Much attention has surrounded 1,3-butadiene because this conjugated diene, the simplest of all, is an ideal candidate to observe the effects of π-electron delocalization, and because the species is of central importance in a wide range of chemistry applications. Butadiene exists in two forms, the most stable planar trans and the long-elusive gauche lying 12 kJ/mol higher in energy. The later was only conclusively detected in the gas phase by some of us recently, in a work that allowed the determination of a partial structure from a combination of pure rotational measurements and high level quantum chemical calculations. In an attempt to detect the ro-vibrational spectrum of gauche-butadiene, we have re-investigated the far-infrared spectrum of the species at both the SOLEIL synchrotron facility and the Canadian light source using three experimental set-ups allowing to reach rotational temperatures ranging from 30 to 350 K. We will present our results on new fundamentals of the trans form, and on the gauche form.

The rotational spectrum of n-propyl nitrate was measured in the frequency range between 6-18 GHz using a Balle-Flygare Fourier transform microwave jet/cavity spectrometer. Parent, ¹³C and ¹⁵N isotopologue transitions for the lower-energy anti-gauche (AG) conformer were found using this instrument. The search for spectra from other conformers was performed using a broadband chirped-pulse jet spectrometer. Transitions from the anti-anti (AA) conformer were observed in this manner. Parent, ¹³C and ¹⁵N isotopologue transitions for the AA conformer were rescanned using the cavity instrument to improve resolution and the signal-to-noise ratio. Rotational, centrifugal distortion, and nuclear electric quadrupole coupling constants for all conformers/isotopologues were fit using Pickett’s SPFIT program. The structure of n-propyl nitrate will be discussed in light of these results.

The rotational spectrum of n-butyl nitrate was measured with a chirped pulse Fourier transform microwave spectrometer over the range of 7-13 GHz. This allowed for the identification and assignment of five conformers, AGA, GAA, GGA, AAA, and GG'A with intensity ratios of approximately 1 : 0.2 : 0.2 : 0.1 : 0.1. These conformers were subsequently investigated by Balle-Flygare Fourier transform microwave spectroscopy in order to resolve the nuclear electric quadruople coupling splitting and for further transitions to be collected, allowing improved fitting. Isotopic substitutions at natural abundance were also observed for the AGA conformer. Quantum chemical calculations reveal that the AGA conformer is the lowest in energy. A variety of computational methods were used to explore the energy ordering and barriers between inter-conversion of the lowest lying energy conformers. This work extends a series of other alkyl nitrate works studied by microwave and mm-wave spectroscopy.Methyl Nitrate; W.B. Dixon, E.B. Wilson, J. Chem. Phys., 35, 191 (1961)Ethyl Nitrate; D.G. Scroggrin, J.M. River, E.B. Wilson, J. Chem. Phys., 60, 1376 (1974); J. Thomas, I. Medvedev, D. Dolson, ISMS 2014 n-Propyl Nitrate; W. Orellana, S.L. Stephens, S.E. Novick, S.A. Cooke, C. Brauer, T.A. Blake, ISMS 2019center Figure

The rotational spectra of two conformers, GAA and AAA, of 1-iodopentane have been recorded using both Fourier transform microwave cavity and chirp spectrometers. Iodine has a large nuclear electric quadrupole coupling tensor, χ, and is heavy enough to require a relativistic calculation for ab initio field gradient calculations. We will present a comparison of the experimental results, a calculation involving a clever scaling of the χ tensor,W.C. Bailey and F.M. Gonzalez J. Mol. Spectro., 651-653, 689-695 (2003)nd a direct relativistic ab initio calculation.L. Cheng, J. Gauss, J. Chem. Phys., 134, 244112 (2011)html:<hr /><h3>Footnotes: W.C. Bailey and F.M. Gonzalez J. Mol. Spectro., 651-653, 689-695 (2003)a L. Cheng, J. Gauss, J. Chem. Phys., 134, 244112 (2011)

