Infrared Spectra of Gas-Phase 1- and 2-Propenol Isomers

Article abstract of DOI:10.1021/acs.jpca.7b02323

Fourier transform infrared spectra of isolated 1-propenol and 2-propenol in the gas-phase have been collected in the range of 900-3800 cm^-1, and the absolute infrared absorption cross sections reported for the first time. Both cis and trans isomers of 1-propenol were observed with the trans isomer in greater abundance. Syn and anti conformers of both 1- and 2-propenol were also observed, with abundance consistent with thermal population. The FTIR spectrum of the smaller ethenol (vinyl alcohol) was used as a benchmark for our computational results. As a consequence, its spectrum has been partially reassigned resulting in the first report of the anti-ethenol conformer. Electronic structure calculations were used to support our experimental results and assign vibrational modes for the most abundant isomers, syn-trans-1-propenol and syn-2-propenol.

Full text of DOI:10.1021/acs.jpca.7b02323

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Article
Infrared Spectra of Gas-Phase 1- and 2-Propenol Isomers
Miranda F. Shaw, David L. Osborn, Meredith J. T. Jordan, and Scott H. Kable
J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017
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Infrared Spectra of Gas-Phase 1- and 2-Propenol Isomers
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Miranda F. Shaw,1 David Osborn,2 Meredith J. T. Jordan,1 Scott H. Kable,3,*
1. School of Chemistry, University of Sydney, New South Wales 2006, Australia
2. Combustion Research Facility, Sandia National Laboratories, Livermore, California
94551, USA
3. School of Chemistry, University of New South Wales, New South Wales 2052,
Australia
Abstract
Fourier transform infrared spectra of isolated 1ꢀpropenol and 2ꢀpropenol in the gasꢀphase
have been collected in the range of 900ꢀ3800 cm−1, and the absolute infrared absorption cross
sections reported for the first time. Both cis and trans isomers of 1ꢀpropenol were observed
with the trans isomer in greater abundance. Syn and anti conformers of both 1ꢀ and 2ꢀ
propenol were also observed, with abundance consistent with thermal population. The FTIR
spectrum of the smaller ethenol (vinyl alcohol) was used as a benchmark for our
computational results. As a consequence, its spectrum has been partially reꢀassigned resulting
in the first report of the antiꢀethenol conformer. Electronic structure calculations were used to
support our experimental results and assign vibrational modes for the most abundant isomers;
syn-transꢀ1ꢀpropenol and synꢀ2ꢀpropenol.
*Author to whom correspondence should be addressed: Email: s.kable@unsw.edu.au
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Introduction
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Enols have become increasingly recognised as important reactive intermediates in
atmospheric1,2 and combustion2ꢀ6 chemistry, and have recently been implicated as precursors
to atmospheric carboxylic acids. 1,7,8 Known sources of enols are aldehydes9ꢀ11 and ketones,12
which are an abundant organic constituent of the troposphere and themselves intermediates in
the photoꢀoxidation of volatile organic compounds (VOCs).13 There are two photochemical
routes for gasꢀphase carbonyls to form enols: the wellꢀknown Norrish Type II mechanism,
and the more recently reported photoꢀtautomerisation. Both reactions begin with absorption of
actinic UV light via an (π*ꢀn) electronic transition to the S1 surface.
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The Norrish Type II reaction (NTꢀII) is one of the two major photolysis reactions of longer
chain aldehydes and ketones.14 Following excitation to the excited surfaces, the carbonyl
oxygen atom abstracts a hydrogen atom from the γꢀcarbon, and subsequent bond cleavage
yields the enol plus an alkene. This process occurs on both the singlet and triplet surfaces.15
Scheme 1 shows the concerted NTꢀII reaction for butanal.16
Scheme 1. Norrish Type II reaction of butanal.
The NTꢀII reaction occurs in competition with the Norrish Type I reaction (NTꢀI, homolytic
C–C σꢀbond cleavage adjacent to the carbonyl) and a minor cyclisation reaction.
Photoꢀtautomerization can occur if the molecule returns to a vibrationallyꢀexcited, electronic
ground state (S0) via internal conversion (IC) from S1, or intersystem crossing (ISC) from T1.
Isomerization to the enol is then possible via a tight transition state on S0.10 An example is
shown in Scheme 2, for the photoꢀtautomerization of acetaldehyde, the quantum yield for
which is both pressure and wavelength dependent.11
Scheme 2. Phototautomerisation of acetaldehyde to vinyl alcohol.
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The importance of this process has been increasingly recognized in recent years, given the
high atmospheric concentrations of small carbonyls such as acetaldehyde, propanal and
acetone. In addition, the NTꢀII reaction is not possible in these smaller carbonyls because they
lack γꢀhydrogens.
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The smallest enol, vinyl alcohol (ethenol) is a tautomer of acetaldehyde, and has been studied
in detail. However, there is no spectroscopic data in the gasꢀphase for the tautomers of
propanal and acetone: 1ꢀ and 2ꢀpropenol. Matrix infrared spectra of 2ꢀpropenol17 and some
larger substituted enols18ꢀ20 have been obtained, but studies of propenols have been limited to
mass spectrometry21 and NMR.22 To the best of our knowledge, vibrational spectroscopy of
enols in the gas phase is limited to the spectrum of vinyl alcohol.23,24 Infrared spectroscopy is
an especially useful technique for the observation of shortꢀlived, gasꢀphase enols, as they can
be readily observed in situ, at a range of photolysis wavelengths and pressures, including
simulated atmospheric conditions. We have previously used this technique to study the
synthesis and spectrum of vinyl alcohol, and now report the gas phase infrared spectra of 1ꢀ
and 2ꢀpropenol, produced by the NTꢀII reactions of 2ꢀmethylbutanal and 2ꢀpentanone
respectively, summarised in Schemes 3a and 3b respectively.
