Binding water to a PEG-linked flexible bichromophore: IR spectra of diphenoxyethane-(H2O)n clusters, n = 2-4.

The single-conformation infrared (IR) and ultraviolet (UV) spectroscopies of neutral 1,2-diphenoxyethane-(H2O)n clusters with n = 2-4 (labeled henceforth as 1:n) have been studied in a molecular beam using a combination of resonant two-photon ionization, IR-UV holeburning, and resonant ion-dip infrared (RIDIR) spectroscopies. Ground state RIDIR spectra in the OH and CH stretch regions were used to provide firm assignments for the structures of the clusters by comparing the experimental spectra with the predictions of calculations carried out at the density functional M05-2X/6-31+G(d) level of theory. At all sizes in this range, the water molecules form water clusters in which all water molecules engage in a single H-bonded network. Selective binding to the tgt monomer conformer of 1,2-diphenoxyethane (C6H5-O-CH2-CH2-O-C6H5, DPOE) occurs, since this conformer provides a binding pocket in which the two ether oxygens and two phenyl ring π clouds can be involved in stabilizing the water cluster. The 1:2 cluster incorporates a water dimer "chain" bound to DPOE much as it is in the 1:1 complex [E. G. Buchanan et al., J. Phys. Chem. Lett. 4, 1644 (2013)], with primary attachment via a double-donor water that bridges the ether oxygen of one phenoxy group and the π cloud of the other. Two conformers of the 1:3 cluster are observed and characterized, one that extends the water chain to a third molecule (1:3 chain) and the other incorporating a water trimer cycle (1:3 cycle). A cyclic water structure is also observed for the 1:4 cluster. These structural characterizations provide a necessary foundation for studies of the perturbations imposed on the two close-lying S1/S2 excited states of DPOE considered in the adjoining paper [P. S. Walsh et al., J. Chem. Phys. 142, 154304 (2015)].

Solvent-mediated internal conversion in diphenoxyethane-(H2O)n clusters, n = 2-4.

1,2-diphenoxyethane (DPOE) is a flexible bichromophore whose excited states come in close-lying pairs whose splitting and vibronic coupling can be modulated by solvent. Building on the ground state infrared spectroscopy of DPOE-(H2O)n clusters with n = 2-4 from the adjoining paper [Walsh et al., J. Chem. Phys. 142, 154303 (2015)], the present work focuses on the vibronic and excited state infrared spectroscopies of the clusters. The type and degree of asymmetry of the water cluster binding to DPOE is reflected in the variation in the magnitude of the S1/S2 splitting with cluster size. Excited state resonant ion-dip infrared spectroscopy was performed at the electronic origins of the first two excited states in order to explore how the water clusters' OH stretch spectra report on the nature of the two excited states, and the interaction of the S2 state with nearby S1 vibronic levels mediated by the water clusters. The data set, when taken as a whole, provides a state-to-state view of internal conversion and the role of solvent in mediating conversion of electronic excitation between two chromophores, providing a molecular-scale view of Kasha's rule.

Binding water clusters to an aromatic-rich hydrophobic pocket: [2.2.2]paracyclophane-(H2O)n, n = 1-5.

