High resolution infrared spectroscopy of [1.1.1]propellane: The region of the ν9 band

The region of the infrared-active band of the ν₉ CH₂ bending mode [1.1.1]propellane has been recorded at a resolution (0.0025 cm⁻¹) sufficient to distinguish individual rovibrational lines. This region includes the partially overlapping bands ν₉ (e′) = 1459 cm⁻¹, 2ν₁₈ (l = 2, E′) = 1430 cm⁻¹, ν₆ + ν₁₂ (E′) = 1489 cm⁻¹, and ν₄ + ν₁₅ (A₂″) = 1518 cm⁻¹. In addition, the difference band ν₄ − ν₁₅ (A₂″) was observed in the far infrared near 295 cm⁻¹ and analyzed to give good constants for the upper ν₄ levels. The close proximities of the four bands in the ν₉ region suggest that Coriolis and Fermi resonance couplings could be significant and theoretical band parameters obtained from Gaussian ab initio calculations were helpful in guiding the band analyses. The analyses of all four bands were accomplished, based on our earlier report of ground state constants determined from combination differences involving more than 4000 pairs of transitions from five fundamental and four combination bands. This paper presents the analyses and the determination of the upper state constants of all four bands in the region of the ν₉ band. Complications were most evident in the 2ν₁₈ (l = 2, E′) band, which showed significant perturbations due to mixing with the nearby 2ν₁₈ (l = 0, A₁′) and ν₄ + ν₁₂ (E′) levels which are either infrared inactive as transitions from the ground state, or, in the latter case, too weak to observe. These complications are discussed and a comparison of all molecular constants with those available from the ab initio calculations at the anharmonic level is presented.     

High resolution infrared spectroscopy of [1.1.1]propellane

The infrared spectrum of [1.1.1]propellane has been recorded at high resolution (0.002 cm⁻¹) with individual rovibrational lines resolved for the first time. This initial report presents the ground state constants for this molecule determined from the analysis of five of the eight infrared-allowed fundamentals ν₉(e′), ν₁₀(e′), ν₁₂(e′), , as well as of several combination bands. In nearly all cases it was found that the upper states of the transitions exhibit some degree of perturbation but, by use of the combination difference method, the assigned frequencies provided over 4000 consistent ground state difference values. Analysis of these gave for the parameters of the ground state the following values, in cm⁻¹: B₀ = 0.28755833(14), D_J = 1.1313(5) × 10⁻⁷, D_JK = −1.2633(7) × 10⁻⁷, H_J = 0.72(4) × 10⁻¹³, H_JK = −2.24(13) × 10⁻¹³, and H_KJ = 2.25(15) × 10⁻¹³, where the numbers in parentheses indicate twice the uncertainties in the last quoted digit(s) of the parameters. Gaussian ab initio calculations, especially with the computed anharmonic corrections to some of the spectroscopic parameters, assisted in the assignments of the bands and also provided information on the electron distribution in the bridge-head carbon-carbon bond.     

The vibrational spectrum of thiazole between 600 and 1400 cm revisited: A combined high-resolution infrared and theoretical study

The Fourier transform infrared gas-phase spectrum of thiazole, C₃H₃NS, has been recorded in the 600-1400 cm⁻¹ wavenumber region with a resolution around 0.0030 cm⁻¹. Nine fundamental bands (ν₅(A′) to ν₁₁(A′), ν₁₅(A″), and ν₁₆(A″)) are analysed employing the Watson model. Ground-state rotational and quartic centrifugal distortion constants as well as upper state spectroscopic constants have been obtained from the fits. A detailed analysis of perturbations identified in the ν₁₁(A′) band at 866.5 cm⁻¹ enables a definitive location of the very weak ν₁₀(A′) and ν₁₄(A″) bands at 879.3 and 888.7 cm⁻¹, respectively. The three levels are analysed simultaneously by a model including Coriolis resonance using an ab initio predicted first order c-Coriolis coupling constant; second and higher order Coriolis parameters are determined. Qualitative explanations in terms of Coriolis resonances are given for a number of crossings observed in ν₅(A′), ν₆(A′), and ν₇(A′) at 1383.7, 1325.8, and 1240.5 cm⁻¹, respectively. The rotational constants, anharmonic frequencies, and vibration-rotation constants (alphas, ) calculated by quantum chemical calculations using a cc-pVTZ and TZ2P basis with B3LYP methodology, have been compared with the present experimental data. The rotation constant differences for each vibrational state, from the ground state values, are closer to experiment from the TZ2P calculations relative to those using cc-pVTZ. The values for Δ_J, Δ_JK, Δ_K, δ_J, and δ_K are close to experiment with both basis sets.     

