New molecules formed by the non-covalent interaction of cyclopropane (c-C₃H₆) with MCl, where M is either Ag or Cu, have been detected and characterized by means of broadband rotational spectroscopy. They were synthesized by laser ablation of a silver or copper rod in the presence of a gaseous sample containing 1% each of c-C₃H₆ and CCl₄, with the remainder argon. Spectra of several isotopologues of each complex have been analysed. The title molecules are found to have C_2v symmetry, and the geometry can be described by the MCl subspecies coordinating “edge on” to the cyclopropane ring. Experimental structures will be compared with those from ab initio calculations and those of related species.

Fluoroalcohols are used as common cosolvents for studies of the secondary and tertiary substructures of polypeptides and proteins in aqueous solution. It has been proposed that small fluoroalcohol aggregates are crucial for the protein structural altering process.[1] A rotational spectroscopic study of the monofluoroethanol (MFE) dimer was reported by our group before.[2] In this presentation, we report our recent results on the MFE trimer and MFE-water clusters. We analyze the competitive formation of intra- and intermolecular hydrogen bonds, processes that may be crucial for the changes in protein structure that occur in fluoroalcohol-water solution. We show that the MFE trimer takes on a much different binding topology from the recently reported phenol trimer.[3] The results will also be compared to the closely related 2,2,2-trifluoroethanol systems. [1] H. Reiersen, A. R. Rees, Protein Eng. 2000, 13, 739 – 743. [2] X. Liu, N. Borho, Y. Xu, Chem. Eur. J. 2009, 15, 270 – 277. [3] a) N. A. Seifert, A. L. Steber, J. L. Neill, C. Pérez, D. P. Zaleski, B. H. Pate, A. Lesarri, Phys. Chem. Chem. Phys., 2013, 15, 11468; b) T. Ebata, T. Watanabe, N. Mikami, J. Phys. Chem., 1995, 99, 5761.

Clusters of carboxylic acids with water, sulfuric acid, and other atmospheric species potentially increase the rate of new particle formation in the troposphere.F. Riccobono, et al., Science, 344, 717 (2014).^,R. Zhang, et al., Science, 304, 1487 (2004).ere, we present high−resolution pure rotational spectra of o−toluic acid and its complex with water in the range of 5−14 GHz, measured with a cavity−based molecular beam Fourier−transform microwave spectrometer. In both the monomer and the complex, the carboxylic acid functional group adopts a syn- conformation, with the acidic proton oriented away from the aromatic ring. In the complex, water participates in two hydrogen bonds, forming a six−membered intermolecular ring. Despite its large calculated c−dipole moment, no c−type transitions were observed for the complex, because of a large amplitude “wagging” motion of the unbound hydrogen of water, similar to the case of the benzoic acid−water complex.E. G. Schnitzler and W. Jäger, Phys. Chem. Chem. Phys., 16, 2305 (2014).o methyl internal rotation splittings were observed, consistent with a high barrier (7 kJ mol^-1 R. Zhang, et al., Science, 304, 1487 (2004).HE. G. Schnitzler and W. Jäger, Phys. Chem. Chem. Phys., 16, 2305 (2014).N

Rovibrational transitions associated with tunneling states in the water bending vibration in OC-H₂O complex have been recorded using a supersonic jet quantum cascade laser spectrometer at 6.2 μm. Analysis of the resulting spectra is facilitated by incorporating fits of previously recorded microwave and submillimeter data accounting for Coriolis coupling to obtain the levels of the ground vibrational state. The results were then used to confirm assignment of the vibration and explore the nature of tunneling dynamics in associated vibrationally excited states of the complex. A seven-dimension ab initio interaction potential is constructed for the complex. The available spectroscopic data is used to generated a morphed potential. Previous prediction of the D₀ of the complex will be incorporated in the analysis.

