Nuclear quadrupole coupling interactions in the rotational spectrum of tryptamine

Journal of Molecular Spectroscopy 269 (2011) 41–48

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Nuclear quadrupole coupling interactions in the rotational spectrum of tryptamine J.L. Alonso ⇑, V. Cortijo, S. Mata, C. Pérez, C. Cabezas, J.C. López, W. Caminati 1 Grupo de Espectroscopía Molecular (GEM) Edificio Quifima, Área Química Física, Campus Miguel Delibes, Parque Científico, Universidad de Valladolid, E-47005 Valladolid, Spain

a r t i c l e

i n f o

Article history: Received 1 March 2011 In revised form 4 April 2011 Available online 23 April 2011 Keywords: Tryptamine Rotational spectroscopy Nuclear quadrupole coupling interactions Conformation Molecular structure

a b s t r a c t Four conformers of tryptamine have been detected in a supersonic expansion and characterized by laser ablation molecular beam Fourier transform microwave spectroscopy LA-MB-FTMW in the 5–10 GHz frequency range. The quadrupole hyperfine structure originated by two 14N nuclei has been completely resolved for all conformers and used for their unambiguous identification. Nuclear quadrupole coupling constants of the nitrogen atom of the side chain have been used to determine the orientation of the amino group involved in N–H  p interactions: to the p electronic system of the pyrrole unit in the GauchePyrrole conformers (GPy) or to the phenyl unit in the Gauche-Phenyl ones. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction Tryptamine (TRA) is a monoamine alkaloid that is formed in plant and animal tissues from tryptophan. Its biochemical relevance is outlined by the fact that the word ‘‘tryptamine’’ appears in the title of more than 500 scientific papers from 1990 to date. The study of the molecular conformations that TRA can adopt could be important for the understanding its biological function and selectivity. This is especially true for biological processes involving molecular recognition, since the active sides in receptors and enzymes are chiral and will interact only with molecules of the correct conformation. The conformational manifold of TRA is composed of nine plausible conformers shown in Table 1 predicted to be the most stable ones. It is related to the orientation of the NH2 group and to the values of the dihedral angles of the ethane chain shown in Scheme 1. The conformational signatures of TRA have been investigated with several spectroscopic techniques combined with supersonic expansion [1–8]. Philips and Levy observed a partially resolved rotational structure in the fluorescence excitation spectrum in a supersonic jet [1]. They found six features, assigned to six different conformers, and they were able to obtain rotationally resolved details for five of them. Connell et al. [2] performed conformational analysis of TRA with rotational coherence spectroscopy. They determined B + C values of the rotational constants for five conformers. Seven conformers of TRA have been more recently investigated by Carney and Zwier with resonant ion-dip infrared and ⇑ Corresponding author. Fax: +34 983186349. E-mail address: [email protected] (J.L. Alonso). Present address: Dipartimento di Chimica ‘‘G. Ciamician’’ dell’Università, via Selmi 2, I-40126 Bologna, Italy. 1

0022-2852/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2011.04.009

UV–UV hole burning spectroscopies, and density functional theory (DFT) calculations [3,4]. They could, mainly based on the theoretical calculations, find the relative energies of structural minima. Caminati observed two gauche conformers by free jet millimetrewave absorption spectroscopy [5]. Nguyen et al. [6,8] and Schmitt et al. [7] assigned the rotationally resolved laser-induced fluorescence spectra of up to seven conformers. In these studies, the conformational assignment was based on the comparison between the theoretically predicted and the observed sets of rotational constants. Table 1 shows the predicted spectroscopic parameters for the nine conformers of TRA. As can be seen, the values of the rotational constants are very close for some of the conformers, making it a challenge to assign them. Here, we present a different approach to definitely identify the conformers of TRA. This is based on the presence of two 14N nuclei with a nonzero nuclear quadrupole moment (I = 1) which interact with the electric field gradient at the site of the nuclei. The nuclear spin of the two 14N nuclei couples to the rotational angular moment resulting in a complicated hyperfine structure with many components. This hyperfine structure depends strongly of the electronic environment of the amino nitrogen atom and its orientation with respect to the principal inertial axis. Therefore, each one of the adopted conformations of TRA would show a different hyperfine structure reflected in different values of the quadrupole coupling constants as shown in Table 1. It has been shown that the determination of those constants constituted the key to assign unambiguously conformers or tautomers of biomolecules when rotational constants are not conclusive enough [9]. Thus, the analysis of the nuclear quadrupole coupling interaction in the rotational spectra of TRA can be used as a structural tool to provide an independent approach for a conclusive identification of its conformations. Molecular beam Fourier transform microwave

