Water induces the same crown shapes as Li+ or Na+ in 15-crown-5 ether: a broadband rotational study.

15-Crown-5 ether (15C5) and its complexes with water have been studied using broadband Fourier transform microwave spectroscopy in a supersonic jet. A new conformer of 15C5 has been observed and established as the new global minimum out of a total of nine isolated structures. In addition, two 15C5-H2O and two 15C5-(H2O)2 clusters have been observed. The cluster structures have been unambiguously identified through the observation of water 18O isotopologue spectra. In all the clusters, at least one water molecule, located close to the axis of the 15C5 ring, interacts through two simultaneous hydrogen bonds to the endocyclic oxygen atoms. This interaction reshapes the 15C5 ring to reduce its rich conformational landscape to only two open structures, related to those found in complexes with Li+ or Na+ ions. In the most abundant 15C5-(H2O)2 form, the two water molecules repeat the same interaction scheme while binding to opposite sides of the ring. In the second most abundant dihydrated form the two water molecules lie on the same side of the ring. This finding is exceptionally rare because water-water interactions typically prevail over the formation of additional solute-water contacts, and it showcases the particular binding features of crown ethers.

Rotational spectroscopy and three-wave mixing of 4-carvomenthenol: A technical guide to measuring chirality in the microwave regime.

We apply chirality sensitive microwave three-wave mixing to 4-carvomenthenol, a molecule previously uncharacterized with rotational spectroscopy. We measure its rotational spectrum in the 2-8.5 GHz range and observe three molecular conformers. We describe our method in detail, from the initial step of spectral acquisition and assignment to the final step of determining absolute configuration and enantiomeric excess. Combining fitted rotational constants with dipole moment components derived from quantum chemical calculations, we identify candidate three-wave mixing cycles which were further tested using a double resonance method. Initial optimization of the three-wave mixing signal is done by varying the duration of the second excitation pulse. With known transition dipole matrix elements, absolute configuration can be directly determined from a single measurement.

Identifying enantiomers in mixtures of chiral molecules with broadband microwave spectroscopy.

Chirality-sensitive broadband microwave spectroscopy was performed on mixtures of carvone enantiomers and conformers to distinguish enantiomers, measure enantiomeric excesses, and determine the absolute configurations of the enantiomers. This method uses microwave three-wave mixing and is inherently well-suited to the analysis of mixtures-a unique advantage over other techniques. In contrast to conventional microwave spectroscopy, the phase of the received signal is also exploited. This phase depends upon the signs of the molecules' dipole-moment components and is used to identify the excess enantiomer. The measured signal amplitude determines the size of the excess. The broadband capabilities of the spectrometer were used to simultaneously excite and measure two conformers of carvone, demonstrating the analysis of a sample with multiple chiral species. Employing quantum chemical calculations and the measured phases, the absolute configurations of the enantiomers are determined.

Photoelectron imaging of several 5d and 6p Rydberg states Xe2 and improving the Xe2(+) I(1∕2g) potential.

Velocity map photoelectron imaging was used to study the photoionization of Xe(2) in several low-lying 5d and 6p Rydberg states. The Rydberg states were prepared by two-photon excitation and ionized by either one additional photon from the pump laser (2+1 ionization), or by one photon of a second color (2+1(') ionization). The 2+1 images and associated photoelectron spectra were consistent with previous results, although some adjustment of previously proposed equilibrium bond lengths was necessary to fit the spectra with Franck-Condon factor calculations. The 2+1(') images provided higher resolution photoelectron spectra and, in conjunction with the Xe(2)(+) potentials reported by Zehnder and co-workers [J. Chem. Phys. 128, 234306 (2008)] and the 6p and 5d Xe(2)∗ potentials calculated by Jonin and Spiegelmann [J. Chem. Phys. 117, 3059 (2002)], provided a means for improving the Xe(2)∗ potentials. New experimental data are also presented for photoionization populating the Xe(2)(+) I(1∕2g) state, and are used to provide a better description of its potential curve.

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.