High-resolution FTIR measurement and analysis of the ν3 band of C2H3D

The Fourier transform infrared (FTIR) spectrum of the ν₃ band of C₂H₃D was measured at an unapodized resolution of 0.0063 cm⁻¹ in the 1240-1340 cm⁻¹ region. Rovibrational constants for the upper state (ν₃ = 1) up to five quartic and two sextic centrifugal distortion terms had been obtained by assigning and fitting a total of 1037 infrared transitions using a Watson’s A-reduced Hamiltonian in the I^r representation. The root-mean-square deviation of the fit was 0.00051 cm⁻¹. The ground state rovibrational constants were also determined by a fit of 674 combination differences together with 21 microwave frequencies from the present infrared measurements with a root-mean-square deviation of 0.00040 cm⁻¹. The upper state (ν₃ = 1) and ground state rovibrational constants of C₂H₃D represent the most accurate values obtained so far. The A-type ν₃ band, centred at 1288.788826 ± 0.000044 cm⁻¹ was found to be relatively free from local frequency perturbations. From the ν₃ = 1 rovibrational constants obtained, the inertial defect Δ₃ was 0.1619724 ± 0.0000001 μŲ.     

High resolution Fourier transform infrared spectra and analysis of the ν14, ν15 and ν16 bands of azetidine

Rotationally resolved vibrational spectra of the three lowest frequency bands of the four-membered heterocycle azetidine (c-C₃H₆NH) have been collected with a resolution of 0.00096 cm⁻¹ using the far infrared beamline at the Canadian Light Source synchrotron. The modes observed correspond principally to motions best described as: β-CH₂ rock (ν₁₄) at 736.701310(7) cm⁻¹, ring deformation (ν₁₅) at 648.116041(8) cm⁻¹, and the ring puckering mode (ν₁₆) at 207.727053(9) cm⁻¹. A global fit of 14 276 rovibrational transitions from the three bands provided an accurate set of ground state spectroscopic constants as well as excited state parameters for each of the three vibrational modes. The ground state structure was determined to be that of the puckered conformer having the NH bond in an equatorial arrangement.     

The ν12 band of C2D4

The Fourier transform infrared (FTIR) absorption spectrum of the ν₁₂ fundamental band of ethylene-d₄ (C₂D₄) was recorded in the 1017-1137 cm⁻¹ region with an unapodized resolution of 0.0063 cm⁻¹. Upper state (v₁₂ = 1) rovibrational constants consisting of three rotational and five quartic constants were improved by assigning and fitting 2103 infrared transitions using Watson’s A-reduced Hamiltonian in the I^r representation. The band centre of the A-type ν₁₂ band is found to be 1076.98480 ± 0.00002 cm⁻¹. The present analysis covering a wider wavenumber range and higher J and K_c values yielded upper state constants including the band centre which are more accurate than previously reported. The rms deviation of the upper state fit is 0.00045 cm⁻¹. Improved ground state rovibrational constants were also determined from the fit of 1247 ground state combination differences (GSCD) from the presently-assigned infrared transitions of the ν₁₂ band of C₂D₄. The rms deviation of the GSCD fit is 0.00049 cm⁻¹. In the rovibrational analysis, local frequency perturbations were not detected even at high J and K_a values. The calculated inertial defect Δ₁₂ is 0.32551 ± 0.00001 μŲ. The line intensities of the individual transitions in the ν₁₂ band were measured and the band strength of 39.8 ± 2.0 cm⁻² atm⁻¹ was derived for the ν₁₂ band of C₂D₄.     

