High-resolution infrared spectra of bicyclo[1.1.1]pentane

Infrared spectra of bicyclo[1.1.1]pentane (C₅H₈) have been recorded at a resolution (0.0015 cm⁻¹) sufficient to resolve for the first time individual rovibrational lines. This initial report presents the ground state constants for this molecule determined from the detailed analysis of three of the ten infrared-allowed bands, ν₁₄(e′) at 540 cm⁻¹, ν₁₇ (a₂″) at 1220 cm⁻¹, ν₁₈(a₂″) at 832 cm⁻¹, and a partial analysis of the ν₁₁(e′) band at 1237 cm⁻¹. The upper states of transitions involving the lowest frequency mode, ν₁₄(e′), show no evidence of rovibrational perturbations but those for the ν₁₇ and ν₁₈ (a₂″) modes give clear indication of Coriolis coupling to nearby e′ levels. Accordingly, ground state constants were determined by use of the combination-difference method for all three bands. The assigned frequencies provided over 3300 consistent ground state difference values, yielding the following constants for the ground state (in units of cm⁻¹): B₀ = 0.2399412(2), D_J = 6.024(6) × 10⁻⁸, D_JK = −1.930(21) × 10⁻⁸. For the unperturbed ν₁₄(e′) fundamental, more than 3500 transitions were analyzed and the band origin was found to be at 540.34225(2) cm⁻¹. The numbers in parentheses are the uncertainties (two standard deviations) in the values of the constants. The results are compared with those obtained previously for [1.1.1]propellane and with those computed at the ab initio anharmonic level using the B3LYP density functional method with a cc-pVTZ basis set.     

High-resolution infrared and theoretical study of gaseous oxazole in the 600-1400 cm region

The Fourier transform gas-phase IR spectrum of oxazole, C₃H₃NO, has been recorded with a resolution of ca. 0.0030 cm⁻¹ in the wavenumber region 600-1400 cm⁻¹. The rotational structures of 10 fundamental bands (four of a-type, three of b-type and three of c-type) have been analysed using 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 number of perturbations have been identified in the bands. From a local crossing observed in ν₁₅ we located the very weak ν₁₄ band at 858.19(1) cm⁻¹. Also ν₁₃ is definitively located at 899.3 cm⁻¹. The three global c-Coriolis interacting dyads ν₉/ν₁₀, ν₁₀/ν₁₁, and ν₁₂/ν₁₃ have each been analysed by a model including first and second order Coriolis resonance using ab initio predicted first order Coriolis coupling constants; second order Coriolis interaction parameters are determined. The rotational constants, harmonic and anharmonic frequencies, intensities, and vibration-rotation constants (alphas, ) have been predicted by quantum chemical calculations using a cc-pVTZ basis at the MP2 and B3LYP methodology levels, and compared with the present experimental data. Both the rotational constants and frequencies are marginally closer to experiment from the B3LYP calculations. In order to make more significant comparisons between theory and experiment for the alphas, we take differences between ground and vibronic state values; under these circumstances, the B3LYP definitely have a closer fit to experiment.     

The high-resolution infrared spectrum of thiophene between 600 and 1200 cm: A spectroscopic and theoretical study of the fundamental bands ν6, ν7, ν13, and the c-Coriolis interacting dyad ν5, ν19

The Fourier transform infrared spectrum of gaseous thiophene, C₄H₄S, has been recorded in the 600-1200 cm⁻¹ spectral region with a resolution of ca. 0.0030 cm⁻¹. Five fundamental bands ν₁₃ (B₁, 712.1 cm⁻¹), ν₇ (A₁; 840.0 cm⁻¹), ν₆ (A₁; 1036.4 cm⁻¹), ν₅ (A₁; 1081.5 cm⁻¹) and ν₁₉ (B₂; 1084.0 cm⁻¹) have been analysed by the standard Watson model (A-reduction). Ground state rotational and quartic centrifugal distortion constants have been obtained from a simultaneous fit of ground state combination differences from four of these bands and previous microwave transitions. Upper state spectroscopic constants have been obtained for all five bands from single band fits using the Watson model. A strong c-Coriolis resonance perturbs the close lying ν₅ and ν₁₉ bands. We have analysed this dyad system by a model including first and second order Coriolis resonance using the theoretically predicted Coriolis coupling constant . From this analysis we locate the previously unobserved ν₁₉ band at 1083.969 cm⁻¹. The rotational constants, ground state quartic centrifugal distortion constants, anharmonic frequencies, and vibration-rotational constants (α-constants) predicted by quantum chemical calculations using a cc-pVTZ basis with B3LYP methodology, are compared with the present experimental data, where there is generally good agreement. A complete set of anharmonic frequencies and α-constants for all fundamental levels of the molecule is given.     

