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.

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.

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.

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.

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.

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.

High-Resolution Infrared and Millimeter-Wave Study of D(3)SiF: The Ground and v(3)=1 States of the (29)Si and (30)Si Species, and the v(3)=v(6)=1 and v(3)=2 States of D(3) (28)SiF.

The nu(3) band of D(3)SiF near 890 cm(-1) recorded with a resolution of 2.4x10(-3) cm(-1) has been explored for the (29)Si and (30)Si isotopic species. Moreover, the nu(3)+nu(6)-nu(6) and 2nu(3)-nu(3) bands for the main (28)Si isotopomer have been assigned. For this purpose the nu(3)+nu(6) and 2nu(3) bands at 1435.697 and 1769.531 cm(-1) have been studied. Ground state parameters of the (29)Si and (30)Si species have been determined by merging newly measured MMW frequencies and ground state combination differences. In addition, v(3)=1 excited state parameters for these species have been obtained. While for the (28)Si species the v(3)=v(6)=1 state is locally perturbed by levels of the v(2)+v(5)=2 polyad, the v(3)=2 state appears to be unperturbed, its parameters being predictable from those of the v(3)=1 state. Anharmonicity constants x(33)=-4.1334 cm(-1) and x(36)=-3.6547 cm(-1) have been determined. Copyright 2001 Academic Press.

Experimental and ab Initio Equilibrium Structure and Harmonic Force Field of 1,2,5-Oxadiazole.

The equilibrium structure of 1,2,5-oxadiazole has been calculated ab initio at the CCSD(T) level using a polarized valence quadruple zeta basis set. The harmonic force field has also been calculated at the MP2/cc-pVTZ, B3LYP/6-311++G(3df, 2pd), and B3LYP/cc-pVQZ levels. These force fields have been subsequently scaled and further refined by fitting them to the experimental values of the vibrational fundamentals of three isotopomers and the centrifugal distortion constants of the parent molecule. The specific refinement of those scaled force constants particularly sensitive to the experimental data set was decisive for obtaining a more reliable harmonic potential. The resulting force fields are presented and used, together with the ground state rotational constants, to calculate an r(z) structure. The experimental r(0), r(s), and r(m) structures have also been determined. The different results have been compared and it is concluded that the ab initio structure is a good approximation of the equilibrium structure. It is also shown that the magnetic correction is not negligible, particularly for the inertial defect. Another interesting conclusion is that the anharmonicity of the C-H stretching might be unusually small. Copyright 2001 Academic Press.

Rovibrational and Rotational Spectroscopy of Levels of Propyne around 1000 cm(-1).

Four vibrational levels in the energy region around 1000 cm(-1) were studied. These were the v(5)=1 and v(8)=1 fundamental levels, both components of the v(9)=v(10)=1 combination level (l(9)=l(10)=+/-1 and l(9)=-l(10)=+/-1), and both components of the v(10)=3 overtone level (l(10)=+/-1 and +/-3). New FTIR spectra with a synchrotron radiation source were recorded in the region of the "superhot" v(10)=3<--2 bands, which made possible the first assignment of levels of the v(10)=3(+/-1) sublevel. More than 330 new rotational transitions in the combination and overtone levels were measured by millimeter-wave spectroscopy betwen 50 and 360 GHz. The new data were analyzed simultaneously together with the previously assigned rovibrational data for the fundamental and combination levels and rotational data for the fundamental levels using a global model with all anharmonic, Coriolis, l-type, and alpha-resonances. Significant improvement of data reproduction and very good consistency with the Hamiltonian parameters of the lower vibrational levels v(9)=1 and v(10)=1, 2 were achieved. A strong dependence of the A(v) constant on the l(10) quantum number is found for propyne: this is shown to be characteristic of skeleton C-C identical withC or C-C identical withN bending modes in H(3)CCCH, H(3)CCN, and their fully deuterated species. Copyright 2001 Academic Press.

The Ground and First Excited Torsional States of Acetic Acid.

A global fit of microwave and millimeter-wave rotational transitions in the ground and first excited torsional states (v(t) = 0 and 1) of acetic acid (CH(3)COOH) is reported, which combines older measurements from the literature with new measurements from Kharkov, Lille, and NIST. The fit uses a model developed initially for acetaldehyde and methanol-type internal rotor molecules. It requires 34 parameters to achieve a unitless weighted standard deviation of 0.84 for a total of 2518 data and includes A- and E-species transitions with J

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, 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.

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.

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.

Radiofrequency, Centimeter-Wave, Millimeter-Wave, and Infrared Spectra of SiHF(3): Investigation of the Ground, v(4) = 1, and v(6) = 1 Vibrational States.

The present paper deals with the analysis of the microwave, millimeter-wave, and infrared spectra of (28)SiHF(3) in its ground, v(6) = 1 and v(4) = 1 excited states. The former was observed up to 1055 GHz leading to the determination of one octic centrifugal distortion constant, L(J) = -0.0749(55) µHz. Furthermore the interaction term ||h(3,GS) || = 1.1032(70) mHz has been fitted from splittings of six K = 3 lines. The excited states have been regarded as isolated ones. This enabled fits according the Q-, D-, and QD-reduction schemes proposed by E. I. Lobodenko, O. N. Sulakshina, V. I. Perevalov, and Vl. G. Tyuterev, (J. Mol. Spectrosc. 126, 159-170 (1987)) and further developed by J. K. G. Watson, C. Gerke, H. Harder, and K. Sarka, (J. Mol. Spectrosc. 187, 131-141 (1997)) and Harder (J. Mol. Spectrosc. 194, 145 (1999)). A multiple fit analysis was performed confirming the assumption that the excited states are not affected by intervibrational resonances. Finally the millimeter spectrum of (29)SiHF(3) and (30)SiHF(3) in their ground state was also measured up to 460 GHz and accurate rotational and centrifugal distortion parameters were derived. 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.