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J.L. Alonso et al. / Journal of Molecular Spectroscopy 269 (2011) 41–48

Table 1 Calculated spectroscopic parameters and ab initio zero point energies at MP2/6-311++G(d,p) level of theory for the nine low-energy conformers of tryptamine. Parameter A (MHz) B (MHz) C (MHz) vaa Nch (MHz) vbb Nch (MHz) vcc Nch (MHz) vaa Npy (MHz) vbb Npy (MHz) vcc Npy (MHz) la (D) lb (D) lc (D) DE (cm1)

GPy-out

GPy-up 1699 696 561 0.86 0.89 1.69 1.28 1.69 2.99 0.6 1.2 0.7

GPy-in 1674 696 561 1.10 0.27 0.82 1.29 1.67 2.96 0.5 2.9 0.7

2315 574 469 -0.45 1.73 2.17 1.59 1.80 3.38 2.0 1.1 0.1

0 cm1

184 cm1

878 cm1

Anti-up

Anti-ph

Anti-py

A (MHz) B (MHz) C (MHz)

1742 618 478

1748 621 479

1755 617 477

vaa Nch (MHz) vbb Nch (MHz) vcc Nch (MHz)

1.94 0.94 2.88

2.70 0.33 2.37

0.77 2.14 1.37

vaa Npy (MHz) vbb Npy (MHz) vcc Npy (MHz) la (D) lb (D) lc (D)

1.53 1.75 3.28 0.8 2.6 0.9

1.56 1.77 3.32 0.1 2.8 1.3

1.54 1.77 3.31 0.9 1.0 0.7

DE (cm1)

792 cm1

791 cm1

GPh-out A (MHz) B (MHz) C (MHz) vaa Nch (MHz) vbb Nch (MHz) vcc Nch (MHz) vaa Npy (MHz) vbb Npy (MHz) vcc Npy (MHz) la (D) lb (D) lc (D) DE (cm1)

GPh-up 1585 763 565 1.94 0.42 1.51 1.46 1.63 3.09 0.2 2.8 1.2

306 cm1

794 cm1 GPh-in 1604 739 566 0.43 2.24 1.81 1.37 1.57 2.95 1.0 2.5 0.7

516 cm1

MB-FTMW [10] spectroscopy provides the high resolution power required for the observation of the hyperfine structure. Although TRA could be investigated in gas phase using heating methods for vaporization, it seemed adequate to take advantage of the benefits of our laser ablation LA-MB-FTMW technique [11] to efficiently vaporize the solid TRA.

1569 750 547 4.68 2.74 1.94 1.51 1.70 3.21 1.4 1.7 0.3 1311 cm1

pressing tryptamine powder mixed with minimum quantities of a commercial binder, was vaporized by the third harmonic (355 nm) of a picoseconds (150 ps) Q-switched pulsed Nd:YAG laser (20 mJ/pulse). The rotational lines of the three conformers previously observed with the heating method were found to have now a better signal-to-noise ratio and, as a consequence, we could assign the rotational spectrum of a fourth conformer of TRA.

