Infrared spectra and UV-induced photochemistry of methyl aziridine-2-carboxylate isolated in argon and xenon matrices

Vibrational Spectroscopy 81 (2015) 68–82

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Infrared spectra and UV-induced photochemistry of methyl aziridine-2-carboxylate isolated in argon and xenon matrices Susy Lopes* , Igor Reva, Rui Fausto CQC, Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal

A R T I C L E I N F O

A B S T R A C T

Article history: Received 18 August 2015 Received in revised form 9 October 2015 Accepted 10 October 2015 Available online 22 October 2015

Methyl aziridine-2-carboxylate (MA2C) has been isolated in low temperature argon and xenon matrices and its structure and photochemistry were studied by FTIR spectroscopy. The reactant as well as the main photoproducts were characterized by comparison of their experimental IR spectra with spectra calculated at the DFT(B3LYP)/6-311++G(d,p) level. The theoretical calculations predicted the existence of N dihedral angle. Both two low energy MA2C conformers, differing by the orientation of the O¼CC conformers were identified in the studied matrices. Both narrowband tunable and broadband UV irradiations of matrix-isolated MA2C yielded isomerization photoproducts resulting from cleavage of the C C and weakest C N bonds of the aziridine ring. Irradiation with UV laser-light at l = 235 nm resulted in the formation of the E isomer of methyl 2-(methylimino)-acetate (MMIA) and the Z isomer of methyl 3-iminopropanoate (M3IP). Subsequent irradiation at 290 nm led to observation of new bands resulting from E ! Z isomerization of MMIA, while bands due to M3IP remained unchanged. The photoproduced Z isomer of MMIA could be subsequently consumed upon higher-wavelength irradiation (l = 330 nm). The initially produced MMIA conformer was found to obey the nonequilibrium of excited rotamers (NEER) N bond of the MA2C principle. No photoproducts resulting from the cleavage of the strongest C aziridine ring were observed, nor that of methyl 3-aminoacrylate (M3AA), which could in principle be obtained also by cleavage of the weakest CN bond of the MA2C aziridine ring, but would imply a different H-atom migration simultaneous with the ring opening process. These results indicate that both N bonds of substituted aziridine rings and the type of the differential electronic characteristics of the C required H-atom migration are major factors in determining the specific photochemistries of substituted aziridines. Photofragmentation reactions of MA2C were also observed, through identification of various related products, e.g., acetonitrile, methanol, methane, CO and CO2. ã 2015 Elsevier B.V. All rights reserved.

Keywords: Methyl aziridine-2-carboxylate Matrix isolation infrared spectroscopy DFT calculations Photoisomerization Photofragmentation

1. Introduction Aziridines are saturated three-membered heterocycles containing a nitrogen atom, which occur naturally and have innumerable applications [1,2]. The best known examples of natural products containing an aziridine ring include biologically active compounds such as mitomycins, carzinophilin and azinomycins, which possess strong antibiotic and antitumor properties [3–6]. Aziridines are also extremely versatile compounds used in the synthesis of molecules such as amino acids, nitrogencontaining larger-ring heterocycles and alkaloids, chiral ligands, natural products, pharmaceutical intermediates, etc., via ringopening or ring-expansion reactions [1–18]. In particular, aziridine-2-carboxylates and their derivatives have been used as

* Corresponding author. E-mail address: [email protected] (S. Lopes). http://dx.doi.org/10.1016/j.vibspec.2015.10.003 0924-2031/ ã 2015 Elsevier B.V. All rights reserved.

intermediates in the synthesis of a- and b-amino acids, both natural and non-natural, by stereospecific ring-opening reactions of the heterocyclic ring with nucleophiles, including organometallic reagents [19–24]. The reactivity of aziridines towards ring-opening and ringexpansion relates to their extremely high ring strain (above 110 kJ mol1), the CN bond cleavage chemistry dominating the reactivity of these compounds [1,25,26]. The nature of the substituent at the N-atom plays a crucial role in the nucleophilic ring-opening of aziridines through CN bond cleavage. According to their trend to undergo CN bond cleavage, aziridines can be classified into two groups, “activated” and “non-activated” [1,2,9,26]. The first group aziridines bear electron-withdrawing substituents at the N-atom such as tosyl or acyl functional groups [15,21], which can stabilize the negative charge of the nitrogen atom and increase the reactivity of the aziridine ring towards nucleophiles. Non-activated aziridines have non-oxygenated substituents at the N-atom, such as the hydrogen atom, alkyl or aryl

S. Lopes et al. / Vibrational Spectroscopy 81 (2015) 68–82

functional groups. In this case, the aziridine ring is more stable, and less reactive towards nucleophiles [13,14,18]. The photochemical cleavage of the aziridine C N bonds has also been found to be preferred in the gas phase to the alternative ring-opening through cleavage of the C C bond, particularly for molecules free of substituents at the carbon atoms [27,28]. However, for aziridines bearing electron withdrawing substituents at the ring carbon atoms, the C C bond cleavage reactions become competitive [27]. The photochemical CC ring-opening in aziridines is a disrotatory process, yielding azomethine ylides as primary photoproducts (see Scheme 1) [29,30]. The latter can then undergo intramolecular rearrangements via [1,2] hydrogen-atom shift or participate in 1,3-dipolar cycloadditions; less frequently, in addition reactions to nucleophiles [31–36]. The matrix isolation technique was previously applied in the identification and characterization of azomethine ylides resulting from the photochemical CC bond cleavage of phenyl-substituted aziridines in Freon matrices at 77 K [37]. Nevertheless, in what way different substituents in the aziridine ring influence the preference for the carbon-nitrogen or the carboncarbon bond cleavage reactions is not yet well understood [27,38,39]. In the present study, we report on the UV-induced photochemical behavior of a compound that belongs to the class of nonactivated aziridines (bearing a hydrogen atom at the ring nitrogen), and have a carboxylate substituent at one of the ring carbon atoms. For this compound one can then expect that both types of photochemistries (C N and C C bond cleavages) may operate. Monomers of methyl aziridine-2-carboxylate (MA2C) were isolated in inert cryogenic matrices and their unimolecular photochemistry was followed by the interpretation of the IR spectra of the starting compound and of the resulting photoproducts. In addition to ring-opening photoreactions, UV irradiation using laser light was found to lead to the observation of additional products originating from photofragmentation reactions. A detailed theoretical investigation of the potential energy surfaces of MA2C and of the possible photoproducts resulting from both C C and C N bond cleavages of the aziridine ring was also carried out at the DFT(B3LYP)/6-311++G(d,p) level of theory. To the best of our knowledge, no studies on matrix-isolated MA2C and its photochemistry have been reported hitherto. 2. Experimental and computational methods Methyl aziridine-2-carboxylate (MA2C) (97% purity) was purchased from TCI Europe. Prior to usage, MA2C was additionally purified by the standard freeze-pump-thaw technique. The MA2C vapors were premixed with argon and xenon (N60 and N48, respectively, both supplied by Air Liquide) at a ratio of 1:1000 in a 3 L Pyrex glass reservoir to a pressure of 800 mbar, using the standard manometric procedure. Matrices were prepared by deposition of the mixtures onto a CsI substrate cooled to 15 K (Ar) or 30 K (Xe) assembled inside the cryostat (APD Cryogenics closed-cycle helium refrigeration system, with a DE-202A expander). The temperature of the (MA2C : inert gas) mixture