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Scheme 3. Synthesis of (a) 1ꢀpropenol, and (b) 2ꢀpropenol.
Experimental
The experimental procedure for photolysis and collection of Fourier transform infrared
(FTIR) spectra have been described previously.11 In brief, experiments were conducted in a
12.4 × 12.4 cm Teflonꢀcoated, crossꢀshaped stainless steel cell. The cell was placed in an
evacuated FTIR spectrometer (Bruker IFS 66 v/S) and aligned such that the IR beam
propagated along one axis through polished KBr windows, while monochromatic UV
photolysis laser pulses propagated along the perpendicular axis through fused silica windows.
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FTIR scans were taken using a liquidꢀnitrogenꢀcooled HgCdTe detector in the range of 820 −
3800 cm−1. The detector is not sensitive to radiation below 820 cm−1. The cell was evacuated
and flushed with N2 between experiments. No photochemical deposits on the fused silica
windows were observed during these experiments. 2ꢀMethylbutanal and 2ꢀpentanone liquids
(Aldrich, ≥99.5%) were purified by several freeze−pump−thaw cycles and delivered directly
to the evacuated cell as neat vapour. The pressure of the gas, typically 2 – 10 torr, was
monitored using two MKS Baratron gauges. At the instrumental resolutions used (0.25 and 1
cm−1), pressure broadening does not contribute to the spectral line shape. Photolysis was
conducted using a tunable OPO laser pumped by a pulsed Nd:YAG laser at a 10 Hz repetition
rate. The photolysis beam (5 mm diameter, 3 – 5 mJ/pulse of UV light) was reflected through
the cell four times. We do not expect multiphoton absorption to be important in the unfocused
beam.
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Computational Methods
Geometry optimisations and frequency calculations were performed with a range of
computational methods that have been reported to accurately reproduce experimental infrared
spectra.25 Vibrational frequencies, including anharmonic corrections for MP2 and DFT
methods, were evaluated for MP2,26 CCSD(T),27,28 and the B3LYP, 29 BLYP,30 B97ꢀ1,31 and
EDF232 functionals, with a 6ꢀ31+G(d,p)33 basis set
according to literature scaling factors.34
.
Harmonic frequencies were adjusted
To examine isomer ratios and reaction mechanisms, geometry minima and transition states
were optimised with B3LYP/6ꢀ31+G(d,p), and single point energy calculations performed on
optimised structures with CCSD(T) and a ccꢀpVTZ35 basis set, with additional diffuse
functions on carbon and oxygen atoms. Reported energies include zeroꢀpoint vibrational
energy corrections.
Frequency calculations using the EDF2 functional were performed with the QChem software
package,36 the Gaussian 0937 suite was used for all other optimisation and frequency
calculations, and Molpro38 used for CCSD(T) single point energy calculations. PGOPHER39
was used to simulate the rotational structure of predicted infrared bands.
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Results and Discussion
Ethenol (Vinyl Alcohol)
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We have previously published the gasꢀphase infrared cross sections of vinyl alcohol (VA),
from the NTꢀII reaction of butanal.11 A comparison of our previously published experimental
spectrum,11 those from the literature, 23,24 and the predicted spectra from several
computational methods was first used to benchmark an appropriate level of theory to
accurately predict the VA spectrum. Because the vibrational modes arising from the alkene
and hydroxy functional groups are very similar for VA and the larger enols, the most accurate
method could then be used to aid assignment of spectra for 1ꢀ and 2ꢀpropenol.
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Vinyl alcohol has two geometric minima, with the hydroxyl group in a syn or anti orientation
with respect to the double bond. The syn geometry is energetically favoured by 4.2 kJ mol−1 at
the CCSD(T)//B3LYP level of theory. In this orientation there is a weak interaction between
the hydroxyl hydrogen and the relatively electron rich C=C bond; no such interaction is
possible in the anti geometry. The barrier for this σꢀbond rotation, however, is relatively small
(15ꢀ20 kJ mol−1).40
Figure 1 shows the FTIR spectrum of VA measured by us previously.11 The spectrum is
dominated by the COH inꢀplane bending (
experimental spectrum shows a splitting of both of these bands, which has been attributed to
Fermi resonance between (
9) and the (ν14 +ν15) combination band at 1082 cm−1, and between
5) and the 2ν13 overtone at 1626 cm−1.41,42 Rotational constants from the most intense bands
ν9) and C=C stretching (ν5) modes. The
ν
(ν
have been analysed previously and found to correspond to the lowerꢀenergy syn conformer.24
The observed VA bands have been assigned accordingly throughout the literature.23,24
The vibrational frequencies for synꢀVA were calculated with a range of density functionals
and ab initio methods listed earlier, and compared to experimental literature frequencies for
synꢀVA (excluding ν8, vide infra). Mean absolute deviations (MADs) between calculated and
literature frequencies ranged from 10 to 40 cm−1. Of the methods tested here, the best results
were seen for B3LYP and B97ꢀ1 anharmonic frequencies, with MADs of only 9.7 and 10.7
cm−1, respectively.
As shown in Table 1, the observed band at 1260 cm−1 had previously been assigned as the ν8
fundamental, 23,24 one of the COH bending modes. However this band was poorly predicted
by every computational method, for both wavenumber and intensity. It appears to be one of
the stronger bands in the experimental spectrum (see Figure 1), but is calculated to have nearꢀ
zero intensity.
