Conformational Space and Photochemistry of Tyramine Isolated in Argon and Xenon Cryomatrixes

Transcription

1 pubs.acs.org/jpca Conformational Space and Photochemistry of Tyramine Isolated in Argon and Xenon Cryomatrixes Barbara M. Giuliano, Sonia Melandri,*,, Igor Reva, and Rui Fausto*, Department of Chemistry, University of Coimbra, P Coimbra, Portugal Department of Chemistry, University of Bologna, I Bologna, Italy *S Supporting Information ABSTRACT: The infrared spectra of tyramine monomers trapped in low-temperature argon and xenon matrixes were recorded. The presence of the flexible ethylamino side chain gives rise to a complex conformational surface that contains several minima of relatively low energies, some of them stabilized by a weak N H π hydrogen bond interaction between the amino group and the phenyl ring. The experimental infrared spectra confirm the presence of at least two stable conformers isolated in the matrixes. Annealing experiments performed on the xenon matrix revealed a change in the relative population of the experimentally relevant conformers upon isolation in this polarizable matrix, compared to the gas phase. The general interpretation of the spectra was based on harmonic and anharmonic quantum chemical calculations, undertaken at the DFT/B3LYP and MP2 levels of theory with the G(d,p) basis set. The photochemical behavior of the matrix-isolated compound upon narrow-band UV irradiation was also investigated. Identification of ketene species in the spectra of the irradiated matrixes suggests the occurrence of a ring-opening reaction, which in the xenon matrix occurs concomitantly with the conformational isomerization of tyramine. 1. INTRODUCTION Tyramine (4-hydroxyphenethylamine; TYR) is a biogenic amine derived from the decarboxylation of the amino acid tyrosine. It can be found in aged foods as a degradation product 1,2 and can act as a releasing agent. 3,4 The presence of the flexible ethylamino group as a side chain of its phenolic aromatic ring is an essential structural feature of tyramine, leading to a complex conformational surface, which contains several minima of relatively low energy. This substance has already been investigated by microwave rotational spectroscopy and laser spectroscopy in the supersonic jet environment. 5 9 Such experiments have evidenced the presence of a minimum of four, up to seven, different conformers in the gas phase, depending on the detection technique employed. In particular, the dispersed fluorescence and IR spectra of tyramine are reported in ref 7, where the vibrational spectra of seven different conformers were observed. All the gas phase experiments agree that the most stable conformers are stabilized by weak intramolecular hydrogen bond interactions and exhibit a folded geometry in which the amino hydrogen atoms interact with the π cloud of the aromatic ring. To the best of our knowledge, the infrared spectra of tyramine isolated in cryogenic matrixes have never been reported before, nor has its photochemistry been investigated hitherto. In order to fill these gaps, we present in this article a study on the structure and infrared spectra of tyramine isolated in argon and xenon matrixes, exploring its conformational landscape with the help of harmonic and anharmonic quantum chemical calculations, and analyzing its reactivity following narrow-band UV irradiation. 2. EXPERIMENTAL AND CALCULATION METHODS A sample of tyramine (99% purity) was purchased from Sigma- Aldrich and used without further purification. Matrixes were prepared by codeposition of tyramine vapors from a heated glass furnace containing a sample of the compound, and a large excess of the matrix gas (argon N60 from Air Liquide; scientific xenon 5.0 from Linde), onto the CsI optical substrate of the cryostat kept at 15 and 30 K, respectively, for argon and xenon matrixes. An APD Cryogenics closed-cycle helium refrigeration system with a DE-202A expander was used. The temperature was measured directly at the sample holder by a silicon diode temperature sensor, connected to a digital controller (Scientific Instruments, Model ), with an accuracy of 0.1 K. The IR spectra were recorded using a Nicolet 6700 Fourier transform infrared spectrometer, equipped with a deuterated triglycine sulfate (DTGS) detector and a Ge/KBr beam splitter, with 0.5 cm 1 resolution. In the photochemical experiments, narrow-band tunable UV radiation was provided by the frequency doubled signal beam of the Quanta-Ray MOPO-SL optical parametric oscillator (full width at half-maximum (fwhm) 0.2 cm 1, repetition rate 10 Hz, pulse energy 3 mj) pumped with a pulsed Nd:YAG laser. The quantum chemical calculations were performed with the Gaussian 03 software package. 10 Equilibrium geometries for all studied species were fully optimized at the DFT level of theory, using the standard G(d,p) basis set and the threeparameter density functional B3LYP, which includes the Becke gradient exchange correction and the Lee, Yang, and Parr Received: May 13, 2013 Revised: September 2, 2013 Published: September 8, American Chemical Society 10248

2 Figure 1. Relaxed B3LYP/ G(d,p) potential energy scans around the C8 C7 C4 C3 (left) and C2 C1 O H21 (right) dihedral angles in tyramine. Along the C aryl CH 2 scan (left), the optimized C2 C1 O H, N C8 C7 C4, and Lp N C8 C7 dihedral angles stayed around cis, anti, and gauche orientations, respectively. Along the C aryl OH scan (right), the C8 C7 C4 C3, N C8 C7 C4, and Lp N C8 C7 dihedral angles stayed around perpendicular, anti, and gauche orientations, respectively. Lp stands for lone pair of electrons of the nitrogen atom, and its orientation is defined to be in the plane bisecting the C8 N9 H19 and C8 N9 H20 planes. Center: optimized structure of Gg conformer with atom numbering. correlation functional. The geometries and relative energies of the TYR minimum energy conformations were also computed using the MP2 method, with the G(d,p) basis set. Both harmonic and anharmonic vibrational calculations were undertaken. The first were obtained at both the B3LYP and MP2 levels of theory. Anharmonic frequencies were obtained just at the B3LYP level of approximation. The theoretical normal modes were analyzed by carrying out potential energy distribution (PED) calculations. Transformations of the force constants with respect to the Cartesian coordinates to the force constants with respect to the molecule fixed internal coordinates allowed the PED analysis to be carried out as described by Schachtschneider and Mortimer. 