Ground Singlet Electronic States Research Articles

A second rhomboidal isomer of SiC3

A second low-lying cyclic isomer of SiC3 has been detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. Calculated to lie about 5 kcal above the ground state rhomboid, the new isomer is also a planar rhomboid with a transannular bond, C2v symmetry, and a singlet electronic ground state. The transannular bond, however, is between the Si and the opposite C, and the rotational spectrum as a result is that of an oblate, not a prolate, asymmetric rotor. Both rhomboidal isomers of SiC3 are produced with comparable abundance under a wide range of experimental conditions, which suggests that cyclic isomers of longer silicon carbides may now be observable with the present techniques. Oblate SiC3 is a plausible molecule for astronomical detection because it is calculated to be fairly polar, and because radio emission lines of the ground state rhomboid have already been detected in the circumstellar shell of the evolved carbon star IRC+10216.

Rhomboidal SiC3

During experiments to detect carbon chain molecules in the laboratory, an electrical discharge through a dilute mixture of silane and diacetylene was found to produce a molecule of unusual structure that is composed of only silicon and carbon. On spectroscopic analysis this molecule has been shown to have the elemental formula SiC3, a closed-shell singlet electronic ground state, and a planar rhomboidal geometry that consists of atoms in the shape of a distorted four-membered ring with a transannular carbon–carbon bond. Rhomboidal SiC3 is isovalent with rhombic C4, a long predicted low-lying isomer of C4 of similar structure which has so far eluded spectroscopic detection. Strong lines of SiC3 can be observed under a wide range of experimental conditions, suggesting that this molecule is quite stable, and that kinetic and thermodynamic factors favor its formation. Radio emission lines of SiC3 have now been detected in space in the molecular envelope of the carbon-rich star IRC+10216.

Laboratory Detection of the Ring‐Chain Carbenes HC4N and HC6N

The highly polar ring-chain carbenes HC4N and HC6N, formed by substituting either CN or CCCN for a hydrogen atom in cyclopropenylidene (c-C3H2), were detected in a supersonic molecular beam with a Fourier transform microwave spectrometer. Seven a- and four b-type rotational transitions of HC4N and 11 a-type transitions of HC6N, each with resolved nitrogen nuclear quadrupole hyperfine structure, were measured between 6 and 21 GHz, yielding precise values for the three rotational constants, the leading centrifugal distortion constants, and the quadrupole coupling constants. Like the hydrocarbon carbenes C5H2, C7H2, and C9H2, both new molecules have a planar ring-chain structures and singlet electronic ground states. The strongest lines of HC4N can be detected with a signal-to-noise ratio exceeding 10 in a total integration time of less than 1 s, but the lines of HC6N were nearly 100 times weaker.

Ab initio calculations of radiationless transitions between excited and ground singlet electronic states of ethylene

General expressions for internal conversion (IC) rate constant calculations have been derived by taking into account displacements, distortions, and rotation (mixing) of normal modes. The electronic part of the rate constant has been computed through the ab initio calculations of vibronic coupling. The corresponding expressions for the simplest two-mode case as well as for the general n-mode case have been derived. We demonstrate the effect of rotated (mixed) normal modes on the IC rate constants based on a model consisting of one promoting and two mixed modes. The dynamics of excited states of C2H4 has been investigated based on the internal conversion mechanism. The calculated rate of internal conversion show that the lifetimes of the excited π–3p and π–π* states of C2H4 are on the picosecond scale. We predict that if the molecule is excited to a Rydberg π–3p state, it relaxes to the ground state via the cascade mechanism, π–3p→π–3s(1B3u)→π–π*(1B1u)→1Ag.

Stereoselective Chemical Ionization Mass Spectrometry: Reactions of CH3OPOCH3+ with Cyclic Vicinal Diols

The phosphenium ion CH3OPOCH3+ readily attacks hydroxyl groups of neutral substrates in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer. The electrophilic character of CH3OPOCH3+ is in agreement with molecular orbital calculations (Becke3LYP/6-31G(d) + ZPE) that predict a singlet electronic ground state for this species. The observed reactions provide a convenient synthetic route to various larger phosphenium ions in the gas phase. Most importantly, however, CH3OPOCH3+ was found to be extremely sensitive to the stereochemical structure of the neutral substrate. The dramatically different reaction product distributions obtained for diastereomeric cyclic vicinal diols suggest that CH3OPOCH3+ provides a powerful chemical ionization reagent for the mass spectrometric determination of the stereochemistry of diols.