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Figure 2 shows the displacement vectors for ν8 and ν9 for both the syn and anti conformers.
Both modes involve a COH bending motion but ν8 causes only a small change in dipole
moment for synꢀVA, whereas ν9 results in the high intensity band at 1118 cm−1. For antiꢀVA,
it is ν8 that has the larger change in dipole moment and corresponding high intensity. The
calculated frequency for ν8 of antiꢀVA is 1233 or 1254 cm−1 from B3LYP and B97ꢀ1
respectively, which is very close to the observed 1260 cm−1 band in the experimental
spectrum. We therefore conclude, on the basis of our anharmonic calculations, that the
experimental gasꢀphase spectrum contains both the syn and anti VA conformers, and the
observed band at 1260 cm−1 is in fact the strong transition involving ν8 of the anti conformer.
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It follows that the actual ν8 band of synꢀVA is instead the observed band at 1300 cm−1, which
had previously been assigned as ν7. From the low calculated intensity and weak signal in the
matrix, it is likely that the actual ν7 band is too weak to be easily observed in the gasꢀphase.
By reassigning the ν7 and ν8 bands in the gasꢀphase spectrum of synꢀVA, this also resolves the
relatively large differences in these vibrations between the gasꢀphase and matrix spectra. This
difference in assignments of ν7 and ν8 had been previously noted, but were ascribed to matrix
effects rather than to the presence of an additional conformer.24
We have calculated the relative populations of the two VA conformers from their Boltzmann
weight at 300 K, using the calculated energy difference of 4.2 kJ mol−1 at the
CCSD(T)//B3LYP level of theory. This analysis assumes room temperature distributions of
both isomers. Implicit in this assumption is that collisionꢀinduced thermalisation occurs on a
much faster time scale than our experiments.7 At equilibrium, 85% of VA is calculated to be
in the syn conformation and 15% is in the anti conformation. This ratio is consistent with the
microwave spectrum obtained by Rodler et al,43 in which an energy difference of 4.5 kJ mol−1
was predicted by the ratio of signal intensities. Our predicted syn and anti room temperature
infrared spectra of VA using this ratio are shown below our experimental, gasꢀphase spectrum
in Figure 1. The rotational band countours are determined by the symmetries of the vibrations,
with inꢀplane modes appearing as A/Bꢀtype hybrid bands and the outꢀofꢀplane modes
appearing as Cꢀtype bands. Comparing the synꢀ and antiꢀ contributions to the predicted
spectrum, note the comparable intensities of the ν1 OH bands despite the minor 15% mole
fraction of the anti- conformer. The width and intensity of the observed spectrum in the
isolated OH stretch region agrees well with the 85:15 population ratio and the predicted
infrared intensities, offer further support for our assignments. Overall, Figure 1 shows very
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good agreement for both the calculated frequencies and the ratio of VA conformers. We have
therefore used similar methods to predict and interpret the propenol spectra.
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1-Propenol
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2ꢀMethylbutanal was irradiated at 295 nm. This wavelength was chosen for the large UV
absorption cross section,44 and because the NTꢀI and NTꢀII processes listed below occur with
very high efficiency at this wavelength, resulting in few minor and secondary products.
Figure 3 shows the FTIR spectrum after 5 minutes of irradiation of 2ꢀmethylbutanal, where
absorptions due to 2ꢀmethylbutanal have been subtracted. The major products observed were
1ꢀpropenol, ethene, CO, formaldehyde and butane, with a minor amount of propanal and 3,4ꢀ
dimethylhexane as described by reactions (R1) to (R6).
2ꢀmethylbutanal + hν
2ꢀmethylbutanal + hν
2 HCO
→
→
→
HCO + CH3CH2CHCH3 (NTꢀI)
C2H4 + CH3CH=CHOH (NTꢀII)
CO + H2CO
(R1)
(R2)
(R3)
(R4)
(R5)
(R6)
HCO + CH3CH2CHCH3
2 CH3CH2CHCH3
CH3CH=CHOH
→
→
→
CO + CH3(CH2)2CH3
CH3CH2CH(CH3)CH(CH3)CH2CH3
CH3CH2CHO (tautomerization)
The contributions from all stable photoproducts and products of bimolecular reactions, other
than 1ꢀpropenol, were quantified and subtracted using scaled reference spectra for these pure
compounds acquired at the same resolution as the photochemical spectrum and under similar
conditions. With the relatively low yields and pressures involved (<100 mTorr of products),
absorbance and number density are related linearly as described by the BeerꢀLambert Law.
The residual signal in the FTIR spectrum is shown in Fig. 4, which we argue below can be
assigned to 1ꢀpropenol isomers.
