Vibration-rotation Fourier transform infrared spectrum of the C-D and CC stretching fundamental, ν1 and ν2, band systems of deuterated monobromoacetylene

A high resolution vibration-rotation spectrum of deuterated monobromoacetylene (DCCBr) has been recorded with a Bruker IFS 120 Fourier Spectrometer in the wavenumber region 1700-2800 cm⁻¹, which covers the C-D and CC stretching fundamental (ν₁ and ν₂, respectively) and the CC and C-Br stretching vibrational combination (ν₂ + ν₃) band systems. The analysis of the spectrum provides accurate vibrational term values and rotational constant for 20 vibration-rotation bands for both isotopic species, DCC⁷⁹Br and DCC⁸¹Br.     

Rotational spectra of isotopic species of silyl fluoride. Part II: Theoretical and semi-experimental equilibrium structure

The equilibrium structure of silyl fluoride, SiH₃F, has been reinvestigated using both theoretical and experimental data. With respect to the former, quantum-chemical calculations at the coupled-cluster level have been employed together with extrapolation to the basis set limit, consideration of higher excitations in the cluster operator, and inclusion of core correlation as well as relativistic corrections (r(Si-F) = 1.5911 Å, r(Si-H) = 1.4695 Å, and ∠FSiH = 108.30°). A semi-experimental equilibrium structure has been determined based on the available rotational constants for the various isotopic species of silyl fluoride (²⁸SiH₃F, ²⁸SiD₃F, ²⁹SiH₃F, ²⁹SiD₃F, ³⁰SiH₃F, ³⁰SiD₃F, ²⁸SiH₂DF, and ²⁸SiHD₂F) together with computed vibrational corrections to the rotational constants (r(Si-F) = 1.59048(6) Å, r(Si-H) = 1.46948(9) Å, and ∠FSiH = 108.304(9)°).     

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.     

Rotational analysis of 6ν3 and 6ν3+ν2−ν2 bands of N2O from ICLAS spectra between 12,760 and 12,900 cm

The absorption spectrum of nitrous oxide (N₂O) has been recorded by Intracavity Laser Absorption Spectroscopy between 12,760 and 12,900 cm⁻¹. The rotational analysis led to an improved determination of rovibrational parameters of the 6ν₃ and 6ν₃+ν₂-ν₂ bands of ¹⁴N₂¹⁶O. The high J rotational levels of the (0 0 ⁰6) and (0 1 ¹6) upper states were found perturbed by an anharmonic interaction. Line intensity values of the 6ν₃ band are provided and the main effective dipole moment parameter has been determined.     

The microwave spectrum and structure of the argon trifluoroacetonitrile complex

The rotational spectrum of argon trifluoroacetonitrile complex has been studied by pulsed-nozzle, Fourier transform microwave spectroscopy. Both a-type and b-type transitions have been observed. The rotational constants are A = 3053.0903(2) MHz, B = 1039.9570(2) MHz, and C = 895.5788(1) MHz. The ¹⁴N nuclear quadrupole hyperfine components of the rotational transitions have been resolved, the ¹⁴N nuclear quadrupole coupling constants are χ_aa = 1.746(1) MHz, and χ_bb − χ_cc = −6.426(2) MHz. The complex is T-shaped, with the argon atom located 3.73 Å from the center of mass of the trifluoroacetonitrile molecule.     

Velocity modulation spectroscopy of molecular ions I: The pure rotational spectrum of TiCl (XΦr)

The pure rotational spectrum of the TiCl⁺ ion in its X³Φ_r ground state has been measured in the frequency range 323-424 GHz, using a combination of direct absorption and velocity modulation techniques. The ion was created in an AC discharge of TiCl₄ and argon. Ten, eleven, and nine rotational transitions were recorded for the ⁴⁸Ti³⁵Cl⁺, ⁴⁸Ti³⁷Cl⁺, and ⁴⁶Ti³⁵Cl⁺ isotopomers, respectively; fine structure splittings were resolved in every transition. The rotational fine structure pattern was irregular with the Ω = 4 component lying in between the Ω = 2 and 3 lines. This result is consistent with the presence of a nearby ³Δ_r state, which perturbs the Ω = 2 and 3 sub-levels, shifting their energies relative to the Ω = 4 component. The data for each isotopomer were analyzed in a global fit, and rotational and fine structure parameters were determined. The value of the spin-spin constant was comparable to that of the spin-orbit parameter, indicating a large second-order spin-orbit contribution to this interaction. The bond length established for TiCl⁺, r₀ = 2.18879 (7) Å, is significantly shorter than that of TiCl, which has r₀ = 2.26749 (4) Å. The shorter bond length likely results from a Ti²⁺Cl⁻ structure in the ion relative to the neutral, which is thought to be represented by a Ti⁺Cl⁻ configuration. The higher charge on the titanium atom shortens the bond.     

