Millimeter-Wave Spectra of the CO Dimer: Three New States and Further Evidence of Distinct Isomers

The millimeter-wave spectrum of the normal isotope of the CO dimer, (¹²C¹⁶O)₂, has been systematically surveyed in the regions 75-105 and 131-174 GHz, with additional measurements covering the entire 60-176 GHz range. By combining these results and using the technique of combination differences based on previously known energy levels, 14 new rotational levels have been assigned and precisely (≈0.1 MHz) located. They belong to 3 completely new states, 1 with A⁺ symmetry and 2 with A⁻ symmetry. The position of the lowest energy A⁻ state results in a new and lower value for the effective tunneling splitting of the CO dimer, 3.73 cm⁻¹. The observation of dramatically different intensities for different bands supports the concept of two isomeric forms for (CO)₂, the ground state having a larger intermolecular separation (≈4.4 Å) with most likely a C-bonded configuration, and the low-lying (0.88 cm⁻¹) excited state having a smaller separation (≈4.0 Å) and an O-bonded geometry.     

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 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.     

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.     

Microwave spectra, molecular structure and theoretical calculation of two isotopic species of acetyl isocyanate CD3C(O)NCO and CH3C(O)NCO

The microwave spectra of two isotopic species of acetyl isocyanate, ¹³CH₃C(O)NCO and CD₃C(O)NCO, were observed in order to determine the r_o structure and confirmation of the molecular conformation. These isotopic species were prepared by reacting acetyl-2-¹³C-chloride or acetyl-d₃ chloride with sliver cyanate. The rotational spectra of A-level in 26.5-60.0 GHz region have been observed by Stark-modulated microwave spectrometer. Some absorption lines in E-level were observed in ¹³CH₃C(O)NCO. The rotational constants in the ground vibrational state were determined to be A = 10654.8(18), B = 2177.32(2), and C = 1827.65(2) MHz for ¹³CH₃C(O)NCO, and A = 9713.90(6), B = 2042.04(2), and C = 1722.78(2) MHz for CD₃C(O)NCO, respectively. The values of ΔI (= I_c − I_a − I_b) of the ¹³C species (−3.024(13) uŲ) and the d₃ species (−6.163(3) uŲ) indicate that the molecule has C_s symmetry. The r_s coordinates of the carbon atom in the methyl group were determined to be |a| = 2.183(3), |b| = 0.706(9), and |c| = 0.080(87) Å. The determined coordinates were in agreement with those calculated for the cis form, in which the carbonyl group is eclipsed by the NCO group. The six structural parameters of the cis form were adjusted by fitting to the observed rotational constants. The observed rotational constants of the cis form were in better agreement with those calculated using the QCISD/6-31G (d, p) level rather than those calculated using the MP2/6-31G (d, p) level. The barrier of internal rotation of the methyl group was determined as 4.283(16) kJ mol⁻¹ in ¹³CH₃C(O)NCO. The structural tendencies and the relationship between ∠RNC and ¹⁴N quadrupole coupling constants (χ_cc) were discussed.     

Global effective Hamiltonians of OCO and OCO improved from CW-CRDS observations in the 5900-7000 cm region

The parameters of the polyad models of the effective Hamiltonian of the ¹⁶O¹³C¹⁷O and ¹⁶O¹³C¹⁸O isotopologues of carbon dioxide have been refined by the least-squares fittings to the line positions collected from the literature. Such refinement has become necessary as the observed dataset has been significantly extended by our CW-CRDS observations in the 5900-7000 cm⁻¹ region. In the case of the ¹⁶O¹³C¹⁷O isotopologue, 1151 line positions of 11 bands have been used to refine the effective Hamiltonian parameters published by Chédin [A. Chédin, J. Mol. Spectrosc. 76 (1979) 430-491]. With the obtained set of parameters, the collected line positions are reproduced with a RMS (root mean squares of the residuals) equal to 0.0013 cm⁻¹. In the case of the ¹⁶O¹³C¹⁸O isotopologue, 61 parameters of the effective Hamiltonian were fitted to more than 6410 line positions. A weighted standard deviation of χ = 1.77 and a global RMS of 0.0017 cm⁻¹, close to the experimental accuracy, were achieved. However, several rotational levels of the 31113 state (P = 10) could not be reproduced in the frame of this polyad model and were then excluded from the fit. We found that these levels are affected by an anharmonic resonance interaction with the 51106 vibrational state (P = 11) leading to energy shifts up to 0.060 cm⁻¹ and significant intensity transfer to several extra lines which could be detected. The coupling matrix element has been estimated to 0.11 cm⁻¹ from the detailed analysis of the experimental spectrum. This is the first evidence of an interpolyad resonance interaction in the case of the carbon dioxide molecule. In order to extend the input spectroscopic information, the weak lines left unassigned in our previous analysis of the CW-CRDS spectrum of the ¹³C enriched carbon dioxide [Y. Ding, P. Macko, D. Romanini, V.I. Perevalov, S.A. Tashkun, J.-L. Teffo, S.-M. Hu, A. Campargue, J. Mol. Spectrosc. 226 (2004) 146-160.] have been revisited. Thirteen ¹³C¹⁶O₂ bands, one ¹⁶O¹³C¹⁷O band and two ¹⁶O¹³C¹⁸O bands could be newly assigned together with a number of transitions corresponding to high J values of previously observed bands. The spectroscopic constants G_v, B_v, and D_v for the unperturbed bands have been fitted to the observed line positions.     