There are four geometry minima for 1ꢀpropenol: cis or trans about the C=C bond, and syn or
anti with respect to the CCOH dihedral angle (see Fig. 4). We calculated anharmonic IR
frequencies and harmonic intensities of all four isomers using the B3LYP and B97ꢀ1
functionals that were benchmarked against ethenol above. The syn and anti conformers are
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analogous to those discussed for ethenol, and we have assumed that a 300 K thermal
population determines their relative abundance. The syn conformers of each structural isomer
are again lower in energy and expected to account for 81% of the trans isomer and 62% of the
cis isomer abundance. Steric interference from the cis methyl group reduces the stability of its
syn conformer, decreasing its ratio of syn:anti compared to either transꢀ1ꢀpropenol or vinyl
alcohol. This effect is even more pronounced in polar solvent, where the antiꢀconformer
apparently becomes the dominant stereochemistry of cisꢀ1ꢀpropenol, as observed by NMR.22
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We have not made any assumptions about the relative abundance of the cis or trans isomers,
because we calculate the cis-trans interconversion barrier to be 284 kJ mol−1, and hence the
two isomers should not interconvert at 300 K. Their abundance likely reflects the dynamics
of their formation, not their thermodynamic stabilities. Therefore the relative abundance of
each isomer was adjusted to best match the experimental spectrum, with a 1:9 ratio of
cis:trans reproducing experiment very well. Figure 4 compares the experimental spectrum to
the four predicted spectra, weighted to best reproduce the experimental spectrum.
The synꢀtrans isomer of 1ꢀpropenol accounts for the majority of the activity in the spectrum;
we were able to assign most of the observed bands by comparing to the calculated anharmonic
frequencies, which are listed in Table 2. All ground state propenol isomers have Cs point
group symmetry. The fundamental modes are comprised of 16 inꢀplane vibrations with A'
symmetry, and 8 outꢀofꢀplane vibrations with A'' symmetry. The A'' symmetries correspond to
Cꢀtype bands, and A' symmetries result in Aꢀtype, Bꢀtype or A/B hybrid bands, depending on
the orientation of the transition moment with respect to the two inꢀplane inertial axes. These
rotational band contours were simulated using the rotational constants calculated at the
B3LYP/6ꢀ31+G(d,p) level of theory. Where possible, the observed rotational band contours
were used to confirm the vibrational assignments. The five overlapping CꢀH stretching bands
from 2900 – 3000 cm−1 were left unassigned. The calculated frequencies of the other isomers
are available in Supporting Information.
Figure 5 shows the most intense bands of 1ꢀpropenol at higher resolution. Although Figure 5a
and Figure 5b are dominated by the most abundant isomer, the OH stretching band in 5c
strongly suggests overlapping A/Bꢀhybrid type bands, with a similar appearance to the OH
stretching region of the VA spectrum. The OH stretch in the syn conformers of both VA and
transꢀ1ꢀpropenol is redꢀshifted due to the attractive interaction between the OH group and the
double bond. The OH in the anti conformer does not have this interaction, and therefore
vibrates at a slightly higher frequency. The sharp features in Figure 5c are observed from
3493ꢀ3744 cm−1, with a spacing that increases steadily from 6.7 cm−1 to 8.7 cm−1. However,
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we were unable to assign the carrier of this spectrum from the rotational constant (assuming a
linear molecule). The spectral resolution is insufficient for Doppler broadening to assist in the
assignment.
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We explored the relative abundance of cis and trans isomers as a kinetic effect, determined by
the height of the transition state of the NTꢀII reaction. The γꢀhydrogen transfer to the oxygen
atom creates a “chairꢀlike” sixꢀmembered ring, which shows a small reduction in energy if the
methyl group is in an equatorial rather than axial position. Figure 6 shows our calculation of
the transition state structures and the preceding geometries as they occur on the T1 surface, for
the (S) enantiomer of 2ꢀmethylbutanal. The aldehyde and transition state structures exist in
both (R) and (S) stereoisomeric forms but produce identical, achiral products. An energy
difference in the barrier height of 4 to 6 kJ mol−1 between the equatorial vs. axial saddle point
structures was consistent for several levels of theory. A 5 kJ mol−1 difference in barrier height
produces an approximately 7ꢀfold difference in rate of reaction over the barrier (assuming a
constant preꢀfactor). This ratio is quite consistent with the observed 9ꢀfold enhancement of
the trans isomer over the cis isomer.
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2-Propenol
The photolysis of 2ꢀpentanone was conducted and analyzed as described above for 2ꢀ
methylbutanal. As well as ethene, 2ꢀpropenol and acetone, the major products arising from the
NTꢀI process were biacetyl, ethenone (ketene), and alkanes, as described by Equations (R7) to
(R11). 2ꢀPentanone can also undergo a variation of the NTꢀI reaction in which cleavage
occurs on the other side of the carbonyl, producing CH3CH2CH2CO and ĊH3 radicals. This
reaction was unlikely to be occurring as the calculated barrier is 16 kJmolꢀ1 higher than the
barrier for (R7), and we did not see evidence of methane or ethane molecules that would arise
from methyl radicals.
2ꢀpentanone + hν
2ꢀpentanone + hν
2 CH3CO
→
→
→
→
CH3CO + CH3CH2CH2 (NTꢀI)
C2H4 + H2C=C(OH)CH3 (NTꢀII)
CH3COCOCH3
(R7)
(R8)
(R9)
CH3CO + CH3CH2CH2
H2C=C=O + CH3CH2CH3
(R10)
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2 CH3CH2CH2
H2C=COHCH3
→
→
CH3(CH2)4CH3
CH3COCH3 (tautomerization)
(R12)
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After subtraction of reference spectra for all stable products in (R7) – (R12), other than 2ꢀ
propenol, the spectrum in Fig. 7 was obtained, which we attribute to 2ꢀpropenol.
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2ꢀPropenol also has syn and anti conformations with the syn (OH towards C=C) being lower
energy, analogous to vinyl alcohol and 1ꢀpropenol. With a calculated energy difference of 5.8
kJ mol−1, over 90% of the enol is expected to be in the syn conformation at equilibrium at 300
K. The simulated spectrum with this ratio of conformers is shown in Figure 7 and again shows
good agreement with the experimental spectrum. Like 1ꢀpropenol, the inꢀplane, A' symmetry
vibrations produce A/Bꢀhybrid contours and the outꢀofꢀplane, A'' symmetry vibrations
produce Cꢀtype bands, distinctive due to their very strong Qꢀbranch Kꢀstacks.
