Comparison of the experimental, semi-experimental and ab initio equilibrium structures of acetylene: influence of relativisitic effects and of the diagonal Born-Oppenheimer corrections.

The equilibrium structure of acetylene (also named ethyne) has been reinvestigated to resolve the small discrepancies noted between different determinations. The size of the system as well as the large amount of available experimental data provides the quite unique opportunity to check the magnitude and relevance of various contributions to equilibrium structure as well as to verify the accuracy of experimental results. With respect to pure theoretical investigation, 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, inclusion of core correlation effects as well as relativistic and diagonal Born-Oppenheimer corrections. In particular, it is found that the extrapolation to the complete basis set limit, the inclusion of higher excitations in the electronic-correlation treatment and the relativistic corrections are of the same order of magnitude. It also appears that a basis set as large as a core-valence quintuple-zeta set is required for accurately accounting for the inner-shell correlation contribution. From a pure experimental point of view, the equilibrium structure has been determined using very accurate rotational constants recently obtained by a "global analysis" (that is to say that all non-negligible interactions are explicitly included in the Hamiltonian matrix) of rovibrational spectra. Finally, a semi-experimental equilibrium structure (where the equilibrium rotational constants are obtained from the experimental ground state rotational constants and computed rovibrational corrections) has been obtained from the available experimental ground-state rotational constants for ten isotopic species corrected for computed vibrational corrections. Such a determination led to the revision of the ground-state rotational constants of two isotopologues, thus showing that structural determination is a good method to identify errors in experimental rotational constants. The three structures are found in a very good agreement, and our recommended values are r(CC) = 120.2958(7) pm and r(CH) = 106.164(1) pm.

(12)C2H2-Ar van der Waals complex.

New theoretical and experimental results on the acetylene-Ar van der Waals complex are presented and the literature is reviewed. New ab initio calculations at the MP2 level were performed using large basis sets with diffuse functions and taking into account the basis set superposition error. It was found that the structure of acetylene is not significantly altered by the complexation and that its vibrational frequencies are only slightly lowered. Finally, it was observed that the calculated properties of the complex (structure, vibrational spectrum, bond dissociation energy) are not sensitive to the structure imposed on acetylene. Experimentally, acetylene-Ar was produced in a supersonic expansion under experimental conditions corresponding to 9 K rotational temperature. Thanks to the performances of CW-CRDS detection, the K(a) = 0 <-- 1, 1 <-- 0, and 2 <-- 1 sub-bands of the nu(1) + nu(3) band could be recorded and resolved and most of their lines assigned. Upper-state rotational constants were fitted, however not including the upper K(a) = 2 state, which shows K-doubling the opposite of the expected. The Lorentzian width of most line profiles sets the mean lifetime to some 7.5 ns. Local perturbations affecting line positions and/or line widths are demonstrated. Additional series of lines tentatively attributed to acetylene-Ar are discussed.

Gas-phase vibrational spectroscopy and ab initio study of organophosphorus compounds: discrimination between species and conformers.

Gas phase vibrational spectra of dimethyl methylphosphonate (DMMP), trimethyl phosphate (TMP), and triethyl phosphate (TEP) have been measured using FTIR spectroscopy. For DMMP, TMP, and TEP, most of the infrared active vibrational modes have been observed in the 50-5000 cm (-1) spectral range, allowing an unambiguous discrimination between the three molecules. The vibrational analysis of the spectra was performed by comparing with MP2 and B3LYP harmonic and anharmonic force field ab initio calculations. The extension to anharmonic calculations provides the best agreement for the mid-infrared and the near-infrared spectra, but they do not improve the harmonic frequency predictions in the far-infrared domain. This part of the vibrational spectra associated with collective and nonlocalized vibrational modes presents the largest frequency differences between the two lowest energy conformers of DMMP and TMP. These two conformers were taken into account in the vibrational assignment of the spectra. Their experimental evidence was obtained by deconvoluting vibrational bands in the mid-infrared and in the far-infrared regions, respectively. For TEP, the conformational landscape appears very complicated at ambient temperature, and a further analysis at low temperature is required to explain the vibrational features of each conformer.