High-resolution infrared analysis of the ν7 band of cis-ethylene-d2 (cis-C2H2D2)

The spectrum of the ν₇ band of cis-ethylene-d₂ (cis-C₂H₂D₂) has been recorded with an unapodized resolution of 0.0063 cm⁻¹ in the 740-950 cm⁻¹ region using a Bruker IFS 125 HR Fourier transform infrared spectrometer. By fitting 2186 infrared transitions of ν₇ with a standard deviation of 0.00060 cm⁻¹ using a Watson’s A-reduced Hamiltonian in the I^r representation, accurate rovibrational constants for ν₇ = 1 state have been derived. The band center of ν₇ has been found to be 842.20957 ± 0.00004 cm⁻¹. In a simultaneous fit of 1331 infrared ground state combination differences from the present ν₇ transitions, together with 22 microwave frequencies, ground state constants have been improved. The rms deviation of the ground state fit was 0.00027 cm⁻¹.     

The ν12 band of ethylene-1-C (CCH4) by high-resolution FTIR spectroscopy

The Fourier transform infrared (FTIR) spectrum of the ν₁₂ fundamental band of ethylene-1-¹³C (or ¹³C¹²CH₄) was recorded with an unapodized resolution of 0.0063 cm⁻¹ in the wavenumber region of 1360-1520 cm⁻¹. Rovibrational constants for the upper state (ν₁₂ = 1) up to five quartic and two sextic centrifugal distortion terms were derived for the first time by assigning and fitting a total of 879 infrared transitions using a Watson’s A-reduced Hamiltonian in the I^r representation. The root-mean-square deviation of the fit was 0.00066 cm⁻¹. The ground state rovibrational constants were also determined by a fit of 523 combination-differences from the present infrared measurements, with a rms deviation of 0.00090 cm⁻¹. The A-type ν₁₂ band which is centred at 1439.34607 ± 0.00004 cm⁻¹ was found to be relatively free from local frequency perturbations. From the ν₁₂ = 1 rovibrational constants obtained, the inertial defect Δ₁₂ was found to be 0.242826 ± 0.000002 μŲ.     

Improved rovibrational constants for the ν12 band of C2H3D

The Fourier transform infrared absorption spectrum of the ν₁₂ fundamental band of ethylene-d (C₂H₃D) was recorded at an unapodized resolution of 0.0063 cm⁻¹ in the 1330-1475 cm⁻¹ region. Upper state (ν₁₂ = 1) rovibrational constants inclusive of three rotational, five quartic, and four sextic centrifugal distortion constants were improved by assigning and fitting 1444 infrared transitions using Watson’s A-reduced Hamiltonian in the I^r representation. The present analysis yielded more higher-order upper state constants than previously reported. The rms deviation of the fit is 0.00055 cm⁻¹. Improved ground state rovibrational constants were also determined from the combined fit of 2026 ground state combination differences (GSCD) from the assigned infrared transitions of the ν₁₂, ν₃ and ν₆ bands and 21 microwave frequencies of C₂H₃D. The rms deviation of the GSCD fit is 0.00047 cm⁻¹. The A-type ν₁₂ band is centered at 1400.76262 ± 0.00004 cm⁻¹. Local frequency perturbations were not detected in the analysis. The calculated inertial defect Δ₁₂ is 0.20809 ± 0.00003 μŲ.     

