Low frequency backbone vibrations of individual conformational isomers: Tryptamine

Schmitt, Michaël; Ke, Feng; Böhm, Marcel; Kleinermanns, Karl

doi:10.1063/1.2357593

Cited by 18 publications

(26 citation statements)

References 20 publications

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“…Under supersonic jet-cooled expansion conditions, ethylbenzene exists as a single conformer. 30 These preferences also extend to alkyl chains attached to other aromatic rings such as indole in tryptamine, 9,68 3-indole-propionic acid, 69 and tryptophan. 60 This conformation, with the C X -C Y bond perpendicular to the ring, both minimizes steric repulsion and allows hyperconjugation between the C X -C Y bond of the chain and aromatic system.…”

Section: The Conformational Preferences Of Related Single-chain mentioning

confidence: 99%

“…[1][2][3][4][5][6][7][8][9][10][11][12] By cooling the molecules in a supersonic expansion, ultraviolet transitions due to single conformers can often be resolved. One of the fruitful strategies used to study these molecules is to incorporate into the molecule an ultraviolet chromophore that can be readily detected using either laser-induced fluorescence or resonant two-photon ionization.…”

Section: Introductionmentioning

confidence: 99%

“…The preponderance of studies in this field have been directed at molecules with biological relevance, leading to impressive progress in the spectroscopy of amino acids, 3,5,6,9,30,31 DNA bases, [32][33][34][35][36][37][38][39][40] and small polypeptides. 3,31,[41][42][43][44][45][46][47][48][49][50] The present study has a somewhat different focus and goal.…”

Section: Introductionmentioning

confidence: 99%

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Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O-(2-acetamidoethyl)-N-acetyltyramine

Shubert

Baquero

Clarkson

et al. 2007

The Journal of Chemical Physics

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Resonant two-photon ionization (R2PI), resonant ion-dip infrared (RIDIR), and UV-UV hole-burning spectroscopies have been employed to obtain conformation-specific infrared and ultraviolet spectra under supersonic expansion conditions for O-(2-acetamidoethyl)-N-acetyltyramine (OANAT), a doubly substituted aromatic in which amide-containing alkyl and alkoxy side chains are located in para positions on a phenyl ring. For comparison, three single-chain analogs were also studied: (i) N-phenethyl-acetamide (NPEA), (ii) N-(p-methoxyphenethyl-acetamide) (NMPEA), and (iii) N-(2-phenoxyethyl)-acetamide (NPOEA). Six conformations of OANAT have been resolved, with S(0)-S(1) origins ranging from 34,536 to 35,711 cm(-1), denoted A-F, respectively. RIDIR spectra show that conformers A-C each possess an intense, broadened amide NH stretch fundamental shifted below 3400 cm(-1), indicative of the presence of an interchain H bond, while conformers D-F have both amide NH stretch fundamentals in the 3480-3495 cm(-1) region, consistent with independent-chain structures with two free NH groups. NPEA has a single conformer with S(0)-S(1) origin at 37,618 cm(-1). NMPEA has three conformers, two that dominate the R2P1 spectrum, with origin transitions between 35,580 and 35,632 cm(-1). Four conformations, one dominate and three minor, of NPOEA have been resolved with origins between 35,654 and 36,423 cm(-1). To aid the making of conformational assignments, the geometries of low-lying structures of all four molecules have been optimized and the associated harmonic vibrational frequencies calculated using density functional theory (DFT) and RIMP2 methods. The S(0)-S(1) adiabatic excitation energies have been calculated using the RICC2 method and vertical excitation energies using single-point time-dependent DFT. The sensitivity of the S(0)-S(1) energy separation in OANAT and NPOEA primarily arises from different orientations of the chain attached to the phenoxy group. Using the results of the single-chain analogs, tentative assignments have been made for the observed conformers of OANAT. The RIMP2 calculations predict that interchain H-bonded conformers of OANAT are 25-30 kJ/mol more stable than the extended-chain structures. However, the free energies of the interchain H-bonded and extended structures calculated at the preexpansion temperature (450 K) differ by less than 10 kJ/mol, and the number of extended structures far outweighs the number of H-bonded conformers. This entropy-driven effect explains the presence of the independent-chain conformers in the expansion, and cautions future studies that rely solely on relative energies of conformers in considering possible assignments.