Predissociation and dissociative ionization of Rydberg states of Xe2 and the photodissociation of Xe2+.

The Rydberg states of Xe(2) in the region between 76,000 and 84,000 cm(-1) were studied by using a combination of two-photon excitation and velocity map ion imaging. The electronic states in this region are based on the Xe((1)S(0))+Xe 6p and 5d dissociation limits, and the large number of states leads to numerous curve crossings and distorted potentials. These Rydberg states can decay by predissociation or fluorescence or can be photoionized, dissociatively photoionized, or photodissociated by the absorption of a single additional photon. Furthermore, the molecular ion can be photodissociated as well. While numerous other techniques have been applied to this problem, velocity map ion imaging provides a high resolution approach to determine the operative processes. When combined with existing data obtained by other methods, the present experiments allow a more complete understanding of the assignment and behavior of these states.

Single-conformation and diastereomer specific ultraviolet and infrared spectroscopy of model synthetic foldamers: alpha/beta-peptides.

Resonant two-photon ionization (R2PI), UV hole-burning (UVHB), and resonant ion-dip infrared (RIDIR) spectroscopies have been used to record single-conformation infrared and ultraviolet spectra of three model synthetic foldamers with heterogeneous backbones, alpha/beta-peptides Ac-beta(3)-hAla-L-Phe-NHMe (betaalphaL), Ac-beta(3)-hAla-D-Phe-NHMe (betaalphaD), and Ac-L-Phe-beta(3)-hAla-NHMe (alphabetaL), isolated and cooled in a supersonic expansion. BetaalphaL and betaalphaD are diastereomers, differing only in the configuration of the alpha-amino acid residue; betaalphaL and alphabetaL contain the same residues, but differ in residue order. In all three alpha/beta-peptides the beta(3)-residue has S absolute configuration. UVHB spectroscopy is used to determine that there are six conformers of each molecule and to locate and characterize their S(0)-S(1) transitions in the origin region. RIDIR spectra in the amide NH stretch region reflect the number and strength of intramolecular H-bonds present. Comparison of the RIDIR spectra with scaled, harmonic vibrational frequencies and infrared intensities leads to definite assignments for the conformational families involved. C8/C7(eq) double-ring structures are responsible for three conformers of betaalphaL and four of betaalphaD, including those with the most intense transitions in the R2PI spectra. This preference for C8/C7(eq) double rings appears to be dictated by the C7(eq) ring of the alpha-peptide subunit. Three of the conformers of betaalphaL and betaalphaD form diastereomeric pairs (A/A', C/C', and G/G') that have nearly identical S(0)-S(1) origin positions in the UV and belong to the same conformational family, indicating no significant change associated with the change in chirality of the alpha-peptide subunit. However, betaalphaL favors formation of a C6/C5 conformer over C11, while the reverse preference holds in betaalphaD. Calculations indicate that the selective stabilization of the lowest-energy C11(g(+)) structure in betaalphaD occurs because this structure minimizes steric effects between the beta(2) methylene group and C=O(1). In the alpha/beta-peptide alphabetaL, two conformers dominate the spectrum, one assigned to a C5/C8 bifurcated double-ring, and the other to a C5/C6 double-ring structure. This preference for C5 rings in the alpha/beta-peptide occurs because the C5 ring is further stabilized by an amide NH...pi interaction involving an NH group on the adjacent amide, as it is in the alpha-peptides. Comparison of the NH stretch spectra of C8/C7(eq) structures in betaalphaL with their C7(eq)/C8 counterparts in alphabetaL shows that the central amide NH stretch is shifted to lower frequency by some 50-70 cm(-1) due to cooperative effects associated with the central amide accepting and donating a H-bond to neighboring amide groups. This swaps the ordering of the C8 and C7 NH stretch fundamentals in the two molecules.

Water's role in reshaping a macrocycle's binding pocket: infrared and ultraviolet spectroscopy of benzo-15-crown-5-(H(2)O)(n) and 4'-aminobenzo-15-crown-5-(H(2)O)(n), n = 1, 2.