2. Experimental details 3. Theoretical calculations The rotational spectra of tryptamine were recorded in the range 5–10 GHz using a molecular beam Fourier transform microwave spectrometer MB-FTMW, with the molecular beam travelling parallel to the axis of the Fabry-Pérot resonator [12], described in detail elsewhere [13]. TRA (Sigma Aldrich) is a white crystalline solid with a melting point 114 °C, and with a negligible vapour pressure at room temperature. Then, its vaporization required some ad hoc tools. In a first set of experiments, the vaporization of the sample was obtained with our heated nozzle, which has been modified with respect to its previous design [14] in order to improve its performance. The spectra of three different conformers were observed by a good signal-to-noise ratio with a temperature of about 200 °C. In a second set of experiments, our LA-MB-FTMW spectrometer was used [11]. A solid rod of tryptamine, formed by

Although several sets of theoretical calculations are already available in the literature [5–7], we performed MP2/6311++G(d,p) ab initio calculations [15] for the nine most stable conformers in order to estimate, besides rotational constants, other parameters which are relevant to our experiments, such as the electric dipole moment components and the nuclear quadrupole coupling constants of the two 14N nuclei. The predicted spectroscopic parameters of the nine conformers and their shapes are collected in Table 1. The nomenclature is the one used in previous works [1–8]. We label as Npy and Nch the nitrogen atoms of the pyrrole ring and of the side chain, respectively. The labels ‘‘G’’ or ‘‘A’’ correspond to the configuration of the alkyl chain: Gauche (G, folded) or Anti (A, extended). The gauche conformers are divided

J.L. Alonso et al. / Journal of Molecular Spectroscopy 269 (2011) 41–48

in two groups: ‘‘GPy’’ (when the chain is oriented towards the pyrrole side of indole) and ‘‘GPh’’ (when the chain is oriented towards the phenyl ring of indole). To distinguish between the amino group orientations in Gauche conformers three additional labels are used: ‘‘out’’, ‘‘up’’ and ‘‘in’’. The Anti conformers are divided in three groups corresponding to three possible arrangements of the side chain amino group upon rotation about the Nch–C bond: ‘‘up’’, ‘‘Py’’ and ‘‘Ph’’ which refer to the orientation of the Nch lone pair. One can see from Table 1 that only the diagonal elements of the quadrupole coupling tensor of the Nch allows for the conformational discrimination. 4. Results 4.1. Gauche-Pyrrole conformers The first frequency scan was performed in the spectral region where the transitions 61,6 50,5 of the GPy-out and GPy-up conformers were expected according to the values calculated with the rotational constants reported by Caminati [5]. This scan was repeated while changing the nozzle temperature from 120 to 200 °C. The two transitions at 7368 and 7343 MHz, respectively, appeared at ca. 170 °C. The best signal-to-noise ratio was obtained, however, with a temperature around 200 °C. As expected, the quadrupole hyperfine structure originated by the two 14N nuclei is completely different for the two transitions, as shown in Fig. 1. More transitions were measured for these two conformers: four la and five lb-R-type transitions for the GPy-out conformer and seven lb-R-type transitions for the GPy-up species. The 56 and of 36 hyperfine components obtained respectively for GPy-out and for the GPy-up were fitted [16] using a Watson’s A-reduced semirigid

Fig. 1. Experimental quadrupole hyperfine structures of the 61,6 predicted ab initio structures (below).

43

rotor Hamiltonian (Ir representation), [17] supplemented with a term to account for the nuclear quadrupole coupling contribution [18], set up in the coupled basis set (I1 I2 I J F), I1 + I2 = I, I + J = F [19]. The energy levels involved in each transition are thus labelled with the quantum numbers J, K1, K+1, I, F. The determined rotational constants A, B and C and the quadrupole coupling constants vaa, vbb, and vcc for both conformers are given in Table 2. The observed transition frequencies are given in Table 3. One can note that it is not possible to distinguish among the GPy-out and GPy-up conformers only on the basis of the values of the rotational constants. These conformers differ only in the orientation of the light NH2 group, so they have similar values of the rotational constants and pyrrole ring Npy atom quadrupole coupling constants. Vice versa, the values of the 14N nuclear quadrupole coupling constants side chain Nch atom, which depend on the orientation of the amino group with respect to the principal inertial axis system, provide a conclusive assignment of the GPyout and GPy-up conformers. Looking at Table 2, one can see that vaa and vcc have opposite signs for the two conformers. This conformational assignment is supported also by the calculated values of the la and lb dipole moment components. Only lb-type transitions could be measured, indeed, for the GPy-up conformer, consistently with its low la and lc predicted values (see Table 1). The most intense lines (those with the higher signal-to-noise ratio) belong to the conformer predicted to be the most stable, GPy-out. 4.2. Gauche-Phenyl conformers In this case, the predictions of the spectra were based on the experimental rotational constants determined with rotationally resolved LIF spectroscopy [6,8] and on the theoretical values of the

