atom with DMB. The DFT calculations predict that the reaction of a Cl atom with DMB will have little tertiary selectivity toward chlorination consistent with experimental results for photochlorination of alkanes in inert solvents. However, the DFT calculations indicate that the reaction of the S&dbd;C&dbd;S···Cl complex with DMB is different with the primary H abstraction reaction being noticeably more difficult than the tertiary H abstraction reaction. This indicates there will be significant tertiary selectivity for the reaction of the S&dbd;C&dbd;S···Cl complex with DMB and this is consistent with experimental results for chlorination of DMB in CS2 solvent. We examine the reaction mechanism and properties of the S&dbd;C&dbd;S···Cl complex and explore potential implications for tertiary selectivity for other related photochlorination reactions.

We report a systematic density functional theory investigation for the reactions of LiCH2I, IZnCH2I, iodomethylzinc phenoxide (PhOZnCH2I), 2,4,6-F3-PhOZnCH2I, and 2,4,6-Cl3-PhOZnCH2I with CH2CH2, trans-CH3CHCHCH3, and PhCHCH2. All of the reactions examined displayed similar concerted mechanisms for the cyclopropanation of these reagents. The zinc carbenoids had similar barriers for the cyclopropanation reaction (in the 17−21 kcal/mol range), while the LiCH2I carbenoid had a much lower barrier for the cyclopropanation reactions (in the 6−7 kcal/mol range). The properties and chemical reactivity of the carbenoids investigated here were compared to each other and to recent results for the isodiiodomethane (CH2I−I) species.

In the present work, photodissociation of HN3 at 248 nm and longer wavelength is investigated with the complete active space SCF (CASSCF) molecular orbital method. The stationary points on the ground- and excited-state potential energy surfaces are fully optimized at the CASSCF level with cc-pVDZ and cc-pVTZ basis sets. The potential energy profiles, governing HN3 dissociation to NH + N2 and H + N3, are characterized with the multireference MP2 (CASPT2) algorithm. The pathways leading to different products are determined on the basis of the obtained potential energy surfaces of dissociation and their crossing points. A comparison is made among the present and previous theoretical results and experimental findings. The present study provides an insight into the mechanism of the UV photodissociation of HN3 at a wavelength range from 355 to 248 nm.

The repair of 12-, 27-, 62-, and 216-nucleotide unpaired insertion/deletion heterologies has been demonstrated in nuclear extracts of human cells. When present in covalently closed circular heteroduplexes or heteroduplexes containing a single-strand break 3' to the heterology, such structures are subject to a low level repair reaction that occurs with little strand bias. However, the presence of a single-strand break 5' to the insertion/deletion heterology greatly increases the efficiency of rectification and directs repair to the incised DNA strand. Because nick direction of repair is independent of the strand in which a particular heterology is placed, the observed strand bias is not due to asymmetry imposed on the heteroduplex by the extrahelical DNA segment. Strand-specific repair by this system requires ATP and the four dNTPs and is inhibited by aphidicolin. Repair is independent of the mismatch repair proteins MSH2, MSH6, MLH1, and PMS2 and occurs by a mechanism that is distinct from that of the conventional mismatch repair system. Large heterology repair in nuclear extracts of human cells is also independent of the XPF gene product, and extracts of Chinese hamster ovary cells deficient in the ERCC1 and ERCC4 gene products also support the reaction.

The minimum-energy structure of the 7HQ−(H2O)n (n = 1−3) complexes (where 7HQ is 7-hydroxyquinoline) and the potential energy surfaces governing triple proton transfer in 7HQ−(H2O)2 and quadruple proton transfer in 7HQ−(H2O)3 have been characterized by means of different ab initio quantum mechanical methods. The solvent effects on the structures and reactivity of the complexes have been studied through the use of Onsager self-consistent reaction field models. A comparison was made between the calculated results and experimental data where available. The role of electron correlation in describing geometric features and the proton-transfer reactions was discussed in detail.

An efficient algorithm is presented for computing the two-dimensional discrete cosine transform (2-D DCT) whose size is a power of a prime. Based on a generalised 2-D to one-dimensional (1-D) index mapping scheme, the proposed algorithm decomposes the 2-D DCT outputs into three parts. The first part forms a 2-D DCT of a smaller size. The remaining outputs are further decomposed into two parts, depending on the summation of their indices. The latter two parts can be reformulated as a set of circular correlation (CC) or skew-circular correlation (SCC) matrix–vector products by utilising the recently addressed maximum coset decomposition. Such a decomposition procedure can be repetitively carried out for the 2-D DCT of the first part, resulting in a sequence of CC and SCC matrix–vector products of various sizes. Employing fast algorithms for the computation of these CC/SCC operations can substantially reduce the numbers of multiplications compared with those of the conventional row–column decomposition approach. In the special case where the data size is a power of two, the proposed algorithm can be further simplified, calling for computations comparable with those of previous works.