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Propiolic Acid in Solid Nitrogen: NIR- and UV-Induced Cis#Trans Isomerization and Matrix-Site Dependent Trans#Cis Tunneling Susy Lopes, Timur Nikitin, and Rui Fausto J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b11319 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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The Journal of Physical Chemistry

Propiolic Acid in Solid Nitrogen: NIR- and UV-Induced CisTrans Isomerization and Matrix-Site Dependent TransCis Tunneling

Susy Lopes,1, Timur Nikitin1 and Rui Fausto1 1CQC,

Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal

ABSTRACT: Propiolic acid (HCCCOOH, PA) was studied experimentally by infrared spectroscopy in a nitrogen matrix and by ab initio calculations. The vibrational spectra of the cis and trans monomers (O=C–O–H dihedral equal to 0 and 180º, respectively) were measured and assigned. The trans-PA monomer was produced by selective vibrational excitation of the cis-PA monomer molecules trapped in different matrix sites. Broadband in situ UV irradiation (  235 nm) of matrix-isolated PA yielded as product the higher energy trans conformer, with no other photoproducts being detected. Two cis-cis dimers were also identified in the matrices and characterized structurally and vibrationally. trans-PA was found to decay back to cis-PA in the dark, by tunneling, and the different lifetimes of the higher-energy PA conformer resulting from pumping different matrix sites and different experimental conditions (using a filter blocking the higher-energy IR radiation of the spectrometer source, and without using such a filter) were discussed.



Corresponding author e-mail: [email protected]

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1. INTRODUCTION Propiolic acid (HCCCOOH, PA) is the simplest carboxylic acid having a triple bond in its structure. PA is a known intermediate in organic synthesis. It has been used in the synthesis of diarylalkynes, conjugated 1,3-diynes, 1,2-diketones, diarylalkynones, thioketones, substituted -pyrones,1–7 coumarins and its derivatives,8–10 which are used as fragrances, pharmaceuticals and agrochemicals. PA is also believed to be present in interstellar space, as several fragments and molecules related to PA have been observed.11–15 Some aspects of the structure and reactivity of the compound were investigated before, both experimentally and theoretically, which are summarized below.16,17,26–34,18–25 PA has two planar conformers (cis and trans) that differ by the orientation of the hydroxyl group and can interconvert to each other through internal rotation around the C—O bond (in the conformers, the conformationally determining O=C–O–H dihedral is equal to 0 and 180º, respectively; Figure 1). Furet et al.32 used quantum chemical calculations performed at several levels of theory with different basis sets to obtain the equilibrium geometry of the most stable conformer of PA (cis form) (and of other molecules of a series of acetylene derivatives). In another publication the thermodynamic properties of saturated and unsaturated carboxylic acids including PA were determined.33 The trans conformer has been shown to be higher in energy than the cis form by 1216 kJ mol—1 (10001364 cm—1),21,25,34 the calculated cis-to-trans barrier staying in the 3148 kJ mol—1 range (25853987 cm—1),21,25,34 depending on the level of theory and basis sets used. In consonance with the theoretical predictions, the microwave spectrum of PA allowed identification of the cis-PA conformer in the gas phase.30,31

5

1

2

3

4

7 6

trans-propiolic acid

cis-propiolic acid


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Figure 1. DFT(B3LYP)/6-311++G(d,p) optimized structures of cis- and trans-propiolic acid including numbering of atoms adopted in this work. Colors: Cgrey, Hwhite, Ored.

The vibrational spectra of PA in the liquid phase and in noble gas matrices have also been reported.25,28,29 In the study by Ndip,29 only the ground state conformer of PA was investigated in argon, carbon monoxide and nitrogen matrices, whereas, Isoniemi et al.25 studied the unimolecular 193-nm photolysis of PA in argon, krypton and xenon matrices and characterized four distinct photolysis products, including the higher-energy trans conformer, which was observed for the first time. The fragmentation reactions of PA have been investigated in several conditions by different groups.16,17,26,27,18–25 A systematic study performed in a series of carboxylic acids bearing single, double and triple carbon-carbon bonds directly connected to the carboxylic moiety, allowed to conclude that the acids which possess a CC bond in their backbone, such as propiolic acid, decarboxylate the fastest at hydrothermal conditions.20 In the ground electronic state, two possible unimolecular decomposition channels have been described: decarboxylation and decarbonylation, which lead to acetylene and carbon dioxide, and ethynol and carbon monoxide, respectively. These processes have been studied theoretically by Ndip et al., using different levels of theory and basis sets.27 On the other hand, the photodissociation dynamics of PA after excitations at 193 and 212 nm was found to lead to CO bond cleavage, generating as primary product the OH radical.21,23,24 Carboxylic acids in general form strong hydrogen bonds with each other or other molecules, such as water or carbon dioxide. Different conformers of a given molecule may show different reactivity (e.g., photochemical) or exhibit distinct aggregation patterns (or, in a more general way, show special tendencies to interact with other species). Propiolic acid has two hydrogendonor sites (a strong hydroxyl hydrogen and one weak acetylenic hydrogen) and two acceptor sites (hydroxyl and carbonyl oxygen atoms) in the molecule. In addition, in propiolic acid, participation of the triple bond as acceptor moiety in Hbond interactions can also be expected. These complexes can be characterized by different types of hydrogen bonds, forming different size-membered rings, including OH…O=, OH…OH, CH…O, and OH…(CC) bonds. The infrared spectra of PA (neutral and deuterated) centrosymmetric dimers in the gas phase were investigated by Bournal and Maréchal, in 1971.35 More recently, Xue and Suhm36 published a study on the cyclic dimer of propiolic acid by Raman jet spectroscopy recorded in the low frequency hydrogen bond mode region.



