Millimeter wave measurement and assignment of the rotational spectrum of 2-aminopyridine

Previous work involving the rotational spectrum of 2-aminopyridine was limited to the lower frequencies of 4-40 GHz with very few lines being assigned. This work extends this earlier study. Here we present a much more extensive measurement and assignment of the rotational spectrum of 2-aminopyridine in the frequency range of 75-110 GHz. The observed frequencies have been assigned to the ground (0⁺ state) and the first excited state in the inversion vibration (0⁻ state). Measurements of these two states have been extended up to J=46. With the newly assigned lines, significantly improved rotational constants and all five centrifugal distortion constants have been obtained.     

Submillimeter-wave spectroscopy of HCN in excited vibrational states

Measurements of the rotational spectrum of HCN in excited vibrational states have been extended to higher-J values. The transitions reach J=8←7 around 710 GHz for most vibrational states studied in this investigation and J=22←21 near 2 THz for the (020) and (030) vibrational states. Using a pure sample of gaseous HCN at 350 K, selected states up to one quantum in the C–H stretching vibration at 3311.5 cm⁻¹ have been investigated. Even transitions having two quanta in the C–H stretch could be studied employing a glow discharge in a gas mixture of CH₄ and N₂. Molecular constants in 13 vibrational states have been obtained, several of which have been studied for the first time by rotational spectroscopy. The vibrational temperature in the discharge system is found to be about 1500 K for the stretching vibrational modes and about 600 K for the bending states.     

High-temperature infrared spectrum of HCN near 3300 cm−2

The infrared absorption of HCN near the fundamental band at 3311 cm^?1 has been measured at temperatures up to 1200 K. Transitions involving high rotational states (up to J = 62) have been measured. These give an improved value for the sextic centrifugal distortion term H₀. Many hot-band transitions have been observed and assigned to transitions originating in vibrationally excited states up to 4000 cm^?1 above the ground state. These measurements give new data on vibrational states involving moderately high bending quantum numbers and indicate that new terms are needed to fit the ro-vibrational energy levels.     

Frequency measurement of pure rotational transitions in the v2 = 1 state of H2O

Frequencies of pure rotational transitions in the v₂ = 1 vibrationally excited state of H₂¹⁶O were measured with a tunable far-infrared spectrometer in the frequency range of 0.5-5 THz. Molecular parameters of Watson’s A-reduced Hamiltonian have been obtained to reproduce the observed frequencies.     

The millimeter wave rotational spectrum of lactic acid

The results of a comprehensive investigation of the rotational spectrum of lactic acid over the frequency region 171-318 GHz are reported. Some supersonic expansion measurements at 8-16 GHz have also been made. A complete set of octic level constants in the asymmetric rotor Hamiltonian has been determined for the ground vibrational state from a fit to over 1000 measured transition frequencies. Spectroscopic constants have also been determined for the first five excited states of the low frequency, 60 cm⁻¹, torsional vibrational mode, and for four other vibrationally excited states. Vibrational states become rather crowded above 200 cm⁻¹, with seven different states only in the next 100 cm⁻¹, and almost all of the measured states in this energy region show evidence of perturbations. The analysis was carried out with the newly developed AABS software package for Assignment and Analysis of Broadband Spectra.     

Far-infrared spectrum and ground state constants of methyl amine

The far-infrared spectrum of methyl amine has been studied in the region 40 to 350 cm⁻¹ by Fourier transform spectroscopy with an apodized resolution of 0.005 cm⁻¹ or better. Both the pure rotational spectrum and the spectrum of the fundamental torsional band have been assigned. This paper reports the ground state constants obtained from a global fitting of a data set including ground state microwave transitions from the literature, as well as far-infrared pure rotational ground state transitions and ground state combination differences from the torsional band obtained in this work. Slightly over 1000 energy differences for the ground state with 0 ≦ K ≦ 19 and K ≦ J ≦ 30 were fit to 30 molecular parameters from a group theoretical formalism developed earlier, and a standard deviation of ±0.00063 cm⁻¹ was obtained. An ambiguity (noted earlier in the microwave literature) in the determination of the structural parameter ϱ, which arises when two large amplitude motions are present in the molecule, can be understood and resolved using the present formalism.     

