Article pubs.acs.org/JPCA
Quantitative Infrared Absorption Spectra and Vibrational Assignments of Crotonaldehyde and Methyl Vinyl Ketone Using Gas-Phase Mid-Infrared, Far-Infrared, and Liquid Raman Spectra: s‑cis vs s‑trans Composition Confirmed via Temperature Studies and ab Initio Methods Rodica Lindenmaier,† Stephen D. Williams,‡ Robert L. Sams,† and Timothy J. Johnson*,† †
Pacific Northwest National Laboratory, Richland, Washington 99354, United States A. R. Smith Department of Chemistry, Appalachian State University, Boone, North Carolina 28618, United States
‡
ABSTRACT: Methyl vinyl ketone (MVK) and crotonaldehyde are chemical isomers; both are also important species in tropospheric chemistry. We report quantitative vapor-phase infrared spectra of crotonaldehyde and MVK vapors over the 540−6500 cm−1 range. Vibrational assignments of all fundamental modes are made for both molecules on the basis of far- and mid-infrared vapor-phase spectra, liquid Raman spectra, along with density functional theory and ab initio MP2 and high energy-accuracy compound theoretical models (W1BD). Theoretical results indicate that at room temperature the crotonaldehyde equilibrium mixture is approximately 97% s-trans and only 3% s-cis conformer. Nearly all observed bands are thus associated with the s-trans conformer, but a few appear to be uniquely associated with the s-cis conformer, notably ν16c at 730.90 cm−1, which displays a substantial intensity increase with temperature (70% upon going from 5 to 50 o C). The intensity of the corresponding mode of the s-trans conformer decreases with temperature. Under the same conditions, the MVK equilibrium mixture is approximately 69% s-trans conformer and 31% s-cis. W1BD calculations indicate that for MVK this is one of those (rare) cases where there are comparable populations of both conformers, approximately doubling the number of observed bands and exacerbating the vibrational assignments. We uniquely assign the bands associated with both the MVK s-cis conformer as well as those of the s-trans, thus completing the vibrational analyses of both conformers from the same set of experimental spectra. Integrated band intensities are reported for both molecules along with global warming potential values. Using the quantitative IR data, potential bands for atmospheric monitoring are also discussed.
1. INTRODUCTION We have recently undertaken studies to provide quantitative infrared spectra of a series of (oxygenated) compounds arising from biogenic emissions including biomass burning.1,2 During the past few years, we have used the Pacific Northwest National Laboratory (PNNL) gas-phase database to complete vibrational analyses of a series of small organics such as isoprene,3 glycolaldehyde,4 glyoxal/methylglyoxal,5 acetol,6 and diiodomethane.7 Using such data, many of these species have also been quantified in recent biomass burning studies.8−10 In this paper we focus on two important organic unsaturated carbonyl compounds, namely crotonaldehyde (CH3CHCHCHO), and methyl vinyl ketone (MVK, CH3C(O)CHCH2, an unsaturated ketone). Our studies on characterizing crotonaldehyde and MVK revealed that the existing vibrational analyses sometimes have discrepancies between theory and experiment11 or are incomplete for various reasons, such as incomplete mid-IR or far-IR data,12−16 or perhaps due to a lack of Raman spectra.16,17 © 2016 American Chemical Society
Crotonaldehyde and MVK are both important oxygenated volatile organic compounds (OVOCs). In addition to other sources, they arise during biomass burning events and are typically oxidized further to form species such as acetaldehyde, glycolaldehyde, methylglyoxal, and formaldehyde along with peroxyacylnitrates.18−21 These are of interest due to their role in the formation of secondary organic aerosols,22−24 and their role in the overall NOx cycle.20,25 The two species are isomers of one another and both have been observed during biomass burning events along with other biogenic sources feeding into the atmosphere. Both also have known anthropogenic sources.18,26 In terms of vapor-phase emission effects, crotonaldehyde is an environmental contaminant known to be both a mutagen and a carcinogen.27,28 It has been identified in the exhaust of Received: October 28, 2016 Revised: December 14, 2016 Published: December 16, 2016 1195
DOI: 10.1021/acs.jpca.6b10872 J. Phys. Chem. A 2017, 121, 1195−1212
Article
The Journal of Physical Chemistry A
Figure 1. Conformers of trans-crotonaldehyde and methyl vinyl ketone (MVK), with absolute energies in Hartrees as predicted by the W1BD method. See text.
