title: Liquid hot NAGMA cooled to 0.4 K: benchmark thermochemistry of a gas-phase peptide.
abstract: Vibrational spectroscopy and helium nanodroplet isolation are used to determine the gas-phase thermochemistry for isomerization between conformations of the model dipeptide, N-acetylglycine methylamide (NAGMA). A two-stage oven source is implemented to produce a gas-phase equilibrium distribution of NAGMA conformers, which is preserved when individual molecules are captured and cooled to 0.4 K by He nanodroplets. With polarization spectroscopy, the IR spectrum in the NH stretch region is assigned to a mixture of two conformers having intramolecular hydrogen bonds composed of either five- or seven-membered rings, C5 and C7, respectively. The C5 to C7 interconversion enthalpy and entropy, obtained from a van't Hoff analysis, are -4.52 ± 0.12 kJ/mol and -12.4 ± 0.2 J/(mol · K), respectively. The experimental thermochemistry is compared to high-level electronic structure theory computations.
authors.affiliation: Department of Chemistry and ‡Center for Computational Quantum Chemistry, University of Georgia , Athens, Georgia 30602, United States.
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Leavitt
authors.forename: Kevin B
authors.initials: KB
authors.lastname: Moore
authors.forename: Paul L
authors.initials: PL
authors.lastname: Raston
authors.forename: Jay
authors.initials: J
authors.lastname: Agarwal
authors.forename: Grant H
authors.initials: GH
authors.lastname: Moody
authors.forename: Caitlyne C
authors.initials: CC
authors.lastname: Shirley
authors.forename: Henry F
authors.initials: HF
authors.lastname: Schaefer
authors.forename: Gary E
authors.initials: GE
authors.lastname: Douberly
mesh_terms: D019277:Entropy; D005740:Gases / Q000737:chemistry*; D006371:Helium; D006860:Hydrogen Bonding; D008958:Models, Molecular; D036103:Nanotechnology; D010455:Peptides / Q000737:chemistry*; D013057:Spectrum Analysis; D013816:Thermodynamics; D014732:Vibration
chemical_list: D005740:Gases; D010455:Peptides; D006371:Helium