High-resolution,near-infrared CW-CRDS,and ab initio investigations of N2O–HDO

We have investigated the N₂O–HDO molecular complex using ab initio calculations at the CCSD(T)-F12a/aug-cc-pVTZ level of theory and using cavity ring-down spectroscopy to probe an HDO/N₂O/Ar supersonic jet around 1.58 μm. A single a-type vibrational band was observed, 13 cm^?1 redshifted compared to the OH+OD excited band in HDO, and 173 vibration-rotation lines were assigned (T_rot ≈ 20 K). A weighted fit of existing microwave and present near infrared (NIR) data was achieved using a standard Watson's Hamiltonian (σ = 1.26), producing ground and excited states rotational constants. The comparison of the former with those calculated ab initio suggests a planar geometry in which the OD rather than the OH bond in water is almost parallel to NNO. The equilibrium geometry and dissociation energy (D_e = –11.7 kJ/mol) of the water–nitrous oxide complex were calculated. The calculations further demonstrate and allow characterising another minimum, 404 cm^?1 (ΔE₀) higher in energy. Harmonic vibrational frequencies and dissociation energies, D₀, were calculated for various conformers and isotopic forms of the complex, in both minima. The absence of N₂O–D₂O from dedicated NIR experiments is reported and discussed.     

Ab initio determination of the torsional spectra of acetic acid

The torsional potential energy surface and the favourite geometries of acetic acid are determined with MP4/cc-p VTZ ab initio calculations. The molecule shows two planar trans and cis conformers whose energy difference is 1882.7 cm^?1. Both minimum energy geometries are separated by a barrier of 4432.1 cm^?1. The most stable trans-conformer shows a quite low methyl torsion barrier of 169.8 cm^?1. The roto-torsional energy levels have been calculated up to J = 10. The two torsional fundamental frequencies of the trans-conformer, the methyl and the OH torsion are 82.857 (A²) and 77.050cm^?1 (E) and 568.532 (A²) and 568.418cm^?1 (E). The V₃ barrier causes a splitting of 0.315cm^?1 in the ground vibrational state where the quartic centrifugal distortion constants have been predicted to be D_J = 90.4kHz, D_JK = ?301.5kHz and D_K = 165.4kHz. Finally, the far-infrared spectra of two isotopomers have been simulated from ab initio calculations.     

Rovibrational states of H+ 3. Part 1: The energy region below 9000 cm−1 and modelling of the non-adiabatic effects

This group's variational method for computing rovibrational energies using hyperspherical coordinates and harmonics has been applied to all H⁺ ₃ states below 13000 cm^?1 (J ≤ 10) for which accurate energies based on a submicrohartre accuracy potential energy surface have been obtained. A comparison with a recent comprehensive compilation of experimental data below 9000 cm^?1 shows deviations of up to 1.2 cm^?1. First it is shown that these deviations exert a systematic influence on the vibrational band but depend to a much lesser extent on rotational excitation. Then the remaining discrepancies can be attributed to the neglect of non-adiabatic effects, for which a useful correction formula based on ab initio results is obtained. The scatter in individual bands can thus be reduced to ~0.1 cm^?1 such that these corrected results are consistent with the accuracy of the potential energy surface itself.     

Theoretical rovibrational spectroscopy beyond fc-CCSD(T): the cation CNC+

An accurate near-equilibrium potential energy surface (PES) for CNC⁺ is constructed based on a high-level composite ab initio method. By combining explicitly correlated all-electron CCSD(T)-F12b with scalar relativistic effects and higher order correlation up to coupled cluster theory with singles, doubles, triples and quadruples (CCSDTQ) we achieve convergence in the wavenumbers of the fundamentals to ca. 1 cm^?1. Rovibrational energies are calculated in a variational approach and vibrational term energies and rotational constants are in excellent agreement with available experimental data. Accurate values for centrifugal distortion constants of CNC⁺ in different vibrational states are predicted. Especially the centrifugal distortion constants in the vibrational ground state of D₀ = 0.563 · 10^?6 cm^?1 and H₀ = 0.188 · 10^?10 cm^?1 should be superior to experimentally derived values. Reassignments of some experimentally observed transitions are suggested based on a comparison of experimental and calculated term differences. The bending part of the PES appears to be almost quartic and the band origin of the bending vibration is predicted at 94.2 cm^?1. Absolute line intensities are calculated for various transitions in CNC⁺. For the bending vibration, an intensity is predicted that is three orders of magnitude smaller than for the antisymmetric stretching vibration.     

