Absorption cross sections of n-butane, n-pentane, cyclopentane and cyclohexane

Highlights

• High resolution infrared Fourier transform spectroscopy.

• Absorption cross sections of n-butane, n-pentane, cyclopentane and cyclohexane.

• For remote sensing in planetary atmospheres.

Abstract

The infrared absorption cross sections of pure n-butane, n-pentane, cyclopentane and cyclohexane have been measured in the 3.3 µm region (and 6.8 µm region for cyclopentane) by high resolution Fourier transform spectroscopy. The samples had temperatures of 294 K and 230 K for n-butane, 294 K and 218 K for n-pentane, 296 K and 235 K for cyclopentane, and 294 K and 222 K for cyclohexane. These spectra are important for molecular identification in extraterrestrial environments such as Saturn's moon Titan, giant planets and exoplanets. All spectra are provided as data point files.

Introduction

The oxidation of non-methane hydrocarbons plays an important role in the production of tropospheric ozone and aerosols, with these hydrocarbons largely coming from fugitive emissions of fossil fuel production [1]. Simple hydrocarbons have also been observed in the atmospheres of the giant planets [2] and Saturn's moon Titan [3] which has an organic-rich atmosphere and is considered to be like that of pre-biotic Earth [4,5]. Organic molecules are produced on Titan by dissociation and ionization of nitrogen and methane in the upper atmosphere. Reactions between two methyl radicals produce ethane and other photochemical processes form larger hydrocarbons. Organic photochemistry also produces an orange aerosol haze on Titan [3,6]. The atmospheres of giant planets are likely reservoirs of complex hydrocarbon species.

Numerous non-methane hydrocarbons (C₂H₂, C₂H₄, C₂H₆, CH₃C₂H, C₃H₆, C₃H₈, C₄H₂ and C₆H₆) [3] have been detected in the stratosphere of Titan mainly using the CIRS (Composite Infrared Spectrometer) Fourier transform instrument on the Cassini spacecraft that was in orbit around Saturn. The most recent hydrocarbons detected on Titan are allene (propadiene, CH₂CCH₂) from TEXES (Texas Echelon-cross-Echelle Spectrograph) on the NASA IR Telescope Facility by Lombardo et al. [7] and cyclopropenylidene (c-C₃H₂) found with ALMA (Atacama Large Millimeter/submillimeter Array) by Nixon et al. [8]. Titan's stratosphere has a temperature of about 80 to 200 K at pressure of about 10 to 0.01 Torr of mainly nitrogen [3]. Hydrocarbon fractional abundances range from 0.2 ppb for benzene (C₆H₆) to a few percent for methane (CH₄). None of the hydrocarbons studied in this paper have been detected on Titan yet, although they are all potentially present.

The detection of hydrocarbon species in the Earth's atmosphere and the atmospheres of planets, moons and exoplanets relies on reliable spectroscopic line lists and cross sections. Most existing spectroscopic cross sections for hydrocarbon species were collected at relatively low resolution and are often broadened by N₂. Higher resolution spectroscopic cross sections that can resolve sharper spectroscopic features are necessary in order to identify and quantify these species under conditions where pressure broadening is small. Our present work includes absorption cross sections in the 3000 cm⁻¹ (CH stretching) region for n-butane, n-pentane, cyclopentane and cyclohexane along with 1460 cm⁻¹ (6.8 µm) cross sections for cyclopentane.

Butane has two isomers: isobutane and n-butane. Hydrocarbons like ethane and propene have been observed in Titan [9] and giant planets [10,11] and the photochemical models of Titan [12] and Saturn [13] predict butane to be as abundant as other small hydrocarbons. Curtis et al. [14] suggest the possibility that some of the haze particles in Titan's atmosphere may be coated with a layer of butane at the tropopause and it may influence cloud formation mechanisms. Navarro-Gonzalez et al. [15] have experimentally studied the corona discharge of a simulated Titan atmosphere and have identified several hydrocarbons including ethane, propane, n-butane, n-pentane, and cyclopropane. Hewett et al. [16] have estimated upper limits for the abundance of n-butane and isobutane on Titan.

Diethyl or n-butane (C₄H₁₀) has two main conformations: s-trans and gauche which have C_2h and C₂ symmetry, respectively, with 36 fundamental vibrational frequencies. The s-trans and gauche conformers have 11 and 21 allowed Raman modes and 10 and 21 infrared active modes, respectively [17]. The lowest energy s-trans conformer and the higher energy (by 234 cm⁻¹) gauche conformations are observed in the infrared spectra of this molecule. In CH stretching region there are 10 vibrational modes. C_2h symmetry includes ν₁-ν₃ (a_g), ν₁₂-ν₁₃ (a_u), ν₂₀-ν₂₁ (b_g), and ν₂₇-ν₂₉ (b_u), while C₂ symmetry has ν₁-ν₃, ν₁₂, and ν₁₃ (a) and ν₂₀, ν₂₁, ν₂₇-ν₂₉ (b) modes in the 3.3 µm region.