Laboratory Detection of the Ring-Chain Molecule C[TINF]5[/TINF]H[TINF]2[/TINF

The highly polar ring-chain C5H2, a carbene with a singlet electronic ground state, has been detected with a Fourier transform molecular beam microwave spectrometer in the same diacetylene-neon discharge in which a carbene with the same elemental formula but a linear backbone was recently found. Thirteen a- and b-type rotational transitions between 6 and 27 GHz were measured to 1-2 kHz, and precise values of the three rotational constants and two centrifugal distortion constants were determined. With these, the entire microwave and millimeter-wave spectrum of C5H2 can be calculated to an accuracy of 0.2 km s-1 or better. The identity of the new molecule as C5H2, which is fairly certain because of the close agreement between the measured rotational constants and those calculated ab initio, is conclusively confirmed by detection of the totally deuterated and two partially deuterated isotopic species at exactly the expected isotope shifts.

Detection and Characterization of the Cumulene Carbenes H 2 C 5 and H 2 C 6

Two cumulene carbenes, H2C5 and H2C6, were detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. Their rotational and leading centrifugal distortion constants were determined with high accuracy, such that the entire radio spectrum can now be calculated. Like the known carbenes H2C3 and H2C4, both molecules have singlet electronic ground states and linear carbon-chain backbones. They can be produced in sufficiently high concentrations in the laboratory that their electronic spectra, expected to lie in the visible, should be readily detectable by laser spectroscopy. The microwave spectra of other, more exotic isomers may be detectable as well.

Silaacetylene: A possible target for experimental studies

The equilibrium geometries and transition states for interconversion of the CSiH2 isomers in the singlet electronic ground state are optimized at the MP2 and CCSD(T) levels of theory using a TZ2P basis set. The heats of formation, vibrational frequencies, infrared intensities, and rotational constants are also predicted. There are three energy minima on the CSiH2 potential energy surface. Energy calculations at CCSD(T)/TZ2P(fd) + ZPE predict that the global energy minimum is silavinylidene (1), which is 34.1 kcal mol−1 lower in energy than trans-bent silaacetylene (2) and 84.1 kcal mol−1 more stable than the vinylidene isomer (3). The barrier for rearrangement 2→1 is calculated at the same level of theory to be 5.1 kcal mol−1, while for the rearrangement 3→2 a barrier of 2.7 kcal mol−1 is predicted. The natural bond orbital (NBO) population scheme indicates a clear polarization of the C(SINGLE BOND)Si bonds toward the carbon end. A significant ionic contribution to the C(SINGLE BOND)Si bonds of 1 and 2 is suggested by the NBO analysis. The C(SINGLE BOND)Si bond length of trans-bent silaacetylene (2) is longer than previously calculated [1.665 Å at CCSD(T)/TZ2P)]. The calculated carbon-silicon bond length of 2 is in the middle between the C(SINGLE BOND)Si double bond length of 1 (1.721 Å) and the C(SINGLE BOND)Si triple bond of the linear form HCSiH (4), which is 1.604 Å. Structure 4 is a higher-order saddle point on the potential energy surface. © 1996 by John Wiley & Sons, Inc.

Falloff Behavior in the Thermal Dissociation Rate of N2O

The striking difference in falloff behavior (pressure dependence) observed between N{sub 2}O and Ar as collision partners in the thermal dissociation rate of N{sub 2}O is noted. A mechanism proposed by Lindars and Hinshelwood involves transitions, both unimolecular and collision-induced, from the ground singlet electronic state to a dissociating triplet state. The mechanism was fit to a wide range of experimental data for neat N{sub 2}O and N{sub 2}O-Ar mixtures (273 individual data points), accounting quantitatively for the observed behavior [0.006 average rms deviation in ln(k)]. Results derived for the limiting high- and low-pressure (M = Ar) rate constants exceed current recommended values by factors of approximately 1.4 and approximately 2, respectively, over the temperature range 925-2000 K. An alternative mechanistic interpretation involving a finite rate for intramolecular vibrational relaxation is discussed. 29 refs., 8 figs., 1 tab.