11 The internal coordinates used in this analysis were defined as recommended by Pulay et al. 12 and are listed in Table S1 (Supporting Information). 3. RESULTS AND DISCUSSION The molecule of tyramine (Figure 1) has four internal rotational degrees of freedom. Two of them are associated with the spatial orientation of the ethylamino side chain and originate from rotations around the CH 2 CH 2 and CH 2 N bonds. The remaining two define the relative positions of the ethylamino moiety and hydroxyl group in relation to the aromatic ring, and correspond to internal rotations about the C aryl CH 2 and C aryl O bonds, respectively. The systematic inspection of the PES of the molecule indicated that the optimized orientations of both the ethylamino and hydroxyl substituents remained essentially perpendicular and parallel to the ring plane, respectively, for all minimum energy conformations, independently of the conformation within the ethylamino moiety. Illustrative relaxed potential energy scans around the C aryl CH 2 and C aryl OH bonds are shown in Figure 1. We choose, as example, the case where the conformations about the CH 2 CH 2 and CH 2 N bonds of the ethylamino group are anti (C aryl C C N dihedral angle is 180 ) and gauche (C C N Lp is 60 ; Lp = lone electron pair of N). As can be seen in Figure 1, one full rotation around the C aryl CH 2 or C aryl OH bond results in two minima, designated as Ag and Ag (first letter indicates the conformation about the CH 2 CH 2 bond; the second indicates that about the CH 2 N bond). If the conformation around the C C N Lp dihedral angle was chosen as anti ( 180 ), the two minima would be mirror reflections of each other (Aa), corresponding to reflection either through the plane of the phenyl ring (flip of the ethylamine group) or in the plane bisecting the phenyl ring (flip of the OH group). According to the B3LYP calculations, the transformation between Ag and Ag via rotation about the C aryl CH 2 bond is associated with a barrier lower than 9 kj mol 1, while that obtained via rotation of the OH group involves a larger barrier (about 15 kj mol 1 ). It is important to note that transformations between the symmetry-equivalent minima can be accomplished via internal rotation either of the ethylamino fragment or, with the same effect, of the hydroxyl group. Then, to characterize a set of unique conformations of tyramine, one of these internal rotations does not need an explicit consideration. Since the OH group internal rotation is related to a higher energy barrier, in the further analysis we have oriented the OH group to have the cis value of the C2 C1 O H dihedral angle. We kept this OH group orientation as a reference (or, equivalent to say, as an asymmetry marker of the phenyl ring) while analyzing all possible orientations of the ethylamino side chain. For a more detailed conformational characterization of the ethylamino fragment, we have performed a two-dimensional potential energy scan where the driving coordinates correspond to the dihedral angles related to the internal rotations around the CH 2 CH 2 and CH 2 N bonds. These angles were incrementally fixed with a step of 20, in all possible 324 combinations, while all other geometric parameters were fully optimized. The resulting potential energy surface is represented in Figure 2. The complete potential energy surface contains nine different minima. According to the adopted naming scheme, these minima are designated using two letters, appearing on the top and right sides of the surface representation in Figure 2. The first (capital) letter refers to the position of the amino group with respect to the phenyl ring, defined by the C aryl C C N dihedral angle. The second (lower case) letter specifies the orientation of the amino group with respect to the ethylamino chain, defined by the C C N Lp dihedral angle. This naming 10249

3 Figure 2. Relaxed potential energy surface of tyramine for rotation around CH 2 CH 2 and CH 2 NH 2 bonds calculated at the B3LYP/ G(d,p) level. The names of conformers correspond to the combinations of letters (G/A/G ) and (g/a/g ) shown at the edges of the surface. The global energy minimum is Gg, and its energy was chosen as the relative zero level. The isoenergy levels are presented by continuous thin lines and are spaced by 3 kj mol 1. Two additional isoenergy levels are shown at 2 (bold line) and 16 (dotted line) kj mol 1. Lp stands for lone pair of the nitrogen atom, and its orientation was chosen to be in the plane bisecting the two CNH planes. system is very similar to that implemented previously in the study of tryptamine, 13 a molecule that also bears an ethylamino group connected to an asymmetric aromatic ring. The CH 2 CH 2 bond rotation gives rise to two types of conformers: the gauche conformers (denoted with the capital letters G and G ), adopting a folded position of the NH 2 group with respect to the aromatic ring, and the anti forms (denoted with A) which assume an unfolded geometry. The internal rotation of the NH 2 group produces three more orientations (denoted with g, a, and g ) for each gauche or anti arrangement around the CH 2 CH 2 bond. Depending on the orientation of the amino group, some of the folded conformers may involve the presence of a weak N H π hydrogen bond interaction, while in the case of the unfolded conformations such an interaction is impossible. The structures of the nine different conformers of tyramine are presented in Figure 3. Their calculated energies are listed in Table 1. According to both the B3LYP and MP2 calculations, the lowest energy conformations are those stabilized by the intramolecular hydrogen bonding between one of the amino hydrogen atoms and the π electron cloud of the aromatic ring (Gg, G g, Ga, and G a; see also Figure 3). In the two folded conformations, where the nitrogen lone pair is pointing toward the