Microwave spectra and centrifugal distortion constants of formic acid containing 13C and 18O: Refinement of the harmonic force field and the molecular structure

Pure rotational spectra of H¹³COOH, HC¹⁸OOH, and HCO¹⁸OH have been measured in the frequency region 8–185 GHz. Analysis of the spectra has given improved rotational constants and quartic and sextic centrifugal distortion constants. The quartic distortion constants have been combined with previously published distortion constants of four other isotopic species, and with the vibrational wavenumbers of seven isotopic species, to produce a refined harmonic force field. An improved substitution structure and the ground state average structure have been obtained. Some unmeasured transition frequencies which may be of importance in radioastronomy are also presented.     

The microwave spectrum of silyl isocyanate, SiH3NCO: Spectra of isotopically substituted species and correction of the rs structure

Microwave spectra have been measured for 10 isotopic species of silyl isocyanate, SiH₃NCO, in the ground vibrational state, and in several excited states of the lowest frequency bending vibration, ν₁₀. This vibration is highly anharmonic, with a potential hump of 31.5 cm⁻¹ at the linear configuration, and its effects have been removed from the rotational constants to produce effective ground-state rotational constants B₀^* for each isotopic species. These B₀^* constants have been used to determine the structural parameters, which are now in good agreement with earlier electron diffraction values. Excellent predictions have been made of the centrifugal distortion constants for different isotopic species and vibrational states, as well as of the l-type doubling constants of the various isotopic species.     

Infrared spectra of matrix-isolated sodium and potassium cyanides

The infrared spectra of matrix-isolated sodium and potassium cyanides in neon, argon, krypton, and nitrogen matrices were examined over the range 4000 cm^?1 to 33 cm^?1. The effect of temperature cycling, which normally results in migration of trapped molecules to generate polymeric species, has been pursued. A careful study of the effect of superheating the vapor species on the spectra has also been carried out. By these means it has been possible to identify the bands in the spectrum due to monomeric and polymeric species. Isotopic frequency shifts were measured for a ¹³C-enriched sample of sodium cyanide and ¹³C- and ¹⁵N-enriched samples of potassium cyanide. From the measured isotopic shifts it was possible to determine the structure of these two compounds as being linear NaCN and KCN. Accurate force constants were calculated using the measured frequency values for the three fundamental vibrational modes of the different isotopic molecules of NaCN and KCN. Bands due to sodium and potassium cyanates were also identified in some of these systems. ¹³C and ¹⁵N isotopic frequency shifts were also observed for NaOCN and KOCN.     

Microwave spectra of mono-C substituted acetone, (CH3)2CO

The microwave spectra of the two mono-¹³C isotopic forms of acetone are reported for the first time. Measurements were carried out from 11 to 25 GHz with a pulsed-beam Fourier-transform microwave spectrometer. Because overall rotation interacts with the internal rotations of the methyl groups, the spectra were analyzed with an effective rotational Hamiltonian for molecules with two periodic large-amplitude internal motions. In acetone-2-¹³C, the methyl groups are equivalent; in acetone-1-¹³C, they are not. The molecular structure has been re-examined by including rotational constant data on other isotopic forms reported previously. An equilibrium structure for acetone has been derived from the observed rotational constants and vibration-rotation constants calculated from ab initio force fields.     

Submillimeter wave spectrum and molecular constants of N2O

The submillimeter wave spectrum of the N₂O molecule has been investigated within the 375–565 GHz frequency range with a sensitivity better than 10^?8 cm^?1. The measured frequencies include 161 lines with intensities γ ? 10^?6 cm^?1 belonging to 22 spectroscopically different species of the molecule (specifically, the ground and some excited vibrational states of the five most abundant isotopic species of the molecule in natural abundance) with a statisticall and systematic error of the order of magnitude 10^?8. Rotational and two centrifugal stretching constants could be determined for each spectroscopic species. For each isotopic species observed, we have made a general analysis of the spectrum in different vibrational states bearing in mind resonance effects. The total number of the rotational and rovibrational constants obtained exceeds 40.     

The Rotational Spectra of H2CCSi and H2C4Si

The microwave rotational spectra of two singlet chains H₂CCSi and H₂C₄Si, have been observed in a pulsed supersonic molecular beam by Fourier transform microwave spectroscopy. Following detection of the singly substituted rare isotopic species and D₂CCSi, an experimental structure (r₀) has been determined to high accuracy for H₂CCSi by isotopic substitution. In addition, the rotational transitions of the ²⁹Si and the two ¹³C isotopic species of H₂CCSi exhibit nuclear spin-rotation hyperfine structure. The component of the spin-rotation tensor along the a-inertial axis is abnormally large for each of these isotopic species, especially for that with ²⁹Si, where the magnitude of C_aa is in excess of 700 kHz.     

