Matrix isolation infrared spectroscopic study of 4-Pyridinecarboxaldehyde and of its UV-induced photochemistry

    Matrix ¡!–[INS][I]–¿i¡!–[/INS]–¿solation ¡!–[INS][I]–¿i¡!–[/INS]–¿nfrared ¡!–[INS][S]–¿s¡!–[/INS]–¿pectroscopic ¡!–[INS][S]–¿s¡!–[/INS]–¿tudy of 4-Pyridinecarboxaldehyde and of its UV-¡!–[INS][I]–¿i¡!–[/INS]–¿nduced ¡!–[INS][P]–¿p¡!–[/INS]–¿hotochemistry Liesel Cluyts, Archna Sharma, Nihal Kus¸, Kristien Schoone, Rui Fausto PII: DOI: Reference:

S1386-1425(16)30448-6 doi: 10.1016/j.saa.2016.08.002 SAA 14588

To appear in: Received date: Revised date: Accepted date:

6 June 2016 25 July 2016 2 August 2016

Please cite this article as: Liesel Cluyts, Archna Sharma, Nihal Ku¸s, Kristien Schoone, Rui Fausto, Matrix ¡!–[INS][I]–¿i¡!–[/INS]–¿solation ¡!–[INS][I]–¿i¡!–[/INS]– ¿nfrared ¡!–[INS][S]–¿s¡!–[/INS]–¿pectroscopic ¡!–[INS][S]–¿s¡!–[/INS]–¿tudy of 4Pyridinecarboxaldehyde and of its UV-¡!–[INS][I]–¿i¡!–[/INS]–¿nduced ¡!–[INS][P]–¿p¡!– [/INS]–¿hotochemistry, (2016), doi: 10.1016/j.saa.2016.08.002

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ACCEPTED MANUSCRIPT Matrix Isolation Infrared Spectroscopic Study of 4-Pyridinecarboxaldehyde and of its UV-Induced Photochemistry

Department of Chemical Engineering, Catholic University Leuven, Celestijnenlaan 200F - box 2424, B-3001 Heverlee, Belgium. b CQC, Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal. c Department of Physics, Anadolu University, TR-26470 Eskişehir, Turkey. d Thomas More University, Kleinhoefstraat 4, B-2440 Geel, Belgium.

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a

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Liesel Cluyts,a,b Archna Sharma,b Nihal Kuş,b,c Kristien Schooned and Rui Faustob,*

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Abstract

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The structure, infrared spectrum, barrier to internal rotation, and photochemistry of 4-pyridinecarboxaldehyde (4PCA) were studied by low-temperature (10 K) matrix isolation infrared spectroscopy and quantum chemical calculations undertaken at

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both Moller-Plesset to second order (MP2) and density functional theory

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(DFT/B3LYP) levels of approximation. The molecule has a planar structure (Cs point group), with MP2/6-311++G(d,p) predicted internal rotation barrier of 26.6 kJ mol–1, which is slightly smaller than that of benzaldehyde (~30 kJ mol–1), thus

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indicating a less important electron charge delocalization from the aromatic ring to the aldehyde moiety in 4PCA than in benzaldehyde. A complete assignment of the infrared spectrum of 4PCA isolated in an argon matrix has been done for the whole

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4000-400 cm–1 spectral range, improving over previously reported data. Both the geometric parameters and vibrational frequencies of the aldehyde group reveal the relevance in this molecule of the electronic charge back-donation effect from the oxygen trans lone electron pair to the aldehyde C–H anti-bonding orbital. Upon in situ UV irradiation of the matrix-isolated compound, prompt decarbonylation was observed, leading to formation of pyridine.

Keywords: 4-Pyridinecarboxaldehyde; Matrix Isolation Infrared Photochemistry; Quantum Chemical Calculations.

* Corresponding author e-mail: [email protected]

1

Spectroscopy;

Narrowband

UV-Induced

ACCEPTED MANUSCRIPT 1. Introduction

Substituted pyridines have attracted much attention due to their multiple applications. 4-Pyridinecarboxaldehyde (4PCA; Figure 1) was shown to be an efficient building block for synthesis of

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Schiff bases using a Korich-type reaction [1], an intermolecular reductive synthetic procedure based on

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the condensation of nitro-substituted arenes and aromatic aldehydes in the presence of iron powder and

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dilute acid [2]. Metal complexes of several of these Schiff bases were shown to exhibit good activity against different types of bacteria (e.g., Staphylococcus aureus, E. coli, Klebsiella, Pneumonia) and fungi (Candida albicans, Apergillus niger and Pencillium sp) [1,3], and also moderate nuclease activity [3]. 4PCA and some of its derivatives have also been reported as useful transamination reagents to introduce

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ketone or aldehyde groups onto the N-termini of antibodies [4,5] for subsequent site-specifically conjugate aminooxy-functionalized molecules (including fluorescent dyes, polyethylene glycol, or

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porphyrins) to these entities [5-7].

