Fourier-transform microwave and submillimeter-wave spectroscopy of chloroiodomethane, CH2ICl

Guided by a previous microwave study (9–35 GHz), the rotational spectrum of both chlorine isotopologues of chloroiodomethane in its vibrational and electronic ground state has been re-investigated in the microwave region and extended to the millimeter/submillimeter-wave region. Weak a-type transitions have been recorded by Fourier transform microwave spectroscopy below 20 GHz whilst strong b-type rotational transitions have been recorded between 15 and 646 GHz, corresponding to energy levels with J″ ≤ 108 and . Molecular constants including those describing the hyperfine structures owing to the two halogen atoms were accurately determined for both species from the least-squares analysis of a total of 1475 distinct transition frequencies (of which 906 belong to the CH₂I³⁵Cl isotopologue). The two sets of rotational constants allowed us to derive an r₀ structure of CH₂ICl.     

The microwave spectrum of cyanophosphine, H2PCN

The rotational spectra of cyanophosphine, H₂PCN, have been measured between 10 and 42.5 GHz by Fourier transform microwave spectroscopy. The rotational constants, centrifugal distortion constants, the ¹⁴N quadrupole coupling constant, and the nuclear spin-rotation coupling constants of ³¹P have been determined. Density functional ab initio calculations were performed, and the calculated values of the molecular constants are in excellent agreement with our experimentally determined results. The spectra of three isotopomers were measured, H₂P¹²C¹⁴N, H₂P¹³C¹⁴N, and H₂P¹²C¹⁵N. The derived r₀ structure is quite comparable to the ab initio predicted H₂PCN equilibrium geometry.     

Infrared Transitions of HCN and HCN between 500 and 10 000 cm

We have measured the Fourier transform spectrum (FTS) of two isotopomers of hydrogen cyanide (H¹²C¹⁴N and H¹²C¹⁵N) from 500 to 10 000 cm⁻¹. The infrared data have been combined with earlier published microwave and submillimeter-wave measurements. From this analysis new vibration–rotation energy levels and constants are given, based on the observation of a number of new vibrational levels, especially for H¹²C¹⁵N. The Coriolis interaction involving Δv₃= −1, Δv₂= 3, and Δl= ±1 has been observed for a great many levels and in some cases the assignments of laser transitions allowed by this interaction are more clearly shown. New vibration–rotation constants are given that allow one to predict the transition wavenumbers for most of the transitions below 10 000 cm⁻¹with accuracies of about 0.5 cm⁻¹or better. Values are given for the power series expansion of thel-type resonance constants and for the centrifugal distortion constants, as well as the usual vibrational and rotational constants.     

The infrared spectrum of 13C2H2 in the 60–2600 cm−1 region: bending states up to v 4 + v 5 = 4

The vibration–rotation spectra of ¹³C substituted acetylene, ¹³C₂H₂, have been recorded in the region between 60 and 2600?cm^?1 at an effective resolution ranging from 0.001 to 0.006?cm^?1. Three different instruments were used to collect the experimental data in the extended spectral interval investigated. In total 9529 rotation vibration transitions have been assigned to 101 bands involving the bending states up to v _tot?=?v ₄?+?v ₅?=?4, allowing the characterization of the ground state and of 33 vibrationally excited states. All the bands involving states up to v _tot?=?3 have been analyzed simultaneously by adopting a model Hamiltonian which takes into account the vibration and rotation l-type resonances. The derived spectroscopic parameters reproduce the transition wavenumbers with a RMS value of the order of the experimental uncertainty. Using the same model, larger discrepancies between observed and calculated values have been obtained for transitions involving states with v _tot?=?4. These could be satisfactorily reproduced only by adopting a set of effective constants for each vibrational manifold, in addition to the previously determined parameters, which were constrained in the analysis.     