69

prior to deposition was 298 K, this temperature then defining the conformational composition of MA2C monomers. The IR spectra, in the 4000–400 cm1 range, were obtained using a Mattson (Infinity 60AR Series) or a Thermo Nicolet 6700 Fourier transform infrared spectrometer, equipped with a deuterated triglycine sulphate (DTGS) detector and a Ge/KBr beam splitter, with 0.5 cm1 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. The broadband irradiation was carried out with UV light provided by a 500 W Hg (Xe) lamp (Newport, Oriel Instruments), with output power set to 200 W, through the outer KBr window of the cryostat. A series of longpass optical filters, transmitting UV-light with l > 397, 367, 328, 313 and 288 nm, was used. No change was observed in the spectra of MA2C upon irradiation under these conditions. Photochemical changes were observed only when irradiation was performed without using any filter, i.e., for l > 235 nm (this value is defined by the absorbance edge of KBr in UV). Matrices were also irradiated with tunable UV light provided by the frequency doubled signal beam of the Quanta-Ray MOPO-SL pulsed (10 ns) optical parametric oscillator (FWHM 0.2 cm1, repetition rate 10 Hz, pulse energy 1 mJ) pumped with a pulsed Nd:YAG laser. The quantum chemical calculations were performed using the Gaussian 03 program package [40] at the DFT level of theory, using the split valence triple-z 6-311++G(d,p) basis set [41] and the three-parameter B3LYP density functional, which includes Becke’s gradient exchange correction [42] and the Lee-Yang-Parr correlation functional [43]. In the case of MA2C, the structures of conformers were optimized at both the DFT and MP2 levels of theory. Relaxed one-dimensional (1-D) potential energy scans and two-dimensional (2-D) potential energy maps were calculated to locate the minima of the possible photoproducts resulting from MA2C ring-opening reactions (for both C C and CN bond cleavages). Transition state structures were located using the synchronous transit-guided quasi-Newton (STQN) method [44]. The nature of all described stationary points on the studied potential energy surfaces (PES) was characterized through the analysis of the corresponding Hessian matrices. Calculated vibrational frequencies and IR intensities were used to assist the analysis of the experimental spectra. The computed harmonic wavenumbers were scaled down by a single factor (0.978) to correct them mainly for the effects of basis set limitations, neglected part of electron correlation and anharmonicity effects. Normal coordinate analysis was undertaken in the internal coordinates space, as described by Schachtschneider and Mortimer [45], using the optimized geometries and harmonic force constants resulting from the DFT(B3LYP)/6-311++G(d,p) calculations. The internal coordinates used in this analysis were defined following the recommendations of Pulay et al. [46]. 3. Results and discussion 3.1. Calculated structures and energies for MA2C

Scheme 1. The CC bond cleavage in aziridine yields azomethine ylide. The ringopening processes induced by thermolysis (bottom) and photolysis (top) are conrotatory and disrotatory, respectively.

The MA2C molecule has one chiral center, carbon atom C3 (see Fig. 1 for atom numbering). The geometric parameters of MA2C reported in this work refer to the S-enantiomer. There are three intramolecular degrees of freedom in MA2C that lead to different conformers: (i) rotation of the ester group around the exocyclic C1C3 bond; (ii) rotation of the methoxy group around the C1O10 bond; (iii) inversion at the pyramidal nitrogen atom. The three above listed coordinates can assume two minimum-energy orientations each. They correspond approximately to cis and trans orientations around the (i) O2¼C1 C3 N4, (ii)

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S. Lopes et al. / Vibrational Spectroscopy 81 (2015) 68–82 Table 2 DFT(B3LYP)/6-311++G(d,p) and MP2/6-311++G(d,p) calculated values for the conformationally relevant dihedral angles ( ) of the eight conformers of MA2C.a Conformer

I II III IV V VI VII VIII

O2¼C1C3N4

O2¼C1O10C11

H9N4C3C1

DFT

MP2

DFT

MP2

DFT

MP2

21.2 158.3 42.3 141.6 12.5 100.6 173.8 140.9

22.4 157.5 40.3 141.6 11.6 100.9 166.5 –

0.7 1.4 0.2 2.3 175.3 173.8 165.9 147.8

0.6 1.8 0.3 2.3 173.2 171.5 160.6 –

3.6 4.8 145.9 143.0 2.8 146.3 146.1 12.1

4.1 5.1 147.6 144.7 2.8 147.7 147.5 –

a See Fig. 1 for the optimized structures of the two most stable conformers of MA2C and for atom numbering; see Figure S1 for high-energy conformers of MA2C.

Fig. 1. DFT(B3LYP)/6-311++G(d,p) optimized structures of the two most stable conformers of methyl aziridine-2-carboxylate (MA2C), with atom numbering adopted in this work. Colors: C—grey, H—white, O—red, N—blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

O2¼C1 O10 C11, and (iii) H9 N4 C3 C1 dihedral angles, respectively. The latter coordinate is related with the sp3 hybridization of N4 that renders two possible orientations of the NH group with respect to the plane of the aziridine ring. The combinations of all possible orientations produce eight unique minima for the MA2C monomer. The relative calculated energies of the eight conformers are presented in Table 1, and their conformationally relevant dihedral angles are presented in Table 2. The internal coordinates for calculated optimized geometries of the eight conformers are given in Table S1 (Supporting information). Optimized geometries of the two most stable MA2C conformers are depicted in Fig. 1. Those of the higher energy conformers are shown in Fig. S1 (Supporting information). The theoretical calculations predicted the existence of two lowenergy minima, both belonging to the C1 symmetry point group (see Fig. 1). In both conformers, the O¼C O CH3 group is in the cis orientation and the N H bond is positioned in the antiorientation regarding the C3 H8 bond of the aziridine ring. Such orientation signifies that the NH and the ester groups are positioned on the same side of the aziridine ring and the NH group is in a close vicinity of one of the oxygen atoms. This fact

results in a stabilizing interaction of the NH group with either the C¼O oxygen lone electron pairs (in conformer I) or the C O oxygen lone electron pairs (in conformer II). The different strengths of the NH  O¼ and NH  O interactions is a factor of stabilization of conformer I in relation to form II, since it is wellknown that a carbonyl oxygen is a better H-bond acceptor than an ester oxygen atom [47,48]. Higher energy structures (see Fig. S1) were obtained when the ester group is in the trans orientation (O¼C O CH3 dihedral angle is about 180 ; conformers V–VIII) or the NH bond is in the cis position regarding the C3 H8 bond of the aziridine ring (conformers III, IV, VI, VII). A relevant structural feature, common to both low energy conformers of MA2C is the fact that the two C N bond lengths are considerably different: the C3 N4 bond being longer (i.e., weaker) than the C5 N4 one (1.470 vs. 1.458 Å in form I, and 1.472 vs. 1.459 Å in form II). According to the DFT calculations, the energy (including the zero-point correction) of form II is by 6.4 kJ mol1 higher than that of I. At the MP2 level, the difference in energy between I and II was predicted as 5.8 kJ mol1. Their calculated relative Gibbs free energies at room temperature (298 K) are similar: 6.3 and 5.9 kJ mol1, at the DFT and MP2 levels, respectively. Considering the calculated relative stabilities of I and II, their expected gas phase equilibrium populations at room temperature were estimated as 92.7% and 7.3%, respectively. Fig. 2 shows the potential energy profile associated with the internal rotation of the ester group around the exocyclic C1 C3 bond in MA2C, corresponding to the interconversion between the two most stable conformers. The calculated energy barrier for this process is no less than 20 kJ mol1 in both directions. Therefore, the relatively high energy barriers shall prevent occurrence of conformational cooling during deposition of

Table 1 DFT(B3LYP)/6-311++G(d,p) and MP2/6-311++G(d,p) calculated relative energies (DE/kJ mol1) including zero-point contributions and calculated relative Gibbs energy difference at 298 K (DG/kJ mol1) for the eight conformers of MA2C. Conformera

DEDFT

DEMP2

DGDFT

DGMP2

I II III IV V VI VII VIII

0.0 6.4 13.6 14.6 33.1 50.4 54.8 73.5

0.0 5.8 13.1 14.4 33.5 50.0 57.5 –

0.0 6.3 12.6 13.3 33.4 48.5 52.2 73.1

0.0 5.9 12.2 13.6 33.6 48.5 56.3 –

a

See Fig. 1 and Fig. S1 for optimized structures of the conformers of MA2C.

Fig. 2. Calculated DFT(B3LYP)/6-311++G(d,p) potential energy profile for internal rotation of the ester group around the exocyclic C1C3 bond corresponding to the interconversion between the two most stable conformers of MA2C.

