V. Alvin Shubert

Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O-(2-acetamidoethyl)-N-acetyltyramine

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.

Photodissociation of acetaldehyde and the absolute photoionization cross section of HCO.

Photodissociation of acetaldehyde (CH3CHO) at 266 nm produced CH3 and HCO radicals, and single-photon vacuum ultraviolet ionization was used to record velocity map ion images of both CH3+ and HCO+. Comparison of the translational energy distributions from both species indicates that secondary fragmentation of HCO is negligible for 266 nm photodissociation. Thus, the relative photoion signals for CH3+ and HCO+ in the mass spectrometer, combined with the recently measured absolute photoionization cross section of CH3, allowed the determination of the absolute photoionization cross section of HCO (σ(HCO) = 4.8 ± 1.5(2.0), 5.9 ± 1.6(2.2), and 3.7 ± 1.2(1.6) Mb at 10.257, 10.304, and 10.379 eV, respectively). The observed values are quite small but consistent with the similarly small value at threshold for the isoelectronic species NO. This behavior is discussed in terms of the character of the HOMO in both molecules.

Photodissociation of 2-Iodoethanol within the A Band

The photodissociation of 2-iodoethanol was studied within the A (sigma* <-- n) absorption band at several wavelengths between 253 and 298 nm, and the velocity distributions and angular distributions of the photofragments were characterized by using velocity-map ion imaging. The two dominant dissociation channels correspond to the production of the 2-hydroxyethyl radical, C2H4OH, and I(2P(3/2)) and I*(2P(1/2)), and in both channels, approximately 50% of the available excess energy is partitioned into translational energy of the fragments. The branching fractions for the I and I* channels at 266 nm were determined by using a combination of (1) the translational energy distributions for the separate I and I* channels determined by two-photon resonant, three-photon ionization, (2) the distributions for the combined I + I* channels determined by single-photon ionization at 118 nm, and (3) the relative photoionization cross sections of I and I* at 118 nm. Evidence was observed for either the secondary decomposition of C2H4OH, the photodissociation of C2H4OH, or the dissociative ionization of the C2H4OH radicals produced in the I channel. These mechanisms are also discussed.