Vibration-rotation Spectra Research Articles

The anomeric effect in 1,3-benzodioxole: additional evidence from the rotational, vibration–rotation and rovibronic spectra

The millimetre wave pure rotation and vibration–rotation spectrum of 1,3-benzodioxole has been studied and the vibrational spacing between the two lowest vibrational states in the five membered ring puckering potential has been determined with considerable accuracy, at ΔE01 = 259 726.035(10) MHz or 8.663 5280(4) cm−1. In addition, rotationally resolved analysis of two hot bands near the reported origin band in the laser-induced fluorescence spectrum has been carried out and was found to rule out the current literature assignment of this spectrum. The new information has been used to reassign the lowest vibrational transitions in the far-infrared and the Raman spectrum of 1,3-benzodioxole in terms of a one dimensional potential with a central barrier of 108 cm−1, which successfully accounts for all states up to υ = 5, and for the observed variation in rotational constants. Considerable reassignment of the published spectra, as well as new experimental work is still necessary for a confident determination of the low-energy region of the molecular potential of 1,3-benzodioxole.

Vibration-rotation spectra of hydrogen halides in rare-gas liquids: Q-branch absorption

Abstract Near-infrared spectra of HCl highly diluted in liquid Ar show intense absorption in the P-R interbranch region, so-called Q-branch absorption. In spite of its relevance for the shape of the bands, its physical origin has been elusive to date. We employ molecular dynamics simulations to study the influence of some physical effects that could contribute to Q-branch absorption. We check that multipole-induced dipole induction mechanisms are not quantitatively relevant in this spectral region. We show that the particular characteristics of accurate HCl-Ar anisotropic potentials and the peculiar hindered rotational motion they provoke on the diatomic probe are essential to understand Q-branch absorption.

A computer assisted procedure of assignments of vibration–rotation bands of asymmetric and symmetric top molecules

An advanced graphically oriented interactive program package for assignments of complex (perturbed) vibration–rotation spectra of asymmetric and symmetric top molecules has been developed. In addition to the well known Loomis–Wood algorithm, the new procedure takes advantage of a precise knowledge of the lower (e.g. ground) vibrational state energies, works with a realistic approximation of effective Hamiltonians for lower as well as upper vibrational states, and allows an instant combination difference inspection of spectral lines by the graphical representation of the appropriate parts of the analyzed experimental spectrum. Being constrained to the combination difference checking, the new algorithm can directly assign the correct rotational quantum numbers as well as ‘quality weights’ estimating relative accuracies of the identified lines.

The inversion of diatomic Born–Oppenheimer-breakdown corrections

The inversion of diatomic vibration–rotation data to give the r dependence of operators is considered. For the Born–Oppenheimer energy levels E vJ and for the expectation values 〈 vJ| X( r)| vJ〉 of an operator X( r), the inversion is unambiguous if the reduced mass is known, leading to the potential energy function V( r) and to X( r). The breakdown of the Born–Oppenheimer approximation is represented in terms of three functions Q i ( r), R i ( r), and S i ( r) for each atom i. The function Q i ( r) can be removed by a transformation that changes the other functions to R ̃ i(r) and S ̃ i(r) , which still have one degree of indeterminacy. The effects of this indeterminacy in fits to truncated Hamiltonians is considered for the extensive data for isotopomers of the LiH molecule. This discussion is relevant to a recent claim that the dipole moment and rotational g J factor of LiH can be determined from a fit to field-free vibration–rotation spectra. The values obtained for these quantities are unreliable because they are strongly dependent on the constraints used in the Born–Oppenheimer-breakdown functions.

The ν3 and 2ν3 bands of 32S16O3, 32S18O3, 34S16O3, and 34S18O3

This sixth of a series of publications on the high-resolution rotation–vibration spectra of sulfur trioxide reports the results of a systematic study of the ν3 and 2ν3 infrared bands of the four symmetric top isotopomers 32S16O3, 32S18O3, 34S16O3, and 34S18O3. An internal coupling between the l=0(A1′) and l=2(E′) levels of the 2ν3 states was observed. This small perturbation results in a level crossing between |k−l|=9 and 12, in consequence of which the band origins of the A1′,l=0 “ghost” states could be determined to a high degree of accuracy. Ground and upper state rotational constants as well as vibrational anharmonicity constants are reported. The constants for the center-of-mass substituted species 32S16O3 and 34S16O3 vary only slightly, as do the constants for the 32S18O3, 34S18O3 pair. The S–O bond lengths for the vibrational ground states of the species 32S16O3, 34S16O3, 32S18O3, and 34S18O3 are, respectively, 141.981 99(1), 141.979 38(6), 141.972 78(8), and 141.969 93(8)pm, where the uncertainties, given in parentheses, are two standard deviations and refer to the last digits of the associated quantity.

