Second torsional state of methylamine from high resolution IR spectra

https://doi.org/10.1016/j.jqsrt.2018.06.008Get rights and content

Highlights

  • The second excited torsional band of methylamine has been reassigned.

  • A global fit of the torsional hot and overtone transitions has been carried out.

  • The assigned transitions were fit to a single state model.

  • The model is based on the group theoretical formalism of Hougen and Ohash.

  • The standard deviation is 0.006 cm−1.

Abstract

The high resolution infrared spectrum of methylamine has been analyzed in the 40–720 cm−1 region (the ν15 = 2←1 hot and ν15 = 2←0 overtone bands) with a resolution of 0.00125 cm−1. More than 14,400 rovibrational transitions (out of 133,000 spectral lines) for 0 ≤ K ≤ 13 and K ≤ J ≤ 40 have been assigned using an original Loomis–Wood program dedicated to methylamine. A global fit of the torsional hot and overtone transitions has been carried out. The assigned transitions were fit to a single state model based on the group theoretical formalism of Hougen and Ohashi, which for the second torsional state seems to be at the limit of applicability. The standard deviation of the fit with K ≤ 11 and J ≤ 40 to 94 parameters was 0.006 cm−1.

Introduction

Methylamine CH3NH2, the simplest primary amine, was first detected in the interstellar medium in 1974 [1], [2], then in 2011 in a spiral galaxy [3] and observed in cometary samples of the Stardust mission [4]. Apart from astrophysical importance, methylamine plays also significant roles in gas phase chemistry [5].

From a spectroscopic point of view, the molecule has been of great interest for many years, since its internal dynamics are determined by two large amplitude internal motions: hindered internal rotation of the methyl group about the Csingle bondN bond (torsional vibration) and inversion of the amino group NH2 (wagging vibration). These two large amplitude motions are strongly coupled and give rise to a rotation-inversion-torsion structure in the vibrational states. The tunnelling splittings are considerable and lead to very complex and dense spectra. The assignment and interpretation of such spectra is rather challenging. In the present study, high resolution FTIR spectra of the torsional overtone band ν15 = 2←0 were investigated together with the hot torsional band ν15 = 2←1.

The rovibrational spectra of the methylamine molecule have been examined extensively over the years. In 1989 Oda and Ohashi assigned transitions in the second excited torsional state for the first time [6]. Line assignment included the ν15 = 2←1 torsional hot band (87 transitions) and the pure rotational spectrum (35 pure rotational transitions) of the second torsional state. Both, the ν15 = 2←1 band and the rotational transitions in the second torsional state were strongly overlapped with very intense ν15 = 0←0, 1←1 and 1←0 transitions (the region of 40–350 cm−1). A least-squares fit was carried out and a couple of the second torsional state molecular parameters were obtained using the group theoretical formalism [7]. Because of the insufficient number of spectral data the results were not satisfactory. The assigned transitions were only of B and E1 symmetry (the B symmetry of pure rotational transitions and B and E1 of ν15 = 2←1 torsional band transitions). Thus, the inversion tunnelling splitting parameters were not determined. These problems were expected to be solved from the study of the ν15 = 2←0 torsional band, which was supposed to be free from serious overlapping with other bands.

A few years later, the far-infrared FT spectrum of methylamine was recorded in the region 340–640 cm−1 with a resolution of 0.006 cm−1 [8]. The study was carried out in order to obtain more detailed information on the second torsional band from the analysis of the ν15 = 2←0 overtone spectrum. Although the assignment of the 2ν15 band seemed to be straightforward, only 450 transitions could be assigned to 13 sub-bands in the second torsional band, to 8 sub-bands in the ν15 = 3←1 band and to 1 in the ν15 = 4←1. For some transitions (K = 0←1, 1←0 etc.) no lower state combination differences were available, thus their assignments were confirmed by the combination differences involving ν15 = 2←1 transitions from the previous study [6]. The same method was used in the present work, where on the basis of the assigned hot transitions, many overtone transitions were successfully assigned. The procedure will be described in more detail in following parts of this paper. In the work from 1992 [8], a global fit of the ν15 = 2←0 band was not satisfactory. Instead, only single sub-bands were fitted individually to polynomials.