In all previously studied complexes between protic acids and chlorofluoroethylenes in our laboratory, the acidic hydrogen atom forms the primary intermolecular interaction with a fluorine atom on the ethylene subunit. This has been rationalized by the greater electronegativity of the fluorine atom leading to a stronger, hydrogen-bond like interaction, than would be formed with the chlorine atom. With (Z)-1-chloro-2-fluoroethylene, however, ab initio calculations for its complex with acetylene indicate that participation of the chlorine atom in the intermolecular interaction leads to lower energy configurations. This is confirmed by observation of the rotational spectrum of the complex by chirped-pulse and Balle-Flygare Fourier transform microwave spectroscopy. The complex is determined to be planar with one interaction between an acetylenic hydrogen and the chlorine atom and a second between the triple bond and the hydrogen atom geminal to chlorine.

The microwave spectrum of the vinyl chloride-hydrogen chloride complex, presented at last year's symposium, is greatly complicated by the presence of two chlorine nuclei as well as an observed, but not fully explained tunneling motion. Indeed, although it was possible at that time to demonstrate conclusively that the complex is nonplanar, the chlorine nuclear quadrupole hyperfine splitting in the rotational spectrum resisted analysis. With higher resolution, Balle-Flygare Fourier transform microwave spectra, the hyperfine structure has been more fully resolved, but appears to be perturbed for some rotational transitions. It appears that knowledge of the quadrupole coupling constants will provide essential information regarding the structure of the complex, specifically the location of the hydrogen atom in HCl. Our progress towards obtaining values for these constants will be presented.

l0pt Figure The covalently linked polyfluorene molecules F1-F6 (see left) are prototypical molecular wires by virtue of their favorable electron/hole transport properties brought about by π-stacking. To understand the role of the covalent linker in facilitating electron transport in these systems, we have investigated several van der Waals (vdW) analogues by resonant mass spectroscopy. Electronic spectra and ion yield curves are reported for jet-cooled vdW clusters containing up to six fluorene units. The near-coincidence of the electronic band origins for the dimer and larger clusters suggests that a structure containing a central dimer chromophore is the predominant conformational motif. As for F1-F6, the threshold ionization potentials extracted from the ion yield measurements decrease linearly with inverse cluster size. Importantly, however, the rate of decrease is significantly smaller in the vdW clusters, indicating more efficient hole stabilization in the covalently bound systems. Results for similar vdW clusters that are locked into specific conformations by steric effects will also be reported.

A generalized formulation of explicit transformations is introduced to investigate the concept of a canonical potential in both fundamental chemical and intermolecular bonding. Different classes of representative ground electronic state pairwise interatomic interactions are referenced to a single canonical potential illustrating application of explicit transformations. Specifically, accurately determined potentials of the diatomic molecules \textH_\text2, \textH_\text2^\text+, HF, LiH, argon dimer, and one-dimensional dissociative coordinates in Ar-HBr, OC-HF, and \textOC−Cl_\text2 are investigated throughout their bound potentials. The advantages of the current formulation for accurately evaluating equilibrium dissociation energies and a fundamentally different unified perspective on nature of intermolecular interactions will be emphasized. In particular, this canonical approach has relevance to previous assertions that there is no very fundamental distinction between van der Waals bonding and covalent bonding or for that matter hydrogen and halogen bonds.

In a recent effort our group investigated the vibrational mechanics of water using the cage of 20 water molecules surrounding an alkali ion as a paradigm system. The M⁺(H₂O)₂₀ clusters are well known “magic number” species (for the larger alkali metals) and are thought to form a pentagonal dodecahedral web encapsulating the ion. We are attracted to these systems because they are sufficiently large to display broad OH fundamental envelopes in a manner similar to that found in bulk water, but do so with a relatively small number of structurally distinct, three coordinated sites in a finite assembly that, although challenging, can be analyzed with electronic structure calculations in the context of a “supermolecule”. We show how this arrangement can provide an ideal platform on which to unambiguously identify the spectral signatures of particular binding sites, information that is invoked to explain the bulk (and interface) spectrum of water but cannot be directly measured in bulk water.
Although this behavior is most relevant to simulations of interfacial water, a future direction of this study will be gaining site-specific information for water in an extended two dimensional structure, and the elucidation of the paths of spectral diffusion associated with this arrangement. This unprecedented work will clarify a number of open questions regarding the site-specificity of ground and vibrationally excited state dynamics.

