Equilibrium Structures of Three-, Four-, Five-, Six-, and Seven-Membered Unsaturated N-Containing Heterocycles

Abstract

Up to six different techniques are utilized to estimate the equilibrium structures (r(e)) of a series of mostly unsaturated, N-containing heterocycles. Accurate Born-Oppenheimer (r(e)(BO)) and, if allowed, semiexperimental (r(e)(SE)), as well as empirical (r(m)-type) estimates of the equilibrium structures of three-membered (1H- and 2H-azirine, aziridine), four-membered (azete), five-membered (pyrrole, pyrazole, imidazole), six-membered (pyridine, pyrimidine, uracil), and seven-membered (1H-azepine) rings, containing usually one but in some cases two N atoms, are determined. The agreement among the structural results of the different techniques is very satisfactory. It is shown that it is possible to use the CCSD(T) electronic structure method with the relatively small wCVTZ basis set, with all electrons correlated, and the effect of further basis set enlargement, wCVTZ → wCVQZ, computed at the MP2 level, to obtain reliable equilibrium structures for the semirigid molecules investigated. Extension to larger basis sets does not significantly improve the accuracy of the computed results. Although all molecules investigated are oblate, and their principal axis system is subject to large rotations upon isotopic substitution, the semiexperimental method, when applicable, provides accurate results, though in the difficult cases it must be augmented with the mixed regression method. Finally, it is noteworthy that the empirical mass-dependent (r(m)) method also delivers surprisingly accurate structures for this class of compounds.

Bond Length Alternation Observed Experimentally: The Case of 1H‐Indazole

“…As indazole is the result of a benzene ring fused to a pyrazole unit, the first structural consideration is the comparison of indazole to benzene and pyrazole. In the case of pyrazole, there is a previous microwave spectroscopy study in the literature, which includes data on all 2 H, 15 N and 13 C singly substituted isotopologues, which was already used in an r e SE geometry evaluation . Presently, we used pyrazole both for comparisons with indazole and also to calibrate the more modest computational conditions adopted for evaluating the r e SE geometry of the much larger indazole molecule, including the assumptions made for structural parameters to the six hydrogen atoms (Table and section 3 in the Supporting Information).…”

“…We note that the computational level adopted here for the vibrational corrections has a minimal effect on the r e SE pyrazole geometry, with average differences of less than 1 mÅ and 0.05° relative to ref. . At the same time the greater (but still acceptable) parameter errors in the present r e SE evaluation for pyrazole are purely the result of more complete inertial defect cancellation in ref.…”

“…At the same time the greater (but still acceptable) parameter errors in the present r e SE evaluation for pyrazole are purely the result of more complete inertial defect cancellation in ref. by means of an additional electronic correction term.…”

“…try evaluation. [52] Presently,w eu sed pyrazoleb oth for comparisons with indazole and also to calibrate the more modestcomputational conditions adopted fore valuating the r e SE geometry of the much larger indazole molecule, including the assumptions made for structural parameters to the six hydrogen atoms (Table 4a nd section 3i nt he Supporting Information).…”

Rotational spectroscopy of imidazole: improved rest frequencies for astrophysical searches

“…A high-level ab initio calculation of the molecular structure of imidazole has been reported previously (Császár et al 2015). Here, we have performed a new computation aimed at obtaining a comprehensive set of theoretical estimates for its spectroscopic parameters.…”