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In the present work, we studied PA isolated in N2 matrix. After deposition, the matrix contains mainly the lower energetic cis conformer (trapped in two different major matrix sites), but also small amounts of cis-cis dimers. The higher energy trans conformer has subsequently been prepared in situ by selective vibrational excitation of the lower-energy cis conformer, using tunable narrowband IR light. Selective production of the trans conformer by pumping the cis form in each one of the two major sites was achieved. Annealing the matrix to higher temperatures led to an increase in the amount of cis-cis dimers, whose major components could be successfully characterized both structurally and vibrationally. trans-PA was found to decay back to cis-PA in the dark, by tunneling, and the different lifetimes of the higher-energy PA conformer resulting from pumping different matrix sites and different experimental conditions (using a filter blocking the higher-energy IR radiation of the spectrometer source, and without using such a filter) were discussed. The interpretation of the experimental data is supported by quantum chemical calculations undertaken at both the DFT(B3LYP) and MP2 levels of theory. 2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1 Experimental. Propiolic acid (PA) (97% purity) was purchased from Sigma-Aldrich and used without further purification. The sample was placed in a glass tube connected to the vacuum chamber of the cryostat through a needle valve (NUPRO SS-4BMRG). Matrices were prepared by co-deposition of the vapor of the compound and large excess of N2 (N60, supplied by Air Liquide) onto a CsI substrate assembled at the cold-tip (12 K) of the cryostat (APD Cryogenics closed-cycle helium refrigeration system, with a DE-202A expander). Small bands due to both decarboxylation and decarbonylation products of PA were observed, resulting from minor decomposition of PA upon sublimation. These include the bands at 2139.5 cm1 (CO),37,38 2349.0 and 662.5 cm-1 (CO2),39,40 2283.0, 742.0 and 747.5 cm-1 (acetylene),41 while the most intense band of ketene (C=C=O as) shall contribute to the total intensity on the 21002164 cm-1.42 No bands due to propynol (another possible product of decarbonylation) and other bands of the remaining decomposition products were observed, with all probability due to their low intensity (propynol might even not be present in the matrix, since its transformation to ketene appears highly probable). However, from the intensity of the bands of the decomposition reactions, we can easily conclude that they are present in trace amounts and are then irrelevant in the context of the study. The mid-IR spectra, in the 4000400 cm−1 range, were obtained using a Thermo Nicolet 6700 Fourier transform infrared spectrometer, equipped with a deuterated triglycine sulphate 4 ACS Paragon Plus Environment

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(DTGS) detector and a Ge/KBr beam splitter, with 0.5 cm−1 spectral resolution. The near-IR spectrum was recorded in the same spectrometer but using a cadmium telluride (MCT/B) detector (cooled by liquid N2) and a CaF2 beam splitter (with 1 cm1 spectral resolution). To avoid interference from atmospheric H2O and CO2, a stream of dry air was continuously purging the optical path of the spectrometer. Matrices were irradiated using two sources through the outer KBr window of the cryostat: broadband irradiation was carried out with UV light (λ > 235 nm) provided by a 500 W Hg(Xe) lamp (Newport, Oriel Instruments), with output power set to 200 W; narrowband (FWHM 0.2 cm−1) irradiation was done using tunable light provided by the idler beam of a Spectra Physics MOPO-SL optical parametric oscillator pumped by a pulsed (pulse energy 10 mJ, duration 10 ns, repetition rate 10 Hz) Quanta Ray Pro-Series Nd:YAG laser. 2.2 Computational Methodology. The quantum chemical calculations at the DFT(B3LYP)43–45 and MP246 levels of theory, with the 6-311++G(d,p), 6-311++G(2d,2p), aug-cc-pVDZ and aug-cc-pVTZ basis sets,47–54 were performed using the Gaussian 09 program (Rev. A.02 and Rev. D.01).55,56 Relaxed one-dimensional potential energy scans were calculated to locate the minimum energy conformations of PA. The optimized structures of the conformers are confirmed to be true energy minima by inspection of the corresponding Hessian matrix. The geometries were optimized using the TIGHT convergence criteria, and the vibrational frequencies and infrared intensities were also calculated at the DFT(B3LYP) and MP2 levels of theory.43–46 The basis set superposition error (BSSE) was corrected by the counterpoise method proposed by Boys and Bernardi.57 The zero-point vibrational energy (ZPVE) was also taken into account. Vibrational calculations were performed both using harmonic and anharmonic potentials, and the zero-point vibrational energy (ZPVE) corrections were also accounted for. Anharmonic IR spectra were computed using a fully automated second order vibrational perturbative approach of Barone and co-workers,58,59 allowing for the evaluation of anharmonic infrared intensities up to 2 quanta, including overtones and combination bands.59–61 Transition state structures were located using the synchronous transitguided quasi-Newton (STQN) method.62 Calculated vibrational frequencies and IR intensities were used to assist the analysis of the experimental spectra. A normal coordinate analysis was undertaken in the internal coordinates space, as described by Schachtschneider and Mortimer, using the optimized geometries and anharmonic force constants resulting from the



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DFT(B3LYP)/6-311++G(d,p) calculations.63 The internal coordinates used in this analysis were defined following the recommendations of Pulay et al.64 For a graphical comparison of theoretical spectra with experiment, the calculated anharmonic frequencies, together with the calculated infrared intensities, were used to convolute each peak with a Lorentzian function having a full width at half-maximum (FWHM) of 2 cm1, so that the integral band intensities correspond to the calculated infrared absolute intensity.65 3. RESULTS AND DISCUSSION 3.1. Computational results. As stated above, PA is characterized by one conformationally relevant internal rotation axis defined by the O5=C4O6H7 torsional angle, i.e., internal rotation within the carboxylic group (CO bond). In agreement with the previously available information, the calculations carried out at the DFT(B3LYP) and MP2 levels of theory (with the basis sets indicated above) predict the existence of two planar conformers of the compound, both belonging to the Cs symmetry point group (Figure 1). They correspond to cis and trans orientations around the O=C−O−H dihedral angle. The potential energy profiles for internal rotation about the CO bond are presented as Supporting Information in Figures S1 and S2 [for DFT(B3LYP) and MP2 levels, respectively)]. Table 1 lists the relative energies of the two conformers, with zero-point corrections, calculated at different levels of theory. Table 1. DFT(B3LYP) and MP2 calculated relative energies (E/ kJ mol1) including zeropoint corrections for the two conformers of PA. Method and basis set Conformers cis trans DFT(B3LYP)/ 6-311++g(d,p) 0.0 13.7 6-311++g(2d,2p) 0.0 11.9 aug-cc-pVDZ 0.0 11.5 aug-cc-pVTZ 0.0 11.3 MP2/ 6-311++g(d,p) 6-311++g(2d,2p) aug-cc-pVDZ aug-cc-pVTZ