The high-resolution infrared spectrum of pyrrole between 900 and 1500 cm revisited

The Fourier transform gas-phase infrared spectrum of pyrrole, C₄H₅N, has been recorded with a resolution of ca. 0.003 cm⁻¹ in the 900-1500 cm⁻¹ spectral region. Four fundamental bands, ν₈(A₁; 1016.9 cm⁻¹), ν₂₃(B₂; 1049.1 cm⁻¹), ν₇(A₁; 1074.6 cm⁻¹), ν₂₀(B₂; 1424.4 cm⁻¹) and the overtone band 2ν₁₆(A₁; 962.7 cm⁻¹) have been analysed using the Watson model. The ν₈ and 2ν₁₆ bands are unperturbed; the ν₇ and ν₂₃ bands are locally perturbed, while the ν₂₀ band is globally perturbed by weak c-Coriolis resonance. Upper state vibrational term values, and rotational and centrifugal distortion constants, have been obtained from fits using S-reduction and I^r-representation as well as A-reduction and III^r-representation. A set of ground state rotational and centrifugal distortion constants using A-reduction was obtained from a simultaneous fit of ground state combination differences from all five bands and previous microwave and millimetre-wave data.     

High resolution analysis of the ethylene-1-C spectrum in the 8.4-14.3-μm region

Fourier transform spectra of mono-¹³C ethylene have been recorded in the 8.4-14.3-μm spectral region (700-1190 cm⁻¹) using a Bruker 120 HR interferometer at a resolution of 0.0017 cm⁻¹ allowing the extensive study of the set of resonating states {10¹, 8¹, 7¹, 4¹, 6¹}. Due to the high resolution available as well as the extended spectral range involved in this study, a much larger set of line assignments are now available. The present analysis has lead to the determination of more accurate spectroscopic constants, including interaction constants, than were obtained in earlier studies. In particular, the following band centers were derived: ν₀(ν₁₀) = 825.40602(30) cm⁻¹, ν₀(ν₈) = 932.19572(15) cm⁻¹, ν₀(ν₇) = 937.44452(10) cm⁻¹, ν₀(ν₄) = 1025.6976(14) cm⁻¹. Finally a synthetic spectrum was generated leading to the assignment of a number of ¹³C¹²CH₄ lines observed in an earlier heterodyne spectroscopic study.     

High-resolution infrared spectra of the two nonpolar isomers of 1,4-difluorobutadiene

High-resolution (0.0013 cm⁻¹) infrared spectra have been recorded for trans,trans-1,4-difluorobutadiene (ttDFBD) and cis,cis-1,4-difluorobutadiene (ccDFBD). The rotational structure in two C-type bands (ν₁₀ and ν₁₂) and one A-type band (ν₂₂) for ttDFBD and in two C-type bands (ν₁₁ and ν₁₂) for ccDFBD has been analyzed. Ground state and upper state rotational constants, except for ν₁₀ of ttDFBD, have been fitted. Band centers are 934.1 cm⁻¹ (ν₁₀), 227.985 cm⁻¹ (ν₁₂), and 1087.919 cm⁻¹ (ν₂₂) for ttDFBD. Band centers are 762.891 cm⁻¹ (ν₁₁) and 327.497 cm⁻¹ (ν₁₂) for ccDFBD. The small inertial defects in the ground state confirm that both isomers are planar. Obtaining the ground state rotational constants for the two isomers of DFBD is a first step toward determining their semi-experimental equilibrium structures.     

The high-resolution infrared spectrum of isoxazole vapor between 800 and 1700 cm

The Fourier transform gas-phase IR spectrum of isoxazole, C₃H₃NO, between 550 and 1700 cm⁻¹ was measured with a resolution of ca. 0.003 cm⁻¹. Ten fundamental bands in the region 800-1700 cm⁻¹ have been analyzed by the Watson Hamiltonian model to yield upper state spectroscopic constants. A number of local resonances have been identified in the bands and explained qualitatively, and the unobserved ν₁₄(A″) fundamental band has been located at 897.5(5) cm⁻¹ from its perturbation effects on the neighboring fundamentals.     