Direct dissociative recombination of ground-state HeH+

[sup 3]HeH[sup +] molecular ions have been stored in an ion storage ring and allowed to interact with electrons from an electron cooler. Neutral reaction products have been counted and an absolute cross section for dissociative recombination has been measured. The spectrum reveals a prominent peak at about 18 eV and a weaker one at about 30 eV. The origin of the low-energy dissociative recombination of [sup 3]HeH[sup +] is discussed, and the mechanism responsible for the high-energy peak is modeled by calculating the vibrational overlap between the ground state of the molecular ion and several repulsive states. It is concluded that ions populate primarily the singlet electronic ground state, and that the peaks at 18 and 30 eV arise predominantly from direct dissociative recombination of vibrationally cold [sup 3]HeH[sup +].

Small Rings. Part 81. Chlorinated Cyclopropenylidenes, Vinylidenecarbenes, and Propargylenes: Identification by Matrix Isolation Spectroscopy

Matrix-inolated mono- and dichlorocyclopropenylidene have been generated by high-vacuum flash pyrolysis from di- and trichlorocyclopropenes. By subsequent photolysis they have been isomerized to the corresponding vinylidenecarbenes and propargylenes. The identification of six new carbenes in based on the comparison of their experimental and calculated (MP2/6-31G * ) infrared spectra. All have singlet electronic ground states

Photoluminescence study of β relaxation of polyethylene blends

AbstractSecondary β relaxation of polyethylene blends (50/50 HDPE/LDPE) were studied by photoluminescence of anthracene molecules dissolved in the polymer bulk. The temperature of the β relaxation has been determined as Tβ = −40°C by the dependence of the ratio of vibronic components of the fluorescence band on the temperature. The molecular mechanism of this relaxation has been discussed considering the possibility of the energy migration involving anthracene molecules in the singlet electronic excited and ground states. © 1993 John Wiley & Sons, Inc.

Electrophilically catalyzed splitting of dimethoxyamine to methoxynitrenium ion and its reactions

The electrophilically catalyzed splitting of the N-0 bond of dimethoxyamine results in the formation of methoxynitrenium ion, which in reaction with nucleophiles exhibits properties of an ambidentate cation — it methylates rigid nucleophiles and epiminates olefins. The stereospecificity of the addition of the nitrenium ion to cis-2-butene indicates its existence in a singlet electronic ground state.

Germanium-germanium multiple bonds: The singlet electronic ground state of Ge 2H 2

We have investigated the singlet states of the potentially GeGe triply bonded parent compound Ge 2H 2 by ab initio theoretical techniques. The stable minima and their relative energies are surprisingly similar to those found in previous studies of Si 2H 2. Thus, we find a dibridged structure to be lowest lying, with germylidene and trans-bent structures about 10 and 18 kcal/mol higher, respectively. As in Si 2H 2, the linear structure is not a minimum on the potential energy surface.

Simulation of magnetic moments in a dilute induced-moment Heisenberg magnet

The author considers a diamagnetically diluted induced-moment Heisenberg magnet in which the active sites have a singlet electronic ground state and an excited doublet state. Moments may be induced at the active sites by mixing the doublet into the singlet through a magnetic exchange interaction or an external magnetic field Hz applied parallel to the z direction. The single-ion Hamiltonian describing this system is -H= Sigma ijJijSiSj- Delta Sigma i(Siz)2-Hz Sigma iSiz, where Delta is the single-doublet separation. Using self-consistent mean-field calculations on computer-generated lattices in one, two and three dimensions, and with connectivities z=2, 3, 4, 6 and 8, he studies the distribution of moments at T=0 as a function of the concentration of diamagnetic impurities, the ratio Jij/ Delta and H. When Hz=0, diamagnetic atoms rapidly reduce the value of the mean induced moment, leaving moments of significant size only in regions composed of active sites that are completely surrounded by other active sites. The sensitivity of the lattice towards dilution increases as z increases. The application of Hz compensates to some extent for the effect of the diamagnetic impurities, raising the mean-moment and reducing the variance of the distribution of the size of individual moments.

