Centrifugal Distortion Effects Research Articles

The Shape of the Simplest Non-proteinogenic Amino Acid α-Aminoisobutyric Acid (Aib).

The simplest non-proteinogenic amino acid α-aminoisobutyric acid (Aib), an analogue of glycine and alanine, has been vaporized by laser ablation and probed by high-resolution Fourier transform microwave spectroscopic techniques. Comparison of the experimental rotational and 14 N nuclear quadrupole constants with that predicted ab initio has allowed the identification of three conformers of Aib exhibiting three types of hydrogen-bond interactions I (NH⋅⋅⋅O=C, cis-COOH), II (OH⋅⋅⋅N, trans-COOH), and III (N-H⋅⋅⋅O-H, cis-COOH) within the amino acid backbone. The observation of conformer III, not detected previously for related proteinogenic amino acids with a nonpolar side chain in a supersonic expansion, indicates that the presence of the methyl groups should restrict the conformational relaxation from conformer Aib-III to Aib-I. For conformer Aib-II, the rotational spectra of the 13 C isotopomers reveal a tunneling motion arising from the two equivalent methyl groups in the molecule. The observation of a single spectrum at the midpoint between those predicted for the two 13 C of the methyl groups has been explained by considering a double-minimum potential function with a low-energy interconversion barrier for a large amplitude internal motion. This singular fact has been corroborated by the anomalous centrifugal distortion effects determined in conformer Aib-II.

Comparative Centrifugal Distortion Analysis in Rotational Spectra of Propanol and Isopropanol Molecules

We have compared the effect of centrifugal distortion on the rotational spectra of isopropanol and propanol molecules in the microwave frequency range. We identified 61 rotational transitions of the propanol molecule with rotational quantum numbers up to J = 36 inclusive. We show that in order to satisfactorily take into account centrifugal distortion in the isopropanol molecule, it is sufficient to use only the quartic and sextic terms, while in calculating the rotational transitions of the propanol molecule we need to include all the way up to dectic centrifugal distortion constants.

Unified parameter for localization in isotope-selective rotational excitation of diatomic molecules using a train of optical pulses

We obtained a simple theoretical unified parameter for the characterization of rotational population propagation of diatomic molecules in a periodic train of terahertz optical pulses around the condition of so-called quantum resonance. The parameter comprises the peak intensity and interval between the pulses, and the level energies of the initial and final rotational states of the molecule. Using the unified parameter, we can predict the upper and lower boundaries of probability localization on the rotational level network, including the effect of centrifugal distortion. The unified parameter was tentatively derived from an analytical expression obtained by performing rotating-wave approximation and spectral decomposition of the time-dependent Schr\"odinger equation under an assumption of time-order invariance. The validity of the parameter was confirmed by comparison with numerical simulations for isotope-selective rotational excitation of KCl molecules.

Hyperfine structure of the hydroxyl free radical (OH) in electric and magnetic fields

We investigate single-particle energy spectra of the hydroxyl free radical (OH) in the lowest electronic and rovibrational level under combined static electric and magnetic fields, as an example of heteronuclear polar diatomic molecules. In addition to the fine-structure interactions, the hyperfine interactions and centrifugal distortion effects are taken into account to yield the zero-field spectrum of the lowest manifold to an accuracy of less than 2 kHz. We also examine level crossings and repulsions in the hyperfine structure induced by applied electric and magnetic fields. Compared to previous work, we found more than 10% reduction of the magnetic fields at level repulsions in the Zeeman spectrum subjected to a perpendicular electric field. In addition, we find new level repulsions, which we call Stark-induced hyperfine level repulsions, that require both an electric field and hyperfine structure. It is important to take into account hyperfine structure when we investigate physics of OH molecules at micro-Kelvin temperatures and below.