To the best of our knowledge, there are no experimental data on the structure of 4PCA molecule. However, its geometry and infrared (IR) spectrum have been investigated using both Density Functional

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Theory (DFT) and Hartree-Fock (HF) theoretical approaches [8,9], the molecule being predicted to be planar at both levels of approximation. The IR spectrum of 4PCA isolated in an argon matrix at 15 K,

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within the range 1900-600 cm–1, has been obtained by Ohno and coworkers [8], and IR and Raman spectra of the liquid compound, at room temperature, have been reported earlier by Green and Harrison

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[10]. Ohno and coworkers [8] described also the ultraviolet-visible (UV-Vis) spectrum of 4PCA in hexane solution at room temperature. The observed intense UV absorption ( = 2800 L mol–1 cm–1) with maximum at 283.3 nm was assigned to the ,* S2←S0 transition, while the low-intensity bands due to

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the n,* transitions to S1 ( = 7.1 L mol–1 cm–1) and T1 (spin-forbidden;  = 0.1 L mol–1 cm–1) were observed at 381.7 and 411.4 nm, respectively [8].

Figure 1. 4PCA minimum energy structure with atom numbering used in this study.

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ACCEPTED MANUSCRIPT In the present study, (i) the equilibrium geometry of 4PCA and the barrier of internal rotation of the aldehyde group were obtained at a higher-level of theory than previously reported (Moller-Plesset to second order with a triple- basis set, completed with polarization functions in both heavy atoms and hydrogens as well as with diffuse functions: MP2/6-311++G(d,p)), (ii) the infrared spectrum of the

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compound isolated in an argon matrix was studied in an extended frequency range (4000-400 cm–1) and

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fully assigned based on normal coordinates’ calculations, and (iii) the UV-induced photochemistry

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exhibited by the matrix-isolated compound was investigated. The present investigation thus expands the results obtained in previous studies on the structure and spectroscopy of the compound [8-10] and provides new data on its photochemistry.

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2. Experimental and Computational Methods

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2.1. Experimental details

4PCA was provided by Koch-Light Laboratories, Ltd. (purity >99 %) and was further purified by the standard freeze-pump-thaw technique before experiments to remove traces of volatile impurities.

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Vapors of the compound at room temperature were co-deposited together with a large excess of argon (N60, supplied by Air Liquide) onto the CsI optical substrate of the cryostat kept at a temperature of 10

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K. The gaseous argon was introduced in the cryostat from a separate line. The temperature of the cold substrate was measured by a silicon diode sensor connected to a digital controller (Scientific Instruments,

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Model 9650-1) with accuracy of ±0.1 degree. The low temperature equipment was based on a closedcycle helium refrigerator (APD Cryogenics) with a DE-202A expander. Infrared spectra were registered with 0.5 cm−1 resolution, in the range 4000–400 cm−1, using a

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Thermo Nicolet Nexus 670 FT-IR spectrometer, equipped with a deuterated triglycine sulphate (DTGS) detector and a KBr beamsplitter. UV irradiations of the matrix-isolated 4PCA were performed using the narrowband (0.2 cm−1 spectral width) light provided by a Spectra Physics Quanta-Ray MOPO-SL optical parametric oscillator pumped with a pulsed Spectra Physics Quanta-Ray PRO-230-10 Nd:YAG laser (repetition rate, pulse energy and duration were 10 Hz, ~5 mW and 10 ns, respectively).

2.2 Computational details Calculation of structures and IR spectra were performed using the Gaussian 03 program package [11]. Geometries were obtained at both the DFT and MP2 levels of theory, with the 6-311++G(d,p) basis set [12,13]. In the DFT calculations, the B3LYP hybrid functional [14-16] was used. The IR spectra were obtained at the DFT(B3LYP)/6-311++G(d,p) level, the calculated wavenumbers being subsequently scaled down by the factor 0.978 to account for the approximations inherent to the used theoretical model and anharmonicity.

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ACCEPTED MANUSCRIPT Normal coordinate analysis was performed in the internal coordinates space, as described by Schachtshneider and Mortimer [17], using the structural data and Cartesian force constants resulting from the B3LYP/6-311++G(d,p) calculations. The Cartesian force constants were converted to the space of symmetry coordinates shown in Table 1 in order to allow for the calculation of the normal modes of

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vibration and potential energy distributions (PEDs). These calculations were performed using a locally

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modified version of the program BALGA [18]. For the purpose of modeling IR spectra, the calculated

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wavenumbers, together with the calculated IR intensities, served to simulate the spectra shown in the figures by convoluting each peak with a Lorentzian function with a full-width-at-half-maximum (FWHM) of 2 cm–1.

