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The study employs several techniques and methodologies to investigate and analyze the free jet absorption millimeterwave spectrum of the gas phase 4-hydroxypyrimidine system that is in tautomeric equilibrium with two different ketonic forms. The analysis reveals that equilibrium normally shifts in favor of the keto form, as its stability with respect to the enolic form increases with the number of ring nitrogen atom.

Conventional and molecular beam Fourier transform microwave spectroscopy is used to measure the rotational spectra of the main isotopomer of the [.sup.29.Si] and of all [.sup.13.C] isotopologues of axial and equatorial forms of 1-fluoro-silacyclohexane. The rotational spectra of some, the most intense, vibrational satellites are measured and they are found to belong to the ring-puckering motions.

The free-jet rotational spectra of methylsalicylate (MS), a molecule with a possible tautomeric and conformational equilibrium, are described. The molecule has adopted a form stabilized by an intramolecular hydrogen bond between the phenolic hydrogen and the carbonylic oxygen and all the rotational transitions are split because of the internal rotation of the methyl group.

The molecular beam Fourier transform microwave spectrum of 1,4-dioxane-trifluoromethane is assigned and measured. The two subunits form a cage stabilized by one C-H...O and two C-H...F weak hydrogen bonds.

The rotational spectrum of pyridine–CF4has beeninvestigated by molecular beam Fourier transform microwave spectroscopyin a supersonic expansion. The CF4moiety is located asa cap over the pyridine nitrogen, and the two parts are freely rotatingwith respect to each other. For this reason, in a first approximation,in the m= 0 state only the pyridine ring is rotatingalong the a-axis, and the value of rotational constant A′ is nearly the same of the constant Aof isolated pyridine. The N···CCF4distance is 3.372(1) Å. The dissociation energy has beenestimated, from the centrifugal distortion, to be ca. 10 kJ/mol. [ABSTRACT FROM AUTHOR]

The rotational spectra of five isotopologues (normaland all monosubstituted 13C species) of methylpyruvatehave been measured with thepulsed jet Fourier transform microwave technique. Rotational transitionsare split into quintets due to the internal rotations of the two methylgroups. The corresponding barriers to internal rotation have beendetermined to be V3(H3C–O)= 4.883(8) kJ mol–1and V3(H3C–C) = 4.657(8) kJ mol–1,respectively. Information on the skeletal heavy atom structure hasbeen obtained from the 15 available rotational constants. [ABSTRACT FROM AUTHOR]

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 μb 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. [ABSTRACT FROM AUTHOR]

The rotational spectra of five conformers of 13- butandiol have been measured by pulsed jet Fourier transform microwave spectroscopy. All of them are stabilized by an internal hydrogen bond and all of them have a GG' or a G'G arrangement of the two hydroxyl oxygens, which means that both oxygen atoms are on the same side with respect to the C1C2C3 plane. Apart from the spectroscopic constants, the relative abundances in the supersonic expansion are provided. [ABSTRACT FROM AUTHOR]

The rotational spectra of five isotopologues of the molecular adduct 1,1,1-trifluoroacetone--water have been assigned using pulsed-jet Fourier-transform microwave spectroscopy. All rotational transitions appear as doublets, due to the internal rotation of the methyl group. Analysis of the tunneling splittings allows one to determine accurately the height of the 3-fold barrier to internal rotation of the methyl group and its orientation, leading to V3 = 3.29 kJ ⋅mol-1 Z (a,i) = 67.5molorespectively. The water molecule is linked to the keton molecule on the side of the methyl group through a O--H- …O hydrogen bond and a C-H* …O intermolecular contact, lying in the effective plane of symmetry of the complex. [ABSTRACT FROM AUTHOR]

The rotational spectra of two tautomers of purine have been measured by pulsed jet Fourier transform microwave spectroscopy coupled to a UV ultrafast vaporization system. The population ratio of the two main tautomers [N(7)H]/[N(9)H] is about 1/40 in the gas phase. It contrasts with the solid state where only the N(7)H species is present, or in solution where a mixture of both tautomers is observed. For both species, a full quadrupolar hyperfine analysis has been performed. This has led to the determination of the full sets of diagonal quadrupole coupling constants of the four (14)N atoms, which have provided crucial information for the unambiguous identification of both species. This work shows the great potential of microwave spectroscopy to study isolated biomolecules in the gas phase. All the work was supported by theoretical calculations.

The rotational spectra of three isotopologues, CH2(35)Cl2···(20)Ne, CH2(35)Cl(37)Cl···(20)Ne, and CH2(35)Cl2···(22)Ne, of the complex dichloromethane-neon have been assigned and measured by molecular beam Fourier transform microwave spectroscopy. The corresponding tunnelling splittings-due to the motion of Ne from above to below the ClCCl plane-have been determined as ΔE0+0- = 6.8900(5), 6.6630(4), and 6.3724(7) MHz, respectively. From these data the barrier to planarity has been obtained, B2 = 68.7 cm(-1). In addition, the structure and the (35)Cl (or (37)Cl) quadrupole coupling constants have been determined.

The rotational spectrum of the 1:1 complex between formic acid and formaldehyde shows that the two units are linked together through a "classical" (OH···O) and a weak (CH···O) hydrogen bond. The molecular system appears quite rigid, and no effects of the internal motions have been observed in the spectrum.

The rotational spectrum of trifluoroacetylacetone shows that the molecule exists in an enolic C(s) form and displays the features of internal rotations of the CH3 and CF3 groups, whose barriers to internal rotation were determined to be V3 = 379 and 30.8 cm(-1), respectively. Its internal dynamics appears to be intermediate between those of acetylacetone, where proton tunneling and low-barrier internal rotation of the two methyl groups make the spectrum quite complex, and hexafluoroacetylacetone, a perfectly "rigid" molecule on the time scale of microwave spectroscopy.

The free jet millimeter-wave absorption spectrum of 1,3-cyclohexanedione has been investigated in the 59.6-74.4 GHz frequency range, and the rotational spectra of two conformational species, the chair-diketo and boat-diketo, and probably one excited vibrational state belonging to the chair-diketo form have been assigned. Quantum chemical calculations, performed at the B3LYP/6-311++G** and MP2/6-311++G** levels, were used to characterize the potential energy surface minima. The potential energy surface related to the interconversion of the observed diketonic species was modeled at the DFT level.

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.