MAGNETIC AND THERMAL PROPERTIES OF HoF3-ORDERING IN A SINGLET GROUND STATE

Holmium trifluoride, HoF3 has Clh symmetry at H O ~ + sites, resulting in a singlet electronic ground state and low-lying excited states. Magnetic, thermal and spectroscopic experiments show that HoF3 orders antiferromagnetically at T N = 0.53 K , which can be interpreted in terms of molecular field theory applied to a non-Kramers doublet S' = 112.

Coupled electron-nuclear magnetism and neutron diffraction

It often occurs that, in a rare-earth compound having a singlet electronic ground state, a long-range magnetically orderedstate is formed by coupling of the nuclear spins with the electrons through the hyperfine interaction, at millikelvin temperatures. Neutron scattering has been used to measure the details of the magnetization processes of both the electrons and the nuclear spins in the coupled state in PrSn3, HoVO4, and PrCu2. The polarized nuclear spins could be observed through spin dependent nuclear scattering by 141Pran 165Ho. The experimental results for PrSn3 and HoVO4 can be explained very well based on mean field considerations. For PrCu2, a recent experiment indicates that a sinusoidal screw structure is realized simultaneously both in the electronic and nuclear spin systems of this material. Such a magnetic structure requires a more refined theoretical explanation.

Coupled electron-nuclear magnetism and neutron diffraction

It often occurs that, in a rare-earth compound having a singlet electronic ground state, a long-range magnetically orderedstate is formed by coupling of the nuclear spins with the electrons through the hyperfine interaction, at millikelvin temperatures. Neutron scattering has been used to measure the details of the magnetization processes of both the electrons and the nuclear spins in the coupled state in PrSn 3 , HoVO 4 , and PrCu 2 . The polarized nuclear spins could be observed through spin dependent nuclear scattering by 141 Pran 165 Ho. The experimental results for PrSn 3 and HoVO 4 can be explained very well based on mean field considerations. For PrCu 2 , a recent experiment indicates that a sinusoidal screw structure is realized simultaneously both in the electronic and nuclear spin systems of this material. Such a magnetic structure requires a more refined theoretical explanation.

Nuclear spin ordering observed by neutron diffraction (invited)

A review is given of recent neutron scattering experiments which have been carried out at milli-Kelvin temperatures to study the hyperfine-enhanced nuclear-spin ordering in the singlet electronic ground state compounds PrSn3, PrCu2, and HoVO4. Bragg peaks were observed in PrSn3 which are due solely to the nuclear spin polarization of 141Pr caused by the hyperfine field from the antiferromagnetically ordered electronic moment. The temperature dependencies of the nuclear polarization and the additional electronic polarization induced by the nuclear polarization have been measured in detail. A magnetic phase transition in PrCu2 which is believed to correspond to a simultaneous ordering of both the nuclear and electronic spin systems has been observed at 58 mK. The positions of the magnetic diffraction peaks in reciprocal space are represented by G±τ, where G is a vector of the reciprocal lattice and τ=0.24a*±0.68c*, indicating that the magnetic periodicity is incommensurate with the chemical lattice. In HoVO4 magnetic diffraction peaks due to the antiferromagnetic ordering of the nuclear spin system were observed below 4.5 mK. The observed magnetic structure and the dependence of the neutron diffraction intensities on magnetic field are consistent with the predictions of Bleaney on the basis of dipolar energy considerations.

Coherent amplification in fluorescent dye solutions. II. The inverse pre-resonance Raman spectrum of rhodamine 6G

Abstract Inverse pre-resonance Raman spectra of rhodamine 6G in solution in methanol have been recorded over the concentration range 10−1 to 10−4 mol l−1. The solute and solvent transitions are found to display perfectly normal lorentzian profiles over the whole of this concentration range. Intensities and depolarization ratios of the pre-resonance Raman features are recorded. It is shown that the spectra arise from monomeric rhodamine 6G molecules in the singlet electronic ground state and the relative merits of the CARS and inverse Raman methods for recording resonance Raman spectra are discussed.