50,5 transition, due to the presence of two

14

N nuclei, for the GPy conformers (top), and sketches of

44

J.L. Alonso et al. / Journal of Molecular Spectroscopy 269 (2011) 41–48

Table 2 Experimental spectroscopic constants of the Gauche-Pyrrole conformers of tryptamine. GPy-out A (MHz) B (MHz) C (MHz) DJ (kHz)b vaa(Nch) (MHz) vbb(Nch) (MHz) vcc(Nch) (MHz) vaa(Npy) (MHz) vbb(Npy) (MHz) vcc(Npy) (MHz) Nc r (kHz)d

a b c d

GPy-up

1730.20222(38) 681.87630(22) 551.42723(26) 0.0570(33) 0.725(19) 0.576(19) 1.301(16) 1.491(17) 1.529(14) 3.020(14) 56 3.7

a

1709.43510(21) 681.87913(14) 550.81379(11) 0.0584(11) 0.759(10) 0.018(9) 0.777(9) 1.343(10) 1.6459(82) 2.9889(82) 36 1.6

Error in parentheses are expressed in units of the last digit. The remaining quartic centrifugal distortion constants were fixed to zero in the fit. Number of quadrupole hyperfine components in the fit. RMS deviation of the fit.

Table 3 Observed frequencies and residuals (in MHz) for the nuclear quadrupole coupling hyperfine components of conformers Gpy-out and Gpy-up of tryptamine. Gpy-out

Gpy-up

J0 ðK 01 ; K 0þ1 Þ  J00 ðK 001 ; K 00þ1 Þ

I0

F0

I00

F00

m

J0 ðK 01 ; K 0þ1 Þ  Jð K 00 ðK 001 ; K 00þ1 Þ

I0

F0

I00

F00

m

3(2, 1)  2(1, 2)

2 2 2 2 2 2 1 1 1 0 2 2 2 1 2 2 1 0 2 2 2 2 1 1 1 2 2 2 1 1 2 2 2 1

5 4 3 3 2 1 4 3 2 3 7 6 5 6 5 4 4 5 7 6 5 4 6 5 4 8 7 6 7 6 7 5 4 7

2 2 2 1 2 2 1 1 1 0 2 2 2 1 2 2 1 0 2 2 2 2 1 1 1 2 2 2 1 1 2 2 2 1

4 3 2 3 1 0 3 2 1 2 6 5 4 5 4 3 3 4 6 5 4 3 5 4 3 7 6 5 6 5 6 4 3 6

7292.971 7291.827 7292.024 7293.205 7293.959 7293.322 7293.438 7293.683 7291.658 7292.878 5960.272 5960.459 5960.363 5960.199 6435.788 6435.663 6435.832 6435.741 6395.741 6396.224 6396.035 6395.498 6395.555 6395.698 6396.245 7070.329 7070.502 7070.420 7070.263 7070.345 6933.050 6932.968 6932.991 6932.934

3(2, 1)  2(1, 2)

