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 case of the weak N-X bond: Ab initio, semi-experimental, and experimental equilibrium structures of XNO (X = H, F, Cl, OH) and FNO2

The equilibrium structures of FNO, ClNO, HONO, and FNO2 have been determined using three different, somewhat complementary methods: a completely experimental, a semi-experimental (where the equilibrium rotational constants are derived from the experimental effective ground-state rotational constants and an ab initio cubic force field), and an ab initio, where geometry optimizations are usually performed at the coupled cluster level of nonrelativistic electronic structure theory using small to very large Gaussian basis sets. For the sake of comparison, the equilibrium structures of HNO and N2O have also been redetermined, confirming and extending earlier results. The semi-experimental method gives structural parameters in good agreement with the reliable experimental results for each compound investigated. Because of inadequate treatment of electron correlation, the single-reference CCSD(T) method gives N-X (X[double bond]F, Cl, OH) bonds that are too strong and associate bond lengths that are significantly too short. The discrepancy increases with increase in the size of the basis set. A much more elaborate treatment of electron correlation at the CCSDTQ level solves this problem and results in increased bond lengths, correctly representing the weakness of the N-X bond in these XNO and XNO2 species. The equilibrium structures determined are accurate to better than 0.001 A and 0.1 degrees .     

Ab initio and equilibrium bond angles. Structures of HNO and H2O2

The possibility of calculating accurate ab initio bond angles is examined using a sample of 29 molecules (35 independent angles) containing only first row atoms and whose equilibrium structures are known. Three different correlated methods are compared: MP2, CCSD(T), and DFT, using the hybrid functional B3LYP. The convergence of Dunning's correlation consistent polarized valence basis sets, cc-pVnZ is also studied. It is found that the CCSD(T) method is consistently the most accurate; the DFT/B3LYP being slightly less reliable than MP2. It is shown that when convergence of the basis set is achieved (which is dependent on the kind of bonding) and when the effect of diffuse functions on electronegative atoms is taken into account, a high accuracy may be obtained: 0.03° for the median of absolute deviations or 0.07° for the mean absolute deviation. It does not exclude the possibility that the ab initio method may fail in some particular case, for instance when a large amplitude motion is involved. The MP2/cc-pVQZ method gives a mean absolute deviation of 0.22° to be compared with the 0.07° of the CCSD(T) method. To obtain these results, it was necessary to reanalyze the structure of a few molecules, particularly, a new and more accurate structure is proposed for nitroxyl, HNO and hydrogen peroxide, H₂O₂.     

Microwave spectra of dimethylsulfide-d6, (CD3)2S,ground and excited torsional states

The microwave spectra of the molecular isotope (CD₃)₂S in the ground state and the first and second excited states of methyl top torsion (internal rotation) and of CSC deformation as well as the ground-state spectra of the ¹³C and ³⁴S substituted forms have been measured. The rotational constants and centrifugal distortion and rotation-vibration interaction constants could be determined. The rotational lines in the excited torsional states (1₁, 1₂, 2₁, 2₂, 2₃) were found to be split into quartets due to the interaction between molecular rotation and methyl top internal rotation. The experimental multiplet splittings were fitted to those calculated from a rotation-internal rotation Hamiltonian in order to obtain values for the internal rotation barrier V₃ and the top-top interaction potential coefficients V₁₂ and V′₁₂. V₁₂ was too highly correlated with V₃ for a separate determination. The values following from the least-squares adjustment are discussed.     

Microwave spectra of propyne and its [13C] isotopic species: Refined molecular structure of propyne

The ground state microwave spectrum of propyne is remeasured between 60 and 240 GHz with a molecular beam spectrometer. These high resolution measurements allows us to accurately determine the rotational constant and all the determinable quartic and sextic centrifugal distortion constants: B = 8 545 877.12 (12) kHz; D_J = 2.9423 (13) kHz; D_JK = 163.423 (20) kHz; H_J = 0.0097 (40) Hz; H_JK = 0.935 (136) Hz; and H_KJ = 5.23 (27) Hz (95% confidence limits are shown in parentheses). The spectra of the [¹³C]isotopic species are also reinvestigated between 30 and 240 GHz with a video spectrometer. With the accurate rotational constants we derive from those spectra a new r₈ structure is calculated for the CC bond lengths: $\text{r}{}_8\text{(}\text{CC}\text{) = 1.4586 (2)}\text{A}\text{?}$ and $\text{r}{}_8\text{(}\text{CC}\text{) = 1.2066 (2)}\text{A}\text{?}$.     