Intracavity laser absorption spectroscopy of AuO: Identification of the bΠ3/2-XΠ3/2 transition

The visible electronic spectrum of AuO has been recorded at rotational resolution using intracavity laser absorption spectroscopy. Five vibrational bands have been analyzed and assigned as the (0, 0), (1, 0), (2, 0), (3, 0), and (4, 0) bands of the b⁴Π_3/2-X²Π_3/2 transition of AuO. The molecular parameters for the newly identified b⁴Π_3/2 state are presented.     

Torsional barrier and equilibrium structure of ethyl cyanide

The quadratic, cubic and semi-diagonal quartic force field of ethyl cyanide has been calculated at the B3LYP level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quadruple-ζ quality and a core correlation correction. The empirical structures are also determined and their accuracy is discussed. The potential barrier V₃ hindering internal rotation of the methyl group has been calculated from 23 rotational transitions of CH₃CH₂C¹⁵N which were found split into doublets, giving V₃ = 3074(27) cal mol⁻¹.     

Ground state rotational spectrum of nitrogen trifluoride: The K = 3 splittings of NF3 and NF3

The ground state rotational spectrum of the ¹⁴NF₃ and ¹⁵NF₃ isotopic species of nitrogen fluoride has been observed in the ∼450-810 GHz frequency range. This investigation allowed us to improve the rotational parameters for both isotopologues. In particular, for the first time the K = 3 line splitting parameter and the sextic centrifugal distortion constants have been determined for ¹⁵NF₃.     

Vibrational and rotational analyses of bands between 636 and 660 nm in the red system of CuCl2

The long wavelength end of the electronic spectrum of CuCl₂, between 636 and 660 nm, has been recorded in the gas phase by laser-excitation spectroscopy using a sample prepared at low temperatures (ca. 10 K) in a free-jet expansion. Under these conditions, it is possible to resolve vibrational, rotational, and even Cu hyperfine structure. The (0, 0) band of the E²Π_u-X²Π_g transition has been identified with an origin at 15546.286(3) cm⁻¹ for ⁶³Cu³⁵Cl₂. The observation and analysis of bands involving vibrationally excited levels has allowed the determination of all three vibrational intervals for the E²Π_u state (ν₁ = 335.88 cm⁻¹, ν₂ = 112.42 cm⁻¹, and ν₃ = 482.17 cm⁻¹, ⁶³Cu³⁵Cl₂). In addition, two other, unrelated transitions have been identified in the same narrow wavelength region. This, combined with the observation of local perturbations of the rotational structure in various bands, reveals the presence of other closely lying electronic states in the same energy region.     

Infrared emission studies of the AΣ-XΠ electronic transition of the SiC radical

The gas phase infrared emission spectrum of the A³Σ⁻-X³Π electronic transition of SiC has been observed using a high resolution Fourier transform spectrometer. Three bands ν′ − ν″ = 0-1, 0-0, and 1-0 have been observed in the 2770, 3723, and 4578 cm⁻¹ regions, where the 0-1 and 0-0 bands were observed for the first time. The SiC radical was generated by a dc discharge in a flowing mixture of hexamethyl disilane [(CH₃)₆Si₂] and He. A total of 1074 rotational transitions assigned to the 0-1, 0-0, and 1-0 bands have been combined in a simultaneous analysis with previously reported pure rotational data to determine the molecular constants for SiC in the two electronic states. The principal equilibrium molecular constants for the A³Σ⁻ state are: B_e = 0.6181195(18) cm⁻¹, α_e = 0.0051921(20) cm⁻¹, r_e = 1.8020884(26) Å, and T_e = 3773.31(17) cm⁻¹, with one standard deviation given in parentheses. The effect of a perturbation was recognized between the ν = 4 level of X³Π and the ν = 0 level of A³Σ, and the analysis was carried out to determine the interaction parameter between the two states.     

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.     

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.     

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.     

Velocity modulation spectroscopy of molecular ions II: The millimeter/submillimeter-wave spectrum of TiF (XΦr)

The pure rotational spectrum of the molecular ion TiF⁺ in its ³Φ_r ground state has been measured in the range 327-542 GHz using millimeter-wave direct absorption techniques combined with velocity modulation spectroscopy. TiF⁺ was made in an AC discharge from a mixture of TiCl₄, F₂ in He, and argon. Ten transitions of this ion were recorded. In every transition, fluorine hyperfine interactions, as well as the fine structure splittings, were resolved. The fine structure pattern was found to be regular with almost equal spacing in frequency between the three spin components, in contrast to TiCl⁺, which is perturbed in the ground state. The data were fit with a case (a) Hamiltonian and rotational, fine structure, and hyperfine constants were determined. The bond length established for TiF⁺, r₀ = 1.7775 Å, was found to be shorter than that of TiF, r₀ = 1.8342 Å—also established from mm-wave data. The hyperfine parameters determined are consistent with a δ¹π¹ electron configuration with the electrons primarily located on the titanium nucleus. The nuclear spin-orbit constant a indicates that the unpaired electrons are closer to the fluorine nucleus in TiF⁺ relative to TiF, as expected with the decrease in bond length for the ion. The shorter bond distance is thought to arise from increased charge on the titanium nucleus as a result of a Ti²⁺F⁻ configuration. A similar decrease in bond length was found for TiCl⁺ relative to TiCl.     

