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The conformational preferences of a series of capped peptides containing the helicogenic amino acid aminoisobutyric acid (Aib) (Z-Aib-OH, Z-(Aib)-OMe, and Z-(Aib)-OMe) are studied in the gas phase under expansion-cooled conditions. Aib oligomers are known to form 3-helical secondary structures in solution and in the solid phase. However, in the gas phase, accumulation of a macrodipole as the helix grows could inhibit helix stabilization. Implementing single-conformation IR spectroscopy in the NH stretch region, Z-Aib-OH and Z-(Aib)-OMe are both observed to have minor conformations that exhibit dihedral angles consistent with the 3-helical portion of the Ramachandran map (ϕ, ψ = -57°, -30°), even though they lack sufficient backbone length to form 10-membered rings which are a hallmark of the developed 3-helix. For Z-(Aib)-OMe three conformers are observed in the gas phase. Single-conformation infrared spectroscopy in both the NH stretch (Amide A) and C[double bond, length as m-dash]O stretch (Amide I) regions identifies the main conformer as an incipient 3-helix, having two free NH groups and two C10 H-bonded NH groups, labeled an F-F-10-10 structure, with a calculated dipole moment of 13.7 D. A second minor conformer has an infrared spectrum characteristic of an F-F-10-7 structure in which the third and fourth Aib residues have ϕ, ψ = 75°, -74° and -52°, 143°, Ramachandran angles which fall outside of the typical range for 3-helices, and a dipole moment that shrinks to 5.4 D. These results show Aib to be a 3-helix former in the gas phase at the earliest stages of oligomer growth.

Broadband microwave spectra over the 2-18 GHz range have been recorded for the resonance-stabilized 2-furanyloxy radical, formed in the first step of pyrolysis of the second-generation biofuel 2-methoxyfuran by methyl loss. Using a flash pyrolysis source attached to a pulsed valve, a 0.7% mixture of 2-methoxyfuran in argon was pyrolyzed at a series of temperatures ranging from 300 to 1600 K. Subsequent cooling in a supersonic expansion produced rotational temperatures of ∼2 K in the interrogation region. Using chirped-pulse Fourier transform microwave (CP-FTMW) methods, combined with strong-field coherence breaking (SFCB), a set of transitions due to the radical were identified and assigned. The experimental rotational constants ( A = 8897.732(93), B = 4019.946(24), C = 2770.321(84)), centrifugal distortion constants, and spin-rotation coupling constants have been determined for the radical and compared with ab initio predictions at the CCSD(T) level of theory. Compared to the 2-methoxyfuran precursor, the 2-furanyloxy radical has allylic C-C bond lengths intermediate between single and double bonds, a shortened C(5)-O(6) bond characteristic of partial double-bond character, and an O(1)-C(5)-O(6) bond angle of 121°, which resembles the O-C-O angle of an ester. Atomic spin densities extracted from the calculations confirm that the 2-furanyloxy radical is best viewed as a carbon-centered allylic lactone radical, with 80% of the spin density on the two allylic carbons and 20% on the pendant O(6) atom.

We report on rotationally resolved laser induced fluorescence (LIF) and vibrationally resolved resonance enhanced multiphoton ionization (REMPI) spectroscopy of the chiral molecule 1-indanol. Spectra of the S1 ← S0 electronic...