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°.     

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)°.     

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.     

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.     

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.     

The Electric Dipole and the Nuclear Quadrupole Moment of Methyl Bromide in the Ground Vibrational State

A set of high-resolution Stark measurements at millimeter-wave frequencies is reported for CH₃⁷⁹Br and CH₃⁸¹Br. These results are analyzed together with previous data available in the literature to find new sets of molecular (rotational, hyperfine, and dipole moments) constants for both isotopic species. A particular improvement is obtained in the evaluation of the dipole moments, whose values are μ(79) = 1.82171 (26) 0, μ(8l) = 1.82185 (26) D, and for the hyperfine-structure parameters, which are estimated by assuming the most recent model for centrifugal distortion (M. R. Aliev and J. T. Hougen, J. Mol. Spectrosc.106, 110-123 (1984)), obtaining eqQ⁷⁹₀ = 577.1088 (57) MHz, χ⁷⁹_J = −0.63 (16) kHz, χ⁷⁹_K = 12.6 (16) kHz and eqQ⁸¹₀ = 482.1030 (94) MHz, χ⁸¹_J = −0.57 (17) kHz, χ⁸¹_K = 9.3 (22) kHz.     

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.     

Geometry,strength of binding and Br2 charge redistribution in the complex OC ··· Br2 determined by rotational spectroscopy

The ground-state rotational spectra of the six isotopomers ¹⁶O¹²C ··· ⁷⁹Br⁷⁹Br, ¹⁶O¹²C ··· ⁸¹Br⁷⁹Br, ¹⁶O¹²C ··· ⁸¹Br⁸¹Br, ¹⁶O¹²C ··· ⁷⁹Br⁸¹Br, ¹⁶O¹³C ··· ⁷⁹Br⁷⁹Br, ¹⁶O¹³C ··· ⁸¹Br⁷⁹Br, were observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. The spectroscopic constants B _O, D _J, χ _aa (Br_i), χ _aa (Br_o), M_bb (Br_i) and M _bb (Br_o), where i = inner and o = outer, were determined for each isotopomer. The complex is linear, with the weak bond between the C atom of CO and Br_i. The rotational constants were used to determine the distance r(C ··· Br_i) = 3.1058Å and to show that the Br—Br bond lengthens by ～0.005–0.01Å on complex formation. The intermolecular stretching force constant kσ = 5.0 Nm^?1 was obtained from D_J and the Br nuclear quadrupole coupling constants were interpreted to reveal that a fraction δ = 0.02 of an electronic charge is transferred from Br_i to Br_o when Br₂ is subsumed into the complex. Properties of the two series OC ··· XY and H₃N ··· XY, where XY = C1₂, Br₂ and BrC1, are compared.     

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, 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.     

Submillimeter spectrum of methyl bromide (CH3Br)

Methyl bromide is a ubiquitous component of the atmosphere, but has yet to be remotely detected in the upper atmosphere. Due to the strong ozone depletion capability of the activated bromine species, the total atmospheric bromine load needs to be carefully monitored. Combined analysis of precise measurements and cataloging of the rotational spectrum of methyl bromide may enable its concentration to be monitored with future remote sensing instrumentation. In an effort to extend and improve previous work for this molecule, the spectrum of CH₃Br has been measured at JPL. Using an isotopically enriched ¹³CH₃Br (90%) sample, spectra have been recorded from 750 to 1200 GHz. Quantum number assignments cover the CH₃⁷⁹Br, CH₃⁸¹Br, ¹³CH₃⁷⁹Br and ¹³CH₃⁸¹Br isotopologues with J < 66 and K < 17 for the ground and ν₃ vibrational states. The dataset for the ¹²C isotopologues is more precise than previous THz measurements resulting in reductions of rotational and distortion parameter uncertainties by factors of 2-15. Parameters of the ν₃ state of the ¹²C isotopologues are improved by 2-105. The spectra of the ¹³C isotopologues are the first reported beyond J = 2.     

Rotational spectra of isotopic species of SO2F2: experimental and ab initio structures

The rotational spectra of ³⁴SO₂F₂ and S¹⁸O¹⁶OF₂ have been measured in their ground vibrational state between 9 and 110 GHz. Accurate rotational constants have been derived. Various experimental structures including the average structure have been determined. The ab initio structure has been calculated at the CCSD(T) level of theory. The different structures are compared and the best equilibrium structure is the ab initio structure: r_e(SO)=1.401 (3) Å, r_e(SF)=1.532 (3) Å, ∠_e(OSO)=124.91(20)°, ∠_e(FSF)=95.53 (20)°.     