Effect of Resolution on the Wavenumber Determination of a Putative Standard to Be Used for near Infrared Diffuse Reflection Spectra Measured on Fourier Transform near Infrared Spectrometers

A mixture of powdered Er2O3, Dy2O3, Ho2O3 and talc has been proposed as a wavenumber standard for near infrared (NIR) diffuse reflection (DR) measurements. Previous measurements were made at a resolution of 2 cm−1 which is significantly less than the full-width at half-height of the peaks in the spectrum. Since many NIR DR spectra are measured at a much lower resolution, some of the peaks are observed to fuse and/or shift. In this paper, the behaviour of the band positions is studied as a function of resolution. The factors that allow some of the peaks to be useful as wavenumber standards for NIR DR measurements while others should not be used are discussed. The instrument should first be calibrated with certain lines in the vibration–rotation spectrum of water vapour. Because most of the lines in the water spectrum are located less than 1 cm−1 from their nearest neighbours, only two of the 3768 lines in the region between 5750 and 7990 cm−1 can be used for calibration and only then if the spectrum is converted to its second derivative and interpolated so that there are at least five data points per resolution element and the wavenumber of maximum absorption is found by fitting the line to a quartic function. For the spectrum of the putative wavenumber standard measured at a resolution of 3–10 cm−1, the band centre is best measured after baseline correction of the spectrum with a polynomial function, while lower resolution spectra are better converted to the second derivative. As might be expected, when choosing peaks as wavenumber standards for instruments operating at different resolutions, one should avoid bands that either become fused at resolutions close to the value being used or whose position varies rapidly with resolution. Peaks should be selected that, besides being in the wavenumber range of interest, can be measured with both high precision and high accuracy.

Effect of Resolution on the Wavelength Determination of a Putative Standard to Be Used for near Infrared Diffuse Reflection Spectra Measured on Grating Spectrometers

Diffuse reflection (DR) Fourier transform near infrared spectra of a powdered mixture of Er2O3, Dy2O3, Ho2O3 and talc were measured at a constant resolution of at least 2 cm−1 on four different combinations of spectrometers and sampling accessories. The wavenumber scale of each of these spectra was corrected with lines in the vibration–rotation spectrum of water vapour so that the accuracy was better than 0.02 cm−1. DR spectra of the powdered sample were then calculated at constant-wavelength resolution to simulate spectra that would have been measured on a grating monochromator. The precision and accuracy of the bands in the spectrum of this sample were then estimated. It is believed that the accuracy of the reported positions of the bands in the spectrum of this putative wavelength standard is about ten times better than the corresponding values in the DR spectrum of a mixture of Er2O3, Dy2O3 and Ho2O3 that had previously been reported by the US National Institute of Standards and Technology.

High resolution spectroscopy of the ν3 band of the van der Waals complex Ar—DCOOH

The vibrational-rotational spectrum of the van der Waals complex Ar-DCOOH has been recorded in the spectral region of the carbonyl stretch band. This work represents the first rotationally resolved vibrational spectrum for this complex. A full set of molecular constants could be deduced by fitting 105 lines with a standard S-reduced Watson Hamiltonian. The band origin was determined to be 1725.8440(1)cm−1 which corresponds to a small red shift of 0.0309cm−1 compared to the monomer band. The rotational constants deviate only slightly from the rotational constants in the ground state, as determined by microwave spectroscopy (I. I. Ioannou and R. L. Kuczkowski, 1993, J. phys. Chem., 98, 2231).

High-resolution Fourier-transform infrared spectroscopy of the ν1 and ν8 fundamental bands of sulphur tetrafluoride (SF4)

The vibration-rotation spectra of the ν1 and ν8 fundamental bands of 32SF4 have been observed using Fourier-transform infrared spectroscopy. The band centre of the c-type ν1 symmetric sulphur-equatorial-fluorine stretching vibration was observed at 891.6 cm−1 and that for the b-type ν8 asymmetric sulphur-equatorial-fluorine stretching vibration at 864.6 cm−1. In total, 2044 rovibrational transitions have been assigned. Analysis of the spectra showed that the rotational states of the ν1 = 1 and ν8 = 1 upper vibrational levels are coupled by an a-type Coriolis interaction. This coupling has been treated both using perturbation theory and by the explicit inclusion of an appropriate Hamiltonian matrix element in a combined fit of the data for both bands. Spectroscopic parameters have been determined for the ground, ν1 = 1 and ν8 = 1 vibrational levels. Weaker transitions resulting from difference bands and the fundamental bands of the 34SF4 isotopomer have been identified but could not be assigned, because of the density of lines in the room-temperature spectrum. The possibility that discrepancies between the observed and predicted spectra of the ν1 fundamental may result from either a Coriolis interaction with the states of another vibrational level, or the effects of intramolecular exchange of axial and equatorial fluorine atoms is considered. The discussion is supported by theoretical calculations which show that the likely path for intramolecular exchange is via a C 4v transition state.

