Ab Initio Anharmonic Force Field Research Articles

Lowest vibrational states of acrylonitrile from microwave and synchrotron radiation spectra

The high resolution Fourier-transform spectrum of acrylonitrile covering the 40–700cm−1 spectral region was recorded at the AILES infrared beamline of the SOLEIL synchrotron. The spectrum allowed assignment of vibration–rotation transitions in four different fundamental bands, five hot bands, one overtone band, as well as of some pure rotational transitions. The new infrared data and previous measurements made with microwave techniques have been combined into a single global fit encompassing over 31000 measured transitions. Precise vibrational term values have been determined for the eight lowest excited vibrational states, including ν11=228.299930(4), ν15=332.678207(4), ν10=560.716701(5), and ν14=681.793862(13)cm−1. The new values are compared with those obtained previously entirely on the basis of rotational perturbations. Several anharmonicity coefficients are determined and compared with abinitio anharmonic force field calculations. The assignment of the ν10 mode is also clarified.

Laboratory characterization and astrophysical detection of vibrationally excited states of vinyl cyanide in Orion-KL

New laboratory data of CH$_2$CHCN (vinyl cyanide) in its ground and vibrationally excited states at the microwave to THz domain allow searching for these excited state transitions in the Orion-KL line survey. Frequency-modulated spectrometers combined into a single broadband 50-1900 GHz spectrum provided measurements of CH$_2$CHCN covering a spectral range of 18-1893 GHz, whose assignments was confirmed by Stark modulation spectra in the 18-40 GHz region and by ab-initio anharmonic force field calculations. For analyzing the emission lines of CH$_2$CHCN species detected in Orion-KL we used the excitation and radiative transfer code (MADEX) at LTE conditions. The rotational transitions of the ground state of this molecule emerge from four cloud components of hot core nature which trace the physical and chemical conditions of high mass star forming regions in the Orion-KL Nebula. The total column density of CH$_2$CHCN in the ground state is (3.0$\pm$0.9)x10$^{15}$ cm$^{-2}$. We report on the first interstellar detection of transitions in the v10=1/(v11=1,v15=1) dyad in space, and in the v11=2 and v11=3 states in Orion-KL. The lowest energy vibrationally excited states of vinyl cyanide such as v11=1 (at 328.5 K), v15=1 (at 478.6 K), v11=2 (at 657.8 K), the v10=1/(v11=1,v15=1) dyad (at 806.4/809.9 K), and v11=3 (at 987.9 K) are populated under warm and dense conditions, so they probe the hottest parts of the Orion-KL source. Column density and rotational and vibrational temperatures for CH$_2$CHCN in their ground and excited states, as well as for the isotopologues, have been constrained by means of a sample of more than 1000 lines in this survey. Moreover, we present the detection of methyl isocyanide (CH$_3$NC) for the first time in Orion-KL and a tentative detection of vinyl isocyanide (CH$_2$CHNC) and give column density ratios between the cyanide and isocyanide isomers.

Far-Infrared Spectrum of S(CN)2 Measured with Synchrotron Radiation: Global Analysis of the Available High-Resolution Spectroscopic Data

The high resolution Fourier transform spectrum of the chemically challenging sulfur dicyanide, S(CN)2, molecule was recorded at the far-infrared beamline of the synchrotron at the Canadian Light Source. The spectrum covered 50-350 cm(-1), and transitions in three fundamentals, ν4, ν7, and ν8, as well as in the hot-band sequence (n + 1)ν4 - nν4, n = 1-4, have been assigned and measured. Global analysis of over 21,300 pure rotation and rotation vibration transitions allowed determination of precise energies for 12 of the lowest vibrationally excited states of S(CN)2, including the five lowest fundamentals. These results constitute an extensive set of benchmarks for ab initio anharmonic force field calculations and the observed and calculated vibration-rotation constants and anharmonic frequencies are compared. The semiexperimental equilibrium, r(e)(SE), geometry of S(CN)2 has also been evaluated. In the course of the measurements, new information concerning the physical chemistry of S(CN)2 has been obtained.

Broadband rotational spectroscopy of acrylonitrile: Vibrational energies from perturbations

The coverage of the room-temperature rotational spectrum of acrylonitrile has been expanded to a total of 1170GHz, by recording broadband spectral segments at frequencies ranging 90–1900GHz. This corresponds, in total, to 61.6% coverage of the rotational spectrum up to 1.9THz and facilitated an in depth study of rotational transitions in the lowest vibrational states of acrylonitrile and up to large values of rotational quantum numbers. Multiple perturbations between the four lowest vibrational states of the molecule have been identified and successfully fitted within the framework of a coupled four state Hamiltonian. The fit encompasses over 12500 measured transition frequencies, and delivers precise wavenumbers for the three lowest excited vibrational states entirely on the basis of perturbations in the rotational spectrum: ν11=228.29986(2), ν15=332.67811(2), and 2ν11=457.17496(2)cm−1. The new results are compared with ab initio anharmonic force field calculations and the techniques used to deal in an efficient manner with a broadband, high-resolution spectrum of this type are also described in some detail.