Microwave spectrum of 3-bromopropene in the excited torsional states

The microwave spectra in the excited states of the CC torsion for the ⁷⁹Br and ⁸¹Br isotopic species of 3-bromopropene were measured in the frequency region 15.3–23.7 GHz. The a-type R-branch and b-type Q-branch rotational transitions in the first and second excited states of one conformer, skew, have been assigned and analyzed. Analysis of the spectrum yields the rotational constants and the nuclear quadrupole coupling constants. From relative intensity measurements the energy differences associated with the CC torsion, between the ground and first excited state, the ground and second excited state have been found to lie 109 and 206 cm^?1, respectively.     

The infrared spectrum of HC13F3

The infrared spectrum of HC¹³F₃ is reported for the first time. Fundamental frequency shifts due to carbon-13 substitution have been measured and rotational analyses of several of the fundamental and combination bands are given. Coriolis x-y and Fermi perturbations are evident in several of the bands. Table VI summarizes the data obtained in this study.     

Molecular beam electric resonance study of KCN,K13CN and KC15N

The microwave spectra of the isotopic species K¹³CN and KC¹⁵N have been investigated by molecular beam electric resonance spectroscopy, using the seeded beam technique. For both isotopic species about 20 rotational transitions originating in the ground vibrational state were observed in the frequency range 9–38 GHz. The observed transitions were fitted to an asymmetric rotor model to determine the three rotational, as well as the five quartic and three sextic centrifugal distortion constants. The hyperfine spectrum of KCN has been unravelled with the help of microwave-microwave double-resonance techniques. One hundred and forty hyperfine transitions in 11 rotational transitions have been assigned. The hyperfine structures of K¹³CN and KC¹⁵N were also studied. For all three isotopic species the quadrupole coupling constants and some spin-rotation coupling constants could be deduced. The rotational constants of the ¹³C and ¹⁵N isotopically substituted species of potassium cyanide, combined with those of the normal isotopic species (determined more accurately in this work), allowed an accurate and unambiguous evaluation of the structure, which was confirmed to be T shaped. Both the effective structure of the ground vibrational state and the substitution structure were evaluated. The results for the effective structural parameters are $\text{r}{}_{\mathrm{<mtext>CN</mtext>}}\text{= 1.169(3)}\text{A}\text{?}$, $\text{r}{}_{\mathrm{<mtext>KC</mtext>}}\text{= 2.716(9)}\text{A}\text{?}$, and $\text{r}{}_{\mathrm{<mtext>KN</mtext>}}\text{= 2.549(9)}\text{A}\text{?}$. The values obtained for the principal hyperfine coupling constant eQq_z(N), the angle between the CN axis and z_N, and the bond length r_CN indicate that in gaseous potassium cyanide the CN group can be considered as an almost unperturbed CN^? ion.     

Millimeter wave spectrum of chlorine nitrate

New measurements of the millimeter wave spectra of the ³⁵Cl and ³⁷Cl isotopic forms of chlorine nitrate in the ground and lowest vibrational states have been made in the region 80–228 GHz. These measurements allow accurate frequency predictions of all strong transitions up to 300 GHz. A comparison of rotational line intensities with those of ClO, which has already been observed in the stratosphere, is provided. The measured and calculated frequencies of ClNO₃ are available on magnetic tape.     

Submillimeter-wave spectrum of carbonyl sulfide: rare isotopic species

Rotational spectra of four rare isotopic species of OCS were observed in the 110-690 GHz region with a source-modulated submillimeter-wave spectrometer. Spectral lines of ¹⁸O¹³CS, O¹³C³⁶S, ¹⁸OC³⁶S, and ¹⁸O¹³C³⁴S were measured in their natural abundances, 21.4, 1.51, 0.29, and 0.95 ppm, respectively. The rotational constants and centrifugal distortion constants were precisely determined from observed line frequencies by least squares methods. In order to facilitate sensitivity examination in the submillimeter-wave region, transition frequencies and peak absorption coefficients were calculated and tabulated for the spectral lines of the four species up to the 1000 GHz region.     

Ground state spectroscopic constants of H15NCS,HN13CS,and HNC34S,and the molecular structure of isothiocyanic acid

The rotational spectrum of isothiocyanic acid was measured for the isotopically enriched species H¹⁵NCS and HN¹³CS as well as HNC³⁴S in natural abundance. In the frequency range from 8 to 240 GHz the a-type R-branch transitions were measured for all three isotopic species. The ^qQ₁ transitions were identified in the microwave region for H¹⁵NCS and HN¹³CS. The rotational and centrifugal distortion constants were determined using Watson's Hamiltonian in the S reduction, extended empirically to higher order terms in the angular momentum. The molecular structure of isothiocyanic acid was reevaluated using a modified substitution method and the NCS chain was found to be bent: $\text{r(}\text{NH}\text{) = 0.993}\text{A}\text{?}$, $\text{r(}\text{NC}\text{) = 1.207}\text{A}\text{?}$, $\text{r(}\text{CS}\text{) = 1.5665}\text{A}\text{?}$, ∠HNC = 131.7°, and ∠NCS = 173.8°. The molecule has the trans conformation.     