The high-resolution FTIR spectrum of the ν6 band of C2H3D

The absorption spectrum of the ν₆ band of C₂H₃D centered near 1125.27674 cm⁻¹ in the 1100-1250 cm⁻¹ region was recorded with an unapodized resolution of 0.0063 cm⁻¹ using a Fourier transform infrared (FTIR) spectrometer. A total of 947 infrared transitions of the A-B hybrid-type band were assigned and fitted to upper-state (ν₆ = 1) rovibrational constants using a Watson’s A-reduced Hamiltonian in the I^r representation up to eighth-order centrifugal distortion terms. The b-type infrared transitions of the band were analyzed for the first time. The root-mean-square deviation of the fit was 0.00062 cm⁻¹. The ground-state rovibrational constants up to eighth-order terms were also obtained by a fit of 617 combination differences from the present infrared measurements, simultaneously with 21 microwave frequencies with a root-mean-square deviation of 0.00055 cm⁻¹. From this work, the upper-state (ν₆ = 1) and ground-state constants of C₂H₃D were derived with the highest accuracy, so far. The a- and b-type transitions of the hybrid ν₆ band were found to be relatively free from local frequency perturbations. The ratio of the a- to b-type vibrational dipole transition moments (μ_a/μ_b) was found to be 1.05 ± 0.10. From the ν₆ = 1 rovibrational constants obtained, the inertial defect Δ₆ was calculated to be 0.3570 ± 0.0008 μŲ.     

Analysis of rotational structure in the high-resolution infrared spectrum and assignment of vibrational fundamentals of butadiene-2,3-C2

The 2,3-¹³C₂ isotopomer of butadiene was synthesized, and its fundamental vibrational fundamentals were assigned from a study of its infrared and Raman spectra aided with quantum chemical predictions of frequencies, intensities, and Raman depolarization ratios. For two C-type bands in the high-resolution (0.002 cm⁻¹) infrared spectrum, the rotational structure was analyzed. These bands are for ν₁₁ (a_u) at 907.17 cm⁻¹ and for ν₁₂ (a_u) at 523.37 cm⁻¹. Ground state and upper state rotational constants were fitted to Watson-type Hamiltonians with a full quartic set of centrifugal distortion constants and two sextic ones. For the ground state, A₀ = 1.3545088(7) cm⁻¹, B₀ = 0.1469404(1) cm⁻¹, and C₀ = 0.1325838(2)  cm⁻¹. The small inertial defects of butadiene and two ¹³C₂ isotopomers, as well as for five deuterium isotopomers as previously reported, confirm the planarity of the s-trans rotamer of butadiene.     

The high-resolution, jet-cooled infrared spectrum of pentafluoroethane

The jet-cooled spectrum of pentafluoroethane (C₂HF₅) has been recorded between 1100 and 1325 cm⁻¹ at a resolution of 0.0022 cm⁻¹. A rotational temperature of approximately 10 K was achieved by expanding 50 Torr of C₂HF₅ in 500 Torr of helium. Transitions belonging to five different fundamental vibrations have been assigned and fit to a Watson Hamiltonian: the ν₃ band at 1309.880494(189) cm⁻¹, ν₄ at 1200.734645(67) cm⁻¹, ν₅ at 1142.78147(33) cm⁻¹, ν₁₃ at 1223.334098(115) cm⁻¹, and ν₁₄ at 1147.394185(163) cm⁻¹. The fit of the ν₄ band has an rms deviation of 0.000436 cm⁻¹ compared to the uncertainty in the experimental line position of 0.0002 cm⁻¹. Satisfactory fits were achieved for the other four bands (ν₃, ν₅, ν₁₃, ν₁₄) at this cold temperature, with most of the centrifugal distortion constants fixed at the ground state values. Joint fits with previous work were attempted for the ν₄ and ν₁₃, successfully in the former case and unsuccessfully in the latter.     