Isotopic substitution shifts in methane and vibrational band assignment in the 5560-6200 cm region

The absorption spectra of the ¹²CH₄ and ¹³CH₄ molecules have been recorded and assigned in the 5560-6200 cm⁻¹ region. The effects of isotopic substitution for ¹²C by ¹³C on the methane vibrational energy levels have been calculated from an ab initio potential energy surface and compared with experiment. Comparison of the results obtained for two isotopic species allows us to confirm the vibrational assignment for the strongest bands of ¹²CH₄ in this region. Good agreement of ab initio calculations with observed energy levels has been demonstrated. A list of the assigned ¹³CH₄ lines valuable in atmospheric applications is reported.     

Vibrational spectroscopy investigation using ab initio and density functional theory on flucytosine

The FT Raman and FTIR spectra of flucytosine were recorded in the region 3500–100 cm⁻¹ and 4000–400 cm⁻¹, respectively. The optimized geometry, wavenumber and intensity of the vibrational bands of flucytosine were obtained by ab initio and density functional theory (DFT) levels with complete relaxation in the potential energy surface using the 6‐31G(d,p) and 6‐311G(d,p) basis sets. A complete vibrational assignment aided by the theoretical harmonic frequency analysis is proposed. The harmonic vibrational wavenumbers calculated are compared with experimental FTIR and FT Raman spectra. The observed and the calculated wavenumbers are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretically constructed bar‐type spectrograms. Copyright © 2007 John Wiley & Sons, Ltd.     

Quantum states of hydrogen transfer and vibration in malonaldehyde

New ab initio results on the 21D potential energy surface of malonaldehyde and a quantum mechanical treatment of the hydrogen transfer motion and its interaction with all vibrations are presented. An explicit approximate reaction path, close to the minimum energy path but matching the reactive normal mode near equilibrium, allows one to predict the ground state tunnelling frequency even when using small basis sets. With a barrier of 1144 cm^?1 (3.27kcal mol-¹) the tunnelling splitting is predicted to be 22.0cm^?1 for the parent species and 3.8 cm^?1 for the species deuterated in the hydrogen bond, in good agreement with the observed values 21.6 and 2.9 cm^?1, respectively. Predicted energy levels for excited states of the hydrogen transfer motion and for the non-reactive vibrations suggest a re-examination of the vibrational spectra and an extension of the number of vibrational factors in the basis set to improve the results for the vibrationally excited states.     

Electronic second hyperpolarizability of the carbon tetrachloride molecule

The electronic second hyperpolarizability γ of the carbon tetrachloride molecule is calculated by the ab initio molecular orbital method considering electron correlation with large basis sets. The static electronic γ value with the CCSD(T) method is 1.65 times the Hartree-Fock value, indicating a considerable electron correlation effect. Taking account of the frequency dispersion and vibrational effects, we estimate the most probable theoretical γ value at 800 nm to be around 17900au (9.0 × 10^?36 esu). On the other hand, the experimental value recently observed by optical Kerr effect method is 10.6 × 10^?36 esu at 800nm. It is concluded that the major part of the experimental χ⁽³⁾ value of carbon tetrachloride can account for the static electronic hyperpolarizability.     

Potential Energy Surface of the Ring Puckering Motion in 1-Chloro-1-silacyclobutane

The equatorial–axial conformational equilibrium of 1-chloro-1-silacyclobutane has been investigated by microwave spectroscopy andab initiocalculations. The potential energy function of the ring puckering motion has been determined: four vibrational states are localized in the equatorial well and two vibrational states in the axial well, all the remaining higher energy states lying above the interconversion barrier. This study has shown that the equatorial conformer is more stable than the axial one by 185(40) cm⁻¹. This value is in good agreement with that provided byab initiocalculations using the 6-31G* basis set.     