S. Lopes et al. / Vibrational Spectroscopy 81 (2015) 68–82

the matrices [49–51] under the used experimental conditions, and the two conformers present in the gas phase are expected to be effectively trapped in the samples. Other MA2C conformers have calculated energies higher than 12 kJ mol1, and are not experimentally relevant (see Table 1). 3.2. Infrared spectra of matrix isolated MA2C (as-deposited matrices) The infrared spectra of MA2C monomers isolated in freshly deposited argon and xenon matrices are presented in Fig. 3 (traces a and b). On the whole, the two experimental spectra are quite similar. The DFT calculated spectra of the two most stable conformers are also shown as stick spectra for comparison in Fig. 3d. Intensities in these stick spectra were not scaled. However, in the simulated sum spectrum (Fig. 3c) intensities were weighted by the expected equilibrium populations of the two conformers in the gas phase at 298 K (0.927 for conformer I and 0.073 for conformer II). The good agreement between the experimental and the calculated spectra allowed an easy assignment of the fundamental bands (Table 3). The definition of the internal symmetry coordinates adopted in the vibrational analysis for the two most stable conformers of MA2C is provided in Table S2 (Supporting information). The calculated wavenumbers, infrared intensities and potential energy distributions resulting from normal mode analysis carried out for these conformers are presented in Tables S3 and S4 (Supporting information). In agreement with the predicted relative energies, conformer I is the main species present in the matrices. Nevertheless, it is possible to observe several bands which show unequivocally that the less populated conformer II is also present in the matrices.

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Fig. 4 displays selected spectral regions of the infrared spectrum of MA2C isolated in xenon (T = 30 K), compared with the theoretical calculated spectra of conformers I and II. This Figure shows more precisely the position of the bands ascribed to the more energetic conformer. The bands observed at 1752, 1355/1353, 1080, 1031 and 824 cm1 in the xenon matrix are ascribable to n(C¼O), g (CH8), wag(CH2), n(CN), n(O CH3) and n(C3C5) vibrations of conformer II (see also Table 3). These vibrations have strong predicted intensities in infrared and do not overlap with vibrations of conformer I. Their counterparts were also observed for MA2C monomers isolated in argon (1758/1755, 1352, 1083, 1039/1037 and 822 cm1), clearly testifying the presence of conformer II in that matrix as well. Some additional less intense bands ascribable to conformer II, located at 1268, 892/887 and 625 cm1 in the spectrum of the compound in xenon, are also observed. These bands have counterparts in the argon matrix spectrum at 1270 (shoulder), 891/889 and 633/629 cm1, respectively. In the 1760–1720 cm1 spectral region, two doublets of bands, at 1758/1755 and 1748/1742 cm1 are observed in the argon matrix spectrum. These bands are assigned to the n(C¼O) vibration of conformers II and I, respectively. The doublets are the result of matrix-site effects. In the xenon matrix spectrum, however, the n(C¼O) vibrations give rise to single bands at 1752 cm1 (form II) and 1740 cm1 (form I). Another interesting feature concerning the n(C¼O) vibration is the observation of the first overtone band for this mode at ca. 3468 cm1 (Ar) and 3462 cm1 (Xe) [52]. The triplet of bands at ca. 2910/2862/2821 cm1 (Ar) and their counterparts for the molecule isolated in xenon (2898/2854/ 2818 cm1) can also be assigned as an overtone (2dsCH3). The n(NH) vibration gives rise to bands of very weak intensity at 3301/3298 and 3289/3286 cm1, for argon and xenon matrices,

Fig. 3. Experimental FTIR spectra of methyl aziridine-2-carboxylate (MA2C) isolated in an argon matrix at 15 K (a), and in a xenon matrix at 30 K (b), compared with the infrared spectrum simulated for the gas-phase equilibrium mixture of conformers I and II (c), theoretical infrared spectra of individual conformers (d) calculated at the DFT (B3LYP)/6-311++G(d,p) level. The sum spectrum (c) was simulated using Lorentzian functions (fwhm = 2 cm1) centered at the calculated wavenumbers (scaled by a single factor of 0.978). Calculated intensities of the bands due to individual conformers were weighted by their expected populations (0.927 for I and 0.073 for II), corresponding to the calculated relative Gibbs energy difference (6.3 kJ mol1) at room temperature. Intensities in the stick spectra (d) are not scaled.

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S. Lopes et al. / Vibrational Spectroscopy 81 (2015) 68–82

Table 3 Experimental (matrix-isolation) and DFT(B3LYP)/6-311++G(d,p) calculated infrared data for MA2C and vibrational assignments based on the results of normal coordinate analysis.a

a Wavenumbers (cm1, scaled by 0.978), calculated intensities (km mol1), s: symmetric; a: antisymmetric; n: stretching; d: in-plane bending; g: out-of-plane bending; t: torsion; rock: rocking; wag: wagging; twist: twisting, n.obs.: not observed; n.i.: not investigated. See Table S2 for definition of symmetry coordinates and Tables S3 and S4 for potential energy distributions. bWhen different types of vibrations in the two conformers are assigned to a common band, the assignments are separated by semi-colon, the first description corresponding to conformer I and the second to conformer II.

S. Lopes et al. / Vibrational Spectroscopy 81 (2015) 68–82

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Fig. 4. (a) Selected regions of the experimental FTIR spectrum of methyl aziridine-2-carboxylate (MA2C) isolated in a xenon matrix at 30 K, compared with (b) the theoretical infrared spectra of individual conformers I and II calculated at the DFT(B3LYP)/6-311++G(d,p) level of theory. The calculated frequencies were scaled by a single factor (0.978). Calculated intensities were weighted by the expected populations of the individual conformers (0.927 for I and 0.073 for II) at room temperature. The ordinate scales are expanded to afford a better visibility of the bands due to the minor conformer II (thus the high intensity bands in the spectra of conformer I are truncated).

1,2-A

4

H

N

H 5

1

H

C R

3

H5

N

7

2C

R

H6

H4

1

C

3

2

C

H 4

H

1,2

6

R

1,3

H

H4

N

1

C

H

3

1,3-B

H6

H7

1,2-B

1

R

7

C

C2 H5

H5

2

N

H C 3

1,3-A

4

N1 2

R

6

3.3. Calculated structures and energies for putative photoproducts of MA2C

C

3

H

6

C H5 H7

H7

2,3 2,3-A

2,3-B 1

R

2

N

C H

5

H4

1 3

C H

H6 7

R

2

C H5

N

3

H

6

C H4 H

7

Scheme 2. Possible structural rearrangements in 2-substituted aziridines resulting from different ring-opening reactions. Pairs of numbers next to the arrows specify the atom numbers where the bond cleavage occurs. Considering the case of MA2C, R equals to C(¼O)OCH3 for the reactant and products resulting only from the ringopening reactions, while R equals to OCH3 or CH3 in the case of occurrence of decarbonylation or decarboxylation, respectively, of MA2C or of its open-ring photoproducts.

respectively. Similar (very weak) n(NH) bands were observed at 3390–3360 cm1 for structurally similar molecule of proline [53,54] where the NH group is inserted in a ring, and interacts with a COOH group attached to the a-carbon atom. The position of the n(NH) experimental bands of MA2C is a clear indication of formation of intramolecular H-bonds involving the NH group, since these bands show greater frequency red-shifts compared to proline, or other amino acids [55]. This is in agreement with the structural results discussed above, which showed that in the two lowest energy conformers of MA2C the specific orientation of the NH and the ester groups, in particular their position on the same side of the aziridine ring, favors the establishment of an intramolecular H-bond of N H  O¼C type in conformer I and of N H  O C type in conformer II.

Upon UV-excitation in gas phase, the unsubstituted parent aziridine yields photoproducts such as ethylene, ethane, methane, atomic and molecular hydrogen and nitrogen, ammonia, and the methyl radical [28,56], testifying the complexity of the photochemistry of this compound. As mentioned in the Introduction, two main reaction mechanisms regarding ring-opening of aziridines are possible: through C C or C N bond cleavages. Scheme 2 shows different possibilities of the ring-opening reaction for the 2-substituted aziridine system. The possible reaction paths are depicted by pairs of numbers which specify where the bond cleavage occurs. Two reaction paths for the cleavage of the CN bonds are shown (1,2 and 1,3) and one for the C C bond rupture (2,3). Atom numbering in Scheme 2 differs from that adopted for MA2C, since Fig. 5 has a more general character, and is proposed as a general complete set of possibilities for 2-substituted aziridines. According to Scheme 2, the starting compound undergoes H-atom shifts which lead to a total of six potential photoproducts, numbered as 1,2-A and 1,2-B (C2 N1 bond cleavage); 1,3-A and 1,3-B (C3 N1 bond cleavage), and 2,3-A and 2,3-B (C2 C3 bond cleavage). For MA2C, where R is a methyl ester group, only structures with the usually most stable cis orientation (O¼COCH3 dihedral angle is about 0 ) will be here considered. Furthermore, the general character of Scheme 2 easily accounts for the possibilities of decarbonylation (R = OCH3) or decarboxylation (R = CH3) of MA2C and of the corresponding photoproducts. 3.3.1. Aziridine ring C C bond cleavage As already mentioned, considering the structure of MA2C, which is a non-activated aziridine for the C N bond cleavage bearing an electron withdrawing ester substituent at a ring carbon atom, one can expect that both C N and CC photochemical ringopening reactions take place upon UV irradiation of the matrix isolated compound [27,28]. According to the mechanism generally assumed, the initial step in the cleavage of the C C bond of the aziridine ring (2,3 in Scheme 2) shall result in formation of the planar intermediate, azomethine ylide (see also Scheme 1). Subsequently, a [1,2] hydrogen atom shift shall occur, leading to the possible formation of two species: methyl 2-(methylene amino)-acetate, MMAA (or 2,3-A in Scheme 2) and methyl 2-(methylimino)-acetate, MMIA (or 2,3-B). In channel (A) the hydrogen atom of the N H bond migrates towards the C H group, while in channel (B) the hydrogen atom shift occurs in the

