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The rotational spectra of five isotopic species of the Ar–SiO complex have been observed at high-spectral resolution between 8 and 18 GHz using chirped Fourier transform microwave spectroscopy and a discharge nozzle source; follow-up cavity measurements have extended these measurements to as high as 35 GHz. The spectrum of the normal species is dominated by an intense progression of a-type rotational transitions arising from increasing quanta in the Si–O stretch, in which lines up to v = 12 (∼14 500 cm−1) were identified. A structural determination by isotopic substitution and a hyperfine analysis of the Ar–Si17O spectrum both suggest that the complex is a highly fluxional prolate symmetric rotor with a vibrationally averaged structure between T-shaped and collinear in which the oxygen atom lies closer to argon than the silicon atom, much like Ar–CO. To complement the experimental studies, a full dimensional potential and a series of effective vibrationally averaged, two-dimensional potential energy surfaces of Ar + SiO have been computed at the CCSD(T)-F12b/CBS level of theory. The equilibrium structure of Ar–SiO is predicted to be T-shaped with a well depth of 152 cm−1, but the linear geometry is also a minimum, and the potential energy surface has a long, flat channel between 140 and 180°. Because the barrier between the two wells is calculated to be small (of order 5 cm−1) and well below the zero-point energy, the vibrationally averaged wavefunction is delocalized over nearly 100° of angular freedom. For this reason, Ar–SiO should exhibit large amplitude zero-point motion, in which the vibrationally excited states can be viewed as resonances with long lifetimes. Calculations of the rovibrational level pattern agree to within 2% with the transition frequencies of normal and isotopic ground state Ar–SiO, and the putative Ka = ±1 levels for Ar–28SiO, suggesting that the present theoretical treatment well reproduces the salient properties of the intramolecular potential. [ABSTRACT FROM AUTHOR]

The structure and bonding of H2NNO, the simplest N-nitrosamine, and a key intermediate in deNOx processes, have been precisely characterized using a combination of rotational spectroscopy of its more abundant isotopic species and high-level quantum chemical calculations. Isotopic spectroscopy provides compelling evidence that this species is formed promptly in our discharge expansion via the NH2 + NO reaction and is collisionally cooled prior to subsequent unimolecular rearrangement. H2NNO is found to possess an essentially planar geometry, an NNO angle of 113.67(5)°, and a N—N bond length of 1.342(3) Å; in combination with the derived nitrogen quadrupole coupling constants, its bonding is best described as an admixture of uncharged dipolar (H2N—N=O, single bond) and zwitterion (H2N+=N—O-, double bond) structures. At the CCSD(T) level, and extrapolating to the complete basis set limit, the planar geometry appears to represent the minimum of the potential surface, although the torsional potential of this molecule is extremely flat. [ABSTRACT FROM AUTHOR]

A systematic search for carbon-chain cumulenones beyond H2C4O has been undertaken using microwave spectral taxonomy in combination with a pulsed jet discharge source. No evidence was found for the C2V isomer of H2C5O or its longer derivatives, but HC(O)C4H, the longer variant of propynal, HC(O)CCH, was identified instead. Its rotational and leading centrifugal distortion constants have been derived to high accuracy from detection of both a- and b-type lines; those below 40 GHz were measured using a Fabry-Perot cavity, while lines between 40 and 72 GHz were recorded by double resonance techniques. Overwhelming evidence for the identification is provided by detection of HC(O)C4D, DC(O)C4H, and HC(18O)C4H at the expected frequencies using isotopically enriched samples. Because HC(18O)C4H is produced with comparable abundance when using either 18O2 or C18O as the source of oxygen, and because H13C(O)C4H is not preferentially formed when starting from 13CO, atomic oxygen appears to be a key reactant in formation, plausibly viaOinsertion, e.g., H2CC2n+2H + O → HC(O)C2n+2H + H. Under the same experimental conditions, HC(O)CCH is more than 10 times more abundant than H2C3O, regardless of the source of oxygen, and no evidence is found for cyclopropenone, c-C3H2O. Taken together, these observations indicate that propynal and longer chains with an odd number of carbon atoms are either energetically more stable than cumulenones of the same size, are kinetically favored products, or both. On the basis of the HC(O)C4H discovery, searches for the isovalent sulfur species, HC(S)C4H, and HC(O)C6H have been conducted. Guided by new quantum chemical calculations, the rotational spectra of both were observed in the centimeter-wave band with the same spectrometer. [ABSTRACT FROM AUTHOR]