Millimeter-wave spectroscopy of rare isotopomers of HC5N and DC5N: determination of a mixed experimental-theoretical equilibrium structure for cyanobutadiyne

Cyanobutadiyne has been produced by gas phase copyrolysis of pyridine and phosphorus trichloride in a flow reactor. The yield of the reaction is sufficiently good to allow the detection of rotational transitions of the ¹³C and ¹⁵N containing species in natural abundance. Normal pyridine and its fully deuterated variant have been used as precursors, making it possible to study the ground-state rotational spectra of 14 isotopomers in the millimeter wave region. Very accurate values of the rotational and quartic centrifugal distortion constants have been obtained for all the isotopic species investigated, and in addition the sextic distortion constant has been precisely determined for the most abundant variants H¹²C₅¹⁴N and D¹²C₅¹⁴N, for which the measurements have been extended up to 460 GHz. A mixed experimental-theoretical equilibrium structure has been evaluated for cyanobutadiyne combining experimental ground-state rotational constants with theoretically computed zero-point contributions. The r_e geometry is compared with operationally defined purely experimental structures, namely r₀, r_s, and r_m⁽¹⁾ molecular structures.     

Analysis of the nuclear quadrupole interaction of Disulfur Dichloride, S2Cl2

Pure rotational spectra of S₂³⁵Cl₂ and S₂³⁵Cl³⁷Cl have been observed using a Fourier-transform microwave spectrometer. An analysis of the hyperfine structure made by considering the nuclear spin statistics showed that S₂Cl₂ has C₂symmetry, where the hyperfine splittings due to the two Cl nuclei were analyzed precisely. The nuclear quadrupole coupling constants including the off-diagonal (χ_ab, χ_ac, χ_bc) components and the nuclear spin–rotation interaction constants associated with the two Cl nuclei have been determined for the first time. We have shown that the nuclear quadrupole interaction plays an important role in the ortho–para mixing.     

Analysis of ν2 of H2Se

The infrared absorption spectrum of ν₂ of H₂Se in the region from 885 to 1347 cm^?1 was obtained with a resolution limit of 0.025 cm^?1 on the University of Denver 50-cm FTIR spectrometer system. We have assigned 1604 lines for the six isotopomers of H₂Se, including 25 lines for the H₂⁷⁴Se isotopomer, and have analyzed them using Watson's A-reduced Hamiltonian in the I^r rotational representation. Ground state constants for each of the five most abundant isotopomers were obtained from fits of microwave transitions combined with weighted averaged ground state combination differences formed from the infrared bands (010), (020), (100), and (001). Upper state constants for each of the five most abundant isotopomers were obtained from least-squares fits of the spectral lines of ν₂, keeping the ground state constants fixed to the values determined from our ground state fits. An alternate set of ground state constants together with isotopic mass adjustment constants for all six isotopomers was determined by simultaneously fitting all available microwave transitions and infrared ground state combination differences. Keeping this set of ground state constants fixed, a single set of upper state constants and isotopic mass adjustment constants for the ν₂ band was determined by a simultaneous fit of infrared spectral lines from all six isotopomers.     

The rotational spectrum of methyl trifluoroacetate

ABSTRACT

The pulsed supersonic jet expansion microwave spectra of the parent and all three ¹³C mono-substituted isotopologues of methyl trifluoroacetate have been measured in the 6.5–18 GHz range. All observed transitions are split into two component lines, due to the internal rotation of the methyl group. The corresponding barrier has been determined to be V ₃ = 4.379(3) kJ/mol. Structural information has been obtained from the 12 available rotational constants.     