phenyl ring (Gg and G g), this interaction is not possible, and their energies relative to the most stable conformers are predicted to be much higher. The energies of the anti forms about the CH 2 CH 2 bond were predicted by the MP2 method to be only slightly lower than those of the Gg and G g forms and considerably higher than those of the folded conformations bearing the stabilizing N H π interaction. At the B3LYP level Figure 3. The nine conformers of tyramine and their optimized geometries. Note that positions of the conformers in this figure correspond to their positions on the two-dimensional potential energy surface shown in Figure 2. The dashed lines specify the amino group hydrogen atoms involved in the stabilizing interaction with the aromatic ring. of theory the relative energies of the anti forms about the CH 2 CH 2 bond were estimated to be smaller, with the Aa conformer being predicted to correspond to the second most stable form of tyramine. We can expect the relative energies of the conformers to be better predicted by the MP2 calculations, since this method is well-known to describe more consistently the intramolecular correlation energy than DFT based methods. 5,14,15 From the MP2 calculated Gibbs energies at 298 K, the relative abundances of the different tyramine conformers in the gas phase equilibrium at that temperature were estimated (Table 1). The abundance of the conformers in a low-temperature matrix may, however, differ substantially from those, because of conformer interconversion either during deposition of the matrix orinthematrixitself.theprobabilityoftheoccurrenceofthe conformational relaxation is related to the energy barriers between the conformers. In matrixes, conformational cooling has been demonstrated to occur in compounds possessing barriers to intramolecular rotation of a few kilocalories per mole The B3LYP and MP2 predicted barriers for interconversions involving the nine conformers of tyramine can be seen in 10250

4 Table 1. Zero Point Corrected Relative Energies (ΔE ZPE /kj mol 1 ), Dipole Moments (μ/d), Relative Gibbs Energies at 298 K (ΔG 298 /kj mol 1 ), and Relative Populations (P 298,%;atT = 298 K) of the Nine Different Conformers of TYR, Calculated at the B3LYP and MP2 Levels of Approximation with the G(d,p) Basis Set ΔE ZPE μ ΔG 298 P 298 b conformer a B3LYP MP2 B3LYP MP2 MP2 MP2 Gg 0.00 c 0.00 d e 23.9 G g Ga G a Aa Ag Ag Gg G g a Conformer structures are shown in Figure 3. b Estimated using the calculated ΔG 298 values. c Absolute energy: E h. d Absolute energy: E h. e Absolute Gibbs energy: E h. Figure 2 and in Scheme 1, respectively. As a general feature, the barriers for the NH 2 group torsion are lower than those for the Scheme 1. Relative Energies (ΔE/kJ mol 1 ) of Transition States between the Nine Conformers of TYR Calculated at the MP2/ G(d,p) Level of Theory a a Data enclosed in squares represent names, relative energies (kj mol 1 ), and net dipole moments (debye, in brackets) of all the minima, positioned in the same way as on the map shown in Figure 2. Data between squares represent the energies of the respective transition states. Values on the perimeter show the transition state energies between the minima on the opposite edges, and for emphasis, they are repeated on the right/left and top/bottom. b Absolute energy: E h. torsion of the ethylamino backbone. The conversion of Gg into the global minimum Gg shows the smallest calculated barrier, less than 3 kj mol 1. This value is small enough to assume that the Gg form will be converted into the Gg form during matrix deposition. The same assumption can be made for the conversion of G g into G g. Likewise, the barriers for the interconversions between the anti CH 2 CH 2 conformers (Ag, Ag, and Aa), which are between 6 and 10 kj mol 1, can also be considered low enough to lead us to expect that only one of these conformers (either the most stable Ag form or the Ag one which possesses the highest value of the dipole moment, according to the MP2 calculations; see Scheme 1) will be present in the low-temperature matrixes after deposition. Similarly, the barriers for the amino group rotation converting the Ga form into the most stable Gg, and accordingly the G a into the G g, have been calculated to be slightly higher ( 12 kj mol 1 ), which is in agreement with the presence of the weak N H π interaction in these conformers. Hence, the presence of Ga and G a conformers in the matrixes is not expected. Also, we will present below the experimental features for postulating the absence of these two conformers in the deposited matrixes. Note that in alanine, as in tyramine, many conformers differing by the NH 2 group orientation are possible. The corresponding barriers for the NH 2 group internal rotation in alanine are of the order of 10 kj mol 1. The less stable forms of alanine readily collapse to the most stable conformer around this coordinate during deposition of the matrixes, and only two forms of alanine with different heavy atom backbone orientations could be trapped upon the matrix deposition. 21 Conformational isomerization about the CH 2 CH 2 bond involves rotation of heavy atoms. The associated barriers were calculated to be no less than 16 kj mol 1 (Scheme 1), being the highest barriers for conformational isomerization in the studied molecule. This result can be used in favor of the presence of both gauche and anti type conformers in the deposited matrixes. Finally, the barrier for the rotation of the OH substituent was calculated to be around 15 kj mol 1, independently of the conformation of the ethylamino substituent. Our recent studies on cytosine 22 showed that two hydroxy conformers could convert to each other in the matrix even over the barrier of 30 kj mol 1. Rotation of the OH group interconverts the Gg, Ga, Ag, and Gg conformers into the G g, G a, Ag, and G g forms, respectively. It is interesting to note that the conformers that differ only in the orientation of the OH group are almost isoenergetic (as can be seen from Table 1 and Scheme 1) but the conformer with the highest dipole moment should be favored in the matrix. Nevertheless, since their calculated spectra are practically coincident, at