0.0 0.0 0.0 0.0

14.9 12.2 11.7 11.9


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From these results, it is found that the relative energy difference between the two forms predicted by the DFT(B3LYP) and MP2 methods is similar, staying in the 1112 kJ mol1 range [(except for the 6-311++G(d,p) basis set where the energy difference is predicted to be slightly higher by both methods: ~14 kJ mol1 (DFT(B3LYP); ~15 kJ mol1 (MP2)]. The calculated transcis energy barrier is around ~30 kJ mol1 regardless the level of theory and the basis set used (4346 kJ mol1, in the reverse direction). Considering both the relative conformational energies between the two conformers and the relatively high energy barriers obtained theoretically, it is expected that only the most stable (cis) conformer is present in the matrices after deposition.66–69 The geometries of the two PA conformers were fully optimized at the different levels of theory used and are presented in Tables S1 and S2. In general, the bond lengths are predicted slightly shorter at the DFT(B3LYP) than at the MP2 level, the same trend being also observed for the CC=O, CCO and O=CO angles, which are predicted smaller by the DFT(B3LYP) calculations. On the other hand, the COH angle are predicted to be smaller at the MP2 than at the DFT(B3LYP) level. For the cis conformer, the calculated geometries are in good general agreement with that obtained by microwave spectroscopy (HC: 1.055 Å, CC: 1.209 Å, CC: 1.445 Å, C=O: 1.202 Å, CO: 1.343 Å, OH: 0.972 Å; CC=O: 124.8º, CCO: 110.7º O=CO: 124.5º, COH: 106.9º),30 with the average error in the structural parameters calculated at the different levels of theory staying in the range 0.44-0.97%. The smallest error was obtained at the DFT(B3LYP)/aug-cc-pVTZ level (0.44%) and the largest one at the MP2/aug-cc-pVDZ (0.97%), with the DFT(B3LYP) calculated geometries showing systematically smaller deviations from the experimental one than those obtained at the MP2 level with the same basis set (see Tables S1 and S2).30 The comparison of the calculated geometries of the two conformers can be used to understand their relative stabilities. The C=O bond length and the COH angle are the structural parameters that differ the most between the two conformers, the C=O bond being longer for cis-PA (e.g., 1.205 vs. 1.198 Å at DFT(B3LYP)/6-311++G(d,p) level) and the COH angle being smaller for this conformer (106.0 vs. 108.5º). The longer C=O bond length in cis-PA demonstrates the existence of a more important -electron delocalization within the carboxylic group in this conformer, which is also confirmed by the slightly shorter CO bond length in cis-PA (1.351 Å) compared to trans-PA (1.356 Å). This electron delocalization, together with the anti-parallel alignment of the bond dipoles associated with the C=O and O–H bonds in cisPA (in opposition to the nearly parallel alignment of these dipoles in trans-PA) are the main 7 ACS Paragon Plus Environment


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factor justifying the greater stability of cis-PA compared to trans-PA. In this regard, the observed trends follow those previously found for other simple carboxylic acids.70–72 It is interesting to note that the COH angle has been often found to be larger in the cis conformers of other carboxylic acids than in the corresponding trans forms.70–72 The opposite result obtained for PA indicates that in this acid the repulsive interactions between the hydroxyl hydrogen atom and the propargylic moiety in trans-PA are rather strong, as compared for instance with those observed in the trans conformers of acetic (CH3COOH) or propionic (CH3CH2COOH) acids.73,74 3.2 Infrared spectra of matrix-isolated PA. The infrared spectrum of PA monomers deposited in solid nitrogen at 12 K is presented in Figure 2. The good correspondence between the DFT(B3LYP)/6-311++G(d,p) calculated anharmonic spectrum of the most stable (cis) conformer and the experimental spectrum is striking, and, in agreement with the predicted relative energies for the two conformers (see Table 1), doubtlessly demonstrates that cis-PA is the only monomeric species present in the matrices after deposition. The assignment of the fundamental bands of cis-PA is presented in Table 2. The definition of the internal symmetry coordinates adopted in the vibrational analysis for the two conformers is provided in Table S3 (Supporting Information), and the calculated wavenumbers, infrared intensities and potential energy distributions resulting from that analysis are presented in Tables S4 and S5. Table S6 shows the results of harmonic and anharmonic vibrational calculations for the fundamental transitions for cis- and trans-PA forms obtained at the DFT(B3LYP)/6-311++G(d,p) level.


0.3

0.2

0.2

0.1

0.1

0.0

0.0 120

Relative Intensity

0.3

30

0

0

Relative Intensity

1 2 3 4 5 6 7 8 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


Absorbance

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30

120

(a)

PA : N2, 12 K

(b)

20 60 10

(c)

20 60 10 0 3600 3500 3400

0 1800 1600 1400 1200 1000 800 600 400

Wavenumber / cm1

Figure 2. Experimental FTIR spectrum of PA isolated in a N2 matrix at 12 K after deposition (a); theoretical infrared spectra with the anharmonic frequencies of cis-PA (b) and trans-PA (c) conformers calculated at the DFT(B3LYP)/6-311++G(d,p) level.

In the experimental spectra, most of the strongest bands appear as doublets (in some cases as multiplets), indicated that the molecules occupy two main sites in the studied matrices. As it could be expected, these bands are associated with vibrations predicted to have high intensity, which are essentially localized modes belonging to groups which can be expected to be more sensitive to matrix effects. Bands due to the OH stretching mode are found at 3534.5 and 3523.5/3522.5 cm1, the C=O stretching vibration appears at 1753.5 and 1750.0/1748.0 cm1, and the CO stretching mode at 1165.5/1164.0/1159.5 and 1152.5 cm1. The torsional mode around the CO bond ((CO)), predicted to appear at 582.0 cm1, was detected at 608.0 and 606.0 cm1. These bands have counterparts observed in argon matrices at 3550.8, 1753.9, 1149.5 and 571.3 cm1 (see Table 2).25 Table 2. Experimental (matrix-isolation) and DFT(B3LYP)/6-311++G(d,p) calculated infrared data of cis-PA isolated in N2 matrices, and vibrational assignments based on the results of normal coordinate analysis.a Approximate Experimental Calculated Description N2 (this work)

N221

Ar34

anharm

IIR 9

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(OH)

Site 1b 3534.5

(CH)

3404.5

(CC)

2147.0 2142.5

(C=O) (COH)

1750.0 1748.0 1326.5

(CO)

1152.5

(CC) (C=O) (CCH) (CCH)

822.0 760.0 760.0

(CC=O) (CO) (OCC) (CCC) (CCC)

Site 2c 3523.5 3522.5 3314.0 3309.0 2153.0 2129.0 2127.5 2118.5 1753.5 1329.5

665.5 586.7 606.0 534.5

1165.5 1164.0 1159.5 820.0 760.0 760.0 668.0 665.5 587.0 608.0 536.0

n.i. n.i.

n.i. n.i.