Sub-Doppler and Doppler spectroscopy of DCN isotopomers in the terahertz region: ground and first excited bending states (v1v2v3)=(0 1 0)

The rotational spectra of the deuterium cyanide isotopic species DCN, D¹³CN, DC¹⁵N, and D¹³C¹⁵N were recorded in the vibrational ground and first excited bending state (v₂=1) up to 2 THz. The R-branch transitions from J=3←2 to J=13←12 were measured with sub-Doppler resolution. These very high resolution (∼70 kHz) and precise (±3-10 kHz) saturation dip measurements allowed for resolving the underlying hyperfine structure due to the ¹⁴N nucleus in DCN and D¹³CN for transitions as high as J=10←9. Additional high JR-branch (J=25←24 to J=28←27) transitions around 2 THz and direct l-type (ΔJ=0, J=19 to J=25) transitions from 66 to 118 GHz were recorded in Doppler-limited resolution. For the ground state of D¹³C¹⁵N, the J=1←0 transition was measured for the first time. The transition frequency accuracies for the other deuterated species were significantly improved. These new experimental data, together with the available infrared rovibrational data and previously measured direct l-type transitions, were subjected to a global least squares analysis for each isotopomer. This yielded precise sets of molecular constants for the ground and first excited vibrational states, including the nuclear quadrupole and magnetic spin-rotation coupling constants of the ¹⁴N nucleus for DCN and D¹³CN. The hyperfine structure due to the D, ¹³C, and ¹⁵N nuclei have not been resolved, but led to a broadening of the observed saturation dips.     

Ground state spectrum of methylcyanide

The rotational spectrum of methylcyanide (acetonitrile) in the ground vibrational state was measured in the spectral region from 91 to 810 GHz using the Cologne and Tsukuba spectrometers operated in the Doppler-limited and sub-Doppler saturation layouts. The resolution of the saturation Lamb-dip measurements is estimated to be about 1 kHz at the best of circumstances and the measuring accuracy of 10-60 kHz depending very sensitively on the quality of the spectrum. In the cases of rotational transitions with the low quantum number J (J<18) and with a low difference of the rotational quantum numbers J−K, the resolved or partly resolved hyperfine structures of the rotational transitions were observed. Together with the most accurate data from the literature, the newly measured experimental data were analyzed using the traditional polynomial energy formula as well as the Padè approximant for the effective rotational Hamiltonian. The resulting rotational, centrifugal distortion, and hyperfine structure spectroscopic constants were obtained with a significantly higher accuracy than the ones listed in the literature. In addition, an anomalous accidental resonance was detected between the K=14 ground state levels and the K=12, +l levels in the excited v₈=1 vibrational state.     

Pyrolysis of ortho-cyanotoluene and PCl3 mixtures: the millimeter and submillimeter-wave spectrum of NCCCCP

The unstable NCCCCP molecule has been detected in the pyrolysis products of phosphorus trichloride and ortho-cyanotoluene mixtures. Its rotational spectrum has been investigated in the millimeter and submillimeter-wave regions for the ground and various vibrationally excited states which approximately lie below 400 cm⁻¹, namely (v₈,v₉)=(0,1),(0,2),(0,3),(0,4),(1,0), and (1,1). Transitions up to J=241←240 were measured for the ground state, making a precise evaluation of the sextic centrifugal distortion constant possible. The l-type resonances between the different sublevels of the bending states have been taken into account in the analysis of the spectra, which yielded accurate determinations of the x_L(99) and x_L(89) anharmonicity constants.     