our level of experimental resolution it is impossible to discriminate between these conformers. For this reason in the spectral analysis presented below, only one member of each of these classes of conformers will be considered and it will be the one with the highest dipole moment. We will then use G g for the couple Gg/G g, Ag for Ag/Ag, Ga for Ga/G a, and G g for G g/gg. After all these considerations, taking into account all the calculated values for the isomerization barriers, and the dipole moments, the species that are expected to be predominant in the matrix after deposition are the Ag and G g conformers. With the assumptions resulting from the analysis of the potential energy landscape of tyramine and taking into account the expected conformational conversions between conformers during matrix deposition, we can now roughly estimate the relative abundance (%) of the conformers in the as-deposited matrixes, using the theoretically predicted (MP2) populations for the conformers in the gas phase before deposition presented in Table 1. The obtained value, G g :Ag = 71:29 (i.e., 2.4), is, as could be anticipated (see Figure 4), somewhat smaller than that found in ref 6 for jet cooled tyramine using R2PI laser spectroscopy (in the spectrum shown in Figure 3 of ref 6, the relative intensity of the bands assigned to the gauche 10251

5 The experimental spectra of TYR isolated in argon and xenon matrixes are shown in Figure 5. The simulated spectrum obtained by adding the frequency scaled calculated harmonic spectra of conformers G g and Ag in a 2.4:1 ratio is also shown in Figure 5. The sum of the predicted spectra of these two Figure 4. Calculated Boltzmann relative populations of G- and A-type conformers as a function of temperature, according to the MP2 calculated Gibbs energies. conformers is ca. 3 times larger than the intensity of the bands assigned to the anti forms) Band Assignments. Due to the characteristics of the molecular system under study (high flexibility of the side chain, relatively large molecule, similar spectra for the various conformers), the assignment of the main spectral features to individual species is difficult to undertake unequivocally. The most intense bands are split into various components that can be assigned to the presence of different conformers as well as to matrix site effects, which are particularly common for argon matrixes. A further complication arises from the fact that the most conformation-sensitive vibrational modes have a highly anharmonic character. Therefore, anharmonic predictions of the spectra have been computed for the seven most stable conformers of tyramine. In other cases, 23,24 where the assignments are clearer, the B3LYP predicted frequencies have been found to reproduce better the experimental spectra than the MP2 ones, and therefore those calculations have been used to make the assignments also in the case of tyramine. In the case of ref 7 the IR spectra of different conformers are obtained by IR dip spectroscopy or laser induced fluorescence and the conformers are collected in groups based on the spectral similarities. The conformers belonging to the G or A groups show similarities in the C H out of plane bending in the benzene ring (around cm 1 ), while groupings a and g are based on the C H methylene stretching region ( cm 1 ). The main features of the observed spectra are well reproduced by B3LYP/aug-cc-pVDZ calculations. In our case the overlap of bands does not allow a straightforward assignment to a single conformation; nevertheless, information on populations and population changes can be extracted from the spectra. In this case the most informative region of the spectra is the fingerprint region. A detailed comparison of the B3LYP harmonic and anharmonic predictions will be provided in the next paragraphs; briefly, the predicted harmonic frequencies, scaled by a common factor of 0.978, have been found to reproduce better the experimental spectra in the fingerprint region, and have been used to make the detailed assignment in this region of the spectra, while the anharmonic predictions have been very effective in reproducing the position of the OH stretching vibration for the various conformers. 25 Figure 5. Experimental infrared spectra of TYR isolated in freshly deposited (a) Ar and (b) Xe matrixes, at 15 and 30 K, respectively. (c) Simulated theoretical spectrum which includes population-weighted contributions of the G g (71%) and Ag (29%) conformers. Before simulation, all frequencies were calculated at the B3LYP/ G(d,p) level of theory and scaled by a factor of Then, the absorption bands were simulated using Lorentzian functions centered at the calculated (scaled) frequencies and with the full width at halfmaximum (fwhm) equal to 2 cm 1. conformers is the minimum set of data necessary to satisfactorily reproduce the recorded spectra. This result is compatible with the expected presence in the matrixes of both G g and Ag forms (vide supra). The small differences between the spectra obtained in Ar and Xe matrixes are due to the different trapping sites which result in a slightly different splitting pattern. In addition, the different polarizabilities of the two matrixes may induce different stabilization orders of the trapped conformers. The experimentally measured frequencies and their comparison with the B3LYP predictions, together with the PED analysis, for the G g and Ag conformers are reported in Table 2. The calculated frequencies and PEDs for the remaining seven conformers are listed in Tables S2 and S3 (Supporting Information). A more detailed discussion about the vibrational assignments proposed for the different regions of the spectrum is given below cm 1 Region (ν(oh), ν(nh 2 ), and ν(ch) Stretching Modes). The most intense band in this region is due to ν(oh), which appears as an intense sharp band at 3631 and 3617 cm 1 in Ar and Xe matrixes, respectively (see Figure 8). Since the OH group of tyramine is not involved in any intramolecular interaction, the ν(oh) band appears at almost the same position as in matrix-isolated phenol. 26 In argon, the band shows different components, possibly due to matrix site splitting effects (similarly as it is for phenol), in addition to the presence of different conformers of tyramine in the matrix. As can be seen in Table 2, the anharmonic predictions for the ν(oh) mode are much closer to the observed frequencies and, 10252

