Microwave Quantum Research Articles

Intramolecular hydrogen bonding in (1-fluorocyclopropyl)methanol as studied by microwave spectroscopy and quantum chemical calculations

(1-Fluorocyclopropyl)methanol has been studied by microwave spectroscopy in the 12–61 GHz spectral region. The rotational spectra of the ground and of four vibrationally excited states belonging to three different normal modes of one rotamer have been assigned. Most other cyclopropylmethanol derivatives prefer conformations stabilized by an internal hydrogen bond with the pseudo-π electrons along the edges of the ring. This is not the case for the title compound. The conformer assigned in this work has an internal hydrogen bond formed between the fluorine atom and the hydrogen atom of the hydroxyl group. This rotamer is at least 4 kJ/mol more stable than any other form of the molecule. It is pointed out that electrostatic interaction between the O–H and C–F bond dipoles can largely explain the conformational preference of this compound. The microwave work has been assisted by gas-phase infrared spectroscopy and quantum chemical calculations made at the MP2/6-311++G** and B3LYP/6-311++G** levels of theory.

The Structural and Conformational Properties of Formic Hydrazide (Formylhydrazine) Studied by Microwave Spectroscopy and Quantum Chemical Calculations

Formic hydrazide (formylhydrazine) has been investigated by microwave spectroscopy in the 8-62 GHz spectral range, as well as by quantum chemical ab initio and density functional theory calculations made at several levels of theory. Both the ab initio and the density functional theory calculations predict that two stable forms exist for this compound, a more stable conformer having a syn-periplanar arrangement for the heavy atoms, and a less stable form where these atoms are anti-periplanar. The latter rotamer is calculated to be 10-14 kJ/mol less stable than the former, depending on the theoretical approach. The barrier height separating the two forms is calculated to be 92.2 kJ/mol at the B3LYP/cc-pVTZ level. The microwave spectra of the ground and four vibrationally excited states of the lowest torsional mode of the syn-periplanar conformer were assigned. The lowest torsional frequency was determined to be 77(15) cm -1 by relative intensity measurements. The variation of the rotational constants upon excitation of this mode and the lowest torsional frequency were fitted to a potential function of the form V(z) ) 15.6(〈z 4 〉 + 4.9〈z 2 〉 )c m -1 , indicating that the heavy atoms are effectively planar at the equilibrium conformation. The dipole moment was determined to be Ia ) 1.54(2), Ib ) 1.85(2), Ic ) 0 (assumed), and Itot ) 2.41(3) D [8.04(8) 10 -30 C m] by Stark effect measurements.

Equilibrium structures for the cis and trans isomers of 1,2‐difluoroethylene and the cis,trans isomer of 1,4‐difluorobutadiene

AbstractMicrowave spectra for seven isotopomers of cis‐1,2‐difluoroethylene have been reinvestigated and extended with the pulsed‐molecular‐beam Fourier transform method. Rotational Hamiltonians that include a full set of quartic centrifugal distortion constants have been fit for each isotopomer. Ground‐state rotational constants for the trans isomer were available from an investigation of the high‐resolution infrared spectra of several isotopomers. Equilibrium rotational constants have been extracted for the cis and trans isomers from the experimental ground‐state constants and the vibration–rotation constants computed at the MP2/6‐31G(d) level. Equilibrium structures have been fit to the equilibrium principal moments of inertia for both isomers, and isomeric differences in the bond lengths and bond angles have been interpreted in terms of the strong influence of fluorine substitution. For cis,trans‐1,4‐difluorobutadiene a similar derivation of the equilibrium structure has been achieved through the combination of published microwave data and quantum chemical calculations. This substance provides an opportunity to examine the influence of cis and trans fluorine substitution within a single molecule. Evidence is weighed for the adequacy of vibration–rotation constants computed with the MP2/6‐31G(d) model. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Four conformers characterized in allyl nitrite.

Allyl nitrite, O=NOCH(2)CH=CH(2), can exist in conformations which include syn (S) and anti (A) configurations of O=N-O-C, anti or gauche (G) configurations of N-O-C-C, and syn or skew (Sk) configurations of O-C-C=C. Four of them have been observed and characterized by their microwave spectra and quantum chemical calculations to have AGSk' (0.0 kcal/mol), AGSk (0.2(2) kcal/mol), SASk (0.2(2) kcal/mol), and SAS (0.9(2) kcal/mol) configurations about the three successive dihedral angles. All four are observed in microwave studies of the vapor at 300 and 200 K and the first three are seen in pulsed-jet beams. The SAS form apparently relaxes to the SASk conformer during expansion in the pulsed-jet. Quantum chemical calculations reveal that the two possible conformers with anti-anti configurations of the O=N-O-C-C fragment, AAS and AASk, are transition states, unstable configurations, or of marginal stability, consistent with structures observed in this and other previously studied alkyl nitrites. The remaining four possible conformers are predicted to have relative energies within 1 kcal/mol of the observed conformers but were not observed experimentally.