Vibrational energy levels of methyl chloride calculated from full dimensional ab initio potential energy surface

The vibrational levels for two isotopic species of methyl chloride have been calculated in the region 0-3500 cm⁻¹ from the ab initio potential energy surface (PES). The isotopic shift of vibrational levels of molecules ¹²CH₃³⁵Cl and ¹²CH₃³⁷Cl has been calculated. The correlation consistent basis sets cc-pVTZ and cc-pVQZ are employed to calculate energy values for 7957 and 3758 points correspondingly from a large domain of the nine-dimensional internal coordinate space. The analytic global PES is fitted with the standard deviation of 4.5 cm⁻¹.     

A refined estimate of the bond length of methane

The atmospheric reaction of H₂S with Cl was investigated using high level ab initio calculations and Canonical Variational Transition State Theory (CVTST). The adduct formation step is the dynamical bottleneck, and the rate constant was calculated to be 1.2 × 10^?9 cm³ molecule^?1 s^?1, which is around ten times greater than the upper experimental value. Additional ab initio classical trajectory calculations show that the adduct formed in the initial collision can easily dissociate, recrossing the variational transition state. The stabilization of this species depends on the vibrational excitation of H₂S molecule, which requires an almost collinear SH-Cl collision. These dynamical effects provide an explanation for the substantial error in the rate constant obtained using CVTST.     

FTIR spectroscopy of the 2v3 overtone band for different BrCN isotopomers: an improved evaluation of the anharmonic force field of cyanogen bromide

The 2v₃ overtone band of six different isotopomers of cyanogen bromide has been recorded in the range from 4200 to 4400 cm^?1 with a resolution of 0.02 cm^?1 using a Fourier transform infrared (FTIR) spectrometer. This allowed us to achieve complete knowledge of the energies for all levels corresponding to double vibrational excitation. An improved evaluation of the quartic force field of cyanogen bromide has been performed using the new data obtained together with those already known from previous works. Theoretical values derived from a recent ab initio calculation have been used to constrain the potential terms which cannot be determined with sufficient precision by the least-squares analysis of the available experimental data.     

An ab initio study of the hyperfine structure in the X2 Π electronic state of HCCS–calculation of vibronically averaged components of the anizotropic hyperfine tensor

The results of ab initio calculations of the vibronically averaged components of the anisotropic magnetic hyperfine tensor in the low-lying vibronic species of the X²Π electronic state of the HCCS radical are reported. The electronically averaged hyperfine coupling constants for hydrogen, deuterium, ¹³C and ³³S are obtained as functions of two bending vibrational modes by the density functional theory method. The vibronic wave functions are calculated with the help of a variational approach which takes into account the Renner–Teller effect and spin-orbit coupling. The results of ab initio calculations are compared to the corresponding experimental findings.     

Rate coefficients and kinetic isotope effects of the abstraction reaction of H atoms from methylsilane

ABSTRACT

Thermal rate constants for chemical reactions using improved canonical variational transition state theory (ICVT) with small-curvature tunnelling (SCT) contributions in a temperature range 180–2000 K are reported. The general procedure is used with high-quality ab initio computations and semi-classical reaction probabilities along the minimum energy path (MEP). The approach is based on a vibrational adiabatic reaction path and is applied to the multiple-channel hydrogen abstraction reaction H + SiH₃CH₃ → products and its isotopically substituted variants. All the degrees of freedom are optimised and harmonic vibrational frequencies and zero-point energies are calculated at the MP2 level with the cc-pVTZ basis set. Single-point energies are calculated at a higher level of theory; CCSD(T)-F12a/VTZ-F12. ICVT/SCT rate constants show that the quantum tunnelling contributions at low temperatures are relatively important and the H-abstraction channel from SiH₃ group of SiH₃CH₃ is the major pathway. The total rate constants are given by the following expression: k_tot(ICVT/SCT) = 2.29 10^?18 T^2.42 exp(?350.9/T) cm³ molec^?1 s^?1. These calculated rates are in agreement with the available experiments. The ICVT/SCT method is further exploited to predict primary and secondary kinetic isotope effects, respectively).     

