The tautomerism of 2-hydroxypyridinef2-pyridinone has been investigated by microwave spectroscopy using both a conventional spectrometer and also a jet-cooled millimeter-wave spectrometer. We have observed spectra attributable to both the (Z)-hydroxy tautomer and the pyridinone tautomer and also their monodeutero isotopomers, the relative abundances in both spectrometers being about 3:l in favor of the hydroxy form. From relative intensity and dipole moment measurements, we estimate the energy difference between the vibrational ground states of the two tautomers to be = 270 (30) cm-I. The rotational constants for 2-pyridinone yield an inertial defect that demonstrates the planarity of this tautomer, in contrast to recent reports of the nonplanarity of pyridinone. We can detect no lines that could be assigned to such a conformer. There are no lines correspondingto the E isomer of 2-hydroxypyridine, indicating that its relative abundance in the jet is less than -5% of the 2 isomer. The rotational spectra of several vibrational satellites have also been assigned. Identification of species is based on comparison of rotational constants determined experimentally with the values predicted from molecular orbital calculations and independently confirmed by comparison of observed and theoretically predicted hyperfine multiplets. The observed rotational constants for both tautomers are within 2% of thevalues predicted by ab initio molecular orbital calculations at the basis set level of 6-31G*, a degree of agreement similar to that found in some previous studies of nitrogen heterocycles. Nitrogen quadrupole coupling constants have been determined for both species: (Z)-2-hydroxypyridine, xOO = 0.4 (3) MHz, Xbb = -2.3 (3) MHz; 2-pyridinone, xcc = -2.8 (2) MHz. These values provide an independent confirmation of the identification of the two species of the molecule.
Completely planar is the hydrogen-bonded complex of pyrazine and water (see sketch), which was obtained by supersonic expansion and investigated by rotational spectroscopy. The water molecule lies in the plane of the aromatic ring, and the lone pair of electrons on the nitrogen atom functions as the acceptor in the N⋅⋅⋅H-O hydrogen bond, not-as in the corresponding benzene complex-the π electrons.
The dimer of acrylic acid can exist in two forms, depending on the entgegen or zusammen orientations of the two allyl groups. The latter one (zusammen) has a permanent value of the μ(b) dipole moment component, which allowed measuring its pulsed jet Fourier transform microwave (MW) spectrum. From the tunneling splitting, originating in the concerted proton transfer of the two carboxylic hydrogen atoms and measured for four isotopologues of such a bimolecule, we could determine the barrier and dynamics of the proton transfer.
4-Hydroxypyrimidine (4HP) has two conformational forms (the hydroxyl hydrogen cis or trans with respect to the adjacent nitrogen), which are in tautomeric equilibrium with two ketonic forms, 4-pyrimidinone (4PO) and 6-pyrimidinone (6PO). We have investigated the free jet absorption millimeterwave spectrum of this system, assigning the rotational spectra of 4HPcis and 4PO; the latter species is more stable by 2.0(9) kJ/mol. No lines corresponding to the trans isomer of 4-hydroxypyridine and to 6PO have been observed.
We report the free-jet rotational spectra of methylsalicylate, a molecule with a possible tautomeric and conformational equilibrium. In the ground electronic state, the molecule adopts a form stabilized by an intramolecular hydrogen bond between the phenolic hydrogen and the carbonylic oxygen, and this structure is characterized as the lowest-energy form by quantum chemical calculations. All rotational transitions are split because of the internal rotation of the methyl group, and the value of the barrier for this motion was determined to be V(3) = 5.38 kJ mol(-1).
The structural and energetic features of the C À H···N interaction and the internal dynamics of the pyridine-trifluoromethane molecular complex in its normal, 15 N and 13 C species are here reported. They have been obtained from analysis of the pure rotational spectra of the complex generated in a supersonic expansion.Weak hydrogen bonds (WHB) are a major topic in hydrogen-bond research. The energies of these interactions lie within a few kJ mol À1 and approach those of van der Waals forces, but have the same directional properties of "classical" hydrogen bonds.[1] This behavior was recently shown for the CÀH···O, CÀH···FÀC, CÀH···S, and CÀH···p linkages by investigating the rotational spectra of several hydrogenbonded molecular complexes generated in supersonic jets. [2][3][4] With this technique, precise information on the energetics and on the structural and dynamical aspects of such interactions is obtained in an environment free from the intermolecular interactions that take place in condensed media.Investigation of the rotational spectrum of benzene-trifluoromethane has shown that this complex is a symmetric top, with the two moieties held together through a C À H···p interaction, [4] and thus provided information on this kind of WHB. When replacing benzene with pyridine, two sites of high electronic density become available in the ring, so the two adducts shown in Figure 1 could be plausibly formed for the pyridine-trifluoromethane (Py-CHF 3 ) complex.Conformation I resembles the shape of the complex formed between benzene and trifluoromethane, with a CÀ H···p linkage, [4] while species II is similar to the complexes that the three diazines form with water.[5] Before the search of the rotational spectrum was started, full geometry optimizations of the monomers and dimers were carried out at the MP2/6-311 + + G** level of theory by using the Gaussian 03 suite of programs.[6] All energies were corrected for basis set superposition error (BSSE) by using the counterpoise procedure.[7] The calculations suggest that form I is not a minimum, while for species II, we obtained the estimated values of the spectroscopic constants listed in the second column of Table 1. These values were used as a guide in the assignment of the spectrum of species II.In Figure 2 we give the atom numbering, the principal axes system, and the parameters of the internal rotation of the CHF 3 group.Following these predictions, two frequency scans of a few hundred MHz each were carried out in the region of the J = 8-7 and 9-8 m a -type R bands. The corresponding K À1 = 0, 1 transitions were easily assigned. Then, seven m a -type R bands, typical of a near-prolate top, evenly spaced by the B + C value and with J in the range from 7 to 15 were measured on the whole, with K À1 ranging from 0 to 3. Most of [a] Dr.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.