The thermal decomposition of syn-ethanal-oxide (syn-CH3CHOO) through vinyl hydrogen peroxide (VHP) leading to hydroxyl radical is characterized using a modification of the HEAT thermochemical protocol. The isomerization step of syn-CH3CHOO to VHP via a 1,4 H-shift, which involves a moderate barrier of 72 kJ/mol, is found to be rate determining. A two-dimensional master equation approach, in combination with semi-classical transition state theory, is employed to calculate the time evolution of various species as well as to obtain phenomenological rate coefficients. This work suggests that, under boundary layer conditions in the atmosphere, thermal unimolecular decomposition is the most important sink of syn-CH3CHOO. Thus, the title reaction should be included into atmospheric modeling. The fate of cold VHP, the intermediate stabilized by collisions with a third body, has also been investigated. [ABSTRACT FROM AUTHOR]

Owing to its unparalleled structural specificity, rotational spectroscopy is a powerful technique to unambiguously identify and characterize volatile, polar molecules. We present here a new experimental approach, automated microwave double resonance (AMDOR) spectroscopy, to rapidly determine the rotational constants of these compounds without a priori knowledge of elemental composition or molecular structure. This task is achieved by rapidly acquiring the classical (frequency vs. intensity) broadband spectrum of a molecule using chirped-pulse Fourier transform microwave (FTMW) spectroscopy and subsequently analyzing it in near real-time using complementary cavity FTMW detection and double resonance. AMDOR measurements provide a unique "barcode" for each compound from which rotational constants can be extracted. To illustrate the power of this approach, AMDOR spectra of three aroma compounds - trans-cinnamaldehyde, α-, and β-ionone - have been recorded and analyzed. The prospects to extend this approach to mixture characterization and purity assessment are described. [ABSTRACT FROM AUTHOR]

HOCO is an important intermediate in combustion and atmospheric processes because the OH + CO → H + CO2 reaction represents the final step for the production of CO2 in hydrocarbon oxidation, and theoretical studies predict that this reaction proceeds via various intermediates, the most important being this radical. Isotopic investigations of trans- and cis-HOCO have been undertaken using Fourier transform microwave spectroscopy and millimeter-wave double resonance techniques in combination with a supersonic molecular beam discharge source to better understand the formation, chemical bonding, and molecular structures of this radical pair. We find that trans- HOCO can be produced almost equally well from either OH + CO or H + CO2 in our discharge source, but cis-HOCO appears to be roughly two times more abundant when starting from H + CO2. Using isotopically labelled precursors, the OH + C18O reaction predominately yields HOC18O for both isomers, but H18OCO is observed as well, typically at the level of 10%-20% that of HOC18O; the opposite propensity is found for the 18OH + CO reaction. DO + C18O yields similar ratios between DOC18O and D18OCO as those found for OH + C18O, suggesting that some fraction of HOCO (or DOCO) may be formed from the back-reaction H + CO2, which, at the high pressure of our gas expansion, can readily occur. The large 13C Fermi-contact term (aF) for trans- and cis-HO13CO implicates significant unpaired electronic density in a σ-type orbital at the carbon atom, in good agreement with theoretical predictions. By correcting the experimental rotational constants for zero-point vibration motion calculated theoretically using second-order vibrational perturbation theory, precise geometrical structures have been derived for both isomers. [ABSTRACT FROM AUTHOR]

Because of its structural specificity, rotational spectroscopy has great potential as an analytical tool for characterizing the chemical composition of complex gas mixtures. However, disentangling the individual molecular constituents of a rotational spectrum, especially if many of the lines are entirely new or unknown, remains challenging. In this paper, we describe an empirical approach that combines the complementary strengths of two techniques, broadband chirped-pulse Fourier transform microwave spectroscopy and narrowband cavity Fourier transform microwave spectroscopy, to characterize and assign lines. This procedure, called microwave spectral taxonomy, involves acquiring a broadband rotational spectrum of a rich mixture, categorizing individual lines based on their relative intensities under series of assays, and finally, linking rotational transitions of individual chemical compounds within each category using double resonance techniques. The power of this procedure is demonstrated for two test cases: a stable molecule with a rich spectrum, 3,4-difluorobenzaldehyde, and products formed in an electrical discharge through a dilute mixture of C2H2 and CS2, in which spectral taxonomy has enabled the identification of propynethial, HC(S)CCH. [ABSTRACT FROM AUTHOR]

