Search Results - (Author, Cooperation:M. C. McCarthy)

2013-12-18

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

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2018-01-12

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Geosciences

Computer Science

Medicine

Natural Sciences in General

Physics

Astronomy, Chemistry

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Rotational transitions from seven low-lying vibrational states of HCCN and five low-lying vibrational states of DCCN have been detected in the frequency range from 100 to 400 GHz with a sensitive millimeter-wave spectrometer. The CCH bending states ν5±1, 2ν5±2, and 3ν5±3, and the CCN bending state ν4±1 have been assigned. In addition, transitions from three vibrational states in HCCN and one in DCCN with zero orbital angular momentum (l) were also detected. These states in all likelihood originate from the three lowest l=0 excited states, i.e., (ν4+ν5)−0, (ν4+ν5)+0, and 2ν05. Analysis of the high-accuracy millimeter-wave frequency data establishes that HCCN is not a near-rigid bent molecule and intensity measurements confirm that the CCH bending states are much lower in energy than in typical well-behaved linear molecules. The low barrier to linearity in HCCN and DCCN of ∼235 cm−1, estimated from intensity measurements and the ab initio calculations of Malmquist et al. [Theor. Chim. Acta 73, 155 (1988)], confirms that HCCN is quasilinear. © 1995 American Institute of Physics.

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1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The fundamental (N = 1 – 0) rotational transitions of the ground 2Σ+ electronic state of the four singly substituted 13C isotopomers of CCCCH have been measured by pulsed-jet Fourier transform microwave spectroscopy. In each isotopomer this transition is split into many well-resolved hyperfine components owing to interaction between the electron spin and the molecular rotation, the proton spin, and the 13C nuclear spin. Here, the hyperfine transition frequencies are analyzed with the higher rotational millimeter-wave frequencies described in the previous paper of McCarthy et al. to produce a precise set of rotational, centrifugal distortion, spin-rotation, and hyperfine coupling constants. In particular, the Fermi-contact interaction of the 13C nucleus has been measured at each substituted position, yielding information on the distribution of the unpaired electron spin density along the carbon chain. The Fermi-contact constants, bF(13C), of 396.8(6), 57.49(5), −9.54(2), and 18.56(4) MHz, for successive 13C substitutions starting furthest from hydrogen indicate that the electronic structure is essentially acetylenic with alternating triple and single bonds. © 1995 American Institute of Physics.

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1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The millimeter-wave rotational spectra of the 13C isotopic species of the CCCCH and CCCN radicals and CCC15N were measured and the rotational, centrifugal distortion, and spin-rotation constants determined, as previously done for the normal isotopic species [Gottlieb et al., Astrophys. J. 275, 916 (1983)]. Substitution (rs) structures were determined for both radicals. For CCCN, an equilibrium structure derived by converting the experimental rotational constants to equilibrium constants using vibration–rotation coupling constants calculated ab initio was compared with a large-scale coupled cluster RCCSD(T) calculation. The calculated vibration–rotation coupling constants and vibrational frequencies should aid future investigations of vibrationally excited CCCN. Less extensive RCCSD(T) calculations are reported here for CCCCH. The equilibrium geometries, excitation energies (Te), and dipole moments of the A2Π excited electronic state in CCCN and CCCCH were also calculated. We estimate that Te=2400±50 cm−1 in CCCN, but in CCCCH the excitation energy is very small (Te=100±50 cm−1). Owing to a large Fermi contact interaction at the terminal carbon, hyperfine structure was resolved in 13CCCCH. Measurements of the fundamental N=0→1 rotational transition of CCCCH with a Fourier transform spectrometer described in the accompanying paper by Chen et al., yielded precise values of the Fermi contact and dipole–dipole hyperfine coupling constants in all four 13C species. The Fermi contact interaction is approximately two times larger in CCCN, allowing a preliminary estimation of hyperfine coupling constant bF in 13CCCN and C13CCN from the millimeter-wave rotational spectra. © 1995 American Institute of Physics.