[2.2.2]Paracylcophane (tricyclophane, TCP) is a macrocycle with three phenyl substituents linked by ethyl bridges (-CH2CH2-) in the para-position, forming an aromatic-rich pocket capable of binding various substituents, including nature's solvent, water. Building on previous work [Buchanan, E. G.; et al. J. Chem. Phys. 2013, 138, 064308] that reported on the ground state conformational preferences of TCP, the focus of the present study is on the infrared and ultraviolet spectroscopy of TCP-(H2O)n clusters with n = 1-5. Resonant two-photon ionization (R2PI) was used to interrogate the mass selected electronic spectrum of the clusters, reporting on the perturbations imposed on the electronic states of TCP as the size of the water clusters bound to it vary in size from n = 1-5. The TCP-(H2O)n S0-S1 origins are shifted to lower frequency from the monomer, indicating an increased binding energy of the water or water network in the excited state. Ground state resonant ion-dip infrared (RIDIR) spectra of TCP-(H2O)n (n = 1-5) clusters were recorded in the OH stretch region, which probes the H-bonded water networks present and the perturbations imposed on them by TCP. The experimental frequencies are compared with harmonic vibrational frequencies calculated using density functional theory (DFT) with the dispersion-corrected functional ωB97X-D and a 6-311+g(d,p) basis set, providing firm assignments for their H-bonding structures. The H2O molecule in TCP-(H2O)1 sits on top of the binding pocket, donating both of its hydrogen atoms to the aromatic-rich interior of the monomer. The antisymmetric stretch fundamental of H2O in the complex is composed of a closely spaced set of transitions that likely reflect contributions from both para- and ortho-forms of H2O due to internal rotation of the H2O in the binding pocket. TCP-(H2O)2 also exists in a single conformational isomer that retains the same double-donor binding motif for the first water molecule, with the second H2O acting as a donor to the first, thereby forming a water dimer. The OH stretch infrared spectrum reflects a cooperative strengthening of both π-bound and OH···O H-bonds due to binding to TCP. The TCP-(H2O)n, n = 3-5 clusters all form H-bonded cycles, retaining their preferred structures in the absence of TCP, but distorted significantly by the presence of the TCP molecule. TCP-(H2O)3 divides its population between two conformational isomers that differ in the direction of the H-bonds in the cycle, either clockwise or counterclockwise, which are distinguishable by virtue of the C2 symmetry of the TCP monomer. TCP-(H2O)4 and TCP-(H2O)5 have OH stretch IR spectra that are close analogues of their benzene-(H2O)n counterparts in the H-bonded OH stretch region, but differ somewhat in the free and π OH stretch regions as the tetramer and pentamer cycles begin to spill out of the pocket interior. Lastly, excited state RIDIR spectroscopy in the OH stretch region is used to probe the response of water cluster to ultraviolet excitation, showing how the proximity of a given water molecule to the aromatic-rich π clouds affects the infrared spectrum of the water network.

Ground state conformational preferences and CH stretch-bend coupling in a model alkoxy chain: 1,2-diphenoxyethane.

1,2-Diphenoxyethane (C6H5-O-CH2-CH2-O-C6H5, DPOE) is a flexible bichromophore in which the two phenyl rings are separated from one another by an -O-CH2-CH2-O- chain with five flexible dihedral angles about which hindered rotation can occur. As such, it is a phenyl capped analog of dimethoxyethane (DMOE), which has served as a model compound for development of force fields for polyethylene glycol (PEG). The ground state conformational energy landscape of DPOE is explored using a combination of single-conformation spectroscopy of the jet-cooled molecule and calculations of the conformational minima and transition states. In the experimental UV spectrum, ultraviolet hole-burning establishes the presence of just two conformations with significant population in the supersonic jet expansion. Fluorescence dip infrared (FDIR) spectroscopy is used to record infrared spectra of the two conformers in the alkyl CH stretch, CH bend, and CO stretch regions. When compared with harmonic vibrational frequency calculations, the two isomers are determined to be of C2h and C2 symmetry, and labeled ttt and tgt to denote the three central dihedrals as trans or gauche. Infrared population transfer spectroscopy is used to determine fractional abundances for the two conformers (f(ttt) = 0.53 ± 0.01; f(tgt) = 0.47 ± 0.01). Relaxed potential energy curves along the three nonequivalent dihedral angles are used to map out the shape of the potential energy landscape that leads to these preferences. The Fermi resonance in the alkyl CH stretch spectrum is successfully modeled using a recently developed methodology [Buchanan et al., J. Chem. Phys. 2013, 138, 064308] employing a reduced dimension Hamiltonian. The scissor overtones couple to the CH2 symmetric stretch and only indirectly to the asymmetric stretch through symmetric stretch/asymmetric stretch coupling. The presence of the oxygen atoms in the chain shifts the CH scissor overtones to higher frequencies than in pure alkyl chains, qualitatively changing the spectral consequences of the Fermi resonance, with the scissor overtones now appearing as the highest frequency bands in the spectrum. The spectra are contrasted with those in 1,2-diphenylethane, a close analog with a very different appearance to its CH stretch spectrum, in which the scissor overtones appear as the lowest frequency bands.

Cyclic constraints on conformational flexibility in γ-peptides: conformation specific IR and UV spectroscopy.

