Isotopic substitution shifts in methane and vibrational band assignment in the 5560–6200 cm−1 region

doi:10.1016/j.jqsrt.2009.02.016

Elsevier

Journal of Quantitative Spectroscopy and Radiative Transfer

Volume 110, Issue 12, August 2009, Pages 964-973

Journal of Quantitative Spectroscopy and Radiative Transfer

https://doi.org/10.1016/j.jqsrt.2009年02月01日6 Get rights and content

Abstract

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.

Introduction

Numerous theoretical studies have been devoted to calculations of the methane vibrational energy levels [1], [2], [3], [4] and rovibrational energy levels [5]. The analysis of highly excited energy levels and transitions of the methane molecule is a difficult problem due to the complex structure of vibrational levels and the high dimensionality of the calculation models [6], [7], [8], [9]. Furthermore, the structure of vibrational levels for the tetradecade of methane in the 5300–6200 cm⁻¹ region is still unclear. The results presented in Refs. [1], [2], [3] significantly differ from those obtained in Refs. [6], [8]. In present work, we compare the ab initio calculations with experiment and determine the structure of the methane vibrational energy levels more precisely. The energy levels in the 5800–6200 cm⁻¹ region are very valuable for atmospheric applications, because this region covers the strongest band of the tetradecade 2ν₃ (F₂). But even the 2ν₃ (F₂) identification is questionable today [10]. Systematic identification of ¹³CH₄ has been published only up to 3200 cm⁻¹ [11], [12].

In this paper, the ab initio points calculated in Ref. [13] have been used to determine the ¹²CH₄ and ¹³CH₄ potential energy surface (PES) in the internal mass-dependent coordinates. The vibrational energy levels up to 6200 cm⁻¹ have been calculated for both molecules. The technique used for calculations of energy levels is described in Ref. [13].

The ¹²CH₄ and ¹³CH₄ spectra have been recorded with high-resolution Fourier transform spectrometers (FTS) at the Toray Research Center Inc., Otsu, Japan. The spectra were recorded at different temperatures that allowed us to estimate the value of the lower rotational quantum number J for transitions with small J values. As a rule, the comparison of the strongest lines of ¹²CH₄ and ¹³CH₄ allowed us to establish a one-to-one correspondence between spectral lines of those molecules.

One of the main goals of the present research is to perform the vibrational assignment based on the calculated isotopic shifts. This is possible to realize for the strongest lines with small J values. A similar research of four lines of the 2ν₃ (F₂) band has been conducted in Ref. [14]. However, the complete identification of all lines in the considered region is not a goal of our research. It is questionable whether this problem can be resolved without the detailed modeling of an effective Hamiltonian.

This paper is composed of three sections. The first section provides a brief description of the technique used to calculate vibrational energy levels and isotopic shifts from an ab initio PES. The second section describes the experimental spectra. The assignments of the ¹²CH₄ and ¹³CH₄ spectra, as well as the comparison of the calculated isotopic shifts with experiment, are presented in the third section.

Section snippets

PES and energy level calculation

The GAUSSIAN 98 program [15] was used to perform the geometry optimization using the CCSD(T) method with the cc-pVQZ basis set. The internal electron correlation effect was not taken into account. The minimum energy −40.450888 h was reached at r_ithat is equal to

r_{e} = 1.0879031 Å

. Near the equilibrium geometry, the interbond angles

q_{e} = ArcCos (- \frac{1}{3})

. To simplify the kinetic energy operator [16], the PES has been calculated using the internal mass-dependent coordinates [17]:

{\overset{\Rightarrow}{r}}_{i}^{'} = {\overset{\Rightarrow}{r}}_{i} + d \sum_{j = 1}^{4} {\overset{\Rightarrow}{r}}_{j}, where d = - \frac{1}{4}

Experiment

The spectra of methane were recorded with two high-resolution FTS. The ¹²CH₄ spectra were recorded with the Bruker IFS 120 HR spectrometer at the Toray Research Center Inc. The ¹³CH₄ spectra were recorded with the Bruker IFS 125 HR spectrometer at the Toray Research Center Inc. The coolable gas cells with an 8.75 cm absorption path length and ZnSe windows were used in these measurements. In FTS, the CaF₂ beam splitter, the InSb detector, and a tungsten halogen lamp as the light source were used.