Microwave and Infrared Spectra,ab InitioCalculation, and Two-Dimensional Model of Amino Group Inversion and Ring Puckering in 2,5-Dihydropyrrole

The microwave spectra of 2,5-dihydropyrrole and 2,5-dihydropyrrole-1-d₁have been measured with Stark and Fourier transform spectrometers in the range 10–39 GHz. Rotational constants, centrifugal distortion constants, and¹⁴N quadrupole coupling constants have been determined from the observed transition frequencies for the ground vibrational state. In addition, two satellites of the normal species and one satellite of the deuterated species have been identified and measured. Splittings of the rotational transitions due to amino group inversion tunneling have been observed and analyzed. Infrared transitions of the amino group inversion mode have been measured in the range 490–720 cm⁻¹. The effect of ring puckering on the inversion motion of the amino group in 2,5-dihydropyrrole and 2,5-dihydropyrrole-N-d₁has been investigated byab initiocalculations and two-dimensional flexible model calculations from the results of microwave and infrared spectroscopy. The observed molecular properties have been reproduced by a model which involved adjustable parameters for the potential energy surface and the structural relaxation of the CCC valence angles. Additional parameters have been transferred from theab initiocalculations. The adjustment of the model to the experimental data has yielded an equatorial equilibrium conformation with slightly larger CCC valence angle than in the most stable axial conformation. Excitation of the first ring puckering state has been found to enhance the inversion tunnel splittings.     

High-resolution infrared analysis of the ν7 band of cis-ethylene-d2 (cis-C2H2D2)

The spectrum of the ν₇ band of cis-ethylene-d₂ (cis-C₂H₂D₂) has been recorded with an unapodized resolution of 0.0063 cm⁻¹ in the 740-950 cm⁻¹ region using a Bruker IFS 125 HR Fourier transform infrared spectrometer. By fitting 2186 infrared transitions of ν₇ with a standard deviation of 0.00060 cm⁻¹ using a Watson’s A-reduced Hamiltonian in the I^r representation, accurate rovibrational constants for ν₇ = 1 state have been derived. The band center of ν₇ has been found to be 842.20957 ± 0.00004 cm⁻¹. In a simultaneous fit of 1331 infrared ground state combination differences from the present ν₇ transitions, together with 22 microwave frequencies, ground state constants have been improved. The rms deviation of the ground state fit was 0.00027 cm⁻¹.     

Comprehensive analysis of the FASSST rotational spectrum of S(CN)2

Results are reported of a comprehensive investigation of an almost continuous rotational spectrum of S(CN)₂ recorded over the frequency region 110-374 GHz by means of the FASSST spectroscopic technique. The spectrum was analysed in detail and over 22 000 transitions were assigned in total. Precise, octic order spectroscopic constants in the asymmetric rotor Hamiltonian have been determined for the ground state and 12 different vibrationally excited states of the parent isotopologue, including first excited states of five different normal modes. Three states near 370 cm⁻¹ and four states near 490 cm⁻¹ above the ground state were found to be mutually interacting and were successfully analysed in terms of a triad and a tetrad of coupled states, respectively. Rotational transitions in the ³⁴S, ¹³C, and ¹⁵N isotopologues of S(CN)₂ have also been assigned and fitted, and newly determined rotational constants were used to derive the geometry of the molecule. The complex multistate analysis of the spectrum was carried out with the newly developed AABS software package for Assignment and Analysis of Broadband Spectra.     

Microwave spectra of the Cl and Cl isotopomers of dichloromethylene: Nuclear quadrupole-, spin-rotation-, and nuclear shielding constants from the hyperfine structures of rotational lines

The rotational spectra of the isotopomers C³⁵Cl³⁷Cl and C³⁷Cl₂ of dichloromethylene in the ground vibronic state were recorded in the range 10-33 GHz using a molecular beam Fourier transform microwave spectrometer. CCl₂ was generated by flash pyrolysis using different precursors. The observed spectra were analyzed to yield rotational and centrifugal distortion constants, as well as the complete Cl nuclear quadrupole coupling tensors and the spin-rotation interaction constants from the hyperfine structure of the rotational lines. With inclusion of data from previous work on the most abundant species C³⁵Cl₂ [N. Hansen, H. Mäder, F. Temps, Phys. Chem. Chem. Phys. (3) (2001) 50-55.] a refined r₀ structure was determined. The spin-rotation interaction constants of all three isotopomers were used to derive ³⁵Cl and ³⁷Cl principal inertial axis nuclear magnetic shielding components which have not yet been determined by NMR spectroscopy.     

