Conformation-specific UV-IR double resonance spectra are presented for ethyl, n-propyl, and n-butylbenzene. With the aid of a local mode Hamiltonian that includes the effects of stretch-scissor Fermi resonance, the spectra can be accurately modeled for specific conformers. These molecules allow for further development of a first principles method for calculating alkyl stretch spectra. Across all chain lengths, certain dihedral patterns impart particular spectral motifs at the quadratic level. However, the anharmonic contributions are consistent from molecule to molecule and conformer to conformer. This transferability of anharmonicities allows for the Hamiltonian to be constructed from only a harmonic frequency calculation, reducing the cost of the model. The phenyl ring alters the frequencies of the CH2 stretches by about 15 cm(-1) compared to their n-alkane counterparts in trans configurations. Conformational changes in the chain can lead to shifts in frequency of up to 30 cm(-1).

Ultrafast time-resolved ion yield (TR-IY) and velocity map imaging spectroscopies are employed to reveal the relaxation dynamics after photoexcitation in ethyl 4-hydroxy-3-methoxycinnamate (ethyl ferulate, EF), an active ingredient in commercially available sunscreens. In keeping with a bottom-up strategy, the building blocks of EF, 2-methoxy-4-vinylphenol (MVP) and 4-hydroxy-3-methoxycinnamyl alcohol (coniferyl alcohol, ConA), were also studied to assist in our understanding of the dynamics of EF as we build up in molecular complexity. In contrast to the excited state dynamics of MVP and ConA, which are described by a single time constant (>900 ps), the dynamics of EF are described by three time constants (15 ± 4 ps, 148 ± 47 ps, and >900 ps). A mechanism is proposed involving internal conversion (IC) between the initially excited S(1ππ*) and S(1nπ*) states followed by intramolecular vibrational redistribution (IVR) on both states, in competition with intersystem crossing onto neighbouring triplet states (15 ± 4 ps). IVR and IC within the triplet manifold then ensues (148 ± 47 ps) to populate a low-lying triplet state (>900 ps). Importantly, the fluorescence spectrum of EF at the S origin, along with the associated lifetime (6.9 ± 0.1 ns), suggests that population is trapped, during initial IVR, on the S(1ππ*) state. This serves to demonstrate the complex, competing dynamics in this sunscreen filter molecule.

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.