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3531 3520 3310 3307

3550.8

3550.7

85.6

3313.1

3348.6

50.3

2190.5

62.6

2138 2136 2127 2117 1752.7 1748.5 1328.4 1326.8 1163.0 1153.0

2137.3

1753.9

1757.2

366.7

1302.2

1289.1

71.6

1149.5

1126.1

371.9

820.5 758.5 708.0 666.0 665.0 586.3 607.0 533.8 n.i. n.i.

818.1 755.0 692.8 650.3

814.8 764.5 763.1 674.2

17.8 39.9 40.5 41.2

585.8 571.3 528.4

593.8 581.9 527.6 269.2 191.1

6.0 85.8 22.7 13.9 6.2

n.i. n.i.

Anharmonic wavenumbers (cm1), calculated intensities (km mol1); ν = stretching;  = in-plane bending; γ = out-of-plane bending; τ = torsion; n.obs. = not observed; n.i. = not investigated. See Table S3 for definition of symmetry coordinates and Table S4 for potential energy distributions. b Bands consumed after excitation at 6898.6 cm1. c Bands consumed after excitation at 6874.9 cm1. a

3.3 Identification of cis-PA Dimers, D1, D2 and D3. Alongside with bands assigned to cis-PA monomer, several additional bands are visible in the IR spectrum of the as-deposited matrix, which can be ascribed to cis-PA dimers. The comparison of the experimental and calculated theoretical spectra indicates that these dominant dimers correspond to the most stable cis-PA dimeric structures (dimers D1, D2 and D3; see Figure 3 and Tables 3, 4 and 5). Dimer D1 (the classic centrosymmetric dimeric form) belongs to the C2h symmetry point group, with only 18 fundamental vibrations active in infrared (and has a dimerization energy of 58.3 kJ mol1 with BSSE and zero-point vibrational energy corrections). For dimer D1, all the predicted bands with higher intensity were experimentally observed, except the CH and CC modes. The CH stretching vibrations are usually considerably less intense under matrix isolation conditions than predicted theoretically, though both modes of this dimer are overlapped with the bands due to the cis-PA monomer.


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Table 3. Experimental (matrix-isolation) and MP2/6-311++G(2d,2p) calculated infrared data of the centrosymmetric dimer of PA (Dimer D1) isolated in N2 matrices and vibrational assignments based on the results of normal coordinate analysis.a Approximate Experimental Calculated Description (CH) (OH)

(CC) (C=O) (COH) (CO) (CO) (CC) (C=O) (CCH) (CCH) (CC=O) (OCC) (CCC) (CCC) ring 1 ring 2 ring 3

N2 (this work) n.obs. 3062.0 3020.0 2991.0 2963.5 2875.0 2864.0 n.obs. 1707.0 1702.0 1411.5 1410.5 1312.5 1048.5 n.obs. n.obs. n.obs. n.obs. n.obs. n.obs. n.i. n.i. n.i. n.i. n.i.

b 465.0 507.0 536.0 564.0 652.0 663.0 43.5 48.1 +86.5 +82.5 +152.0 +441.5

harm

c

3482.2 3298.9

IIR 102.3 3856.4

+6.8 473.6

2144.9 1737.8

179.1 831.4

+11.9 25.9

1470.7

137.7

+105.8

1313.2 1024.5 871.5 773.7 684.2 670.9 607.9 571.3 260.4 250.6 91.8 74.4 36.7

736.8 171.4 51.4 25.3 62.7 69.2 47.0 6.6 70.5 5.8 6.8 0.2 2.5

+143.2 +441.2 +52.6 +6.0 +4.4 +8.8 +24.6 +46.0 +91.6 +4.6

Wavenumbers (cm1), calculated intensities (km mol1); ν = stretching;  = in-plane bending; γ = out-of-plane bending; τ = torsion; n.obs. = not observed; n.i. = not investigated. b Average shift in relation to the observed bands of the PA monomer (cis-PA). c Average shift in relation to the calculated PA monomer (cis-PA; see Table S7).

a

The bands observed at 3062.0/3020.0/2991.0/2963.5/2875.0/2864.0 (major peaks of a broad split feature, see Figure S3), 1707.0/1702.0, 1411.5/1410.5, 1312.5 and 1048.5 cm1 correspond to the OH, C=O, COH, CO and CO modes of dimer D1 and show frequency shifts regarding their position in cis-PA in very good agreement with the theoretically predicted shifts (experimental shifts: 465.0/507.0/536.0/564.0/652.0/663.0, 43.5/48.1, +86.5/+82.5, +152.0 and +441.5 cm1, respectively; calculated shifts: 473.6, 25.9, +105.8, +143.2 and +441.2 cm1). On the other hand, dimers D2 and D3 belong to the Cs symmetry point group, with 36 fundamental vibrations active in infrared (and have dimerization energies of 25.3 and 24.8 kJ mol1 with BSSE and zero-point vibrational energy corrections). Dimer 2 is characterized by one strong OH…O=C hydrogen bond and a weaker OH…OH hydrogen 11 ACS Paragon Plus Environment


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bond showing two strong red shifts (113.9 and 208.1 cm1), however, only one band is observed at 3372.0 cm1 (experimental shift: 157.0 cm1); the less intense band being overlapped with the OH vibration of the cis-PA monomer. The two C=O stretching modes are observed at 1758.5 and 1731.0/1730.0 cm1 (experimental shifts: +8.0 and 19.5/20.5 cm1) and predicted shifts at +9.4 and 23.4 cm1. The observed bands at 1241.0 and 1128.0/1126.0/1122.0 cm1 are assigned to the CO stretching modes (experimental shifts: +81.0 and 32.5/34.5/38.5 cm1; predicted shifts: +70.3 and 2.5 cm1). For the C–O torsional mode, the calculations predict a large blue shift of +232.5 cm1, observed at 778.0 cm1 with an experimental shift of +171.0 cm1. On the other hand, dimer D3 exhibits two distinctive Table 4. Experimental (matrix-isolation) and MP2/6-311++G(2d,2p) calculated infrared data of dimer D2 of PA isolated in N2 matrices and vibrational assignments based on the results of normal coordinate analysis.a Approximate Experimental Calculated Description (OH) (1) (OH) (2) (CH) (1) (CH) (2) (CC) (1) (CC) (2) (C=O) (2) (C=O) (1) (COH) (1) (COH) (2) (CO) (1) (CO) (2) (CC) (1) (CO) (1) (CC) (2) (C=O) (C=O) (CO) (2) CCH (2) CCH (1) CCH (1) CCH (2) (CC=O) (1) (CC=O) (2) (OCC) (1)

