Hindered Internal Rotation and Ortho-H2 Enrichment in Trans-Stilbene--H2/D2 Complexes

A supersonic free jet expansion has been used to prepare trans-stilbene--H2 and D2 complexes. The cooling in the jet collapses most of the ortho and para H2 and D2 rotational population to the lowest rotational levels of a given nuclear spin symmetry: j = 0 and j = 1. The laser-induced fluorescence excitation spectrum of stilbene--D2 shows a well-resolved doublet at the origin due to stilbene--D2( j = 0) and stilbene--D2( j = 1) complexes. The 4.9 cm-1 splitting of these transitions indicates that the D2 molecule is undergoing hindered internal rotation in the complex and that the barrier to internal rotation changes upon electronic excitation. The relative intensities of the stilbene--D2( j = 0) and stilbene--D2( j = 1) origins depend on the D2concentration in the jet. At low D2 flows the transitions arising from stilbene--D2( j = 1) are favored while at high D2 flows the ( j = 0)/(j = 1) transition intensities approach the 2:1 intensity ratio given by their nuclear spin statistical weights. By contrast, in stilbene--H2 we observe only a single transition at the origin which we assign to stilbene--H2( j = 1). We are able to place an upper bound on the stilbene--H2( j = 0) transition intensity of 5% of the stilbene--H2( j = 1) intensity. Dispersed fluorescence spectra are used to bracket the binding energies of the stilbene--H2/D2 complexes in both ground and excited states

INFRARED SPECTROSCOPY OF SINGLE-TURN AND DOUBLE-TURN TETHERED ALPHA-HELICES IN THE GAS PHASE: DON'T LET YOUR LEFT HAND KNOW WHAT YOUR RIGHT HAND IS DOING.

This talk will describe single-conformation IR and UV spectroscopy of a series of single-turn and double-turn alpha helices as cryo-cooled, gas phase ions. Synthesized samples of tethered pentapeptides are known to form single-turn alpha helices in aqueous solution. When L-amino acids are used, a right-handed single-turn helix is formed while D-amino acids produce a left-handed helix. Due to the tether, these structures are remarkably stable in aqueous solution over a wide range of temperatures, pH, and denaturant. They can be concatenated, making LL, DD, LD, and DL double-turn forms. When a methylated arginine is placed at the C-terminal end of the tethered peptide, single-turn helices are the most stable structure, and are observed exclusively. The spectra in the NH stretch and amide I regions show distinct effects that depend on position along the helix, and the presence or absence of a kink due to concatenation of two opposite-handed helices. We will discuss these spectra and the prospects they offer as scaffolds for studying a wide range of interesting structural and dynamical problems

BROADBAND MICROWAVE AND COMPUTATIONAL STUDY OF HEXAFLUORO-O-XYLENE: HIGHLY COUPLED CF3 ROTORS

The rotational constants and quartic centrifugal distortion coefficients of hexafluoro-o-xylene and all singly 13C isotopologues were precisely determined from the 8 to 18GHz gas phase microwave spectrum. A preliminary r0 structure was determined, reproducing the experimental rotational constants with deviations of no more than 15kHz. Interestingly, rather than the C2v symmetry structure expected intuitively, as in o-xylene, calculations with a variety of methods (B3LYP, CAM-B3LYP, $\omega$B97XD, MP2, and CCSD(T)) predict a C2 symmetry structure in which the two CF3 groups rotate in opposite directions by about 16 degrees. Analysis of the interactions between the two CF3 groups using an effective fragment potential (EFP) approach identified two major contributions to their interaction, due to exchange repulsion and electrostatic repulsion, with electrostatic repulsion responsible for the barrier at the C2v geometry. SH, SMF, PM and TSZ gratefully acknowledge support for this work from the Department of Energy Basic Energy Sciences Gas Phase Chemical Physics program under Grant No. DE-FG02-96ER14656. YK and LVS gratefully acknowledge support from the National Science Foundation (NSF CHE-1800505). Present address for TSZ: Combustion Research Facility, Sandia National Laboratory, Livermore, CA 94550

State Mixing and Vibrational Predissociation in Large Molecule Van Der Waals Complexes: Trans‐Stilbene–X Complexes Where X=He, H2, Ne, and Ar