High-resolution infrared spectroscopy of trans- and cis-H(18)ON(18)O: equilibrium structures of the nitrous acid isomers.

In this paper, we present the first high-resolution spectra and analysis of the nu 4 fundamental bands of fully (18)O-substituted nitrous acid, trans- and cis-H(18)ON(18)O. These bands are not perturbed by neighboring vibrational levels and were used to determine for the first time accurate rotational and centrifugal distortion constants of the ground and nu 4 = 1 states of trans- and cis-H(18)ON(18)O. The ground-state rotational constants were then used, together with the rotational constants of other HONO isotopic species and with rotation-vibration parameters from ab initio calculations, to determine accurate semi-experimental equilibrium structures of trans- and cis-HONO. Our study confirms the results of a recent work by Demaison et al. (J. Phys. Chem. A 2006, 110, 13609-13617) concerning the structure of trans-HONO, whereas the new structure of cis-HONO obtained in this paper is a significant improvement compared with the previous work of Cox et al. (J. Mol. Struct. 1994, 320, 91-106). The recommended parameters for the equilibrium structure of cis-HONO are r(e)(ON) = 1.1816(10) A, r(e)(N-O) = 1.3887(10) A, r(e)(O-H) = 0.9744(7) A, angle(e)(ONO) = 113.18(1) degrees, and angle(e)(HON) = 104.67(4) degrees.

Anharmonic force field of cis- and trans-formic acid from high-level ab initio calculations, and analysis of resonance polyads.

The quadratic, cubic, and semidiagonal quartic force fields of cis- and trans-formic acid have been calculated using three different levels of theory. They all give satisfactory results, including the one at the lowest level of theory which is the MP2 method employing a basis set of triple-zeta quality. The results are used to theoretically analyze resonance polyads, including the one involving the 4(1), 5(1), 6(1), 8(1), 7(1)9(1), and 9(2) vibrational states. A semiexperimental equilibrium structure is derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. The ab initio structure calculated at the CCSD(T) level of theory using a basis set of quintuple-zeta quality is in excellent agreement with the semiexperimental structure.

Equilibrium structure of sulfuric acid.

The equilibrium structure of the more stable conformer of H2SO4, of C2 symmetry, has been calculated ab initio using the CCSD(T) method and taking into account the core correlation correction. The accuracy of this structure has been checked by comparing it to that of similar molecules and by estimating the effects of basis set enlargement and of diffuse functions. Furthermore, the quadratic, cubic, and quartic force fields have been calculated at the MP2 level of theory using a basis set of triple-zeta quality. The spectroscopic constants derived from the force field are in satisfactory agreement with the experimental ones. The resulting band origins are compared to literature infrared values, including those for overtone and combination bands. Normal modes of vibration are pictured. Using this force field, semiexperimental equilibrium rotational constants are determined which allows us to check the accuracy of the ab initio structure and to refine it using a mixed regression method.

The chiral molecule CHClFI: first determination of its molecular parameters by Fourier transform microwave and millimeter-wave spectroscopies supplemented by ab initio calculations.

The rotational spectrum of chlorofluoroiodomethane (CHClFI) has been investigated. Because its rotational spectrum is extremely crowded, extensive ab initio calculations were first performed in order to predict the molecular parameters. The low J transitions were measured using a pulsed-molecular-beam Fourier transform spectrometer, and the millimeter-wave spectrum was measured to determine accurate centrifugal distortion constants. Because of the high resolution of the experimental techniques, the analysis yielded accurate rotational constants, centrifugal distortion corrections, and the complete quadrupole coupling tensors for the iodine and chlorine nuclei, as well as the contribution of iodine to the spin-rotation interaction. These molecular parameters were determined for the two isotopologs CH35ClFI and CH37ClFI. They reproduce the observed transitions within the experimental accuracy. Moreover, the ab initio calculations have provided a precise equilibrium molecular structure. Furthermore, the ab initio molecular parameters are found in good agreement with the corresponding experimental values.

