Carbon suboxide as a quasilinear molecule with a large amplitude bending mode: Determination of the molecular constants and the ν7 potential function

The model of a quasilinear molecule with a large amplitude bending mode is used to treat C₃O₂. The Hamiltonian operator, including the rotation-vibration interaction, is derived allowing only a single vibrational degree of freedom, namely, the ν₇ mode corresponding to the bending at the central carbon atom. The CCO angle is constrained to be 180°. With this model the rotational energy levels and, thus, the molecular constants can be computed for any ν₇ level once the ν₇ potential is specified. The l-doubling is included only for π states. The model contains three adjustable parameters: the rotational constant in the linear configuration and two terms in the potential function, and these are determined by fitting three experimental quantities: the rotational constants in and the separation between the ground and 2ν7⁰ states. The resulting ν₇ potential has a 30.56 cm^?1 barrier at α = 0 with a minimum at α = 11.04°, where 2α is the angular deviation from linearity. The model gives a good fit to the 2ν₇ Raman data and to the rotational and centrifugal distortion constants in all of the nν7^l states which have been analyzed. A similar analysis is applied with equal success to the states with ν₄, the asymmetric CC stretch mode at 1587 cm^?1, simultaneously excited with a ν₇ mode. The potential in this case has a 56.58 cm^?1 barrier at α = 0 with a minimum at α = 13.02°.     

Microwave spectrum,structure, and nuclear quadrupole coupling of cis-1-chlorobutadiene-1,3

The microwave spectra of the two natural isotopic species of cis-1-chlorobutadiene-1,3, CH³⁵ClCHCHCH₂ and CH³⁷ClCHCHCH₂, together with all monosubstituted species with deuterium or ¹³C isotopes have been measured and assigned and the complete substitution structure has been determined. The spectral region investigated was between 18 and 40 GHz. The molecule was found to be planar and the following values were found for the principal parameters: r(CC)_chlorovinyl = 1.327(6), $\text{r(}\text{CC}\text{)}{}_{\mathrm{<mtext>vinyl</mtext>}}\text{= 1.343(2)}\text{A}\text{?}$, <(CCC)_chlorovinyl = 126.5(3)° and <(CCC)_vinyl = 123.0(7)°. The nuclear quadrupole coupling constants, χ_aa and χ_bb, for the parent molecule were calculated and further used to estimate the symmetry of the field gradient around the CCl bond. Force-field calculations were used to predict the centrifugal distortion constants and inertial defect of some isotopic species. Thermodynamic functions were calculated for cis- and trans-1-chlorobutadiene-1,3 and used to estimate the energy difference between them.     

Microwave spectra of deuterated ethylenes: Dipole moment and rz structure

The pure rotational spectra of three deuterated ethylenes, CH₂CD₂, CH₂CHD, and cis-CHDCHD, were observed by microwave spectroscopy, and the rotational and centrifugal distortion constants were determined precisely. The dipole moment of CH₂CD₂ was calculated from the Stark effects to be 0.0091 ± 0.0004 D. From the observed rotational constants the average structure was calculated to be $\text{r}{}_{\mathrm{z}}\text{(CC)}\text{= 1.3391 ± 0.0013}\text{A}\text{?}$, $\text{r}{}_{\mathrm{z}}\text{(CH)}\text{= 1.0869 ± 0.0013}\text{A}\text{?}$, θ_z(CCH) = 121.28 ± 0.10°, and $\text{r}{}_{\mathrm{z}}\text{(CH)}\text{- r}{}_{\mathrm{z}}\text{(CD)}\text{= 0.00137 ± 0.00037}\text{A}\text{?}$, where the errors include one standard deviation in the fitting and errors due to an uncertainty (±0.03°) in θ_z(CCH) - θ_z(CCD).     

Microwave spectrum of 2-propene-1-imine,CH2CHCHNH

The reactive species, 2-propene-1-imine, has been identified by its microwave spectrum as a pyrolysis product of diallylamine vapor (100 mTorr, 400°C). Two entirely planar forms are observed, both with an “s-trans” CCCN configuration. The lower energy rotamer has an “anti” CCNH configuration, with $\text{r}\text{CC = 1.453(3)}\text{A}\text{?}$, $\text{r}\text{CC = 1.336(4)}\text{A}\text{?}$, and ∠ CCC = 122.9(3)° and a dipole moment of 2.01(2) D with 1.13(1) D and 1.66(2) D “a” and “b” components. The higher energy rotamer has a “syn” CCNH configuration and a dipole moment of 2.51(2) D with 2.39(2) and 0.77(3) D the “a” and “b” components. From relative intensity measurements, the ground state energy difference is determined to be 0.9 ± 0.1 kcal mole^?1.     

