In a quest for improving current laser-ablation devices in combination with molecular-beam Fourier-transform microwave spectroscopy (LA-MB-FTMW)Bermúdez, C.; Mata, S.; Cabezas, C.; Alonso, J. L., Tautomerism in Neutral Histidine. Angew. Chem. Int. Ed. 53, 11015–11018 (2014)^, Sanz, M. E.; Cabezas, C.; Mata, S.; Alonso, J. L. Rotational Spectrum of Tryptophan. J. Chem. Phys., 140 (20), 204308 (2014).w

The structure of the Gly-Pro and Pro-Gly dipeptides have been studied by laser ablation chirped pulse and molecular beam Fourier transform microwave (LA-CP-FTMW and LA-MB-FTMW) spectroscopy.Bermúdez, C.; Mata, S.; Cabezas, C.; Alonso, J. L., Tautomerism in Neutral Histidine. Angew. Chem. Int. Ed. 53, 11015–11018 (2014).^, J. L. Alonso and J. C. López, in Gas-Phase IR Spectroscopy and Structure of Biological Molecules, eds. A. M. Rijs and J. Oomens, Topics in., 2015, vol. 364, pp. 335–402.W

Last year we presented preliminary results on three methylamine substituted pyridines, 2-, 3-, and 4-picolylamine. After helpful feedback, the microwave spectra of all three molecules were recollected over the frequency range of 7-18 GHz using by zero-fitting the free induction decay. Each molecule showed a distinctive quadrupole splitting, which is representative of the local electronic environment around the two different ¹⁴N nuclei, with the pyridine nitrogen being particularly sensitive to the pi-electron distribution within the ring. An extended Townes and Dailey analysis was used to determine the lone pair density around each nitrogen and compared to that of benzylamine and pyridine. Results of this analysis and how it explains the configuration of the methylamine group with respect to the pyridine ring in each of the picolylamines will be discussed.

Nitrobenzonitrile consists of two electron-withdrawing groups, which have negative inductive and mesomeric effects on the phenyl ring resulting in interesting physical properties. The rotational spectra of 2- and 3-nitrobenzonitrile were recorded via chirped-pulse Fourier transform microwave spectroscopy in the frequency range of 2−8 GHz. For both molecules, the main isotopologues and all isotopologues of the respective ¹³C-, ¹⁵N-, ¹⁸O-monosubstituted species in their natural abundance were assigned. These assignments allowed for the structural determination of 2- and 3-nitrobenzonitrile via Kraitchman's equations as well as a mass-dependent least-squares fitting approach. Structural changes caused by steric interaction and competition for the electron density of the phenyl ring highlight how these strong electron-withdrawing substituents affect one another according to their respective positions on the phenyl ring.

The rotational spectrum of methyl pivalate (t-BuC(O)OCH₃) in the ground vibrational state was observed by molecular beam-Fourier transform microwave spectroscopy. Thirty b-type rotational transitions were assigned. Some high-K_a lines were found to be split and we have interpreted these splittings in terms of the internal rotation of the methyl group. Some forbidden transitions were also observed in case where K_a = 2 levels were involved in the internal rotation with E state. The observed spectra including the forbidden transitions were analyzed using XIAM program. Spectral assignment confirmed that the observed spectral splitting was due to the methyl rotation of the methoxy group. In comparison with the theoretical rotational constants, observed rotational constants indicated the molecule took a trans-zigzag conformation for C-C-C-O-C skeleton with C_s symmetry.

The rotational spectra of methoxycarbonyl isocyanate ( CH₃OC(O)NCO) and methyl azidoformate (CH₃OC(O)N₃) in the ground vibrational state were observed by molecular beam-Fourier transform microwave spectroscopy. Observed spectral lines for a-type transitions were assigned. Splittings of the spectral lines were observed by the internal rotation of the CH₃ group and the hyperfine structure of the ¹⁴N atom. Comparison of the observed spectroscopic constants with the calculated ones led to the conclusion that the assigned spectrum was due to the syn-syn form that NCO or N₃ group. Determined parameters by the spectral analysis of methoxycarbonyl isocyanate were rotational, centrifugal distortion and nuclear electric quadrupole coupling constants including the potential barrier V₃ to internal rotation of the methyl group. At this stage, we could not analyze the complicate nuclear electric quadrupole coupling splittings of the observed spectrum of methyl azidoformate due to the hyperfine structure.