As was seen for the other enols, the most intense band in the predicted anti spectrum is the
COH bending mode, which appears ~100 cm−1 higher than the equivalent syn band. It is
therefore likely that the broad band observed from 1260 – 1330 cm−1 contains both the anti
COH bending mode, and some of the weaker CH bending modes from the syn conformer. The
absorbance of the 2ꢀpropenol spectrum was too low to distinguish any weaker bands from the
anti conformer.
Table 3 lists the observed and calculated frequencies, as well as the observed frequencies of
matrix isolated 2ꢀpropenol.17 B97ꢀ1 anharmonic frequencies are shown for the CH and OH
stretching modes as these gave slightly better agreement with experiment than did B3LYP for
this molecule. This region shows relatively poor agreement with the matrix values. An
additional band was observed in the matrix spectrum at 2835 cm−1 and assigned as ν5,
whereas our assigned ν2 band at 3110 cm−1 was not reported for the matrix spectrum. The rest
of the spectrum shows good agreement between the present gasꢀphase spectrum and that
collected in the matrix. Argonꢀmatrix and gasꢀphase spectra typically differ by 1ꢀ10 cm−1, and
up to 20 cm−1 for OH stretching bands,45 which is consistent with the data in Table 3. The
gasꢀphase enol had previously been identified only by reduced transmittance at 3628 cm−1,12
which is well within the range of the broad absorption band that we see here (approximately
3610 – 3660 cm−1).
There are some key differences in the spectra of 1ꢀ and 2ꢀpropenol. 2ꢀPropenol shows a more
intense CꢀH bending mode at 1386 cm−1, whereas the COH bend at 1185 cm−1 is both less
intense and shifted 70 cm−1 higher than the corresponding band in 1ꢀpropenol. An additional
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weak band is observed at 1606 cm−1, attributed to Fermi resonance between the 2ν20 overtone
and the high intensity C=C stretching band. The ν20 fundamental is below our observed
wavenumber range, but the overtone is estimated to be in the range of 1600 – 1640 cm−1 using
the calculated and matrix values. Intensity sharing in the Fermi resonance may also explain
why the intensity of the experimental C=C stretching band is lower than in the simulated
spectrum; the relative intensities are otherwise well predicted. Fermi resonance is seen in this
region in the spectrum of vinyl alcohol, with the equivalent 2ν13 overtone. The active
vibration in both cases is the outꢀofꢀplane “wag” of the terminal methylene group which is not
possible for 1ꢀpropenol.
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Quantitative absorption cross-sections
One of the motivations for the present research is to measure the quantum yield of
photochemical processes that produce the 1ꢀ and 2ꢀpropenol molecules, for example via the
NTꢀII pathway or by photoꢀtautomerization from a 3ꢀcarbon carbonyl molecule. The
measurement of quantitative concentrations and therefore quantum yields requires knowledge
of the infrared absorption crossꢀsection for each enol, which is reported below.
The number density of enol produced in the NTꢀII reaction was calculated using the measured
changes in number density of the parent, and of the ethene product, which is formed in a 1:1
stoichiometric ratio with the enol. Two kinetic models were used: a simpler model where the
concentration of the parent, methylbutanal, is assumed to remain unchanged through the
course of the experiment and a more complex model where the slow decrease in parent
concentration is explicitly included. The observed loss of parent was consistently 2ꢀ5% for all
photolysis wavelengths and the inferred crossꢀsection for the enol varies by only 1% between
the complex and simple model. Therefore, only the simple kinetic model is explained here;
the derivation of the more complex model can be found in Supplementary Material.
Because the amount of parent photolyzed is only a small percentage of the total, the NTꢀII
reactions (R2) and (R7) can be considered pseudoꢀzeroth order, with an effective rate constant
of k1′:
ꢁꢂ′ = ꢁꢂ [ꢃꢄꢅꢆꢇꢈ]
[ꢉ] = ꢁꢂ′ꢈ
(K1)
(K2)
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where [E] is the ethene concentration. Under this assumption, [E] rises linearly with time
during photolysis, consistent with the experimental data in Fig. 8a for 2ꢀmethylbutanal and
Figure 8b for 2ꢀpentanone photolysis.
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Although propenol (P) is produced at an equal rate to ethene (E), it also undergoes a wallꢀ
catalyzed, first order decay to the keto form (propanal or acetone), with rate constant k2,
throughout the duration of the experiment. The overall change in propenol concentration can
therefore be described by the rate equation K3, which integrates to K4:
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ꢊ
ꢊ
ꢊꢋ [ꢌ] = ꢊꢋ [ꢉ] − ꢁꢍ[ꢌ]
(K3)
(K4)
ꢎ ’
ꢏ
[1 − ꢆꢑꢎ ꢋ
]
ꢐ
[ꢌ] =
ꢎ
ꢐ
The rate constant for enol production, k1′, is equal to the fitted, approximately linear rate of
increase in ethene over time.
The rate constant for the tautomerization reaction (R6 or R12), k2, was found by observing the
timeꢀdependent decrease in absorbance of the major enol bands in the ten minutes following
irradiation (after the laser was turned off), and fitting these to a first order exponential decay.
Both rate constants depend on experimental factors such as photolysis wavelength, laser
power, and gas pressure for k1, and cell coating for k2.
