Aqueous pyridine plays an important role in a variety of catalytic processes aimed at harnessing solar energy. In this work, the pyridine-water interaction is studied by microwave spectroscopy and density functional theory calculations. Water forms a hydrogen bond to the nitrogen with the oxygen tilted slightly toward either of the ortho-hydrogens of the pyridine, and a tunneling motion involving in-plane rocking of the water interconverts the resulting equivalent structures. A pair of tunneling states with severely perturbed rotational spectra is identified and their energy separation, ΔE, is inferred from the perturbations and confirmed by direct measurement. Curiously, values of ΔE are 10404.45 and 13566.94 MHz for the H2O and D2O complexes, respectively, revealing an inverted isotope effect upon deuteration. Small splittings in some transitions suggest an additional internal motion making this complex an interesting challenge for theoretical treatments of large amplitude motion. The results underscore the significant effect of the ortho-hydrogens on the intermolecular interaction of pyridine.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation, Grant Nos. CHE-1266320 and CHE-1563324, a UMN Doctoral Dissertation Fellowship awarded to R.B.M., the Minnesota Supercomputer Institute, and a Lando Summer Undergraduate Research Fellowship awarded to R.D.C. through the University of Minnesota. We thank Prof. Renee Frontiera for bringing this problem to our attention and to Dr. Jon Hougen for some helpful correspondence.