The pure rotational spectrum of allyl isocyanate (CH₂=CHCH₂NCO) was studied using chirped pulse and Balle-Flygare Fourier Transform microwave (FTMW) spectroscopy. Besides the previously reported gauche conformer,S. Maiti, A. I. Jaman, and R. N. Nandi, J. Mol. Spectrosc. 158, 8-13 (1993)he lowest energy conformer was identified for the first time with the assistance of the quantum-chemical calculations performed at the B3LYP-D3(BJ) and MP2 levels of theory with Dunning’s cc-pVQZ basis set. The assignments were confirmed by the resolved hyperfine structure due to the ¹⁴N quadrupole moment and the spectra of the corresponding ¹³C, ¹⁵N and ¹⁸O singly substituted isotopologues in natural abundance. Rotational transitions of the most stable conformer revealed a tunneling splitting due to the interconversion motion between its two mirror images, and the tunneling path was established theoretically. In addition, benchmark calculations of various density functionals with and without dispersion corrections were carried out to investigate the effect of the short-range dispersion energy on the conformational structures. S. Maiti, A. I. Jaman, and R. N. Nandi, J. Mol. Spectrosc. 158, 8-13 (1993)t

A spectroscopic analysis of allyl isothiocyanate (CH₂=CHCH₂NCS) was conducted via chirped-pulse and Balle-Flygare Fourier Transform microwave (FTMW) spectroscopy in the 7-25 GHz range. Rotational transitions associated with the different conformers were assigned based on information gathered from quantum-chemical calculations computed at B3LYP-D3(BJ) and MP2 levels of theory using the Dunning cc-pVTZ basis set. The results showed the existence of two conformers, the gauche conformer, which was reported in previous workMaiti, S.; Jaman, A. I.; Nandi, R. N. J. Mol. Spectrosc. 165 (1994), 168-172 s well as the first reported existence of a second conformer which corresponds to the global minimum. Spectroscopic assignments included transitions due to both parent conformers, ³⁴S, ¹³C and ¹⁵N singly substituted isotopic species as well as hyperfine splitting due to the presence of ¹⁴N. Maiti, S.; Jaman, A. I.; Nandi, R. N. J. Mol. Spectrosc. 165 (1994), 168-172 a

The conformational landscape of mandelic acid (phenyl glycolic acid), the simplest alpha-hydroxy acid containing an aromatic substituent, has been unveiled by laser ablation coupled with broadband Fourier transform microwave spectroscopy LA-CP-FTMW. The four identified conformers provide the first global structural picture of this molecule traditionally used in cosmetics and dermatology as well as in the synthesis of pharmaceuticals. Rotational transitions of singly substituted ¹³C isotopic species of the most stable conformer in its natural abundances were also detected, providing insight on its geometry.

Foldamers are peptides synthesized to adopt specific motifs, for instance, helices. One class of foldamers that has received particular attention are β-peptides,with two carbons between amide groups, or γ-peptides, with three. This extension of the carbon backbone provides additional conformational flexibility and more choice for the position(s) of peptidomimetic substitution. Previous gas-phase jet-cooled conformation specific spectroscopy of the γ-peptide, Ac-γ²-Phe-NHMe, found three different conformers and provided the first spectroscopic signature of amide stacking.[1] The study also utilized infrared population transfer (IRPT) studies that determined the relative population of each conformer in the expansion, and later, the binding of a single water molecule.[2] Here, we expand the study further to include Ac-γ⁴-Phe-NHMe. Repositioning of the peptidomimetic side chains from the 2- to the 4-position of γ-peptides has been shown to induce helix formation in solution. This motivates a study of the intrinsic change in conformational preferences induced by γ²- vs. γ⁴-Phe substitution. Conformation specific IR-UV double resonance spectra were recorded for the jet-cooled γ⁴ conformers in the NH stretch and amide I/II regions. DFT calculations at the B3LYP level of theory (6-31+G(D), D3BJ) were compared with experiment to assign structures. Two structures were assigned as phenyl rotamers of a nine-membered ring closed by an NH…O=C hydrogen bond (C9). Tentative assignment for the third conformer is to a strained seven-membered ring closed by a hydrogen bond. Currently, IRPT is being used to determine the relative populations of the conformers in the expansion. Additionally, Ac-γ⁴-Phe-NHMe•H2O will be studied to compare bare and partially solvated structures. [1] James III, W. H.; Müller, C. W.; Buchanan, E. G.; Nix, M. G.; Guo, L.; Roskop, L.; Gordon, M. S.; Slipchenko, L. V.; Gellman, S. H.; Zwier, T. S., J. Am. Chem. Soc. 2009, 131, 14243-14245. [2] Buchanan, E. G.; James III, W. H.; Gutberlet, A.; Dean, J. C.; Guo, L.; Gellman, S. H.; Zwier, T. S., Faraday Discuss. 2011, 150, 209-226.