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S. Lopes et al. / Vibrational Spectroscopy 81 (2015) 68–82 Table 4 DFT(B3LYP)/6-311++G(d,p) calculated relative energies (DE/kJ mol1) including zero-point vibrational contributions for relevant structures of possible photoproducts resulting from different ring-opening reactions of MA2C (Scheme 2). Molecule

Fig. 5. DFT(B3LYP)/6-311++G(d,p) calculated relaxed potential energy map of MMAA as a function of CCN¼C and O¼CCN dihedral angles. These dihedral angles were incremented in steps of 20 and all remaining internal coordinates were optimized at each point. Across this map, the ester group was kept in a cis orientation (and optimized). Each conformer corresponds to two equivalent-bysymmetry minima represented by squares (&, conformer I), black circles (*, II) and triangles (~, III). First-order transition states interconnecting the conformers are represented by crosses. Energies are relative to the most stable conformer (I). Isoenergy levels are spaced by 2 kJ mol1 (an additional level at 7 kJ mol1 is shown by dotted lines). See Fig. 6 for structures I-III, as well as Fig. 7 for corresponding onedimensional potential energy profiles along the lines indicated in color.

MMAA I II III MMIA I (E) II (E) III (Z) IV (Z) M3IP I (E) II (E) III (E) IV (Z) V (Z) VI (Z) VII (Z) M3AA I (Z) II (Z) III (E) IV (E) M2IP I (Z) II (Z) III (E) IV (E) M2AA I II

DEa

DEb

Symmetry

0.0 1.3 6.0

87.0 88.3 93.0

C1 C1 C1

0.0 2.1 11.3 17.5

74.7 76.8 85.9 92.2

Cs Cs C1 C1

0.0 3.4 6.9 6.8 6.9 7.2 10.8

47.4 50.8 54.2 54.2 54.3 54.6 58.1

C1 C1 C1 C1 C1 C1 C1

0.0 12.4 14.7 19.1

0.0 12.4 14.7 19.1

Cs Cs C1 C1

0.0 1.6 12.0 18.6

35.3 36.9 47.3 53.8

Cs Cs Cs C1

0.0 4.8

37.6 42.4

C1 C1

a

Relative energies to the most stable structure of the specified molecule. Relative energies to the lowest energy structure among all possible forms (M3AA I). b

Fig. 6. DFT(B3LYP)/6-311++G(d,p) optimized structures of the methyl 2-(methyleneamino)-acetate (MMAA) conformers. Color codes: carbon—grey, hydrogen— white, oxygen—red, nitrogen—blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

opposite direction, towards the CH2 group, rearranging into a methyl group. For the putative MMAA product, internal rotation around C C and C N single bonds may afford several conformers, while in case of MMIA both Z–E isomerism about the C¼N bond and conformational isomerism about the CC bond may exist (rotation about the C O bond in both MMAA and MMIA can also give rise to cis and trans O¼COCH3 conformers, but the high-energies of trans forms make them of no practical relevance). Fig. 5 presents the relaxed potential energy of MMAA as a function of the CCN¼C and O¼CCN dihedral angles. In the performed calculations, these dihedral angles were incremented in steps of 20 and all remaining internal coordinates were optimized at each point. The ester group was kept in a cis orientation. The calculations predict three different minimum energy structures with relative energies of 0.0 (I), 1.3 (II) and 6.0 (III) kJ mol1 (see Fig. 6 and Table 4). Conformers I and II possess a trans-like orientation of the ¼CH2 and ester groups connected to the CN bond, being more stable than structure III, where the arrangement of these groups is nearly cis. On the other hand, the relative stability of I and II results mainly from the different strengths of the repulsive interaction between the nitrogen atom

Fig. 7. (a) One-dimensional (1-D) profile along the CCNC coordinate, showing the minimum-energy path (MEP) connecting structures I and III. (b) 1-D profile along the OCCN coordinate, showing the MEP for interconversion between conformers I and II. See positions of these 1-D MEPs on the 2-D map shown in Fig. 5. Zero-point vibrational energy levels along the chosen reaction coordinates are shown as horizontal lines. Note: the linear energy scales in frames (a) and (b) are equal.

S. Lopes et al. / Vibrational Spectroscopy 81 (2015) 68–82

75

Fig. 8. DFT(B3LYP)/6-311++G(d,p) optimized structures of the four most stable conformers of methyl 2-(methylimino)-acetate (MMIA). Color codes: carbon—grey, hydrogen—white, oxygen—red, nitrogen—blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

and the oxygen atoms of the ester substituent, which is stronger when it involves the more voluminous oxygen ester atom ( O ) instead of the carbonyl oxygen atom (¼O) [47,48]. The interaction between the nitrogen atom and the oxygen atoms (¼O and  O ) determines also the non-planarity of this fragment in these two conformers (see Table S5 for conformationally relevant dihedral angles). The structures with completely planar arrangements of all heavy atoms were found to be transition states. The energy barriers for II ! I and III ! I conversions in MMAA were also calculated, because of their potential interest for the interpretation of the experimental results. As shown in Fig. 7, the II ! I conversion implies internal rotation around the C C bond, with an energy barrier of 2.3 kJ mol1. In the case of the III ! I conversion, the driving coordinate is the CCN¼C dihedral angle, the associated energy barrier being 0.3 kJ mol1 only. Noteworthy, the zero-point vibrational energy of structure III, for the coordinate transforming it into structure I (CCN¼C torsion), is equal to 0.3 kJ mol1, i.e., exactly the same as the III ! I energy barrier. Therefore, structure III cannot be treated as an independent conformer, but should be considered only as an excited vibrational state of conformer I, so that, in practical terms MMAA has only two low energy conformers (I and II). In the case of MMIA, calculations allowed identification of four low energy conformers (Fig. 8), two having an E configuration about the C¼N bond (I and II) and two possessing a Z arrangement about this bond (III and IV). Conformer I is the lowest energy MMIA form, with conformer II being only 1.3 kJ mol1 higher in energy than I (see Table 4). The energy barrier separating these two conformers was calculated to be higher than 10 kJ mol1 in both directions. Forms III and IV have relative energies of 11.3 and 17.5 kJ mol1, respectively, comparing to I. The heavy atoms in two E conformers are planar, while the Z forms are non-planar due to the repulsion between the closely located methyl and ester groups in these forms. An interesting point to note is that disrotatory photochemical C C ring-opening in MA2C starting from the most stable conformer of the reactant (MA2C form I) can only give rise to conformers II and III of MMIA, while conformer II of MA2C can only lead to production of MMIA forms I and IV. Comparing now the relative energies of the two putative products resulting from cleavage of the CC bond of the aziridine ring of MA2C, it can be seen that MMIA has lower energy compared to MMAA (the calculated energy difference between the most stable forms of the two compounds is 12.3 kJ mol1; see Table 4). This can be explained taking into account the extended p