High resolution electronic study of ONO, ONO and ONO: A rovibronic survey covering 11 800-14 380 cm

The 11 800-14 380 cm⁻¹ frequency range has been scanned for rotationally resolved rovibronic transitions in the A²B₂-X²A₁ electronic band system of the symmetric (C_2v) ¹⁶O¹⁴N¹⁶O and ¹⁸O¹⁴N¹⁸O isotopologues and in the corresponding electronic band system of the asymmetric (C_s) ¹⁸O¹⁴N¹⁶O isotopologue. The rotational analysis—reflecting minor differences in mass—in combination with symmetry induced spectral differences allows an identification of 68 ¹⁶O¹⁴N¹⁶O vibronic levels, 26 ¹⁸O¹⁴N¹⁸O vibronic levels and 51 ¹⁸O¹⁴N¹⁶O vibronic levels. The bands are recorded using near infrared fluorescence spectroscopy and a piezo valve based pulsed molecular beam expansion of premixed ¹⁸O₂ and ¹⁴N¹⁶O in Ar. The majority of the observed bands is rotationally assigned and can be identified as transitions starting from the vibrational ground state of one of the isotopologues. Numerous hot bands have also been identified. A comparison of the overall spectroscopic features of C_2v vs. C_s symmetric species provides qualitative information on symmetry dependence of vibronic couplings.     

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.     

The AB2-XA1 electronic transition of NO2: A rovibronic survey covering 14 300-18 000 cm

More than 250 rotationally resolved vibrational bands of the A²B₂-X²A₁ electronic transition of ¹⁵NO₂ have been observed in the 14 300-18 000 cm⁻¹ range. The bands have been recorded in a recently constructed setup designed for high resolution spectroscopy of jet cooled molecules by combining time gated fluorescence spectroscopy and molecular beam techniques. The majority of the observed bands has been rotationally assigned and can be identified as transitions starting from the vibrational ground state or from vibrationally excited (hot band) states. An exceptionally strong band is located at 14 851 cm⁻¹ and studied in more detail as a typical benchmark transition to monitor ¹⁵NO₂ in atmospheric remote sensing experiments. Standard rotational fit routines provide band origins, rotational and spin rotation constants. A subset of 177 vibronic levels of ²B₂ vibronic symmetry has been analyzed in the energy range between 14 300 and 17 250 cm⁻¹, in terms of integrated density and using Next Neighbor Distribution. It is found that the overall statistical properties and polyad structure of ¹⁵NO₂ are comparable to those of ¹⁴NO₂ but that the internal structures of the polyads are completely different. This is a direct consequence of the X²A₁-A²B₂ vibronic mixing.     

Elastic constants of solids and fluids with initial pressure via a unified approach based on equations-of-state

The second and third-order Brugger elastic constants are obtained for liquids and ideal gases having an initial hydrostatic pressure p₁. For liquids the second-order elastic constants are C₁₁ = A + p₁, C₁₂ = A − p₁, and the third-order constants are C₁₁₁ = −(B + 5A + 3p₁), C₁₁₂ = −(B + A − p₁), and C₁₂₃ = A − B − p₁, where A and B are the Beyer expansion coefficients in the liquid equation of state. For ideal gases the second-order constants are C₁₁ = p₁γ + p₁, C₁₂ = p₁γ − p₁, and the third-order constants are C₁₁₁ = −p₁(γ² + 4γ + 3), C₁₁₂ = −p₁(γ² − 1), and C₁₂₃ = −p₁ (γ² − 2γ + 1), where γ is the ratio of specific heats. The inequality of C₁₁ and C₁₂ results in a nonzero shear constant C₄₄ = (1/2)(C₁₁ − C₁₂) = p₁ for both liquids and gases. For water at standard temperature and pressure the ratio of terms p₁/A contributing to the second-order constants is approximately 4.3 × 10⁻⁵. For atmospheric gases the ratio of corresponding terms is approximately 0.7. Analytical expressions that include initial stresses are derived for the material ‘nonlinearity parameters’ associated with harmonic generation and acoustoelasticity for fluids and solids of arbitrary crystal symmetry. The expressions are used to validate the relationships for the elastic constants of fluids.     