gasoline- and diesel-engines,29,30 jets, tobacco smoke, and from the combustion of polymers and of wood.18,31−33 However, it occurs naturally in meat, fruits, vegetables, bread, milk, beer, wine, and liquors.28,34 It has even been detected in human milk and the exhaled breath of nonsmokers.35 In terms of sinks, aldehydes such as crotonaldehyde are eliminated from the atmosphere mainly by photolysis or by reaction with either OH (during daylight hours) or NO3 (at night) radicals. Reactions with these two atmospheric oxidants lead to acylperoxy radicals, RC(O)O2, which react rapidly with NOx to form ozone and a stable peroxyacylnitrate, RC(O)O2NO2 or nPAN. nPANs are NOx reservoirs in the atmosphere, being able to transport NO2 over long distances given their relatively long lifetimes in the upper troposphere.25 Methyl vinyl ketone is a primary first-yield product of isoprene oxidation by the OH radical in the presence of NOx.26,36−42 It can also result from the ozonolysis of isoprene.43,44 In terms of other sources, MVK is also produced in significant amounts by automobile engines45 and also by biomass burning.33,46 The main atmospheric loss process for MVK is its oxidation by OH (with a lifetime of ∼10 h), which leads to formation of intermediate alkyl radicals; these react further with O2 to form peroxy radicals (RO2). These in turn will react with, e.g., NO to produce NO2 and other species such as glycolaldehyde (HOCH2CHO), methylglyoxal (CH3COCHO), formaldehyde (CH2O), and peroxyacetyl nitrate (CH3C(O)OONO2).20,47 These are also of interest due to their role in the secondary organic aerosol formation24 and their ability to act as NOx temporary reservoirs. A necessary prerequisite for gas-phase quantification of species such as MVK via IR spectroscopy is the availability of high-quality infrared reference data. Most of the results reported here were originally generated as components of PNNL’s quantitative Northwest Infrared (NWIR; http://nwir.pnl.gov) gas-phase database,1,48,49 which was created to detect and quantify gas-phase molecules for measurements either in the laboratory or in the field near the earth’s boundary layer. For example, infrared spectroscopy has been used during the Tropical Forest and Fire Emissions Experiment33 (TROFFEE) to study emissions from biomass burning. During that study, emission factors (EF) were reported for crotonaldehyde ranging between 0.068 and 0.302 g/kg and for MVK ranging between 0.113 and 0.499 g/kg for different fuel types: tropical forest, grass, and slash. These values, however, were derived from proton transfer reaction mass spectrometer (PTR-MS) measurements rather than IR spectroscopy. In principle, having the PNNL gas-phase database and the correct assignments for these two molecules would make it possible to perform remote or ground-based spectral measurements to quantify such biomass burning products9,10,50,51 as well as for other applications.52,53
For example, the control of the selective hydrogenation of α,β-unsaturated compounds has been extensively studied using a large variety of catalysts54−57 and could be studied via IR spectroscopy. Crotonaldehyde has two structural isomers (cis−trans or Z−E) due to its central CC bond, as well as s-cis and s-trans conformers, as seen in Figure 1. In this paper the crotonaldehyde results, unless stated otherwise, are for the s-cis and s-trans conformers of trans-crotonaldehyde. Spectra for crotonaldehyde were first investigated more than a century ago by Purvis and McCleland58 and revisited a decade later by Lüthy,59 with Gredy and Piaux60 and Blacet et al.61 being the first to consider its cis−trans isomerism. Although Gredy and Piaux60 reported lines that were attributed to traces of the cis-isomer (∼1%), Blacet et al.61 indicated that the commercial crotonaldehyde they analyzed was a pure trans-isomer with no trace of the cisisomer. Other studies followed, further investigating the crotonaldehyde IR spectrum for different purposes, e.g., Blout et al.62 studied the effect of increased conjugation on the spectra of twenty-six compounds containing conjugated double bonds. Bowles et al.17 reported that no change in the IR spectrum of crotonaldehyde is observed with change in temperature for the solution or vapor, again indicating that it exists only in the planar s-trans conformationresults for which we report contradictory data (vide inf ra). They also made vibrational frequency assignments for crotonaldehyde vapor in the mid-infrared only. Durig et al.12 and Oelichmann et al.16 have both