Microwave spectrum, molecular structure, conformational equilibrium, vibrational frequencies and quantum chemical calculations for methyl vinyl sulfide

The microwave (MW) spectrum of methyl vinyl sulfide (CH2CHSCH3) has been investigated in the 20.0–60.5GHz spectral region at dry ice temperature. The most stable rotamer is planar with the methyl and vinyl groups in the syn conformation. A less stable skew rotamer has been assigned for the first time. Its CC–S–C torsional angle is approximately 154° from syn. Isotopic species of all the heavy atoms of the skeleton of the syn conformation have been assigned. This allowed the following structure parameters to be calculated as: C1–C2=133.15 (18) pm, C1–S3=174.11 (9) pm, S3–C4=179.70 (9) pm, C2–C1–S3=130.23 (11)° and C1–S3–C4=101.99 (9)°. The uncertainties are one standard deviation. Ten vibrationally excited states of the syn conformer were assigned, while the ground state and two excited states of the skew conformer were assigned. The frequencies of several of these excited states have been determined by relative intensity measurements. The syn form was found to be 5.0 (3)kJmol−1 more stable than the skew conformer by relative intensity measurements. The uncertainty is one standard deviation. Quantum chemical calculations at HF, MP2 (full), DFT and QCISD levels of theory using the 6-311++G∗∗, cc-pVTZ and AUG-cc-pVTZ basis sets have been made to assist the experimental work.

The structural and conformational properties of 2-methoxyfuran as studied by microwave spectroscopy and quantum chemical calculations

The microwave spectrum of 2-methoxyfuran has been investigated in the 10.0–60.0GHz spectral region at about −35°C. One rotamer denoted Syn was assigned. This conformer is at least 3kJmol−1 more stable than any other form. Syn has a symmetry plane (Cs symmetry). The methyl group is Syn to the nearest CC bond of the ring. The dipole moment components along the principal initial axes and the total dipole moment are (in units of 10−30Cmm): μa=3.62(16), μb=5.98(3), μc=0.0 (for symmetry reasons), and μtot=6.99(12). Four vibrationally excited states belonging to three different normal modes were assigned and their frequencies determined by relative intensity measurements. The barrier to internal rotation of the methyl group is at least 14kJmol−1.The microwave work has been assisted by quantum chemical computations at the MP2/6-311++G∗∗ (frozen core), B3LYP/cc-pVTZ and B3LYP/6-311++G∗∗ levels of theory. These rather advanced calculations were found to predict rather different conformations for a second conformer.

The structural and conformational properties of 1-amino-1-ethynylcyclopropane as studied by microwave spectroscopy and quantum chemical calculations

The microwave spectrum of 1-amino-1-ethynylcyclopropane has been investigated in the 19.0–62.0GHz spectral region at about −30°C. One rotamer denoted Conformer I was assigned in this work. Conformer I has a symmetry plane (Cs symmetry). Both the hydrogen atoms of the amino group are involved in intramolecular hydrogen bonds with the π-electrons of the triple bond. The dipole moment components and the total dipole moment are (in units of 10−30Cm): μa=1.353(22), μb=1.502(9), μc=0.0 (for symmetry reasons), and μtot=2.022(79). Three vibrationally excited states belonging to three different normal modes were assigned and their frequencies determined by relative intensity measurements.The microwave work has been assisted by quantum chemical computations at the MP2/6-311++G∗∗ (frozen core) and B3LYP/6-31G∗ levels of theory.

Coherent resonant transport through a mesoscopic system with quantum ac microwave field

The time-dependent transport through an ultrasmall quantum dot coupling to two electron reservoirs is investigated. The quantum dot is perturbed by a quantum microwave field (QMF) through gate. The tunneling current formulae are obtained by taking expectation values over coherent state (CS), and SU(1,1) CS. We derive the transport formulae at low temperature by employing the nonequilibrium Green function technique. The currents exhibit coherent behaviors which are strongly associated with the applied QMF. The time-dependent currents appear compound effects of resonant tunneling and time-oscillating evolution. The time-averaged current and differential conductance are calculated, which manifest photon-assisted behaviors. Numerical calculations reveal the similar properties as those in classical microwave field (CMF) perturbed system for the situations concerning CS and squeezed vacuum SU(1,1) CS. But for other squeezed SU(1,1) CS, the tunneling behavior is quite different from the system perturbed by a single CMF through gate. Due to the quantum signal perturbation, the measurable quantities fluctuate fiercely.

