Ground-state constants, ab initio anharmonic force field, and equilibrium structure of F2BOH

The millimeterwave spectra of F₂¹⁰BOH and F₂¹¹BOH (difluorohydroxyborane) have been measured in their ground vibrational state. Accurate rotational and centrifugal distortion constants have been determined. The equilibrium geometry and anharmonic force fields have been calculated at the CCSD(T) level of theory. The ab initio centrifugal distortion constants and rotation-vibration interaction constants are compared to the experimental values. Some discrepancies are found and discussed. Particularly, it is explained why the semi-experimental structure is not reliable. The best equilibrium structure is: r_e(BF_cis) = 132.29 pm, r_e(BF_trans) = 131.29 pm, r_e(BO) = 134.48 pm, r_e(OH) = 95.74 pm, ∠_e(FBF) = 118.36°, ∠_e(F_cisBO) = 122.25°, and ∠_e(BOH) = 113.14°.     

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)°.     

Ab initio anharmonic force field and equilibrium structure of carbonyl chlorofluoride

The quadratic, cubic, and semi-diagonal quartic force field of OCFCl has been calculated at the MP2 level of theory employing a basis set of triple-zeta quality. The spectroscopic constants derived from the force field are in excellent agreement with those from previous and new experiments. The equilibrium structure has been derived from experimental ground state rotational constants and ab initio rovibrational interaction parameters. This semi-experimental structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory. This good agreement indicates that the derived structure is accurate. The equilibrium geometry is: r_e(CO)=1.173(1) Å; r_e(C-F)=1.323(1) Å; r_e(C-Cl)=1.721(1) Å; ∠_e(OCF)=124.0 (1)°; and ∠_e(OCCl)=126.4(1)°.     

The Ground State Spectroscopic Parameters and Equilibrium Structure of PD3

Infrared spectra of PD₃ have been measured in the 20-320 cm⁻¹ range and in the region of the ν₂/ν₄ and ν₁/ν₃ fundamental bands near 750 and 1690 cm⁻¹, respectively, with a resolution of ca. 0.0025 cm⁻¹. Furthermore, submillimeter-wave spectra covering the J=4-3, 13-12, and 14-13 clusters in the vibrational ground state were recorded. The observed ΔJ=+1 rotational lines were augmented by about 5500 ground state combination differences formed from transitions belonging to the fundamental bands. Of these, 1300 involved perturbation-allowed lines with ΔK≠0. These data and observations taken from the literature were appropriately weighted and fitted to 14 ground state molecular constants. The A and B reductions of the rotational Hamiltonian were found to be equivalent. Improved effective ground state and equilibrium structures were determined for both PH₃ and PD₃; the equilibrium structures, r_e (PH)=141.1607(83) pm and α_e (HPH)=93.4184(95)° and r_e (PD)=141.1785(57) pm and α_e (DPD)=93.4252(68)°, are in good agreement.     

Ground state spectrum of methylcyanide

The rotational spectrum of methylcyanide (acetonitrile) in the ground vibrational state was measured in the spectral region from 91 to 810 GHz using the Cologne and Tsukuba spectrometers operated in the Doppler-limited and sub-Doppler saturation layouts. The resolution of the saturation Lamb-dip measurements is estimated to be about 1 kHz at the best of circumstances and the measuring accuracy of 10-60 kHz depending very sensitively on the quality of the spectrum. In the cases of rotational transitions with the low quantum number J (J<18) and with a low difference of the rotational quantum numbers J−K, the resolved or partly resolved hyperfine structures of the rotational transitions were observed. Together with the most accurate data from the literature, the newly measured experimental data were analyzed using the traditional polynomial energy formula as well as the Padè approximant for the effective rotational Hamiltonian. The resulting rotational, centrifugal distortion, and hyperfine structure spectroscopic constants were obtained with a significantly higher accuracy than the ones listed in the literature. In addition, an anomalous accidental resonance was detected between the K=14 ground state levels and the K=12, +l levels in the excited v₈=1 vibrational state.     

The millimeter-wave rotational spectrum of CF3CCD in the excited vibrational state v10=2

The millimetre-wave rotational spectra of the excited vibrational state v₁₀=2 of the symmetric top molecule, CF₃CCD, have been recorded for J^′′=12 up to J^′′=25. The l=±2 and l=0 series have been assigned and the spectra analysed to give rotational parameters including x_ll=7716.975 MHz. The main interactions between states of different l are the r_t(2,−1)=0.158 MHz and q_t⁺(2,2)=3.308 MHz. Two type of l-resonance are identified, one of which is due to an avoided crossing between the l=0 and l=+2 series. The spectra are qualitatively similar to the corresponding ones of CF₃CCH.     

