Microwave measurements of ¹⁴N and D quadrupole coupling for (Z)-2-hydroxypyridine and 2-pyridone tautomers

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 eQq _cc(N) values have opposite signs. The ¹⁴N quadrupole coupling strengths for (Z)-2-hydroxypyridine in the principal inertial axis system are as follows: eQq _aa(N) = -0.076(11), eQq _bb(N) = -2.283(6), and eQq _cc(N) = 2.359(6) MHz. The ¹⁴N and D nuclear quadrupole coupling strengths for (Z)-2-deuteriohydroxypyridine in the principal inertial axis are eQq _aa(N) = -0.1465(4), eQq _bb(N) = -2.2045-(4), and eQq _cc(N) = 2.3510(4) MHz and eQq _aa(D) = -0.0250(9), eQq _bb(D) = 0.1699(4), and eQq _ccD) = -0.1449(4) MHz. The ¹⁴N quadrupole coupling strengths for 2-pyridone in the principal inertial axis system are eQq _aa(N) = 1.496(4), eQq _bb(N) = 1.269(4), and eQq _cc(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: eQq _aa(N) = 1.511(2), eQq _bb(N) = 1.249(5), and eQq _cc(N) = -2.759(5) MHz and eQq _aa(D) = -0.110(7), eQq _bb(D) = 0.354(6), and eQq _cc(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) A 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.