Research

More recently, our group pursued the isolation of several cyanobutadiene isomers of astrochemical interest.  While mixtures of the (E)- and (Z)-1-cyano-1,3-butadiene isomers had previously been reported, their separation posed a substantial challenge, particularly due to their lack of stability under many conditions.  As a result, stereoselective and semi-selective synthetic routes were developed to acquire the necessary quantities of each isomer, depicted on the right.  Neither the synthesis nor physical properties of 4-cyano-1,2-butadiene had been previously reported, so a synthesis was developed for this molecule, as well.  All of the cyanobutadiene isomers have been characterized and were used in unpublished rotational spectroscopy studies.

Recent publication:

Kougias, S. M.; Knezz, S. N.; Owen, A. N.; Sanchez, R. A.; Hyland, G. E.; Lee, D. J.; Patel, A. R.; Esselman, B. J.; Woods, R. C.; McMahon, R. J. Synthesis and Characterization of Cyanobutadiene Isomers – Molecules of Astrochemical Significance. J. Org. Chem. 2020, 85 (9), 5787-5798.  View Article

Our group uses synthetic chemistry to access molecules of interest, including isotopically substituted molecules, precursors to high-energy species, and molecules that are themselves not stable at room temperature.  These molecules are studied in our group by matrix isolation and spectroscopy or gas-phase rotational spectroscopy or as part of collaborations with other groups who use these molecules for their experiments.

During our photochemical study of the highly unsaturated MeC₇H, a rearrangement product was clearly present in both argon and nitrogen matrices.  Ab initio and DFT computational predictions suggested that the lowest-energy, open-chain isomer on the C₈H₄ potential energy surface was oct-7-ene-1,3,5-triyne, making it a likely candidate for the rearrangement product.  In order to conclusively confirm this assignment, oct-1-ene-3,5,7-triyne was synthesized, in the manner shown on the left, to use as an authentic standard for one of our matrix isolation photochemistry experiments.

Containing the cold-head and spectroscopic windows (above), our entire matrix-isolation apparatus is pictured below.  A vacuum-sealed glass manifold, is attached to a cyrogenic cold head that can achieve temperatures of about 10 K.  The low temperatures are achieved by use of a helium refrigeration system.  The entire apparatus is mobile, which allows us to conveniently expose a deposited sample on the spectroscopic windows to a lamp or to analyze the sample by IR or UV/Vis spectroscopy.

A specialty of the McMahon group is synthesizing and characterizing highly unsaturated carbon and nitrogen species that are inaccessible to most synthetic organic methods.  These molecules are trapped at very low temperature in an inert matrix as shown on the left.  By analyzing the electronic structure and photochemistry of these species, we advance our understanding of the nature of tunneling and physical organic chemistry.  The geometric and electronic structure of HC₃H, HC₅H, and HC₇H demonstrate the complexity of these highly unsaturated species: HC₃H is a diradical with a bent structure, HC₅H is a linear carbene, and HC₇H is linear diradical.

Recent Publications:

Seburg, R. A.; Patterson, E. V.; McMahon, R. J. Structure of Triplet Propynylidene (HCCCH) as Probed by IR, UV/vis, and EPR Spectroscopy of Isotopomers. J. Am. Chem. Soc. 2009, 131 (26), 9442-9455. View Article

Knezz, S. N.; Waltz, T. A.; Haenni, B. C.; Burrmann, N. J.; McMahon, R. J., Spectroscopy and Photochemistry of Triplet 1,3-Dimethylpropynylidene (Me‒C‒C‒C‒Me). J. Am. Chem. Soc. 2016, 138 (38), 12596-12604.  View Article

Our group has a long-standing interest in the reactivity of molecules at matrix-isolation temperatures and have observed numerous reactions at these very low temperatures which could only be the result of hydrogen atom and heavy-atom tunneling reactions.