Syntheses and Reactions of Some Cyclopropyl-substituted Imines. Part 2. Structural and Conformational Properties of Dicyclopropylketimine as Studied by Microwave Spectroscopy and Quantum Chemical Calculations.

Syntheses and Reactions of Some Cyclopropyl-substituted Imines. Part 2. Structural and Conformational Properties of Dicyclopropylketimine as Studied by Microwave Spectroscopy and Quantum Chemical Calculations.

Thermally induced decay of the atom in a photonic bandgap reservoir

We investigate thermally induced partial decay of a two-level atom into a photonic bandgap reservoir in regimes when the atomic transition is tuned in the forbidden gap or at the band edge. The atom decays into a quasi-equilibrium state different from those given by the Boltzmann statistical distribution. We take values of physical parameters typical for Rydberg atoms and currently existing photonic crystals. We show that specific thermally induced transitions can be significant for such Rydberg atoms even at bandgap frequencies provided that there is a peak in the density of modes at the band edge. We point out a temperature sensitivity of microwave quantum optics devices based on photonic band structures due to temperature-dependent transitions and level shifts in regimes when important quantum transitions occur at energies near to band-edge frequencies.

From photonic band gaps to refractive index engineering

Since the pioneering work of Yablonovitch ten years ago, photonic band gap (PBG) materials have become the subject of a well-established domain of electromagnetism, with well-defined branches dealing with microwave engineering, quantum optics, semiconductor technology or laser theory. The resolution of Maxwell's equations in three-dimensional (3D) resonant structures is more than ever the key point. This paper reviews the state-of-the-art of photonic band gap materials. It seems that, after the early years devoted to understanding simple and standard structures (i.e. highly symmetric, infinite or perfectly two-dimensional, or with simple planar boundaries, …), the way from photonic crystal concepts to real photonic structures requires now to go one step further towards complexity, towards a complete three-dimensional refractive index engineering.

Concepts in high-frequency EPR — Applications to bio-inorganic systems

In this contribution we describe a high-frequency high-field EPR facility which has been developed at the University of Nijmegen. We present the design of a heterodyne quasi-optical bridge based on a millimeter-wave vector network analyzer as source and detection system. The mm-waves are transported in free-space through Gaussian beam optic elements and through a corrugated guide inside the resonator insert. The Fabri-Perot (TEM00n) resonator is coupled through a metallic mesh and because of its bimodal property it can be operated using orthogonal detection leading to substantial improvement in sensitivity. In the first stage of the project, a multifrequency CW-facility is realized covering the 100–500 GHz range. In our initial explorative experiments we demonstrate the advantages of HF-EPR of high-spin systems: Due to the large microwave quantum, transitions which would be undetectable at X-band due to the large zerofield splitting can now be observed in good sensitivity. As a model for biological high-spin systems a sample of metmyoglobin was measured at D-band (130 GHz). Theg = 5.9 perpendicular line from theS = 5/2 ferric heme was detected and its line-width was compared to data previously obtained at Q-, X-, S- and L-band. As a model for biological integer spin systems theS = 1 signal of Ni(II) in nickel Tutton salt (Ni(NH4)2(SO4)2) was studied at 35 and 130 GHz.

Structural and Conformational Properties of 1,1,2,2-Tetrafluoroethyl Methyl Ether as Studied by Microwave Spectroscopy and Quantum Chemical Calculations.

Structural and Conformational Properties of 1,1,2,2-Tetrafluoroethyl Methyl Ether as Studied by Microwave Spectroscopy and Quantum Chemical Calculations.

Structural and Conformational Properties of 1,2-Difluoropropane as Studied by Microwave Spectroscopy and Quantum Chemical Calculations.

Structural and Conformational Properties of 1,2-Difluoropropane as Studied by Microwave Spectroscopy and Quantum Chemical Calculations.

Theory of a type of quantum amplification: Phase-sensitive amplification by frequency upconversion.

It is shown that the spectroscopic bridge, a recently proposed method to achieve high-efficiency microwave oscillators and phase-sensitive detectors, may be used in a spin multiplet of certain paramagnetic ions to obtain a type of microwave quantum amplification: phase-sensitive amplification by frequency upconversion. This operation of a solid-state maser without population inversion is based on a principle similar to that used for achieving amplification in a transistor (triode). The major differences between this amplification and the parametric amplification, or the usual double-resonance maser action, are discussed.