6 Table 2. Experimental Wavenumbers (ν /cm 1 ) and Relative Integral Intensities (I) of the Absorption Bands in the Infrared Spectrum of Tyramine Isolated in Ar and Xe Matrixes, Compared with Wavenumbers (ν /cm 1 ), Absolute Intensities (A th /km mol 1 ), and Potential Energy Distribution (PED, %) Calculated for Tyramine at the B3LYP/ G(d,p} Level of Theory experimental ν a(i b ) calculated ν c (A th ) PED d (%) Ar Xe G g harm. scaled G g anh Ag harm. scaled Ag anh G g Ag ; ; (195.5) ; (170.5) (66.1) (66.0) ν(oh) (100) ν(oh) (100) n.o. e ; (10.2) (2.7) (1.2) ν a (NH 2 ) (99) ν a (NH 2 ) (99) n.o (5.3) (0.7) (0.5) ν x (NH 2 ) (98) ν x (NH 2 ) (98) ; ; ; ; ; ; (271.0) ; ; ; ; ; ; ; (250.9) (5.1) (5.5) ν(c2 H13) (87) + ν(c3 H14) (12) ν(c2 H13) (93) (5.1) (12.5) ν(c3 H14) (86) + ν(c2 H13) (12) ν(c5 H10) (72) + ν(c6 H11) (25) (15.6) (13.0) ν(c5 H10) (71) + ν(c6 H11) (27) ν(c3 H14) (92) (17.6) (18.9) ν(c6 H11) (72) + ν(c5 H10) (27) ν(c6h11) (75) + ν(c5 H10) (25) (21.6) (28.0) ν a (C7H 2 ) (93) ν a (C7H 2 ) (76) + ν(c8 H18) (14) (68.8) (23.1) ν(c8 H18) (89) ν(c8 H18) (80) + ν a (C7H 2 ) (10) (19.0) (27.7) ν a (C7H 2 ) (87) ν a (C7H 2 ) (81) + ν a (C7H 2 ) (15) (67.5) (61.8) ν(c8 H17) (98) ν(c8 H17) (98) ; (49.5) ; (67.8) (47.4) (33.0) scis(nh 2 ) (76) + ν b (CC) (16) scis(nh 2 ) (101) (36.5) (40.2) scis(nh 2 ) (25) + ν b (CC) (47) + δ b (CH) ν b (CC) (63) + δ b (CH) (18) + δ(r2) (10) (13) ; (12.8) (11.4) (14.1) ν c (CC) (65) ν c (CC) (66) (21.3) (198.7) (181.8) (104.7) (107.8) δ c (CH) (50) ν c (CC) (36) δ c (CH) (51) ν c (CC) (37) n.o. n.o (2.2) (3.4) scis(c8h 2 ) (99) scis(c8h 2 ) (88) + scis(c7h 2 ) (13) n.o. n.o (7.5) (2.6) scis(c7h 2 ) (93) scis(c7h 2 ) (86) + scis(c8h 2 ) (13) ; (41.1) (26.7) (17.3) (16.2) ν d (CC) (39) + δ d (CH) (27) ν d (CC) (41) + δ d (CH) (28) ; (12.7) (14.4) (14.6) (15.9) wag(c8h 2 ) (74) + rock(nh 2 ) (16) wag(c8h 2 ) (74) + rock(nh 2 ) (13) n.o (6.1) (26.5) δ a (CH) (44) + ν f (CC) (26) + δ(oh) (19) n.o (3.4) (10.3) wag(c7h 2 ) (47) + δ a (CH) (15) + twist(c8h 2 ) (17) ; (25.2) ; (13.3) (21.4) (1.5) ν f (CC) (31) + δ a (CH) (34) + δ(oh) (18) wag(c7h 2 ) (30) + ν f (CC) (21) + δ a (CH) (14) + twist(c7h 2 ) (13) 1317 (4.9) : (3.9) (4.6) (3.0) ν f (CC) (28) +wag(c7h 2 ) (17) + δ a (CH) twist(c8h 2 ) (24) + ν r (CC) (21) + wag (16) + twist(c7h 2 ) (18) n.o : (3.1) (4.0) (2.6) twist(c8h 2 ) (37) + rock(nh 2 ) (14) + ν f (CC) (13) + δ a (CH) (11) (C7H 2 ) (18) + twist(c7h 2 ) (13) twist(c8h 2 ) (46) + wag(c7h 2 ) (15) n.o. n.o (107.7) ν(c1 O12) (49) + ν a (CC) (15) + δ c (CH) (15) ; ; ; (84.9) (111.0) (3.6) ν(c1 O12) (50) + ν a (CC) (15) + δ c (CH) twist(c7h 2 ) (21) + rock(c8h 2 ) (18) + rock (101.9) (15) (NH 2 ) (15) + wag(c8h 2 ) (13) n.o. n.o (2 3) ν(c4 C7) (33) + ν a (CC) (16) + wag (C7H 2 ) (13) + δ c (CH) (13) ; (19.8) (2.9) (10.9) (1.3) twist(c7h 2 ) (47) ν(c4 C7) (34) + ν a (CC) (18) + δ c (CH) (14) + wag(c7h 2 ) (13) n.o. n.o (5.0) (5.2) δ b (CH) (76) + ν b (CC) (21) δ b (CH) (76) + ν b (CC) (21) ; ; (136.4) : ; : (189.3) (155.7) (169.4) δ(oh) (52) + ν f (CC) (17) δ(oh) (56) + ν f (CC) (11) 10253

7 Table 2. continued experimental ν a(i b ) calculated ν c (A th ) PED d (%) Ar Xe G g harm. scaled G g anh Ag harm. scaled Ag anh G g Ag n.o n.o (1.1) (2.6) rock(nh 2 ) (36) + twist(c8h 2 ) (25) rock(nh 2 (21) + twist(c7h 2 ) (20) + δ d (CH) (16) + twist(c8h 2 ) (14) ; (21.6) ; (24.9) (18.5) (16.3) δ d (CH) (49) + ν d (CC) (27) δ d (CH) (39) + ν d (CC) (25) (3.6) (12.2) (12.7) (32.8) ν(c8 N9) (47) + ν(c7 C8) (23) ν(c8 N9) + (61) + ν(c7 C8) (18) (4.2) n.o (8.1) (3.5) rock(c8h 2 ) (25) + rock(c7h 2 ) (23) + ν(c8 N9) (22) + rock(c8h 2 ) (22) + rock ν(c8 N9) (22) (C7H 2 ) (18) + twist(c7h 2 ) (12) n.o. n.o (0.9) (0.05) δ(r 1 ) (45) + ν c (CC) (37) + δ c (CH) (16) δ(r 1 ) (45) + ν c (CC) (38) + δ c (CH) (16) n.o. n.o (0.1) (0.0) γ h (CH) (102) + γ d (CH) (11) γ d (CH) (70) + ν(c7 C8) (14) n.o. n.o (0.3) (0.4) γ d (CH) (98) + γ b (CH) (10) γ d (CH) (29) + γ b (CH) (25) + ν(c7 C8) (22) + rock(nh 2 ) (11) n.o ; (3.6) (0.6) (0.3) ν(c7 C8) (37) + rock(c8h 2 ) (15) + rock γ b (CH) (85) + γ d (CH) (16) (NH 2 ) (12) + ν a (CC) (10) n.o (3.8); (3.4) (7.5) (12.6) rock(c7h 2 ) (22) + γ c (CH) (13) + ν(c7 C8) (12) + ν(c8 N9) (12) (8.1) 826.1; 822.9; (44.3) (17.2) 808.0; (119.9) (5.4); 808.5; 807.0; 805.7; (122.2) (8.8) 789.2; (14.4); (5.5) (24.6) (28.0) ν a (CC) (41) + ν(c1 O12) (17) + δ(r2) (15) (41.1) ν a (CC) (45) + δ(r2) (11) + ν(c4 C7) (10) γ c (CH) (69) + γ(c1 O12) (11) (91.0) (131.4) wag(nh 2 ) (63) + γ c (CH) (20) wag(nh 2 ) (72) + rock(c7h 2 ) (13) (18.5) (14.6) γ a (CH) (83) γ a (CH) (74) + γ c (CH) (11) 767.9; (35.8) (15.9) (7.7) rock(c8h 2 ) (30) + rock(c7h 2 ) (29) n.o. n.o (24.6) δ(r1) (15) + wag(nh 2 ) (13) + ν(c4 C7) (12) + rock(c8h 2 ) (10) n.o (1.1); (2.5) (4.4) τ(r1) (47) n.o (1.0) (5.5) (0.6) τ(r1) (36) + ν(c4 C7) (19) + δ(r1) (15) τ(r1) (79) + γ(cl O12) (11) n.o. n.o (0.6) (0.4) δ(r3) (75) + ν c (CC) (12) δ(r3) (75) + ν c (CC) (12) n.o ; (14.2) (24.8) (20.3) scis(c4 C7 C8) (17) + γ(c1 O12) (15) + τ(r2) (13) + γ(c4 C7) (13) n.o (9.4) (6.1) (21.5) γ(cl O12) (23) + δ(r2) (23) + τ(r2) (16) + scis(c7 C8 N9) (10) δ(r2) (25) + γ(cl O12) (12) + scis(c4 C7 C8) (12) + τ(r2) (10) δ(r2) (25) + γ(cl O12) (22) + τ(r2) (18) + scis(c7 C8 N9) (10) n.o. n.o (14.0) (7.7) scis(c7 C8 N9) (27) + δ (R2) + (25) δ(cl O12) (51) + δ(c4 C7) (16) + τ(r3) (14) + δ(r3) (14) n.o. n.o (2.8) (2.3) δ(cl O12) (43) + τ(r3) (18) + δ(c4 C7) (11) + δ(r3) (11) τ(r3) (101) n.o. n.o (2.7) (3.2) τ(r3) (96) + δ(c1 O12) (11) τ(r1) (32) + γ(c1 O12) (23) + γ (C4 C7) (18) + scis(c4 C7 C8) (15) n.i. n.i (1.1) (12.5) τ(r1) (16) + γ(c8 N9) (13) + γ(c1 O12) (12) + scis(c7 C8 N9) (12) n.i. n.i (12.9) (3.3) δ(c4 C7) (33) + scis(c7 C8 N9) (14) + δ(c1 O12) (11) scis(c7 C8 N9) (46) + δ(r2) (17) + ν(c4 C7) (15) δ(c4 C7) (55) + δ (C1 O12) (20) + ν d (CC) (11) n.i. n.i (106.2) (0.6) γ(oh) (90) γ(oh) (99) n.i. n.i (62.9) (0.4) τ(nh 2 ) (84) τ(nh 2 ) (87) n.i. n.i (6.9) (20.3) τ(r2) (27) + scis(c4 C7 C8) (23) + τ(r2) (48) + scis(c4 C7 C8) (29) γ(c8 N9) (15) 10254