Direct Observation, Study, and Control of Molecular Superrotors

Extremely fast rotating molecules whose rotational energy is comparable with the molecular bond strength are known as "superrotors." It has been speculated that superrotors may exhibit a number of unique properties, yet only indirect evidence of these molecular objects has been reported to date. Here we demonstrate the first direct observation of molecular superrotors by detecting coherent unidirectional molecular rotation with extreme frequencies exceeding 10 THz. The technique of an "optical centrifuge" is used to control the degree of rotational excitation in an ultrabroad range of rotational quantum numbers, reaching as high as N = 95 in oxygen and N = 60 in nitrogen. State-resolved detection enables us to determine the shape of the excited rotational wave packet and quantify the effect of centrifugal distortion on the rotational spectrum. Femtosecond time resolution reveals coherent rotational dynamics with increasing coherence times at higher angular momentum. We demonstrate that molecular superrotors can be created and observed in dense samples under normal conditions where the effects of ultrafast rotation on many-body interactions, intermolecular collisions, and chemical reactions can be readily explored.

Highly Sensitive Ammonia Probes of a Variable Proton-to-Electron Mass Ratio.

The mass sensitivity of the vibration-rotation-inversion energy levels of ammonia is probed using the nonrigid inverter theory. It is shown that the sensitivity exhibits non-negligible centrifugal distortion dependence, which is currently disregarded. The centrifugal distortion effects are especially important in the case of the Δk = ±3 "forbidden" transitions involving accidentally coinciding ro-inversional states |a,J,K = 3⟩ and |s,J,K = 0⟩ of the ν2 vibrational state of (14)NH3. The energy differences of these states exhibit very anomalous mass sensitivities (see Jansen etal. J. Chem. Phys. 2014 , 140 , 010901 ), thus appearing as new highly sensitive probes of the cosmological variability of the proton-to-electron mass ratio.

Synchrotron Based FT-FIR Pure Rotational Spectroscopy of the NH2 Radical in Its Two Lowest Vibrational States

Six Fourier-transform FIR spectra of the NH2 radical have been recorded at high resolution (0.001 cm(-1)) using synchrotron radiation on the AILES beamline at SOLEIL Synchrotron. Three different experimental discharge setups have been used to observe, in absorption, 1009 pure rotational transitions of NH2 in the vibrational ground state (000) and 170 pure rotational transitions within the first excited vibrational state (010). These results constitute a significant extension of the observed quantum numbers for these two states. The spectra permitted several couplings to be resolved (asymmetric coupling, spin-rotation coupling, hyperfine structure) for relatively highly excited energy levels. An effective fit has been realized using both standard Watson-S and -A reductions despite an abnormal centrifugal distortion effect for this light hydride.

Conformational Equilibria in Bimolecules of Carboxylic Acids: A Rotational Study of Fluoroacetic Acid–Acrylic Acid

The rotational spectra of three conformers of the fluoroacetic acid–acrylic acid bimolecule have been assigned by pulsed jet Fourier Transform microwave spectroscopy. Information on their relative populations in the jet has been obtained by relative intensity measurements. In addition, the Ubbelohde effect has been quantitatively estimated from the changes of the planar moment of inertia Paa upon H→D isotopic substitution of the carboxylic hydrogens. The dissociation energies of the three conformers, as estimated from centrifugal distortion effects, have almost the same values for the three species.

Ab initio effective rotational Hamiltonians: A comparative study

AbstractTwo independent methods to obtain ab initio effective rotational Hamiltonians have been implemented recently. The first one is based on a generalization of perturbation theory to noncommutative rings, the other one on contact transformation (CT) techniques. In principle, both methods are able to give rotational Hamiltonians including centrifugal distortion effects of arbitrary high orders. These methods are compared, for the first time, in this article with regard to calculations of the rotational levels of methane vibrational ground state. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

The millimeter-wave spectrum of methoxyacetonitrile

This paper is the second as part of a study to elucidate cyano group chemistry in the interstellar medium, wherein we present a characterization of the millimeter-wave spectrum of methoxyacetonitrile. Its structural isomer, 2-cyanoethanol was described in paper Braakman and Blake (submitted for publication) [1]. The rotational spectrum of methoxyacetonitrile shows the characteristics of a well-behaved asymmetric rigid-rotor. We assigned nearly 7400 transitions to 2 conformers and 2 excited states. All were fit to Watson A-reduced Hamiltonians, including only the effects of centrifugal distortion. The lowest energy states are fit close to experimental accuracy, with a gradual decrease in quality for higher energy states. No repulsive interactions are apparent. The measurements and predictions from the fits reported here are being used as a basis for observational studies of this species.