Single-conformation spectroscopy has been used to study two cyclically constrained and capped γ-peptides: Ac-γACHC-NHBn (hereafter γACHC, Figure 1a), and Ac-γACHC-γACHC-NHBn (γγACHC, Figure 1b), under jet-cooled conditions in the gas phase. The γ-peptide backbone in both molecules contains a cyclohexane ring incorporated across each Cß-Cγ bond and an ethyl group at each Cα. This substitution pattern was designed to stabilize a (g+, g+) torsion angle sequence across the Cα-Cß-Cγ segment of each γ-amino acid residue. Resonant two-photon ionization (R2PI), infrared-ultraviolet hole-burning (IR-UV HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been used to probe the single-conformation spectroscopy of these molecules. In both γACHC and γγACHC, all population is funneled into a single conformation. With RIDIR spectra in the NH stretch (3200-3500 cm(-1)) and amide I/II regions (1400-1800 cm(-1)), in conjunction with theoretical predictions, assignments have been made for the conformations observed in the molecular beam. γACHC forms a single nearest-neighbor C9 hydrogen-bonded ring whereas γγACHC takes up a next-nearest-neighbor C14 hydrogen-bonded structure. The gas-phase C14 conformation represents the beginning of a 2.614-helix, suggesting that the constraints imposed on the γ-peptide backbone by the ACHC and ethyl groups already impose this preference in the gas-phase di-γ-peptide, in which only a single C14 H-bond is possible, constituting one full turn of the helix. A similar conformational preference was previously documented in crystal structures and NMR analysis of longer γ-peptide oligomers containing the γACHC subunit [Guo, L., et al. Angew. Chem. Int. Ed. 2011, 50, 5843-5846]. In the gas phase, the γACHC-H2O complex was also observed and spectroscopically interrogated in the molecular beam. Here, the monosolvated γACHC retains the C9 hydrogen bond observed in the bare molecule, with the water acting as a bridge between the C-terminal carbonyl and the π-cloud of the UV chromophore. This is in contrast to the unconstrained γ-peptide-H2O complex, which incorporates H2O into both C9 and amide-stacked conformations.

Towards a first-principles model of Fermi resonance in the alkyl CH stretch region: application to 1,2-diphenylethane and 2,2,2-paracyclophane.

The spectroscopy of two flexible hydrocarbons, 1,2-diphenylethane (DPE) and 2,2,2-paracyclophane (TCP) is presented, and a predictive theoretical model for describing the alkyl CH stretch region of these hydrocarbons is developed. Ultraviolet hole-burning spectroscopy identified two isomers of DPE and a single conformation of TCP present in the supersonic jet expansion. Through the analysis of the ground state low-frequency vibronic spectroscopy obtained by dispersed fluorescence, conformational assignments were made for both DPE and TCP. The two isomers of DPE were found to retain the low energy structures of butane, being present in both the gauche and anti structures. TCP forms a C(2) symmetric structure, differing from the predicted lower energy C(3) conformation by the symmetry of the ethano bridges (-CH(2)CH(2)-) linking the phenyl substituents. Resonant ion-dip infrared spectroscopy is used to record single-conformation IR spectra of the two conformers of DPE and the single conformer of TCP in the alkyl CH stretch region and in the mid-IR that covers the CH bend fundamentals. A local mode Hamiltonian that incorporates cubic stretch-bend coupling is developed. Its parameters are obtained from density functional theory methods. Full dimensional calculations are compared to those that use reduced dimensional Hamiltonians in which anharmonic CH stretches and scissor modes are Fermi coupled. Excellent agreement is found. Scale factors of select terms in the reduced dimensional Hamiltonian are determined by fitting the theoretical Hamiltonian to the anti-DPE spectrum. The scaled Hamiltonian is then used to predict successfully structures for the remaining lower symmetry experimentally determined spectra in the alkyl CH stretch region.

Excitonic splitting and vibronic coupling in 1,2-diphenoxyethane: conformation-specific effects in the weak coupling limit.