Anharmonic force field and vibrational dynamics of CH2F2 up to 5000 cm−1 studied by Fourier transform infrared spectroscopy and state-of-the-art ab initio calculations

Difluoromethane (CH(2)F(2), HFC-32) is a molecule used in refrigerant mixtures as a replacement of the more environmentally hazardous, ozone depleting, chlorofluorocarbons. On the other hand, presenting strong vibration-rotation bands in the 9 μm atmospheric window, it is a greenhouse gas which contributes to global warming. In the present work, the vibrational and ro-vibrational properties of CH(2)F(2), providing basic data for its atmospheric modeling, are studied in detail by coupling medium resolution Fourier transform infrared spectroscopy to high-level electronic structure ab initio calculations. Experimentally a full quantum assignment and accurate integrated absorption cross sections are obtained up to 5000 cm(-1). Ab initio calculations are carried out by using CCSD(T) theory and large basis sets of either the correlation consistent or atomic natural orbital hierarchies. By using vibrational perturbation theory to second order a complete set of vibrational and ro-vibrational parameters is derived from the ab initio quartic anharmonic force fields, which well compares with the spectroscopic constants retrieved experimentally. An excellent agreement between theory and experiment is achieved for vibrational energy levels and integrated absorption cross sections: transition frequencies up to four quanta of vibrational excitation are reproduced with a root mean square deviation (RMSD) of 7 cm(-1) while intensities are predicted within few km mol(-1) from the experiment. Basis set performances and core correlation effects are discussed throughout the paper. Particular attention is focused in the understanding of the anharmonic couplings which rule the vibrational dynamics of the |ν(1)>, |2ν(8)>, |2ν(2)> three levels interacting system. The reliability of the potential energy and dipole moment surfaces in reproducing the vibrational eigenvalues and intensities as well as in modeling the vibrational and ro-vibrational mixings over the whole 400-5000 cm(-1) region is also demonstrated by spectacular spectral simulations carried out by using the ro-vibrational Hamiltonian constants, and the relevant coupling terms, obtained from the perturbation treatment of the ab initio anharmonic force field. The present results suggest CH(2)F(2) as a prototype molecule to test ab initio calculations and theoretical models.

The ν 1, ν 2, and ν 3 bands of carbonyl chlorofluoride (COFCl) at 5.3, 9.1, and 13.1 μm: position and intensity parameters and their use for atmospheric studies

A list of line positions and, for the first time, of line intensities was generated for the ν 1, ν 2, and ν 3 fundamental bands of the 12C 16OF 35Cl and 12C 16OF 37Cl isotopologs of carbonyl chlorofluoride, located at 5.3, 9.1, and 13.1 μm, respectively. In addition, for the most abundant isotopolog ( 12C 16OF 35Cl) this linelist includes also the contributions from the first two associated hot bands. The parameters included in this database were generated by combining the results of previous experimental analyses and ab initio calculations [Perrin A, Flaud JM, Bürger H, Pawelke G, Sander S, Willner H. First high resolution analysis of the six fundamental bands ν 1, ν 2, ν 3, ν 4, ν 5 and ν 6 of COF 35Cl in the 340 to 2000 cm −1 region. J Mol Spectrosc 2001;209:122–232; Demaison J, Perrin A, Bürger H. Ab initio anharmonic force field and equilibrium structure of carbonyl chlorofluoride. J Mol Spectrosc 2003;221:47–56]. For the purpose of the present work, a partial re-investigation of the ν 1 of COF 35Cl was performed, together with the first identification of the ν 2 band of COF 37Cl. These parameters were generated in order to improve the quality of remote sensing of the atmosphere in the mid-IR. Analyses of atmospheric solar occultation spectra measured by the JPL MkIV interferometer show that the new linelist not only improves the quality of retrievals of COFCl, but also of several other gases whose absorptions overlap those of COFCl.

Ab Initio Anharmonic Force Field and Rotational Analyses of Infrared Bands of Perchloryl Fluoride

Perchloryl fluoride, FClO(3), has been studied both experimentally and theoretically, using high-resolution Fourier transform spectroscopy and high-level ab initio methods. The geometry, dipole moment, and anharmonic force field of the molecule have been calculated ab initio, using the coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)] level of theory. The infrared spectra of monoisotopic species have been recorded. The nu(1) + nu(2) bands of F(35)Cl(16)O(3), F(37)Cl(16)O(3), F(35)Cl(18)O(3), and F(37)Cl(18)O(3), the nu(2) + nu(3) and nu(2) + nu(3) - nu(3) bands of F(35)Cl(16)O(3) and F(37)Cl(16)O(3), and the 2nu(3) - nu(3) band of F(37)Cl(16)O(3) have been analyzed. The spectroscopic parameters obtained from these and from previous analyses have been compared with the theoretical results.

