Rotational Spectra of CH2DCCH and CH3CCD: Experimental and ab Initio Equilibrium Structures of Propyne

The ground state rotational spectra of CH₂DCCH and CH₃CCD (main species and ¹³C-substituted species) have been measured up to 470 GHz. Accurate rotational and centrifugal distortion constants have been determined. r₀, r_s, r_ε,I, and r^ρ_m, structures of propyne have been calculated. The ab initio structure has also been calculated using three different methods (SCF, MP2, and QCISD) and two basis sets (DZP and TZ2P). Offsets have been derived empirically using molecules containing structural units present in propyne and whose equilibrium structures have been determined previously. A near-equilibrium structure has been estimated to be acetylenic r(C---H) = 1.061 (1) Å, r(CC) = 1.204 (1) Å, r(C---C) = l.458 (2) Å, methyl r(C---H) = 1.089 (1) Å, and (CCH) = 110.7 (5)°.     

The microwave spectrum, structure, chlorine nuclear quadrupole coupling constants, dipole moment, and centrifugal distortion constants of dichlorosilane

The microwave spectra of three isotopic species of dichlorosilane, SiH₂Cl₂, in its ground vibrational state, have been measured in the frequency region 8–40 GHz. The spectra have yielded values for the rotational constants, centrifugal distortion constants, and chlorine nuclear quadrupole coupling constants, as well as the molecular dipole moment, 1.13 ± 0.02 D. The molecule has C_2v symmetry, and the bond lengths and angles r(Si---Cl=2.033±Å, r(Si---H)=1.480±0.015Å, (Cl---Si---Cl)=109°43′±20±, (H---Si---H)=111°18′±40′ The centrifugal distortion constants have been compared with those calculated using a published force field.     

The harmonic force field and ground-state average structure of difluoroborane

An improved harmonic force field of difluoroborane has been calculated using the vibrational wavenumbers and quartic centrifugal distortion constants of four isotopic species. The unidentified vibrational mode ν₅ is predicted at 1049 ± 50 and 775 ± 50 cm⁻¹ for HBF₂ and DBF₂, respectively. The ground-state average structure of HBF₂ has been found to be r_z(BH) = 1.195 ± 0.003 Å; r_z(BF) = 1.315 ± 0.001 Å; (FBF) = 118.0 ± 0.1°.     

The methyl bromide molecule: A critical consideration of perturbations in spectra

This work gives an extensive critique of studies on methyl bromide and all its isotopic varieties with special stress on their rotational, vibrational, and rovibrational spectra. The rotational constants of more than 40 vibrational states of CH₃Br and 20 of CD₃Br, as well as of the ground states of all varieties, were critically examined and corrected where needed. An almost complete set of harmonic and anharmonic constants for CH₃Br was derived. From the set of rotation-vibration interaction constants, new accurate equilibrium constants A_e and B_e have been evaluated for CH₃⁷⁹Br, CH₃⁸¹Br, CD₃⁷⁹Br, CD₃⁸¹Br, from which the following equilibrium structure is obtained: r_e(C---H) = 1.0823 Å; r_e(C---Br) = 1.9340 Å; α(HCH) = 111.157°.     

Doppler-limited dye laser excitation spectroscopy of DCF

The dye laser excitation spectrum of the vibronic transition of DCF was observed between 17 200 and 17 400 cm⁻¹ with the Doppler-limited resolution. DCF was produced by the reaction of microwave-discharged CF₄ with CD₃F. The observed spectra, which were found to be nearly free of perturbations, were assigned to 858 transitions of the K′_a − K″_a = 4−5, 3−4, 2−3, 1−2, 0−1, 1−0, 2−1, 3−2, 3−3, 2−2, 1−1, 0−0, 2−0, and 0−2 subbands, and were analyzed to determine the rotational constants and centrifugal distortion constants for both the and à states. The rotational constants of DCF thus determined were combined with those of HCF to calculate the structural parameters for this molecule: r(C---H) = 1.138 Å, r(C---F) = 1.305 Å, and HCF = 104.1° for the ground state, and r(C---H) = 1.063 Å, r(C---F) = 1.308 Å, and HCF = 123.8° for the excited à state.     