Fundamental vibrational frequencies and spectroscopic constants for the methylperoxyl radical,CH3O2, and related isotopologues 13CH3OO,CH3 18O18O,and CD3OO

Accurate spectroscopic and geometric constants for CH₃O₂, and its isotopologues ¹³CH₃OO, CH₃ ¹⁸O¹⁸O and CD₃OO, are predicted. Employing coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)], we obtain optimized equilibrium geometries using Dunning's cc-pVTZ basis set. A Taylor expansion of the potential energy surface, including all third-order and semidiagonal fourth-order terms in a basis of normal coordinates, yields anharmonic vibrational frequencies and vibrationally-averaged properties including the effects of anharmonicity. We detail the strong influence of Fermi resonances on the problematic ν₆ vibrational mode of CD₃OO, arriving at a value of 993?cm^?1; two previous experimental measurements of this mode appear to have been incorrectly assigned. Our computed energies for the low intensity ν₁₁ transition are in excellent agreement with experimental measurements performed for CH₃ ¹⁸O¹⁸O and CD₃OO, inspiring confidence that our results will serve as a guide for experimental measurement of this yet-unobserved quantity for the CH₃OO and ¹³CH₃OO isotopologues. Given the reliability of our force field, and considering the results of other experiments, we make a number of reassignments to previously recorded spectra, which eliminate large disagreements between experimental observations. The vibrational averaging of the rotational constants and geometries are also discussed for each isotopologue.     

The Equilibrium Structure of Silyl Fluoride

The experimental equilibrium structure of silyl fluoride has been determined using new sets of accurate rotational constants that have recently been obtained by taking into account the most important interactions between the excited vibrational states. The equilibrium structure has also been calculated at the CCSD(T) level of theory with the cc-pVQZ+1 basis set (including corrections for the core correlation). The anharmonic force field up to semidiagonal quartic terms has been calculated at the MP2 level of theory and the equilibrium structure has been derived from the experimental rotational constants and the ab initio rovibrational interaction parameters. Finally, the average structure of both ²⁸SiH₃F and ²⁸SiD₃F has been reevaluated and used to derive the equilibrium structure. These structures are compared and the experimental structure is found to be in slight disagreement with the other ones. The preferred structure is obtained by calculating the median value of the different structures. The results are r_e(SiF)=1.5907 (9) Å, r_e(SiH)=1.4696 (13) Å, ∠_e(HSiF)=108.32(15)°, and ∠_e(HSiH)=110.60(14)°.     

New analysis of the Coriolis-interacting v2 and v5 bands of CH3Br and CH3Br

The and fundamental bands of CH₃⁷⁹Br and CH₃⁸¹Br have been studied by Fourier transform infrared spectroscopy with an unapodized resolution of 0.004 cm⁻¹, corresponding to an improvement of one order of magnitude compared to previous studies. For both isotopomers, some 2427 (2239) lines were newly assigned for the parallel and the perpendicular bands and, in addition, 80 perturbation-allowed transitions were also added. The ground-state axial rotational constants A₀ were redetermined from allowed and perturbation-allowed infrared transitions observed in the v₂ and v₅ bands around the local crossing. The A₀ values obtained for both isotopomers are more accurate but fully compatible with those obtained previously. Using those results, and the variation of the rotational constants with vibration, new accurate equilibrium constants A_e and B_e have been also determined for CH₃⁷⁹Br and CH₃⁸¹Br. The excited states v₂=1 and v₅=1 are coupled by Coriolis-type interactions (Δl=±1,ΔK=±1) and (Δl=?1,ΔK=±2), while the l₅=±1 levels of v₅ interact also through “l(2,2)”-type interaction (Δl=±2,ΔK=±2). The Coriolis coupling term was determined to be for CH₃⁷⁹Br and for CH₃⁸¹Br. All interaction parameters have been determined with higher accuracy, compared to previous studies. A total of 4213 (3704) line positions with J?68(64) and K?16(11) including all available data was fitted using 20 (18) parameters with a root-mean-square deviation of 0.0007 (0.0006) cm⁻¹ for CH₃⁷⁹Br and CH₃⁸¹Br, respectively. Two different but equivalent forms of reduced Hamiltonians with two different sets of constrained constants were successfully applied according to Lobodenko's reduction [J. Mol. Spectrosc. 126 (1987) 159]. The ratio of the transition moments, |d₂/d₅|=1.65, and a positive sign of the Coriolis intensity perturbation d₂×ζ₂₅×d₅ were determined. Therefore, it has been possible to generate an accurate prediction of the whole spectrum between 1200 and 1650 cm⁻¹, including Q branches.     