Coordinate

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Table 1 – Definition of symmetry coordinates used in the normal coordinate analysis of 4PCA. a Definitionb

S21 S22 S23

CC C=O CH CH r1 CH r2 CH r3 CH r4 r1 r2 r3 r4 r5 r6 CH CCO CCC r1 r2 r3 CH r1

H13-(C11O12C3) C11-(C3C4C2) H8-(C2C3C1) + H7-(C1C2N6) + H10-(C5C4N6) + H9-(C4C3C5) H8-(C2C3C1) + H7-(C1C2N6) - H10-(C5C4N6) - H9-(C4C3C5) H8-(C2C3C1) - H7-(C1C2N6) + H10-(C5C4N6) - H9-(C4C3C5) H8-(C2C3C1) - H7-(C1C2N6) - H10-(C5C4N6) + H9-(C4C3C5) C2C3 - C1C2 + N6C1 - C5N6 + C4C5 - C3C4 C2C3 - N6C1 + C5N6 - C3C4 C2C3 - 2C1C2 + N6C1 + C5N6 - 2C4C5 + C3C4 C3C11

CH CC CH r1 CH r2 CH r3 CH r4 r1 r2 r3 CC

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C3C11 C11O12 C11H13 C2H8 C1H7 C4C9 + C5H10 C4C9 - C5H10 2C1C2 + 2C4C5 - C1N6 - N6C5 - C2C3 - C3C4 C1C2 + C4C5 + C1N6 + N6C5 + C2C3 + C3C4 C1C2 - C4C5 C1N6 - N6C5 - C2C3 + C3C4 C1N6 + N6C5 -C2C3 - C3C4 C1N6 - N6C5 + C2C3 - C3C4 O12H13C11 - C3H13C11 2O12C3C11 - O12H13C11 -C3H13C11 C2C11C3 - C4C11C3 C2N6C1 - C1C5N6 + N6C4C5 - C5C3C4 + C4C2C3 - C3C1C2 C2N6C1 - 2C1C5N6 + N6C4C5 + C5C3C4 - 2C4C2C3 + C3C1C2 C2N6C1 - N6C4C5 + C5C3C4 - C3C1C2 C3H8C2 - C1H8C2 + C2H7C1 - C6H7C1 + C4H10C5 - C6H10C5 + C3H9C4 - C5H9C4 C3H8C2 - C1H8C2 - C2H7C1 + C6H7C1 + C4H10C5 - C6H10C5 C3H9C4 + C5H9C4 C3H8C2 - C1H8C2 + C2H7C1 - C6H7C1 - C4H10C5 + C6H10C5 C3H9C4 + C5H9C4 C3H8C2 - C1H8C2 - C2H7C1 + C6H7C1 - C4H10C5 + C6H10C5 + C3H9C4 - C5H9C4

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S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20

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A´

Approximate Description

CH r2 CH r3 CH r4

A´´ S24 S25 S26 S27 S28 S29 S30 S31 S32 S33 a b

For atom numbering, see Figure 1. , stretching; , in-plane bending; , out-of-plane rocking; , torsion; r, ring. Normalization factors not given; angles are defined with the apex atom at the end.

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ACCEPTED MANUSCRIPT 3. Results and Discussion

3.1 Structure and barrier to aldehyde group internal rotation In agreement with previous results [8,9], both B3LYP and MP2 calculations undertaken in the

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present study yield a planar (Cs point group) minimum energy structure for the 4PCA molecule. The

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calculated optimized geometrical parameters are given in Table 2. The geometries predicted by the two

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theoretical modes show the same general trends, though the bond lengths obtained at the B3LYP level are systematically slightly shorter than those obtained at the MP2 level. The calculated angles obtained by the two methods present very similar values, being equal to within ±1º. The most striking geometrical feature is the long aldehyde C–H bond length (1.1098 Å). This is, however, a rather general trend in

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aldehydes, resulting from the electronic charge back-donation effect from the trans-lone electron pair of the oxygen atom to the C–H anti-bonding orbital [19]. As will be stressed later on, this structural feature

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leads also to a considerably low C–H vibrational frequency, as found in other similar molecules [19-22]. A few other interesting structural facets can be noticed in the data shown in Table 2. (i) Among the 3 angles within the aldehyde group, the C–C=O angle is the largest one (~124º), being about 9º larger

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than the C–C–H angle; this is a consequence of the need to open space for the bigger oxygen atom. (ii) The hydrogen atoms in the ortho position to the nitrogen are pulled towards this atom, the

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N–C–H angles being ~116º, i.e. ~4º smaller than the C–C–H ring angles that assume values very close to 120º; simultaneously, the C–C–N angles are ca. 124º, while the C–N–C angle is only ~117º, and the

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C–C–C ring angles stay in the 118-119º range. All these ring angle values are identical (to less than ±0.5º) to the angles calculated at the same levels of calculation for pyridine, indicating that the introduction of the aldehyde group in the molecule does not significantly perturb the geometry of the