High-resolution infrared and theoretical study of the fundamental bands ν2, ν4 and ν9 and the c-Coriolis interacting dyad ν5, ν14 of 1,3,4-thiadiazole

The Fourier transform gas-phase IR spectrum of 1,3,4-thiadiazole, C₂H₂N₂S, has been recorded with a resolution of ca. 0.003 cm⁻¹ in the 800-1500 cm⁻¹ spectral region. Five fundamental bands ν₂(A₁; 1391.9 cm⁻¹), ν₄(A₁; 964.4 cm⁻¹), ν₅(A₁; 894.6 cm⁻¹), ν₉(B₁; 821.5 cm⁻¹), and ν₁₄(B₂; 898.4 cm⁻¹) have been analysed using the Watson model. Ground state rotational and quartic centrifugal distortion constants as well as upper state spectroscopic constants have been obtained from fits. The ν₄ and ν₉ bands are unperturbed while a strong c-Coriolis resonance perturbs the close-lying ν₅ and ν₁₄ bands. This dyad system has been analysed by a model including first and second order c-Coriolis resonance using the theoretically predicted Coriolis coupling constant . The ν₂ band is strongly perturbed by a local resonance, and we obtain a set of spectroscopic parameters using a model including second order a-Coriolis resonance with the inactive ν₁₀ + ν₁₄ band. 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.     

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.     

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 and theoretical study of gaseous 1,2,5-selenadiazole in the 600-1400 cm range

The Fourier transform gas-phase IR spectrum of natural isotopic 1,2,5-selenadiazole, C₂H₂N₂Se, has been recorded with a resolution of ca. 0.0025 cm⁻¹ in the wavenumber region 600-1400 cm⁻¹. The three a-type bands, ν₂ (A₁), ν₄ (A₁), ν₅ (A₁), the two b-type bands ν₁₁ (B₁), ν₁₂ (B₁), and the c-type band ν₁₄ (B₂) for each of the isotopologues C₂H₂N₂⁸⁰Se and C₂H₂N₂⁷⁸Se have been analyzed using the Watson model. Ground state rotational and quartic centrifugal distortion constants as well as upper state spectroscopic constants have been obtained from the fits. The rotational constants, harmonic and anharmonic frequencies, and vibration-rotation constants (alphas, ) have been predicted by quantum chemical calculations using a cc-pVTZ basis at the MP2 and B3LYP methodology levels, and compared with the present experimental data. Although the rotation constants are marginally closer to experiment from the MP2 calculations, in general the B3LYP frequencies and alphas are closer to experiment.     

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.     

High-resolution infrared and microwave study of the ν20 and ν21 levels near 300 cm in 1,2,4-triazine

The gas phase far-IR spectrum of the ν₂₀ (A″, 367.88 cm⁻¹) and ν₂₁ (A″, 311.28 cm⁻¹) bands of 1,2,4-triazine, a five membered ring having the point group C_s, has been studied at a resolution ranging from 0.002 to 0.003 cm⁻¹. From the MW spectrum 58 transitions in the ν₂₀ level and 64 in the ν₂₁ level have been assigned. The ν₂₀ and ν₂₁ modes which are due to non-planar motions of the ring system are found to be nearly unperturbed. From a simultaneous analysis of IR and MW transitions band centers, rotational constants, and the quartic centrifugal distortion constants , and δ_K have been obtained using the Watson Hamiltonian, A-reduction, III^r-representation.     

A high-resolution infrared study of five bands of 1,2,5-thiadiazole in the range 750-1250 cm, together with ab initio and DFT studies

The Fourier transform gas-phase IR spectrum of 1,2,5-thiadiazole, C₂H₂N₂S, has been recorded with a resolution of ca. 0.003 cm⁻¹ in the wavenumber region 750-1250 cm⁻¹. Five fundamental bands in this region, ν₄ (A₁), ν₅ (A₁), ν₁₁ (B₁), ν₁₃ (B₁), and ν₁₄ (B₂), have been analysed by the Watson Hamiltonian model to yield ground-state rotational and quartic centrifugal distortion constants as well as upper-state spectroscopic constants. A global perturbation of the ν₄ level is explained by Fermi resonance with the 2ν₁₅ level which has been located from its resonance effect. Rotational constants, harmonic and anharmonic frequencies have been calculated using a cc-pVTZ basis, at the MP2 and B3LYP methodology levels, and compared with the experimental data.     