Growing experimental evidence suggests that zwitterionic structures are more prevalent in isolated gas phase environments than previously thought. These strong electrostatic interactions can govern the conformational landscape, alter fragmentation pathways, and promote unique chemistry upon UV irradiation. It is therefore crucial to develop methods that specifically probe for zwitterionic interactions and provide a deeper understanding of the structural factors which promote their stability. In this study we probe a series of arginine containing peptides cooled to 10 K using IR-UV double resonance spectroscopy. The main peptide sequences interrogated were YGRAR and FGRAR. Using the IR signatures of the hydride stretch as well as in the amide I/II regions it was possible to identify conformers which were zwitterionic. IR signatures reveal that YGRAR mainly adopts a zwitterionic configuration. The assigned structure for singly protonated YGRAR shows that the tyrosine OH and the middle protonated arginine residue work in tandem to solvate the carboxylate anion, while the C-terminal arginine plays a seemingly minor role in stabilizing the structure. As a consequence of the tyrosine OH engaging in a strong hydrogen bond with a highly basic site, the electronic origin transition of the zwitterionic species is shifted by  400 cm⁻¹to lower energy compared to non-zwitterionic structures. The magnitude of this shift is somewhat lessened by the C-terminal protonated arginine which acts as a hydrogen bond donor to the tyrosine hydroxyl oxygen. A further consequence of this hydrogen bond is the presence of an underlying broad component to the UV spectrum. It is postulated that this broad absorption is caused by an excited state hydrogen transfer between the tyrosine chromophore and carboxylate group. Similar UV induced reactions have been observed in neutral clusters of phenol and ammonia. The importance of the hydrogen bond between the carboxylate and tyrosine OH is showcased by FGRAR, which does not form a zwitterion. These results show that neutral sidechains can play a large role in promoting zwitterionic structure in addition to the presence of highly basic residues.

We present laser-desorbed, jet-cooled, conformation-specific UV and IR data on a series of increasingly complex β-peptide oligomers: Ac-(ACHC)₂-NHBn, Ac-ACHC-m₄ACHC-NHBn, Ac-m₅ACHC-m₄ACHC-NHBn, Ac-m₄ACHC-ACHC-NHBn, Ac-m₄ACHC-m₅ACHC-NHBn, Ac-(ACHC)₃-NHBn, and Ac-(ACHC)₄-NHBn. Synthetic foldamers are polymers composed of non-natural building blocks which either mimic, or expand upon, nature’s preferred secondary structures which are accessible to pure α-amino acid sequences. The ring-constrained β-amino acid, cis-2-aminocyclohexanecarboxylic acid (ACHC), is one such non-natural building block which when polymerized with alternating chirality has been shown to adopt both right- and left-handed 12/10 mixed helices in solution and crystalline form. ACHC may adopt two local minima conformations: one in which the NH is axial (ax) with respect to the cyclohexane chair and the C=O is equatorial (eq), and vice versa. In poly-ACHC sequences, the cooperative conformational isomerization between these two minima switches the screw-sense of the 12/10 helix. The use of the more rigid β-amino acids, cis-2-amino-cis-4-methylcyclohexanecarboxylic acid (m₄ACHC) and cis-2-amino-cis-5-methylcyclohexanecarboxylic acid (m₅ACHC) sterically lock the ACHC residue into one of its two minima, depending on the stereochemical patterning at the ring’s three stereocenters. The isolated, solvent-free conformational preferences will be compared with condensed phase data, and the energetic impact of the benzyl chromophore on preferred structure will be discussed.