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ring. Moreover, these values are in almost perfect agreement with the experimental values of the angles in pyridine, obtained by microwave spectroscopy (C–C–N: 123.80±0.03º, C–N–C: 116.94±0.03º; C–C– C: 118.40±0.03-118.53±0.03º; N–C–H (ortho): 116.01±0.03º; C–C–H (ortho): 120.19±0.03 [23-25]). (iii) Finally, the exocyclic C–C bond connecting the pyridil ring to the aldehyde moiety in 4PCA is predicted to be 1.4887 Å at the MP2 level, a somewhat larger value compared to the value obtained at the same theoretical level for both benzaldehyde (1.4830 Å) and p-anisaldehyde (1.4803 Å) [26] (the B3LYP values show the same trend, being 1.4878, 14808 and 1.47284 Å, respectively in 4PCA, benzaldehyde and p-anisaldehyde [26]; for pyrrole-2-carboxaldehyde, the B3LYP reported values for the exocyclic C–C bond length are 1.4452 and 1.4490 Å, depending on the conformer [19], i.e., considerably shorter than for the remaining aldehydes mentioned here). As it will be shown below, these trends correlate well with the relative size of the barrier for internal rotation of the aldehyde group in these 4 molecules, and thus appear as a good indication of the degree of electron delocalization from the ring to the aldehyde moiety in these compounds. It shall be mentioned that, among the 4 molecules now mentioned, there is only available structural experimental data for benzaldehyde, the range of values reported for the C–C

5

ACCEPTED MANUSCRIPT bond length (1.479-1.492 Å, depending on the experimental technique used [27-29]) agreeing reasonably well with the calculated values given above for that molecule.

MP2

B3LYP

Parameter

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Parameter bond length

MP2

B3LYP

bond angle

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C3C4C5 118.56 C4C5N6 123.65 C5N6C1 116.98 N6C1C2 123.99 N6C1H7 115.64 C2C1H7 120.37 C1C2H8 121.57 C3C2H8 120.18 C2C3C11 121.19 C4C3C11 120.25 N6C5H10 115.88 C4C5H10 120.46 C5C4H9 120.53 C3C4H9 120.91 bond angle C3C11H13 114.96 C1C2C3 118.25 118.33 C3C11O12 123.96 C2C3C4 118.56 118.51 H13C11O12 121.08 a Bond lengths in Å; bond angles in degrees. See Figure 1 for atom numbering.

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1.3902 1.3974 1.3950 1.3944 1.3348 1.3398 1.0858 1.0832 1.0851 1.0860 1.4878 1.2081 1.1098

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1.3960 1.4014 1.3995 1.3991 1.3442 1.3478 1.0877 1.0860 1.0878 1.0878 1.4887 1.2174 1.1095

118.59 123.29 117.69 123.59 115.96 120.45 121.57 120.11 121.30 120.18 116.22 120.49 120.50 120.91 114.72 124.41 120.87

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CE P

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C1C2 C2C3 C3C4 C4C5 C5N6 N6C1 C1H7 C2H8 C4H9 C5H10 C3C11 C11O12 C11H13

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T

Table 2 – Calculated optimized geometrical parameters for 4PCA with MP2 and B3LYP methods and the 6-311++G(d,p) basis set. a

The internal rotation barrier of the aldehyde group in 4PCA obtained at the MP2 level is 26.6 kJ –1

mol . This value is significantly lower than that predicted by the B3LYP method (30.0 kJ mol–1). Since it has been shown that for molecules comprising an aromatic ring with a π-conjugated substituent the rotational barrier around the C(sp2)–C(ring) bond appears systematically overestimated by current DFT methods [30,31], the calculated MP2 barrier is probably a better value. In fact, the presently accepted value for the aldehyde group rotational barrier in benzaldehyde is ca. 32 kJ mol–1, obtained either from experimental data [32] or theoretically by extrapolation of the quantum chemical results to the complete basis set limit [33].† This value is in agreement with the MP2/6-311++G(d,p) calculated value for that molecule (30.3 kJ mol–1 [26]), but far from the B3LYP one obtained with the same basis set, which amounts to 36.7 kJ mol–1 [26]. The substantial height of the barrier for rotation of the aldehyde group in

†

Initial estimations of this barrier height based on analyses of experimental data were of the order of 20 kJ mol– 1 [28,34,35], but latter on found to be underestimated in result of wrong spectral assignments [32,33].