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1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The structures of two linear silicon carbides, SiC4 and SiC6, have been determined by a combination of isotopic substitution and large-scale coupled-cluster ab initio calculations, following detection of all of the singly substituted isotopic species in a supersonic molecular beam with a Fourier transform microwave spectrometer. Rotational constants obtained by least-squares fitting transition frequencies were used to derive experimental structures; except for those nearest the center of mass, individual bond lengths for both chains have an error of less than 0.008 Å. Accurate equilibrium structures were derived by converting the experimental rotational constants to equilibrium constants using the vibration–rotation coupling constants from coupled-cluster calculations, including connected triple substitutions. Equilibrium dipole moments and harmonic vibrational frequencies were also calculated for both chains. On the basis of the calculated vibration–rotation and l-type doubling constants, weak rotational satellites from a low-lying vibrational state of SiC4 were assigned to v6, a bending mode calculated to lie about 205 cm−1 above the ground state. A recommended ab initio equilibrium structure for SiC8 has also been established. © 2000 American Institute of Physics.

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1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

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. © 1999 American Institute of Physics.

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1089-7690

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Physics

Chemistry and Pharmacology

Optical bands from the 3Σu−←X 3Σg− transition of the linear carbon chains HC7H, HC9H, HC11H, and HC13H have been measured in the gas phase by cavity ringdown spectroscopy. The bands exhibit well resolved P and R branches, which for HC7H are partially rotationally resolved. Comparisons between observed and simulated spectra indicate broadening in excess of that expected from the laser linewidth and Doppler width, suggesting rapid radiationless transitions in roughly 0.1–0.01 ns, a time scale consistent with the absence of observed fluorescence from these molecules. The HC7H and HC9H bands are of nearly equal strength, but those of the shorter chain HC5H are too weak to detect at the 1 ppm level. None of the bands observed here lies sufficiently close to any of the optical diffuse interstellar bands to provide positive identification. © 2000 American Institute of Physics.

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1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

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. © 1999 American Institute of Physics.

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1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Following a high-level coupled cluster calculation, the rotational spectrum of the bent HC4N singlet carbene was detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. The three rotational constants, the leading centrifugal distortion constants, and two nitrogen hyperfine coupling constants were determined to high accuracy. The rotational constants agree with those calculated ab initio to better than 0.5%. Like the isoelectronic C5H2 carbene of similar structure, HC4N was found to have fairly large centrifugal distortion and a large inertial defect. The calculated dipole moment of HC4N is 2.95 D. © 1999 American Institute of Physics.

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1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The rotational spectra of the HCCCNH+, NCCNH+, and CH3CNH+ ions have been observed in a supersonic molecular beam by Fourier transform microwave (FTM) spectroscopy. The rotational and centrifugal distortion constants were determined for all three, and the nitrogen quadrupole hyperfine coupling constants for HCCCNH+ and NCCNH+. From the respective Doppler shifts, it is found that the velocities of the ions are 3% larger than those of the parent unprotonated molecules, and the linewidths are greater by about 50%. The concentration of the ions near the nozzle is approximately 1011 cm−3, which is sufficiently high to be detectable in the visible and the IR by present laser techniques. © 2000 American Institute of Physics.

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1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Rhomboidal SiC3, a planar ring with C2v symmetry and a transannular C–C bond, was detected at centimeter wavelengths in a pulsed supersonic molecular beam with a Fourier transform microwave (FTM) spectrometer, and was subsequently observed in a low-pressure dc glow discharge with a free-space millimeter-wave absorption spectrometer. The rotational spectrum of SiC3 is characterized by large harmonic defects and large splitting of the K-type doublets. Lines in the centimeter-wave band were very strong, allowing the singly substituted isotopic species to be observed in natural abundance. Measurements of the normal and five isotopically substituted species with the FTM spectrometer provided conclusive evidence for the identification and yielded an experimental zero-point (r0) structure. Forty-six transitions between 11 and 286 GHz with Ka≤6 were measured in the main isotopic species. Three rotational and nine centrifugal distortion constants in Watson's A-reduced Hamiltonian reproduce the observed spectrum to within a few parts in 107 and allow the most intense transitions up to 300 GHz to be calculated with high accuracy. The spectroscopic constants confirm that SiC3 is a fairly rigid molecule: the inertial defect is comparable to those of well-known planar rings and the centrifugal distortion constants are comparable to molecules of similar size. The number of SiC3 molecules in our supersonic molecular beam in each gas pulse is at least 3×1011, so large that electronic transitions may be readily detectable by laser spectroscopy. © 1999 American Institute of Physics.