Conformation and dipole moment of tetrahydropyran-4-one by microwave spectroscopy

The microwave spectrum of tetrahydropyran-4-one has been studied in the frequency region 18 to 40 GHz. The rotational constants for the ground state and nine vibrationally excited states have been derived by fitting a-type R-branch transitions. The rotational constants for the ground state are (in MHz) A = 4566.882 ± 0.033, B = 2538.316 ± 0.003, C = 1805.878 ± 0.004. From information obtained from the gas-phase far-infrared spectrum and relative intensity measurements, these excited states are estimated to be ～ 100 cm^?1 above the ground state for the first excited state of the ring-bending and ～ 185 cm^?1 for the first excited state of the ring-twisting mode. Stark displacement measurements were made for several transitions and the dipole moment components determined by least-squares fitting of the displacements: (in Debye) |μ_a| = 1.693 (0.001), |μ_b| = 0.0, |μ_c| = 0.300 (0.013) yielding a total dipole moment μ_tot = 1.720 (0.003). A model calculation to reproduce the rotational parameters indicates that the data are consistent with the chair conformation.     

Millimeter and submillimeter wave rotational spectrum of pyridine in the ground and excited vibrational states

The pure rotational spectra of the ground and five excited vibrational states of pyridine were measured, assigned and fit in the 75-110 and 260-370 GHz frequency bands. An improved set of spectroscopic constants was obtained for the ground state, and all the rotational and quartic centrifugal distortion constants were obtained for the excited vibrational states.     

Accurate rotational spectroscopy of sulfur dioxide, SO2, in its ground vibrational and first excited bending states, v2 = 0, 1, up to 2 THz

Approximately 150 pure rotational transitions each have been recorded for SO₂, v₂ = 0 and 1, in selected frequency regions up to 2 THz. The J and K_a quantum numbers reach very high values: 92 and 23, respectively, for the ground vibrational state and 81 and 21, respectively, for the first excited bending state. The highest levels accessed are almost 3000 cm⁻¹ above ground. The relative experimental uncertainties Δν/ν are about 10⁻⁸ for several medium to strong, isolated lines, and generally better than 2.5 × 10⁻⁷. Improved spectroscopic parameters have been obtained for both states, particularly for the excited bending state. In fact, the accuracies with which the energy levels of the v₂ = 1 state are known depend essentially only on the accuracy with which the vibrational spacing is known from infrared spectroscopy.     

The Emission Spectrum of Hot Water in the Region between 370 and 930 cm−1

An emission spectrum of the water molecule at a temperature of 1550°C has been recorded in the range from 373 to 933 cm⁻¹. More than 4000 pure rotational lines were observed with the strongest belonging to the ground state (000) and the first excited bending vibrational level (010). Transitions involving rotational quantum numbersJandK_asignificantly higher than previously recorded have been assigned.     

The rotational spectrum, nuclear field shift effects, Pt nuclear spin-rotation constant, and electric dipole moment of PtSi

The pure rotational spectrum of the ground electronic state of platinum monosilicide has been measured for nine isotopomers. For the most abundant isotopomer, ¹⁹⁴Pt²⁸Si, the J=1-0 and the J=2-1 transitions were recorded up to the fourth vibrationally excited state. The data set obtained enabled a multi-isotopomer fit to a Dunham-type expression and the constants Y₀₁, Y₀₂, Y₁₁, Y₂₁, and Y₃₁ were determined. In the process of fitting the data it was necessary to include Born-Oppenheimer breakdown correction terms and the values and significance of these terms are discussed. Strong evidence is presented indicating within the rotational spectrum the presence of field shift effects due to the finite size of the Pt nucleus. The nuclear spin-rotation constant, C_I(¹⁹⁵Pt) is found to be 30.98(157) kHz in the ground vibrational state. Hyperfine structure arising from the ²⁹Si nucleus was not observed. The measurement of Stark shifts in the rotational spectrum of PtSi has enabled the determination of the dipole moments for the ¹⁹⁴Pt²⁸Si and ¹⁹⁶Pt²⁸Si isotopomers.     

The rotational spectrum of methoxyethyne: Centrifugal distortion and internal rotation analysis

In the rotational spectrum of methoxyethyne 173 new transitions (J ≤ 30) have been measured between 150 and 240 GHz. In the centimeter range 25 new transitions (J ≤ 11) of the first excited torsional state have also been assigned. An overall fit of the measurements using a structure relaxation model has allowed us to accurately determine the internal rotation parameters. For the A substate effective rotational parameters are given which allow the calculation of transition frequencies of possible astrophysical interest.     