N2 (this work) 3372.0 n.obs.c n.obs. n.obs. n.obs. n.obs. 1758.5 1731.0 1730.0 n.obs. n.obs. 1241.0 1128.0 1126.0 1122.0 n.obs. 778.0 n.obs. n.obs. n.obs. n.obs. n.obs. n.obs. n.obs. n.obs. n.obs. n.obs. n.obs.

b 157.0

+8.0 19.5 20.5 +81.0 32.5 34.5 38.5 +171.0

harm

c

3658.6 3564.4 3482.4 3481.3 2144.1 2135.7 1772.8 1740.3

IIR 834.9 370.0 62.5 69.5 75.8 63.3 174.2 521.9

113.9 208.1 +7.0 +5.9 +11.1 +2.7 +9.4 23.4

1421.2 1350.2 1240.2 1167.5

39.4 7.8 678.8 768.5

+56.4 14.7 +70.3 2.5

847.3 841.2 826.3 778.8 761.1 718.7 688.2 684.0 671.1 669.1 599.1 588.7 561.1

18.9 202.7 44.2 0.3 5.4 6.9 31.7 32.5 35.6 32.5 14.2 0.4 34.1

+28.4 +232.5 +7.3 +11.1 6.6 +110.1 +8.4 +4.2 +8.9 +6.9 +15.8 +5.4 +35.8 12


Page 13 of 31 1 2 3 4 5 6 7 8 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


(OCC) (2) (CCC) (2) (CCC) (1) (CCC) (1) (CCC) (2) Ring 1 Ring 2 a Ring 3 Ring 4 a Ring 5 Ring 6 a

n.obs. n.i. n.i. n.i. n.i. n.i. n.i. n.i. n.i. n.i. n.i.

546.8 268.1 250.5 237.2 182.9 121.6 106.3 86.2 51.0 37.0 33.1

10.2 1.3 5.5 21.0 18.9 3.7 1.1 0.4 0.3 1.2 0.1

+21.5 +22.1 +4.5 +68.4 +14.1

Wavenumbers (cm1), calculated intensities (km mol1); ν = stretching;  = in-plane bending; γ = out-of-plane bending; τ = torsion; n.obs. = not observed; n.i. = not investigated. (1) and (2) label the left and right molecules of the dimers. b Average shift in relation to the observed bands of the PA monomer (cis-PA). c Average shift in relation to the calculated PA monomer (cis-PA; see Table S7). c This mode is probably overlapping with the

a

(OH) vibration of cis-PA monomer.

Dimer D1

Dimer D3

Dimer D2

Figure 3. MP2/6-311++G(2d,2p) optimized structures of the three most stable dimers of propiolic acid: dimers D1, D2 and D3. Colors: Cgrey, Hwhite, Ored. Table 5. Experimental (matrix-isolation) and MP2/6-311++G(2d,2p) calculated infrared data of dimer D3 of PA isolated in N2 matrices and vibrational assignments based on the results of normal coordinate analysis.a Approximate Experimental Calculated Description (OH) (2) (OH) (1) (CH) (1) (CH) (2) (CC) (2) (CC) (1) (C=O) (1) (C=O) (2) (COH) (1) (COH) (2) (CO) (1) (CO) (2)

N2 (this work) 3550.0 3236.5 3224.0 n.obs. n.obs. n.obs. n.obs. 1740.0 1738.0 1723.0 1721.5 1389.5 1386.5 1355.0 1352.5 1183.5 1174.5

b

harm

+23.5 290.5 303.0

10.1 12.0 27.5 29.0 +61.5 +58.5 +27.0 +24.5 +23.0 +14.0

c

3771.5 3531.8

IIR 123.6 1067.1

1.0 240.7

3485.5 3481.6 2147.3 2133.6 1758.6

69.5 55.9 85.3 49.9 575.1

+10.1 +6.2 +14.3 +0.5 5.1

1736.3

167.0

27.4

1420.2

13.7

+55.4

1389.1

168.2

+24.2

1229.8 1198.9

460.5 359.8

+59.8 +29.0 13



(CO) (1) (CC) (2) (CC) (1) (C=O) (2) (C=O) (1) CCH (2) CCH (1) CCH (2) CCH (1) (CO) (2) (CC=O) (2) (CC=O) (1) (OCC) (1) (OCC) (2) (CCC) (1) (CCC) (2) (CCC) (1) (CCC) (2) Ring 1 Ring 2 a Ring 3 Ring 4 a Ring 5 Ring 6 a

1173.0 947.5 n.obs. n.obs. n.obs. n.obs. n.obs. n.obs. n.obs. n.obs. n.obs.c n.obs. n.obs. n.obs. n.obs. n.i. n.i. n.i. n.i. n.i. n.i. n.i. n.i. n.i. n.i.

+13.0 +340.5

Page 14 of 31

893.4 844.2 836.2 782.7 768.9 705.4 675.3 671.0 657.1 628.1 597.6 588.0 552.0 534.7 259.0 255.3 199.7 176.1 118.1 85.8 63.6 54.7 30.5 4.5

100.2 42.7 32.0 39.5 10.9 28.4 32.9 35.3 36.5 80.8 13.6 7.3 5.0 28.6 0.2 11.9 17.3 3.3 5.0 0.0 1.1 2.3 1.8 1.2

+310.1 +25.3 +17.3 +15.0 +1.2 +25.6 4.5 +8.8 5.0 +19.5 +14.3 +4.7 +26.7 +9.4 +13.0 +9.3 +30.9 +7.3

Wavenumbers (cm1), calculated intensities (km mol1); ν = stretching;  = in-plane bending; γ = out-of-plane bending; τ = torsion; n.obs. = not observed; n.i. = not investigated. (1) and (2) label the left and right molecules of the dimers. b Average shift in relation to the observed bands of the PA monomer (cis-PA). c Average shift in relation to the calculated PA monomer (cis-PA; see Table S7). c This mode is probably overlapping with the

a

(CCH) vibration of cis-PA monomer.

