Microwave Measurements of 14N and D Quadrupole Coupling for (Z)-2-Hydroxypyridine and 2-Pyridone Tautomers

Abstract

Rotational spectra for the two tautomers (Z)-2-hydroxypyridine and 2-pyridone and their deuterated isotopomers were measured in the microwave range between 4 and 14 GHz using a pulsed beam Fourier transform microwave spectrometer. Nitrogen and deuterium quadrupole hyperfine structure was completely resolved for many of the observed transitions, and the measured 14N quadrupole coupling tensors are quite different for these two tautomers. The eQqcc(N) values have opposite signs. The 14N quadrupole coupling strengths for (Z)-2-hydroxypyridine in the principal inertial axis system are as follows: eQqaa(N) = −0.076(11), eQqbb(N) = −2.283(6), and eQqcc(N) = 2.359(6) MHz. The 14N and D nuclear quadrupole coupling strengths for (Z)-2-deuteriohydroxypyridine in the principal inertial axis are eQqaa(N) = −0.1465(4), eQqbb(N) = −2.2045(4), and eQqcc(N) = 2.3510(4) MHz and eQqaa(D) = −0.0250(9), eQqbb(D) = 0.1699(4), and eQqcc(D) = −0.1449(4) MHz. The 14N quadrupole coupling strengths for 2-pyridone in the principal inertial axis system are eQqaa(N) = 1.496(4), eQqbb(N) = 1.269(4), and eQqcc(N) = −2.765(4) MHz. The 14N and D nuclear quadrupole coupling strengths for 1-deuterio-2-pyridone in the principal inertial axis system are as follows: eQqaa(N) = 1.511(2), eQqbb(N) = 1.249(5), and eQqcc(N) = −2.759(5) MHz and eQqaa(D) = −0.110(7), eQqbb(D) = 0.354(6), and eQqcc(D) = −0.244(6) MHz. New, improved, experimental rotational constants were obtained for the H and D isotopomers of both tautomers. Kraitchman analysis indicates the “tautomeric” hydrogen atom is at a distance of 2.653(2) Å in 2-hydroxypyridine and a distance of 2.124(1) Å in 2-pyridone from the centers of mass of the two tautomers, respectively. The DFT calculated eQq(N) values for both the tautomers and the deuterated tautomers are in good agreement with the present experimental values. The Townes−Dailey model has been used to analyze the new quadrupole coupling data of the tautomers and the results are presented in terms of nitrogen atom p-orbital occupation numbers.