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ACCEPTED MANUSCRIPT 4PCA, as well as in other aromatic aldehydes, may be attributed to conjugation between the π-electron systems of the carbonyl moiety and of the aromatic ring, and the exageration in the predicted barriers by the B3LYP method correlates well with the shorter values for the exocyclic C–C bond length obtained by this method when compared to those obtained at the MP2 level (see Table 2). Interestingly, the error in

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predicting the energy barriers at the B3LYP level seems to be systematic, since for example the relative

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order of the calculated barriers for 4PCA, benzaldehyde [26], p-anisaldehyde [26] and pyrrole-2-

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carboxaldehyde [19] is the same as predicted by the MP2 method (30.0, 36.7, 43.1 and 52.0 kJ mol–1, vs. 26.6, 30.3, 33.9 and 42.5 kJ mol–1). Note also that the relative order of magnitude of the energy barriers

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correlate well with the corresponding exocyclic C–C bond lengths in these 4 molecules (see above).

3.2 IR spectrum and UV-induced photochemistry of 4PCA isolated in solid argon (10 K) The IR spectrum of 4PCA isolated in argon (10 K) is shown in Figure 2, together with the

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B3LYP/6-311++G(d,p) calculated spectrum. The proposed assignments and results of normal coordinates’ analysis are provided in Table 3. The previous partial data reported in [8] (just for the 1900600 cm–1 region) are also shown in this table, the two investigations yielding similar results within this

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spectral range. The higher resolution used in the present study compared to [8] (0.5 vs. 1.0 cm–1) allowed

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to conclude on the existence of at least two spectroscopically different trapping sites for the 4PCA molecules isolated in solid argon. Additional splitting observed for several of the most intense bands may be attributed to discrimination, in these cases, of additional matrix trapping sites and/or to Fermi

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resonances (see Table 3). Among the Fermi resonance interactions, those involving the aldehyde CH stretching vibration (2843-2704 cm–1) and the mode with major contributions from the exocyclic CC stretching and one of the ring deformation (r1) coordinates (giving rise to features within the 1202-1189

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cm–1 range) are remarkable, with Fermi separations as large as ca. 140 and 13 cm–1, respectively. For these two modes, the comparison of the observed band intensities with the calculated ones is also particularly useful to conclude on the involvement of these vibrations in Fermi resonances. Indeed, as it can be seen in Figure 2, the calculated intensities of the bands due to these two modes can only be well fitted by the experimental data upon taking into account the added intensities of the observed Fermi component bands, which, besides, are nearly identically distributed within each set of bands ascribed to each pair of Fermi components. In the case of the other proposed Fermi resonance interactions (see Table 3) the coupling is much weaker, the Fermi splitting being small and the intensities of the Fermi component-bands being considerably asymmetrically distributed. It shall be noticed that the theoretically predicted spectrum fits very well the experimental spectrum, both regarding frequencies and intensities, what makes the assignment of the bands very secure (the calculated frequencies in the high-frequency region are somewhat overestimated because a single scale factor for frequencies was used; use of a second scale factor would allow to reach a very good frequency matching also for this spectral range).

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ACCEPTED MANUSCRIPT A final comment shall be done regarding the frequency of the aldehyde CH stretching vibration. The bands ascribed to this mode appear centered at ca. 2775 cm–1, a rather low CH frequency, which can be explained by taking into account the electronic charge back-donation effect from the trans-lone electron pair of the oxygen atom to the C–H anti-bonding orbital [19] that was already mentioned above

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0.35

argon matrix (10 K)

0.30

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0.20 0.15

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Absorbance

0.25

0.10

D

0.05

1600

1200

CE P

2800

800

400

Calculated B3LYP/6-311++G(d,p)

70 60

O

H

50

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Relative IR Intensity

80

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0.00 3200

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when discussing the effect of this interaction on the C–H bond length.

40

N

30 20 10 0

3200

2800

1600

1200

800

400

Wavenumber/ cm-1 Figure 2. Top: Infrared spectrum of 4PCA isolated in solid argon at 10 K (bands due to traces of water were subtracted). Bottom: B3LYP/6-311++G(d,p) calculated spectrum of 4PCA (wavenumbers scaled by 0.978; calculated intensities – shown in Table 3 – correspond to the areas under the bands, which have been simulated by Lorentzian functions with full-width-at-half-maximum (FWHM) of 2 cm–1.

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ACCEPTED MANUSCRIPT Table 3 – Observed bands of 4PCA isolated in argon, B3LYP/6-311++G(d,p) calculated IR spectra and results of normal coordinate analysis.a Experimental Ar (this study)

[8]







IIR

Assignment

%

3129.6

2.9

3085.1

n.i.

CH r1

A'

3104.8

11.4

3070.9/3067.8/3064.8

n.i.

CH r3

A'

3090.7

14.1

3049.6/3045.0

n.i.

CH r2

A'

3085.4

12.4

3040.4/3036.6

n.i.

A'

2837.6

103.4

n.i.