High-resolution rotational spectrum of methyl cyanide

The ground-state microwave spectrum of methyl cyanide is remeasured between 70 and 240 GHz with a molecular beam spectrometer and by use of the Lamb dip method. It allows us to determine with great accuracy the B rotational constant, the quartic and sextic centrifugal distortion constants, the nuclear quadrupole coupling constant, and the spin-rotation constants of nitrogen. The magnetic shielding constants of nitrogen are also determined.     

High-Resolution Infrared and Millimeter-Wave Study of the Fermi-Coupled v(2) = 1 and v(3) = 2 States of DSiF(3)

The nu(2) (nu(eff.) 854.841 cm(-1)) and 2nu(3) infrared bands (nu(eff.) 840.083 cm(-1)) of DSiF(3) have been studied with a resolution of 2.5 x 10(-3) cm(-1). Moreover, millimeter-wave transitions in the v(2) = 1 and v(3) = 2 states up to J" = 33 have been measured. The assignments and fit of the poorly resolved, compressed cluster-type 2nu(3) IR transitions have been confirmed by a simultaneous study of the 2nu(3)-nu(3) band. The constant W = 5.116 cm(-1) of the Fermi interaction between the v(2) = 1 and v(3) = 2 levels has been determined from frequency effects which are in agreement with relative intensities of the nu(2) and 2nu(3) bands. The deperturbed (B(0) - B(v)) and (C(0) - C(v)) values of the states involved agree with their ab initio predictions within 7% in the worst case. Copyright 2001 Academic Press.     

Microwave and Submillimeter-Wave Measurements of HDCO in the ν4, ν5, and ν6 Bands: Evidence of Vibrational Induced Rotational Axis Switching (“VIRAS”)

The rotational spectrum of HDCO in the 4¹, 5¹, and 6¹ excited vibrational states has been investigated in Lille and Kiel using a sample enriched in deuterium. In Lille, the measurements were performed in the millimeter region (160-600 GHz). The spectra in Kiel were recorded using Fourier transform microwave spectrometers in the regions around 8-18 and 18-26 GHz, employing a rectangular waveguide of length 12 m and a circular waveguide of length 36 m, respectively. These results were combined with the 4¹, 5¹, and 6¹ infrared energy levels which were obtained from a previous analysis of FTS spectra of the ν₄ (CHD bend), ν₅ (CHD rocking), and ν₆ bands (out of plane bend) recorded in the 10-μm region at Giessen (A. Perrin, J.-M. Flaud, M. Smirnov, and M. Lock, J. Mol. Spectrosc.203, 175-187 (2000)). The energy level calculation of the 4¹, 5¹, and 6¹ interacting states accounts for the usual A- and B-type Coriolis resonances in the 5¹⇔6¹ and 4¹⇔6¹ off diagonals blocks. In addition, since the energy levels of the 5¹ and 6¹ states are very strongly resonating, it proved necessary, as in our previous study, to use a {J_x, J_z} nonorthorhombic term in the 5¹ and 6¹v-diagonal blocks of the Hamiltonian matrix in order to reproduce properly the observed microwave transitions and infrared energy levels. Therefore, this work confirms that HDCO is a good example of the vibrational induced rotational axis switching (“VIRAS”) effect.     

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)°.     

Millimeter-Wave Spectroscopy, High-Resolution Infrared Spectrum, ab Initio Calculations, and Molecular Geometry of FPS

The transient thiophosphenous fluoride FPS was produced by pyrolysis of 2.5% F₂PSPF₂ in Ar at 1300–1800°C. High-resolution (≥0.004 cm⁻¹) Fourier transform infrared spectra of the a-type ν₁ and b-type ν₂ bands, centered respectively at 803.249 and 726.268 cm⁻¹, were measured and fitted to rotational and quartic centrifugal distortion parameters. The millimeter-wave spectrum, essentially b-type, was measured between 300 and 370 GHz in the ground state and in the ν₃ excited state for FP³²S and in the ground state for FP³⁴S. The frequencies were fitted to a Watson-type A-reduced Hamiltonian up to sextic distortion terms. High level ab initio calculations with large basis sets were performed on FPS and supported the first identification of its infrared and millimeter wave spectra. The calculated anharmonic force field provided precise ab initio rovibrational α constants which were combined with the experimental molecular parameters to determine an accurate equilibrium structure of the molecule: r_e(PS)=188.86 pm, r_e(PF)=158.70 pm, θ(FPS)=109.28°. The collision-controlled 1/e lifetime measured in a 10-Pa (1 : 20) F₂PSPF₂/Ar mixture was 2 s, more than two orders of magnitude larger than that of FPO under the same experimental conditions.