The H 216O molecule: Line position and line intensity analyses up to the second triad

Accurate line strengths were obtained for the H 2 16O molecule in the 800–1750 cm −1 region. Measurements were carried out for 974 lines including pure rotational transitions and ν 2 , 2 ν 2 - ν 2 , 3 ν 2 - 2 ν 2 , and ν 1 - ν 2 band transitions. The absolute experimental uncertainty on the measurements, in percent of the observed line strength, is 1.5% or better for a number of lines. Line position and line intensity analyses of data pertaining to water and involving the eight first vibrational states of this molecule were also carried out. The data considered in the line position analysis were taken from the literature and involved rotational energy levels, line positions, and microwave frequencies. The data considered in the line intensity analysis involve the line strengths measured in this work, line strengths available from the literature, and Stark coefficients. The theoretical model used in both analyses accounts for anomalous centrifugal distortion effects. The unitless standard deviation achieved in both analyses is 1.2.

Relative Strengths of the OH⋅⋅⋅Cl and OH⋅⋅⋅F Hydrogen Bonds

Which bond goes where? The observed conformer of H2O⋅⋅⋅ClFCH2 displays one OH⋅⋅⋅Cl and two weak CH⋅⋅⋅O hydrogen-bonding interactions. The dissociation energy of this conformer is estimated to be 8.5 kJ mol−1 from the centrifugal distortion effects.

Free-Jet Rotational Spectrum and Tunneling Motion of Difluoromethane···Xenon

The rotational spectra of three isotopologues of difluoromethane...xenon have been investigated by free-jet millimeter-wave absorption spectroscopy. Only mu(c)-type transitions have been observed, all of them evenly split due to the internal motion of Xe relative to the difluoromethane moiety. The vibrational splitting, 39.1(3) MHz, has been used to estimate the tunneling barrier, V(2) = 109 cm(-1). Information on the dissociation energy has been deduced from centrifugal distortion effects (E(B) = 1.8 kJ mol(-1)). The xenon atom lies in the sigma(v) symmetry plane of difluoromethane containing the hydrogen atoms, at an r(0) distance of 3.816 A from its center of mass (cm), and forms a Xe-cm-C r(0) angle of 118 degrees . The observed conformation is in agreement with the minimum found with a distributed polarizability model.

Alignment of symmetric top molecules by short laser pulses

Nonadiabatic alignment of symmetric top molecules induced by a linearly polarized, moderately intense picosecond laser pulse is studied theoretically and experimentally. Our studies are based on the combination of a nonperturbative solution of the Schr\odinger equation with femtosecond time-resolved photofragment imaging. Using methyliodide and tert-butyliodide as examples, we calculate and measure the alignment dynamics, focusing on the temporal structure and intensity of the revival patterns, including their dependence on the pulse duration, and their behavior at long times, where centrifugal distortion effects become important. Very good agreement is found between the experimental and numerical results. This allows us to use our theory and numerical results to provide additional insight into the origin of the experimental findings.

Microwave spectrum and conformation of n-propyltrifluorosilane

The conformational properties of gaseous n-propyltrifluorosilane (CH 3CH 2CH 2SiF 3) have been investigated by microwave spectroscopy and high-level quantum chemical calculations. The microwave spectrum was investigated in the 20–62 GHz spectral range at a temperature of −78 °C. The spectra of the ground vibrational state and three vibrationally excited states of one conformer having an antiperiplanar conformation of the C–C–C–Si chain of atoms were assigned. No evidence for the existence of the synclinal ( gauche) conformer was seen in the microwave spectrum. It is concluded that the synclinal form is at least 3.5 kJ/mol less stable than the antiperiplanar conformer in the gas phase. Density functional theory calculations have been performed for the system mainly to predict the effects of centrifugal distortion. The G3 quantum chemical method has been used to test the ability of this method to predict the energy difference between the synclinal and antiperiplanar conformers.

Free-jet rotational spectrum and tunneling motion in difluoromethane⋯krypton

The free-jet millimeter-wave absorption spectra of four isotopomers of difluoromethane⋯krypton have been assigned. The krypton is in the HCH plane of difluoromethane, 3.616 (3) Å from from its center of mass, and forms an angle of 64° (2) with the FCF plane. Ab initio and distributed polarizability model calculations confirm the observed conformation. The measured transitions are split into two component lines due to the tunnelling between two equivalent positions, with Kr above or below the FCF plane. The vibrational splitting, 79.19 (4) MHz, has been used to estimate the inversion barrier (1.23 (5) kJ mol −1), while an estimate of the dissociation energy (1.9 kJ mol −1) has been deduced from centrifugal distortion effects.