We report the gas-phase optical detection of Si2C near 390 nm and the first experimental investigation of the rovibrational structure of its ¹A1 ground electronic state using mass-resolved and fluorescence spectroscopy and variational calculations performed on a high-level ab initio potential. From this joint study, it is possible to assign all observed Ka = 1 vibrational levels up to 3800 cm−1 with confidence, as well as a number of levels in the Ka = 0, 2, and 3 manifolds. Dixon-dip plots for the bending coordinate (ν2) allow an experimental determination of a barrier to linearity of 783(48) cm−1 (2σ), in good agreement with theory (802(9) cm−1). The calculated (Ka, ν2) eigenvalue lattice shows an archetypal example of quantum monodromy (absence of a globally valid set of quantum numbers) that is reflected by the experimentally observed rovibrational levels. The present study provides a solid foundation for infrared and optical surveys of Si2C in astronomical objects, particularly in the photosphere of N- and J-type carbon stars where the isovalent SiC 2 molecule is known to be abundant. [ABSTRACT FROM AUTHOR]

The molecular structure of trans, planar hydridotrioxygen (HOOO) has been examined by means of isotopic spectroscopy using Fourier transform microwave as well as microwave-millimeter-wave double resonance techniques, and high-level coupled cluster quantum-chemical calculations. Although this weakly bound molecule is readily observed in an electrical discharge of H2O and O2 heavily diluted in an inert buffer gas, we find that HOOO can be produced with somewhat higher abundance using H2 and O2 as precursor gases. Using equal mixtures of normal and 18O2, it has been possible to detect three new isotopic species, H18OOO, HO18O18O, and H18O18O18O. Detection of these species and not others provides compelling evidence that the dominant route to HOOO formation in our discharge is via the reaction OH + O2 → HOOO. By combining derived rotational constants with those for normal HOOO and DOOO, it has been possible to determine a fully experimental (r0) structure for this radical, in which all of the structural parameters (the three bond lengths and two angles) have been varied. This best-fit structure possesses a longer central O-O bond (1.684 Å), in agreement with earlier work, a markedly shorter O-H bond distance (0.913 Å), and a more acute ∠HOO angle (92.4°) when compared to equilibrium (re) structures obtained from quantum-chemical calculations. To better understand the origin of these discrepancies, vibrational corrections have been obtained from coupled-cluster calculations. An empirical equilibrium (reemp) structure, derived from the experimental rotational constants and theoretical vibrational corrections, gives only somewhat better agreement with the calculated equilibrium structure and large residual inertial defects, suggesting that still higher order vibrational corrections (i.e., γ terms) are needed to properly describe large-amplitude motion in HOOO. Owing to the high abundance of this oxygen-chain radical in our discharge expansion, a very wide spectral survey for other oxygen-bearing species has been undertaken between 6 and 25 GHz. Only about 50% of the observed lines have been assigned to known hydrogen-oxygen molecules or complexes, suggesting that a rich, unexplored oxygen chemistry awaits detection and characterization. Somewhat surprisingly, we find no evidence in our expansion for rotational transitions of cis HOOO or from low-lying vibrationally excited states of trans HOOO under conditions which optimize its ground state lines. [ABSTRACT FROM AUTHOR]

The rotational spectra of two small silicon sulfides, silanethione H2SiS and the disilicon sulfide ring Si2S, have been detected in the centimeter band by Fourier transform microwave spectroscopy of a molecular beam; lines of H2SiS were also observed in the millimeter band up to 377 GHz in a glow discharge. Precise rotational and centrifugal distortionconstants have been determined for the normal and a number of the more abundant rare isotopic species of both closed-shell molecules. Theoretical equilibrium (re) structures of H2SiS and Si2S were derived from coupled-cluster calculations that included triple and quadruple excitations, core correlation, and extrapolation to the basis-set limit. The re structures agree to within 5×10-4 Å and 0.1°with empirical equilibrium (reemp) structures derived from the experimental rotational constants, combined with theoretical vibrational and electronic corrections. Both H2SiS and Si2S are good candidates for radioastronomical detection in the circumstellar shells of evolved carbon-rich stars such as IRC+10216, because they are fairly polar and are similar in composition to the abundant astronomical molecule SiS. [ABSTRACT FROM AUTHOR]