Microwave spectrum of CHD2CN and CHD2NC,and the harmonic force field and rz structure of methyl cyanide and isocyanide

The microwave spectra of CHD₂CN and CHD₂NC have been measured from 18 to 40 GHz; about 20 type A and 30 type C transitions have been observed for each molecule. These have been fitted to a Hamiltonian using 3 rotational constants, and 5 quartic and 4 sextic distortion constants, in the I^rS reduction of Watson [in “Vibrational spectra and structure” Vol. 6 (1977)]; the standard error of the fit is 26 kHz. For methyl cyanide the 5 quartic distortion constants have been used to further refine the recent harmonic force field of Duncan et al. [J. Mol. Spectrosc.69, 123 (1978)], but the changes are small. Finally, for both molecules, the harmonic force field has been used to determine zero point average moments of inertia I_z from the ground state rotational constants for many isotopic species, and these have been used to determine an r_z structure. The results are compared with r_s structure calculations.     

Sulfur dioxide: Rotational constants and asymmetric structure of the state

Eight bands of the 2350 Å system of sulfur dioxide have been rotationally analyzed as A-type transitions of a prolate asymmetric rotor, confirming that the electronic transition is ¹B₂ ← ¹A₁[2b₁(π^*) ← 1a₂(π)]. The electronic energy and rotational constants of the 0-0 band are, in cm⁻¹: These constants correspond to the average structure r₀ = 1.560 Å and θ₀ = 104.3°. However, the vibrational structure can only be satisfactorily accounted for on the hypothesis of a double-minimum potential in the antisymmetrical stretching coordinate Q₃, the energies of the fundamental levels in the three modes of the B₂ state being: (100), 960 cm⁻¹; (010), 377 cm⁻¹; and (001), 220 cm⁻¹ The (001) level is not observed in the spectrum but can be calculated from the distortion constants and inertial defect of the rotational analysis: the level (002) = 561 cm⁻¹, obtained directly from the vibrational structure, establishes that there is strong, positive anharmonicity in the first three levels of this vibration, as required by the assumption of a double-minimum potential function. Preliminary values are reported for the barrier to the symmetrical configuration, V/hc 100 cm⁻¹, and for the difference in bond distances in the equilibrium configuration, Δr0.12 Å. Coon and his co-workers have previously considered the possible asymmetry of this state but the Q₃ inversion barrier obtained by them, 656 cm⁻¹, is much higher than in the present work, and reasons for this are discussed.     

Assignment and analysis of the rotational spectrum of bromoform enabled by broadband FTMW spectroscopy

The rotational spectrum of the bromoform molecule is complicated by overlap of extensive hyperfine splitting structure of three bromine nuclei belonging to four isotopic species of comparable abundance. We have been able to achieve an unambiguous assignment of this spectrum on the basis of complete hyperfine patterns of the lowest-J rotational transitions recorded at conditions of supersonic expansion with chirped-pulse, broadband Fourier transform microwave spectroscopy. The mm-wave rotational spectrum was then also studied up to J = 131 and 318 GHz, and extensive measurements are reported for four isotopic species of HCBr₃ and four DCBr₃ species. Precise values of many spectroscopic constants have been determined from global fits of all measurements for a given isotopic species and have been substantiated by comparisons among the various isotopic species and with results of ab initio calculations. The experimental measurements allowed determination of the axial rotational constant C for the symmetric top species, evaluation of nuclear quadrupole hyperfine splitting constants in the principal quadrupole axes of the bromine nucleus, and extension of the range of known values of the h₃ splitting constant. The r_z geometry of HCBr₃ was also determined.     

The microwave spectrum of phosphabutadiyne, H---CC---CP

A detailed rotational analysis of the microwave spectrum between 26.5 and 40 GHz of phosphaethene, CH₂=PH, has been carried out. This molecule is the simplest member of a new class of unstable molecules—the phosphaalkenes. The species can be produced by pyrolysis of (CH₃)₂PH, CH₃PH₂ and also somewhat more efficiently from Si(CH₃)₃CH₂PH₂. Full first-order centrifugal distortion analyses have been carried out for both ¹²CH₂³¹PH and ¹²CH₂³¹PD yielding: A₀ = 138 503.20(21), B₀ = 16 418.105(26), and C₀ = 14 649.084(28) MHz for ¹²CH₂³¹PH. The 1₀₁-0₀₀ μ_A lines have also been detected for ¹³CH₂PH, cis-CDHPH and trans-CHDPH. These data have enabled an accurate structure determination to be carried out which indicates: r(H_cC) = 1.09 ± 0.015 Å, (H_cCP) = 124.4 ± 0.8°; r(H_tC) = 1.09 ± 0.015 Å, (H_tCP) = 118.4 ± 1.2°; r(CP) = 1.673 ± 0.002 Å, (HCH) = 117.2 ± 1.2°; r(PH) = 1.420 ± 0.006 Å, (CPH) = 97.4 ± 0.4°. The dipole moment components have been determined as μ_A = 0.731 (2), μ_B = 0.470 (3), μ = 0.869 (3) D for CH₂PH; μ_A = 0.710 (2), μ_B = 0.509 (10), μ = 0.874 (7) D for CH₂PD.     