Microwave spectrum,quadrupole coupling constants,and barrier to internal rotation of 2-iodopropene

The microwave spectrum of 2-iodopropene has been investigated between 7.7 and 18 GHz. The measured transition frequencies of the ground and two vibrationally excited states have been analyzed using direct diagonalization of the rotational and quadrupole Hamiltonian. The following rotational and quadrupole coupling constants have been determined in a leastsquares fit for the ground state: A = 9285.153(20) MHz; B = 2337.2198(14) MHz; C = 1887.5871(14) MHz; and χ_cc = ?1820.783(33) MHz; χ_ab = 147.5(10) MHz; χ_bb = 957.018(41) MHz; and χ_cc = 863.765(40) MHz. The quadrupole coupling constants have been transformed to their principal axis system. From the splittings of some transitions of the first torsionally excited state a value of V₃ = 905(3) cm^?1 has been found for the threefold barrier hindering the internal rotation of the methyl group.     

Submillimeter-wave spectroscopy of HCO in the excited vibrational states

The pure rotational transitions of HCO⁺ in excited vibrational states located below 5000 cm⁻¹ over the ground state have been investigated with a high-sensitivity frequency/magnetic field double modulation submillimeter-wave spectrometer in the frequency range of 280-810 GHz. The ions were generated in an extended negative glow discharge through a gas mixture of a few millitorrs of H₂ and CO and 12 mTorr of Ar buffer gas. Throughout the experiments, the cell was maintained at liquid nitrogen temperature. In the present study, we have determined accurate molecular constants for the excited vibrational states. Our analysis suggests that there may be a higher order Coriolis interaction between the (0 3 1) and (1 2 0) states. In previous investigations, the Stark effect caused by the electric field present in the discharge plasma was cited as a reason for non-observations of low-J lines in the (02²0) and for the systematic shifts observed for low-J lines in the (01¹0), (02²0), (03¹0), and (04²0) states of HCO⁺ as well as DCO⁺. In the present investigation, some low-J lines in the (02²0) and (04²0) states have been observed in emission. Furthermore, J = 8-7, J = 9-8 lines in (03^1e1) were detected in emission. This finding indicates that missing low-J lines for the Δ sublevel obtained in the past is not due to the Stark effect but due to small population differences in those levels.     

Millimeter-Wave Spectroscopy of ClBS: An Improved Evaluation of the Equilibrium Structure of Chlorothioborine

The rotational spectrum of the unstable ClBS molecule has been investigated in the millimeter-wave region, from 80 to 195 GHz. A high-temperature reaction between crystalline boron and disulfur dichloride vapor was used to produce the molecule in a flow pyrolysis system. Eight different isotopic species were studied measuring lines in the ground and excited vibrational states 01¹0 (ClBS bend), 10⁰0 (ClB stretch), 02⁰0, 02²0, and 00⁰1 (BS stretch). The analysis of the spectra has been performed taking simultaneously into account both the Fermi resonance between the 10⁰0 and 02⁰0 states, and l-type resonance effects in the v₂=2 vibrational state. This procedure allowed us to calculate directly deperturbed rotational constants, from which the equilibrium rotational constant of seven isotopic variants could be accurately determined yielding a much improved evaluation of the equilibrium structure of chlorothioborine: r_e(ClB)=1.6806±0.0001 Å and r_e(BS)=1.6049±0.0001 Å. The equilibrium structures of ClBS and of the related molecules HBS, FBS, HCP, FCP, and ClCP have been also theo-retically evaluated by high-level CCSD(T) calculations performed using cc-pVTZ, cc-pVQZ, and cc-pV5Z basis sets. The different trends respectively observed for the BS and CP bond lengths in the XBS and XCP triatomic molecules are discussed.     

The C–D Stretch of Monodeuterated Propargyl Radical (CH2CCD)

The high-resolution infrared spectrum of the monodeuterated propargyl radical (CH₂CCD) has been obtained in the region of its acetylenic C–D stretch. Lower state rotational constants were determined for the molecule. The upper state was significantly perturbed making the upper state rotational constants determined much more uncertain.