Fig. 9. (a) DFT(B3LYP)/6-311++G(d,p) calculated relaxed potential energy map showing the position of the three conformers of E M3IP as a function of N¼CCC and CCC¼O dihedral angles. These dihedral angles were incremented in steps of 20 and all remaining internal coordinates were optimized at each point. Across this map, the ester group was kept in a cis orientation (and optimized). Each conformer corresponds to two equivalent-by-symmetry minima and is represented by black circles (*). First-order transition states interconnecting the conformers are represented by crosses, X. Energies are relative to the most stable conformer (I) and do not include zero-point vibrational corrections. Isoenergy levels are spaced by 3 kJ mol1 (an additional level at 4 kJ mol1 is shown by dashed lines). The minimum-energy pathways for conformational interconversion, corresponding to one-dimensional profiles (b) and (c) are shown in different colors (I+ ! II+, dark blue and red); (I+ ! III+ ! II–, or I ! III– ! II+, pink); (I+ ! I, green) and (II + ! II, cyan); (b) and (c) minimum-energy paths for interconversion between conformers I, II and III of E M3IP along the relevant coordinates shown in (a). Equivalent by symmetry minima are denoted by + and –. Note: the linear energy scales in (b) and (c) are equal. See Fig. 10 for conformer structures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

delocalization in MMIA, where the C¼O and C¼N bonds are conjugated, while in MMAA the conjugation is interrupted due to the presence of the methylene group between these two double bonds. 3.3.2. Aziridine ring CN bond cleavages The second type of photochemical process analysed is the cleavage of one of the C N bonds of the aziridine ring: channels 1,2 and 1,3 in Scheme 2. The simultaneous migration of the hydrogen atom (H6) towards the nitrogen and formation of the double C¼N bond between C3 and N1 and the rupture of the weakest C N bond (1,2) yields product 1,2-A (methyl 3-iminopropanoate, M3IP). In this case, besides three internal

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S. Lopes et al. / Vibrational Spectroscopy 81 (2015) 68–82

180

HN=CH trans (Z M3IP) 8

N=CCC dihedral angle / degrees

6

VI

2

12 0

-60

4

6

8 12

6

10

2

V−

5 12

10

8 10

10

5

4

4 8

60

8

6

IV

2

120

(a)

5

10

12 12

6

VII V+

6

8

10

5

12

4 -120 5 8 -180 -180

4

2

2

5

6 -120

6 -60

0

8

60

120

180

CCC=O dihedral angle / degree

(b) 4 2

VI

V−

V+

0

Fig. 10. DFT(B3LYP)/6-311++G(d,p) optimized structures of the conformers of methyl 3-iminopropanoate (M3IP) with the HN¼CH fragment in cis (E-M3IP) and trans (Z-M3IP) position. Color codes: carbon—grey, hydrogen—white, oxygen—red, nitrogen—blue. Note that E and Z notation is applied as a function of the HN¼CC (not HN¼CH) dihedral.

degrees of freedom that can give rise to different conformers (the internal configuration of the ester group, and the rotations around the two CC bonds), the arrangement of the HN¼CH moiety itself can give rise to different isomers, as the HN¼CH dihedral angle can be either cis or trans (corresponding to E and Z isomeric M3IP species, respectively, defined as functions of the HN¼CC dihedral). Two independent potential energy scans were performed, taking into consideration the two possible orientations of the HN¼CH fragment. Fig. 9(a) presents the 2-D PES of M3IP with the HN¼CH unit in a cis position as a function of N¼CCC and O¼CCC dihedral angles. The PES shows three different minima, with relative energies within 7 kJ mol1, defined by the conformationally relevant dihedral angles N¼CC C (135.3, 123.9 and 1.9 ) and C CC¼O (21.2, 137.4 and 95.5 ), for conformers I–III, respectively (see also Table S5). As shown in Fig. 9, each conformer has a symmetryequivalent mirror form. Relaxed potential energy profiles interconnecting the three conformers are presented in Fig. 9(b) and (c). The weak hydrogen bond formed between the carbonyl oxygen atoms and the hydrogen atom of the HN¼CH fragment, closing an intramolecular five-membered ring H C C(H) C¼O, is the main factor responsible for the stabilization of the lowest energy form I with respect to conformers II and III (see Fig. 10). It should be noticed that the interaction of the hydrogen atom with the lone pair of the carbonyl oxygen can be expected to be more important than with those of the methoxylic ester oxygen (as in conformer II), because in the first case the hybridization brings the maximum of electron density in the plane of the molecule and in the second case out of the plane.

Relative Energy / kJ mol

−1

120

60

0

N=CCC / º

60 CCC=O / º

(c)

VII

4 2

IV

0 30

90

30

N=CCC / º

-30

CCC=O / º

(d) 4 2 0 -180

VI

-120

IV

-60

0

60

120

CCC=O dihedral angle / º Fig. 11. (a) DFT(B3LYP)/6-311++G(d,p) calculated relaxed potential energy map showing the position of the four conformers of Z M3IP as a function of N¼CCC and CCC¼O dihedral angles. These dihedral angles were incremented in steps of 20 and all remaining internal coordinates were optimized at each point. Across this map, the ester group was kept in a cis orientation (and optimized). Each conformer corresponds to two equivalent-by-symmetry minima and is represented by black circles (*). First-order transition states interconnecting the conformers are represented by crosses, X. Energies are relative to the most stable conformer (IV) and do not include zero-point vibrational corrections. Isoenergy levels are spaced by 2 kJ mol1 (an additional level at 5 kJ mol1 is shown by dashed lines). The minimum-energy pathways for conformational interconversions given by different colors in (a), are shown as one-dimensional profiles of the same color connecting: (b) conformers VI and V; (c) conformers VII and IV; (d) conformers IV and VI. Colors: (VI ! IV), dark blue; (V+ ! V), pink; (V ! VI), green; (IV ! VI), red and (IV ! VII), purple. V+ and V are the two equivalent-by-symmetry forms of conformer V. See Fig. 10 for conformer structures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The energy barriers between the various conformers of E M3IP are rather low. The barriers between the two equivalent-bysymmetry forms I and between the two equivalent-by-symmetry forms II were calculated as being 3 and 6 kJ mol1, respectively. Those interconverting forms I and II, by rotation about the C C(¼O) bond, are ca. 2 and 3.5 kJ mol1 (see Fig. 9(c)). Finally, the energy barriers associated with the III ! I and III ! II processes amount both to 6 kJ mol1 (see Fig. 9(b)).

S. Lopes et al. / Vibrational Spectroscopy 81 (2015) 68–82

Fig. 12. DFT(B3LYP)/6-311++G(d,p) optimized structures of the four most stable conformers of methyl 3-aminoacrylate (M3AA) and relaxed potential energy profiles for internal rotation around the CC bond in both Z (I and II, blue circles) and E (III and IV, red squares) isomers. Color codes: carbon—grey, hydrogen—white, oxygen—red, nitrogen—blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The PES of the Z isomer of M3IP (HN¼CH trans) is shown in Fig. 11. It exhibits four different minima, all belonging to the C1 symmetry point group, with relative energies within 4 kJ mol1 (see Fig. 10 and Table 4). The conformationally relevant N¼CC C and C C—C¼O dihedral angles corresponding to conformer V are (16.0 and 41.2 ), IV (134.6 and 48.2 ), VI (120.4 and 119.4 ) and VII (23.4 and 119.5 ) (see also Table S5, in the Supporting information). The potential energy profiles for interconversion between these conformers are presented in frames (b–d), where one can see that the interconversion energy barriers between the different conformers are low, with energies varying from 1 to 5 kJ mol1. The most striking result is the comparatively close energies of the three most stable Z forms, conformer V being less stable than conformer IV by only 0.1 kJ mol1, whereas for VI the relative energy value is 0.4 kJ mol1. For this set of structures, two weak hydrogen bonds can be formed between the carbonyl oxygen or methoxylic ester oxygen atoms and the hydrogen atoms of the HN¼CH fragment, closing into an intramolecular six-membered ring H N¼C C C¼O or a fivemembered ring HC C C¼O. These are the foremost important factors to consider in the stabilization of these minima. The six-membered ring is present in forms V and VII, where the ¼NH  O distances are 2.2 and ca. 2.3 Å, respectively. In conformers IV and VI, the five-membered ring ¼CH  O distances are ca. 2.7 Å. Compared to the possible products resulting from cleavage of the aziridine ring CC bond (MMAA and MMIA), the energy of

