title: Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O-(2-acetamidoethyl)-N-acetyltyramine.
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 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.
authors.affiliation: Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, USA.
authors.forename: V Alvin
authors.initials: VA
authors.lastname: Shubert
authors.forename: Esteban E
authors.initials: EE
authors.lastname: Baquero
authors.forename: Jasper R
authors.initials: JR
authors.lastname: Clarkson
authors.forename: William H
authors.initials: WH
authors.lastname: James
authors.forename: Jeffrey A
authors.initials: JA
authors.lastname: Turk
authors.forename: Alissa A
authors.initials: AA
authors.lastname: Hare
authors.forename: Kevin
authors.initials: K
authors.lastname: Worrel
authors.forename: Mark A
authors.initials: MA
authors.lastname: Lipton
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Schofield
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Jordan
authors.forename: Timothy S
authors.initials: TS
authors.lastname: Zwier
mesh_terms: D019277:Entropy; D006860:Hydrogen Bonding; D008956:Models, Chemical; D015394:Molecular Structure; D011522:Protons; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared / Q000295:instrumentation / Q000379:methods; D013056:Spectrophotometry, Ultraviolet / Q000295:instrumentation / Q000379:methods; D014439:Tyramine / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: C527552:O-(2-acetamidoethyl)-N-acetyltyramine; D011522:Protons; D014439:Tyramine