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1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The resonance Raman excitation profiles for the ν1, ν2, and ν3 vibrations of violaxanthin in acetone, toluene, and carbon disulfide solvents have been measured. The results are interpreted in terms of a three-mode vibrational theory which includes both homogeneous and inhomogeneous broadening effects. Excellent agreement between calculated and observed excitation profiles and visible spectra was found in all three solvents. The broadening mechanism is exclusively homogeneous. The results also indicate that the excited-state vibrational frequencies may be important in the theoretical model.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Three new silicon–carbon chains, SiC4H, SiC5H, and SiC6H, have been detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. A detailed spectroscopic characterization of these and the previously described chains SiCCH, SiCN, and SiNC is given here. All six radicals are linear chains with 2Π electronic ground states and all have resolvable hyperfine structure in their lower rotational transitions; all except SiC5H have resolved lambda-type doubling. Because transitions of SiCCH, SiCN, and SiNC were also detected by millimeter-wave absorption spectroscopy in both spin components, for these the rotational, centrifugal distortion, and fine structure and hyperfine coupling constants were determined to high precision using the standard Hamiltonian for a molecule in a 2Π state. For SiC4H, SiC5H, and SiC6H, at least seven transitions in the lowest-energy fine structure component were measured between 7 and 30 GHz, and, at most, five spectroscopic constants were required to reproduce their spectra to a few parts in 107. The hyperfine coupling constants of the SiCnH radicals are fairly close to those of isovalent Cn+1H, indicating that the chemical bonding may be similar. The missing radical in the present sequence, SiC3H, may soon be found along with cyclic isomers of SiCCH and SiC4H. If SiC5H possesses strong electronic transitions in the visible like isovalent C6H, its spectrum should be detectable by long path optical spectroscopy. © 2001 American Institute of Physics.

Electronic Resource

1432-1939

Populus tremuloides ; Carbon/nutrient balance ; Choristoneura conflictana ; Plant/herbivore interaction ; Secondary metabolite

Springer Online Journal Archives 1860-2000

Biology

Summary We investigated the effects of nitrogen fertilization upon the concentrations of nitrogen, condensed tannin and phenolic glycosides of young quaking aspen (Populus tremuloides) leaves and the quality of these leaves as food for larvae of the large aspen tortrix (Choristoneura conflictana), a Lepidopteran that periodically defoliates quaking aspen growing in North America. Nitrogen fertilization resulted in decreased concentrations of condensed tannin and phenolic glycosides in aspen leaves and an increase in their nitrogen concentration and value as food for the large aspen tortrix. These results indicate that plant carbon/nutrient balance influences the quality of aspen leaves as food for the large aspen tortrix in two ways, by increasing the concentrations of positive factors (e.g. nitrogen) and decreasing the concentrations of negative factors (eg. carbon-based secondary metabolites) in leaves. Addition of purified aspen leaf condensed tannin and a methanol extract of young aspen leaves that contained condensed tannin and phenolic glycosides to artificial diets at high and low levels of dietary nitrogen supported this hypothesis. Increasing dietary nitrogen increased larval growth whereas increasing the concentrations of condensed tannin and phenolic glycosides decreased growth. Additionally, the methanol extract prevented pupation. These results indicate that future studies of woody plant/insect defoliator interactions must consider plant carbon/nutrient balance as a potentially important control over the nutritional value of foliage for insect herbivores.

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