A'

1740.2

250.0

2842.1/2838.0/2822.8/2792.8/ 2746.6/2730.1/2704.5c 1732.0/1727.7/1726.1/1724.9/ 1723.9d

A'

1593.0

0.8

1595.2

1595

A'

1568.5

24.1

A'

1484.2

3.2

A'

1411.4

19.9

1418.5/1413.4

A'

1382.0

8.5

1387.7/1386.4

A'

1316.6

15.8

1322.8/1321.4

A'

1251.0

12.3

1229.6/1227.1/1226.0

A'

1214.4

20.7

1216.9/1215.9

S7[86] + S5[10]

CH

S3[100]

C=O

S2[90]

r1

S8[63] + S23[22]

1570

r4

S11[74]

1506

CH r1

S10[62] + S12[30]

1414

CH r2

S21[44] + S10[25] + S14[13]

1387

CH

S14[69] + S22[13]

1322

CH r3

S22[70]

1226

r6

S13[74] + S10[16]

1216

CH r4

S23[47] + S8[25] + S12[13]

1191

CC; r1

S1[30] + S9[22] + S17[21] + S23[14]

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D

S5[91]

A'

1184.8

60.2

A'

1078.5

0.6

1077

1085

r3

S21[44] + S10[44]

A'

1058.9

6.4

1051.8/1051.0

1062

r5

S12[38] + S20[21] + S17[20] + S23[12]

A''

1006.5

2.2

1004.3

1004

CH

S24[79]

A'

986.7

3.2

990.3

991

r2

S9[55] + S17[41]

A''

985.2

0.002

n.o.

n.o.

CH r3

S28[70] + S32[17]

A''

961.3

0.1

n.o.

n.o.

CH r4

S29[67] + S30[19]

A''

875.9

0.2

n.o.

n.o.

CH r2

S27[92]

A'

825.7

16.7

835

r1; CC

S1[24] + S15[17] + S9[12] + S17[12]

801

CH r1

S26[82] + S31[11]

739

r1

S30[68] + S29[18] + S25[13]

668

r2

S18[57] + S19[30] + S11[10]

647

CCO

S15[409 + S18]22] + S19[19]

CE P

TE

1201.7/1199.9/1191.0/1189.5

e

S6[89]

CH r4

1725

1491.4

S4[96]

SC R

A'

1571.6/1570.1

PEDb

T

Calculated B3LYP 6-311++G(d,p)

IP

Symmetry

AC

837.7/835.5/834.7

A''

802.5

42.5

A''

724.7

1.4

806.3/803.2/801.6/801.2/800.5 727.5

A'

664.9

1.2

666.9

f

g

A'

643.5

37.8

650.2/646.9/646.5/645.7

A''

469.5

18.6

470.1

n.i.

CC

S25[58] + S31[35]

A'

425.7

1.9

423.5

n.i.

r3

S19[34] + S1[26]

A''

378.0

0.02

n.i.

n.i.

r3

S32[76] + S28]19]

A''

219.9

1.2

n.i.

n.i.

r2

S31[41] + S33[26] + S25[12] + S26[11]

A'

209.9

9.8

n.i.

n.i.

CCC

S16[69] + S15[24]

A''

106.8

12.2

n.i.

n.i.

CC

S33[68] + S31[12]

Frequencies () in cm ; infrared intensities (I ) in km mol . Calculated frequencies were scaled down by 0.978. n.o., not observed; n.i., non-investigated. b For definition of symmetry coordinates see Table 1. Potential energy distributions (PEDs) smaller than 10% are not given. c Fermi resonance with 2xCH. d Fermi resonance with CH r3+r3. e Fermi resonance with r2+CCC. f Fermi resonance with CCO+CCC. g Fermi resonance with 2xr3. a

–1

–1

IR

9

ACCEPTED MANUSCRIPT The matrix-isolated 4PCA was irradiated using the narrowband UV-beam (0.2 cm–1 bandwidth) provided by a Quanta-Ray MOPO-SL optical parametric oscillator. The chosen irradiation wavelength was 285 nm, nearly matching that corresponding to the maximum of the intense ,* S2←S0 absorption of 4PCA observed in hexane solution (283.3 nm [8]). In a preliminary experiment, the irradiation

T

wavelength was initially set at 350 nm and decreased in steps of 5 nm, the first signs of occurrence of a

IP

photo-induced reaction having started to be noticed when the wavelength was 310 nm; the efficiency of

SC R

the process was then found to be maximized for an excitation wavelength in the range 290-280 nm. After 30 min of irradiation at 285 nm, approximately 40 % of 4PCA was consumed (as evaluated by the decrease in intensity of the infrared bands of the 4PCA spectrum), while new bands due to photoproduced species were observed.