By means of Fabry-Pérot Fourier transform microwave spectroscopy, the rotational spectrum of protonated sulfur dioxide in two distinct isomeric forms, a cis- and a trans-geometry, is reported. The search for both isomers was based on theoretical structures obtained at the CCSD(T)/cc-pwCVQZ level of theory corrected for zero-point vibrational effects. At a similarly high level of theory, the cis-isomer is calculated to be the global minimum on the potential energy surface, but the trans-isomer is predicted to lie only a few kcal/mol higher in energy. A total of seven lines, including a- and b-type transitions, has been observed for both isomers, and precise rotational constants have been derived. Because sulfur dioxide, SO2, is a widespread and very abundant astronomical species, and because it possesses a large proton affinity, HOSO+ is an excellent candidate for radioastronomical detection. [ABSTRACT FROM AUTHOR]

Isofulminic acid, HONC, the most energetic stable isomer of isocyanic acid HNCO, higher in energy by 84 kcal/mol, has been detected spectroscopically by rotational spectroscopy supported by coupled cluster electronic structure calculations. The fundamental rotational transitions of the normal, carbon-13, oxygen-18, and deuterium isotopic species have been detected in the centimeter band in a molecular beam by Fourier transform microwave spectroscopy, and rotational constants and nitrogen and deuterium quadrupole coupling constants have been derived. The measured constants agree well with those predicted by ab initio calculations. A number of other electronic and spectroscopic parameters of isofulminic acid, including the dipole moment, vibrational frequencies, infrared intensities, and centrifugal distortion constants have been calculated at a high level of theory. Isofulminic acid is a good candidate for astronomical detection with radio telescopes because it is highly polar and its more stable isomers (HNCO, HOCN, and HCNO) have all been identified in space. [ABSTRACT FROM AUTHOR]

The a- and b-type rotational transitions of the weakly bound complexes formed by molecular hydrogen and OCS, para-H2–OCS, ortho-H2–OCS, HD–OCS, para-D2–OCS, and ortho-D2–OCS, have been measured by Fourier transform microwave spectroscopy. All five species have ground rotational states with total rotational angular momentum J=0, regardless of whether the hydrogen rotational angular momentum is j=0 as in para-H2, ortho-D2, and HD or j=1 as in ortho-H2 and para-D2. This indicates quenching of the hydrogen angular momentum for the ortho-H2 and para-D2 species by the anisotropy of the intermolecular potential. The ground states of these complexes are slightly asymmetric prolate tops, with the hydrogen center of mass located on the side of the OCS, giving a planar T-shaped molecular geometry. The hydrogen spatial distribution is spherical in the three j=0 species, while it is bilobal and oriented nearly parallel to the OCS in the ground state of the two j=1 species. The j=1 species show strong Coriolis coupling with unobserved low-lying excited states. The abundance of para-H2–OCS relative to ortho-H2–OCS increases exponentially with decreasing normal H2 component in H2/He gas mixtures, making the observation of para-H2–OCS in the presence of the more strongly bound ortho-H2–OCS dependent on using lower concentrations of H2. The determined rotational constants are A=22 401.889(4) MHz, B=5993.774(2) MHz, and C=4602.038(2) MHz for para-H2–OCS; A=22 942.218(6) MHz, B=5675.156(7) MHz, and C=4542.960(7) MHz for ortho-H2–OCS; A=15 970.010(3) MHz, B=5847.595(1) MHz, and C=4177.699(1) MHz for HD–OCS; A=12 829.2875(9) MHz, B=5671.3573(7) MHz, and C=3846.7041(6) MHz for ortho-D2–OCS; and A=13 046.800(3) MHz, B=5454.612(2) MHz, and C=3834.590(2) MHz for para-D2–OCS. [ABSTRACT FROM AUTHOR]

