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Evaluation of two-electron integrals forms a substantial part of the CPU time for any ab initio molecular orbital program. This part of the package, "MICROMOL", is parallelized. However, this parallelization leads to only sublinear speedups (typically 3 on a 4-node machine). In view of these results, the task of development of an efficient program for two-electron integrals suitable for the parallel environment has been taken up. The program is written in FORTRAN considering specific symmetry features and application of rigorous bounds. This program is further parallelized with a good load balancing strategy. The molecules used as the test cases are: trans-butadiene, benzene, nitrobenzene, naphtalene and cytosine, with 3 G and 4-31 G basis sets. The results indicate that the parallel version of this program gives a typical speedup of 3.6 for a 3 G basis set and approximately 3.4 for a 4-31 G basis set for all the molecules tested. The sequential version of this program is ~1.2 times faster than the sequential version of MICROMOL, whereas the parallel version is ~ 1.4 times faster than the parallelized MICROMOL. PACS: 31.15. + q; 31.20.Ej.

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Evaluation of the relative importance of electrostatic and orbital effects in determining facial selectivities of n systems (rr-facial selectivity) in nucleophilic and electrophilic additions to trigonal carbon centers is currently a topic of intense debate. 0.R l e R = R ' = C N should approach l a from the face opposite to the more electronrich 0, bond. However, on the basis of PM3 calculations on 7-methylenenorbornyl derivatives 1 a. 1 c, and related systems. it has been proposed[31 that the initial approach of a charged reagent would be along the m t i side, governed by electrostatics, and the subsequent nucleophilic attack (for example, by water) would lead to the observed product stereochemistry.Studies on CCI, and halonium ion additions to these systems were suggested for providing further insights. We now report experimental facial selectivities for the addition of such electrophiles to 7-isopropylidenenorbornane derivatives (2) and provide interpretations on the basis of a b initio molecular electrostatic potentials (MEP) and elec-2 a R = C N tron densities, as well asThe use of the isopropylidene unit offers significant advantages. From an experimental point of view, the double bond becomes more reactive, which enables the study of carbene additions. ['] Further, unlike in 1, the site of electrophilic attack is likely to be at C7 and not at C8, a factor which is crucial for interpretation of the experimental results. Finally, the mechanistic details of electrophilic attack in 2 are unambiguous and the steric neutrality on the two n faces is maintained. 7-Isopropylidenenorbornanes 2 smoothly undergo dichlorocarbene and singlet oxygen addition, epoxidation, and reaction with the bromine(1) cation. The reaction conditions, yields. and products obtained are shown in Scheme 1, and the n-face selec- tivities are summarized in Table 1 . Structures of diastereomers were determined on the basis of ' H and 13C NMR data, but more specifically through the greater deshielding of the C2 exo-proton in the syz-addition products (3, 5, 7, 9) relative to that of the anti-products (4, 6, 8, 10).[2,6,71 The results confirm that even a single electron-withdrawing substituent in the endo position is capable of inducing syn-facial electrophilic addition. thus reinforcing our earlier There is also no ambiguity about the initial direction of attack of Brf. Since the nucleophilic attack on the bromonium ion is followed by an elimination to give the olefins 7 and 8, the loca- Ed.