Search

We demonstrate the efficient transfer of population from the upper to the lower inversion level within the rotational ground state of para-ND 3 . A packet of ND 3 molecules is velocity controlled and state selected using a Stark decelerator, and subsequently transferred with 93 % efficiency using the principles of adiabatic rapid passage. Transitions are induced using microwave chirps both under zero-field conditions as well as in the presence of an electric field to probe M J -resolved transition efficiencies. Simulations of the transfer efficiencies based on the von Neumann equation, taking all hyperfine transitions into account, showed excellent agreement with experimental observations. [ABSTRACT FROM AUTHOR]

The rotational constants of s-trans and s-cis methyl methacrylate and methacrylic acid are determined from microwave spectra. All singly substituted heavy-atom isotopologs of the four species are measured in natural abundance. The experimental rotational constants are combined with anharmonic vibrational corrections using the Coulomb-attenuating method, Becke, three-parameter, Lee-Yang-Parr density functional with Grimme's D3 dispersion corrections and the Becke-Johnson damping function, yielding precise semi-experimental equilibrium rotational constants. These constants are used to determine semi-experimental equilibrium structures with sub-picometer accuracy, suitable for benchmarking purposes. [ABSTRACT FROM AUTHOR]

The microwave spectra of 10 teflic acid isotopologues were recorded in the frequency range of 3-25 GHz using supersonic jet-expansion Fourier transform microwave spectroscopy. Despite being asymmetric in its equilibrium structure, the delocalization of the hydrogen atom leads to a symmetric top vibrational ground state structure. In this work, we present the zero point structure obtained from the experimental rotational constants and an approach to determine the semi-experimental equilibrium structure aided by ab initio data. The Te–O bond length determined in the equilibrium structure is accurate to the picometer and can be used as a benchmark for computational methods treating relativistic effects. [ABSTRACT FROM AUTHOR]

The magnetic hyperfine structure of the non-rigid methanol molecule is investigated experimentally and theoretically. 12 hyperfine patterns are recorded using molecular beam microwave spectrometers. These patterns, along with previously recorded ones, are analyzed in an attempt to evidence the effects of the magnetic spin-torsion coupling due to the large amplitude internal rotation of the methyl group [J. E. M. Heuvel and A. Dymanus, J. Mol. Spectrosc. 47, 363 (1973)]. The theoretical approach setup to analyze the observed data accounts for this spin-torsion in addition to the familiar magnetic spin-rotation and spin-spin interactions. The theoretical approach relies on symmetry considerations to build a hyperfine coupling Hamiltonian and spin-rotation-torsion wavefunctions compatible with the Pauli exclusion principle. Although all experimental hyperfine patterns are not fully resolved, the line position analysis yields values for several parameters including one describing the spin-torsion coupling. [ABSTRACT FROM AUTHOR]

The rotational spectrum of dinitrogen pentoxide (N2O5) has been investigated between 8 to 25 GHz at a rotational temperature of ∼2.5 K using a pulsed-molecular-beam Fourier-transform microwave spectrometer. Two weak b-dipole spectra are observed for two internal-rotor states of the molecule, with each spectrum poorly characterized by an asymmetric-rotor Hamiltonian. The observation of only b-type transitions is consistent with the earlier electron-diffraction results of McClelland et al. [J. Am. Chem. Soc. 105, 3789 (1983)] which give a C2 symmetry molecule with the b inertial axis coincident with the C2 axis. Analysis of the 14N nuclear hyperfine structure demonstrates that the two nitrogen nuclei occupy either structurally equivalent positions or are interchanging inequivalent structural positions via tunneling or internal rotation at a rate larger than ∼1 MHz. For the two internal rotor states, rotational levels with Ka+Kc even have IN=0, 2, while levels with Ka+Kc odd have IN=1, where IN is the resultant nitrogen nuclear spin. This observation establishes that the equilibrium configuration of the molecule has a twofold axis of symmetry. Guided by ab initio and dynamical calculations which show a planar configuration is energetically unfavorable, we assign the spectrum to the symmetric and antisymmetric tunneling states of a C2 symmetry N2O5 with internal rotation tunneling of the two NO2 groups via a geared rotation about their respective C2 axes. Because of the Bose–Einstein statistics of the spin-zero oxygen nuclei, which require that the rotational–vibrational–tunneling wave functions be symmetric for interchange of the O nuclei, only four of the ten vibrational-rotational-tunneling states of the molecule have nonzero statistical weights. Model dynamical calculations suggest that the internal-rotation potential is significantly more isotropic than implied by the electron-diffraction analysis. © 1996 American... [ABSTRACT FROM AUTHOR]

The high sensitivity and resolution of Fourier-transform microwave spectroscopy using a pulsed jet coaxial to a Fabry–Perot resonator have been exploited to measure pure rotational transitions of several isotopomers of the weakly polar Ne–Ar van der Waals dimer in natural abundance. Transitions of the most abundant isotopomer, 20Ne–40Ar, could be observed with an excellent signal-to-noise ratio with a single polarization pulse. The ground-state rotational constants for this species yield a zero-point separation of R0[bar_over_tilde:_approx._equal_to]360.7 pm. Simple model van der Waals potentials have been fit to the microwave transitions for the various isotopomers, providing estimates of the equilibrium spacing at the well minimum of Re=348.0(2) pm. More elaborate potentials based on ab initio calculations or on molecular-beam scattering cross sections and thermodynamic and transport properties have also been tested. The induced electric dipole moment is estimated to be μ0=7.3(1.6)×10-33 C m [0.0022(5) D] by comparison of π/2 polarization pulses with a reference molecule (Ar–CO2) whose dipole moment is known from Stark effect splitting measurements. Uncertainties in parentheses are one standard deviation. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The steric effects imposed by the isopropyl group of valine in the conformational stabilization of the capped dipeptide N-acetyl-l-valinamide (Ac-Val-NH2) have been studied by laser ablation molecular beam Fourier transform microwave (LA-MB-FTMW) spectroscopy. The rotational and quadrupole coupling constants of the two 14N nuclei determined in this work show that this dipeptide exists as a mixture of C7 and C5 conformers in the supersonic expansion. The conformers are stabilized by a C=O…H–N intramolecular hydrogen bond closing a seven- or a five-membered ring, respectively. The observation of both conformers is in good agreement with previous results on the related dipeptides containing different residues, confirming that the polarity/non-polarity of the side chains of the amino acid is responsible for the conformational locking/unlocking. The voluminous isopropyl group is not able to prevent the less stable C5 conformer from forming but it destabilizes the C=O…H–N interaction. [ABSTRACT FROM AUTHOR]