We report a detailed study of vibrational predissociation and intramolecular–intermolecular state mixing in the first excited singlet state of t r a n s‐stilbene van der Waals complexes with helium, hydrogen, neon, and argon. We present evidence that the helium atom in stilbene–He and the H2 molecule in stilbene–H2 possess very low frequency van der Waals bending levels involving delocalization of the complexed species over both phenyl rings. In stilbene–He, the mode‐selective, strong coupling of the out‐of‐plane phenyl ring modes with the pseudotranslation van der Waals modes leads to a dramatic, inhomogeneous broadening of the transitions to several times their breadth in in‐plane vibrations. The observed dispersed fluorescence spectra give product state distributions and internal clock lifetime estimates which can only be made consistent with direct lifetime measurements by assuming extensive state mixing of the intramolecular levels with the van der Waals levels in which the states accessed by the laser are actually only about 30% intramolecular in character. We conclude that in these complexes the processes of intramolecular–intermolecular state mixing (static IVR) and vibrational predissociation are not independent processes but are closely tied to one another. In fact, the vibrational product state distributions observed for the out‐of‐plane phenyl ring levels can best be interpreted as reflecting the percentage van der Waals character in the initially prepared state. In stilbene–H2 the mode selective coupling exhibits itself as a splitting of the out‐of‐plane transitions into a set of 5–6 closely spaced transitions separated by only about 1 cm− 1. The sequence of transitions is suggestive of an in‐plane potential for H2 motion which is nearly flat across the entire length of the stilbene molecule with a small barrier presented by the ethylenic carbons through which the H2 molecule can tunnel. Dispersed fluorescence spectra from these levels point to a two‐tiered coupling scheme with the bound van der Waals levels. In contrast, the out‐of‐plane phenyl transitions in stilbene–Ne and stilbene–Ar possess unusual shifts, but the transitions are narrow once again. In these cases the complexed atom appears to be largely localized over a single phenyl ring

The Infrared and Ultraviolet Spectra of Individual Conformational Isomers of Biomolecules: Tryptamine

Resonant ion-dip infrared (RIDIR) and UV-UV hole-burning spectroscopies are used to record the hydride stretch infrared spectra and S 1 rS 0 ultraviolet spectra of each of seven conformational isomers of tryptamine free from interference from one another. The different conformations of the ethylamine side chain produce unique S 1 rS 0 vibronic spectra, which can serve as the basis for RIDIR spectroscopy. The seven conformers possess unique spectral signatures in the alkyl CH stretch region of the infrared, which aid in the assignment of the observed transitions in the ultraviolet. Density functional theory (DFT) calculations of the structures, relative energies, and harmonic vibrational frequencies of nine low-energy minima are compared with the present and previous experimental data on tryptamine to assign the spectra of all seven conformers, all of which point the R carbon out of the plane of the indole ring. The nine conformers consist of all combinations of the three minimum-energy amino group positions (anti, gauche toward the phenyl side, and gauche toward the pyrrole side of indole) and three amino group orientations (180°, (60°) at each position. All three anti conformers are observed experimentally, whereas only the two lowest-energy of the three orientational conformers at each gauche position are observed. The dominant factor in determining the form of the CH stretch infrared spectrum is the orientation of the amino group, with the amino group position playing a secondary role. The frequencies of the S 1 rS 0 origin transitions, on the other hand, are most sensitive to the position of the amino group, whether it is anti, gauche phenyl, or gauche pyrrole. The prospects for using these methods more generally to characterize the conformations and energetics of flexible biomolecules are discussed

Infrared Spectroscopy Of H-bonded Bridges Stretched Across The Cis-amide Group: I. Water Bridges