Laser-induced fluorescence (LIF), resonant two-photon ionization (R2PI), ultraviolet hole-burning (UVHB), resonant ion-dip infrared (RIDIR), and infrared-infrared ultraviolet hole-burning (IR-IR-UV) spectroscopies were carried out on benzo-15-crown-5 ether-(H(2)O)(n) (B15C-(H(2)O)(n)) and 4'-amino-benzo-15-crown-5 ether-(H(2)O)(n) (ABC-(H(2)O)(n)) clusters with n = 1,2 formed in a supersonic expansion. Two isomers of B15C-(H(2)O)(1) with S(0)-S(1) origins at 35,628 and 35,685 cm(-1) (B15C-(H(2)O)(1)(A) and B15C-(H(2)O)(1)(B), respectively) were identified and, on the basis of the combined evidence from the single-isomer UV and IR spectra, assigned to structures in which the H(2)O molecule donates both its OH groups to H-bonds to the crown oxygens. Both isomers share the same open, chairlike C(s) symmetry structure for the crown ether that exposes the crown oxygen lone pairs to binding to H(2)O on the interior of the crown. This crown conformation is not among those represented in the observed conformers in the absence of the H(2)O molecule, indicating that even a single water molecule is capable of reshaping the crown binding pocket in binding to it. In B15C-(H(2)O)(1)(A), the water molecule takes up a position parallel to the crown plane of symmetry, using one OH group to bind to the two benzo oxygens, while the other OH binds to a single crown oxygen on the opposite side of the crown. The H(2)O molecule in B15C-(H(2)O)(1)(B) binds to the other two crown oxygens, in an orientation perpendicular to the crown's symmetry plane. B15C-(H(2)O)(2) also has two isomers. The first, B15C-(H(2)O)(2)(A) with S(0)-S(1) origin at 35,813 cm(-1), is assigned to a structure in which the two water molecules take up the two positions occupied by individual water molecules in B15C-(H(2)O)(1) A and B. The second isomer, with S(0)-S(1) origin at 35,665 cm(-1), has an OH stretch RIDIR spectrum that reflects a water-water H-bond, with the second water molecule binding to the crown-bound water in the parallel binding site. The combined data from B15C-(H(2)O)(1), ABC-(H(2)O)(1), and ABC-(HOD) complexes is used to deduce the uncoupled OH stretch wavenumber shifts associated with each of the unique binding sites for H(2)O to the crown. Arguments are presented that the binding pocket present in benzo-15-crown-5 ether is of a near ideal size to accommodate strong bidentate binding of individual water molecules to its most open crown conformation.

Jet-cooled electronic and vibrational spectroscopy of crown ethers: benzo-15-crown-5 ether and 4'-amino-benzo-15-crown-5 ether.

Laser-induced fluorescence (LIF), ultraviolet hole-burning (UVHB), and resonant ion-dip infrared (RIDIR) spectroscopies were carried out on isolated benzo-15-crown-5 ether (B15C) and 4'-amino-benzo-15-crown-5 ether (ABC) cooled in a supersonic expansion. Three conformational isomers of B15C and four of ABC were observed and spectroscopically characterized. Full optimizations and harmonic frequency calculations were undertaken for the full set of almost 1700 conformational minima identified in a molecular mechanics force field search. When compared with TDDFT predictions, the S(0)-S(1) origin positions serve as a useful diagnostic of the conformation of the crown ether near the phenyl ring responsible for the UV absorption and to the position of the NH(2) substituent. In-plane orientations for the beta carbons produce red-shifted S(0)-S(1) origins, while out-of-plane "buckling" produces substantial blue shifts of 600 cm(-1) or more. Comparison between the alkyl CH stretch spectra of B15C and ABC divide the spectra into common subgroups shared by the two molecules. The high-frequency CH stretch transitions (above 2930 cm(-1)) reflect the number of CH...O interactions, which in turn track in a general way the degree of buckling of the crown. On this basis, assignments of each of the observed conformational isomers to a class of structure can be made. All the observed structures have some degree of buckling to them, indicating that in the absence of a strong-binding partner, the crown folds in on itself to gain additional stabilization from weak dispersive and CH...O interactions.