Figure 8 shows the measured, experimental number densities for (a) 1ꢀpropenol and (b) 2ꢀ
propenol, including the kinetic fit to equations K2 and K4.
The wallꢀcatalyzed tautomerization described by k2 was considerably faster for 2ꢀpropenol
than either 1ꢀpropenol or vinyl alcohol. Rather than a slow decay of the enol signal as we saw
for the other enols, 2ꢀpropenol was completely absent from the spectrum in less than a minute
Fig. 8 shows this rapid loss of enol during irradiation, where the rate of decay quickly reaches
the rate of production, and we see a small, constant amount of enol after the first two minutes.
Nevertheless, the rate constant for the decay of 2ꢀpropenol was found by fitting Equation (K4)
to the integrated absorbance of the C=C band between 1570 – 1630 cm−1 (shown in Fig. 8b),
and the OH stretching band at 3610 – 3670 cm−1. These two regions were chosen because
there was no overlap with bands from other species.
.
From the calculated enol number densities, the BeerꢀLambert Law was used to calculate the
IR absorption crossꢀsections of 1ꢀ and 2ꢀpropenol at 1 cmꢀ1 resolution, as shown in Fig. 9. We
estimate the total uncertainty in 1ꢀpropenol absorption crossꢀsections to be 10ꢀ15%. Despite
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the much faster decay of 2ꢀpropenol due to tautomerization, the reasonable S/N ratio in the
spectrum produced a quantitative IR absorption crossꢀsection for 2ꢀpropenol with uncertainty
that we estimate at 20%.
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The enol IR crossꢀsection and the decay mechanism for 1ꢀpropenol (R6) was also checked by
recording the time dependence of the propanal (ketoꢀtautomer) spectral signature. The
propanal number density was determined by the same method as for other species, i.e.
measuring a reference spectrum at known pressure. The sum of the number densities of
propanal and 1ꢀpropenol was always within ~5% of the measured ethene number density at all
wavelengths, pressures and irradiation times. This conservation of keto/enol species provided
additional evidence that the calculated crossꢀsection of 1ꢀpropenol is accurate, and that the
enol does indeed decay via tautomerization to its keto form.
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Finally, we also compared the integrated experimental cross sections to the calculated
intensities for the most intense and least overlapping bands of the most abundant conformers
– the OH stretching, C=C stretching, and COH bending modes. The average ratio of
calculated intensities to integrated cross sections for these vibrational modes was 0.84 for synꢀ
transꢀ1ꢀpropenol and 0.68 for synꢀ2ꢀpropenol. We consider these values to indicate quite good
agreement given that we only included one isomer in the calculated intensity, and the likely
error in both experimental cross sections and calculated intensities.
Conclusions
We have measured quantitative infrared spectra of gasꢀphase 1ꢀ and 2ꢀpropenol and assigned
the major constituents as synꢀtransꢀ1ꢀpropenol and synꢀ2ꢀpropenol respectively. We have also
clarified the assignment of the gasꢀphase ethenol spectrum, which includes contributions from
antiꢀethenol. These enol spectra will assist photochemistry studies of enolꢀproducing
carbonyls, including both longer chain molecules that undergo the Norrish Type II reaction,
and smaller, abundant molecules such as acetaldehyde, acetone and propanal that can
tautomerize to their enol isomer. Although these enols react with the walls of our vessel and
decay back to their keto form within 1ꢀ10 minutes under lowꢀpressure laboratory conditions,
their tropospheric lifetime has been estimated from tens of minutes to hours depending on the
surrounding conditions,46,47 allowing competitive OH initiated oxidation to organic acids to
occur.8
We have reassigned part of the vinyl alcohol infrared spectrum, identifying a contribution
from the minor antiꢀ conformer in this spectrum. Finally, our experimental spectrum of 1ꢀ
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propenol is most consistent with a 1:9 ratio of cis:trans isomers. We provide an explanation
for this population ratio based on the Norrish Type II photodissociation mechanism after
intersystem crossing to the lowest triplet state, in which a higher energy transition state for
formation of cis relative to trans controls the dominance of the trans isomer produced.
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Supporting information:
Digitized spectra, reporting both absorbance and absorption cross section, versus wave
number. Derivation of the complete kinetics scheme, and the calculated vibrational
frequencies for all isomers of 1ꢀ and 2ꢀpropenol.
Acknowledgements:
The Australian authors acknowledge financial support of the Australian Research Council
(grant number DP160101792). MFS acknowledges an Australian Postgraduate Award. This
material is based upon work supported by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, who provided the experimental apparatus and support of
DLO.
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42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Electronic Spectra, J. Quant. Spectrosc. Radiat. Transfer, 2016, 186, 221ꢀ242.
Tishchenko, O.; Ilieva, S.; Truhlar, D. G., Energetics of Reaction Pathways for
Reactions of Ethenol with the Hydroxyl Radical: The Importance of Internal
Hydrogen Bonding at the Transition State, J. Chem. Phys. 2010, 133, 021102ꢀ
021104.
(41)
(42)
(43)
(44)
Hawkins, M.; Andrews, L., Reactions of Atomic Oxygen with Ethene in Solid Argon.
The Infrared Spectrum of Vinyl Alcohol, J. Am. Chem. Soc. 1983, 105, 2523ꢀ2530.
Rodler, M.; Blom, C. E.; Bauder, A., Infrared Spectrum and General Valence Force
Field of synꢀVinyl Alcohol, J. Am. Chem. Soc. 1984, 106, 4029ꢀ4035.