The rotational spectra of five weakly bonded hydrogen-OCS complexes (paraH2, orthoH2, HD, orthoD2, and paraD2) are measured. Hyperfine structure is resolved and analyzed in all except the complex with paraH2, where I=0. For the two j=1 species, orthoH2-OCS and paraD2-OCS, nuclear hyperfine coupling constants are found to be da=21.2(2) and 8.4(2) kHz, respectively, indicative of nearly free uniaxial rotation of the hydrogen around the b-inertial axis. Similar analyses for HD-OCS and orthoD2-OCS yield the quadrupole coupling constants eqQa=16(2) and 30(2) kHz, respectively, showing that the internal rotational motions of HD and orthoD2 in the complex are slightly hindered producing a small nonspherical distribution. For orthoD2-OCS, the observed hyperfine structure indicates that the nuclear spin states I=0 and 2 are strongly coupled in the rotation of the complex. [ABSTRACT FROM AUTHOR]

Vibrational excitation and relaxation of five linear polyatomic molecules, OCS, OC3S, HC3N, HC5N, and SiC2S, have been studied by Fourier transform microwave spectroscopy in a supersonic expansion after the application of a low-current dc electric discharge. For each chain, the populations in bending and stretching modes have been characterized as a function of the applied discharge current; for stable OCS and HC3N, vibrational populations were studied as well in the absence of a discharge. With no discharge present the derived vibrational temperatures are slightly below T, the temperature of the gas before the supersonic expansion (i.e., 300 K). In the presence of the discharge, vibrational excitation occurs via inelastic collisions with the electrons and the vibrational temperatures rise as the applied current increases. Global vibrational relaxation is governed by rapid vibration–vibration (VV) energy transfer and slow vibration–translation (VT) energy transfer. The latter process is rate-determining and depends primarily on the wave number of the vibration. Vibrational modes with wave numbers near and below kT/hc (where T=300 K and kT/hc∼210 cm-1) are efficiently cooled by VT transfer because a sufficient number of collisions occur in the initial stages of the supersonic expansion. Vibrational modes with wave numbers around 450 cm-1 appear to be inefficiently cooled in the molecular beam; at these energies VV and VT rates are probably comparable. For high-frequency vibrations, VV energy transfer dominates. For the longer chains OC3S and HC5N, higher-lying modes are generally not detectable and vibrational temperatures of most lower-lying modes were found to be lower than those of OCS and HC3N, suggesting that as the size of the molecules increases, intermode VV transfer becomes more efficient, plausibly due to the higher density of vibrational levels. New high resolution spectroscopic data have been obtained for several vibrationally excited states of OC3S, HC3N, and HC5N. Rotational lines of the 13C and 15N isotopic species of HC5N have been measured, yielding improved rotational and centrifugal distortion constants; 14N nitrogen quadrupole coupling constants for the isotopic species of HC5N with 13C have been determined for the first time. [ABSTRACT FROM AUTHOR]

Fourier transform microwave spectroscopy has been used to investigate vibrational excitation and relaxation of diatomic molecules produced by an electric discharge in the throat of a supersonic nozzle. Rotational transitions of SO, SiO, and SiS, in vibrational states up to v=33 for [sup 32]S[sup 16]O, v=45 for [sup 28]Si[sup 16]O, and v=51 for [sup 28]Si[sup 32]S in their ground electronic states have been detected. The isotopic species [sup 33]S[sup 16]O, [sup 34]S[sup 16]O, [sup 29]Si[sup 16]O, [sup 28]Si[sup 18]O, [sup 29]Si[sup 32]S, and [sup 28]Si[sup 34]S have also been observed in highly excited vibrational states. Microwave transitions include up to v=22 for the second lowest excited electronic state b [sup 1]Σ[sup +] of SO (∼10 510 cm-1 above ground) have also been detected. Effective vibrational temperatures have been derived for each species, and a general model is proposed to qualitatively explain the observations. Vibrational excitation is caused by inelastic collisions with the hot electrons produced in the discharge. The subsequent vibrational populations are largely determined by vibration–vibration energy transfer via molecule–molecule binary collisions. Two regions can be inferred from the data: one characterized by a temperature of around 1000 K and a second region with a temperature of several thousand degrees Kelvin. Improved Dunham coefficients and correction terms for the breakdown of the Born–Oppenheimer approximation have been determined for b [sup 1]Σ[sup +] SO, X [sup 1]Σ[sup +] SiO, and X [sup 1]Σ[sup +] SiS. Nuclear spin-rotation hyperfine structure for the [sup 29]Si isotopic species of SiO and SiS has been observed in all highly excited vibrational states. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