The high-resolution infrared spectrum of BF3 from 400 to 1650 cm

High-resolution infrared spectra of boron trifluoride, enriched to 99.5 at. % ¹¹B, have been measured from 400 to 1650 cm⁻¹. In that region we have identified and analyzed 16 absorption bands attributed to the three fundamental bands, two combination bands, 10 hot bands, and one difference band. All possible states were accessed in this region through direct transitions either from the ground state or as hot bands from thermally populated levels. The spectral resolution of the measurements varied from 0.0015 to 0.0020 cm⁻¹. An improved set of ground state rotational constants and rovibrational constants for the infrared-active fundamental vibrations have been determined from over 32 000 assigned transitions. This study resulted in the first direct characterization of the infrared-inactive ν₁ state of ¹¹BF₃ leading to values for ν₁, , and of 885.843205(24), 0.000678548(53), and 0.000337564(66) cm⁻¹, respectively. The Fermi resonance perturbation between the E′ states ν₃ and 3ν₄ (l = ±1) was further elucidated by observation of hot band transitions to both the 3ν₄ (l = ±1) and 3ν₄ (l = ±3) states. Several other resonances were also found including the weak rotational interaction, between the state 2ν₂ and the E′ state of ν₁ + ν₄.     

Infrared diode laser spectroscopy of the He-N2O van der Waals complex in the 1285 cm region

The rovibrational spectrum of the He-N₂O van der Waals complex has been recorded in the N₂O-monomer ν₁ region (∼1285 cm⁻¹) using an infrared tunable diode laser spectrometer in conjunction with a free supersonic jet expansion and an astigmatic multi-pass absorption cell. Twenty-two lines are assigned to the ν₂-band of He-N₂O. Rotational constants for the ν₂-excited state are derived. The band-origin of the spectrum is determined to be ν₀ = 1285.0666(7) cm⁻¹ and shows a blue-shift of 0.1633(8) cm⁻¹ compared with the N₂O ν₁-band origin.     

A reevaluation of the assignment of the vibrational fundamentals and the rotational analysis of bands in the high-resolution infrared spectra of trans- and cis-1,3,5-hexatriene

Assignments of the vibrational fundamentals of cis- and trans-1,3,5-hexatriene are reevaluated with new infrared and Raman spectra and with quantum chemical predictions of intensities and anharmonic frequencies. The rotational structure is analyzed in the high-resolution (0.0013-0.0018 cm⁻¹) infrared spectra of three C-type bands of the trans isomer and two C-type bands of the cis isomer. The bands for the trans isomer are at 1010.96 cm⁻¹ (ν₁₄), 900.908 cm⁻¹ (ν₁₆), and 683.46 cm⁻¹ (ν₁₇). Ground state (GS) rotational constants have been fitted to the combined ground state combination differences (GSCDs) for the three bands of the trans isomer. The bands for the cis isomer are at 907.70 cm⁻¹ (ν₃₃) and 587.89 cm⁻¹ (ν₃₅). GS rotational constants have been fitted to the combined GSCDs for the two bands of the cis isomer and compared with those obtained from microwave spectroscopy. Small inertial defects in the GSs confirm that both molecules are planar. Upper state rotational constants were fitted for all five bands.     

High-resolution infrared and theoretical study of the fundamental bands ν6, ν7, ν9 and ν13 of 1,2,3-thiadiazole

The Fourier transform gas-phase IR spectrum of 1,2,3-thiadiazole, C₂H₂N₂S, has been recorded with a resolution of ca. 0.003 cm⁻¹ in the 700-1100 cm⁻¹ spectral region. Four fundamental bands ν₆(A^/; 1101.8 cm⁻¹), ν₇(A^/; 1038.8 cm⁻¹), ν₉(A^/, 858.9 cm⁻¹), and ν₁₃(A^//; 746.2 cm⁻¹) have been analyzed using the Watson model in A-reduction. Two additional bands, ν₈ (A^/; 894.6 cm⁻¹) and ν₁₂(A^//; 881.2 cm⁻¹) were assigned by their weak Q-branches. Ground state rotational and quartic centrifugal distortion constants as well as upper state spectroscopic constants have been obtained from fits. A number of weak global and local interactions are present in the bands. The resonances identified were qualitatively explained by Coriolis type perturbations with neighboring levels. Ground state rotational and quartic centrifugal distortion constants, anharmonic frequencies, and vibration-rotational α-constants predicted by quantum chemical calculations using a cc-pVTZ basis and B3LYP methodology, have been compared with the present experimental data, where there is generally good agreement.