2 2 2 2 1 1 1 0 2 2 2 1 0 2 2 2 1 0 2 2 1 0 2 2 2 2 1 2 2 2 2 1 0 2

5 4 3 2 4 3 2 3 7 6 5 6 5 8 7 6 7 6 7 6 7 6 9 8 7 6 8 9 8 7 5 6 7 8

2 2 2 2 1 1 1 0 2 2 2 1 0 2 2 2 1 0 2 2 1 0 2 2 2 2 1 2 2 2 2 1 0 2

4 3 2 1 3 2 1 2 6 5 4 5 4 7 6 5 6 5 6 5 6 5 8 7 6 5 7 8 7 6 4 5 6 7

7232.436 7231.210 7231.025 7232.872 7232.159 7232.548 7231.283 7233.013 6370.122 6370.598 6370.679 6370.193 6370.067 7343.488 7343.859 7343.924 7343.548 7343.460 6641.862 6641.877 6641.999 6642.060 8335.845 8336.113 8336.164 8335.881 8335.889 7869.027 7868.959 7868.970 7869.079 7868.970 7869.064 9057.823

5(0, 5)  4(0, 4)

5(1,4 )  4(1, 3)

5(1, 5)  4(0, 4)

6(0, 6)  5(0, 5)

6(1, 6)  5(1, 5)

5(1, 5)  4(0, 4)

6(1, 6)  5(0, 5)

6(0, 6)  5(1, 5)

7(1, 7)  6(0, 6)

7(0, 7)  6(1, 6)

8(0, 8)  7(1, 7)

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J.L. Alonso et al. / Journal of Molecular Spectroscopy 269 (2011) 41–48 Table 3 (continued) Gpy-out

Gpy-up

J0 ðK 01 ; K 0þ1 Þ  J00 ðK 001 ; K 00þ1 Þ

I0

F0

I00

F00

m

6(1, 6)  5(0, 5)

2 2 2 2 2 1 1 1 2 2 2 2 1 1 1 0 2 2 2 2 2 1

8 7 6 5 4 7 6 5 8 7 6 5 7 6 5 6 9 8 7 6 5 8

2 2 2 2 2 1 1 1 2 2 2 2 1 1 1 0 2 2 2 2 2 1

7 6 5 4 3 6 5 4 7 6 5 4 6 5 4 5 8 7 6 5 4 7

7368.445 7368.816 7368.666 7368.263 7368.427 7368.291 7368.411 7368.824 6634.866 6634.731 6634.750 6634.974 6634.906 6634.948 6634.707 6634.848 8359.303 8359.577 8359.463 8359.176 8359.293 8359.191

6(0, 6)  5(1, 5)

7(1, 7)  6(0, 6)

Fig. 2. Experimental quadrupole hyperfine structures of the 61,6 predicted ab initio structures (below).

J 0 ðK 01 ; K 0þ1 Þ  Jð K 00 ðK 001 ; K 00þ1 Þ

50,5 transition, due to the presence of two

quadrupole coupling constants. Using the same experimental conditions described above, first scans were done around the frequencies predicted for bR-type transitions that were expected to be the most

14

I0

F0

I00

F00

m

1 0

8 8

1 0

7 7

9057.837 9057.852

N nuclei, for the GPh conformers (top), and sketches of

intense ones. After discarding all the signals caused by the transitions of the GPy assigned conformers, one series of weaker lines with the quadrupole pattern expected for the GPh-out conformer was left.

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J.L. Alonso et al. / Journal of Molecular Spectroscopy 269 (2011) 41–48

Table 4 Experimental spectroscopic constants of the Gauche-Phenyl conformers of tryptamine. GPh-out A (MHz) B (MHz) C (MHz) (MHz) DJ (kHz)b vaa(Nch) (MHz) vbb(Nch) (MHz) vcc(Nch) (MHz) vaa(Npy) (MHz) vbb(Npy) (MHz) vcc(Npy) (MHz) Nc r (kHz)d

a b c d

GPh-up a

1592.4573(24) 754.7709(15) 560.69491(13) 0.0261(15) 1.692(33) 0.331(25) 1.361(25) 1.491(33) 1.464(25) 2.955(25) 36 2.3

1604.9884(46) 737.5957(12) 561.27626(23) – 0.507(74) 2.09(13) 1.580(56) 1.3846(87) 1.455(37) 2.840(28) 20 3.4

Error in parentheses are expressed in units of the last digit. The remaining quartic centrifugal distortion constants were fixed to zero in the fit. Number of quadrupole hyperfine components in the fit. RMS deviation of the fit.