Microwave structural studies of organoselenium compounds 1. Microwave spectra, molecular structure, and methyl barrier to internal rotation of dimethyl diselenide

The rotational spectra of nine isotopomers of dimethyl diselenide, CH₃SeSeCH₃, have been measured with a molecular-beam Fourier transform microwave spectrometer. The spectra were complex due to the presence of many isotopomers in natural abundance and the splitting caused by the interactions with two methyl internal rotors. The spectra were assigned and fit to experimental precision to an effective rotational Hamiltonian for molecules with two periodic internal motions. The spectra of the symmetric isotopomers are consistent with a C₂ equilibrium structure. The rotational constants were used to determine the r_s structure of the C-Se-Se-C frame with the results r(SeSe)=2.306(3) Å, r(SeC)=1.954(6) Å, ?(CSeSe)=99.8(2)°, ?(CSeSeC)=85.2(1)°. A barrier to internal rotation of the methyl groups of 395 ± 2 cm⁻¹ was derived from the internal rotation splittings.     

Large amplitude bending motion in CsOH, studied through ab initio-based three-dimensional potential energy functions

The large-amplitude bending motion in CsOH, a ‘classical’ molecule whose microwave spectrum was first recorded in 1967, has been studied ab initio. The three-dimensional potential energy surface has been calculated at the RCCSD(T)_DK3/[QZP + g ANO-RCC (Cs, O, H)] level of theory and employed in MORBID calculations of the rotation-vibration energies and intensities. The ground electronic state is ¹Σ⁺ with the equilibrium structure r_e(Cs-O) = 2.3930 Å, r_e(O-H) = 0.9587 Å, and ∠_e(Cs-O-H) = 180.0°. The O-H moiety is bound to Cs by an ionic bond and the molecule can be described as Cs^δ+(OH)^δ-. Hence, the bending potential is shallow and gives rise to large-amplitude bending motion. The ro-vibrationally averaged structural parameters, determined as expectation values over MORBID wavefunctions, are 〈r(Cs-O)〉₀ = 2.3987 Å, 〈r(O-H)〉₀ = 0.9754 Å, and 〈∠(Cs-O-H)〉₀ = 163°. Although the averaged structure in the vibrational ground state is far from being linear, the Yamada-Winnewissi-linearity parameter for CsOH is γ₀≈-1.0, the value characteristic for a linear molecule.     

Anharmonic force field and equilibrium structure of nitric acid

The quadratic, cubic and semi-diagonal quartic force field of nitric acid has been calculated at the CCSD(T) 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. It is found that the A and B semi-experimental equilibrium rotational constants of the ¹⁸O isotopologues (for which the rotation of principal axes is large) cannot be accurately reproduced. This problem is discussed and a remedy is proposed. Finally, the semi-experimental structure is in agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of at least quadruple-ζ quality and a core correlation correction, except for the long NO single bond for which the CCSD(T) value is too short due to inadequate treatment of electron correlation. The empirical structures are also determined and their accuracy is discussed. The best equilibrium structure is: r_e(NO_syn) = 1.209(1) Å, r_e(NO_anti) = 1.194(1) Å, r_e(NO) = 1.397(1) Å, r_e(OH) = 0.968(1) Å, ∠(ONO_syn) = 115.8(1)°, ∠(ONO_anti) = 114.2(1)° and ∠(NOH) = 102.2(1)°.     