77

Fig. 13. DFT(B3LYP)/6-311++G(d,p) optimized structures of the conformers of methyl 2-iminopropanoate (M2IP) and relaxed potential energy profiles for internal rotation around the CC bond in both Z (I and II, blue circles) and E (III and IV, red squares) isomers. Color codes: carbon—grey, hydrogen—white, oxygen—red, nitrogen—blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

M3IP is much lower (by 39.6 and 27.3 kJ mol1, respectively, when the most stable forms are compared; Table 4). Product 1,2-B (methyl 3-aminoacrylate, M3AA; see Scheme 2) is also obtained by cleavage of the weakest aziridine ring N C bond (1,2), this time followed by formation of a new double bond between carbon atoms 2 and 3. The molecule may exist in two isomeric forms (E and Z) around the C¼C bond, each isomer having two conformers that can be interconverted by internal rotation around the central C C bond. The Z forms are more stable. Conformers II (Z), and III and IV (E) have energies higher than the most stable Z form (I) by 12.4, 14.7 and 19.1 kJ mol1, respectively (see Table 4). In all conformers, the N C¼C C and C¼C C¼O dihedral angles are close to either 0 or 180 . However, only conformers I and II belong to the Cs symmetry point group exhibiting a planar NH2 moiety, while in conformers III and IV the nitrogen atom lies in the molecular plane but is slightly pyramidalized, with the two amino hydrogen atoms twisted somewhat out of the plane (see Table S5 for values of the relevant dihedral angles. Imposing Cs symmetry during optimization of conformers III and IV yielded transition states with energies above the corresponding minima by less than 0.1 kJ mol1 (which is below the zero point level of the minimum energy structure). Conformer I is stabilized by a considerably strong intramolecular hydrogen bond of N H  O¼C type (|H  O| = 2.031 Å) in a six-membered ring arrangement, which justifies its low energy. Conformer II bears a weaker N H  O C interaction (|H  O| = 2.065 Å), while in the two E isomers the relative position

78

S. Lopes et al. / Vibrational Spectroscopy 81 (2015) 68–82 Table 5 Experimental (Ar and Xe matrices) and DFT(B3LYP)/6-311++G(d,p) calculated infrared data and vibrational assignments for the most intense predicted IR bands of the major observed bands of MMIA II (photoproduct of MA2C).a Ar matrix

Xe matrix

MMIA II

n

n

n

IIR

Approx. description

1772/1766 n.o. 1457 1356/1353 1203 1183 n.o. 1035 980 934/932 923 742

1764 1686 1457 1355 1201 1166 1117 1040/1035 993 936 925/924 n.o.

1770.0 1696.1 1460.8 1354.9 1187.4 1165.2 1131.2 1026.6 965.1 944.9 918.6 735.7

217.0 14.7 38.7 68.9 187.9 260.6 6.3 26.8 22.6 13.5 8.3 17.6

n(C¼O) n(N¼C) d2N(CH3) d(CH) d4O(CH3) n(CO) d4N(CH3) n(NCH3) n(OCH3) g (CH) n(CO) d(CC¼O)

a Wavenumbers (cm1, scaled by 0.978), calculated intensities (km mol1), n: stretching; d: in-plane bending; g : out-of-plane bending. See Table S6 for definition of internal coordinates and Table S8 for potential energy distributions for MMIA II.

Fig. 14. DFT(B3LYP)/6-311++G(d,p) optimized structures of the two most stable conformers of methyl 2-aminoacrylate (M2AA) and relaxed potential energy profile for internal rotation around the CC bond. Color codes: carbon—grey, hydrogen— white, oxygen—red, nitrogen—blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of the NH2 and C(¼O)OCH3 fragments precludes the establishment of any intramolecular H-bond between these two fragments, which are replaced by much weaker ¼CH  O contacts. The optimized structures for the four conformers of M3AA and the potential energy profiles for interconversion between the pairs of conformers (I, II) and (III, IV) are shown in Fig. 12. The energy barrier for the II ! I conversion amounts to ca. 50 kJ mol1, a value slightly larger than that corresponding to the IV ! III process (45 kJ mol1). Very interestingly, M3AA was predicted to have the lowest energy among all putative photoproducts of MA2C (see Table 4), with its lowest energy form I lying at least 35 kJ mol1 below the remaining species. The cleavage of the stronger aziridine C N ring bond (1,3) also yields two different products (see Scheme 2): 1,3-A (methyl 2-iminopropanoate, M2IP) is formed by concerted scission of the CN bond and migration of the hydrogen atom (H5) from C2 to C3, with subsequent rearrangement of the initial  CH2 group into a methyl group; and 1,3-B (methyl 2-aminoacrylate, M2AA), which results from the simultaneous migration of one the hydrogen atoms from C3 to N and formation of a C¼C double bond. M2IP may exist in two isomeric forms around the C¼N bond (E and Z), each one having two conformers that can be interconverted by internal rotation around the central C C bond (Fig. 13). The two Z conformers and the E conformer III have Cs symmetry, while conformer IV (E) belongs to the C1 point group. In the most stable form (I), the NH bond is oriented toward the carbonyl oxygen atom, while in form II it is oriented toward the other oxygen atom. Once again, the stronger N H  O¼C intramolecular H-bond forming a five-membered ring in form I, vs. the weaker N H  O C one in form II, is the key factor determining the relative energy of these two forms. Absence of any of these interactions in the E conformers of M2IP (III and IV) justifies their comparatively higher energies. The energy barriers associated with the interconversion between the pairs of M2IP conformers (I, II; Z forms) and (III,

IV; E forms) are also shown in Fig. 13. The II ! I barrier amounts to 15 kJ mol1, being considerably larger than that for the IV ! III isomerization, which has been predicted to be ca. 2 kJ mol1. In turn, the Cs structure separating the two equivalent-by-symmetry minima corresponding to conformer IV has an energy higher than that of the minima by less than 0.1 kJ mol1, which stays below the zero point level of the minima. In practical terms, when this is taken into account, this means that the most probable structure for conformer IV is of Cs symmetry, this conformer showing a comparatively larger conformational entropy associated with the large amplitude torsional vibration about the central C C bond. M2IP has been calculated to be the second species, in the increasing energy order, among all considered putative photoproducts of MA2C with the Z conformer I having an energy 35.3 kJ mol1 above that of the M3AA form I (see Table 4). The M2AA molecule bears only one conformationally relevant internal rotation axis, corresponding to the rotation around the C C bond. Two conformers were found (Fig. 14), both of them non-planar (Cs symmetry structures are first order transition states). Conformer I, having an N H  O¼C intramolecular hydrogen bond is more stable than form II, which bears a weaker N H  O C interaction. The calculated interconversion barrier between these two forms is larger than 27 kJ mol1 in both Table 6 Experimental (Ar and Xe matrices) and DFT(B3LYP)/6-311++G(d,p) calculated infrared data and vibrational assignments for the most intense predicted IR bands of the major observed bands of MMIA III (photoproduct of MA2C).a Ar matrix

Xe matrix

MMIA III

n

n

n

IIR

1747

1752/1749/ /1747/1743 1649 1451/1450 1398 1378 1197 1182/1180/ /1178/1175 1130 1102 1004/1000 926 741/740

1740.5

179.7

n(C¼O)

1679.9 1445.8 1398.3 1376.2 1191.9 1169.3

17.2 18.7 4.2 17.3 289.2 178.5

n(CN) d3N(CH3) d1N(CH3) d(CH) d4O(CH3) n(CO)

1134.8 1101.9 989.2 913.9 751.3

31.5 7.0 36.4 13.5 28.0

d4N(CH3) d5N(CH3) n(OCH3) g (CH) d(CC¼O)

1653 n.o. 1396/1394 1374 1205 1181/1180/ /1177/1175 1145 1102.0 1002.0 910/909 763/762

App. description

a Wavenumbers (cm1, scaled by 0.978), calculated intensities (km mol1), n: stretching; d: in-plane bending; g : out-of-plane bending. See Table S6 for definition of internal coordinates and Table S9 for potential energy distributions for MMIA III.