8 Table 2. continued experimental ν a(i b ) calculated ν c (A th ) PED d (%) Ar Xe G g harm. scaled G g anh Ag harm. scaled Ag anh G g Ag n.i. n.i (2.8) (21.5) 86.6 γ(c8 N9) (58) + τ(r2) (25) γ(c8 N9) (86) n.i. n.i (0.1) (7.7) 73.3 γ(c4 C7) (34) + scis(c4 C7 C8) (28) + γ(c4 C7) (40) + τ(r2) (33) + τ(r2) (24) + γ(c8 N9) (11) scis(c4 C7 C8) (21) n.i. n.i (0.2) (2.3) 30.4 τ(c7 C8) (99) τ(c7 C8) (92) a Wavenumbers of the strongest components of split bands are given in italics. b Relative integrated intensities. c Theoretical positions of absorption bands were scaled using the empirical formula ν scaled = 0.978ν unscaled. d PEDs lower than 10% are not included. Definition of internal coordinates is given in Table S1 in the Supporting Information. e n.o., not observed; n.i., not investigated. in particular, the predicted vibrational shifts of the G g and Ag conformers are in good agreement with the separation of the two observed components in the xenon matrix spectra. The intensity of the ν(nh 2 ) bands is predicted to be very low, and these bands are hardly discernible in the measured spectra. The calculations do not predict a strong effect on the ν(nh 2 ) frequencies caused by the intramolecular N H π hydrogen bond. The shifts in the calculated frequencies for the conformers bearing this interaction in relation to the Gg and G g conformers, in which this interaction is absent, are within a few wavenumbers. The proposed assignments for these modes are then tentative, due to their low intensity. The ν(ch) modes can be roughly separated into two groups, i.e., the aromatic CH stretching, giving rise to bands above 3000 cm 1, and the aliphatic CH 2 stretching vibrations, expected to absorb below 3000 cm 1. Although the theoretical predictions agree fairly well with the experimental findings, these modes cannot distinguish between the different conformations because this region of the spectrum is very congested cm 1 Region. The main vibrational modes observed in this region, though giving rise to very intense bands, are not specific for distinguishing the various conformers. Nevertheless, the general agreement between the calculated (particularly in the case of the scaled harmonic calculations) and experimental data is rather good. As for ν(oh), these bands appear with several components (especially in Ar matrix), which can be ascribed to matrix site splitting and the presence of different conformers in the matrix. The band exhibiting the largest peak intensity in the argon matrix spectrum, observed at 1518 cm 1 (1514 cm 1 in Xe), originates from the all-in-phase in-plane bending CH vibration of the aromatic ring and is predicted at very close values for all conformers. The ν(co) mode is observed at ca (Ar) and 1254 (Xe) cm 1, also in good agreement with the theoretical predictions. The intense band at 1170 cm 1 (1169 cm 1 in Xe) is assigned to the δ(oh) in-plane bending mode. Compared to these three bands, all the remaining features observed in this spectral region are, in agreement with the theoretical predictions, very much less intense (see Table 2 for detailed assignments) cm 1 Region. Unlike the other regions of the spectrum, the differences of the calculated frequencies for the various conformers in this spectral region are adequate to be resolved and allow tentative assignment of bands to different conformers. Figure 6 shows a closer view of the cm 1 region of the experimental spectra and compares it with the predicted spectra of the G g and Ag conformers. To aid in the visual comparison of the calculated and measured spectra, the relative intensity of the calculated bands has been scaled accordingly with the theoretical predictions. The most intense absorption in this region is due to the NH 2 wagging mode, predicted at ca. 800 cm 1. The band at 767 cm 1 is clearly distinguishable from the other components and shall be ascribed to the Ag /Ag conformers exclusively. In turn, the bands at ca. 825 cm 1 are also clearly separated from the others, and are distinctive of the Gg/G g conformers. The bands between 775 and ca. 815 cm 1, on the other hand, are more difficult to doubtlessly assign based only on the comparison of the experimental spectra under discussion and the calculated data. However, as shown in sections 3.2 and 3.3, annealing and UV-irradiation experiments in xenon matrix allowed us also to make additional safe detailed band assignments to individual conformers in this range (see Table 2)

9 destruction of the matrix upon further heating. The xenon matrix was deposited at 30 K and annealed up to 60 K without remarkable alteration of the baseline, which is an indication of the physical state of the matrix, and without signals of tyramine aggregation in the recorded spectrum. In the argon matrix, annealing did not produce any band intensity changes, while in the experiments performed with xenon, changes in the relative intensity of the components of the most intense bands started to occur between 40 and 42 K (Figures 7 9). A similar annealing behavior was observed for Figure 6. Part of the experimental infrared spectra of TYR isolated in freshly deposited (a) Ar and (b) Xe matrixes, at 15 and 30 K, respectively. (c) Simulated infrared spectra of the G g (black line) and Ag (red line) conformers. For the details of the simulation, see the caption of Figure 5. It is interesting to compare tyramine amino group vibrations with those of the model compound n-propylamine. The latter compound, like tyramine, may also form two types of conformers around the CCCN backbone, gauche and anti. As in tyramine, the wagging vibrations of the gauche forms of matrixisolated n-propylamine were found at higher wavenumbers than those due to anti forms. 27 Unlike tyramine, n-propylamine cannot establish intramolecular H-bond interactions. Interestingly, the gauche forms of n-propylamine have their NH 2 wagging modes at lower frequencies than in gauche-tyramine. The increase of frequency of NH 2 wagging modes usually occurs when the corresponding groups are involved in H-bond interactions. 