Timothy S. Zwier

We report the IR spectroscopy and structural analysis of the halide ions F–, Cl–, and Br– bound inside a bicyclic amide-based cryptand cage (1) under cryo-cooled conditions in the gas phase. We record infrared spectra of the cryo-cooled gas-phase ions in the hydride stretch and fingerprint regions using IR-UV double resonance methods. The IR spectra in the NH and CH stretch regions are remarkably simple, reflecting highly symmetric structures for all three 1·X– complexes. We compare the experimental spectra with calculated candidate structures using first-principles predictions of the infrared spectra in the NH, CH, and fingerprint regions. We assign the 1·F– complex to a 9-coordinate, D3 symmetry structure that contains six NH···F– and three aromatic CH···F– H-bonds, much as it does in the crystalline solid, with a trigonal twist angle α = 35°. While the gas-phase and X-ray structures of 1·F– are similar, the crystal structure is compressed by its surroundings, with the distance between the two bridgehead tertiary amines that is 0.58 Å shorter than our assigned gas-phase structure. In contrast, the 1·Cl– and 1·Br– complexes are uniquely assigned based on their infrared spectra to C3h symmetry structures in which the six amide N’s form a 6-coordinate trigonal prism (α = 0°) binding pocket, while the two tertiary N’s form caps on the prism’s triangular faces. To accommodate these larger halide ions, the three aromatic rings rotate so that their closest CH groups are nearly tangential to the spherical halide ion at the cage’s center.

The prospect of using lasers to selectively break chemical bonds in molecules is often thwarted by the intrinsic anharmonic mixing of the vibrational levels that lead to dissociation with a dense manifold of other vibrational states that do not. Using an ultraviolet-infrared (UV-IR) double resonance excitation scheme, we demonstrate both site- and mode-selective photodissociation of the O–H bond in either of the two tyrosine chromophores (Tyr, Y) in a cryogenically cooled, gas phase protonated pentapeptide ion [YGGYL+H]+ with 231 vibrational modes. The two tyrosine’s have unique UV absorptions with electronic origins separated from one another by 324 cm–1, enabling site-specific electronic excitation of the 1ππ* state of the Y1 or Y4 chromophore. Isotopic substitution of 18OH on one of the two tyrosine’s phenol moiety (forming Y(18O)GGYL or YGGY(18O)L) produces a shift in the infrared absorption frequency of the OH stretch fundamental of −11 cm–1. Since the OH stretch fundamental shifts by −72 cm–1 in the 1ππ* state relative to the ground state, UV-IR spectra as a function of IR-UV time delay map out the excited state IR spectra and provides a means for promoting either OH group to OH(v = 1) in their 1ππ* excited state. Using UV-IR excitation at short IR-UV delay times, we observe site-selective and mode-selective loss of the OH hydrogen atom, which is efficiently lost only when both UV and IR excitation are on the same Tyr chromophore, with a mode-selectivity of greater than 100:1. No other combination of UV and IR excitation produces any measurable H atom loss. Since the OH stretch coordinate is the reaction coordinate for H atom loss, we identify tunneling underneath the 1ππ*/1πσOH* conical intersection (CI) as responsible for the mode-selectivity of the photofragmentation, and argue that tunneling through the CI following OH(v = 1) excitation is fast enough that it competes favorably over vibrational energy scrambling, making it possible to selectively fragment either OH bond in this large molecule with high fidelity.

We model the spectroscopy of the two methyl torsional degrees of freedom coupled to the OH torsional motion in 2,6-dimethylphenol. Recent gas-phase rotational transitions [Welsh et al., J. Phys. Chem. Lett. 17, 3749–3758 (2026)] have been interpreted to be the result of certain symmetry states of the methyl groups leading to the quenching of the OH torsional motion. Both a three-dimensional model and an adiabatic model, in which the OH torsion is treated as the slow mode, are developed to test these conjectures. Good agreement is found between the exact and approximate adiabatic models. The adiabatic model is developed to interpret and elucidate the key couplings leading to the quenching of the tunneling splitting.