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.     

Infrared spectra of the polar and nonpolar N2O dimers in the 1280 cm region of the ν3 fundamental

The high-resolution infrared spectrum of the polar N₂O dimer has been observed in the region of the N₂O ν₃ fundamental (∼1280 cm⁻¹) using a tunable diode laser to probe a pulsed supersonic slit jet. About 120 rotational transitions were assigned in terms of an a/b hybrid band of a planar asymmetric top molecule with a slipped parallel structure. The vibrational origin was determined to be 1290.21 cm⁻¹, showing a blue shift of 5.31 cm⁻¹ with respect to the monomer band origin. In addition, the spectrum of the nonpolar isomer at 1279.71 cm⁻¹ has been remeasured and analyzed in improved detail. Small but widespread perturbations are noted in this band, which appear somewhat similar to larger effects observed previously in the ν₁ + ν₃ region for nonpolar (N₂O)₂.     

The infrared spectrum of the CO dimer

The infrared spectrum of (¹²C¹⁸O)₂ has been studied for the first time using a tunable diode laser spectrometer in the 2095 cm⁻¹ region to probe a pulsed supersonic jet expansion. Very dilute gas mixtures of CO in He were used, resulting in small consumption of ¹²C¹⁸O sample gas, as well as cold and simple spectra. The results were analyzed using a term value scheme to obtain model-independent energies for 7 rotational levels belonging to 2 stacks in the lower state, v_CO=0, and 22 levels belonging to 7 stacks in the upper state, v_CO=1. The two ground state isomers of the CO dimer were found to be separated by only 0.639 cm⁻¹ for (¹²C¹⁸O)₂. These results provide a foundation for future studies of the millimeter wave spectrum.     

The CO dimer: new light on a mysterious molecule

The present state of research on the CO dimer is reviewed. In recent years, both infrared and millimeter-wave spectra have been measured and partially assigned by the use of combination differences. A microwave-millimeter wave double resonance experiment, reported here for the first time, provides independent confirmation of these assignments and the resulting (CO)₂ energy level scheme. In the double resonance experiment, the OROTRON spectrometer functions as a supersensitive intra-cavity millimeter-wave detector. We update the continuing, but difficult, experimental efforts in recording the spectra, the quest for secure assignments, and the construction of a consistent and reliable energy level scheme. Although at present we have only limited knowledge of some aspects of the CO dimer, such as its geometrical structure, we have succeeded in characterizing unambiguously nine “stacks” of ground state energy levels with “microwave accuracy” (∼0.1 MHz). Every energy level within a given stack exhibits the same symmetry: either A⁻ or A⁺. Only transitions between A⁺ and A⁻ levels are allowed, and consequently ordinary pure rotational transitions within a stack are forbidden. Transitions between stacks can be thought of as tunneling transitions, and the separation of the lowest energy A⁺ and A⁻ states corresponds to a value of for the effective “tunneling splitting” of the CO dimer. The stacks tend to fall into two groups, corresponding to “isomers” with effective inter-molecular separations of either 4.0 or 4.4 Å. The larger inter-molecular separation of the true ground state (4.4 Å) likely corresponds to a C-bonded configuration, while the low-lying excited state with the smaller separation (4.0 Å) likely displays an O-bonded geometry.     

