The geometry of pyrazole: A test for ab initio calculations

Abstract

A b initio calculations on the structure of pyrazole have been carried out at different levels of accuracy. At the Hartree-Fock (HF) level, the performance of several basis sets, namely 3-21G, 6-31G, 6-31G**, and 6-311G* * was investigated.The influence of electron correlation effects also was studied by carrying out geometry optimizations at the MP2, MP4, and QCISD levels. The performance of a density functional method also was evaluated. We have also investigated the possible influence of the frozen core approximation on the final optimized geometry. Three different statistical analyses were considered in determining which geometry is closest to the experimental microwave geometry-namely Paul Curtin's diagrams, cluster analysis, and multidimensional scaling. From these analyses, we conclude that there is no asymptotic approach to the experimental geometry by increasing the quality of the theoretical model, although, as expected, the more reliable structures are those obtained at the MP2, MP4, and QCISD levels, as well as those obtained by the B3LYP density functional method. We have also found that the values of the rotational constants are a tight criterion to define the quality of a molecular geometry. 0 1995 by

A Theoretical Evaluation of the pKHBand Δ$H{{{\,\ominus}\hfill \atop {\rm HB}\hfill}}}$ Hydrogen‐Bond Scales of Nitrogen Bases

“…Geometries of the molecular systems were optimised with the B3LYP functional, also used to evaluate the harmonic vibrational frequencies and their total energies. [26,27] The 6-31+GA C H T U N G T R E N N U N G (d,p) Pople basis set was selected as the minimum basis set recommended to give accurate geometries and vibrational frequencies of the monomers [28,29] and realistic relative estimations of the hydrogen-bond energies of the complex formation [19,30,31] without the cost of MP2 calculations. Structures were confirmed as energetic minima through harmonic frequency calculations.…”

Statistical analysis of noncovalent interactions of anion groups in crystal structures. I. Hydrogen bonding of sulfate anions

“…The 2D (two-dimensional) substructure searches and 3D geometrical searches were carried out using the QUEST3D (Cambridge The geometrical criteria which are selected to characterize hydrogen bonding have been the subject of some discussion (Taylor, Kennard & Versichel, 1983Jeffrey & Maluszynska, 1990;G6rbitz & Etter, 1992;Steiner & Saenger, 1992;Llamas-Saiz, Foces-Foces, Mo, Yanez & Elguero, 1992 No preference for hydrogen bonding of sulfate with oxygen or nitrogen donors was observed: the proportion of potential O--H (35%) and N--H (65%) donors corresponds to the proportion of observed hydrogen bonds. In a first approach these two hydrogen-bonding groups were treated together.…”

Five‐Membered Heterocycles: 1,2‐Azoles. Part 1. Pyrazoles

“…Note that the 15 N chemical shifts are very sensitive to hydrogen bonds and, obviously, to protonation. ) 3524 (n NH gas) [4] Source: [11] X-ray (CSD) [12] PYRZOL INDAZL 1 H NMR Source: [13] Source: [13,14] 13 C NMR Source: [15] Source: [16] 15 N NMR Source: [17] Source: [ Source: [22] Source: [23] Best theoretical calculations Source: [24] Source: [25] Aromaticity (benzene ¼ 0.991) 0.900: [26] 0.808: [26] a) a) Naphthalene: 0.811, 2H-indazole: 0.792 (using as criteria the HOMA) [27]. 111.9 NMR information on the non-aromatic derivatives of pyrazoles is very abundant, particularly on D 2 -pyrazolines and on pyrazolones [4,7].…”