An S2Fluorescence Model for Interpreting High‐Resolution Cometary Spectra. I. Model Description and Initial Results

A new versatile model providing S2 fluorescence spectrum as a function of time is developed with the aim of interpreting high-resolution cometary spectra. For the S2 molecule, it is important to take into account both chemical and dynamic processes because S2 has a short lifetime and is confined in the inner coma, where these processes are most important. The combination of the fluorescence model with a global coma model allows for the comparison with observations of column densities taken through an aperture and for the analysis of S2 fluorescence in different parts of the coma. Moreover, the model includes the rotational structure of the molecule. Such a model is needed for interpreting recent high spectral resolution observations of cometary S2. A systematic study of the vibrational-rotational spectrum of S2 is undertaken, including relevant effects, such as nonequilibrium state superposition and the number density profile within the coma due to dynamics and chemistry, to investigate the importance of the above effects on the scale length and abundance of S2 in comets.

Vibration–rotation spectra of HCl solutions in SF 6 at isochoric conditions

We report here new data on the fundamental and the first overtone band shapes measured in the IR spectra of HCl solutions in SF 6 at several constant densities in the range of ρ∼220–260 amagat and at temperatures T∼280–340 K, i.e. below and above the critical point of the solvent. The experimental data will be discussed in terms of a recently developed ‘modified rotor’ model, that assumes solute species to rotate in a quasi-free manner once their rotational and translational kinetic energies exceed the barrier to rotation caused by a slowly varying anisotropic potential of interaction between the solute and solvent molecules.

An expert system for identification of lines in vibrational-rotational spectra

An expert system for automatic identification of the complex vibrational-rotational spectra of molecules has been developed. An iteration approach is implemented in this system, in which employment of the exact combination rule is combined with determination of the spectroscopic constants by solving of the inverse problems and comparison of the calculated parameters of spectral lines with the corresponding measured values. In order to calculate the energy levels and the frequencies and intensities of lines, the Watson Hamiltonian, the Pade-Borel approximants, and generating functions are used. The system is based on the application of pattern-recognition algorithms. Recognition training makes it possible to obtain the required flexibility of the system and to use different methods of identification based on the application of combination rules both for the analysis of strong bands and for the assignment of weak single lines. The system developed can be used to analyze the spectra of the C s and C 2V molecules, as well as employ the calculated spectrum of a molecule of any type prepared in advance. This system was successfully used to identify the H 2 16 O, H 2 17 O, H 2 18 O, D2O, HDO, H 2 32 S, H 2 34 S, and H 2 33 S and molecules.

High resolution spectroscopy of the ν3 band of DCOOD

The rotation-vibration spectrum of DCOOD has been recorded in the carbonyl stretch (ν3) region. Using a standard S-reduced Watson Hamiltonian in the Ir representation, 225 lines could be fitted to a vibrational–rotational band. A full set of molecular constants was obtained. The ν3 band is found to be strongly perturbed in the Ka: 1←1 and Ka: 2←2 subband. The perturbation is attributed to a Fermi resonance with the 2ν8 overtone band and Coriolis coupling to a combination band (ν4+ν7). The band center is determined to be 1725.1218(1) cm−1 which is more than 10 cm−1 shifted compared to previous studies.

The anharmonic resonances in the infrared spectrum of 12C13CH2

The vibration-rotation spectrum of 13C mono-substituted acetylene, 12C13CH2, has been recorded in the region between 1100 and 3500 cm−1 at instrumental (4 × 10−3 cm−1) or Doppler limited (6–8 × 10−3cm−1) resolution. Hot and cold bands involving the stretchings v1, v2 and v3 and the stretching-bending combination bands with ν2 = 1, ν4 ⩽ 1 and ν5 ⩽ 2 have been observed and analysed. The anharmonic resonances within the dyads ν1/(v2 + 2ν5) and ν3/(ν2 + ν4 +ν5), which strongly perturb the spectrum of 13C2H2 and 12C2H2, respectively, have been analysed. From the simultaneous treatment of all the assigned transitions a set of deperturbed molecular parameters, containing also the coupling coefficients K1,255 and K3,245, was derived. They fully characterize the ro-Vibrational pattern of the observed stretching and stretching-bending combination states and provide an accurate description of the anharmonic resonances in the molecule.

Application of the DVR method to the vibration-rotation spectrum of N 2O: derivation of the dipole moment derivatives in Radau coordinates

The rovibrational line intensities of the N 2O molecule in its ground electronic state have been used to determine its dipole moment derivatives using a fitting procedure. The wavefunctions calculation have been made using a Discrete Variable Representation method in Radau coordinates and an available potential energy surface. The resulting dipole moment derivatives are compared to previous ones obtained using normal coordinates approach.

