Ab initio studies of crystal field effects. VII. Structure of 2,3‐diketopiperazine using a 13‐molecule cluster, a calculation involving 1092 basis functions

Abstract

The structure of 2,3-diketopiperazine in a crystal phase (P21/c), modeled by a 13-molecule cluster surrounded by point charges, was completely optimized using standard gradient procedures. The (ab initio) MIA approach was used to perform the SCF step, which, using a 4-216 basis set, involves calculations using 1092 basis functions. Results are in very good agreement with experimental (X-ray) data, in contrast to a previous study in which a model was used consisting of only point charges. 0

Solids modeled by ab initio crystal field methods, part 22: The Fock matrix transformed supermolecule model and the structure determination of D‐erythronic acid‐3,4‐carbonate

“…The cyclization procedure developed will be explained using a 13 molecule cluster (see Fig. 1 and Table 1 for the numbering and relative symmetry of the molecules, molecule 1 is the central molecule in this cluster), constructed in the usual way,9 of DEC as an example.…”

“…To include the interactions between the central molecule and its nearest neighbours quantum chemically, the supermolecule (SM) model9 was introduced, in which the central molecule, together with its 10–15 nearest neighbors, are described by a wave function. The molecules surrounding the wave function cluster are represented by point charges, as in the PC model.…”

“…In this model the nearest neighbor interactions are described at full ab initio level and the long‐range Coulomb interactions are taken into account. This method gives a very good description of the crystal field effects on the central molecule 9–15…”

Solids modeled by ab initio crystal field methods. Part 16: An ab initio study of the geometry of 2‐(2‐methyl‐3‐chloroanilino) nicotinic acid: gas phase and solid state calculations

“…In this model, a molecule is surrounded by other molecules, geometrically arranged in accordance with space group symmetry. There are two important variations of the cluster model: the point charge model (PC) [4] in which the central molecule and the neighboring molecules interact purely electrostatically and the more advanced super molecule model (SM) [5] in which a group of several molecules is treated quantum mechanically while the rest of the environment is treated electrostatically. When the overlap between the central molecule and the nearest neighbors is negligible, the PC model is sufficient for the description of the system, otherwise the SM model is preferred.…”

Semiempirical and Ab Initio Calculations of Tautomerism in 2,3-Dihydroxypyrazine

“…A similar compound, 2,3dioxopiperazine, has been studied both experimentally and theoretically and it was found that the dioxo form is the most stable species in the solid state. 29 In this regard, it appeared interesting to study tautomerism in 2,3-dihydroxypyrazine. This compound can exist in three tautomeric forms, two of which exist in more than one conformer (Fig.…”