Seven bR-type transitions were measured and a total of 36 hyperfine components were fitted as described previously. Even when accumulating many experiments, it was not possible to detect rotational transitions belonging to other conformers. However, we noted that when vaporizing the sample with the third harmonic of a Q-switched pulsed Nd:YAG laser (20 mJ/pulse) it was possible to detect additional weaker transitions of the three assigned conformers. The signal-to-noise ratio with this vaporizing method was better than that observed with the heating system, so that we decided to search for GPh-up, predicted to be the fourth conformer in order of energy. Hence, frequency scans in the region predicted for the 61,6 50,5 rotational transition of this fourth conformer allowed to observe, when using a high number of cycles, a weak set of signals around 7336 MHz. To obtain completely resolved lines it was necessary to accumulate between 5000 and 9000 cycles. Five bR-type rotational transitions of a new spectrum were measured in the frequency range 5–7.5 GHz, likely belonging to conformer GPh-up. This assignment was confirmed, once again, by the agreement between the experimental and theoretically predicted hyperfine pattern of the rotational transitions, as shown in Fig. 2 for the 61,6 50,5 transition. A total of 20 hyperfine components were fitted with the same procedure adopted for the three previous conformers. The experimental values of the rotational constants and quadrupole coupling constants are collected in Table 4 for both conformers. Measured transitions are given in Table 5. Conformer GPh-out is structurally similar to conformer GPh-up. The only different between them is the orientation of the amino group. This slight structural difference, reflected in similar values of the rotational constants, has a great effect on the values of their 14 N quadrupole coupling constants; vaa and vbb of the 14Nch atom have opposite signs for the two conformers (see Table 4). Conformers GPh-out and GPh-up could not have been distinguished without the analysis of the quadrupole coupling effect in the rotational spectra, which constitutes a unique identifier for conformers.

NH2 H 2C CH2

N H Scheme 1. Sketch of tryptamine, showing the hindered single bond rotations which govern the conformational equilibrium.

Relative intensity measurements were carried out on three

lb-type transitions of each conformer in order to determine the relative populations in the supersonic jet. Intensity measurements for conformer GPh-up are not so reliable due to the low intensity of its spectrum. The resulting population trend NGPy-out > NGPy-up > NGPh-out > NGPh-up is consistent with the predicted values of relative energies of Table 1. Further frequency scans for the search of higher energy conformers did not produce any result. Such a failure is not surprising because the calculated energies of the remaining conformers are at least 700 cm1 above that of the most stable species, GPy-out.

5. Conclusions The rotational spectra of the four low-energy conformers of TRA, two Gauche-Pyrrole and two Gauche-Phenyl, have been observed using LA-MB-FTMW spectroscopy. Conclusive identification of these conformers was achieved by comparing the experimental values of the rotational and nuclear quadrupole coupling constants

47

J.L. Alonso et al. / Journal of Molecular Spectroscopy 269 (2011) 41–48 Table 5 Observed frequencies and residuals (in MHz) for the nuclear quadrupole coupling hyperfine components of conformers GPh-out and GPh-up of tryptamine. GPh-out J

0

ðK 01 ; K 0þ1 Þ

GPh-up J

00

ðK 001 ; K 00þ1 Þ

3(2, 1)  2(1, 2)

5(1, 5)  4(0, 4)

6(1, 6)  5(0, 5)

6(0, 6)  5(1, 5)

7(1, 7)  6(0, 6)

7(0, 7)  6(1, 6)

8(0, 8)  7(1, 7)