On the basis of extensive coupled cluster calculations, the rotational spectra of the linear silicon- and sulfur-containing carbon chains SiC[sub 2n] S (n = 1 -3) in their singlet electronic ground state and SiC[sub 3]S in its triplet electronic ground state have been detected and characterized by means of molecular beam Fourier transform microwave spectroscopy. Rotational and centrifugal distortion constants have been determined to high accuracy as well as the spin-spin coupling constant for triplet SiC[sub 3] S. In addition, the [sup 29]Si, [sup 34]S, and both [sup 13]C isotopic species of SiC[sub 2]S have been detected, allowing the determination of both an effective r[sub 0] structure, as well as a mixed experimental-theoretical structure resulting from the combination of the measured rotational constants with the vibrationrotation coupling constants calculated ab initio. Several rotational satellite lines have also been observed for SiC[sub 2]S and, on the basis of their predicted vibration-rotation and l-type doubling constants, were assigned to the two highest-frequency stretching modes υ[sub 1] and υ[sub 2], and to a progression in the second lowest-frequency bending mode υ 4 up to 3 υ 4. Equilibrium structures and various spectroscopic properties are predicted for all SiC[sub n]S species with n = 1 - 8. [ABSTRACT FROM AUTHOR]

Uses the sideband optical-optical double resonance Zeeman effect spectroscopy to study the electronic spectrum of the gas-phase platinum-hydrogen compound. Analysis of the sub-Doppler Zeeman spectrum of each feature using a systematic quasi-two-dimensional approach; Radio frequency magnetic resonances.

Applies the sideband optical-optical double resonance Zeeman effect spectroscopy to study transition metal hydrides. Resolution of the extremely small ground state photon hyperfine splittings; Record of the sub-dopper spectra; Deviation of the g values in each state from the nominal Hund's coupling case.

Several variants of the one-laser, two-color technique of sideband optical–optical double resonance Zeeman (SOODRZ) spectroscopy, which enables Zeeman and hyperfine splittings to be measured at sub-Doppler resolution, are described; a detailed theory of the line shape and signal-to-noise ratio for each variant is also presented. In the single-frequency variant of SOODRZ spectroscopy, radio frequency sidebands are imposed onto a single-frequency continuous wave laser beam to determine energy splittings between pairs of closely spaced levels. Each pair of closely spaced levels is coupled to a third isolated level by optical transitions and is tuned into resonance by the Zeeman effect. A theoretical treatment of the SOODRZ effect is presented here. It is shown that both the real and imaginary contributions to the complex third-order optical susceptibility tensor χ(3) can be observed and that the SOODRZ signal originates from a cross term between the amplitude of the unperturbed frequency-modulated beam of the laser and the nonlinear signal amplitude Re{EL*·ENL}. The SOODRZ signal is directly proportional to ΔNμ4MI20 (where M is the modulation index), rather than (ΔN)2, μ8, and I30 as in degenerate four-wave mixing and other nonlinear spectroscopies, which means the detection sensitivity of SOODRZ spectroscopy is considerably higher for low laser intensities and at lower number densities.Moreover, the linewidth of the sub-Doppler feature is independent of the both the Doppler width of the optical transition and the natural width of the common linked level and, hence, is immune from the effects such as upper state predissociation. Two different types of modulators, acousto-optic and electro-optic, can be used in the single-frequency variant of SOODRZ spectroscopy. The unique features of SOODRZ schemes incorporating both types of modulators are described. SOODRZ spectroscopy can also be performed with a multimode broadband... [ABSTRACT FROM AUTHOR]

The technique of frequency-modulation enhanced magnetic rotation spectroscopy (FM-MRS) is applied here to the study of the electronic spectra of gas-phase transition metal hydrides and deuterides. Experimental FM-MR results for several bands of PdH, PdD, NiH, and CuH are presented. In the case of PdH, an Ω=0.5–XΩ=0.5 band at 4371 Å has been observed and assigned. Prior to this work, although several bands of PdD had been reported previously, no electronic transitions of PdH had been assigned. All of the PdH, PdD, and NiH bands studied in the 4200–4600 Å wavelength region are strongly predissociated, and in the case of PdH and PdD, strongly and multiply perturbed as well. [ABSTRACT FROM AUTHOR]