OH stretching modes, attributed to the free and intermolecularly bound OH groups. The first has a very small predicted red shift in relation to the frequency of the OH mode in monomeric cis-PA (1 cm1) observed at 3550.0 cm1 (shift: +23.5 cm1), while the second is predicted as an intense vibration deviated from the band of the monomer by 240.7 cm1 and was observed as a broad multiplet with major maxima at 3236.5/3224.0 cm1 (experimental shifts: 290.5/303.0 cm1, see Figure S3). The two C=O stretching modes are observed at 1740.0/1738.0 and 1723.0/1721.5 cm1, with experimental shifts of 10.1/12.0 and 27.5/29.0 cm1 (theory: 5.1 and 27.4 cm1). The COH(1) and COH(2) modes are reported as doublets at 1389.5/1386.5 and 1355.0/1352.5 cm1, with experimental shifts of +61.5/+58.5 and +27.0/+24.5 cm1 (calculated shifts of: +55.4 and +24.2 cm1). Finally, the CO stretching modes were found at 1183.5 and 1174.5/1173.0 cm1 (shifts: +23.0 and +14.0/+13.0 cm1) with predicted shifts of +59.8 and +29.0 cm1. The C–O torsional mode,


Page 15 of 31 1 2 3 4 5 6 7 8 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


predicted to appear at 893.4 cm1 (shift: +310.1 cm1) was detected at 947.5 cm1 (shift: +340.5 cm1). 3.4 Narrowband Selective NIR-Infrared Irradiation Experiments: Observation of trans-PA. As in previous studies on carboxylic acids,75–79 narrowband vibrational excitation of the lower energy conformer of PA (cis, dihedral angle O=COH equal to 0º) was used to produce the higher energy trans conformer, by exciting the first overtone corresponding to the fundamental absorption of the OH stretching mode of cis-PA. The trans-PA conformer was found to subsequently convert to the lower energy conformer via dissipative proton tunneling through the torsional barrier, as described in detail below. As mentioned before, in the spectrum of the as-deposited matrix, two bands are assigned to the OH stretching fundamental, which are originated in cis-PA molecules trapped in two different matrix sites. The same pattern is observed for the 1st OH stretching overtone, in the NIR spectral region (see Figure S4), which allowed to perform individual excitation of these two types of molecules. In situ NIR irradiation at 6898.6 (site 1) and 6874.9 cm1 (site 2), led to generation of trans-PA. The irradiations were found to be site selective: the bands belonging to each excited site were promptly consumed, while the bands belonging to the second site remained unchanged (or experienced a very slight reduction of intensity). In addition, the positions of the new bands, due to trans-PA, were shifted according to which site of cis-PA was excited, indicating that the photogenerated trans-PA molecules could be located within the initially occupied sites (or the transformation requires only a very minor matrix reorganization). The fundamental absorptions of trans-PA are reported in Table 6. Figure 4 displays the infrared spectrum after excitation at 6874.9 cm1 (a), which corresponds to the overtone of the OH stretching mode of cis-PA observed at 3523.5/3522.5 cm1 (site 2), and after excitation at 6898.6 cm1 (b), which corresponds to the overtone of the OH stretching vibration of cis-PA observed at 3534.5 cm1 (site 1).



0.3

PA : N2, 12 K

Absorbance

0.2

Relative Intensity

1 2 3 4 5 6 7 8 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

0.1

Page 16 of 31

0.8

0.050

0.4

0.025 0.000

0.0

0.0

0.4

0.8

0.2

0.2

0.4

0.1

0.0

0.0

0.0

30

120

8

60

4

20 10

(a)

0 0 3650 3600 3550 3500 1800178017601740

0 840

(b)

(c)

800

760

720

Wavenumber / cm1 Figure 4. Experimental FTIR spectrum of PA isolated in a N2 matrix at 12 K after deposition (black) and after irradiation at 6874.9 cm1 (red) (a); after deposition (black) and after irradiation at 6898.6 cm1 (blue) (b); theoretical infrared spectra with the anharmonic frequencies of cis-PA (black) and trans-PA (orange) (c) calculated at the DFT(B3LYP)/6-311++G(d,p) level. Table 6. Experimental (matrix-isolation) and DFT(B3LYP)/6-311++G(d,p) calculated infrared data of conformer trans-PA isolated in N2 matrices and vibrational assignments based on the results of normal coordinate analysis.a Approximate Experimental Calculated Description

N2 (this work) Site 1b 3575.5 3298.0 2121.0 1781.0 1776.5

Ar34

COH (CO) (CC) (C=O)

1248.0 n.obs. 832.0 749.5

(CCH) (CCH) (CC=O)

703.5 n.obs. 591.0

(OCC) (CO)

n.obs. 508.0

Site 3568.5 3302.5 2124.5 1781.0 1779.5 1776.0 1265.0 n.obs. 833.5 750.0 748.0 703.5 671.0 591.5 591.0 n.obs. n.obs.

(CCC)

n.i.

n.i.

(OH) (CH) CC (C=O)

2c

anharm

IIR

3600.2 3308.9 2127.1 1787.4

3616.0 3346.7 2172.8 1793.1

55.9 55.0 47.7 285.9

1287.1 1157.7 826.8 743.9

1267.4 1117.5 823.3 751.1

387.7 58.3 24.1 13.1

689.3 655.4 588.9

748.0 687.0 595.5

41.3 38.4 14.6

n.obs. 477.9 475.5 n.i.

538.9 528.2

3.6 121.8

251.8

0.2 16


Page 17 of 31

(CCC)

n.i.

n.i.

n.i.

188.0

3.7

Wavenumbers calculated intensities (km s = symmetric; a = antisymmetric; ν = stretching;  = in-plane bending; γ = out-of-plane bending; τ = torsion; n.obs. = not observed; n.i. = not investigated. See Table S3 for definition of symmetry coordinates and Table S5 for potential energy distributions. b New bands appeared after excitation at 6898.6 cm1. c New bands appeared after excitation at 6874.9 cm1. (cm1),

a

mol1),

These experiments allowed a clear assignment of the bands of cis-PA due to the two sets of molecules, those occupying site 1 and those occupying site 2. Irradiation at 6874.9 cm1 (site 2) led to a decrease of intensity of the cis-PA bands observed at e.g., 3523.5/3522.5 (OH), 1753.5 (C=O), and 1165.50/1164.0/1159.5 (CO) cm1 (see Table 2), whereas irradiation at 6898.6 cm1 (site 1) led to decrease of the intensity of the bands at 3534.5, 1750.0/1748.0, and 1152.5 cm1 (Table 2). Concomitantly to the selective decrease of intensity of the bands of cisPA molecules occupying each matrix site, new bands due to trans-PA emerge in the spectra, whose frequencies, as already mentioned, depends on which specific matrix site was excited. For example, trans-PA bands at 3568.5 and 1781.0/1779.5/1776.0 cm1 were observed upon excitation of cis-PA in site 2 (Table 5), corresponding to the OH and C=O stretching vibrational modes of trans-PA in this site, while the bands due to these modes in trans-PA molecules trapped in site 1 were observed at 3575.5 and 1781.0/1776.5 cm1, upon pumping cis-PA molecules in site 1. A clearer picture of the changes produced in the matrix after the performed NIR irradiations is show in Figure 5, in selected spectral regions.