A theoretical study of FeCN in the Δ electronic ground state

The three-dimensional potential energy and dipole moment surfaces for the electronic ground state ⁶Δ of FeCN have been computed at the MR-SDCI + Q + E_rel/[Roos ANO (Fe), aug-cc-pVQZ (C, N)] level of theory, where MR-SDCI means ‘multi-reference single and double excitation configuration interaction’ and ANO means ‘atomic natural orbital’. Based on these potential energy and dipole moment surfaces, the spectroscopic parameters, rovibronic energies, structural parameters, vibrational transition moments, and the wavenumbers and intensities of selected rotation-vibration transitions have been calculated. The equilibrium structure is linear with r_e(Fe-C) = 2.048 Å and r_e(C-N) = 1.168 Å, and the zero-point averaged structure is bent with 〈r(Fe-C)〉₀ = 2.082 Å, 〈r(C-N)〉₀ = 1.172 Å, and 〈∠(Fe-C-N)〉₀ = 170(5)°. At all the MR-SDCI + Q and the size-extensive multi-reference averaged quadratic coupled-cluster (MR-AQCC) levels of theory, with and without relativistic correction E_rel, that were employed in the present work, ⁶Δ FeCN is predicted to be slightly more stable than ⁶Δ FeNC. For example, the energy difference between the two isomers is approximately 150 cm⁻¹ at the highest level of theory employed, MR-AQCC + E_rel/[Roos ANO (Fe), aug-cc-pVQZ (C, N)] with zero-point energy correction. The electronic structure of ⁶Δ FeCN has also been compared with that of ⁶Δ FeNC. At present, no experimental spectroscopic data are available for ⁶Δ FeCN. It is hoped that the present work will stimulate experimental investigations of this molecule.     

Microwave and high resolution infrared spectra of vinyl chloride, ab initio anharmonic force field and equilibrium structure

The quadratic, cubic, and semi-diagonal quartic force field of vinyl chloride has been calculated at the MP2 level of theory employing a basis set of triple-ζ quality. The spectroscopic constants derived from this force field are compared with the experimental values. To make this comparison more complete, the rotational constants of the lowest excited state, v₉ = 1 at 395 cm⁻¹ have been determined by microwave spectroscopy and the ν₁₂ band (around 618 cm⁻¹) has been investigated by high-resolution infrared Fourier transform spectroscopy. The equilibrium structure has been derived from experimental ground state rotational constants and ab initio rovibrational interaction parameters. This semi-experimental structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quintuple-ζ quality and a core correlation correction. The experimental mass-dependent r_m structures are also determined and their accuracy is discussed. The recommended equilibrium geometry is: r (CC) = 1.3262(10), r (CCl) = 1.7263(10), r (CH_g) = 1.0784(10), r (CH_c) = 1.0795(10), r (CH_t) = 1.0797(10), ∠(CCCl) = 122.77(10)°, ∠(CCH_g) = 123.86(10)°, ∠(CCH_c) = 121.80(10)°, ∠(CCH_t) = 119.29(10)°.     

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.     

The Lamb-dip spectrum of phosphine: The nuclear hyperfine structure due to hydrogen and phosphorus

For the first time, the hyperfine structure of the rotational J = 1 ← 0 (K = 0) and J = 2 ← 1 (K = 0, 1) transitions of phosphine has been resolved by using microwave spectroscopy. To this purpose, the Lamb-dip technique has been employed. In addition, the J = 3 ← 2 (K = 0, 1, 2) transition has been recorded at Doppler resolution. The present investigation allowed us to provide accurate values for most of the hyperfine constants as well as ground state rotational parameters.     

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)°).     

A new joint analysis of the ground and first excited torsional states of methylformate

A global fit within experimental accuracy of microwave rotational transitions in the ground and first excited torsional states (v_t = 0 and 1) of methylformate (HCOOCH₃) is reported, which combines older measurements from the literature with new measurements from Kharkov. In this study the so-called ‘‘rho axis method’’ that treats simultaneously both A and E species of the ground and first excited torsional states is used. The final fit requires 55 parameters to achieve an overall unitless weighted standard deviation of 0.71 for a total of 10533 transitions (corresponding to 9298 measured lines) with rotational quantum numbers up to J ? 62 and K_a ? 26 in the ground state and J ? 35 and K_a ? 23 in the first excited torsional state. These results represent a significant improvement over past fitting attempts, providing for the first time a fit within experimental accuracy of both ground and first excited torsional states.     

The rotational spectrum and hyperfine structure of arsenic monophosphide, AsP

Arsenic monophosphide has been prepared by laser ablation of arsenic in the presence of PH₃. The J = 2-1 and 1-0 transitions in both the ground and first excited vibrational states have been measured with a cavity pulsed jet Fourier-transform microwave spectrometer. An improved equilibrium internuclear distance (r_e) has been obtained. Hyperfine structure in the rotational spectrum of AsP has been resolved and has led to the first determinations of the As nuclear quadrupole coupling constant and both nuclear spin-rotation constants. The data enable the electronic structure of AsP to be compared with those of other mixed Group 15 diatomic molecules.     