The rotational spectra of the υ8 = υ9 = 1 and υ6 = υ7 = 1 interacting vibrational states of nitric acid (HNO3)

The analysis of the rotational spectrum of HNO₃ has been extended to include the υ₈ = υ₉ = 1 state at 1205.7 cm⁻¹ and the υ₆ = υ₇ = 1 state at 1223.4 cm⁻¹. Based on 78-519 GHz data, the assignments in the 8¹9¹ vibrational state have been significantly expanded from the previously reported microwave measurements [T.M. Goyette, F.C. De Lucia, J. Mol. Spectrosc. 139 (1990) 241-243]. A new microwave analysis is also reported for the 6¹7¹ vibrational state. A simultaneous analysis takes into account the localized ΔK_a = ±2 Fermi resonances between the vibrational states, describes the torsional splitting of 3.3 and 1.4 MHz for the 8¹9¹ and 6¹7¹ states respectively, and fits to experimental accuracy over 1500 rotational transition frequencies that extend up to J = 59. Infrared energy levels [A. Perrin, J.-M. Flaud, F. Keller, A. Goldman, R. D. Blatherwick, F. J. Murcray, C. P. Rinsland, J. Mol. Spectrosc. 194 (1999) 113-123] were also included in the analysis and fit to experimental accuracy. Measurement of strongly perturbed transitions in each vibrational state provide a determination of the band origin difference of 17.733184(17) cm⁻¹. The rotational constants agree well with those predicted by vibrational-rotational constants of the fundamental modes. Furthermore, the analysis will provide a very accurate simulation of the infrared spectrum of HNO₃ in the 8.3 μm region.     

High resolution analysis of the rotational levels of the (0 0 0), (0 1 0), (1 0 0), (0 0 1), (0 2 0), (1 1 0) and (0 1 1) vibrational states of SO2

A high resolution (0.0018 cm⁻¹) Fourier transform instrument has been used to record the spectrum of an enriched ³⁴S (95.3%) sample of sulfur dioxide. A thorough analysis of the ν₂, 2ν₂ − ν₂, ν₁, ν₁ + ν₂ − ν₂, ν₃, ν₂ + ν₃ − ν₂, ν₁ + ν₂ and ν₂ + ν₃ bands has been carried out leading to a large set of assigned lines. From these lines ground state combination differences were obtained and fit together with the existing microwave, millimeter, and terahertz rotational lines. An improved set of ground state rotational constants were obtained. Next, the upper state rotational levels were fit. For the (0 1 0), (1 1 0) and (0 1 1) states, a simple Watson-type Hamiltonian sufficed. However, it was necessary to include explicitly interacting terms in the Hamiltonian matrix in order to fit the rotational levels of the (0 2 0), (1 0 0) and (1 0 1) states to within their experimental accuracy. More explicitly, it was necessary to use a ΔK = 2 term to model the Fermi interaction between the (0 2 0) and (1 0 0) levels and a ΔK = 3 term to model the Coriolis interaction between the (1 0 0) and (0 0 1) levels. Precise Hamiltonian constants were derived for the (0 0 0), (0 1 0), (1 0 0), (0 0 1), (0 2 0), (1 1 0) and (0 1 1) vibrational states.     

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)°.     

Rotational spectrum and structure of asymmetric dinitrogen trioxide, N2O3

The rotational spectra of the ground vibrational state and the ν₉ = 1 torsional state have been reinvestigated and accurate spectroscopic constants have been determined. The torsional frequency, ν₉ = 70(15) cm⁻¹, has been determined by relative intensity measurements. The assignment of the infrared spectrum has been slightly revised and an accurate harmonic force field has been calculated. The equilibrium structure has been determined using different, complementary methods: experimental, semi-experimental and ab initio, leading to r(NN) = 1.870(2) Å, in particular.     

The pure rotational spectrum of TiS (XΔr) at submillimeter wavelengths

The pure rotational spectrum of TiS in its X³Δ_r ground state has been measured using millimeter-wave direct-absorption techniques in the frequency range of 313-425 GHz. This free radical was created by the reaction of titanium vapor, produced in a high-temperature Broida-type oven, with H₂S. Eight to ten rotational transitions were recorded for the main titanium isotopologue, ⁴⁸TiS, in the v = 0 and v = 1 levels, as well as for the v = 0 state of ⁴⁶TiS, observed in natural abundance (⁴⁸Ti:⁴⁶Ti = 74:8). All three Ω components were observed in almost every recorded transition, with no evidence for lambda-doubling. The data were fit with a Hund’s case(a) Hamiltonian, and rotational, spin-orbit, and spin-spin constants were determined, as well as equilibrium parameters for ⁴⁸TiS. Relatively few fine structure parameters were needed for the analysis of TiS (A, A_D, and λ), unlike other 3d metal species. The rotational pattern of the three fine structure components suggests the presence of a nearby excited ¹Δ state, lying ∼3000 cm⁻¹ higher in energy. From the equilibrium parameters, the dissociation energy for TiS was estimated to be ∼5.1 eV, in reasonable agreement with past thermochemical data.