Variation of Br quadrupole coupling with isotopic substitution in methyl bromide

The J = 0 ← 1 transitions in CH₃⁷⁹Br (I), CH₃⁸¹Br (II), CD₃⁷⁹Br (III), and CD₃⁸¹Br (IV) were measured using a Stark-cell spectrometer constructed from C-band waveguide. High-resolution spectra yielded precise values for the bromine quadrupole coupling strength. Values obtained were eqQ(I) = ?577.08 ± 0.15 MHz, eqQ(II) = ?482.18 ± 0.15 MHz, eqQ(III) = ?575.66 ± 0.15 MHz, and eqQ(IV) = ?480.89 ± 0.15 MHz. The observed center frequencies for the J = 0 ← 1 transitions are ν₀(I) = 19136.35 ± 0.03 MHz, ν₀(II) = 19063.62 ± 0.03 MHz, ν₀(III) = 15429.23 ± 0.03 MHz, and ν₀(IV) = 15362.41 ± 0.03 MHz. A 0.26 ± 0.02% decrease in bromine quadrupole coupling is observed when the methyl group is fully deuterated. This is in agreement with, and supports interpretations given for, previous results on methyl chloride.     

The a-Type K = 0 Microwave Spectrum of the Methanol Dimer

The rotational spectrum of (CH₃OH)₂ has been observed in the region 4-22 GHz with pulsed-beam Fabry-Perot cavity Fourier-transform microwave spectrometers at NIST and at the University of Kiel. Each a-type R(J), K_a = 0 transition is split into 15 states by tunneling motions for (CH₃OH)₂, (¹³CH₃OH)₂, (CH₃OD)₂, (CD₃OH)₂, and (CD₃OH)₂. The preliminary analysis of the methyl internal rotation presented here was guided by the previously developed multidimensional tunneling theory which predicts 16 tunneling components for each R(J) transition from 25 distinct tunneling motions. Several isotopically mixed dimers of methanol have also been measured, namely ¹³CH₃OH, CH₃OD, CD₃OH, and CD₃OD bound to ¹²CH₃OH. Since the hydrogen bond interchange motion (which converts a donor into an acceptor) would produce a new and less favorable conformation from an energy viewpoint, it does not occur and only 10 tunneling components are observed for these mixed dimers. The structure of the complex is similar to that of water dimer with a hydrogen bond distance of 2.035 Å and a tilt of the acceptor methanol of 84° from the O-H-O axis. The effective barrier to internal rotation for the donor methyl group of (CH₃OH)₂ is ν₃ = 183.0 cm⁻¹ and is one-half of the value for the methanol monomer (370 cm⁻¹), while the barrier to internal rotation of the acceptor methyl group is 120 cm⁻¹.     

Submillimeter-wave spectroscopy of HN2 and DN2 in the excited vibrational states

The pure rotational transitions of HN₂⁺ and DN₂⁺ in the first excited vibrational states for all the fundamental vibrational modes have been observed in the range of 300-750 GHz. The molecular constants determined are much more accurate compared with those obtained from the infrared spectroscopy. The equilibrium rotational constants, B_e = 46832.45 (71) MHz for HN₂⁺ and B_e = 38708.38 (58) MHz for DN₂⁺, have been determined by correcting for the higher-order vibration-rotation interaction effects, γ_ij, obtained by an infrared investigation. The equilibrium bond lengths are derived from these equilibrium rotational constants: r_e(H-N) = 1.03460 (14) Å and r_e (N-N) = 1.092698 (26) Å.     

The Coriolis interaction between the fundamentals ν2 and ν5 of CD3Br

The bands ν₂ and ν₅ of CD₃Br have been measured at a resolution of 0.010 cm^?1. They were analyzed simultaneously by taking into account the xy-Coriolis interaction. More than 1600 transitions were assigned for each isotopic species CD₃⁷⁹Br and CD₃⁸¹Br. The Coriolis coupling term proved to be ζ_2,5^y = 0.559. The band centers are (in cm^?1) ν₂: 991.401 (CD₃⁷⁹Br), 991.390 (CD₃⁸¹Br); ν₅: 1055.474 (CD₃⁷⁹Br), 1055.471 (CD₃⁸¹Br).     

The microwave spectrum and structure of carbonyl fluoride

The ground state microwave rotational spectra of four isotopic species of carbonyl fluoride have been measured between 18 GHz and 77 GHz, and analyzed to obtain the quartic and some sextic centrifugal constants. The rotational constants have been used to obtain a r₀ structure and, using a harmonic force field, a r_z structure is also obtained: r_z(C---F) = 1.3166 (10) Å, r_z(C---O) = 1.1700 (26) Å.These values are considerably more precise than those of the previously estimated average structure.