The Millimeter-Wave Spectrum of 2,3-Dihydrofuran

The millimeter-wave spectrum of 2,3-dihydrofuran in the ground and five ring-puckering excited states has been measured in the frequency range 100-250 GHz. The ground and first ring-puckering excited states have been fitted to a two-state Hamiltonian including Coriolis coupling interaction. The determined energy difference of 18.684(7) cm-1 between these states and the a and b type coupling parameters are consistent with the ring-puckering potential function and the previously observed dependence of the centrifugal distortion constants DeltaJK, DeltaK, and deltaK. A small ring-puckering dependence of the quartic centrifugal distortion constants DeltaJ and deltaJ has been also observed. This dependence is well accounted for in terms of the ring-puckering potential function and the vibrational dependence of the rotational constants.

Structural and conformational properties of 2-propynylphosphine (propargylphosphine) as studied by microwave spectroscopy supplemented by quantum chemical calculations.

The microwave spectrum of 2-propynylphosphine (propargylphosphine), H-C triple bond C-CH2-PH2, has been investigated in the 18-26.5 and 32-48 GHz spectral regions at about -50 degrees C. Two conformers with different orientation of the phosphino group, denoted conformer I and conformer II, respectively, were assigned. Conformer I has a symmetry plane (Cs symmetry) with both hydrogen atoms of the phosphino group pointing toward the triple bond (C-C-P-H dihedral angles approximately 47 degrees from syn-periplanar (0 degrees )). The C-C-P-H dihedral angles are 73 and 167 degrees, respectively, from syn-periplanar in conformer II. Only one of the hydrogen atoms of the phosphino group points toward the triple bond in this rotamer. Conformer I is 1.5(20) kJ/mol more stable than II. The dipole moment of II was determined to be (in units of 10(-30) C m) mu(a) = 0 (assumed), mu(b) = 3.05(7), mu(c) = 1.60(9), and mu(tot) = 3.44(9) [mu(tot) = 1.03(3) D]. Two vibrationally excited states were assigned for each of the two rotamers I and II. Their frequencies were determined by relative intensity measurements. Many of the transitions of conformer II were split into two components presumably because of tunneling of the phosphino group. The tunneling frequency was determined to be 0.814(42) MHz for the ground vibrational state and 11.49(18) MHz for the first excited state of the C-P torsional vibration. Quantum chemical calculations at the B3LYP and MP2 levels of theory using the 6-311++G(3df,2pd) basis set reproduced experimental rotational constants, quartic centrifugal distortion constants, and dipole moment components within a few percent. The energy difference between the two conformers was calculated using the Gaussian-2 theory, and conformer I was found to be more stable than conformer II by 2.1 kJ/mol.

High-Resolution Infrared Spectrum of D3Si79Br in the nu1/nu4 Region: The Structure of Silyl Bromide

FTIR spectra of monoisotopic D3Si79Br covering the bands nu1 (a1, 1580.637 cm-1) and nu4 (e, 1615.085 cm-1) have been recorded with a resolution of 3 x 10(-3) cm-1. The rovibrational analysis revealed severe perturbations of the -5

The Submillimeter-Wave Spectrum and Quantum Chemical Calculations of Glyoxylic Acid.

Glyoxylic acid is a possible candidate for interstellar detection. Many transitions of the submillimeter wave spectrum of the ground vibrational state of its most stable conformer have been measured for the first time. These transitions have been used together with microwave transitions measured previously to obtain accurate spectroscopic constants that should facilitate a search for this compound in interstellar space. High-level quantum chemical calculations of the structure, quartic centrifugal distortion constants, inertial defect, and energy difference between the two low-energy conformers of glyoxylic acid have also been made. Accurate predictions of the equilibrium structures of the most stable forms of glyoxylic, as well as of formic acid, are reported. Copyright 2001 Academic Press.