The present work reports the complete structure of the second torsional state on the basis of the analysis of the ν15 = 2←0 overtone band and ν15 = 2←1 hot torsional band. The overall number of transitions of the overtone and hot torsional bands exceeds 14,400. A global fit was carried out including these transitions, which resulted in a standard deviation of 0.006 cm−1. Such a complete analysis of the second torsional state was possible thanks to the very precise knowledge of the ground [9] and first torsional [10] states, which could be used simultaneously in a Lower State Combination Differences method (LSCD).

The precise structure of the excited torsional states is crucial for the understanding of resonances observed in the rovibrational spectrum of methylamine. The analyses of infrared bands such as the inversion of the amino group or Csingle bondN stretch revealed significant perturbations from highly excited torsional states [11], [12], [13]. In Fig. 1 one can see the energy level structure for the ground state, four lowest excited torsional states ν15 = 1, 2, 3, 4, the first excited wagging state ν9, and the Csingle bondN stretching state ν8. The first and second excited states are isolated from other states, so there are no significant perturbations in that region. The energy region of the inversion state overlaps with excited torsional states, i.e., the upper component of the third excited torsional state 3ν15 and the lower component of the fourth excited torsional state 4ν15.

The available theoretical models [14] do not allow for precise calculation of rotational structure of highly excited torsional states. We believe that this information can be extracted from experimental spectra. The region where ν15 = 3←1 and ν15 = 4←1 can be observed covers the range of the overtone band ν15 = 2←0. However, the spectrum is so congested (Fig. 2) that it is necessary to assign the stronger ν15 = 2←0 band first and then to look for weaker bands. This procedure should deliver energies of ν15 = 3 and 4 excited torsional states and consequently untangle the perturbation problem in the inversion state.

Section snippets

Experimental data

The high-resolution spectra of methylamine have been recorded at the University of Oulu in Finland using a Bruker IFS-120HR Fourier Transform spectrometer in the range from 40 to 360 cm−1 (hot band) and from 260 to 720 cm−1 (overtone band) at a pressure of 0.036 Torr with the path length of 3.2 m in the optimized White cell (Fig. 2). The operating temperature of the bolometer was 1.4–2 K.

The resolution due to MOPD is 0.00125 cm−1 and the relative wavenumber precision is almost one order of

Theoretical model

In most studies concerning methylamine spectra, the phenomenological Hamiltonian based on the group-theoretical high-barrier tunneling formalism was used [7]. This formalism is based on the assumption that a complete set of vibration-rotation states, which are well localized in one minimum of the potential surface, is in interaction with the equivalent complete set of vibration-rotation states localized in neighboring minima. The electronic ground state potential energy surface of methylamine

Line assignment

The present study aimed to assign extensively lines for ν15 = 2←0 and ν15 = 2←1 torsional bands in the IR spectrum. This is the first attempt to assign simultaneously two IR bands which lead to the second excited torsional state. The second torsional state, 2ν15, is well isolated from other vibrational states (Fig. 1), thus no vibrational perturbations are expected to occur.

Previous analyses of the IR bands [6], [8], in which the overtone band and the hot band were analyzed independently,

Global fitting

In the previous studies on the second hot and overtone bands, global fits of the ν15 = 2←1 and ν15 = 2←0 torsional transitions using a group theoretical formalism [7] could not be satisfactory [6], [8] because of insufficient number of data. In the IR torsional hot band [6] and in the overtone band [8] of methylamine only 87 and 450 lines were assigned, respectively. In this paper the numbers are 6632 and 7771, respectively. But it is not only the number which matters. In the present work the

Conclusions

For the first time, the second torsional fundamental band of methylamine has been completely assigned in a high resolution infrared spectrum. The spectrum was recorded with a higher resolution and precision than previously which was crucial for the analysis of the spectrum of very high density of lines.