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Section: The Conformational Preferences Of Related Single-chain mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O-(2-acetamidoethyl)-N-acetyltyramine

Shubert

Baquero

Clarkson

et al. 2007

The Journal of Chemical Physics

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“…The approximate descriptions of the modes given in Table 2 have been taken from ref. 37. Table 2 compares the vibrational frequencies below 300 cm À1 of the seven Anti(OMe) conformers.…”

Section: Computational Resultsmentioning

confidence: 99%

The conformational landscape of 5-methoxytryptamine studied by rotationally resolved fluorescence spectroscopy and resonant ionization spectroscopy

Kalkman

Meerts

et al. 2009

Phys. Chem. Chem. Phys.

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Rotationally resolved electronic spectra of three different conformers of 5-methoxytryptamine were recorded in a molecular beam. 5-Methoxy substitution reduces the number of observed conformers to three compared to seven that have been reported for tryptamine. Quantum chemical calculations indicate that anti-rotamers of the methoxy-group are more stable relative to the syn-forms. Two gauche structures (Gpy(in) and Gph(in)) of the ethylamino group with respect to the indole chromophore were found to be less stable than the other seven. The lowest electronically excited state has been identified as the (1)L(b) one for all observed conformers which was confirmed by quantum-chemical calculations. Based on the comparison of rotational constants obtained from fits using evolutionary algorithms with those from calculations, the three observed conformers were determined to be the Gpy(up), Gph(up), Gpy(out) ethylamino side-chain conformations.

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“…Recently, dispersed fluorescence spectroscopy has been used to assign the low frequency torsional structure in a TRA monomer. 43 The lowest frequency vibration in TRA is a torsional mode at 44 cm −1 ͑S 0 ͒, which becomes 41 cm −1 in S 1 . We have therefore modeled the observed product contour as a series of sequence bands involving this torsional mode, split off to lower frequency by −3 cm −1 for each succeeding vЉ level.…”

Section: Internal Energy Fittingmentioning

confidence: 99%

Infrared photodissociation of a water molecule from a flexible molecule-H2O complex: Rates and conformational product yields following XH stretch excitation

Clarkson

Herbert

Zwier

2007

The Journal of Chemical Physics

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Infrared-ultraviolet hole-burning and hole-filling spectroscopies have been used to study IR-induced dissociation of the tryptamine.H2O and tryptamine.D2O complexes. Upon complexation of a single water molecule, the seven conformational isomers of tryptamine collapse to a single structure that retains the same ethylamine side chain conformation present in the most highly populated conformer of tryptamine monomer. Infrared excitation of the tryptamine.H2O complex was carried out using a series of infrared absorptions spanning the range of 2470-3715 cm-1. The authors have determined the conformational product yield over this range and the dissociation rate near threshold, where it is slow enough to be measured by our methods. The observed threshold for dissociation occurred at 2872 cm-1 in tryptamine.H2O and at 2869 cm-1 in tryptamine.D2O, with no dissociation occurring on the time scale of the experiment (approximately 2 micros) at 2745 cm-1. The dissociation time constants varied from approximately 200 ns for the 2869 cm-1 band of tryptamine.D2O to approximately 25 ns for the 2872 cm-1 band of tryptamine.H2O. This large isotope dependence is associated with a zero-point energy effect that increases the binding energy of the deuterated complex by approximately 190 cm-1, thereby reducing the excess energy available at the same excitation energy. At all higher energies, the dissociation lifetime was shorter than the pulse duration of our lasers (8 ns). At all wavelengths, the observed products in the presence of collisions are dominated by conformers A and B of tryptamine monomer, with small contributions from the other minor conformers. In addition, right at threshold (2869 cm-1), tryptamine.D2O dissociates exclusively to conformer A in the absence of collisions with helium, while both A and B conformational products are observed in the presence of collisions with helium. Using resolution-of-identity approximation to second-order Moller-Plesset binding energies extrapolated to the complete basis set limit and harmonic vibrational frequencies and transition states calculated at the density functional limit B3LYP/6-31+G* level of theory, Rice-Ramsperger-Kassel-Marcus (RRKM) predictions for the dissociation, isomerization, and water shuttling rates as a function of energy are made. At threshold, the experimental dissociation rate is almost 10(3) faster than RRKM predictions. Reasons for this apparent non-RRKM behavior will be discussed.

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Low frequency backbone vibrations of individual conformational isomers: Tryptamine

Cited by 18 publications

References 20 publications

Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O-(2-acetamidoethyl)-N-acetyltyramine

Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O-(2-acetamidoethyl)-N-acetyltyramine

The conformational landscape of 5-methoxytryptamine studied by rotationally resolved fluorescence spectroscopy and resonant ionization spectroscopy

Infrared photodissociation of a water molecule from a flexible molecule-H2O complex: Rates and conformational product yields following XH stretch excitation

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