The technique of frequency-modulation enhanced magnetic rotation spectroscopy (FM-MRS) is described. An experimental comparison between FM-MRS, frequency-modulation, and magnetic rotation spectroscopies clearly demonstrates that the double null absorption-based FM-MR technique is more sensitive than either FM or MRS separately. The theoretical basis for this sensitivity enhancement can be attributed to a 1/θ (where θ is the small uncrossing angle between two nearly crossed polarizers) improvement in the sensitivity of the combined FM-MR technique relative to that of standard FM spectroscopy. The sensitivity improvement of FM-MRS relative to MRS is even better than that for FM spectroscopy. In practical terms, this enhancement can improve the experimentally achievable sensitivity of FM and magnetic rotation spectroscopies by factors of O(101) to O(102) and O(102) to O(104), respectively, levels that approach the theoretical quantum noise limit. In addition, because FM-MRS is based on the Zeeman effect, Zeeman information is encoded into FM-MR line profiles in the form of relative phase labels and line shapes. The relative phases and line shapes of rotational lines can be used to group features by rotational branch and to differentiate between magnetically active (Λ≠0, Σ≠0) and inactive (Λ=0, Σ=0) features. The theory of FM-MRS, including a discussion of line shapes and a sensitivity analysis, is reviewed here; applications of the FM-MR technique are presented separately in the following paper. [ABSTRACT FROM AUTHOR]

The application of magnetic rotation spectroscopy (MRS) to the electronic spectra of gas phase NiH and CeF is examined. Experimental results for both MRS and laser excitation spectroscopy are presented for the NiH B 2Δ5/2–X 2Δ5/2 (1,0) and 2[uppercase_phi_synonym]7/2–X 2Δ5/2 (0,0) bands. A comparison between these two techniques clearly illustrates the ability of MRS to simplify and presort spectra. The power of MRS originates from its specificity for low-J, ΔΩ=ΔJ transitions and its ability, based on the sign and magnitude of the MRS effect, to partially presort rotational branches. A simple theoretical explanation of the MRS results, based on the structure of the underlying Zeeman pattern, is presented. Also, the extension of MRS to the more complicated and congested spectrum of CeF is presented. Using MRS, a previously unassigned CeF band, near 568 nm, has been definitively assigned as Ω’=4.5←Ω‘=3.5. [ABSTRACT FROM AUTHOR]

The equilibrium structure for the ground state of the HNCN radical is calculated at the levels of the self-consistent field theory (SCF), the second-order Mo\ller–Plesset perturbation approximation (MP2), and the full single and double excitation coupled cluster theory including all connected triples in a noniterative manner [CCSD(T)], using various extended basis sets starting from 6–311 G(d,p). At the CCSD(T) level, the outer C–N bond is more than 0.1 Å shorter than the inner one and the N–C–N group departs from linearity by 6°. The total N–C–N length is in good agreement with the experimental value [Herzberg and Warsop, Can. J. Phys. 41, 286 (1963)], however, the H–N–C angle is about 6° smaller. The N–H bond is very close to a normal N–H bond but is about 0.2 Å smaller than the experimental estimate. Except for the smaller H–N–C angle, the geometrical parameters for HNCN closely parallel those for the triplet HCCN molecule. The dipole moment, harmonic frequencies, electric quadrupole, and Fermi contact coupling constants of HNCN are also calculated. The calculated harmonic frequencies confirm the preliminary assignments of Wu, Hall, and Sears [J. Chem. Soc. Faraday Trans. 89, 615 (1993)]. The quadrupole coupling constants for the inner and outer N atoms are comparable, implying a complex pattern of hyperfine split components in the lowest rotational transitions. The present calculation may thus serve as a useful guide for the interpretation of the rotational spectrum. [ABSTRACT FROM AUTHOR]

