Tautomeric equilibrium and hydrogen shifts in tetrazole and triazoles: Focal-point analysis and ab initio limit

Abstract

High-level ab initio electronic structure calculations, including extrapolations to the complete basis set (CBS) limit, were performed, and highly precise relative energies of five-member N-heterocycles were determined. Nitrogen-containing heterocycles studied included tautomers of tetrazole (CH(2)N(4)) and triazoles (C(2)H(3)N(3)). Valence focal-point analysis of 1H-tetrazole, 2H-tetrazole, 5H-tetrazole, 1H-1,2,3-triazole, 2H-1,2,3-triazole, 1H-1,2,4-triazole, and 4H-1,2,4-triazole and a number of transition state (TS) calculations were performed, using energy values determined by CCSD(T)/aug-cc-pVTZ, MP3/aug-cc-pVQZ, and MP2/aug-cc-pV5Z. An accuracy of 0.10-0.25 kcal mol(-1) (35-87 cm(-1)) was obtained for comparison of tautomer energy differences. Relative CCSD(T)/CBS energies of 2.07, 3.98, and 6.25 kcal mol(-1) for 1H-tetrazole, 1H-1,2,3-triazole, and 4H-1,2,4-triazole, respectively, were calculated. Use of electron correlation methods resulted in markedly different convergence behaviors for triazole and tetrazole tautomers. Similarly, differences in convergence were observed for TSs with respect to corresponding minima structures. It was confirmed that the MP2 method predicts an acyclic structure for 5H-tetrazole. The same was not observed for the corresponding TS geometry. Comparison with density functional theory (B3LYP) and model chemistry methods (CBS-4M and CBS-QB3) is reported.

GIAO/DFT studies on 1,2,4‐triazole‐5‐thiones and their propargyl derivatives

“…[21] Similar observations were also made by Elguero et al [22] and explained in terms of electrostatic proximity effects arising from the nonstabilizing interactions between nitrogen lone pairs and NH pairs. Balabin [24] has also addressed tautomerism in tetrazole and triazole by ab initio calculations. Recently, in a theoretical study, Davari et al have discussed tautomerism in 1,2,4-triazole thiones and some of their disubstituted derivatives.…”

On the relation between basis set convergence and electron correlation: a critical test for modern ab initio quantum chemistry on a “mindless” data set

“…Nevertheless, dCCSD(T) can be regarded as a good approximation of electron correlation effects in molecular systems [19][20][21][22]. In many cases, the corrections above the CCSD(T) level are negligible [22,24,25,28]. Figure 2 represents the results of simple statistical analysis of 165 molecules in terms of BSIE vs. dCCSD(T) correlation.…”

“…Many of the molecules and molecular complexes are, chemically, rather unusual. One expects that this set will provide an unbiased estimation for the whole range of possible organic molecules and even go beyond that [24,25,27]. After the initial geometry optimizations at the DFT-PBE/TZVP level and CCSD/cc-pVDZ single-point calculations for the T1-and D1-diagnostics as described in [27], the CCSD(T) complete basis set (CBS) correlation energy was extrapolated using cc-pVTZ and cc-pVQZ single-point values [19,20,28].…”

“…The need to overcome basis set incompleteness and systematically approach the complete basis set limit has led to the creation of correlation consistent basis sets: (aug-)cc-pVxZ, x = D -6 [23][24][25].…”

Amide Resonance Structure Detected by NMR to Predict Hydroxyl Unit in Protein

“…The amide resonance structure of imidic acid (HO-C=N) has been studied during previous experiments [6], and quantum chemical studies [7]. Note that amide and imidic acid are tautomers of each other, and the tautomeric ratios [8] depend on conditions such as the temperature, solvent, and pH. The imidic acid structures have been proved in Polyamidoamine (PAMAM) by 15N NMR, 1H NMR, 13C NMR, and IR spectra [9].…”