Comparison of the ¹²CH₄ and ¹³CH₄ cold spectra (assignment of spectra)

The 2ν₃ (F₂) bands of the ¹²CH₄ and ¹³CH₄ molecules have been compared in Ref. [14]. One of the purposes of the present research is to verify the identification of the ν₂+ν₃+ν₄ and ν₁+ν₃ bands. The ab initio calculations [1], [3] show a wide disagreement with assignment reported in Ref. [8]. An assumption that the identifications of the ν₂+ν₃+ν₄ and ν₁+ν₃ bands reported in Ref. [8] are not correct was made in Ref. [4]. This assumption is based only on the ab initio calculations. A good

Conclusion

The spectra of the two most abundant isotopologues of the methane have been recorded and analyzed to confirm vibrational assignment of the ¹²CH₄ energy levels. The spectral measurements were performed at different temperatures by means of high-resolution Fourier transform spectrometers. A comparison of the room and low temperature spectra allows us to make the J assignment of low rotational levels for observed lines of ¹²CH₄ and ¹³CH₄. The PES-based calculated vibrational energy levels of both

Acknowledgments

We are grateful for the support provided by the NIES GOSAT (Greenhouse Observing SATellite) project.

These studies were supported by the Russian Foundation for Basic Research (Russia) in the framework of the Grant no. 06-05-65010a. This investigation was made partly within the framework of the Program 2.10.1 "Optical Spectroscopy and Frequency Standards".

A.V.N. acknowledges Prof. J.-P. Champion and Drs. L.R. Brown and V. Boudon for continuous collaboration on the methane spectra studies.

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Empirical low energy values for methane transitions in the 5852–6181 cm⁻¹ region by absorption spectroscopy at 81 K
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Journal of Quantitative Spectroscopy and Radiative Transfer

Abstract

Introduction

Section snippets

PES and energy level calculation

Experiment

Comparison of the ¹²CH₄ and ¹³CH₄ cold spectra (assignment of spectra)

Conclusion

Acknowledgments

References (21)

Vibrational energy levels for CH₄ from an ab initio potential

Spectrochim Acta Part A

Towards accurate ab initio predictions of the vibrational spectrum of methane

Spectrochim Acta Part A

Spherical top data system (STDS) software for the simulation of spherical top spectra

JQSRT

Analysis of the interacting octad system of ¹²CH₄

J Mol Spectrosc

High-resolution FTIR spectroscopy using a jet: sampling the rovibrational spectrum of ¹²CH₄

J Mol Spectrosc

Identification of hot band transitions of CH₄ near 3000 cm⁻¹

J Mol Spectrosc

Preliminary analysis of the pentad of ¹³CH₄ from Raman and infrared spectra

J Mol Spectrosc

Improved algorithms for the modeling of vibrational polyads of polyatomic molecules: application to T_d, O_h, and C_3V molecules

J Mol Spectrosc

Global analysis of the high resolution infrared spectrum of methane ¹²CH₄ in the region from 0 to 4800 cm⁻¹

Chem Phys

Empirical low energy values for methane transitions in the 5852–6181 cm⁻¹ region by absorption spectroscopy at 81 K

J Mol Spectrosc

Cited by (39)

New dipole moment surfaces of methane

Broad-band frequency references in the near-infrared: Accurate dual comb spectroscopy of methane and acetylene

The 2009 edition of the GEISA spectroscopic database

Rotational and vibrational energy levels of methane calculated from a new potential energy surface

GOSAT-2009 methane spectral line list in the 5550-6236cm-1 range

High resolution spectroscopy and the first global analysis of the Tetradecad region of methane 12CH4

Isotopic substitution shifts in methane and vibrational band assignment in the 5560–6200 cm−1 region

Abstract

Introduction

Section snippets

PES and energy level calculation

Experiment

Comparison of the 12CH4 and 13CH4 cold spectra (assignment of spectra)

Conclusion

Acknowledgments

Spectrochim Acta Part A

Spectrochim Acta Part A

JQSRT

J Mol Spectrosc

J Mol Spectrosc

J Mol Spectrosc

J Mol Spectrosc

J Mol Spectrosc

Chem Phys

J Mol Spectrosc

Isotopic substitution shifts in methane and vibrational band assignment in the 5560–6200 cm⁻¹ region

Comparison of the ¹²CH₄ and ¹³CH₄ cold spectra (assignment of spectra)