Relative Intensity Absorbance

1 2 3 4 5 6 7 8 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


0.1 0.0 -0.1

0.3 0.0

0.03 0.00 -0.03

-0.2

-0.3

-0.06

20

100

20

0

0

0

-100 -20 3650 3600 3550 3500

(a)

-20 1800 1780 1760 840

(b)

800

760

720

Wavenumber / cm1 Figure 5. Experimental difference IR spectrum of PA obtained after irradiation at 6874.9 cm1 minus the spectrum of the freshly deposited PA in a N2 matrix at 12 K (red) and after irradiation at 6898.6 cm1 minus the spectrum of the freshly deposited PA in a N2 matrix at 12 K (blue) (a); simulated 17 ACS Paragon Plus Environment


Page 18 of 31

difference infrared spectrum with the anharmonic frequencies of cis-PA minus trans-PA (b) calculated at the DFT(B3LYP)/6-311++G(d,p) level.

Bands due to a few modes of trans-PA could not be observed experimentally, most probably because they appear overlapped with bands of cis-PA or have very broad profiles that could not be distinguished from the base line. These include the bands due to the CO stretching and OCC modes, which were not observed in any of the NIR irradiation experiments, and also those associated with the CCH and COH mode, which were not observed after irradiating at 6898.6 cm1 and 6874.9 cm1, respectively (see Table 5). 3.5 Broadband UV Irradiation (λ > 235 nm). Irradiation of the matrix-isolated PA with UV light (λ > 235 nm) using a Hg/Xe arc lamp was also undertaken. Interestingly, the performed irradiations led to essentially similar observations, compared to the results of selective vibrational excitation of the cis-PA form. The main observation was the production of the higher energy trans-PA conformer, and conversion between different matrix sites of the cis-PA conformer. No other products were formed, namely those resulting from either decarbonylation or decarboxylation of the compound. Figure 6 shows an IR spectrum of PA/N2 after deposition at 12 K, and the results after broadband UV irradiation (λ > 235 nm). Both the appearance of bands due to trans-PA (both sites were detected) and matrix site conversions for cis-PA are clearly shown in this figure. Regarding this last observation, the most significant changes are observed in the characteristic OH, C=O and CO stretching spectral regions and also in the COH lower frequency region, and the general pattern is conversion of site 1 into site 2. For example, in the OH stretching region, the band at 3522.5 cm1 (site 2) increases in intensity, whereas the band at 3534.5 cm1 (site 1) decreases, in the carbonyl region, the band at 1753.5 cm1 (site 2) increases in intensity while the bands belonging to site 1 (1750.0 and 1748.0 cm1) were consumed, and in the case of the torsional mode, the band at 606.0 cm1 (site 1) decreases and converts mainly to the site at 608.0 cm1 (site 2). In the work by Isoniemi et al.,25 the authors studied the 193 nm photolysis of PA in argon, krypton and xenon matrices. Formation of the higher energy trans-PA conformer upon irradiation was also observed in their study, and their proposed assignments for this species in the studied matrices are in good agreement with those presented here for trans-PA in N2 matrix. Interestingly, photofragmentation reactions were also observed to take place under the conditions used in the study by Isoniemi et al.,25 which led to generation of three distinct 18 ACS Paragon Plus Environment

Page 19 of 31

photolysis products (HCCH…CO2, HCCOH…CO and H2O…C3O). Such photolysis products were not observed in the present investigation.

Absorbance

0.2

(a) cis-PA

0.1

trans-PA 0.0 0.6

Absorbance

1 2 3 4 5 6 7 8 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


3580

3560

3540

3520

3500

(b)

cis-PA 0.3

trans-PA 0.0 1780

1760

1740

1720

Wavenumber / cm1

Figure 6. Experimental FTIR spectrum of PA isolated in a N2 matrix at 12 K after deposition (black) and showing the effect resulting from broadband UV irradiation (λ > 235 nm) (red) in the OH (a) and the C=O stretching regions (b). Arrows in blue show the depletion of bands and arrows in red show the results of site-conversion between bands of cis-PA.

3.6 Trans-PACis-PA Tunneling Reaction. After production of trans-PA by the experimental method described above, it was noticed that the amount of the photogenerated trans-PA slowly decreases in the dark, simultaneously with the regeneration of cis-PA. As it has been observed before for other carboxylic acids,76,79–83 the trans-PA conformer can then convert spontaneously by tunneling into cis-PA. The study of the tunneling decay of the trans-PA conformer in an N2 matrix was performed using different experimental conditions. The tunneling kinetic studies were measured: a) with a cut-off filter ( 235 nm; both sites). The lifetime of the trans-PA conformer in an N2 matrix was found to be 21 min (the same time as the recovery of cis-PA, see Figure 7 (a)), after excitation at 6874.9 cm1 (without filter), which increases slightly when the cut-off filter was used (38 min). This means that, with all 19 ACS Paragon Plus Environment


Page 20 of 31

probability, the higher-frequency IR light provided by the Globar source of the spectrometer is also promoting the trans-PAcis-PA reaction (through NIR-induced conversion in a similar way of the cis-PAtrans-PA process used to generate the higher-energy conformer of PA, or by facilitating the tunneling by promoting trans-PA conformer to a vibrationally excited torsional state, thus reducing the effective size of the torsional barrier through which the tunneling takes place). After excitation at 6898.6 cm1 (site 1), the lifetime of trans-PA is about 1 hour longer (1 h 37 min), with the recovery time of the cis form showing the same value ~1 h 37 min (measured both with and without cut-off filter; see Figure 7 (b)). This result shows that the stability of trans-PA molecules in site 1 is ca. 3–4 times higher than in site 2. Significantly different stabilities of trans- carboxylic acid conformers trapped in different matrix-sites were observed before for other carboxylic acids.77,84,85 The average lifetime of trans-PA molecules produced after broadband UV irradiation (λ > 235 nm; both sites were populated using this approach) was found to be 49 min (Figure 7 (c)) without filter, which as it could be expected stays in between the values measured for each one of the individual sites, and reflects also the fact that the UV irradiation is more effective in promoting the cis-PAtrans-PA reaction in site 2 than in site 1. Indeed, both cis-PAtransPA and the reverse reactions seem to be easier in site 2 than in site 1, pointing to the existence of less spatial constraints for the transformation between the two conformers in site 2. It shall be pointed out that the lifetime of trans-PA was found to be substantially shorter in solid Ar at 9 K (~400 s),25 compared to the lifetimes now obtained for the compound in a nitrogen matrix (N2:Ar decay ratio is 14.5, 3.1 and 7.3 for site 1, site 2 and after UV irradiation). In this regard, PA follows the general trend observed for other carboxylic acids, like for example formic and acetic acids (N2:Ar decay ratio equal to 90 and 504, respectively).82,86