NU

Figure 3 (top panel) shows the difference spectrum: UV-irradiated sample spectrum minus asdeposited 4PCA-argon matrix spectrum. The product band appearing at 2138 cm–1 is a distinctive feature

MA

of carbon monoxide [36], indicating occurrence of photodecarbonylation. The accompanying photoproduct should be pyridine, and, accordingly, all the remaining observed bands ascribable to photoproduced species correspond well to both previously experimental data for pyridine isolated in

D

solid argon [37,38] and the B3LYP/6-311++G(d,p) calculated spectrum for this compound (see Figure 3 and Table 4). In Table 4, data obtained by Castellucci, Sbrana and Verderame [39] for pyridine isolated

TE

in N2 matrix are also shown, together with the band assignments. Note that the previously available data for pyridine [37,38] in argon matrix did not consider the high-frequency CH stretching region and have

CE P

the 4 and 13 modes misassigned (here, the mode designation introduced by Zerbi, Crawford, Jr. and Overend [40] is adopted). In [37,38], the 4 mode was assigned to the bands at 1599 and 1592 cm–1, which should be assigned to the 9+10 combination tone, while the band due to 4, observed at 1583

AC

cm–1, was misassigned to 13. Also, in view of the present results the assignments proposed in [39] for the 1, 11, 16 and 6 modes of pyridine isolated in N2 matrix shall also be revised (see Table 4). Specifically, the assignments for 1, 11 shall be interchanged, 6 shall be reassigned to the band observed at 1219 cm–1, and the band at 1208 cm–1, assigned in [39] to 16, corresponds most probably to the first overtone of 10, whose fundamental is observed in N2 matrix at 603 cm–1; the 16 mode has a very low IR intensity, precluding its experimental observation (~0.03 km mol–1; see Table 4). The decarbonylation reaction in 4PCA can be expected to take place through a mechanism similar to that operating in benzaldehyde and p-anisaldehyde [26,41], i.e., after excitation to S2, intersystem crossing to the triplet manifold (possibly preceded by internal conversion to the S1 state) leads to the cleavage of the C–H aldehyde bond; the formed radical can then release CO and recombine with the hydrogen atom within the matrix cage, leading to the observed photoproducts. Note that recombination of radicals is considerably favored in a solid matrix compared to the gas phase, because the mobility of the hydrogen atom is reduced in this case. For benzaldehyde, the intersystem crossing is

10

ACCEPTED MANUSCRIPT known to be highly efficient [42-49] with the lowest excited singlet state possessing a bifurcation on its potential energy surface to the triplet manifold [45-47].

IP

T

0.06

Argon matrix (10 K)

SC R

0.02 0.00

NU

Absorbance

0.04

-0.02

MA

-0.04 -0.06 3200 2800

D

1600

TE

CE P

20

1200

800

400

Calculated B3LYP/6-311++G(d,p)

CO

N

0

AC

Relative IR Intensity

40

2000

-20

O

-40

3200 2800

H

N

2000

1600

1200

800

400

Wavenumber/ cm-1 Figure 3. Top: Infrared difference spectrum: UV-irradiated sample minus as-deposited 4PCA-argon matrix. Bottom: B3LYP/6-311++G(d,p) calculated difference spectrum: spectrum of pyridine minus 4PCA spectrum (wavenumbers scaled by 0.978; calculated intensities – shown in Tables 3 and 4 – correspond to the areas under the bands, which have been simulated by Lorentzian functions with fullwidth-at-half-maximum (FWHM) of 2 cm–1.

11

ACCEPTED MANUSCRIPT Table 4 – Observed bands of pyridine resulting from UV-photolysis of 4PCA isolated in argon, compared with previously observed bands of a genuine sample of pyridine isolated in argon and N2 matrices and with results of B3LYP/6-311++G(d,p) calculations. a Ar [37,38]

N2 [39]









3089

11

3065

2

3042

3

3030 1584

12 4

1578

13

1479.5

1485

5

1440.9

1441

14

n.o.

1352

15

n.o.

1219

16

d

1219 1147.7 1146.2 1144.8

1208 1148

6 17

1073d

7

n.i.

b1

3115.5

25.4

3086.5

n.i.

a1

3100.5

4.9

3059.2

n.i.

a1

3080.4

4.1

3039.3

n.i.

b1 a1

3078.1 1587.3

27.8 23.9

3027.6 1583.0

n.i. 1599.3 1592.2

b1

1581.7

10.2

1578.6

1583.4

a1

1477.0

2.4

1483.4

b1

1437.4

26.9

1441.3

b1

1353.2

0.1

n.o.

b1

1255.9

0.03

n.o.

4.7 2.5

1219.9 1148.5

a1

1068.7

5.3

1073.3

b1

1052.5

0.002

a1

1023.5

6.3

a1

988.0

4.8

b2

984.2

0.003

a2

974.5

0.0

CE P

D

1214.2 1144.3

0.016

MA

3088.3

TE

7.0

IP

1

3123.4

a1 b1

Assignmentc

3077

a1

SC R

IIR

T

B3LYP/6-311++G(d,p)

Experimental Ar (this study)b

NU

Calculated Symmetry

n.o.

n.o.