The millimeterwave spectrum of germyl fluoride: Determination and comparison of the effective, substitution, and equilibrium structures

The ground state rotational spectrum of germyl fluoride was measured up to 1273 GHz (J ≤ 63). The rotational constants and quartic and sextic centrifugal distortion constants have been determined accurately for five isotopic species in natural abundance (^{70/72/73/74/76}Ge). The high accuracy of the rotational constants of these five isotopomers allowed us to study the mass dependence of the substitution coordinate of Ge. Equilibrium rotational constants of ⁷⁴GeH₃F were deduced with the help of the axial rotational constant and the rotation-vibration interaction constants determined by high resolution infrared spectroscopy. The r₀, r_,I, and r_e structures of GeH₃F were determined.     

Microwave spectrum ofcis 3-fluorophenol

The microwave spectrum ofcis 3-fluorophenol involving rotational states up toJ=28 has been observed and analysed in the frequency range 23–25 GHz in the ground vibrational state at room temperature. Analysis yields three rotational and five quartic centrifugal distortion constants. A tentativer ₀ structure has been proposed satisfying the observed rotational constants. The small value of the inertia defect Δ=0·07 confirms the planarity of the conformer.     

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 sub-millimeter and Fourier transform microwave spectrum of HZnCl (X Σ)

The pure rotational spectrum of HZnCl (X ¹Σ⁺) has been recorded using sub-millimeter direct-absorption methods in the range of 439–540 GHz and Fourier transform microwave (FTMW) techniques from 9 to 39 GHz. This species was produced by the reaction of zinc vapor and chlorine gas with H₂ or D₂ in a d.c. glow discharge for the sub-millimeter studies. In the FTMW measurements, HZnCl was created in a discharge nozzle from Cl₂ and (CH₃)₂Zn. Between 5 and 10 rotational transitions were measured in the sub-millimeter regime for four zinc and two chlorine isotopologues; four transitions were recorded with the FTMW machine for the main isotopologue, each consisting of several chlorine hyperfine components. The data are consistent with a linear molecule and a ¹Σ⁺ ground electronic state. Rotational and chlorine quadrupole constants were established from the spectra, as well as an r_m⁽²⁾ structure. The Zn–Cl and Zn–H bond lengths were determined to be 2.0829 and 1.5050 Å, respectively; in contrast, the Zn–Cl bond distance in ZnCl is 2.1300 Å, longer by 0.050 Å. The zinc–chlorine bond distance therefore shortens with the addition of the H atom. The ³⁵Cl electric quadrupole coupling constant of eQq = −27.429 MHz found for HZnCl suggests that this molecule is primarily an ionic species with some covalent character for the Zn–Cl bond.     

The microwave spectrum and rotational structure of the Δ and Σ electronic states of sulfur monoxide

The spectrum of ¹Δ and ³Σ SO has been studied in the millimeter and submillimeter region of the microwave spectrum. This expanded spectral coverage has made possible the measurement of twenty-two previously unobserved transitions, several of which are necessary for an accurate calculation of the energy levels. As a result, it is now possible to calculate the rotational transitions between energy levels for which J ≤ 10 in both the ground ³Σ electronic state and the excited ¹Δ electronic state to an accuracy comparable to that of the microwave measurements themselves ( 1 MHz). Among the molecular constants calculated are; for the ¹Δ state: B₀ = 21 295.405 MHz, D₀ = 0.0350 MHz, ω_e = 1108 cm⁻¹, and r₀ = 1.4920 Å; and for the ³Σ state: B₀ = 21 523.561 MHz, D₀ = 0.03399 MHz, λ₀ = 158 254.387 MHz, γ₀ = −168.342 MHz, ₀ = 0.305 MHz, r₀ = 1.4840 Å, B_e = 21 609.552 MHz, λ_e = 157 779.2 MHz, and r_e = 1.4811 Å.     