The water-containing clusters of oxindole (OI) and 3,4-dihydro-2(1H)-quinolinone (DQ) have been studied in the hydride stretch region of the infrared by the technique of resonant ion-dip infrared spectroscopy (RIDIRS). Both OI and DQ are constrained cis-amides with adjacent N-H and C=O groups between which water can form H-bonded bridges. The hydride stretch fundamentals of OI-W-n with n = 1-3 and DQ-W-n with n = 1, 2 without exception divide up into free OH stretch fundamentals near 3700 cm(-1) and a set of H-bonded bridge fundamentals in the 3200-3450 cm(-1) region. The bridge fundamentals show a distribution of intensities that reflects strong coupling among the XH oscillators in the bridge. When more than one water is involved in the bridge, the bridge fundamentals are unusually broad, with widths of 50-80 cm(-1) full width at half-maximum. Minimum-energy structures, binding energies, vibrational frequencies, and infrared intensities have been calculated by density functional theory with a Becke3LYP functional and a 6-31+G* basis set. The calculated infrared spectra match experiment well, confirming the bridge structures for the clusters. The form of the calculated normal modes provides insight into the nature of the bridge fundamentals

Infrared Spectroscopy of H-Bonded Bridges Stretched across the cis-Amide Group: II. Ammonia and Mixed Ammonia/Water Bridges

Clusters of two model cis amides, oxindole and 3,4-dihydro-2(IH)-quinolinone, containing one and two ammonia molecules have been studied in the IR hydride stretch region using resonant ion-dip IR spectroscopy. The spectra confirm that ammonia is able to form hydrogen-bonded bridges across the adjacent amide N-H and C=O sites in a manner very similar to that of water. Such bridged structures require that ammonia assume the role of a hydrogen bond donor. Further similarities of the hydrogen bonding capabilities of ammonia and water have been revealed by investigations of ternary clusters containing an amide, one ammonia, and one water molecule. Experimentally, two species are observed having IR spectra consistent with a hydrogen-bonded bridge structure. The two species differ only in the relative positions of the ammonia and water molecules within the bridge. These experimental results are well supported by optimized structures, vibrational frequencies, and IR intensities calculated using density functional theory with the Becke3LYP functional. Additionally, the characteristic features of the hydride stretch fundamentals in a hydrogen-bond-donating ammonia molecule can be readily understood using a simple model for the coupled NH oscillators in which the hydrogen-bonded NH has its force constant lowered and its dipole derivative increased, much like in other hydrogen-bonded XH groups

Towards Understanding Photodegradation Pathways in Lignins:The Role of Intramolecular Hydrogen Bonding in Excited States

The photoinduced dynamics of the lignin building blocks syringol, guaiacol, and phenol were studied using time-resolved ion yield spectroscopy and velocity map ion imaging. Following irradiation of syringol and guaiacol with a broad-band femtosecond ultraviolet laser pulse, a coherent superposition of out-of-plane OH torsion and/or OMe torsion/flapping motions is created in the first excited 1ππ* (S1) state, resulting in a vibrational wavepacket, which is probed by virtue of a dramatic nonplanar → planar geometry change upon photoionization from S1 to the ground state of the cation (D0). Any similar quantum beat pattern is absent in phenol. In syringol, the nonplanar geometry in S1 is pronounced enough to reduce the degree of intramolecular H bonding (between OH and OMe groups), enabling H atom elimination from the OH group. For guaiacol, H bonding is preserved after excitation, despite the nonplanar geometry in S1, and prevents O–H bond fission. This behavior affects the propensities for forming undesired phenoxyl radical sites in these three lignin chromophores and provides important insight into their relative “photostabilities” within the larger biopolymer

Wavepacket insights into the photoprotection mechanism of the UV filter methyl anthranilate

Meradimate is a broad-spectrum ultraviolet absorber used as a chemical filter in commercial sunscreens. Herein, we explore the ultrafast photodynamics occurring in methyl anthranilate (precursor to Meradimate) immediately after photoexcitation with ultraviolet radiation to understand the mechanisms underpinning Meradimate photoprotection. Using time-resolved photoelectron spectroscopy, signal from the first singlet excited state of methyl anthranilate shows an oscillatory behavior, i.e. quantum beats. Our studies reveal a dependence of the observed beating frequencies on photoexcitation wavelength and photoelectron kinetic energy, unveiling the different Franck-Condon overlaps between the vibrational levels of the ground electronic, first electronic excited, and ground cationic states of methyl anthranilate. By evaluating the behavior of these beats with increasing photon energy, we find evidence for intramolecular vibrational energy redistribution on the first electronic excited state. Such energy redistribution hinders efficient relaxation of the electronic excited state, making methyl anthranilate a poor choice for an efficient, efficacious sunscreen chemical filter