16 The observed increase of NH 2 wagging frequency in tyramine, compared to n-propylamine, is consistent with the presence of a weak stabilizing interaction in gauchetyramine. CH out-of-plane vibrations in phenol produce a very strong IR band around 750 cm 1. Due to proximity with NH 2 wagging, CH out-of-plane vibrations could also contribute around 800 cm 1 in tyramine, along with NH 2 wag. This may contribute to the complexity of observed spectrum around 800 cm 1. We refer also to spectra of a similar molecule, propylbenzene, isolated in argon matrix. Instead of an NH 2 group, propylbenzene has a CH 3 group at the same position as in tyramine. It shows a strong infrared band around 748 cm 1, and nothing particularly strong around 800 cm 1. This can be proof that in our case the bands around 800 cm 1 are due to the amino group wagging modes. Another interesting analogy with tyramine is that both gauche and anti conformers of propylbenzene were trapped in argon matrixes at 12 K, without any conformational cooling. This should reinforce our interpretation, confirming that it should be possible to trap also gauche (Gx) and anti (Ax) conformers of tyramine Annealing of the Matrixes. Annealing experiments were performed in both argon and xenon matrixes to aid in the interpretation of the spectra, in particular to confirm the presence of different conformers in the matrixes. Due to its physical characteristics, annealing of the argon matrix (deposited at 15 K) was carried out only up to 30 K, to avoid Figure 7. (a) Experimental infrared spectra of TYR in a freshly deposited Xe matrix at 30 K (black line) and after annealing at 42 K (red line). (b) simulated spectra for the G g (black line) and Ag (red line) conformers. For the details of the simulation, see the caption of Figure 5. Figure 8. (a) OH stretching region of the experimental infrared spectra of TYR in a freshly deposited Xe matrix at 30 K (black line) and after annealing at 42 K (red line). (b) Anharmonic prediction of ν(oh) for the G g (black line) and Ag (red line) conformers. Absorption bands were simulated using Lorentzian profiles centered at the B3LYP calculated (nonscaled) anharmonic frequency and having fwhm = 2 cm 1. Infrared intensities were taken from the harmonic B3LYP calculation and then scaled by 0.71 and 0.29, respectively, according to the calculated relative population of the conformers. rearrangements of conformers of trimethyl phosphate (TMP). 28 The rotations of methoxy groups in TMP (similar in size to the ethylamino fragment in tyramine) have calculated barriers of ca

10 window. In the gas phase, tyramine exhibits an absorption band in the UV spectrum with origin at about 282 nm. 7 In the performed experiments, irradiation of the deposited samples was then undertaken starting at 290 nm and ending at 270 nm, with steps of 1 nm. The irradiation time for each step was 2 min. Taking into account previous results in related compounds, for matrix-isolated tyramine the expected photochemical processes should be mainly photoisomerizations (Scheme 2). Depending on the reaction site of the molecule, Scheme 2. Possible Reaction Pathways after UV Irradiation of Matrix-Isolated TYR Figure 9. (a) Experimental infrared spectra of TYR in a freshly deposited Xe matrix at 30 K (black line) and after annealing at 42 K (red line). (b) Simulated anharmonic spectra for the G g (black line) and Ag (red line) conformers. For details of the simulation, see the caption of Figure kj mol 1, and internal conversion around these barriers in TMP in xenon matrixes was observed at temperatures above 40 K. Figure 7 shows the observed changes in the region of the NH 2 wagging vibration, compared with the frequency scaled harmonic predicted spectra for the G g and Ag conformers. Relative intensities are scaled according to the calculated relative population of the conformers. The OH stretching and the OH bending regions are shown in Figures 8 and 9, respectively. In these cases comparison with anharmonic calculations is also presented. The anharmonic prediction in the NH 2 wagging region is still very shifted from the experimental values and does not aid the interpretation. Indeed, the amino group wagging vibrations are known to be problematic for anharmonic simulation. 29,30 The large deviation from the experimental value for this mode is in agreement with anharmonic calculations performed on glycine. 31 The components that increase upon annealing fit well the calculated spectrum of the anti (Ax) conformers. These conformers are predicted by the calculations to be less stable than the gauche forms. Hence, based on the calculated relative energies, the reverse conversion could be expected, since annealing of the matrix must lead the conformational system to approach the low-temperature equilibrium, i.e., as a general rule, to conversion of the less stable conformers into the more stable ones. These results suggest that, at least in the highly polarizable xenon matrix, the anti conformers become more stable than the gauche forms. Nevertheless, the putative conflict between the experimental and calculated data may also be a result of insufficient accuracy of calculations to predict either relative conformational energies or vibrational spectra. Conclusions about the relative stability of the two conformers in the argon matrix could not be extracted due to the lack of observation of any spectral changes in the annealing experiments performed in this matrix. However, we can assume that the barrier is large enough to prevent the conformational interconversion, for those forms that are trapped in the matrix, in the temperature range accessible for experiments with argon Narrow-Band UV Irradiation Experiments. Freshly deposited argon and xenon matrixes of tyramine were irradiated using narrow-band tunable UV laser light, through a quartz isomerization processes in tyramine may lead to three classes of products: isomerization of the ethylamine side chain can deplete one of the gauche or anti conformers in favor of the other one (path A in Scheme 2); isomerization of the phenol ring can lead either to production of a Dewar isomer (path B) or an open ring conjugated ketene (path C). The formation of a Dewar isomer could be easily excluded, since no new bands were observed in the OH stretching region of the spectra of the irradiated matrixes (the OH stretching vibrations for the Dewar forms are calculated at least 16 cm 1 away from the ones of the other conformers of TYR). On the other hand, new bands in the cm 1 region started to appear at the early stages of irradiation at 290 nm, in the spectra of both argon and xenon irradiated tyramine matrixes. The presence of these bands is an unequivocal indication of the formation of a ketene species as can be deduced from the infrared spectra of ketenes reported in the literature. 