Vibrationally and rotationally resolved electronic spectra of 1,2-diphenoxyethane (C6H5-O-CH2-CH2-O-C6H5, DPOE) are reported for the isolated molecule under jet-cooled conditions. The spectra demonstrate that the two excited surfaces are within a few cm(-1) of one another over significant regions of the torsional potential energy surfaces that modulate the position and orientation of the two aromatic rings with respect to one another. Two-color resonant two-photon ionization (2C-R2PI) and laser-induced fluorescence excitation spectra were recorded in the near-ultraviolet in the region of the close-lying S0-S1 and S0-S2 states (36,400-36,750 cm(-1)). In previous work, double resonance spectroscopy in the ultraviolet and alkyl CH stretch regions of the infrared was used to identify and assign transitions to two conformational isomers differing primarily in the central C-C dihedral angle, a tgt conformation with C2 symmetry and a ttt conformation with C2h symmetry [E. G. Buchanan, E. L. Sibert, and T. S. Zwier, J. Phys. Chem. A 117, 2800 (2013)]. Comparison of 2C-R2PI spectra recorded in the m∕z 214 (all (12)C) and m∕z 215 (one (13)C) mass channels demonstrate the close proximity of the S1 and S2 excited states for both conformations, with an upper bound of 4 cm(-1) between them. High resolution spectra of the origin band of the tgt conformer reveal it to consist of two transitions at 36,422.91 and 36,423.93 cm(-1), with transition dipole moments perpendicular to one another. These are assigned to the S0-S1 and S0-S2 origin transitions with excited states of A and B symmetry, respectively, and an excitonic splitting of only 1.02 cm(-1). The excited state rotational constants and transition dipole coupling model directions prove that the electronic excitation is delocalized over the two rings. The ttt conformer has only one dipole-allowed electronic transition (Ag→Bu) giving rise to a pure b-type band at 36,508.77 cm(-1). Here, the asymmetry induced by a single (13)C atom in one of the rings is sufficient to localize the electronic excitation in one or the other ring. Dispersed fluorescence (DFL) spectra are used to provide assignments for all vibronic structure in the first 200 cm(-1)of both conformers. In the tgt conformer, both "a" and "b" symmetry fundamentals are observed, consistent with extensive vibronic coupling between the two dipole-allowed, nearly degenerate excited states. In the ttt conformer, the lowest frequency vibronic transition located 46 cm(-1) above the Bu origin is assigned to a bu fundamental (labeled R[overline]) built off the dipole-forbidden Ag state origin. The DFL spectrum of the Ag(R[overline](1)) level contains strong transitions to v(")(R[overline]) = 0, 1, and 2, seemingly at odds with vibronic coupling models. Studies of the DFL spectrum of this band as a function of distance from the nozzle reveal that much of the intensity in v(") = 1 arises from collisions of DPOE while in the excited state Ag(vb' = 1) level with He, producing Bu(R[overline] = 1) levels with large collision cross section. The remaining intensity in the fundamental at large x∕D is ascribed to emission from the (13)C isotopomer, for which this emission is dipole-allowed.

Mixed 14/16 helices in the gas phase: conformation-specific spectroscopy of Z-(Gly)n, n = 1, 3, 5.

Single-conformation ultraviolet and infrared spectroscopy has been carried out on the neutral peptide series, Z-(Gly)(n)-OH, n = 1,3,5 (ZGn) and Z-(Gly)(5)-NHMe (ZG5-NHMe) in the isolated environment of a supersonic expansion. The N-terminal Z-cap (carboxybenzyl) provides an ultraviolet chromophore for resonant two-photon ionization (R2PI) spectroscopy. Conformation-specific infrared spectra were recorded in double resonance using resonant ion-dip infrared spectroscopy (RIDIRS). By comparing the experimental spectra with the predictions of DFT M05-2X/6-31+G(d) calculations, the structures could be characterized in terms of the sequence of intramolecular H-bonded rings of varying size. Despite the enhanced flexibility of the glycine residues, a total of only six conformers were observed among the four molecules. Two conformers for ZG1 were found with the major conformation taking on an extended, planar ß-strand conformation. Two conformers were observed for ZG3, with the majority of the population in a C11/C7/C7/π(g-) structure that forms a full loop of the glycine chain. Both ZG5 molecules had their population primarily in a single conformation, with structures characteristic of the first stages of a "mixed" ß-helix. C14/C16 H-bonded rings in opposing directions (N → C and C → N) tie the helix together, with nearest-neighbor C7 rings turning the backbone so that it forms the helix. φ/ψ angles alternate in sign along the backbone, as is characteristic of the mixed, C14/C16 ß-helix. The calculated conformational energies of these structures are unusually stable relative to all others, with energies significantly lower than the PGI/PGII conformations characteristic of polyglycine structures in solution and in the crystalline form, where intermolecular H-bonds play a role.