Electron paramagnetic resonance identification of an Fe-Ag pair in CdTe

Electron paramagnetic resonance measurements on bulk CdTe crystals revealed the presence of a new defect with monoclinic symmetry in addition to the isolated Fe3+ (3d5, S=5/2). The angular dependence of the resonance lines can only be explained by assuming a spin S=5/2, an isotropic g-value of g=2.12 and additional fine structure terms D and E with a ratio E/D of 0.091. D is assumed to be much larger than the microwave quantum h nu . On the basis of resolved hyperfine interactions we identify the defect as an Fe-Ag pair.

Quantum dots as a tunable microwave quantum detector

In arrays of field-effect confined quantum dots, we observe a novel resonant photovoltaic response to microwave and far-infrared radiation. A maximum photosignal of 5 μV at an incoming microwave power of less than 1 mW is detected when the frequency of the radiation matches the resonance condition of the electrons confined in the quantum dots. The response frequency is tunable by means of the gate voltage or an external magnetic field. This observation leads to an efficient quantum detector working up to temperatures of about T=6 K and to a novel spectroscopic tool for the investigations of low-dimensional electronic systems.

Redox properties and EPR spectroscopy of the P clusters of Azotobacter vinelandii MoFe protein.

In Azotobacter vinelandii MoFe protein the oxidation of the P clusters to the S = 7/2 state is associated with a redox reaction with Em,7.5 = +90 +/- 10 mV (vs the normal hydrogen electrode), n = 1. A concomitant redox process is observed for a rhombic S = 1/2 EPR signal with g = 1.97, 1.88 and 1.68. This indicates that both S = 1/2 and S = 7/2 signals are associated with oxidized P clusters occurring as a physical mixture of spin states. The maximal intensity of the S = 1/2 and S = 7/2 signals in the mediated equilibrium redox titration is similar if not identical to that of solid-thionine-treated samples. Summation of the spin concentration of the S = 1/2 spin state (0.25 +/- 0.03 spin/alpha 2 beta 2) and the S = 7/2 spin state (1.3 +/- 0.2 spin/alpha 2 beta 2) confirms that the MoFe protein has absolutely no more than two P clusters. In spectra of enzyme fixed at potentials around -100 mV a very low-intensity g = 12 EPR signal was discovered. In parallel-mode EPR the signal sharpened and increased > 10-fold in intensity which allowed us to assign the g = 12 signal to a non-Kramers system (presumably S = 3). In contrast with the non-Kramers EPR signals of various metalloproteins and inorganic compounds, the sharp absorption-shaped g = 12 signal is not significantly broadened into zero field, implying that the zero field splitting of the non-Kramers doublet is smaller than the X-band microwave quantum. The temperature dependence of this g = 12 EPR signal indicates that it is from an excited state within the integer spin multiplet. A bell-shaped titration curve with Em,7.5 = -307 +/- 30 mV and +81 +/- 30 mV midpoint potentials is found for the g = 12 EPR signal. We propose that this signal represents an intermediate redox state of the P clusters between the diamagnetic, dithionite-reduced and the fully oxidized S = 7/2 and S = 1/2 state. Redox transitions of two electrons (-307 +/- 30 mV) and one electron (+90 +/- 10 mV) link the sequence S = 0<-->S = 3<-->(S = 7/2 and S = 1/2). We propose to name the latter paramagnetic oxidation states of the P clusters in nitrogenase POX1 and POX2, and to retain PN for the diamagnetic native redox state.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantum Features of Microwave Propagation in a Rectangular Waveguide

Abstract The formal analogy between the distribution of the electromagnetic field in waveguides and microwave cavities and quantum mechanical probability distributions is put into evidence. A waveguide of a cut-off frequency ωc acts on an electromagnetic wave as a quantum potential barrier Ug = hωc . A non-habitual time independent Schrödinger equation, describing guided wave propagation, is established

Low-field microwave absorption and quantum oscillations in type-I superconductors

The effects of low-magnetic field microwave absorption (LFMA) and quantum oscillations (QO) discovered earlier in high-temperature superconductors can also be seen in conventional type-I superconductors such as lead, tin, indium and mercury. We have found that both these effects arise from surface irregularity of a sample. LFMA appears when an oxide layer of a few hundred Ångström; covers the surface and QO are observed in irregular samples with sharp boundaries.