Rovibrational Spectroscopy of the v(5) = 1 Level of (28)SiDF(3).

The nu(5) fundamental band of trifluorosilane-d (SiDF(3)) at 627 cm(-1) was studied for the first time by high-resolution FTIR spectroscopy at a resolution of 2.4 x 10(-3) cm(-1). The analysis was performed simultaneously with available microwave and newly measured submillimeter-wave data in the approximation of an isolated degenerate fundamental level of a C(3 Kv) symmetric top molecule leading to a standard deviation of 0.22 x 10(-3) cm(-1) for the reproduction of the infrared wavenumbers, 36 kHz for the microwave, and 198 kHz for the submillimeter-wave frequencies, respectively. The unitary equivalence between the two reductions (Q and D) of the effective Hamiltonian applied in the analysis is demonstrated. Copyright 2000 Academic Press.

High-Resolution Infrared Spectrum of BrCN in the nu(2) and nu(1)/2nu(2) Regions.

FT infrared spectra of BrCN have been recorded in the region of the nu(2) band near 340 cm(-1), the nu(1) band near 580 cm(-1), and the 2nu(2) band near 690 cm(-1) with a resolution between 2.9 and 4.7 x 10(-3) cm(-1). The vibrational levels (01(1)0), (10(0)0), (02(0)0), (02(2)0), (11(1)0), and (20(0)0) have been analyzed employing cold bands, hot bands, and new millimeter-wave transitions. Band-by-band polynomial analyses and a combined fit of all data relevant to the 2v(1) + v(2) = 2 polyad levels have been performed. The latter fit considered l-resonance interactions between the (02(0)0), e and (02(2)0), e levels and Fermi resonance between the two Sigma states (10(0)0) and (02(0)0). Altogether about 1000 pieces of data up to J = 100 were fitted for each of the two isotopic species with rms of the residuals of 2-8 x 10(-4) cm(-1) for the infrared and 10-120 kHz for the pure rotational data. Copyright 2000 Academic Press.

High-Resolution Infrared and Millimeter-Wave Study of the v(3) = 1 State of HSiF(3) and DSiF(3).

Millimeter-wave spectra of HSiF(3) and DSiF(3) in the v(3) = 1 excited state have been measured from 100 to 490 GHz. Infrared spectra have been recorded in the nu(3) regions, nu(0) 424.0301 and 420.9320 cm(-1) in HSiF(3) and DSiF(3), respectively, with a resolution of 2.4 x 10(-3) cm(-1). Since in both species the parameters alpha(B)(3) and alpha(C)(3) have very similar values, no K structure could be resolved in the (Q)P and (Q)R clusters for low-to-medium K values. For high J the effect of the ground state D(JK) term more and more dominates and spreads the J clusters into opposite directions such that medium-to-high K components, particularly those with K = 3p, are resolved. Rotational and infrared data have been fitted together using a model up to sextic centrifugal distortion constants. No perturbations were indicated. Hot bands (nu(3) + nnu(6))-nnu(6) with n = 1, 2, and 3 have been detected and analyzed. Copyright 2000 Academic Press.

Centrifugal Distortion Analysis of a Near-Spherical Top, SO(2)F(2): The First Determination of All Six Quartic Centrifugal Distortion Constants for an Asymmetric Top.

The rotational spectrum of the near-spherical top molecule SO(2)F(2) (sulfuryl fluoride) has been investigated by microwave Fourier transform spectroscopy and by millimeter-wave spectroscopy. The ground state spectrum has been measured from 10 to 472 GHz. One of the reasons for studying this molecule is that it is a nearly spherical top and we wanted to verify our theoretical prediction that for such a molecule all six quartic centrifugal distortion constants (and nine sextic distortion constants) should be determinable, while for a standard asymmetric rotor, Watson has shown that only five quartic and seven sextic distortion constants are determinable. The analysis of the spectra confirmed our predictions, because all six quartic constants were well determinable. The results have been confirmed independently by ab initio calculations of the force field and quartic distortion constants. Because the molecule is relatively heavy, contributions of some sextic constants are too small and we have not been able to determine all nine sextic constants predicted by theory. Copyright 2000 Academic Press.