The microwave spectrum of 1-phosphabut-3-ene-1-yne, CH2=CHCP

The new molecule 1-phosphabut-3-ene-1-yne, CH₂=CHCP, produced by pyrolyzing prop-1-ene-3-phosphorus dichloride, CH₂=CHCH₂PCl₂, was detected by microwave spectroscopy. The analysis of the rotational transitions indicates that the molecule is planar with constants: A₀ = 46 694(24), B₀ = 2807.7100(21), and C₀ = 2645.8356(21) MHz. These rotational constants indicate that the structure of the vinyl group is essentially the same as that in CH₂=CHCN and CH₂=CHCCH; r(C---C) = 1.432 Å and (C=C---C) = 123.9°. The dipole moment parameters are μ_A = 1.181(2), μ_B = 0.074(1), and μ = 1.183(2) D. The vibrational satellite spectra for the C---CP bending modes indicate that ν₁₁(a′) = 184 ± 30 cm⁻¹ and ν₁₅(a″) = 263 ± 30 cm⁻¹.     

The structure of H2CSe from microwave spectroscopy

The microwave spectra of six isotopic species of selenoformaldehyde, H₂¹³C^78,80Se, D₂C^78,80Se, and DHC^78,80Se have been assigned. The resultant rotational constants, together with those of previous work yield the following substitution structure: r(C=Se) = 175.31 pm, HCH = 117.93°, and r(C---H) = 109.04 pm. This structure is compared with the equilibrium structure derived from ab initio calculations. The fundamental vibrational frequencies and centrifugal distortion constants of H₂C⁸⁰Se have been calculated.     

The rotational spectrum of acrylonitrile in excited states of the two low-frequency CCN bending vibrational modes

The strongest vibrational satellites in the rotational spectrum of acrylonitrile have been assigned and frequencies of μ_a- and μ_b-type transitions in the frequency range 27–184 GHz are reported for the first two excited states in the lowest frequency in-plane CCN bending vibrational mode and the first excited state in the out-of-plane CCN bending mode. The values of the rotational constants, the quartic and sextic centrifugal distortion constants, and one octic centrifugal distortion constant are determined for each of these states. Less extensive results are also presented for the third quantum of the in-plane bend. The data set for the ground state has been extended by a number of new measurements and the improved ground state constants are used in a discussion of changes in rotational and centrifugal distortion constants with vibrational state where all constants associated with P_zⁿ and P²P_z⁽ⁿ⁻²⁾ terms in the Hamiltonian are found to reflect the common origin of the two CCN bends.     

The microwave spectrum of cyanophosphaacetylene, H2PCCCN

The a type transitions of the microwave rotational spectra of cyanophosphaacetylene, H₂PCCCN, have been investigated in the frequency region between 5 and 26.5 GHz by Fourier transformation microwave (FTMW) spectroscopy. Rotational, centrifugal distortion and ¹⁴N nuclear quadrupole coupling constants have been determined. Density functional theory level ab initio calculations were performed to predict the molecular constants, and the predicted values are in good agreement with our experimentally determined results. The ¹³C and ¹⁵N isotopomer transitions were also observed. The derived r₀ structure is quite comparable to the calculated H₂PCCCN equilibrium geometry.     

The harmonic force field and rz structure of HNCO

The presently available microwave, millimeter wave, and far-infrared data of five isotopic species of isocyanic acid, namely, HNCO, H¹⁵NCO, HN¹³CO, HNC¹⁸O, and DNCO, have been used to obtain improved values of the ground-state rotational constants, the five quartic distortion constants, and some higher-order distortion constants in the I^rS reduced Hamiltonian of Watson. The appropriate planarity relation among the quartic centrifugal distortion constants has been imposed in the fitting procedure. The general harmonic force field of isocyanic acid has been determined using all existing data, and assuming a trans bent equilibrium geometry of the molecule with an NCO angle of 170°. Finally an r_z structure has been obtained using the A_z, B_z, and C_z rotational constants of five isotopic species. The bending of the NCO chain is found to be 8° in the trans configuration.     

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.