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.     

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.     

High-resolution infrared spectroscopy of the interacting ν9, ν5 + ν6 and 3ν6 levels of CH2BrF

The high-resolution (0.0030 cm⁻¹) Fourier transform infrared spectrum of CH₂⁷⁹BrF has been studied in part of the atmospheric window between 910 and 980 cm⁻¹, the region of the ν₉ (935.847 cm⁻¹) and ν₅ + ν₆ (961.239 cm⁻¹) bands. The ν₉ fundamental consists of a pseudo a-type band induced by Coriolis coupling with ν₅ + ν₆, in turn exhibiting a predominant a-type structure. Several interactions connecting these levels and the dark state 3ν₆ have been assessed. The whole data set is treated using Watson’s A-reduced Hamiltonian in the I^r representation implemented with first order a- and b- and c-type Coriolis terms. A detailed analysis of the rotational structure yields a set of accurate upper-state parameters up to quartic distortion terms for ν₉ and ν₅ + ν₆. In addition, spectroscopic information about the dark ternary overtone of ν₆ has been obtained.     

The formaldehyde cation: Rovibrational energy level structure and Coriolis interaction near the adiabatic ionization threshold

Rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the 0⁰, 6¹ and 4¹ vibrational levels of the ground electronic state of the formaldehyde cation were recorded using a resonant three-color three-photon excitation scheme. The first adiabatic ionization energy of CH₂O (87793.33(1.30) cm⁻¹) and the rigid-rotor rotational constants (A⁺ = 8.874(8) cm⁻¹, B⁺ = 1.342(15) cm⁻¹, C⁺ = 1.148(18) cm⁻¹) of the vibronic ground state of CH₂O⁺ were derived. A strong a-type Coriolis interaction between the 6¹ and 4¹ vibrational levels was observed. The Coriolis coupling parameter and the deperturbed fundamental vibrational frequencies of the in-plane-rocking mode ν₆ and the out-of-plane bending mode ν₄ were determined to be 8.70(10) cm⁻¹, 823.67(30) cm⁻¹ and 1036.50(30) cm⁻¹, respectively. The intensity distribution of the photoelectron spectra was analyzed in the realm of a simple photoionization model.     

High-resolution infrared spectra of spiropentane, C5H8

Infrared spectra of spiropentane (C₅H₈) have been recorded at a resolution (0.002 cm⁻¹) sufficient to resolve for the first time individual rovibrational lines. This initial report presents the ground state rotational constants for this molecule determined from the detailed analysis of the ν₁₆ (b₂) parallel band at 993 cm⁻¹. In addition, the determination included more than 2000 ground state combination-differences deduced from partial analyses of four other infrared-allowed bands, the ν₂₄(e) perpendicular band at 780 cm⁻¹ and three (b₂) parallel bands at 1540 cm⁻¹ (ν₁₄), 1568 cm⁻¹ (ν₅ + ν₁₆), and 2098 cm⁻¹ (ν₅ + ν₁₄). In each of the latter four cases, the spectra show complications; in the case of ν₂₄, these complications are due to rotational l-type doublings, and in the case of the parallel bands, the spectral complexities are due to Fermi resonance and Coriolis interactions of the upper states with nearby levels. The unraveling of these is underway but the assignment of many of these transitions permit the confident use of the ground state differences in determining the following constants for the ground state (in units of cm⁻¹): B₀ = 0.1394741(1), D_J = 2.461(1) × 10⁻⁸, D_JK = 8.69(3) × 10⁻⁸. For the unperturbed ν₁₆ fundamental, more than 3000 transitions were fit and the band origin was found to be at 992.53793(3) cm⁻¹. The numbers in parentheses are the uncertainties (two standard deviations) in the value of the last digit of the constants. Surprisingly, the very accurate B₀ value measured here is lower than the value (0.1418 cm⁻¹) calculated from an electron diffraction structure, instead of being higher, as expected. Where possible, the rovibrational results are compared with those computed at the anharmonic level using the B3LYP density functional method with a cc-pVTZ basis set. These too suggest that the electron diffraction results are in question.     