In total, 14,403 lines have been assigned and the assignment was confirmed through the lower state combination differences (LSCD). In this paper the lower state means the energy structure of the

References (20)

  • M. Oda et al.

    J. Mol. Spectrosc.

    (1989)
  • N. Ohashi et al.

    J. Mol. Spectrosc.

    (1987)
  • N. Ohashi et al.

    J. Mol. Spectrosc.

    (1992)
  • V.V. Ilyushin et al.

    J. Mol. Spectrosc.

    (2005)
  • I. Gulaczyk et al.

    J. Mol. Spectrosc.

    (2017)
  • I. Gulaczyk et al.

    J. Mol. Spectrosc.

    (2011)
  • M. Kręglewski

    J. Mol. Spectrosc.

    (1989)
  • N. Ohashi et al.

    J. Mol. Spectrosc.

    (1987)
  • N. Ohashi et al.

    J. Mol. Spectrosc.

    (1989)
  • W. Łodyga et al.

    J. Mol. Spectrosc.

    (2007)
There are more references available in the full text version of this article.

Cited by (8)

  • Large amplitude inversion tunneling motion in ammonia, methylamine, hydrazine, and secondary amines: From structure determination to coordination chemistry

    2021, Coordination Chemistry Reviews
    Citation Excerpt :

    In this analysis, some series of lines for the third and fourth excited torsional bands were also assigned, i.e. for ν15 = 3–1 and ν15 = 4–1, respectively. A complete structure of the second torsional state of methylamine was presented by Gulaczyk et al. [159] where rovibrational transitions belonging to the overtone band ν15 = 2–0 as well as to the hot torsional band ν15 = 2–1 were assigned and fitted to a theoretical model based on the Hougen-Ohashi formalism with a standard deviation of 0.006 cm−1. Fig. 11 shows the high-resolution infrared spectrum of methylamine from 40 to 720 cm−1.

  • IR rotational spectrum of methylamine

    2020, Journal of Quantitative Spectroscopy and Radiative Transfer
    Citation Excerpt :

    Since it is not possible to calculate rotational structures of highly excited torsional states with the available theoretical models [28], hopefully this information will be obtained from systematic analysis of experimental spectra. We believe that after removing from the congested spectrum of methylamine all previously assigned rovibrational transitions for the first excited torsional band ν15=1←0 [17], overtone ν15=2←0 and hot band ν15=2←1 [24] together with pure rotational transitions in all three states (ground state, first excited and second excited states), we will be able to assign more and much weaker spectral lines belonging to the ν15=3←1 andν15=4←1 transitions in order to explain perturbations in the inversion band region of methylamine. The high-resolution spectra of methylamine that were used in the analysis of pure rotational transitions were recorded by Veli-Mati Horneman at the University of Oulu in Finland using a Bruker IFS-120HR Fourier Transform spectrometer in the range from 40 to 360 cm−1, at a pressure of 0.036 Torr with the path length of 3.2 m in the optimized White cell.

  • A simultaneous fit of v<inf>t</inf> = 0 and 1 torsion-wagging-rotational levels of CH<inf>3</inf>NH<inf>2</inf> using a hybrid “tunneling and non-tunneling” Hamiltonian formalism

    2020, Journal of Molecular Spectroscopy
    Citation Excerpt :

    It is thus clear that there is some trouble either in the theoretical model (e.g., one or more missing important terms in the Hamiltonian, in spite of our extensive searches), or in some of the experimental high-K assignments (e.g., some inconsistency between MW and IR measurements at high K). We will leave this question unresolved until a global fit involving the vt = 2 ← 1 and vt = 2 ← 0 infrared bands [20–22] and the unpublished new microwave measurements [15] is attempted. It can be said, however, that the present hybrid fit fits almost all of the available data in the literature with K < 9 nearly to experimental error.

  • Understanding (coupled) large amplitude motions: The interplay of microwave spectroscopy, spectral modeling, and quantum chemistry

    2021, Theoretical and Computational Chemistry: Applications in Industry, Pharma, and Materials Science
View all citing articles on Scopus
View full text