Rodler, M., Microwave Spectrum, Dipole Moment, and Structure of antiꢀVinyl
Alcohol, J. Mol. Spectrosc. 1985, 114, 23ꢀ30.
Tadić, J. M.; Moortgat, G. K.; Bera, P. P.; Loewenstein, M.; Yates, E. L.; Lee, T. J.,
Photochemistry and Photophysics of nꢀButanal, 3ꢀMethylbutanal, and 3,3ꢀ
Dimethylbutanal: Experimental and Theoretical Study, J. Phys. Chem. A, 2012, 116,
5830ꢀ5839.
(45)
(46)
(47)
Leszczynski, J. Computational Chemistry: Reviews of Current Trends Volume 3;
World Scientific: Singapore, 1998.
da Silva, G., Carboxylic acid Catalyzed KetoꢀEnol Tautomerizations in the Gas
Phase, Angew. Chem. Int. Ed. 2010, 49, 7523ꢀ7525.
Karton, A., Inorganic AcidꢀCatalyzed Tautomerization of Vinyl Alcohol to
Acetaldehyde, Chem. Phys. Lett. 2014, 592, 330ꢀ333.
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The Journal of Physical Chemistry
FTIR spectroscopy of propenols
Shaw et al
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2
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TABLES:
4
5
6
Table 1. Observed and predicted (anharmonic, B3LYP/6ꢀ31+G(d,p)) vibrational frequencies
for vinyl alcohol.
7
8
9
synꢀVA
Calc.
antiꢀVA
Calc.
Literature
expt gas
Expt
Calc.
freq
Calc.
freq
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Ar matrix
intensity
intensity
(Debye2Å2
amu−1)
/ cm−1
/ cm−1
(cm−1) (Debye2Å2
amu−1)
(cm−1)
a,b
c
ν1
3634 m
3122 w
ꢀ
3620 s
ꢀ
3609
3112
3050
2999
1669
1405
1324
1284
1103
946
0.69
0.25
0.15
0.17
1.88
0.23
0.21
0.02
4.86
0.32
0.30
0.85
1.87
0.04
2.52
3681
1.65
0.19
0.00
0.67
2.59
0.00
0.01
5.23
0.30
1.30
0.09
0.68
1.61
0.46
2.18
ν2
3129
3041
3023
1677
1400
1323
1234
1116
941
ν3
ꢀ
ν4
ꢀ
ꢀ
ν5
1663d m
1412 w
1300 w
1260 m
1118d vs
948 w
ꢀ
1662d vs
1409 w
1326 w
1300 m
1121d s
943 w
ꢀ
ν6
ν7
ν8
ν9
ν10
ν11
ν12
ν13
ν14
ν15
486
480
960m
817 s
699 w
ꢀ
971 m
813 s
698 w
413 s
962
940
816
844
693
699
379
351
(a) Experimental intensities described by w=weak, m=medium, s=strong, vs=very strong.
(b) Gas phase frequencies from Koga et al24 and Joo et al23(c) Matrix frequencies from
Hawkins et al41 and Rodler et al.42 (d) Experimental frequencies corrected for Fermi
resonance perturbations, as described in Koga et al24 and Rodler et al.42
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The Journal of Physical Chemistry
Page 20 of 40
FTIR spectroscopy of propenols
Shaw et al
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Table 2. synꢀtransꢀ1ꢀPropenol observed and calculated B3LYP/6ꢀ31+G(d,p) anharmonic
frequencies.
5
6
7
8
Calc. freq Obs. freq
Band Sym.
type(a) (Cs)
Mode description(b)
/ cm−1
3615
3078
2952
2949
2898
1692
1464
1395
1351
1307
/ cm−1
3639
3080
2955
ꢀ
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
A/B
(B)
A'
A'
OꢀH stretch
9
alkene CꢀH stretch
alkene CꢀH stretch
IP CH3 CꢀH stretch
OOP CH3 all CꢀH sym stretch
C=C stretch
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(A/B) A'
(B)
(A)
A/B
(B)
(B)
A
A'
A'
A'
A'
A'
A'
A'
2888
1684
1467
1400
1362
1311
CH3 HꢀCꢀH bend (scissors)
CH3 def. (umbrella)
IP alkene CꢀH bend
IP both alkene CꢀH bend
A
IP CꢀOꢀH bend, alkene CꢀH
bend
ν11
1232
1105
1083
919
ꢀ
(A)
A
A'
A'
A'
ν12
ν13
ν14
ν15
ν16
ν17
ν18
ν19
ν20
ν21
ν22
ν23
ν24
1108
CꢀO stretch, CꢀOꢀH bend
IP CꢀOꢀH bend, CH3 CꢀH bend
CH3 wag
ꢀ
(A)
915
(A/B) A'
536
ꢀ
(B)
(A)
(C)
(C)
(C)
C
A'
IP CꢀCꢀCꢀO bend
IP CꢀCꢀC bend
297
ꢀ
A'
2919
1443
1039
927
2932
A''
A''
A''
A''
A''
A''
A''
A''
OOP CH3 CꢀH antisym. stretch
CH3 CꢀH OOP bend
CH3 rock
ꢀ
ꢀ
932
OOP alkene CꢀH bend
OOP alkene CꢀH bend
OOP OꢀH bend
785
ꢀ
ꢀ
ꢀ
ꢀ
(C)
(C)
(C)
(C)
355
247
OOP OꢀH bend
173
CH3 torsion
(a) No parentheses: calculated and observed. In parentheses: calculated only, as the
experimental peak is too weak or obscured.