completed vibrational assignments for the mid-IR, far-IR, and Raman spectra of trans-crotonaldehyde (indicating that this is the form in which crotonaldehyde most often exists) but do not agree on several mode assignments including those for the CH3 out-of-plane rock, and those for the C−CHO stretch and C−CH3 stretch. In their study in the early 1980s Oelichmann et al.16 reported not only frequencies but also vibrational intensities for crotonaldehyde and MVK, but their experimental results did not always agree with their theoretical calculations. This is likely due to the limited basis sets and limited processor capabilities of the day; the IR or Raman intensities are of course far more difficult to predict than the frequency eigenvalues alone. More recently, Jayaprakash et al.63 compared experimental results obtained from the neat liquid infrared and Raman spectra of crotonaldehyde with their theoretical calculations. However, that study does not include gas-phase or far-IR results. It is also important to note that other works18,19,32,64 have used quantitative IR spectroscopy to derive products of the reaction of crotonaldehyde with species such as OH, NO3, O3, but although Magneron et al.18,32 do report quantitative UV−visible cross sections, none of these authors explicitly reports quantitative IR spectra of crotonaldehyde. 1196
DOI: 10.1021/acs.jpca.6b10872 J. Phys. Chem. A 2017, 121, 1195−1212
Article
The Journal of Physical Chemistry A Methyl vinyl ketone, thought to exist in both the s-cis and s-trans forms (Figure 1), was investigated as early as the 1960s. Noack and Jones15 demonstrated that MVK exists as a mixture of s-cis and s-trans conformers at room temperature and made the first liquid IR and Raman assignments for this molecule using a CHCl3/CCl4/C2Cl4 solvent. By contrast, the study using microwave spectroscopy by Foster et al.65 failed to identify any proof of the s-cis conformer in the vapor-phase spectrum. Bowles et al.,17 Krantz et al.,14 Oelichmann et al.,16 and Durig and Little13 have all further investigated the conformers of MVK and made vibrational assignments that distinguished between the two conformers. It is worthy of note, however, that the spectra of the MVK conformers were still poorly characterized at that time, save for the intense absorption lines associated with the CO stretching motion for the two conformers and some of the lines at frequencies below 1400 cm−1. De Smedt et al.,66 and later Sankaran and Lee67 added theoretical calculations, shedding more light on the structure and conformer states of the MVK molecule. During their study of the photooxidation processes of methyl vinyl ketone Raber and Moortgat26 used MVK reference spectra to determine several photooxidation parameters (e.g., photolytic rate constant, product yields, etc.). In their study, a quantitative MVK infrared reference spectrum was used to monitor its concentration (along with formation products) via IR spectroscopy during photooxidation processes onset by UV photolysis into the 330 nm n → π* absorption band. Quantitation was achieved via the MVK ν16c band at 598.3 cm−1. Tuazon and Atkinson21 explicitly report a quantitative MVK spectrum in their Figure 1, from which we can estimate cross sections as discussed in section 4.4 below. On the basis of these previous works, the present paper reports the high-resolution, high signal/noise vapor-phase midIR and far-IR spectra, along with liquid Raman spectra, as well as theoretical calculations for both crotonaldehyde and MVK. The objective of the paper is to improve the vibrational assignments of these two species. Equally important, we report seminal measurement of quantitative crotonaldehyde and MVK mid-infrared vapor-phase cross sections using the vetted methods of the PNNL gas-phase database. During the course of these studies, however, the temperature dependencies of certain IR bands revealed several interesting phenomena about the conformers of the two molecules that will be discussed below.
photoconductive mercury cadmium telluride (HgCdTe) detector were used to acquire the interferograms. Each single channel spectrum in the 540−6500 cm−1 range was derived from 256 averaged interferograms acquired at 0.112 cm−1 resolution, boxcar apodized and zerofilled by a factor of 2 prior to Fourier transformation. A single pass sample cell with a length of 19.96 cm, a 5.1 cm inner diameter, and KBr wedged windows was used for analysis. The cell was filled with the analyte vapor using a gold-plated stainless steel manifold evacuated to