Microwave, Submillimeter-Wave, and High-Resolution FTIR Study of SO(2)F(2) in the nu(8) State.

The high-resolution infrared spectrum of the nu(8) band of SO(2)F(2) (nu(as) SF(2)) centered at 887.2 cm(-1) has been recorded with a resolution of 2.4 x 10(-3) cm(-1). More than 8000 transitions of the C-type band with DeltaK(a) = +/-1 (and in addition some DeltaK(a) = +/-3 transitions) have been assigned. Microwave and millimeter-wave spectra of the v(8) = 1 state up to 450 GHz have been recorded, and 177 pure rotational transitions have been measured. Rotational and rovibrational data have been combined, and excited state parameters up to sextic centrifugal distortion constants have been determined using a Watson-type Hamiltonian in S-reduction. No perturbation was indicated. Copyright 2000 Academic Press.

The Rotational Spectrum of H2Te

In the present work, we study the spectrum of the H2Te molecule in the submillimeter-wave and far infrared region. An important aim of this investigation is the further experimental characterization of the anomalous "four-fold cluster effect" exhibited by the rotational energy levels in the vibrational ground state of H2Te. The spectrum in the region 90-472 GHz was measured with a source-modulated millimeter-wave spectrometer and that between 600 and 1600 GHz with a far-infrared sideband spectrometer. The far infrared spectrum from 30 to 360 cm-1 was measured with a Bruker IFS 120 HR interferometer attached to a 3 m long cell. We have assigned 224 submillimeter-wave lines and 1695 FIR lines. These observed data were supplemented by a large number of ground state combination differences derived from rotation-vibration bands of H2Te, and the resulting large data set was analyzed by means of a modified Watson Hamiltonian. Accurate sets of rotational and centrifugal distortion constants for all eight tellurium isotopomers were obtained.

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.

Rotational Spectrum of Vinylarsine.

The rotational spectrum of vinylarsine in the ground state has been studied in the range 7-320 GHz. The spectra of a syn conformer and a gauche conformer have been unambiguously assigned on the basis of the existence of a b-type or a c-type spectrum. Rotational constants, quartic, and some sextic centrifugal distortion constants were derived. For the syn form, measurements of low J aR0,1 transitions in a pulsed-nozzle Fourier transform microwave spectrometer (FTMWS) enabled the determination of the diagonal elements of the quadrupole tensor, as well as two spin-rotation constants. Ab initio calculations performed at the MP2 level using the 6-311++G(3df, 3pd) basis set reproduced experimental rotational constants within 0.2%. Copyright 1998 Academic Press.

Rotational Spectra of the Isotopic Species of Chloroform: Experimental and Ab Initio Structures.

The millimeter-wave spectra of three different samples of chloroform (CHCl3, CDCl3, and 13CHCl3) have been measured between 145 and 470 GHz which corresponds to J values between 22 and 70. We report accurate rotational and centrifugal distortion constants for the ground vibrational states of 11 isotopic species. The experimental ro, rs, rIepsilon, rrhom, and rz structures have been determined using the determined rotational constants. The structure has also been calculated ab initio at the SCF, MP2, RQCISD, and B3LYP levels using triple zeta polarized basis sets. The experimental results are found in excellent agreement with the ab initio predictions. An approximate equilibrium structure has been obtained by combining the experimental results and the ab initio calculations: re(C-H) = 1.080 (2) Å, re(C-Cl) = 1.760 (2) Å, and anglee(HCCl) = 108.23 (2) degrees. Copyright 1998 Academic Press.