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

Shubert, VA and Baquero, EE and Clarkson, JR and James, WH and Turk, JA and Hare, AA and Worrel, K and Lipton, MA and Schofield, DP and Jordan, KD and Zwier, TS (2007) Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O -(2-acetamidoethyl)- N -acetyltyramine. Journal of Chemical Physics, 127 (23). ISSN 0021-9606

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Abstract

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 S0 - S1 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 S0 - S1 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 S0 - S1 adiabatic excitation energies have been calculated using the RICC2 method and vertical excitation energies using single-point time-dependent DFT. The sensitivity of the S0 - S1 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 kJmol 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 kJmol, 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. © 2007 American Institute of Physics.


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Details

Item Type: Article
Status: Published
Creators/Authors:
CreatorsEmailPitt UsernameORCID
Shubert, VA
Baquero, EE
Clarkson, JR
James, WH
Turk, JA
Hare, AA
Worrel, K
Lipton, MA
Schofield, DP
Jordan, KDjordan@pitt.eduJORDAN
Zwier, TS
Date: 1 December 2007
Date Type: Publication
Journal or Publication Title: Journal of Chemical Physics
Volume: 127
Number: 23
DOI or Unique Handle: 10.1063/1.2803076
Schools and Programs: Dietrich School of Arts and Sciences > Chemistry
Refereed: Yes
ISSN: 0021-9606
MeSH Headings: Entropy; Hydrogen Bonding; Models, Chemical; Molecular Structure; Protons; Quantum Theory; Spectrophotometry, Infrared--instrumentation; Spectrophotometry, Infrared--methods; Spectrophotometry, Ultraviolet--instrumentation; Spectrophotometry, Ultraviolet--methods; Tyramine--analogs & derivatives; Tyramine--chemistry
PubMed ID: 18154390
Date Deposited: 03 May 2013 14:43
Last Modified: 02 Feb 2019 15:56
URI: http://d-scholarship.pitt.edu/id/eprint/18251

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