Ab initio anharmonic force field and equilibrium structure of vinyl fluoride and vinyl iodide

The quadratic, cubic and semi-diagonal quartic force field of vinyl fluoride and vinyl iodide have been calculated at the MP2 level of theory employing a basis set of triple- ζ quality including a relativistic pseudopotential on iodine. 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. Finally, the structure of different vinyl derivatives is compared.

Infrared absorption spectra of matrix-isolated cis, cis-HOONO and its ab initio CCSD(T) anharmonic vibrational bands

The infrared absorption spectra of matrix-isolated cis, cis-peroxynitrous acid (HOONO and DOONO) in argon have been observed. Six of the nine fundamental vibrational modes for cis, cis-HOONO have been assigned definitively, and one tentatively. Coupled-cluster, ab initio anharmonic force field calculations were used to help guide some of the assignments. The experimental matrix frequencies (cm(-1)) for cis, cis-HOONO are (a' modes) nu1 = 3303+/-1, nu2 = 1600.6+/-0.6, nu3 = 1392+/-1, nu4 = 922.8+/-0.5, nu5 = 789.7+/-0.4, nu6 = 617+/-1; and (a" mode) nu8 = 462+/-1. The fundamentals for the deuterated isotopomer, cis, cis-DOONO, are (a' modes) nu1 = 2447.2+/-0.6, nu2 = 1595.7+/-0.7, nu3 = 1089.1+/-0.4, nu4 = 888.1+/-0.4, nu5 = 786.6+/-0.5, nu6 = 613.9+/-0.9; and (a" mode) nu8 = 456.5+/-0.5.

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, v 9 = 1 at 395 cm −1 have been determined by microwave spectroscopy and the ν 12 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 (C C) = 1.3262(10), r (C Cl) = 1.7263(10), r (C H g ) = 1.0784(10), r (C H c ) = 1.0795(10), r (C H t ) = 1.0797(10), ∠(CCCl) = 122.77(10)°, ∠(CCH g ) = 123.86(10)°, ∠(CCH c ) = 121.80(10)°, ∠(CCH t ) = 119.29(10)°.

Ground state rotational spectrum, K = 3 splittings, ab initio anharmonic force field and equilibrium structure of trifluoroamine

The submillimetre-wave spectrum of 14NF3 has been measured and the ground state rotational spectrum has been reanalysed, including the K=3 splittings. The quadratic, cubic and semidiagonal quartic force field has been calculated at the CCSD(T) level of theory employing a basis set of at least polarized valence triple-zeta quality. This force field has been used to predict the spectroscopic constants, including the parameters specific to the doubly degenerate vibrational states. The calculated values are found to be in good agreement with the available experimental data. The equilibrium structure has been derived from the experimental ground state rotational constants and either the ab initio or the experimental rovibrational interaction parameters. These experimental and semiexperimental structures are in excellent agreement with the ab initio equilibrium geometry.

Tunable Diode Laser Spectroscopy of cis-1,2-Difluoroethylene: Rovibrational Analysis of the ν9 and ν5+ν10 Bands and Anharmonic Force Field

cis-1,2-Difluoroethylene was synthesized, and the gas-phase infrared spectrum was investigated in the ν9 and ν5+ν10 band region (1350−1391 cm-1), at a resolution of about 0.002 cm-1, employing a tunable diode laser spectrometer. These vibrations of symmetry species B1 yield strong a-type bands. Most of the J and K structure was recorded in different subbranches and the rovibrational analysis led to the assignment of 2313 (J ≤ 64, Ka ≤ 26) and 1172 (J ≤ 57, Ka ≤ 20) transitions of the ν9 and ν5+ν10 bands, respectively. Using Watson's A-reduction Hamiltonian in the Ir representation a set of accurate spectroscopic parameters, up to four sextic centrifugal distortion terms, was obtained for both the excited states v9 = 1 and v5 = v10 = 1. A Fermi resonance coupling constant W = 4.2(5) cm-1 was estimated for the perturbation between the ν9 and ν5+ν10 levels. Correlated harmonic force constants were obtained from coupled cluster CCSD(T) calculations with the cc-pVTZ basis set while the anharmonic force constants were computed at MP2 level using the same basis set. There is a good agreement with the available experimental data, in particular the equilibrium geometry compared with the experimental r0 structure and the fundamental modes. In addition, the ab initio anharmonic force field of cis-1,2-difluoroethylene provided a critical assessment of the experimental spectroscopic parameters, especially in the treatments of strong Fermi interactions.