Page 21 of 31

1.0

trans-PA cis-PA

0.8 0.6 0.4

(a)

0.2

trans-PA with filter cis-PA with filter

0.0 00:00

Relative Intensity

1 2 3 4 5 6 7 8 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


00:15

00:30

00:45

01:00

1.0 trans-PA cis-PA

0.8 0.6 0.4

(b)

0.2

trans-PA with filter cis-PA with filter

0.0 1.0 0.8 0.6 0.4

(c)

trans-PA cis-PA

0.2 0.0 00:00 00:30 01:00 01:30 02:00 02:30

Time (hours)

Figure 7. Tunnelling decay of the trans-PA monomer and formation of the cis-PA form in a nitrogen matrix at 12 K. The decay of trans-PA was evaluated using bands (a) at 1779.5/1776.0 cm1 (C=O stretching mode), and the recovery of cis-PA was evaluated using the bands at 3523.5/3522.5 cm1 (OH stretching mode) after irradiation at 6874.9 cm1 (site 2; with and without cut-off filter, circles and squares, respectively); (b) at 1781.0/1776.0 cm1 (C=O stretching of trans-PA), 1750.0 and 3534.5 cm1 (C=O and OH stretching modes of cis-PA) were used (without filter  squares) while the bands at 1781.0/1776.5 and 1750.0 cm1 for trans-PA and cis-PA were used when the filter was inserted (circles) after irradiation at 6898.6 cm1 (site 1); (c) at 1781.0/1779.5/1776.0 cm1 (C=O stretching mode of trans-PA) while the recovery of cis-PA was evaluated using the multiplet at 1753.5/1750.0/1748.0 cm1 (C=O stretching mode) after broadband UV irradiation (λ > 235 nm; both sites) without filter. The results were fitted with single exponential functions.

On the other hand, comparing the present results with data obtained for other carboxylic acids, such as formic and acetic acids, previously measured in solid nitrogen, (~11 and ~7 h, respectively for formic and acetic acids),87 it can be concluded that the lifetimes for transpropiolic acid are considerably shorter. This result agrees with the relative heights of the transto-cis barriers calculated for these acids. At the MP2/6-311++G(2d,2p) level of theory, thee barriers are 4072, 4142 and 2732 cm1 (48.7, 49.5 and 32.7 kJ mol1) for formic, acetic and propiolic acids, respectively.



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4. CONCLUSIONS In the present work, propiolic acid (PA) was investigated by IR spectroscopy in a nitrogen matrix and by ab initio calculations carried out at DFT and MP2 levels of theory. Besides the two monomeric species (cis-PA and trans-PA), the cis-cis dimers D1 and D2 were also observed in the experiments. The higher-energy conformer, trans-PA, was produced by narrowband vibrational excitation of the lower-energy form, cis-PA. Once formed, the transPA form decays in time via the hydrogen atom tunneling mechanism, leading to regeneration of cis-PA. The decay time of trans-PA was found to vary according to the different experimental conditions applied in the experiments (measuring of the decay times with or without using a cut-off filter blocking the higher-frequency radiation of the Globar source of the spectrometer). The lifetime of the trans-PA conformer was also found to be considerably influenced by the local environment in a N2 matrix, being 3–4 times longer in the case of one of the major matrix sites (site 1), compared to the second major site (site 2). Indeed, the lifetime of the trans-PA conformer in an N2 matrix was found to be 38 min (21 min in the absence of the filter) after irradiation at 6874.9 cm1 (molecules of trans-PA produced in site 2), while it amounts to ca. 1 h 37 min when irradiation is performed at 6898.6 cm1 (trans-PA generated in site 1). The obtained results allowed for the detailed assignment of the infrared spectra of cis and transPA, and cis-cis dimers D1 and D2, determination of the lifetimes of trans-PA in a nitrogen matrix and study its dependence on the matrix trapping site and experimental conditions for measuring the decay times. Data has been compared with similar data obtained for formic and acetic acids.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI… See supporting information Figures S1 and S2 with calculated DFT(B3LYP) and MP2 potential energy profiles for internal rotation of the carboxylic group around the CO bond corresponding to the interconversion between the two conformers of PA; Figure S3 with the experimental infrared spectrum of PA isolated in a N2 matrix at 12 K after deposition and after 22 ACS Paragon Plus Environment

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annealing; Figure S4 with fragment of near-infrared experimental spectrum of propiolic acid isolated in a nitrogen matrix; Tables S1 and S2 with calculated bond lengths and angles at the DFT(B3LYP) and MP2 levels and reference values reported in the microwave study for the cis-PA form; Table S3 with definition of symmetry coordinates used in the normal mode analysis of the two conformers of PA; Tables S4 and S5 with calculated [DFT(B3LYP)/6311++G(d,p)] anharmonic wavenumbers, IR intensities and Potential Energy Distributions (PED) for the two conformers of PA; Table S6 with the results of harmonic and anharmonic vibrational calculations carried out at the DFT(B3LYP)/6-311++G(d,p) level for the fundamental transitions for cis- and trans-PA; Table S7 with the calculated fundamental frequencies and intensities for cis-PA at the MP2/6-311++G(2d,2p) level of theory. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Authors ORCID ID Number: Susy Lopes, https://orcid.org/0000-0002-7631-4597 Timur Nikitin, https://orcid.org/0000-0002-0907-5936 Rui Fausto, https://orcid.org/0000-0002-8264-6854

Author Contributions All the authors have discussed the results and contributed to the final manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors acknowledge the financial support from the Portuguese “Fundação para a Ciência e a Tecnologia” (FCT) (Post-doctoral Grant ref. SFRH/BPD/77276/2011 and Project 23 ACS Paragon Plus Environment


Page 24 of 31

PTDC/QEQ-QFI/3284/2014 – POCI-01-0145-FEDER-016617). T. N. is grateful to FCT for financial support through the project PTDC/QEQ-QFI/3284/2014. The Coimbra Chemistry Centre (CQC) is also supported by FCT (Project UI0313/QUI/2013) and COMPETE-UE. We thank Dr. R. R. F. Bento for his technical assistance in the early stages of this work.


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Propiolic Acid in Solid Nitrogen: NIR- and UV ... - ACS Publications

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