The rotational spectrum of protonated vinyl cyanide, CH2CHCNH+, a prototypical branched nitrile species and likely intermediate in astronomical sources and in the planetary atmosphere of Titan, has been detected in a pulsed-discharge supersonic molecular beam by means of Fourier transform microwave spectroscopy. Fifteen lines arising from 11 a-type rotational transitions have been observed between 9 and 46 GHz, several with partially resolved nitrogen hyperfine structure. From this data set, the leading spectroscopic constants, including all three rotational constants, have been determined to high accuracy. The agreement between experimental rotational constants and those calculated at the CCSD(T) level of theory is of order 0.1%. An even better estimate was obtained through empirical scaling using calculated and experimental rotational constants of isoelectronic vinyl acetylene. Measurement of a small nitrogen quadrupole coupling constant in protonated vinyl cyanide is consistent with a quadruply bound nitrogen atom and a H+-N≡C-R type structure. Because vinyl cyanide is abundant in molecule-rich astronomical sources and possesses a high proton affinity, and because protonated vinyl cyanide is unreactive with hydrogen and other well-known interstellar species, this cation is an excellent candidate for astronomical detection. The present work suggests that other organic molecules containing the nitrile group and closely related species such as protonated vinyl acetylene can probably be detected with the same instrumentation. [ABSTRACT FROM AUTHOR]

The structure and bonding of H 2 NNO, the simplest N-nitrosamine, and a key intermediate in deNO x processes, have been precisely characterized using a combination of rotational spectroscopy of its more abundant isotopic species and high-level quantum chemical calculations. Isotopic spectroscopy provides compelling evidence that this species is formed promptly in our discharge expansion via the NH 2 + NO reaction and is collisionally cooled prior to subsequent unimolecular rearrangement. H 2 NNO is found to possess an essentially planar geometry, an NNO angle of 113.67(5)°, and a N-N bond length of 1.342(3) Å; in combination with the derived nitrogen quadrupole coupling constants, its bonding is best described as an admixture of uncharged dipolar (H 2 N-N=O, single bond) and zwitterion (H 2 N + =N-O - , double bond) structures. At the CCSD(T) level, and extrapolating to the complete basis set limit, the planar geometry appears to represent the minimum of the potential surface, although the torsional potential of this molecule is extremely flat.

The thermal decomposition of syn-ethanal-oxide (syn-CH 3 CHOO) through vinyl hydrogen peroxide (VHP) leading to hydroxyl radical is characterized using a modification of the HEAT thermochemical protocol. The isomerization step of syn-CH 3 CHOO to VHP via a 1,4 H-shift, which involves a moderate barrier of 72 kJ/mol, is found to be rate determining. A two-dimensional master equation approach, in combination with semi-classical transition state theory, is employed to calculate the time evolution of various species as well as to obtain phenomenological rate coefficients. This work suggests that, under boundary layer conditions in the atmosphere, thermal unimolecular decomposition is the most important sink of syn-CH 3 CHOO. Thus, the title reaction should be included into atmospheric modeling. The fate of cold VHP, the intermediate stabilized by collisions with a third body, has also been investigated.

By means of Fabry-Pérot Fourier transform microwave spectroscopy, the rotational spectrum of protonated sulfur dioxide in two distinct isomeric forms, a cis- and a trans-geometry, is reported. The search for both isomers was based on theoretical structures obtained at the CCSD(T)/cc-pwCVQZ level of theory corrected for zero-point vibrational effects. At a similarly high level of theory, the cis-isomer is calculated to be the global minimum on the potential energy surface, but the trans-isomer is predicted to lie only a few kcal/mol higher in energy. A total of seven lines, including a- and b-type transitions, has been observed for both isomers, and precise rotational constants have been derived. Because sulfur dioxide, SO(2), is a widespread and very abundant astronomical species, and because it possesses a large proton affinity, HOSO(+) is an excellent candidate for radioastronomical detection.

The rotational spectra of five weakly bonded hydrogen-OCS complexes (paraH(2), orthoH(2), HD, orthoD(2), and paraD(2)) are measured. Hyperfine structure is resolved and analyzed in all except the complex with paraH(2), where I=0. For the two j=1 species, orthoH(2)-OCS and paraD(2)-OCS, nuclear hyperfine coupling constants are found to be d(a)=21.2(2) and 8.4(2) kHz, respectively, indicative of nearly free uniaxial rotation of the hydrogen around the b-inertial axis. Similar analyses for HD-OCS and orthoD(2)-OCS yield the quadrupole coupling constants eqQ(a)=16(2) and 30(2) kHz, respectively, showing that the internal rotational motions of HD and orthoD(2) in the complex are slightly hindered producing a small nonspherical distribution. For orthoD(2)-OCS, the observed hyperfine structure indicates that the nuclear spin states I=0 and 2 are strongly coupled in the rotation of the complex.