Microwave spectroscopic study of the conformations,relative energies,and dipole moments of ethyl cyanoformate

The microwave spectrum of ethyl cyanoformate displays a-type band spectra from three nearly prolate conformers. High-resolution spectra of the two more stable species have been assigned. One form, designated extended, has rotational constants A″ = 6453.3(4) MHz, B″ = 1500.47(6) MHz, C″ = 1236.36(6) MHz, which are consistent with a syn-anti [τ₁ (OCOC) = 0°, τ₂ (COCC) = 180°] structure. The second form, labeled compact, has rotational constants A″ = 6787.8(7) MHz, B″ = 1549.38(8) MHz, C″ = 1406.80(8) MHz, which are consistent with a syn-gauche [τ₁ (OCOC) = 0°, τ₂ (COCC) ～ 90°] structure. The extended form is marginally more stable, ΔE = 55 ± 27 cm^?1. The extended conformer has dipole moment components μ_a = 4.44(7), μ_b ～ 0 D and the compact conformer has dipole moment components μ_a = 4.25(7), μ_b = 0, μ_c = 1.08(23) D. The third conformer (relative energy 600 ± 140 cm^?1) has the most intense band series even at ?63°C. the bands of this conformer are unresolvable into individual rotational transitions.     

Microwave spectrum and substitution structure of methylene chloride

The microwave spectrum of methylene chloride has been reinvestigated in order to obtain a complete substitution (r_s) structure of well-defined precision. Measurements on the ¹³CH₂Cl₂ species have yielded the following rigid-rotor rotational constants: A = 30746.20 ± 0.10 MHz, B = 3320.63 ± 0.11 MHz, and C = 3053.44 ± 0.10 MHz. These data, combined with revised values reported earlier for other isotopic species, yields the following r_s structural parameters: CCl = 1.767 ± 0.002 Å, CH = 1.085 ± 0.002 Å, ∠HCH = 112.1 ± 0.2°, and ∠ClCCl = 112.2 ± 0.1°.     

The microwave spectrum,substitution structure,and dipole moment of thioketen,H2CCS

The microwave spectrum of the unstable thiocarbonyl thioketen, H₂CCS, has been investigated in the region 26.5–40 GHz. All singly substituted species as well as D₂CCS have been studied and the derived rotational constants yield the following structural parameters: $\text{r}{}_{\mathrm{<mtext>s</mtext>}}\text{(CS) = 1.554 ± 0.003}\text{A}\text{?}$, $\text{r}{}_{\mathrm{<mtext>s</mtext>}}\text{(CC) = 1.314 ± 0.003}\text{A}\text{?}$, $\text{r}{}_{\mathrm{<mtext>s</mtext>}}\text{(CH) = 1.090 ± 0.006}\text{A}\text{?}$, ∠_s(HCH) = 120.3 ± 0.5°. The dipole moment is μ = 1.02 ± 0.01 D. Four low frequency vibrational modes have been observed and their assignments are discussed.     

The microwave spectrum of P-cyanophosphaethene,CH2PCN

The new molecule P-cyanophosphaethene, produced by the pyrolysis of chlorocyanotrimethyl-silylmethylphosphane, Me₃SiCH₂P(CN)Cl, was detected by microwave spectroscopy. Analysis of the microwave spectrum gave rotational constants A₀ = 20 127.0 (16), B₀ = 4067.758 (11), C₀ = 3377.722 (11) MHz. Use of these values and an assumed geometry for the rest of the molecule provided estimates of 101.4(6)° for the (CPC) bond angle and 1.788(7) Å for the (PC) single bond length.     

Microwave spectra and barriers to internal rotation of trifluoroacetic acid and trifluoroacetyl fluoride

The rotational transitions of trifluoroacetic acid and trifluoroacetyl fluoride were identified with radiofrequency-microwave and microwave-microwave double resonance spectroscopy. Isotopic substitution of the hydrogen atom in trifluoroacetic acid showed that the hydrogen atom is cis with respect to the CO bond. A second conformation was not found. From A,E splittings in higher vibrational levels the internal rotation barriers were calculated: for trifluoroacetic acid, V₃ = 241.8 ± 0.5 cm^?1 (v = 4); for trifluoroacetyl fluoride V₃ = 383.6 ± 0.5 cm^?1 (v = 5).     