New vibrational assignments in the Ā1 Au-[Xtilde] 1Σ+ g electronic transition of acetylene,C2H2: the v′1 frequency

New laser-induced fluorescence spectra of supersonic jet cooled acetylene (C₂H₂) in the wavelength region 230–205 nm have led to an improved understanding of the vibrational structure of the A ¹A_u state. Among the new bands observed are two weak perturbed bands at 46008 cm^?1 and 46116 cm^?1. Rotational analyses of these bands, together with the corresponding ‘hot’ bands arising from the ground state v4 fundamental, have shown that the upper states have asymmetric top K structure that is unaffected by a axis Coriolis coupling; this means that they do not involve overtones of the low frequency bending vibrations and therefore must be combinations of a_g vibrational normal modes. From their positions in the manifold, their vibrational assignments can only be 2² ₀3¹ ₀; and 1¹ ₀;3¹ ₀. These assignments lead to values of x ₂₂, x₁₃, and a revised value for the symmetric CH stretching frequency, ν₁ = 2880.5cm^?1; this revised value is 160cm^?1 lower than the previously accepted value, but consistent with new ab initio calculations that we performed at the EOM-CCSD level using a TZ2P (triple-zeta plus double polarization) basis set.     

Thermochemical properties of small open-shell systems: experimental and high-level ab initio results for NH2 and

The first adiabatic ionization energy and the first singlet–triplet splitting of the amidogen radical (NH₂) have been determined by high-level ab initio quantum chemistry based on the coupled-cluster approach (90?041 and 10?319?cm^?1, respectively) and by high-resolution pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy (90 083.8 ± 1.0 and 10 222.0 ± 1.3?cm^?1, respectively). A comparison between the theoretical and experimental values demonstrates the predictive powers of high-level ab initio theory in the derivation of the thermochemical properties of small molecular systems. The absolute accuracy of better than 100?cm^?1 alleviates the experimental search for the relevant spectral features.     

Theoretical determination of the vibrational levels of NH+ 3 and its isotopomers

The vibrational levels associated with the electronic ground state X²A₂″ of NH⁺ ₃ have been determined up to 5000 cm^?1 by perturbation and variational calculations with full dimensionality of the molecule. For the variational part a new version of MULTIMODE was used which uses the ab initio electronic energy and its first derivative to define the potential energy function. These quantities were generated by the B97-1 density functional and RCCSD(T) approaches. For ND⁺ ₃, ND₂H⁺ and NDH⁺ ₂ the vibrational levels were calculated only by perturbation theory. The rotational constants for all the isotopomers were determined and the first transition dipole moments for NH⁺ ₃ and ND⁺ ₃ were plotted. A critical comparison of the perturbation and variational techniques suggests a possible further modification to the MULTIMODE algorithm for large systems.     

FT‐Raman and FT‐IR spectral and quantum chemical studies on some flavonoid derivatives: Baicalein and Naringenin

In this study, the experimental and theoretical results on the molecular structures of some flavonoid derivatives (Baicalein and Naringenin) are presented. The FT‐IR and FT‐Raman spectra of the compounds have been recorded together for the first time between 4000–400 cm⁻¹ and 3500–5 cm⁻¹ regions, respectively. The molecular geometry and vibrational wavenumbers of the compounds have been also calculated in their ground states by using ab initio HF and DFT/B3LYP functional with 6‐31G(d,p) basis set used in calculations. The calculations were utilized to the C₁ symmetries of the molecules. All calculations were performed with Gaussian 98 software. The obtained vibrational wavenumbers and optimized geometric parameters were seen to be in good agreement with the experimental data. Scale factors have been used in order to compare how the calculated and experimental data are in agreement. Theoretical infrared intensities were also reported. Copyright © 2008 John Wiley & Sons, Ltd.     

The integrated number of vibrational states in acetylene (12C2H2, 13C2H2, 12C2D2)

A calculated exhaustive set of vibrational state energies in ¹²C₂H₂, ¹³C₂H₂ and ¹²C₂D₂ has been used to analyse the evolution of the integrated number of states with increasing vibrational energy N(E) up to 15000 cm^?1, 12000cm^?1 and 10000 cm^?1 in each isotopomer, respectively. The regular contribution to N(E) was modelled analytically and numerical parameters were fitted. The other expected contribution to N(E), which is of oscillatory nature, was quantified and is discussed using energyand time-dependent theories. Related periods of oscillation and temporal recurrences are interpreted consistently in terms of the constant of the motion N_r = 5v₂ + 3v₂ + 5v₃ + v₄ + v₅ and of an average vibrational quantum. More pragmatically, the vibrational dynamics appear to be dominated by the bending vibrations, i.e., by the slowest oscillators.