High-resolution Fourier-transform infrared spectroscopy of the ν2 fundamental band of thionyl fluoride, SOF2Electronic supplementary information (ESI) available: Tables S1 and S2. See http://www.rsc.org/suppdata/cp/b2/b210153j/

The high-resolution vibration–rotation spectrum of the ν2 fundamental band of gas-phase thionyl fluoride, SOF2, has been observed centred at 808 cm−1. Spectra were recorded in absorption using a Fourier-transform spectrometer with the sample of SOF2 held in a gas cell at a temperature of 294 K. In total, 1929 a- and c-type vibration–rotation transitions of the most abundant isotopomer, 32S16OF2, have been assigned and included in a least-squares fit. The least-squares analysis, performed using a standard semi-rigid rotor Hamiltonian, has yielded precise spectroscopic parameters for the v2=1 level. The analysis confirms the assignment of the band to excitation of the symmetric S–F stretching vibration. Additional, partially resolved features are attributed to the Q-branches of the ν2 fundamental band of 34S16OF2 and of ν2 difference bands from low-lying excited vibrational levels.

The Infrared Spectrum of 13C 2D 2: The Bending States up to v4+ v5=2

The vibration–rotation spectrum of 13C 2D 2 has been recorded in the infrared region between 420 and 1100 cm −1 with an effective resolution ranging from 0.004 to 0.006 cm −1, and in the millimeter-wave region between 68 and 518 GHz. A total of about 1400 rovibrational transitions (66 of which have been measured in the millimeter-wave region) have been assigned to 8 bands with 15 l-vibrational components involving the bending states up to v t = v 4+ v 5=2. The ground state and nine vibrationally excited states have been characterized. All the measured transitions have been analyzed simultaneously by adopting a model Hamiltonian which takes into account the usual vibration and rotation l-type resonances, together with the Darling–Dennison coupling between the v 4=2 and v 5=2 bending states. The derived spectroscopic parameters reproduce the transition wavenumbers with a standard deviation of the fit of the order of the experimental uncertainty.

High-Resolution IR Spectroscopy of the N 2O–H 2O and N 2O–D 2O van der Waals Complexes

We report high-resolution infrared vibrational–rotational spectra of the weakly bound complexes N 2O–H 2O and N 2O–D 2O in the higher frequency N 2O stretching mode region (ν 3=2223.756693(124) cm −1). The measurements were carried out using a free jet expansion in combination with a lead salt diode laser spectrometer. Rotational constants, quartic centrifugal distortion constants, and band origins have been derived for both isotopomers. The geometrical structure is determined using isotopic substitution. The deduced structure shows evidence for a second hydrogen bond interaction within the complex. The nonrigidity of the complexes gives rise to an internal rotation of the water molecule around its own C 2 v symmetry axis. For N 2O–H 2O, a tunneling splitting arising from this internal motion has been observed in the spectra. According to symmetry considerations, the observed splitting in the spectrum of N 2O–H 2O corresponds to the difference between the tunneling frequencies in the ground and vibrationally excited states.

Nuclear spin selection rule in the photochemical reaction of CH3 in solid parahydrogen

Photolysis of a methyl radical CH3 in solid parahydrogen produces a methane molecule CH4 via the reaction between an intermediate singlet methylene CH21 and a parahydrogen molecule H2. Conservation of nuclear spin during the reaction has been investigated by the intensity distribution of the rotation-vibration spectrum of methane produced by the reaction. It was found that the population of each nuclear spin state of methane just after the reaction was different from that of the statistical ratio, which indicates that a nuclear spin selection rule does exist in the reaction. However, the observed population was significantly different from the theoretically predicted ratio. The discrepancy between the experiment and the theory may indicate a breakdown of the nuclear spin conservation during the reaction, if the reaction mechanism in solid parahydrogen is the same as in the gas phase.

The Infrared Spectrum of 12C13CH2: The Bending States up to v4+v5=4

The vibration–rotation spectra of 13C monosubstituted acetylene, 12C13CH2, have been recorded in the region between 450 and 3200 cm−1 with an effective resolution ranging from 0.004 to 0.006 cm−1. A total of about 5300 rovibrational transitions have been assigned to 53 bands involving the bending states up to vt=v4+v5=4, allowing the characterization of the ground state and of 30 vibrationally excited states. All the bands involving states up to vt=3 have been analyzed simultaneously by adopting a model Hamiltonian which takes into account the vibration and rotation l-type resonances. The derived spectroscopic parameters reproduce the transition wavenumbers with a RMS value of the order of the experimental uncertainty. Using the same model larger discrepancies between observed and calculated values have been obtained for transitions involving states with vt=4. These could be satisfactorily reproduced by only adopting, in addition to the previously determined parameters which were constrained in the analysis, a set of effective constants for each vibrational manifold.