The high resolution FTIR spectrum of the ν2 band of formaldehyde isotopomer H2CO

The Fourier transform infrared (FTIR) absorption spectrum of the ν₂ fundamental band of the formaldehyde isotopomer H₂¹³CO was recorded at an unapodized resolution of 0.0063 cm⁻¹ in the 1630–1780 cm⁻¹ region. Upper state (ν₂ = 1) rovibrational constants inclusive of three rotational, five quartic, and six sextic centrifugal distortion constants were accurately determined by assigning and fitting 447 unperturbed infrared transitions with a rms deviation of 0.00056 cm⁻¹ using Watson’s A-reduced Hamiltonian in the I^r representation. Analysis of new transitions measured in this work yielded more higher-order upper state constants with greater accuracy than previously reported. The band center of the A-type ν₂ band was found to be 1707.980943 ± 0.000058 cm⁻¹ while the calculated inertial defect Δ₂ of the H₂¹³CO molecule was 0.09581 ± 0.00004 μŲ.     

The pure rotational spectra of the two lowest energy conformers of the asymmetric ether C4H9OC2H5

An experimental study has been performed shedding light on the conformational energies of the asymmetric ether n-butyl ethyl ether. Rotational spectroscopy between 7.8 GHz and 16.2 GHz has identified two conformers of n-butyl ethyl ether, C₄H₉OC₂H₅. In these experiments spectra were observed as the target compound participated in an argon expansion from high to low pressure causing molecular rotational temperatures to be below 4 K. For one conformer, 95 pure rotational transitions have been recorded, for the second conformer, 20 pure rotational transitions were recorded. Rotational constants and centrifugal distortion constants are presented for both butyl ethyl ether conformers. The structures of both conformers have been identified by exploring the multi-dimensional molecular potential energy surface using ab initio calculations. From the numerous low energy conformers identified using ab initio methods, the three lowest conformers were pursued at increasingly higher levels of theory, i.e. complete basis set extrapolations, coupled cluster methods, and also taking into consideration zero point vibrational energies. The two conformers observed experimentally are only revealed to be the two lowest energy conformers when high levels of quantum chemical methodologies are employed.     

Microwave spectra of the ground vibrational state of formaldehyde substituted with O and O

Pure rotational transitions in the ground vibrational state have been measured for H₂¹²C¹⁸O, H₂¹²C¹⁷O, H₂¹³C¹⁸O, and H₂¹³C¹⁷O in the frequency region 8–75 GHz. These have included both Q- and R-branch transitions, and have permitted accurate evaluation of rotational constants and several quartic centrifugal distortion constants for each species. These in turn have permitted the prediction of several transitions of possible use in radioastronomy.     

High resolution rotational spectrum of FCO2 radical (extension to lower frequencies)

We have subsequently extended the measurement of the FC¹⁶O₂ radical to the lower frequency region, between 210 and 245 GHz with the recently developed Prague microwave spectrometer. About 60 new a-type R-branch transitions have been observed. Our present analysis enabled us to improve significantly the determination of the ground state constants: most of the parameters are now determined with higher precision up to an order of magnitude better than the previous study. In addition eight new constants could be determined or nearly determinable for the first time.     