CW-cavity ringdown spectroscopy of carbon dioxide isotopologues near 1.5 μm

The absorption spectra of carbon dioxide in natural isotopic abundance and with 99% enrichment in ¹³C have been recorded by CW-cavity ringdown spectroscopy in two specific spectral regions: 5957-6122 and 6745-6833 cm⁻¹. The spectra were obtained at Doppler limited resolution by using a CW-CRDS spectrometer based on fibered DFB lasers. The typical sensitivity of 5 × 10⁻¹⁰ cm⁻¹, allowed for the detection of lines with intensity as weak as 5 × 10⁻²⁹ cm/molecule. More than 2900 line positions of the six major isotopologues contributing to the spectra (¹²C¹⁶O₂, ¹⁶O¹²C¹⁷O, ¹⁶O¹²C¹⁸O, ¹³C¹⁶O₂, ¹⁶O¹³C¹⁷O and ¹⁶O¹³C¹⁸O), were measured and assigned on the basis of their respective global effective Hamiltonian models. For comparison, only 507 lines are provided by the HITRAN database in these spectral regions. The band by band analysis has led to the determination of the rovibrational parameters of a total of 52 bands, 30 of them being newly reported. Most of the observed line positions show an agreement close to the experimental uncertainty (1-2 × 10⁻³ cm⁻¹) with the predictions of their respective effective Hamiltonian models. However, the quality of the predictions degrades for the minor isotopologues reaching maximum deviations of 0.35 cm⁻¹ in one specific case. For several bands, rovibrational transitions with J values between 60 and 90 could be newly detected. While an excellent agreement is observed with the line positions predicted by the Hamiltonian models, the comparison of these observations with the line positions listed in the HITRAN database or extrapolated by using the best FTS rotational constants available in the literature has evidenced significant deviations.     

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.     

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.     

ESR and optical study of Mn doped potassium hydrogen sulphate

The ESR spectrum of Mn²⁺ doped potassium hydrogen sulphate at liquid nitrogen temperature (77 K) has been analyzed and site of entered Mn²⁺ in the lattice has been discussed. The values of the zero field parameters that give good fit to the observed ESR spectra have been obtained. The obtained g, A, B, D, E and a values are 2.0002, 66×10⁻⁴ cm⁻¹, 26×10⁻⁴ cm⁻¹, 59×10⁻⁴ cm⁻¹, 32×10⁻⁴ cm⁻¹ and −8×10⁻⁴ cm⁻¹, respectively. The percentage of covalency of the metal-ligand bond has also been estimated. From the optical absorption study at room temperature, the distortion has been suggested. The observed bands are assigned as transitions from the ⁶A_1g(S) ground state to various excited quartet levels of Mn²⁺ ion in a cubic crystalline field. The electron repulsion and crystal field parameters B, C, Dq and α providing good fit to the observed optical spectra have been evaluated and the values obtained for the parameters are B=627 cm⁻¹, C=2580 cm⁻¹ , Dq=790 cm⁻¹ and α=76 cm⁻¹.     

Microwave spectrum and structure of methyl phosphonic difluoride

The rotational spectrum of methyl phosphonic difluoride has been reinvestigated using a pulsed-molecular-beam Fabry-Perot cavity microwave spectrometer. The enhanced resolution of the Fourier transform microwave (FTMW) spectrometer (compared to the original work done in a conventional Stark spectrometer) has allowed the measurement of small A-E splittings of many of the rotational transitions caused by the internal rotation of the methyl top. The barrier to internal rotation, V₃ = 676 (25) cm⁻¹, has been determined experimentally from the A-E splittings of the rotational transitions in the ground vibrational state. This barrier height is substantially lower than the previously determined value for the barrier, which was 1252 (14) cm⁻¹. High-level ab initio calculations at the MP2/aug-cc-pVTZ level predict a barrier to internal rotation of 638 cm⁻¹, in agreement with the experimentally determined value found here. The high sensitivity of the FTMW spectrometer has also permitted the measurement of the ¹³C and ¹⁸O isotopomers in natural abundance. The addition of these two isotopomers has allowed an improved structural determination.     

New analysis of the Comet-Tail (AΠi − XΣ) system in the CO isotopic molecule

The emission spectra of the 0-2, 4-2, and 6-1 bands of the Comet-Tail (A²Π_i − X²Σ⁺) system in the ¹⁴C¹⁶O⁺ isotopic molecule, comprising nearly 600 lines, have been recorded and analyzed for the first time. The spectra have been photographed under high resolution by using conventional spectroscopy, and it was possible to separate and observe most of the lines of all the 12 branches of this transition. The reduction of the individual bands’ spectra has been performed by nonlinear least-squares procedure and by means of effective Hamiltonians of Brown et al. the rovibronic structure parameters have been obtained. The currently investigated bands of the Comet-Tail system and the earlier analyzed bands of the A − X and B − A systems in the ¹⁴C¹⁶O⁺ molecule have been merged together. The results of this global fit made it possible to derive a new set of the equilibrium molecular constants for the A and X states. Then the RKR potential curve parameters for both A and X states and Franck-Condon factors as well as r-centroids for the A − X transition have been calculated for the ¹⁴C¹⁶O⁺ molecule.