(b) IP = in plane, OOP = out of plane.
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Page 21 of 40
The Journal of Physical Chemistry
FTIR spectroscopy of propenols
Shaw et al
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Table 3. 2ꢀPropenol observed, literature and calculated anharmonic frequencies.
4
5
6
7
8
Calc.
freq /
cm−1
Obs.
freq /
cm−1
Matrix
Band
Sym
type
(Cs)
Mode description
freq (a)
cm−1
/
type(b)
ν1
ν2
ν3
ν4
3638*
3106*
3000*
2988*
3642
3110
3014
2977
3622 w
2992 w
2979 w
2950 w
A/B
(A/B)
(A)
A'
A'
A'
A'
OꢀH stretch
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CH2 asym stretch
CH2 sym stretch
IP CH3 CꢀH stretch
(B)
CH3 all CꢀH sym
stretch
ν5
ν6
ν7
ν8
ν9
2899*
1676
2902
1678
2835 w
1673 w
(A)
A'
A'
A/B
C=C stretch
OOP CH3 bend
(scissors)
1460
1424
1462
ꢀ
1467 m
ꢀ
(B)
A'
A'
(A/B)
CH2 bend (scissors)
CH3 (umbrella), CH2
scissor
1386
1386
1379 m
(B)
A'
CꢀOꢀH bend, CꢀO
stretch
ν10
1323
1171
1006
967
1317
1331 w
1181 s
1002 m
963 w
849 w
478 w
ꢀ
(B)
A/B
(B)
A'
A'
A'
A'
A'
A'
A'
ν11
ν12
ν13
ν14
ν15
ν16
1185
CꢀOꢀH bend
1001
IP CH3 rock
ꢀ
ꢀ
ꢀ
ꢀ
(B)
IP CH2 CꢀH rock
IP CꢀC stretch
IP CH2ꢀCꢀOH bend
IP CꢀCꢀC bend
849
(B)
491
(A/B)
(B)
388
OOP CH3 CꢀH asym
stretch
ν17
2938*
1433
1048
794
2949
2921 w
1439 w
1050 w
822 w
697 w
494 w
395 s
ꢀ
(C)
C
A''
A''
A''
A''
A''
A''
A''
A''
ν18
ν19
ν20
ν21
ν22
ν23
ν24
1435
OOP CH3 bend
OOP CꢀCꢀC bend
OOP CH2 wag
CH2 torsion
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
(C)
(C)
(C)
(C)
(C)
(C)
695
495
OOP CꢀCꢀC bend
OOP OꢀH bend
CH3 torsion
420
218
* ν1ꢀ ν5, ν17 from B97ꢀ1.
(a) Literature matrix values from Zhang et al.17
(b) No parentheses: calculated and observed. In parentheses: calculated only, as the
experiment is too weak or obscured.
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Page 22 of 40
FTIR spectroscopy of propenols
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FIGURES:
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56
57
58
59
60
Figure 1. Experimental and simulated VA spectra. The experimental band marked with an
asterisk is discussed in the text. The colored traces show calculated B3LYP/6ꢀ31+G(d,p)
anharmonic spectra of syn:anti conformers in an 85:15 ratio. OOP=out of plane, IP=in plane,
FR=Fermi resonance, ν = stretching mode, δ = bending mode. The perturbing species for the
FR are indicated as assigned in previous work.23
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Page 23 of 40
The Journal of Physical Chemistry
FTIR spectroscopy of propenols
Shaw et al
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Figure 2. COH bending modes for syn and anti VA.
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Page 24 of 40
FTIR spectroscopy of propenols
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Figure 3: FTIR spectrum of all major photoproducts from 295 nm irradiation of 2ꢀ
methylbutanal. Some less abundant species are not labelled.
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Page 25 of 40
The Journal of Physical Chemistry
FTIR spectroscopy of propenols
Shaw et al
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Figure 4. Simulated and experimental spectra of 1ꢀpropenol. Rotational contours were
simulated in PGOPHER, using vibrational origins, rotational constants, and dipole derivatives
from anharmonic B3LYP/6ꢀ31+G(d,p) calculations. The relative fraction of each isomer is
discussed in the text.
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The Journal of Physical Chemistry
Page 26 of 40
FTIR spectroscopy of propenols
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59
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Figure 5. Highest intensity bands of 1ꢀpropenol at 0.25 cm−1 resolution. (a) COH bend at 1108
cm−1 (b) C=C stretch at 1684 cm−1 (c) OH stretching bands at 3639 cm−1 and 3693 cm−1.
Major contributing isomers/conformers are shown.
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Page 27 of 40
The Journal of Physical Chemistry
FTIR spectroscopy of propenols
Shaw et al
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Figure 6. NTꢀII reaction on the T1 surface. Energies are reported in kJ mol−1 and are relative
to the S0 minimum, calculated at CCSD(T)/aug’ꢀccꢀpVTZ//B3LYP/6ꢀ31+G(d,p).
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FTIR spectroscopy of propenols
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Figure 7. Simulated and experimental spectra of 2ꢀpropenol. The simulated spectra contain
vibrational origins from electronic structure calculations, and rotational contours from
PGOPHER,37 using calculated rotational constants and dipole derivatives.
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Page 29 of 40
The Journal of Physical Chemistry
FTIR spectroscopy of propenols
Shaw et al
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Figure 8. Fitted and predicted number densities of ethene and enol in the photolysis of (a) 2ꢀ
methylbutanal, and (b) 2ꢀpentanone, using Equations (K2) and (K4).
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