Fourier transform infrared spectrum of thioketene,CH2CS

The FTIR spectrum of the unstable species thioketene, CH₂CS, has been detected in a vapor-phase flow pyrolysis system. The region 800–3500 cm^?1 has been surveyed with a resolution of 1 cm^?1, enabling the frequencies of the six fundamentals which lie above 800 cm^?1 to be determined. Certain bands have been studied under very high resolution and the results of the analyses of the perpendicular bands ν₇ and ν₃ + ν₈, observed with a resolution of 0.01 and 0.005 cm^?1 respectively, are presented. The ground state constant, A₀, is determined as 286 453.60(58) cm^?1.     

Microwave spectrum of dimethyldichlorosilane

The microwave spectrum of dimethyldichlorosilane has been observed and the rotational constants and centrifugal distortion constants have been determined for ³⁵Cl₂ and ³⁵Cl³⁷Cl species. From these constants, the molecular structure is determined as $\text{r(}\text{SiCl}\text{) = 2.055 ± 0.003}\text{A}\text{?}$, $\text{r(}\text{SiC}\text{) = 1.845 ± 0.005}\text{A}\text{?}$, ∠ClSiCl = 107.2 ± 0.3°, ∠CSiC = 114.7 ± 0.3°. An analysis of the ³⁵Cl₂ quadrupole splittings leads to quadrupole coupling constants of χ_aa = ?19.6 ± 0.3 MHz, χ_bb = ?3.7 ± 1.4 MHz, χ_cc = 23.3 ± 1.4 MHz, χ_bond = ?38.0 ± 1.6 MHz, and η_bond = 0.22 ± 0.08.     

Deuteron chemical shift and EFG tensors in α-glycine

By computer fitting of resonance lines in deuteron spectra of perdeuterated crystals, deuteron chemical shifts in crystals are measured with an accuracy of ±0.3 ppm. Second-order quadrupole shifts must be taken into account. An important advantage of the method is that the measured chemical shifts and the shift tensors are automatically related to EFG tensors which can be assigned easily. The method is applied to α-glycine for which a shift anisotropy Δσ = 21 ± 9 ppm has been reported for the methylene protons. Full EFG and chemical-shift tensors are reported at both methylene deuteron sites, and motionally averaged tensors are reported for the ND₃⁺ sites. The shift anisotropies Δσ for the methylene deuterons are 4.7 and 9.4 ppm, respectively. Significant differences of the isotropic shifts and quadrupole coupling constants are also observed for these two deuterons. They are interpreted as evidence for the formation of a weak CH … O hydrogen bond in α-glycine.     

Centrifugal distortion and internal rotation analysis of the rotational spectra of N-methylmethanimine d0 and d5

The ground state millimeter-wave spectra of CH₃NCH₂ and CD₃NCD₂ have been measured. The rotational constants, centrifugal distortion constants, and barrier hindering internal rotation of the methyl group have been determined for both species. For the parent species I_α and ?(i,a) were also obtained, and for the perdeuteriated species the quadrupole coupling constants of ¹⁴N were determined.     

Microwave and photoelectron study of cis- and trans-isocyanato ethene,CH2CHNCO (vynyl isocyanate)

The microwave and photoelectron spectra of isocyanato ethene CH₂CHNCO have been studied. The microwave results indicate that the species is planar and possesses both a cis and a trans form. The appearance of dense and complicated vibrational satellite lines indicates that the molecule is quite flexible, a general property of molecules containing the isocyanate group. The rotational constants are:

$\text{cis: A}{}_0\text{= 20 146.8, B}{}_0\text{= 3107.267, C}{}_0\text{= 2689.513}\text{MHz}\text{; trans: A}{}_0\text{= 62 584.051, B}{}_0\text{= 2437.730, C}{}_0\text{= 2346.507}\text{MHz}$

These constants are shown to be consistent with structures in which $\text{r(}\text{CN}\text{) = 1.382 ± 0.005}\text{A}\text{?}$, ∠(CCN) = 122 ± 1° (for both conformers), and ∠(CNC) = 142.4 ± 0.5° (cis) and 138.4 ± 1.5° (trans). The dipole moments are μ(cis) = 2.120 ± 0.015 and μ(trans) = 2.207 ± 0.007 D. Several distinct peaks are observed in the photoelectron spectrum; however, the structure is not resolved into features belonging to the different isomers. The first ionization potential lies at 9.80 ± 0.1 eV. The spectrum has been assigned with the aid of theoretical calculations.     