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.

Photodissociation of i-C3H7I within the A band and anisotropy-based decomposition of the translational energy distributions.

The photodissociation of i-propyl iodide in the A absorption band was studied by using velocity map ion imaging following excitation between 304 and 253 nm. The translational energy distributions and translational energy dependence of the angular distributions of the I (2)P(3/2) and (2)P(1/2) photofragments were recorded as a function of the photodissociation wavelength. These distributions are used to decompose the i-C(3)H(7)+I (2)P(3/2) channel into contributions from two processes: Excitation to the (3)Q(0(+)) state followed by crossing onto the (1)Q(1) surface, and direct excitation to the (3)Q(1) surface followed by dissociation on that surface. As in the case of methyl iodide, the former process dominates; the latter process contributes only in the red wing of the absorption band, with its contribution peaking at approximately 287 nm with an absorption of approximately 1% of the band maximum. The data for the i-C(3)H(7)+I(*) (2)P(1/2) channel display a smooth behavior across the full energy range of the present study, and are consistent with direct excitation to the (3)Q(0(+)) surface followed by dissociation on that surface.

Chiral Analysis Using Broadband Rotational Spectroscopy.

broadband microwave spectroscopy is a proven tool to precisely determine molecular properties of gas-phase molecules. Recent developments make it applicable to investigate chiral molecules. Enantiomers can be differentiated, and the enantiomeric excess and, indirectly, the absolute configuration can be determined in a molecule-selective manner. The resonant character and high resolution of rotational spectroscopy provide a unique mixture compatibility. Future directions, such as extending the technique to chemical analysis, are discussed.

Phase Dependence of Double-Resonance Experiments in Rotational Spectroscopy.

We here report on double-resonance experiments using broadband chirped pulse Fourier transform microwave spectroscopy that can facilitate spectral assignment and yield information about weak transitions with high resolution and sensitivity. Using the diastereomers menthone and isomenthone, we investigate the dependence of pumping a radio frequency transition on both the amplitude and phase of the signal from a microwave transition with which it shares a common rotational level. We observe a strong phase change when scanning the radio frequency through molecular resonance. The direction of the phase change depends on the energy level arrangement, that is, if it is progressive or regressive. The experimental results can be simulated using the three-level optical Bloch equations and described with the AC Stark effect, giving rise to an Autler-Townes splitting.

Exploring the conformational landscape of menthol, menthone, and isomenthone: a microwave study.

The rotational spectra of the monoterpenoids menthol, menthone, and isomenthone are reported in the frequency range of 2-8.5 GHz, obtained with broadband Fourier-transform microwave spectroscopy. For menthol only one conformation was identified under the cold conditions of the molecular jet, whereas three conformations were observed for menthone and one for isomenthone. The conformational space of the different molecules was extensively studied using quantum chemical calculations, and the results were compared with molecular parameters obtained by the measurements. Finally, a computer program is presented, which automatically identifies different species in a dense broadband microwave spectrum using calculated ab initio rotational constants as initial input parameters.

IR-IR-UV hole-burning: conformation specific IR spectra in the face of UV spectral overlap.

A new technique, IR-IR-UV hole-burning, is reported for obtaining conformation specific IR spectra when the electronic spectra are too closely overlapped to obtain clean spectra free from interference from other conformations via standard ion dip or fluorescence dip methods. The 4'-aminobenzo-15-crown-5 ether-(HDO) complex is used as an example, on which the method was applied to prove the presence of two conformations having overlapped electronic spectra and to assign IR transitions belonging to the same conformation.

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.

Laser-initiated shuttling of a water molecule between H-bonding sites.

The two-step laser excitation scheme of stimulated emission pumping (SEP) induces shifts of a single water molecule between two remote hydrogen bonding sites on trans-formanilide. This reaction can be initiated by selective excitation of either isomer (CO-bound or NH-bound) with different SEP excitation wavelengths. Energy (E) thresholds for isomerization in both directions have been measured [796 wave numbers NH) CO)