Quantitative Study of Enantiomer-Specific State Transfer.

We here report on a quantitative study of enantiomer-specific state transfer, performed in a pulsed, supersonic molecular beam. The chiral molecule 1-indanol is cooled to low rotational temperatures (1-2 K) and a selected rotational level in the electronic and vibrational ground state of the most abundant conformer is depleted via optical pumping on the S_{1}←S_{0} transition. Further downstream, three consecutive microwave pulses with mutually perpendicular polarizations and with a well-defined duration and phase are applied. The population in the originally depleted rotational level is subsequently monitored via laser-induced fluorescence detection. This scheme enables a quantitative comparison of experiment and theory for the transfer efficiency in what is the simplest enantiomer-specific state transfer triangle for any chiral molecule, that is, the one involving the absolute ground state level, |J_{K_{a}K_{c}}⟩=|0_{00}⟩. Moreover, this scheme improves the enantiomer enrichment by over an order of magnitude compared to previous works. Starting with a racemic mixture, a straightforward extension of this scheme allows one to create a molecular beam with an enantiomer-pure rotational level, holding great prospects for future spectroscopic and scattering studies.

Bond Length Alternation and Internal Dynamics in Model Aromatic Substituents of Lignin.

Broadband microwave spectra were recorded over the 2-18 GHz frequency range for a series of four model aromatic components of lignin; namely, guaiacol (ortho-methoxy phenol, G), syringol (2,6-dimethoxy phenol, S), 4-methyl guaiacol (MG), and 4-vinyl guaiacol (VG), under jet-cooled conditions in the gas phase. Using a combination of 13 C isotopic data and electronic structure calculations, distortions of the phenyl ring by the substituents on the ring are identified. In all four molecules, the rC(1)-C(6) bond between the two substituted C-atoms lengthens, leading to clear bond alternation that reflects an increase in the phenyl ring resonance structure with double bonds at rC(1)-C(2) , rC(3)-C(4) and rC(5)-C(6) . Syringol, with its symmetric methoxy substituents, possesses a microwave spectrum with tunneling doublets in the a-type transitions associated with H-atom tunneling. These splittings were fit to determine a barrier to hindered rotation of the OH group of 1975 cm-1 , a value nearly 50 % greater than that in phenol, due to the presence of the intramolecular OH⋅⋅⋅OCH3 H-bonds at the two equivalent planar geometries. In 4-methyl guaiacol, methyl rotor splittings are observed and used to confirm and refine an earlier measurement of the three-fold barrier V3 =67 cm-1 . Finally, 4-vinyl guaiacol shows transitions due to two conformers differing in the relative orientations of the vinyl and OH groups.

High-resolution UV spectroscopy of 1-indanol.

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 transition are recorded in a jet-cooled, pulsed molecular beam. Using two time-delayed pulsed lasers, the lifetimes of the S1 state of the two most stable conformers, referred to as eq1 and ax2, have been determined. The S1← S0 origin bands of these conformers as well as the transition to a vibrationally excited level in the S1 state of eq1 are recorded with full rotational resolution (25 MHz observed linewidth) by measuring the LIF intensity following excitation with a tuneable, narrowband cw laser. On selected rotationally resolved electronic transitions, Lamb-dips are measured to confirm the Lorentzian lifetime-contribution to the observed lineshapes. The rotationally resolved S1← S0 origin band of a neon-complex of eq1 is measured via LIF as well. The fit of the rotationally resolved LIF spectra of the origin bands to those of an asymmetric rotor yields a standard deviation of about 6 MHz. The resulting spectroscopic parameters are tabulated and compared to the outcome of ab initio calculations. For both conformers as well as for the Ne-eq1 complex, the geometric structures in the S0 and S1 states are discussed. For all systems, the transition dipole moment is mainly along the a-axis, the contributions along the b- and c-axes being about one order of magnitude smaller.

Propagating molecular rotational coherences through single-frequency pulses in the strong field regime.

In the weak-field limit in which microwave spectroscopy is typically carried out, an application of a single-frequency pulse that is resonant with a molecular transition will create a coherence between the pair of states involved in the rotational transition, producing a free-induction decay (FID) that, after Fourier transform, produces a molecular signal at that same resonance frequency. With the advent of chirped-pulse Fourier transform microwave methods, the high-powered amplifiers needed to produce broadband microwave spectra also open up other experiments that probe the molecular response in the high-field regime. This paper describes a series of experiments involving resonant frequency pulses interrogating jet-cooled molecules under conditions of sufficient power to Rabi oscillate the two-state system through many Rabi cycles. The Fourier-transformed FID shows coherent signal not only at the applied resonant frequency but also at a series of transitions initially connected to the original one by sharing an upper or lower level with it. As the duration of the single-frequency excitation is increased from 250 to 1500 ns, the number of observed off-resonant, but dipole-allowed, molecular coherences grow. The phenomenon is quite general, having been demonstrated in Z-phenylvinylnitrile, E-phenylvinylnitrile (E-PVN), benzonitrile, guaiacol, and 4-pentynenitrile. In E-PVN, the highest power/longest pulse duration, coherent signal is also present at energetically nearby but not directly connected transitions. Even in molecular samples containing more than one independent species, only transitions due to the single species responsible for the original resonant transition are present. We develop a time-dependent model of the molecular/photon system and use it in conjunction with the experiment to test possible sources of the phenomenon.

Broadband Microwave Spectroscopy of 2-Furanyloxy Radical: Primary Pyrolysis Product of the Second-Generation Biofuel 2-Methoxyfuran.

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.

Structural Characterization of Phenoxy Radical with Mass-Correlated Broadband Microwave Spectroscopy.

A combination of broadband microwave spectroscopy and VUV photoionization time-of-flight mass spectra has been used to record rotational spectra of the prototypical phenoxy radical, its per-deuterated isotopomers, and the full set of singly 13C-substituted analogues. Rotational parameters associated with the fits to the full set of isotopomers produce a highly accurate r0 structure for the phenoxy radical. High-level ab initio calculations accurately reproduce the rotational constants and spin-rotation parameters. The structure of the phenoxy radical is distinctly quinoidal, with delocalization of the unpaired electron spin density on the oxygen and phenyl ring. The fitted Fermi contact terms for the 13C atoms reflect a weighting of resonance structures that is 27% on the O atom, 21.5% on each of the two ortho C's, and 30% on the para C, providing a quantitative measure of its sites for subsequent reactions that will control its abundances in combustion and atmospheric environments.