Rotational spectrum of three conformers of 3,3-difluoropentane: Construction of a 480 MHz bandwidth chirped-pulse Fourier-transform microwave spectrometer

The rotational spectra for three conformers of 3,3-difluoropentane have been measured using both a newly constructed narrow bandwidth chirped-pulse Fourier-transform microwave spectrometer and a Balle-Flygare resonant cavity Fourier-transform microwave spectrometer. The chirped-pulse instrument produces a microwave pulse spanning up to 480 MHz bandwidth in the 7-18 GHz region by mixing a 1 μs chirped pulse (of up to 240 MHz bandwidth) from an arbitrary function generator with the output from a microwave synthesizer.Rotational spectra for the normal isotopic species and all possible ¹³C single substitutions were observed for the gauche-gauche and anti-gauche conformers, allowing a Kraitchman substitution structure and an inertial fit structure to be determined. ¹³C isotopic species and dipole moment components were not measurable for the less intense anti-anti species as a result of partially resolved fine splitting. Details of the new chirped-pulse instrument will be described and the structural results will be presented and compared with ab initio data for 3,3-difluoropentane.     

Millimeter-wave spectrum of BrCN produced by dc discharge

Ground state (v=0) and first excited state (v=1) millimeter-wave rotational absorption spectra of cyanogen bromide (BrCN) and some of its isotopic species, have been investigated in the frequency region: 40.0-75.0 GHz using a source-modulated millimeter-wave spectrometer. Millimeter-wave radiation has been produced using a frequency multiplier, the fundamental radiation source being klystrons. BrCN has been produced by applying a dc glow discharge through a mixture of 3-bromobenzonitrile and trifluoromethylbromide (CF₃Br) at low pressure. The quadrupole hyperfine structure of ⁸¹Br and ⁷⁹Br have been resolved, measured, and analyzed. Finally, internuclear distances of BrCN have been determined.     

Rotational analysis of 3664 Å band of NiCl

Nickel chloride was excited in a high frequency discharge source and the band at 3664 Å was photographed at an inverse dispersion of 0.59 Å/mm on a two metre plane grating spectrograph. The rotational analysis was carried out and the molecular constants of the upper state are reported. The rotational isotopic shifts due to³⁷Cl support the rotational analysis. The electronic transition involved is identified to beβ ²Δ_5/2 →X ₁ ²Δ_5/2.     

Microwave spectra of the ground vibrational state of formaldehyde substituted with 17O and 18O

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.     

Rotational analysis of 4730 Å band of MgCl

Magnesium chloride was excited in a high frequency discharge source and the 4730 Å band was photographed at an inverse dispersion of 0.48 Å mm^?1 in a two meter plane grating spectrograph. The rotational analysis of the band has been carried out and the molecular constants of the upperB ²Σ⁺ state are reported for the first time. Observation of rotational isotopic shifts due to Cl³⁷ supports the rotational analysis. The electronic transition involved is identified to beB ²Σ⁺ →A ²Π_1/2(a) (inverted).     

Ground state rotational spectrum of nitrogen trifluoride: The K = 3 splittings of NF3 and NF3

The ground state rotational spectrum of the ¹⁴NF₃ and ¹⁵NF₃ isotopic species of nitrogen fluoride has been observed in the ∼450-810 GHz frequency range. This investigation allowed us to improve the rotational parameters for both isotopologues. In particular, for the first time the K = 3 line splitting parameter and the sextic centrifugal distortion constants have been determined for ¹⁵NF₃.     

Millimeter-wave spectrum of Ne–CO: new measurements

The b-type rotational transitions of the van der Waals complex, Ne–CO have been measured using the intracavity OROTRON jet spectrometer in the frequency range of 80–115 GHz. The high sensitivity of this technique enabled us to detect all three Ne isotopic modifications of the complex, ²⁰Ne–CO, ²²Ne–CO, and ²¹Ne–CO in natural abundance. The observed and assigned transitions belong to the Q-branch of the K = 1–0 subband and include also R (0) and P (2) lines. The newly obtained data were analysed together with previously observed millimeter-wave b-type and microwave a-type rotational transitions.     

Microwave spectrum,conformation, and quadrupole coupling constants of cyclopentyl chloride

The microwave spectra of the normal and two isotopic species of cyclopentyl chloride have been observed and analyzed. For the normal isotopic species the rotational constants (in MHz) are A = 4547.77 ± 0.01, B = 2290.22 ± 0.01, and C = 2073.34 ± 0.01. From the rotational constant data, it has been shown that the stable molecular conformation is the bent axial form. Quadrupole coupling constants have been measured for the ³⁵Cl nucleus, the values being (in MHz) χ_aa = ?23.70 ± 0.10, χ_bb = 32.33 ± 0.36, and χ_cc = ?8.63 ± 0.37. When transformed to the CCl bond axis system, the coupling constants confirm the axial structure. Extensive vibrational satellite structure, presumably arising from the pseudorotational ring mode with a fundamental frequency of 52 ± 5 cm^?1, has been observed and assigned. No spectral evidence has been observed for a second stable molecular conformer.