The broadband microwave spectrum of perfluorophenol (C₆F₅OH) was recorded over the 7.5-17.5 GHz and 26-40 GHz frequency ranges under jet-cooled conditions in the gas phase. 895 lines were assigned to the ¹²C₆F₆OH parent molecule. We also detect torsion-rotation transitions of the singly ¹³C and ¹⁸O-substituted isotopologues in natural abundance and fit a total of 405 transitions of the isotopologues. Tunneling due to OH internal rotation in a V₂ potential leads to a 0₊/0_- tunneling splitting of ΔE = 24.826 MHz that appears on the a-type transitions. Using a properly symmetrized relaxed potential energy curve, we carry out discrete-variable reference (DVR) calculations to predict both the tunneling splitting and the Coriolis-like coupling term F_ab[J_aJ_b + J_bJ_a] to an accuracy of 3% from first principles. We compare calculated transition frequencies using the best-fit F_ab to those with F_ab = 0 to identify sets of rotational levels in the 0₊ and 0_- tunneling levels that are shifted significantly by Coriolis-like mixing between them, which is maximized for K_c = 1,2 in this oblate symmetric top limit. Finally, in the singly substituted isotopologues of PFP (¹³C or ¹⁸OH), the potential energy surface for tunneling is still symmetric, but asymmetry in the kinetic terms of the Hamiltonian that are introduced by asymmetric ¹³C-substitution at the positions ortho and meta to the OH group quench the OH tunneling. While the v = 0₊ and 0_- torsional wave functions for these levels are completely localized in ortho-¹³C PFP, we predict partial delocalization in v = 0₊ and 0_- torsional substates of meta-¹³C PFP. However, the predicted transitions between tunneling levels in meta-¹³CC₅F₅OH could not be assigned experimentally.

2,6-Dimethylphenol has three internal rotors: two methyl rotors taking up positions ortho to the phenol–OH group and the OH rotor itself. We recorded the broadband microwave spectrum of 2,6-dimethylphenol over the 7.5–17.5 GHz range under jet-cooled conditions in the gas phase. In contrast to the tunneling doublets observed in phenol, a-type rotational transitions in 2,6-dimethylphenol appear as equally spaced triplets, with the two outer components split by ± 48.54 MHz relative to the central line. We fit the spectrum using a model in which OH tunneling either occurs or is quenched, depending on whether the two methyl rotors are in AA/EE states (tunneling present) or AE/EA states (tunneling quenched). Tunneling of the OH group is quenched in the AE/EA methyl rotor states due to OH/CH₃ coupling that modulates the methyl rotor barrier height by almost a factor of 2, depending on the OH orientation relative to the methyl group. This, in turn, changes the energy of the AE and EA methyl rotor states, producing an effective asymmetry in the OH tunneling coordinate that is significantly greater than the tunneling splitting. This localizes the OH torsional wave functions in one or the other of the two wells. By contrast, the a-type rotational transitions in the AA and EE methyl rotor states possess a tunneling splitting similar to that observed in phenol. We compare and contrast the case of state-dependent quenching of tunneling encountered here to the reduction in tunneling splitting that can occur in asymmetric vibrational states.

Carbamate formation is an elementary step that governs nitrogen-centered nucleophilic CO₂ capture across diverse environments, yet a direct, structure-specific understanding has been lacking. Here, we report the first gas-phase characterization of closed-shell carbamate formation by deprotonated amides, using cryogenic ion trap vibrational spectroscopy combined with quantum chemical calculations. Reactions of deprotonated benzamide and isophthalamide anions with CO₂ form carbamate species, with the amide nitrogen serving as the nucleophilic site. Diagnostic, strongly red-shifted antisymmetric CO₂ stretching vibrational bands, supported by a bonding analysis, establish chemisorption with substantial charge transfer. In the multiamide system, an intramolecular N-H···O hydrogen bond provides synergistic stabilization, correlating with larger red shifts and more exothermic binding. These structure-assigned benchmarks provide molecular-level insights into the binding motif, charge redistribution, and hydrogen bond-mediated stabilization of amide carbamates, which aid the characterization of carbamate formation in condensed phase.