Microwave spectrum,dipole moment,and structure of anti-vinyl alcohol

The rotational spectra of the anti conformer of vinyl alcohol (ethenol, H₂CCHOH) and its OD modification have been studied by microwave spectroscopy. The compounds have been generated by very-low-pressure pyrolyses of the appropriate isotopic species of 3-thietanol. In both cases the 25 measured μ_a- and μ_b-type transitions allowed the rotational constants and all five quartic centrifugal distortion constants to be determined. Stark effect measurements have yielded the electic dipole moment: μ_a = 0.547(2), μ_b = 1.702(1), and μ = 1.788(1) D. By relative intensity measurements it has been found that the vibrational ground state of the anti conformer lies 4.5±0.6 kJ mol^?1 above the syn conformer. In addition, ab initio calculations at the 6–31G⁷ level have been performed to obtain the structure, relative energy, and dipole moment of both rotamers.     

The rotation-inversion spectrum of ketenimine,H2CCNH

High-resolution measurements of μ_c-type transitions of ketenimine in the 4- to 7-GHz region revealed a splitting of each line caused by the inversion motion of the imino hydrogen. The inversion splitting amounts to 66 kHz. Data from ab initio molecular orbital calculations for the changes of the geometrical parameters during the inversion motion have been used to carry out semirigid bender calculations. A barrier height of 4700 ± 200 cm⁻¹ and an equilibrium HNC angle of 115.4 ± 0.6° have been derived. In addition, the microwave spectrum of the N-deuterated species has been assigned. The inversion splitting could not be resolved but the quadrupole coupling of the deuterium nucleus has been observed. Attempts to detect the inversion splitting in methanimine, H₂CNH, have been unsuccessful, suggesting a higher barrier than in ketenimine.     

Laser spectroscopy of sulfur dioxide: The ν1 band of 32S16O18O and the ν3 band of 32S18O2

The combination of data obtained from infrared-microwave double resonance with those from pure rotational spectroscopy has allowed a full set of constants for the ν₁ band of S¹⁶O¹⁸O to be determined. Tunable diode laser spectroscopy has been carried out on the ν₃ band of S¹⁸O₂ and 112 transitions were measured with a nominal accuracy of ±0.001 cm⁻¹. Band constants have been determined from these data.     

Microwave spectrum,centrifugal distortion constants,and quadrupole coupling constants of 3-chloropyridine

The microwave spectrum of 3-chloropyridine has been measured in the frequency region of 8.2 to 18 GHz. The rotational constants, centrifugal distortion constants, and the quadrupole coupling constants for the ³⁵Cl species are A = 5839.448 ± 0.027 MHz, B = 1604.152 ± 0.005 MHz, C = 1258.327 ± 0.004 MHz, Δ_J = 0.10 ± 0.01 KHz, Δ_JK = 0.36 ± 0.09 KHz, Δ_K = 1.18 ± 0.07 KHz, δ_J = ?0.008 ± 0.005 KHz, δ_K = 0.88 ± 0.20 KHz, χ_aa = ?70.04 ± 0.38 MHz, χ_bb = 36.68 ± 0.19 MHz. The values of rotational constants and quadrupole coupling constants for the ³⁷Cl species are A = 5840.052 ± 0.034 MHz, B = 1559.354 ± 0.01 MHz, C = 1230.739 ± 0.016 MHz, χ_aa = ?54.20 ± 1.26 MHz, χ_bb = 29.49 ± 0.48 MHz. The double bond character in the CCl bond is found to be 2%. The smaller than expected value of rotational constant A points to a “fattening” of the pyridine ring about the a-axis in contrast to 2-chloropyridine, where no such substitution effect was observed.     

Molecular structure of phosgene as studied by gas electron diffraction and microwave spectroscopy: The rs,rm, and re structures

The microwave spectra of eight isotopic species of COCl₂ have been observed, and the following rotational constants have been obtained: An analysis of the rotational constants has resulted in the r_s and r_m structures. The equilibrium structure, r_e, has been estimated by combining the r_m parameters derived according to Watson's method and the r_e bond distances estimated in our recent electron-diffraction and spectroscopic studies to be $\text{r}{}_{\mathrm{e}}\text{(CO}\text{) = 1.1756 ± 0.0023}\text{A}\text{?}$, $\text{r}{}_{\mathrm{e}}\text{(CCl}\text{) = 1.7381 ± 0.0019}\text{A}\text{?}$, ∠_eClCCl = 111.79 ± 0.24°.     

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.