We report here the FTIR spectra of 1:1 complexes of eight fluorophenol derivatives with water measured under matrix isolation condition. In all the complexes, oxygen of water is the hydrogen bond acceptor and phenolic O-H the hydrogen bond donor. The attributes of the O-H...O linkage in the complexes are tuned remotely by fluorine substitutions at different aromatic sites of phenol. The goal of the study is to find the intermolecular interactions that correlate best with the sequence of spectral shifts of the donor O-H stretching (ν_OH) frequencies. Measurements reveal that the probe phenolic ν_OH shifts vary by nearly 90% from unsubstituted phenol to pentafluorophenol. Interestingly, this large variation correlates poorly with the predicted binding energies of the complexes. Secondly, although electrostatic interaction is considered to dominate the overall stabilization of such classical hydrogen bonds, we see that the shifts do not display any correlation with this interaction at the hydrogen bonding sites. On the other hand, the purely quantum mechanical charge-transfer interaction energies, as obtained from Natural Bond Orbital analysis,are found to display very good correspondence with the spectral shifts. Thus, we propose that such local charge-transfer type interactions are better descriptors of weakening of the hydrogen bond donor than electrostatic energy parameters.

O-H stretching infrared fundamentals (ν_OH) of phenol and a series of fluorophenol monomers and their 1:1 complexes with benzene have been measured under a matrix isolation condition (8K). For the phenol-benzene complex the measured shift of ν_OH is 78 cm⁻¹and for 3, 4, 5-trifluorophenol it is 98 cm⁻¹. Although the cold matrix isolation environment is very different from an aqueous medium, the measured spectral shifts display an interesting linear correlation with the aqueous phase acid dissociation constants (pK_a) of the phenols. The spectral shifts predicted by quantum chemistry calculations at several levels of theory are consistent with the observed values. Correlations of the shifts are also found with respect to energetic, geometric and several other electronic structure parameters of the complexes. Partitioning of binding energies of the complexes into components following the Morokuma-Kitaura scheme shows that dispersion is the predominant component of attractive interaction, and electrostatics, polarization and charge-transfer terms also have contributions to overall binding stability. NBO analysis reveals that hyperconjugative charge-transfers from the filled π-orbitals of the hydrogen bond acceptor (benzene) to the anti-bonding σ*(O–H) orbital of the donors (phenols) display correlations which are fully consistent with the observed variations of spectral shifts. The analysis also shows that the O-H bond dipole moments of all the phenolic species are nearly the same, implying that local electrostatics has only a little effect at the site of hydrogen bonding.

A comparative study of infrared spectral effects for 1:1 complex formation of acetic acid (AA), trifluoroacetic acid (TFAA) and trichloroacetic acid (TFAA) with water and benzene has been carried out under a matrix isolation environment. Despite the large difference in aqueous phase acidities of the three acids, the measured ν_OHstretching frequencies of the monomers of the three molecules are found to be almost same, and in agreement with gas phase electronic structure calculations. Intrinsic acidities are expressed only in the presence of the proton acceptors, water or benzene. Although electronic structure calculations predict distinct ν_OH red-shifts for all three acids, the measured spectral features for TCAA and TFAA in this range do not allow unambiguous assignments for the 1:1 complex. On the other hand, the spectral changes in the ν_C=O region are more systematic, and the observed changes are consistent with predictions of theory. Components of overall binding energy of each complex have been obtained from energy decomposition analysis, which allows determination of the relative contributions of various physical forces towards overall stability of the complexes, and the details will be discussed in the talk.

The effects of environment on the photophysical properties of a series of newly-developed near infrared emitting dyes was studied spectroscopically. Properties of interest include fluorescence emission, fluorescence lifetime, and quantum yield. Tracking how the photophysics of these compounds are affected in the solid phase, in thin films, in solution, and at the single molecule level with changing environment will provide a deeper insight into how dye structure affects their function.