Absorption cross sections for isobutane have been studied extensively [16,18,19]. Grosch et al. [20] have measured absorption cross sections of n-butane in the CH stretching region at temperatures of 298, 373, and 473 K with nitrogen as a broadening gas. Cross sections of n-butane in the mid infrared region (7–15 µm) with nitrogen as a broadening gas at temperatures of 180–298 K were obtained by Sung et al. [21]. Jolly et al. [22] have recorded absorption cross sections of n-butane at 150 K and 297 K focusing on the bands centered at 733 cm⁻¹ and 966 cm⁻¹.

This work provides the cross sections of pure n-butane in the 3000 cm⁻¹ region obtained at 296 K and 230.3 K without a broadening gas.

n-Pentane (C₅H₁₂) is a straight chain alkane that is an example of a volatile organic compound present in Earth's atmosphere, specifically from rapid increase in natural gas production in recent decades. Studies from Swarthout et al. [23] have looked at the environmental impacts of these volatile organic compounds, including n-pentane, at the Boulder Atmospheric Observatory.

n-Pentane has a number of trans and gauche conformations that are observed in the infrared spectra with the trans, C_2v conformation being the lowest in energy. It contains 45 fundamental vibrational frequencies, 35 being infrared active modes, and all of them being Raman active. This C_2v conformation includes modes of a₁ (ν₁ − ν₁₃), a₂ (ν₁₄ − ν₂₃), b₁ (ν₂₄ − ν₃₂), and b₂ (ν₃₃ − ν₄₅) symmetry. Vibrational spectra of n-pentane have been studied by LaPlante et al. [24], and Raman spectra have also been studied by Kuznetsov et al. [25].

This work provides cross sections of pure n-pentane in the 3000 cm⁻¹ region obtained at 218 K and 292 K.

Cyclopentane (C₅H₁₀) is a fascinating molecule due to its unusual structure and vibrations. It has 10 equivalent bent conformations of C_s symmetry and another 10 equivalent twist conformations of C₂ symmetry. These 20 conformations have such small energy barriers between them that they can readily interconvert in a process known as pseudorotation [26,27]. Additionally there is also a D_5h planar conformation that lies approximately 2100 cm⁻¹ above the bent and twist conformations [27]. It is difficult to conduct a traditional analysis of the 39 vibrational modes for cyclopentane as has been previously discussed for n-butane and n-pentane in Sections 1.2 and 1.3 above because of the effects of pseudorotation. We refer the reader to the work of Ocola et al. for a more traditional normal mode analysis [26]. We also would like to note that a more complete vibrational analysis that includes the coupling of the normal modes with the pseudorotation has just appeared [28].

This present work includes high resolution cross sections of pure cyclopentane in the CH stretching region at 3000 cm^-1 and for the CH₂ scissors bend at 1460 cm⁻¹.

Cyclohexane (C₆H₁₂) is a ring of 6 carbon atoms with multiple conformations. Of these, the “chair” is the minimum energy conformation with its 6 axial and 6 equatorial hydrogen atoms. Local minima occur in the “twist boat” conformation, about 5.5 kcal/mol higher than the “chair conformation.” [29,30] Cyclohexane is almost completely in the chair conformation at room temperature, where less than 0.1% of the molecules are in the twist-boat conformation. Volatile organic compounds including cyclohexane are found in the Earth's atmosphere, produced by biogenic and anthropogenic means [31]. Hydrocarbons such as cyclohexane may also be found in the atmospheres of gas giant planets such as Saturn as well as its moon Titan where it is formed by organic photochemistry and ion chemistry [13,32] .

The chair form of cyclohexane has D_3d symmetry, and the twist boat form has D₂ symmetry. There are 48 normal modes and 32 fundamental vibration frequencies, listed here grouped by symmetry and using numbering of the vibrational modes from the order of irreducible representations in Herzberg's character tables: [33] a_1g (ν₁- ν₆), a_1u (ν₇- ν₉), a_2g (ν₁₀- ν₁₁), a_2u (ν₁₂- ν₁₆), e_g (ν₁₇- ν₂₄) and e_u (ν₂₅- ν₃₂). Only the 3 a_1u and 8 e_u modes are infrared active [34]. The a_1g and e_g modes are Raman active. There have been many previous efforts to make infrared and Raman measurements of cyclohexane in its different phases with accompanying calculations to assist with vibrational assignments [35], [36], [37], [38], [39], [40], [41]. This work builds on the previous investigation into the high-resolution infrared spectra of cyclohexane published by Bernath and Sibert [42].