High-resolution rotational spectroscopy of iminosilylene,HNSi

By means of Fourier transform microwave spectroscopy of a supersonic beam, the fundamental rotational transition of isotopic and vibrationally excited iminosilylene, HNSi, has been detected. In addition to seven isotopic species, vibrational satellite transitions from more than 30 vibrationally excited states, including the three fundamental modes, have been detected. Those from ν₂ are particularly intense, enabling detection of transitions from as high as (0,22⁰,0) (i.e. ～10,000 cm^?1 above ground). At high spectral resolution, well-resolved nitrogen quadrupole structure has been observed in nearly every transition. Excitation of ν₁ or ν₃ changes eQq(N) little, but eQq(N) systematically decreases with increasing excitation of the ν₂ bend, from a value of 0.376(5) MHz for (0,0⁰,0) to ?2.257(5) MHz for (0,20⁰,0). With the large amount of new data in hand, it has also been possible to determine the leading vibration–rotation constants (α_i and γ_i) for ν₂ or ν₃ to high precision, and derive a revised semi-empirical equilibrium structure for this fundamental triatomic molecule. Various electronic and molecular properties of iminosilylene have been calculated at the coupled cluster level of theory, and these generally agree well with experiment and previous calculations. An unsuccessful search for HSiN, a highly polar isomer calculated to lie nearly 3 eV above HNSi, is also reported.     

Analysis of ν2 of H2S

The infrared absorption spectrum of ν₂ of H₂S in the region from 1000 to 1500 cm^?1 was obtained with a resolution limit of <0.05 cm^?1 on the University of Denver 50-cm FTIR spectrometer system. We have assigned 387 lines due to H₂³²S, 75 lines due to H₂³⁴S, and 15 lines due to H₂³³S, and have analyzed them using Typke's reduction of Watson's Hamiltonian. Slightly revised ground-state constants for the 32 isotope were obtained from a simultaneous fit of the microwave transitions observed by Helminger, Cook, and De Lucia, combined with weighted averaged ground-state combination differences formed from the infrared bands (010), (020), (100), (001), (110), (011), (210), and (111). The standard deviation for the fit was 0.0018 cm^?1 for the infrared data and 0.000032 cm^?1 for the microwave lines. Upper-state constants for the 32 isotope were obtained from a least-squares fit of the spectral lines of ν₂, keeping the ground-state constants fixed to the values determined by the combination difference fit. The standard deviation of the (010) line fit was 0.0017 cm^?1 for the 32 isotope. Ground-state and upper-state isotopic mass adjustment constants were determined in a simultaneous fit of lines of H₂³³S and H₂³⁴S, keeping the ground-state and upper-state constants for the 32 isotope fixed.     

The C hyperfine constants of HCS and HSC studied by microwave spectroscopy

The ¹³C hyperfine constants of the H¹³CS and HS¹³C radicals are determined by microwave spectroscopy. For H¹³CS, the 1₀₁-0₀₀ rotational transition is measured at 38.5 GHz with a Fourier transform microwave spectrometer, and two ¹³C hyperfine constants are determined. They are well interpreted in terms of a relatively large HCS bonding angle (132.8°). For HS¹³C, the N=7-6, 9-8, and 10-9 rotational transitions are measured in the 268-384 GHz region by using a source modulation spectrometer combined with a free-space discharge cell, and five ¹³C hyperfine constants including the nuclear spin-rotation constant, C_aa, are determined. From the ¹³C hyperfine constants, the p character of the unpaired electron orbital on the carbon atom is estimated to be 66.5%, supporting a classical resonance picture; .     

Reaction products from a discharge of N2 and H2S: The microwave spectrum of NH2SH

Thiohydroxylamine has been identified as one of the reaction products from the discharge reaction of N₂ + H₂S. Both cis and trans conformers have been observed. The rotational spectra have been studied from 56 to 170 GHz for the normal species and several deuterated isotopic species of each conformer. The electric dipole moments of both conformers have been determined. A number of the transitions of the cis conformer exhibit splittings due to the nuclear quadrupole moment of the ¹⁴N nucleus. A least squares fit of the frequency splittings have led to an analysis of the eQq values. Ab initio calculations using a 4-31G basis set both with and without polarization functions have been carried out to aid in the analysis and to provide a final structural comparison with the microwave results.