The ν7, ν8, and ν11 bands of propynal, C2HCHO, in the 650 cm region

The infrared spectrum of propynal, C₂HCHO, is studied at high resolution (0.003 cm⁻¹) in the range 570-640 cm⁻¹. The relatively intense ν₁₁ (CC-H out-of-plane bend, 693 cm⁻¹) and ν₇ (CC-H in-plane bend, 651 cm⁻¹) fundamental bands are linked by a strong a-type Coriolis interaction. The somewhat weaker ν₈ (CCO in-plane bend, 614 cm⁻¹) fundamental has a significant Fermi-type interaction with the “dark” background state 3ν₉ (∼618 cm⁻¹). About 1400 lines are assigned and analyzed in terms of a four-state fit in order to obtain accurate band origins, rotational and centrifugal distortion parameters, and Fermi and Coriolis interaction parameters. This represents the first systematic high-resolution infrared study of propynal.     

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.     

The 2ν1, 2ν2 and 2ν3 overtones of FClO3

The infrared spectra of the 2ν₁, 2ν₂ and 2ν₃ overtones of perchloryl fluoride, FClO₃, have been recorded at high resolution using monoisotopic pure samples. Four symmetric top species have been investigated: F³⁵Cl¹⁶O₃, F³⁷Cl¹⁶O₃, F³⁵Cl¹⁸O₃ and F³⁷Cl¹⁸O₃. The v_i = 2, i = 1, 2, 3 vibrationally excited states are totally symmetric, so these overtones correspond to parallel bands of medium/weak intensity, centered from 2010 to 2120 cm⁻¹ (2ν₁), from 1390 to 1430 cm⁻¹ (2ν₂) and from 1070 to 1100 cm⁻¹ (2ν₃). Most of the bands are unperturbed and their analysis was straightforward. The band origins, the rotational and centrifugal molecular constants in the v₁ = 2, v₂ = 2 and v₃ = 2 states have been determined, with standard deviations of the fits from 0.00024 to 0.00067 cm⁻¹. The 2ν₁ overtones of F³⁵Cl¹⁶O₃ and F³⁷Cl¹⁶O₃ are perturbed by an A₁/E Coriolis resonance between the v₁ = 2 state and one E component of the v₄ = 1, v₆ = 2 manifold. The 2ν₂ of F³⁷Cl¹⁸O₃ is perturbed by the same kind of interaction involving the v₁ = v₆ = 1 (E) state, at about 1396 cm⁻¹. In these bands the resonance is localized on rotational levels with specific J and K values. As a consequence, a few transitions of the perpendicular bands involving the interacting levels could be identified in the spectra. A simultaneous fit of the transitions assigned to the dyads has been performed and the parameters of the excited states have been determined, including the high order Coriolis interaction coefficient . The anharmonic constants x₁₁, x₂₂, x₃₃ of all the studied isotopologues of FClO₃, x₄₆ of F³⁵Cl¹⁶O₃, x₄₆ + g₄₆ of F³⁷Cl¹⁶O₃ and x₁₆ of F³⁷Cl¹⁸O₃, have been derived.     

The ν1 and ν4 fundamental bands of H3SiCl investigated with high resolution

The gas phase infrared spectrum of monoisotopic H₃Si³⁷Cl has been reinvestigated in the ν₁/ν₄ region near 2200 cm⁻¹, using a Fourier transform spectrometer, with a nominal resolution of 0.0027 cm⁻¹. The rovibrational analysis confirms, besides the weak Coriolis x, y resonance between the (v₁ = 1) and (v₄ = 1) levels, the existence of two strong local perturbations in the ν₄ band. These are caused by rotational (Δk = Δl = ±1) type resonances with and , respectively. Another local perturbation of the 12 ? KΔK ? 14 subbands of the ν₄ band, probably due to a (Δk = Δl = ±1) interaction with , was detected and analyzed. All these local perturbations have been studied individually using a simple model of two interacting sublevels. Without the transitions involved in the local perturbations, more than 2000 lines of the ν₁/ν₄ band system were used to obtain a complete set of vibration-rotation parameters set for the v₁ = 1 and v₄ = 1 states. By means of a band contour simulation, both the transition moment ratio ∣M₄:M₁∣ = 1.25 and a positive sign of the Coriolis intensity perturbation were determined.The present results, together with the accurate existing data for ν₂, ν₃, ν₅, and ν₆ bands, allowed us to derive the experimental values, A_e = 2.8722945(37) cm⁻¹ and B_e = 0.2182248(22) cm⁻¹, which are compared with those of ab initio calculations.     

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.