1072 n.o.

1031.7

1032.2

1032

8

991.4

991.8 n.o.

992 n.o.

9

n.o. n.o.

n.o.

n.o.

18

23 20 24

b2

933.5

a2

872.8

b2 b2

741.0 699.0

11.8 67.2

743.1 703.0 701.2 700.2

744.4 705.6 701.4

745 704

25 26

b1

654.5

0.3

n.o.

n.o.

n.o.

19

a1

603.0

3.6

602.0

602.1

603

10

b2

408.3

4.1

408.0

n.i.

406

27

a2

372.3

0.0

21

AC

0.0

22

Frequencies () in cm ; infrared intensities (I ) in km mol . Calculated frequencies were scaled down by 0.978. b Major bands due to overtones and combinations were observed at 3031.0 (6+12), 3007.6 (14+4), 1599.1 (9+10) and 746.3 (222) cm–1. Frequencies presented in italic correspond to bands misassigned in the references. c Designation of the vibrations according to [40]. d From [38]; not observed in [37]. a

–1

–1

IR

Interestingly, for both benzaldehyde and p-anisaldehyde the radicals resulting from cleavage of the C–H aldehyde bond could be experimentally observed upon photolysis of the matrix-isolated

12

ACCEPTED MANUSCRIPT aldehydes [41], confirming the involvement of the radicals in the decarbonylation reaction. On the other hand, for 4PCA isolated in argon matrix the corresponding radical could not be observed, what points to a shorter lifetime of this species in this case. This is in consonance with the already mentioned smaller degree of conjugation between the aldehyde group in 4PCA compared to benzaldehyde and p-

T

anisaldehyde (as noticed by the longer exocyclic C–C bond and lower aldehyde torsional barrier in

SC R

IP

4PCA; see above).

4. Conclusions

NU

In the present study, 4PCA was studied by low-temperature (10 K) matrix isolation infrared spectroscopy and quantum chemical calculations undertaken at different levels of approximation. In the minimum energy conformation, 4PCA molecule is planar (Cs point group), with an aldehyde rotation

MA

barrier of 26.6 kJ mol–1 (MP2/6-311++G(d,p) predicted value). Such energy barrier is slightly smaller than that of benzaldehyde (~30 kJ mol–1 [26]) and other related aldehydes (e.g., p-anisaldehyde [26];

D

pyrrole-2-carboxaldehyde [19]). These results are in agreement with the longer exocyclic C–C bond in 4PCA than in these other aldehydes, and indicating a comparatively less important electron charge

TE

delocalization from the aromatic ring to the aldehyde moiety in 4PCA. The infrared spectrum of 4PCA isolated in an argon matrix was assigned for the whole 4000-400

CE P

cm–1 spectral range, completing and improving on previously reported data [8]. The low frequency of the aldehyde C–H stretching vibration was found to be a good probe of the relevance in 4PCA of the electronic back-donation effect from the oxygen trans lone electron pair to the aldehyde C–H bond. Such

AC

effect reflects also in the long C–H aldehyde bond length. Upon UV irradiation of the matrix-isolated compound, prompt decarbonylation was observed, leading to formation of pyridine. A mechanism similar to that operating in benzaldehyde and panisaldehyde [26,41] is proposed for this photoreaction, i.e., after excitation to S2, intersystem crossing to the triplet manifold leads to the cleavage of the C–H aldehyde bond, the formed radical then releasing CO and recombining with the hydrogen atom within the matrix cage. On the whole, the present investigation expanded structural and spectroscopic data on the studied compound [8-10] and provides new data on its UV-induced photochemistry.

Acknowledgments: The Coimbra Chemistry Centre (CQC) is supported by the Portuguese “Fundação para a Ciência e a Tecnologia” (FCT), through the project UI0313/QUI/2013, co-funded by COMPETEUE. N. Kuş thanks FCT for the post-doctoral grant ref. SFRH/BPD/88372/2012.

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SC R

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CE P

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D

MA

NU

SC R

IP

T

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16

ACCEPTED MANUSCRIPT Graphical Abstract

Argon Matrix

AC

CE P

TE

D

MA

NU

SC R

IP

T

+ CO

λ = 285 nm

17

ACCEPTED MANUSCRIPT Highlights 

4-Pyridinecarboxaldehyde (4PCA) is studied by matrix isolation IR spectroscopy and quantum chemical calculations. Equilibrium geometry of the molecule and the internal rotation barrier of the aldehyde group are

T



The IR spectrum of the matrix isolated compound (Ar, 10 K) is described and assigned in the full 4000-400 cm–1 spectral range.

The narrowband UV-induced photochemistry of PCA in an argon matrix is investigated and the

CE P

TE

D

MA

NU

bands due to photoproducts have been assigned.

AC



SC R



IP

reported.

18