Single-conformation infrared spectra of model peptides in the amide I and amide II regions: experiment-based determination of local mode frequencies and inter-mode coupling.

Single-conformation infrared spectra in the amide I and amide II regions have been recorded for a total of 34 conformations of three α-peptides, three ß-peptides, four α/ß-peptides, and one γ-peptide using resonant ion-dip infrared spectroscopy of the jet-cooled, isolated molecules. Assignments based on the amide NH stretch region were in hand, with the amide I/II data providing additional evidence in favor of the assignments. A set of 21 conformations that represent the full range of H-bonded structures were chosen to characterize the conformational dependence of the vibrational frequencies and infrared intensities of the local amide I and amide II modes and their amide I/I and amide II/II coupling constants. Scaled, harmonic calculations at the DFT M05-2X/6-31+G(d) level of theory accurately reproduce the experimental frequencies and infrared intensities in both the amide I and amide II regions. In the amide I region, Hessian reconstruction was used to extract local mode frequencies and amide I/I coupling constants for each conformation. These local amide I frequencies are in excellent agreement with those predicted by DFT calculations on the corresponding (13)C = (18)O isotopologues. In the amide II region, potential energy distribution analysis was combined with the Hessian reconstruction scheme to extract local amide II frequencies and amide II/II coupling constants. The agreement between these local amide II frequencies and those obtained from DFT calculations on the N-D isotopologues is slightly worse than for the corresponding comparison in the amide I region. The local mode frequencies in both regions are dictated by a combination of the direct H-bonding environment and indirect, "backside" H-bonds to the same amide group. More importantly, the sign and magnitude of the inter-amide coupling constants in both the amide I and amide II regions is shown to be characteristic of the size of the H-bonded ring linking the two amide groups. These amide I/I and amide II/II coupling constants remain similar in size for α-, ß-, and γ-peptides despite the increasing number of C-C bonds separating the amide groups. These findings provide a simple, unifying picture for future attempts to base the calculation of both nearest-neighbor and next-nearest-neighbor coupling constants on a joint footing.

Competition between amide stacking and intramolecular H bonds in γ-peptide derivatives: controlling nearest-neighbor preferences.

Resonant two-photon ionization (R2PI), IR-UV holeburning (IR-UV), and resonant ion-dip infrared spectroscopy (RIDIRS) have been used to record mass-selected, single-conformation ultraviolet and infrared spectra of three simple diamide derivatives of γ-amino acids as isolated molecules cooled in a supersonic expansion. This work builds on an earlier study of Ac-γ(2)-hPhe-NHMe (James, W. H., III, et al. J. Am. Chem. Soc. 2009, 131, 14243), which showed that this methyl-capped γ-peptide forms amide-stacked conformations that are similar in stability to H-bonded conformations containing a C9 ring and more stable than C7 H-bonded ring structures. Among the γ-peptides discussed here, Ac-γ(2)-hPhe-N(Me)(2) contains an additional methyl group relative to the previously studied Ac-γ(2)-hPhe-NHMe and therefore lacks the amide NH group responsible for C9 ring formation. Three conformations of Ac-γ(2)-hPhe-N(Me)(2) are observed, all of which are amide-stacked structures. In a second new molecule, Ac-γ(2)-hPhe-NH(iPr), the C-terminal NHMe group of Ac-γ(2)-hPhe-NHMe is replaced with an NH(iPr) group. Three conformations of Ac-γ(2)-hPhe-NH(iPr) are observed, all of which are C9 H-bonded structures. The dramatic difference between C-terminal NHMe and NH(iPr) reveals the delicate balance of noncovalent forces within these γ-peptides. The third molecule we examined is a gabapentin-derived diamide (designated 1), which contains a phenylacyl group at the N-terminus and an N(Me)(2) group at the C-terminus; the latter precludes C9 H bonding. Comparison of 1 with Ac-γ(2)-hPhe-N(Me)(2) allows us to examine the impact of the backbone substitution pattern (monosubstitution at carbon-2 vs disubstitution at carbon-3) on the competition between the C7 H-bonded and the amide-stacked conformation. In this case, only C7 rings are observed. The different gas-phase behaviors observed among the molecules analyzed here offer insight on the intrinsic conformational propensities of the γ-peptide backbone, information that provides a foundation for future foldamer design efforts.