36,37 For ketenes, the shapes of absorption bands of the same species are different in different environments, and even in the same environment; the discussion of this effect is not trivial and requires a dedicated study which is beyond the scope of this paper. Ketenes have been found to be main photoproducts resulting from the opening of aromatic six-membered rings bearing oxygen-containing substituents The ketene marker bands in the cm 1 region continuously increased along the performed irradiation experiments. Figure 10 shows this region of the spectra obtained during a series of irradiations of the argon and xenon matrixes. As can be seen from the total intensity of the new bands, the rate of the ringopening reaction is quite low. Irradiation of the xenon matrix at 287 nm has shown, in addition to the bands associated with the ketene species, results similar to those obtained during the annealing experiments. Some components of the most intense bands, assigned to the 10257

11 Figure 10. Region of the C C O antisymmetric stretching vibration in the infrared spectra of TYR isolated in (a) argon and (b) xenon matrixes, in the freshly deposited matrix (black line) and after UV irradiations. Ag conformer, increased, while bands ascribed to the G g form decreased. It can then be concluded that in the xenon matrix the photoisomerization of gauche form into anti form occurs simultaneously with the ring-opening reaction. On the other hand, irradiation of the argon matrix did not lead to a clear observation of the gauche anti isomerization process, with all the components of the recorded bands being observed to decrease simultaneously. In argon matrix, the single reliably observed photoprocess was then the formation of the isomeric open ring conjugated ketene. 4. CONCLUSIONS The infrared spectra of tyramine isolated in low-temperature argon and xenon matrixes were described for the first time. The spectra of the as-deposited matrixes are compatible with the presence of at least two types of conformers: one has been assigned to the most stable gauche forms about the CH 2 CH 2 bond (G g + Gg) and the other one has been assigned to the most stable anti type conformers (Ag + Ag). The most stable conformer in the gas phase (Gg) has a folded structure stabilized by the presence of a weak N H π hydrogen bond interaction. Observation of both gauche and anti forms in the as-deposited matrixes is consistent with the calculated relative values for the different conformational isomerization barriers, with that associated with the rotation around the CH 2 CH 2 bond, involving rotation of heavy atoms, being predicted to be the highest among all interconversion barriers. Annealing experiments performed in the xenon matrix, compared with harmonic and anharmonic predictions for the most stable conformers, seem to confirm the presence in the deposited matrix of the gauche and anti conformers and the occurrence of the gauche anti isomerization reaction upon temperature increase. This last result indicates a change in the relative energies of the two experimentally relevant conformers upon isolation in the polarizable xenon matrix, compared to the gas phase, with the more polar anti form being stabilized in the matrix environment New bands formed after UV irradiation of the matrix-isolated tyramine (both in argon and xenon matrixes) are consistent with the formation of a conjugated ketene species, generated via ring-opening reaction, which has already been shown to occur for similar six-membered-ring compounds bearing oxygencontaining substituents Concomitant occurrence of the gauche anti isomerization reaction was observed in the xenon matrix. For both matrixes, no evidence of photoinduced formation of Dewar isomers of tyramine has been found. ASSOCIATED CONTENT *S Supporting Information Internal coordinates used in normal mode analyses for tyramine (Table S1) as well as calculated harmonic and anharmonic wavenumbers, absolute intensities, and potential energy distribution for seven tyramine conformers at the B3LYP/ G(d,p) level of theory (Tables S2 and S3), and complete ref 10. This material is available free of charge via the Internet at AUTHOR INFORMATION Corresponding Authors * rfausto@ci.uc.pt. Tel.: * sonia.melandri@unibo.it. Tel.: Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The research leading to these results has received funding from the European Community s Seventh Frame Work Programme under Grant Agreement No , and the Portuguese Fundac aõ para a Cie ncia e a Tecnologia (FCT) Projects PTDC/QUI-QUI/111879/2009 and PTDC/QUI-QUI/ /2010, FCOMP FEDER , cofunded by QREN-COMPETE-UE. B.M.G. acknowledges FCT for the postdoctoral grant SFRH/BPD/44689/2008. REFERENCES (1) Karovicǒva, J.; Kohajdova, Z. Biogenic Amines in Food. Chem. Pap. 2005, 59, (2) Silla Santos, M. H. Biogenic Amines: Their Importance in Foods. Int. J. Food Microbiol. 1996, 29, (3) Snyder, S. H. Drugs and the Brain; Scientific American Library: New York, (4) Mycek, M. J.; Harvey, R. A.; Champe, P. C. Farmacologia; Zanichelli: Bologna, Italy, (5) Melandri, S.; Maris, A. Intramolecular Hydrogen Bonds and Conformational Properties of Biogenic Amines: A Free-Jet Microwave Study of Tyramine. Phys. Chem. Chem. Phys. 2004, 6, (6) Yoon, I.; Seo, K.; Lee, S.; Lee, Y.; Kim, B. Conformational Study of Tyramine and Its Water Clusters by Laser Spectroscopy. J. Phys. Chem. A 2007, 111, (7) Makara, K.; Misawa, K.; Miyazaki, M.; Mitsuda, H.; Ishiuchi, S.; Fujii, M. Vibrational Signature of the Conformers in Tyramine Studied by IR Dip and Dispersed Fluorescence Spectroscopies. J. Phys. Chem. 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