13  Computational organic chemistry

Abstract

Conformational analysis remains one of the bread-and-butter activities of the computational organic chemist. The papers reviewed in this section represent a cross-section of this activity.Aziridine-2-carbonitrile. The energies of the cis and trans forms of aziridine-2carbonitrile (2) have been determined at very high levels of theory by Tschumper. 5 This state-of-the-art study included very large basis sets and the use of the coupled cluster CCSD(T) and Brueckner doubles BD(TQ) methods. The latter method included a perturbative treatment of the triple as well as quadruple excitations, as shown. The focal point analysis technique was then used to extrapolate to the basis set limit and obtain results of thermochemical accuracy. The best estimate of ∆G 0 from this study shows that the trans form lies 3.5 kJ mol Ϫ1 higher in energy than the cis. This value is in good agreement with previous theoretical studies, but not with the experimental estimate of the lower bound of 11 kJ mol Ϫ1 . The transition structure connecting the two isomers was also calculated and gives rise to an estimated barrier of 77.15 kJ mol Ϫ1 above the cis isomer. These results have significance in two areas of experimental research: the study of nucleic acid structure, and the determination of rotational spectra in interstellar dust clouds.Fumaric and maleic acids. The (E ) and (Z ) forms of butenedioic acid (fumaric and maleic acid, respectively) have been the subject of a combined molecular orbital and matrix isolation study. 6 The authors used the popular MP2 and B3LYP methods to determine the lowest energy rotational conformers of each structure (see 3). In each case these were shown to be planar, with the maleic acid incorporating an intramolecular hydrogen bond to form a seven-membered ring. The OH ؒ ؒ ؒ O = C hydrogen-bonded conformer was considerably lower in energy than the OH ؒ ؒ ؒ OH hydrogen-bonded conformer. Infrared spectra of the matrix isolated species indicated that the most stable conformers of maleic and fumaric acids were indeed present in the argon matrix.

Theoretical and Computational Modeling of Endohedral Functionalization Energy for Armchair Models of Nanotubes

“…The DFT/B3LYP/6-31g(d) difference between the endohedral and the exohedral reaction energy E k endo/exo = E k endo − E k exo for the (4 4 12), (5 5 12) and (6 6 12) molecular models S of SWCNTs versus the reacting carbon atom position k. The results are in agreement with Eqs (7,. 13).…”

“…6 This computational model is a reasonable compromise between accuracy and computational cost for the gas phase energy calculations. 7 We performed DFT/B3LYP/6-31g(d) calculations using Gaussian 03 software 8 for molecular models and DFT B3LYP 6-31g(d) for the periodical model using Crystal 03 software. 9 We studied the (n n) armchair molecular models of SWNTs (Fig.…”

Theoretical and Computational Modeling of Functionalization Energy for Molecular and Periodical Models of Linear Conjugated Transpolyacetyle

“…12 This computational model is a reasonable compromise between accuracy and computational cost for the isolated systems energy calculations. 13 We performed DFT B3LYP 6-31G(d) calculations using Gaussian 03 software 14 for molecular models of transpolyacetyle and DFT B3LYP 6-31G(d) for the DFT based periodical model of transpolyacetyle using Crystal 03 software. 15 We studied transpolyacetyle molecular models with different number of carbon atoms L, (Fig.…”

Theoretical and Computational Modeling of Functionalization Energy for Highly Symmetrical Molecules: Nanotubes and Fullerenes

“…12 This computational model is a reasonable compromise between accuracy and computational cost for the gas phase energy calculations. 13 We performed DFT B3LYP 6-31g(d) calculations using Gaussian 03 software 14 for molecular models and Crystal 03 software 15 for periodical models.…”