Conformation-specific spectroscopy and populations of diastereomers of a model monolignol derivative: chiral effects in a triol chain.

Single-conformation spectroscopy of two diastereomers of 1-(4-hydroxy-3-methoxyphenyl)propane-1,2,3-triol (HMPPT) has been carried out under isolated, jet-cooled conditions. HMPPT is a close analog of coniferyl alcohol, one of the three monomers that make up lignin, the aromatic biopolymer that gives structural integrity to plants. In HMPPT, the double bond of coniferyl alcohol has been oxidized to produce an alkyl triol chain with chiral centers at C(α) and C(ß), thereby incorporating key aspects of the ß-O-4 linkage between monomer subunits that occurs commonly in lignin. Both (R,S)- and (R,R)-HMPPT diastereomers have been synthesized in pure form for study. Resonant two-photon ionization (R2PI), UV hole-burning (UVHB)/IR-UV hole-burning (IR-UV HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been carried out, providing single-conformation UV spectra in the S(0)-S(1) region (35200-35800 cm(-1)) and IR spectra in the hydride stretch region. Five conformers of (R,S)- and four conformers of (R,R)-HMPPT are observed and characterized, leading to assignments for all nine conformers. Spectroscopic signatures for α-ß-γ, γ-ß-α, and α-γ-ß-π chains and two cyclic forms [(αßγ) and (αγß)] of the glycerol side chain are determined. Infrared ion-gain (IRIG) spectroscopy is used to determine fractional abundances for the (R,S) diastereomer and constrain the populations present in (R,R). The two diastereomers have very different conformational preferences. More than 95% of the population of (R,R) configures the glycerol side chain in a γ-ß-α triol chain, while in (R,S)-HMPPT, 51% of the population is in α-ß-γ chains that point in the opposite direction, with an additional 21% of the population in H-bonded cycles. The experimental results are compared with calculations to provide a consistent explanation of the diastereomer-specific effects observed.

Evolution of amide stacking in larger γ-peptides: triamide H-bonded cycles.

The single-conformation spectroscopy of two model γ-peptides has been studied under jet-cooled conditions in the gas phase. The methyl-capped triamides, Ac-γ(2)-hPhe-γ(2)-hAla-NHMe and Ac-γ(2)-hAla-γ(2)-hPhe-NHMe, were probed by resonant two-photon ionization (R2PI) and resonant ion-dip infrared (RIDIR) spectroscopies. Four conformers of Ac-γ(2)-hPhe-γ(2)-hAla-NHMe and three of Ac-γ(2)-hAla-γ(2)-hPhe-NHMe were observed and spectroscopically interrogated. On the basis of comparison with the predictions of density functional theory calculations employing a dispersion-corrected functional (ωB97X-D/6-311++G(d,p)), all seven conformers have been assigned to particular conformational families. The preference for formation of nine-membered rings (C9) observed in a previous study [James, W. H., III et al., J. Am. Chem. Soc. 2009, 131, 14243] of the smaller analog, Ac-γ(2)-hPhe-NHMe, carries over to these triamides, with four of the seven conformers forming C9/C9 sequential double-ring structures, and one conformer a C9/C14 bifurcated double ring. The remaining two conformers form C7/C7/C14 H-bonded cycles involving all three amide NH groups, unprecedented in other peptides and peptidomimetics. The amide groups in these structures form a H-bonded triangle with the two trimethylene bridges forming loops above and below the molecule's midsection. The structure is a natural extension of amide stacking, with the two terminal amides blocked from forming the amide tristack by formation of the C14 H-bond. Pair interaction energy decomposition analysis based on the fragment molecular orbital method (FMO-PIEDA) is used to determine the nonbonded contributions to the stabilization of these conformers. Natural bond orbital (NBO) analysis identifies amide stacking with a pair of n → π* interactions between the nitrogen lone pairs and π* orbitals on the carbonyl of the opposing amide groups.

Spectroscopy and photophysics of structural isomers of naphthalene: Z-phenylvinylacetylene.