Free and pulsed jet rotational spectra and van der Waals motions of ethanol⋯argon

The rotational spectrum of the ethanol⋯argon van der Waals complex has been investigated by free-jet absorption millimeter-wave and molecular beams Fourier transform microwave spectroscopy. The spectra of normal and OD isotopomers have been assigned, but only for the complex with trans-ethanol. Ar motions produce vibrational splittings of 1.6662(7) and 1.70(2) MHz for the two isotopic species, respectively. The barrier hindering the tunnelling motion has been evaluated to be about 55 cm −1. The van der Waals structural parameters are: r 0(Ar–O) = 3.56(1) Å, α 0(∠ArOC) = 94°(1) and τ 0(∠ArOCC) = 71°(1). The dissociation energy, as obtained from centrifugal distortion effects, is 2.1 kJ mol −1.

Internal Motions of the Rare Gas Atom in Dimethyl Ether−Krypton

The free jet millimeter-wave absorption spectra of two isotopomers of the weakly bonded dimethyl ether−Kr complex have been assigned and measured. The Kr atom lies in the σv symmetry plane of dimethyl ether perpendicular to the COC plane, at a r0-distance of 3.67 Å from its center of mass (cm). The line connecting the krypton atom to cm forms an r0-angle of 70° with the O-cm line. The observed conformation is in agreement with the global minimum as found with a distributed polarizability model. Many rotational transitions are split into two component lines, due to the motion of Kr relative to dimethyl ether in the complex. The corresponding splitting has been used to determine the barrier to the internal motion. Information on the dissociation energy has been deduced from the centrifugal distortion effects.

Rotational spectrum of jet-cooled HfO2 and HfO

The rotational spectrum of jet-cooled hafnium dioxide obtained by laser ablation of a solid ceramic rod has been investigated by Fourier-transform microwave spectroscopy in the 8 to 28 GHz frequency range. Rotational transitions within the ground and several excited vibrational states of the lowest vibrational mode of the molecule have been assigned. The resulting spectra have been fit, yielding rotational parameters for the five most abundant isotopomers of HfO2. Centrifugal distortion effects are noticeable even for the lowest-J transitions. Very large quadrupole coupling effects for the isotopomers with nuclear quadrupole moments (179Hf (I=9/2) and Hf177 (I=7/2)) have been accounted for using the diagonal elements of the nuclear quadrupole coupling tensor. A ground-state effective C2v geometry has been obtained for HfO2, yielding ro(Hf–O)=1.7764(4) Å and ∠(O–Hf–O)=107.51(1)°. The electric dipole moment has been determined for HfO2180 from Stark-effect measurements, giving μ=26.42(3)×10-30C⋅m [7.92(1) D]. Ab initio calculations using density functional theory and relativistic core potentials are in satisfactory agreement with the experimental results. Finally, in the course of this investigation, the rotational spectrum of the diatomic molecule, HfO, has also been reexamined, and new results for vibrational satellites (up to v=18 in some cases) of the J=1←0 rotational transition are reported. A vibrational analysis using Le Roy’s form of the Dunham expansion has allowed the determination of atomic mass-dependent Born–Oppenheimer breakdown parameters for both atoms of HfO.

Pure rotational spectrum of 2-pyridone⋯water and quantum chemical calculations on the tautomeric equilibrium 2-pyridone⋯water/2-hydroxypyridine⋯water

The free-jet millimeter-wave spectrum of the complex 2-pyridone⋯water has been measured. The water is strongly linked to the partner group through a double hydrogen bond. We could not observe the spectrum of 2-hydroxypyridine⋯water (2-hydroxypyridine is more abundant than 2-pyridone in the tautomeric mixture). This failure is explained by density functional and ab initio calculations on the relative stability of the two 1:1 complexes with water. Estimates of the dissociation energy have been obtained both from the centrifugal distortion effects and from quantum chemical calculations.