S. Lopes et al. / Vibrational Spectroscopy 81 (2015) 68–82 Table 7 Experimental (Ar and Xe matrices) and DFT(B3LYP)/6-311++G(d,p) calculated infrared data and vibrational assignments for the most intense predicted IR bands of the major observed bands of E M3IP I (photoproduct of MA2C).a Ar matrix

Xe matrix

E M3IP I

n

n

n

IIR

App. description

1757/1753 1653 1405 n.o. 1315/1310/1303/ /1300/1294/1290 n.o. 1197/1196 1169/1167

1762 1657 1403 1398/1396 1306/1302/ /1300 1253/1249 n.o. 1158/1157

1748.8 1679.9 1412.4 1392.8 1321.1

253.9 17.2 23.8 11.1 154.9

n(C¼O) n(N¼C) d(CH2) d1(HC¼N)

1099/1098 983 951 n.o. 747/746

10.0 198.1 144.4 46.8 7.9 32.9 20.3 8.9 39.5

d2(HC¼N) n(CO)

1097/1096/1094 984 n.o. 903 n.o.

1245.9 1194.6 1146.4 1144.1 1095.7 966.9 952.1 889.7 741.9

wag(CH2)

twist(CH2) d5(CH3) g (CHNH) n(OCH3) n(CC) n(CO) d(CC¼O)

a Wavenumbers (cm1, scaled by 0.978), calculated intensities (km mol1), n: stretching; d: in-plane bending; g : out-of-plane bending; wag: wagging; twist: twisting. See Table S7 for definition of internal coordinates and Table S10 for potential energy distributions for E M3IP (I).

directions (see Fig. 14). As for the E isomeric structures of M3AA, in M2AA the nitrogen atom of the NH2 moiety is somewhat pyramidalized, with both hydrogen atoms twisted out of the plane of the heavy atoms. The energy of the lowest energy conformer of M2AA (I) is similar to those of the Z conformers of M2IP, the alternative product resulting from the cleavage of the stronger aziridine C N ring bond (1,3) of MA2C. 3.4. UV-induced photochemical transformations of matrix isolated MA2C The first observation is that the experiments on the UV-induced photochemistry of matrix isolated MA2C yielded essentially the same results, when (a) using irradiations from the two different light sources (Hg/Xe high-pressure arc lamp and optical parametric oscillator), and (b) working with xenon or argon as matrix hosts. As

79

discussed below, irradiation led mainly to photocleavage of the MA2C aziridine ring (both C C and C N bond cleavages), the main photoproducts being forms II and III of MMIA and form I of M3IP. Other minor photoproducts could also be identified as described in the next sections. The definitions of the internal coordinates adopted in the vibrational analysis for the two observed conformers of MMIA and for form I of M3IP are provided in Tables S6 and S7 (Supporting information). The calculated wavenumbers, infrared intensities and potential energy distributions resulting from normal mode analyses carried out for these species are presented in Tables S8– S10. The general assignments of the bands due to these main photoproducts are presented in Tables 5–7 . 3.4.1. Narrowband UV Irradiation 3.4.1.1. Irradiations at l = 235 nm and l = 290 nm. According to preliminary irradiations of the samples using the Hg/Xe lamp and different cut-off filters, the photochemical reactions of MA2C start to occur for irradiations at 235 nm. Immediately after just 2 min of irradiation at l = 235 nm using UV-laser, new absorptions emerged throughout the spectrum, which are particularly noticeable in the carbonyl stretching region and around 1200 cm1. After 80 min irradiation at 235 nm, approximately 80% of the compound initially isolated in the matrix had been consumed (Fig. 15). Two different photoproducts were initially produced, resulting from C C and C N bond cleavages of the MA2C aziridine ring, which could be identified as methyl 2-(methylimino)-acetate (MMIA) and methyl 3-iminopropanoate (M3IP; E isomer). The infrared spectra of the irradiated matrices reveal that at this stage, only one conformer of MMIA was produced: it was the second most stable conformer (II), which could be assigned based on the calculated theoretical spectra. Interestingly, the most stable conformer of MMIA (I) was not produced (or it was produced in a non-significant amount). This observation can be explained considering that, as mentioned before, conformer II of MMIA is the conformer of this molecule that correlates structurally with the lowest energy conformer of MA2C (I), which was found to be present in the as-deposited matrices in a much larger amount than conformer II (this latter structurally correlated with conformer I of MMIA). The preferential photoproduction of higher energy

Fig. 15. (a) Experimental IR difference spectrum resulting from UV irradiation at 235 nm of MA2C isolated in an argon matrix during 80 min. Growing bands show upward, consumed bands (due to MA2C) are negative; (b) black: simulated infrared spectrum of MA2C (the same as in Fig. 3c, but multiplied by 1), blue: simulated infrared spectrum of conformer II of MMIA, red: simulated infrared spectrum of the most stable conformer of the E form of M3IP (form I). The strongest MA2C bands are truncated in both panels. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 16. Carbonyl stretching region (1790–1720 cm1) and 1225–1180 cm1 spectral region, showing the phototransformation of MA2C into MMIA (conformers II and III) and M3IP I using UV irradiations at different wavelengths (l = 235, 290 and 330 nm). Top: fragments of the infrared spectrum of MA2C monomers isolated in an Ar matrix: (black) recorded after deposition of the matrix (15 K); (red) after irradiation of the matrix with UV (l = 235 nm) laser light; (blue) after irradiation of the matrix with UV (l = 290 nm) laser light; (green) after irradiation of the matrix with UV (l = 330 nm) laser light. Bottom: theoretical spectra, with wavenumbers scaled by 0.978 (intensities were not scaled): full squares (&, MMIA II), open circles (, M3IP I) and full triangles (~, MMIA III). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

conformers has been observed for other compounds, for example, in the photochemistry of trienes, where irradiation of the matrix isolated reactant also results in the formation of thermodynamically less stable conformers [57,58]. Such results obey the nonequilibrium of excited rotamers (NEER) [57] principle, which implies that each conformer yields its own specific assortment of photoproducts, since equilibration of conformers in the excited state does not occur, implying unique photochemistry and photophysics for each initially excited conformer. The most significant bands assigned to MMIA II and M3IP I appear in the carbonyl region and near 1200 cm1 (Fig. 16). Two sets of intense bands stand out. For MMIA II, bands due to the n(C¼O) mode and are observed at 1772/1766 cm1 in argon matrix (1764 cm1, in xenon) and intense bands ascribable to the d4O(CH3) and n(CO) vibrational modes are seen at 1203 and 1183 cm1 (Ar; 1201 and 1166 cm1 in Xe), respectively (see Table 5 for details). In the case of M3IP I, the n(C¼O) vibrational mode gives rise to a doublet at 1757/1753 cm1 in the argon matrix (1762 cm1 in Xe), and the nCO and the twist(CH2) modes appear as doublets at 1197/ 1196 and 1169/1167 cm1 (argon matrix data), respectively. Upon subsequent irradiation at l = 290 nm, bands assigned to conformer II of MMIA decreased in intensity, while new bands emerged in the spectrum. For example, one can clearly see in Fig. 16 that the bands at 1772/1766 cm1 and 1203 cm1 reduced their intensity with simultaneous increase in the intensity of new bands at 1747 cm1 and 1205 cm1 (a shoulder at ca. 1747 cm1 was already present in the spectrum of the as-deposited matrix, but it is clear that this shoulder and the new band emerging upon irradiation at 290 nm have different origins). These results demonstrate the depopulation of form II of MMIA and its conversion into another species, which could be identified as conformer III of MMIA. Conformer III of MMIA is more energetic by 9.2 kJ mol1 than form II (see Table 4) and can be obtained from this latter through E–Z isomerization around the C¼N bond (see

Fig. 8). On the other hand, the intensities of the bands due to M3IP I, such as those of the doublets at 1757/1753 or 1197/1196 cm1, remain practically unchanged (Fig. 16), as this species does not react upon irradiation at 290 nm. 3.4.1.2. Irradiation at l = 330 nm. After the irradiations at 235 and 290 nm, an additional irradiation using longer wavelength laser light (l = 330 nm) was performed. Such irradiation induced new changes in the infrared spectrum of the irradiated matrix. Besides the decrease of bands due to MMIA III, produced upon irradiation at l = 290 nm (e.g., the bands at 1747 and 1205 cm1), appearance of new bands indicating the formation of a new photoproduct was observed. MMIA III converts back to MMIA II upon irradiation at 330 nm, as shown by the increase of the intensity of this latter species (see Fig. 16). On the other hand, the new bands observed upon this irradiation were assigned to photoproducts other than those considered in Scheme 2. Indeed, besides the possible photoproducts resulting from C C and C N bond cleavage of the aziridine ring shown in Scheme 2, other possible photochemical processes, such as decomposition of the starting compound and/or its photoproducts can take place. Some products of fragmentation can be easily identified, due to observation of very characteristic bands of these species. Carbon dioxide bands [59,60] were observed at 2348, 2345, 2340 and 2339 cm1 in the argon matrix (2349, 2346, 2344, 2343, 2339, 2337 and 2334 cm1 in Xe). Bands due carbon monoxide were found at 2143, 2138 and 2137 cm1 in the argon matrix [61,62], and at 2140, 2137, 2134, 2132, 2130 and 2127 cm1 in the xenon matrix. Other bands at 2146, 2134 and 2133 cm1 (Ar) must be due to CO associated with other photoproducts, must probably with methanol, which should be produced together with CO by decarbonylation of MA2C, and whose presence in the photolyzed matrices could also be demonstrated by observation of its characteristic bands [63] at 2847, 1333/1331 and 1034 cm1 (in argon;