Accurate ab initio anharmonic force field and heat of formation for silane

From large basis set coupled cluster calculations and a minor empirical adjustment, an anharmonic force field for silane has been derived that is consistently of spectroscopic quality (±1 cm−1 on vibrational fundamentals) for all isotopomers of silane studied. Inner-shell polarization functions have an appreciable effect on computed properties and even on anharmonic corrections. From large basis set coupled cluster calculations and extrapolations to the infinite-basis set limit, we obtain TAE0 = 303.80 ± 0.18 kcal mol−1, which includes an anharmonic zero-point energy (19.59 kcal mol−1), inner-shell correlation (—0.36 kcal mol−1), scalar relativistic corrections (— 0.70 kcal mol−1) and atomic spin-orbit corrections (—0.43 kcal mol−1). In combination with the recently revised ΔH o f, o[Si(g)], we obtain ΔH o f.o[SiH4(g)] = 9.9 ± 0.4 kcal mol−1 in between the two established experimental values.

Accurate ab initio anharmonic force field and heat of formation for silane

From large basis set coupled cluster calculations and a minor empirical adjustment, an anharmonic force field for silane has been derived that is consistently of spectroscopic quality ($\pm 1 cm^{-1}$ on vibrational fundamentals) for all isotopomers of silane studied. Inner-shell polarization functions have an appreciable effect on computed properties and even on anharmonic corrections. From large basis set coupled cluster calculations and extrapolations to the infinite-basis set limit, we obtain TAE_0=303.80 \pm 0.18 kcal/mol, which includes an anharmonic zero-point energy (19.59 kcal/mol), inner-shell correlation (-0.36 kcal/mol), scalar relativistic corrections (-0.70 kcal/mol), and atomic spin-orbit corrections (-0.43 kcal/mol). In combination with the recently revised \HVSI{0}, we obtain $\Delta H^{\circ}_{f,0}[SiH_4(g)]=9.9 \pm 0.4 kcal/mol$, in between the two established experimental values.

Theoretical Approaches to Anharmonic Resonances

AbstractFour theoretical models for the treatment of anharmonic resonances are discussed, and one particular curvilinear approach is generalized. Theoretical resonance parameters for three of these models are calculated on the basis of ab initio anharmonic force fields. They are compared with experimentally derived effective resonance parameters and with other published theoretical data. 25 anharmonic resonances in 16 different molecules are covered, including the stretch‐bend Fermi resonances in the haloforms CHX3 and the methyl halides CH3X(X = F‐I). These comparisons suggest guidelines for the theoretical treatment of resonances.

The dipole change vector for isolated CH stretches 1. Partially deuterated methyl bromide

The vapour phase infrared contour of the isolated CH stretch fundamental of CHD 2Br, obtained at 0.5 cm −1 resolution, has been simulated using anharmonic rigid asymmetric top calculations. Anharmonic rotational constants were obtained (i) from a simple Morse model, (ii) from fitting the constants to observed rotational fine structure recorded at 0.1 cm −1 resolution, and (iii) from ab initio anharmonic force fields. The Morse model was found to overstimate the anharmonicity; the other two methods were found to give similar results. The simulations indicate that the dipole change vector, (∂ μ / ∂ Q) 0, shows an angle of 44± 2° with the CH bond, while the same angle obtained from atomic polar tensors is equal to 43± 2°.

Ab initio prediction of fundamental, overtone and combination band infrared intensities

Programs have been implemented for calculating ab initio anharmonic force fields which are quartic in displacement coordinates and, likewise, dipole-moment expansions which are cubic in displacement coordinates. Knowledge of these, and the use of second-order perturbation theory, leads to predictions for fundamental, overtone and combination band infrared intensities as well as the usual spectroscopic constants. Here we use the ab initio MP2 method, with large basis sets, applied to H 2O and H 2CO. The results, including those for the overtone and combination band intensities, are in good agreement with the available experimental data.

Theoretical studies of the hydrogen peroxide potential surface. 1. An ab initio anharmonic force field

Ab initio GVB + 1 + 2 calculations, employing a (4s3p2d1f/3s2p) basis set, have been used to characterize the ground-state potential energy surface of hydrogen peroxide. An anharmonic force field that is quartic in the nontorisonal degrees of freedom and higher order in the torsion is reported. A second-order perturbation theory calculation of the vibrational energy levels using the ab initio force field yields fundamental frequencies that are within 25 cm{sup {minus}1} (3%) of experiment. A reaction path analysis of the torsional mode is also presented. The predicted tunneling splitting between the lowest pair of torsional states is 11.214 cm{sup {minus}1}. Also calculated are tunneling splittings for excited vibrational states for both H{sub 2}O{sub 2} and D{sub 2}O{sub 2}.