I

0

2 2 2 1 1 0 2 2 2 2 1 1 0 2 2 2 2 1 1 2 2 2 2 1 2 2 1 1 2 2 2 0 2 2 1 1

0

I

00

F

5 4 3 3 2 3 7 6 5 4 6 5 5 8 7 6 5 7 6 8 7 6 5 7 9 7 8 7 9 8 7 7 10 9 8 7

2 2 2 1 1 0 2 2 2 2 1 1 0 2 2 2 2 1 1 2 2 2 2 1 2 2 1 1 2 2 2 0 2 2 1 1

4 3 2 2 1 2 6 5 4 3 5 4 4 7 6 5 4 6 5 7 6 5 4 6 8 6 7 6 8 7 6 6 9 8 7 6

F

00

m

J 0 ðK 01 ; K 0þ1 Þ  J00 ðK 001 ; K 00þ1 Þ

I0

F0

I00

F00

m

7187.539 7186.173 7185.882 7187.718 7186.268 7188.145 6292.517 6292.815 6292.906 6292.611 6292.593 6292.533 6292.505 7313.857 7314.063 7314.126 7313.933 7313.912 7313.872 7044.946 7044.966 7045.011 7045.039 7044.988 8371.508 8371.691 8371.549 8371.519 8230.379 8230.427 8230.456 8230.411 9386.107 9386.151 9386.121 9386.161

4(1, 4)  3(0, 3)

2 2 2 1 2 2 2 1 1 2 2 0 2 2 2 1 1 1 2 2

6 5 3 5 7 5 4 6 5 7 4 5 8 6 5 7 6 5 6 5

2 2 2 1 2 2 2 1 1 2 2 0 2 2 2 1 1 1 2 2

5 4 2 4 6 4 3 5 4 6 5 4 7 5 4 6 5 4 5 4

5330.476 5330.969 5330.834 5330.902 6321.222 6321.886 6321.510 6321.530 6321.173 5751.415 5751.264 5751.484 7335.857 7336.306 7336.064 7336.064 7335.835 7336.083 6998.144 6998.215

with those predicted ab initio. The nuclear quadrupole coupling constants are of great relevance because they allow discrimination between conformers with similar rotational constants. It constitutes a conclusive tool in the identification of TRA conformers and confirms the assignment of the lower energy conformers of Refs. [6–8], based on the values of the rotational constants. Nuclear quadrupole coupling constants of the nitrogen atom (Nch) of the side chain can be used to establish the orientation of the amino group within the molecule (see Tables 2 and 3). Hence, it is possible to identify an experimentally characterize N–H  p interactions without the need for three dimensional structures obtained by ab initio calculations. The four conformers are stabilized by a weak NH  p interaction between the amino group and the high p density sites of the indole ring: with the p electronic system belonging to the pyrrole unit (GPy conformers) or to the phenyl unit (GPh conformers). The higher stabilities of the GPy conformers suggest that the p electronic density of the pyrrole ring is higher than that of the phenyl ring. This relevant information is not attainable from other spectroscopic techniques. The N–H  p interaction observed in the GPh and GPy conformers of TRA is the same one stabilizing the Gauche conformers of the related PEA molecule. It seems that for this class of biologically important compounds, characterized by an ethylamine groups attached to an aromatic system, the N–H  p weak hydrogen bond is the leading structural motif. Acknowledgments The research was supported by the Ministerio de Ciencia e Innovación (Dirección General de Investigación CTQ2006-05981/BQU),

5(1, 5)  4(0, 4)

5(0, 5)  4(1, 4)

6(1, 6)  5(0, 5)

6(0, 6)  5(1, 5)

Programa Consolider Ingenio 2010 (Dirección General de Investigación CSD2009-00038 Molecular Astrophysics) and the Junta de Castilla y León (Grant VA070A08). V.C., C.P. and C.C. thank the Ministerio de Ciencia e Innovación for FPI grants. W. Caminati thanks the group of molecular spectroscopy of the University of Valladolid for their kind hospitality, and the Minister of Education of the Spanish Government for a grant for a position as Invited Professor.

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