The fluorescence spectroscopy of Z-phenylvinylacetylene (Z-PVA) has been studied under jet-cooled conditions. The laser-induced fluorescence (LIF) spectrum shows vibronic activity up to 600 cm(-1) above the pi pi* electronic origin at 33 838 cm(-1). In contrast, the single vibronic level fluorescence spectrum of the electronic origin shows strong intensity in transitions ending in ground state levels at least 1200 cm(-1) above the ground state zero-point level. The double-resonance technique of ultraviolet depletion (UVD) spectroscopy was used to show that there are strong absorptions in Z-PVA that are not observed in the LIF spectrum due to the turn of a nonradiative process in this electronic state. The LIF and UVD spectra were compared quantitatively to calculate the relative single vibronic level fluorescence quantum yields. Upon inspection, there are some indications of state specific effects; however, the nature of these effects is unclear. Ab initio and density functional theory calculations of the ground and excited states were used to map the first two excited states of Z-PVA along the C[triple bond]CH bending coordinate, determining them to be pi pi* and pi sigma*, respectively, in character. The crossing of these two states is postulated to be the underlying reason for the observed loss in fluorescence intensity 600 cm(-1) above the pi pi* origin. The spectroscopy of Z-PVA has been compared to the previously characterized E isomer of phenylvinylacetylene [Liu, C. P., Newby, J. J., Muller, C. W., Lee, H. D. and Zwier, T. S. J. Phys. Chem. A 2008, 112 (39), 9454.].

Intramolecular amide stacking and its competition with hydrogen bonding in a small foldamer.

Attractive interactions between two carboxamide groups in a "stacked" geometry are explored under isolated molecule conditions. Infrared spectra of single conformations of a small gamma-peptide, Ac-gamma(2)-hPhe-NHMe, reveal the presence of a conformation in which the two amide planes are approximately parallel with the amide dipoles in an antialigned orientation. This stacked conformation is energetically comparable to conformations that contain an intramolecular amide-amide H-bond. Amide stacking interactions can compete with H-bonding in circumstances where the amide groups can be brought into a stacking configuration with minimal strain, opening the way for its use in the design of future foldamer structures.

Solvent Effects on Vibronic Coupling in a Flexible Bichromophore: Electronic Localization and Energy Transfer induced by a Single Water Molecule.

Size and conformation-specific ultraviolet and infrared spectra are used to probe the effects of binding a single water molecule on the close-lying excited states present in a model flexible bichromophore, 1,2-diphenoxyethane (DPOE). The water molecule binds to DPOE asymmetrically, thereby localizing the two electronically excited states on one or the other ring, producing a S1/S2 splitting of 190 cm(-1). Electronic localization is reflected clearly in the OH stretch transitions in the excited states. Since the S2 origin is imbedded in vibronic levels of the S1 manifold, its OH stretch spectrum reflects the vibronic coupling between these levels, producing four OH stretch transitions that are a sum of contributions from S2-localized and S1-localized excited states. The single solvent water molecule thus plays multiple roles, localizing the electronic excitation in the bichromophore, inducing electronic energy transfer between the two rings, and reporting on the state mixing via its OH stretch absorptions.

Single-conformation spectroscopy and population analysis of model γ-peptides: new tests of amide stacking.

Single-conformation ultraviolet and infrared spectra of a series of model gamma-peptides are reported, with the goal of providing new tests of amide stacking as an amide-amide binding motif. The data also serve to illustrate the power and challenges of carrying out single-conformation spectroscopy of neutral molecules of this size in the gas phase under jet-cooled conditions. Building on recent work on Ac-γ2-hPhe-NHMe (James et al., J. Am. Chem. Soc., 2009, 131, 14243), the effects of derivatization and H2O complexation on amide stacking are studied. Ac-γ2-hPhe-N(Me)2 shows only amide stacked structures, blocking the competing position for formation of an amide-amide H-bond. The Ac-γ2-hPhe-NHMe-H2O complex includes structures in which the H2O molecule forms a bridge between the two stacked amide planes, retaining and enhancing amide stacking. IR population transfer methods are also employed to study the dynamics of photodissociation of the amide stacked-H2O complex. Finally, IR ion-gain spectroscopy is introduced as a means of recording infrared spectra containing contributions from all conformers present, based on IR-induced broadening of the UV absorptions. Its role in estimating fractional abundances is tested on Ac-γ2-hPhe-NHMe.