The Equilibrium Structure of Silyl Fluoride

The experimental equilibrium structure of silyl fluoride has been determined using new sets of accurate rotational constants that have recently been obtained by taking into account the most important interactions between the excited vibrational states. The equilibrium structure has also been calculated at the CCSD(T) level of theory with the cc-pVQZ+1 basis set (including corrections for the core correlation). The anharmonic force field up to semidiagonal quartic terms has been calculated at the MP2 level of theory and the equilibrium structure has been derived from the experimental rotational constants and the ab initio rovibrational interaction parameters. Finally, the average structure of both ²⁸SiH₃F and ²⁸SiD₃F has been reevaluated and used to derive the equilibrium structure. These structures are compared and the experimental structure is found to be in slight disagreement with the other ones. The preferred structure is obtained by calculating the median value of the different structures. The results are r_e(SiF)=1.5907 (9) Å, r_e(SiH)=1.4696 (13) Å, ∠_e(HSiF)=108.32(15)°, and ∠_e(HSiH)=110.60(14)°.     

A new analysis of the ν4(E) rovibrational band of the symmetric top molecule PF3

The high-resolution Fourier transform infrared spectrum of phosphorus trifluoride PF₃ have been reinvestigated in the ν₄ perpendicular band region around 347 cm⁻¹. Thanks to recent pure rotational measurements, 595 new infrared transitions of the ν₄ band have been assigned extending the rotational quantum number values up to K_max = 66 and J_max = 67. As a consequence of this extension, a sophisticated model containing a large number of parameters and interaction constants was adopted for the analysis of the IR transitions of the ν₄ fundamental band of PF₃. A merge of the IR transitions and the reported MW/MM/RF data within the v₄ = 1 excited level yielded an accurate rotational ground state C₀ value, 0.159970436 (69) cm⁻¹, which was used to determine an improved GS structure, r₀(P-F) = 1.56324405 (11) Å and ?(FPF) = 97.752232 (29)°. All experimental data have been refined applying various reduction forms of the effective rovibrational Hamiltonian developed for an isolated degenerate state of a symmetric top molecule. The v₄ = 1 excited state of the PF₃ oblate molecule was treated with models taking into account ?- and k-type intravibrational resonances. Parameters up to sixth order have been accurately determined and the unitary equivalence of the derived parameter sets in different reductions was demonstrated.     

Analysis of the first triad of interacting states (0 2 0), (1 0 0), and (0 0 1) of D2O from hot emission spectra

The far-infrared and middle-infrared emission spectra of deuterated water vapour were measured at temperatures 1370, 1520, and 1940 K in the ranges 320-860 and 1750-3400 cm⁻¹. The measurements were performed in an alumina cell with an effective length of hot gas of about 50 cm. More than 3550 new measured lines for the D₂¹⁶O molecule corresponding to transitions from highly excited rotational levels of the (0 2 0), (1 0 0), and (0 0 1) vibrational states are reported. These new lines correspond to rotational states with higher values of the rotational quantum numbers compared to previously published determinations: J_max = 29 and K_a(max) = 22 for the (0 2 0) state, J_max = 29 and K_a(max) = 25 for the (1 0 0) state, and J_max = 30 and K_a(max) = 23 for the (0 0 1) state. The extended set of 1987 experimental rotational energy levels for the (0 2 0), (1 0 0), and (0 0 1) vibration states including all previously available data has been determined. For the data reduction we used the generating function model. The root mean square (RMS) deviation between observed and calculated values is 0.004 cm⁻¹ for 1952 rovibrational levels of all three vibration states. A comparison of the observed energy levels with the best available values from the literature and with the global predictions from molecular electronic potential energy surfaces of water isotopic species [H. Partridge, D.W. Schwenke, J. Chem. Phys. 106 (1997) 4618] is discussed. The latter confirms a good consistency of mass-dependent DBOC corrections in the PS potential function with new experimental rovibrational data.     

Millimeter-wave spectrum of 2,3-difluorobenzonitrile and ab initio DFT calculations

The ground vibrational state rotational spectrum of 2,3-difluorobenzonitrile has been reinvestigated in the frequency range 40.0-99.0 GHz. High J and K₋₁ (J ? 62 and K₋₁ ? 20) transitions have been measured and analyzed to determine accurate rotational and centrifugal distortion constants. Finally, the experimental values were compared with the corresponding values computed at the DFT-B3PW91/6-31g(d,p) level of theory. A very good agreement has been found.