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 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 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.     

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.     

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.     

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 laser spectroscopy of the potassium-argon molecule: The BΣ state

The absorption spectrum of the KAr molecule has been observed with high resolution between 13 032 and 13 077 cm⁻¹ using tunable laser diodes as light sources, a supersonic beam for production of the molecules, and laser-induced fluorescence for detection. In addition, optical-optical double resonance (OODR) experiments have been performed to simplify the spectrum and to get rotational assignment. Altogether, 670 lines due to the transition B²Σ⁺ ← X²Σ⁺ have successfully been assigned with vibrational levels of the B state ranging from v = 0 to v = 6. The corresponding energy values were fitted to the well-known Dunham expansion. In addition, we were able to analyse a local perturbation between the vibrational level v = 1 of the B state and v = 14 of the A²Π_3/2 state. Unexpected extra lines in the OODR spectra are most probably due to a collision-induced population of other levels. For the equilibrium distance and the well-depth of the B state we obtain from the Dunham expansion 7.03 (8) Å and 26.2 (8) cm⁻¹, respectively.     

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 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.