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corresponding to the bands observed at 2849, 1332 and 1035 cm1 in the xenon matrix). The bands observed at ca. 1303, 1310 and 3030 cm1 in the argon matrix are ascribable to methane [64,65], having their counterparts in the xenon matrix at ca. 1302, 1304 and 3012/3018 cm1. Acetonitrile gives rise to the band at the characteristic frequency of 2234 cm1 (in argon matrix; 2232 cm1 in Xe) [66]. 3.4.2. Broadband UV irradiation (l > 235 nm) Irradiation of MA2C with broadband UV light (l > 235 nm; both in argon and xenon matrices) yielded the same photoproducts as upon irradiation with narrowband UV laser light, i.e., MMIA (II and III) and M3IP (I), together with photofragmentation products (Fig. S2, Supporting information). However, all photoproducts are formed promptly and could already be detected after 1 min of irradiation. 4. Conclusion Quantum chemical calculations on MA2C carried out at the DFT/(B3LYP)/6-311++G(d,p) level predicted the existence of two low-energy minima. The energy barrier separating these two forms is above ca. 20 kJ mol1 in both directions. These two low energy forms were found to be present in the as-deposited matrices (argon, xenon) of the compound and their spectra were fully assigned in both matrices. The photochemistry of matrix isolated MA2C submitted to both narrowband tunable laser light irradiation (l = 235, 290 and 330 nm) and broadband UV irradiation (l > 235 nm) was investigated in detail, supported by an extensive series of calculations on the a priori possible photoproducts. The photoproducts obtained in both argon and xenon matrices in result of the two types of irradiations performed (narrowband and broadband) were found to be essentially similar, though the use of selective narrowband tunable UV laser irradiation allowed a more accessible way of assigning the different photoproducts, in particular, facilitating the identification of the two photoproduced conformers (II and III) of MMIA, which was not possible through the analysis of the photochemistry resulting from broadband irradiation. Two main types of photoreactions were observed: isomerization resulting from ring-opening reactions of the MA2C aziridine ring through CC and C N bonds cleavages, and photofragmentations. When narrowband irradiations were used, the first kind of observed photoprocesses led to the initial production of conformer II of MMIA, which correlates structurally with the lowest energy conformer of MA2C (I) that was the dominant species initially present in the as-deposited matrices, and the lowest energy E M3IP conformer (I). The production of conformer II of MMIA results from MA2C aziridine ring CC bond cleavage, and obeys the nonequilibrium of excited rotamers (NEER) [57] principle. This conformer could be subsequently converted into MMIA conformer III via E-Z photoisomerization (through irradiation at l = 290 nm), which in turn could afterward be reverted to form II upon irradiation at l = 330 nm. Among the a priori possible isomerization products resulting from MA2C aziridine ring C C bond cleavage, MMIA is the lowest energy species. Production of M3IP conformer I is the result of the cleavage of the weakest C N bond of the MA2C aziridine ring. Noticeably, no products resulting from the cleavage of the second C N bond of the MA2C aziridine ring were observed, although the calculations predict that these species (M2IP and M2AA) have slightly lower energies than M3IP. The most striking result of the photochemical experiments performed in the present investigation is the absence, among the photoproducts of MA2C, of M3AA, which is predicted by the calculations as the lowest energy putative isomerization product (see Table 4). Both M3IP and M3AA should result from

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cleavage of the weakest C N bond of the MA2C aziridine ring (see Scheme 2). The H-atom migration from the methylene group either to vicinal carbon or vicinal nitrogen atom would form either M3AA or M3IP, respectively. The observation of M3IP and non-observation of M3AA seems to indicate that the H-atom migration to the vicinal carbon is strongly preferred. Several photofragmentation products of MA2C were also observed, including acetonitrile, methanol, methane, CO and CO2, reflecting a complex photodecomposition pattern of the compound. Supporting information Fig. S1, with the high-energy conformers of MA2C optimized at the DFT(B3LYP)/6-311++G(d,p) level of theory; Fig. S2, with the experimental IR difference spectra of irradiated Ar and Xe matrices of MA2C by broadband UV light (l > 235 nm), compared with the relevant theoretical spectra; Table S1—DFT(B3LYP)/6-311++G(d,p) calculated bond lengths and angles of the eight conformers of MA2C; Table S2—definition of symmetry coordinates used in the normal mode analysis of the conformations of MA2C; Tables S3–S4—calculated [scaled, DFT(B3LYP)/6-311++G(d,p)] wavenumbers, IR intensities and Potential Energy Distributions (PED) for the two most stable conformers of MA2C; Table S5—DFT(B3LYP)/6-311 ++G(d,p) calculated conformationally relevant dihedral angles for the putative photoproducts of isomerization, by ring opening, of MA2C; Tables S6–S7—definition of symmetry coordinates used in the normal mode analysis of the conformations of methyl 2-(methylimino)-acetate (MMIA) and methyl 3-iminopropanoate (M3IP; E isomer I); Tables S8–S10—calculated [scaled, DFT(B3LYP)/ 6-311++G(d,p)] wavenumbers, IR intensities and Potential Energy Distributions (PED) for conformers II and III of MMIA and conformer I of M3IP (I). This material is available free of charge via the Internet. Acknowledgements This work was supported by the Portuguese “Fundação para a Ciência e a Tecnologia” (FCT) via Research Project PTDC/QUI-QUI/118078/2010 (FCOMP-01-0124-FEDER-021082), co-funded by QREN-COMPETE-UE. The Coimbra Chemistry Centre is also supported by the FCT through the project Pest-OE/QUI/ UI0313/2014. S. L. and I. R. acknowledge FCT for Grant No. SFRH/ BPD/77276/2011 and the Investigador FCT grant, respectively. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. vibspec.2015.10.003. References [1] A. Padwa, Comprehensive Heterocyclic Chemistry III, Chapter 1.01—Aziridines and Azirines: Monocyclic, 2008, pp. 1–104. [2] Aziridines and Epoxides in Organic Chemistry, in: A.K. Yudin (Ed.), Wiley-VCH, Weinheim, 2006. [3] J.B. Patrick, R.P. Williams, W.E. Meyer, W. Fulmor, D.B. Cosulich, R.W. Broschard, J.S. Webb, J. Am. Chem. Soc. 86 (1964) 1889. [4] R.S. Coleman, J.-S. Kong, T.E. Richardson, J. Am. Chem. Soc. 121 (1999) 9088. [5] E. Vedejs, B.N. Naidu, A. Klapars, D.L. Warner, V.-S. Li, Y. Na, H. Kohn, J. Am. Chem. Soc. 125 (2003) 15796. [6] M. Hashimoto, M. Matsumoto, K. Yamada, S. Terashima, Tetrahedron 59 (2003) 3089. [7] P.T. Trapentsier, I. Ya. Kalvin'sh, E.E. Liepin'sh, E. Ya. Lukevits, Chem. Heterocycl. Compd. 19 (1983) 391. [8] H.M.I. Osborn, J. Sweeney, Tetrahedron: Asymmetry 8 (1987) 1693. [9] W. McCoull, F.A. Davis, Synthesis 10 (2000) 1347. [10] K.J.M. Beresford, N.J. Church, D.W. Young, Org. Biomol. Chem. 4 (2006) 2888. [11] J.L. Vicario, D. Badía, L. Carrillo, J. Org. Chem. 66 (2001) 5801.

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