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 -(2-acetamidoethyl)--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) -phenethyl-acetamide (NPEA), (ii) -(-methoxyphenethyl-acetamide) (NMPEA), and (iii) -(2-phenoxyethyl)-acetamide (NPOEA). Six conformations of OANAT have been resolved, with origins ranging from , denoted , respectively. RIDIR spectra show that conformers each possess an intense, broadened amide NH stretch fundamental shifted below , indicative of the presence of an interchain H bond, while conformers have both amide NH stretch fundamentals in the region, consistent with independent-chain structures with two free NH groups. NPEA has a single conformer with origin at . NMPEA has three conformers, two that dominate the R2P1 spectrum, with origin transitions between 35 580 and . Four conformations, one dominate and three minor, of NPOEA have been resolved with origins between 35 654 and . 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 adiabatic excitation energies have been calculated using the RICC2 method and vertical excitation energies using single-point time-dependent DFT. The sensitivity of the 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 more stable than the extended-chain structures. However, the free energies of the interchain H-bonded and extended structures calculated at the preexpansion temperature differ by less than , 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.
V.A.S., E.E.B., J.R.C., W.H.J., and T.S.Z. acknowledge support from the National Science Foundation (CHE-0551075) for this research. D.P.S. acknowledges the New Zealand Foundation for Research, Science and Technology for a postdoctoral fellowship. K.D.J. acknowledges support from the NSF (CHE-0518253).
II. EXPERIMENTAL METHODS
III. THE CONFORMATIONAL PREFERENCES OF RELATED SINGLE-CHAIN MOLECULES
IV. THEORETICAL METHODS
V. RESULTS AND ANALYSIS
E. Noninteracting side-chain conformers
F. Interchain H-bonded conformations
A. The single-chain conformational preferences
B. Electronic frequency shifts in the alkoxy chains
C. Obtaining accurate relative energies for the conformers of OANAT
D. Entropy-driven population distributions in OANAT
Data & Media loading...
Article metrics loading...
Full text loading...