The rotational spectra of the singly-substituted ¹³C isotopic species of a number of sulfur-terminated carbon chains have been detected between 5 and 40 GHz using a supersonic jet in combination with a cavity Fourier transform microwave spectrometer. The chains include both closed-shell molecules (e.g., H₂C₃S) and radicals (e.g., HCCS, HC₃S, and C₄S). The experiments were carried out with precursors enriched in ¹³C, either H¹³C¹³CH or ¹³CS₂. From the ¹³C hyperfine coupling constants, the unpaired electronic density along the chain can be quantified for the radical species, while precise experimental structures (r₀) can be derived for each molecule by a least-squares fit to the rotational constants. The use of ¹³CS₂ in particular provides clues as to the dominant formation pathway for each chain in our discharge nozzle. Somewhat surprisingly, the ¹³C from this precursor appears to be substituted in one of three distinct ways: random, a specific C site, or not at all. This propensity appears to be molecule specific, implying that both neutral-radical and radical-radical reactions are important.

Produced in a reaction or interaction either between methanol and methoxide or methoxide and oxygen, an unexpected and difficult to identify species has appeared in our mass spectrum. We seek to narrow its molecular formula to one of CH₃ O₃ or C₂ H₇ O₂ using fully deuterated methanol as a precursor to separate the two formulas by mass, and to further use the photoelectron spectrum and a fragmentation spectrum of the fully deuterated species to most accurately determine the molecular identity and completely assign its photoelectron spectrum.

With the development of designed foldamers that that display biological activity and aid in drug delivery processes, there is an increasing interest in exploring the inherent conformational properties of model foldamers to understand better the subtle counter-balance of forces at play. β-amino acids have one additional backbone carbon atom that extends the spacing between amide groups, thereby providing a flexibility of construction, leading to secondary structures that either mimic or are complementary to those found in nature. Here we explore the secondary structures of foldamers in which the position of substitution of the methyl side chains of the Ala residues are switched from the β² to the β³ position. In particular, we present data on a β³-Ala tripeptide, Ac- β³-Ala- β³-Ala-β³-Ala-NHBn under jet-cooled, isolated conditions. This talk will present conformation-specific IR and UV spectra using the techniques of resonant ion-dip infrared (RIDIR) spectroscopy and IR-UV holeburning, respectively. Following an exhaustive computational search of the conformational potential energy surface, low-lying minima are optimized and IR spectra calculated for comparison with the experimental RIDIR spectra. A single conformer is observed experimentally and assigned to a conformer with a C12/C8 hydrogen-bonded architecture that incorporates a 12-membered turn that is the β-peptide analog of a β-turn.

Aminoisobutyric acid (Aib) is an achiral synthetic amino acid with a high propensity to form 3₁₀ helices, the second most biologically abundant helix. Aib is also known to induce helix formation in other peptides, and can persuade helix formation from a consecutive sequence of as little as three residues. Studying polyAib in the gas-phase provides a spectroscopic opportunity to examine the inherent hydrogen bonding that directs this helix formation. Since Aib is achiral, there is no infrared (IR) distinction between left- and right-handed helices, preventing powerful IR-based structural differentiation of left- and right-handed structures. However, the introduction of chirality through a cap offers a means by which the balance between the two can be tipped. However, it could also induce alterations to the structure. This talk describes conformation-specific IR and ultraviolet (UV) double-resonance spectroscopy of a series of three Aib dipeptides with three UV-active C-terminal caps: NH-benzyl (NHBn) as a reference structure due to its common use as a fluorophore in similar studies, NH-p-fluorobenzyl (NHBnF), and alpha-methylbenzylamine (AMBA), a chiral cap. These molecules are brought into the gas phase via laser desorption and cooled in a supersonic expansion, enabling us to probe the inherent conformational preferences of the isolated molecules. For both the NHBn and NHBnF cap, a single conformer is observed, with infrared spectra assignable to nearly identical type II β-turn structure. Additionally, the higher oscillator strength of the NHBnF cap enabled UV-UV holeburning, not readily accomplished with the NHBn cap. For AMBA, two unique conformers were found, one of which was a nearly identical type II β-turn, while the minor conformer possessed two sequential γ-turns formed between adjacent amide groups, stabilized by an NH-π interaction.

The isomeric benzyl and tropylium C₇H₇⁺ cations have been of great interest to physical organic and gaseous ion chemists for many decades. Still, infrared spectroscopic characterization of these ions in the gas phase could so far only be achieved for their methylated derivatives but not for the C₇H₇⁺ ions themselves. Thus, we set out to produce both relevant isomers of this elusive ion in a cold molecular beam experiment through ionization of different precursor molecules comprising the preformed 6- and 7-membered rings. We measured their IR spectra via photodissociation with a tunable OPO/OPA laser system in combination with the argon messenger atom technique. The obtained spectra were assigned with the aid of second order vibrational perturbation theory utilizing dispersion-corrected density functional theory.