Recent publications:

McMahon, R. J. Chemistry: Chemical reactions involving quantum tunneling. Science (Washington, DC, United States) 2003, 299 (5608), 833-834. View Article

Nunes, C. M.; Knezz, S. N.; Reva, I.; Fausto, R.; McMahon, R. J. Evidence of a Nitrene Tunneling Reaction: Spontaneous Rearrangement of 2‑Formyl Phenylnitrene to an Imino Ketene in Low‑Temperature Matrices. J. Am. Chem. Soc. 2016, 138 (47) 15287-15290.  View Article

The millimeter-wave spectra are obtained in a single-pass flow/static cell system with a typical frequency range from 130 to 375 GHz. The signal is generated by a frequency synthesizer and a tone-burst modulation is applied by a frequency modulator. The modulated signal is then amplified and its frequency multiplied via an amplification and multiplication chain to achieve the frequencies mentioned above. The signal is passed through a three-meter Pyrex tube and is detected by a VDI zero biased detector. The detector signal is then sent to a Lock-In Amplifier which determines the signal solely from the signal generation and sends the signal to a computer for visualization and storage.

The spectrometer also has the capability to create an electrical discharge within the three-meter Pyrex tube via cylindrical electrodes at each end while the millimeter-wave spectrum is obtained. The Pyrex chamber operates at mTorr pressures and at temperatures from 200 – 298 K. The temperature is controlled via an ultra low-temp chiller.  The instrument also contains a quadrupole mass spectrometer to sample ions from the discharge. A custom LABVIEW program is used to externally control all components of signal generation and acquisition as well as smooth and suppress background noise from the raw signal. A description of the spectrometer and software can be found in the references below.

Recent Publications:

Esselman, B. J.; Amberger, B. K.; Shutter, J. D.; Daane, M. A.; Stanton, J. F. .; Woods, R.C; McMahon, R. J. Rotational Spectroscopy of Pyridazine and its Isotopologs from 235 – 360 GHz: Equilibrium Structure and Vibrational Satellites.  J. Chem. Phys. 2013, 139 (22), 224304. View Article

Zdanovskaia, M. A.; Esselman, B. J.; Woods, R. C.; McMahon, R. J., The 130 – 370 GHz Rotational Spectrum of Phenyl Isocyanide (C₆H5NC). J. Chem. Phys. 2019, 151, 024301.  View Article

Least-squares fitting of microwave or low-frequency data often results in determination of the rotational constants and some quartic distortion constants.  Data in the relatively high-frequency range that we analyze (130 – 360 GHz) frequently results in determination of rotational constants and distortion constants up to the octic level.  Our group has become proficient at rapidly and thoroughly analyzing simple distorted rotors.

Recent Publications:

Orr, V. L.; Esselman, B. J.; Dorman, P. M.; Amberger, B. K.; Guzei, I. A.; Woods, R. C.; McMahon, R. J. Millimeter-wave Spectroscopy, X-ray Crystal Structure, and Quantum Chemical Studies of Diketene – Resolving Ambiguities Concerning the Structure of the Ketene Dimer. J. Phys. Chem. A. 2016, 120 (39), 7753-7763.  View Article

Esselman, B. J.; Zdanovskaia, M. A.; Woods, R. C.; McMahon, R. J. Millimeter-wave Spectroscopy of the Chlorine Isotopologues of 2-Chloropyridine and Twenty-three of Their Vibrationally Excited States. J. Mol. Spectrosc. 2019, 365, 111206.  View Article

Through a recent collaboration with Prof. Zbigniew Kisiel, we have explored several cases were two or more vibrational states can exhibit Coriolis or Fermi coupling to one another, making their fitting much more complicated.  The group’s first coupled-state fit was the Coriolis-coupled dyad of υ₂₂ and υ₃₃ of benzonitrile.  Successfully least-squares fitting a set of coupled states enables the most precise determination of their energy separation of any current methods.  Since then, we have published several coupled-state fits and a few of the group members are in the process of coupled-state fitting for other molecules of interest.

Recent Publications:

Dorman, P. M.; Esselman, B. J.; Park, J. E.; Woods, R. C.; McMahon, R. J. Millimeter-Wave Spectrum of 4-Cyanopyridine in its Ground State and Lowest-Energy Vibrationally Excited Dyad, ν₂₀ and ν₃₀. J. Mol. Spectrosc. 2020, 369, 111274.  View Article

Zdanovskaia, M. A.; Esselman, B. J.; Woods, R. C.; McMahon, R. J., The 130 – 370 GHz Rotational Spectrum of Phenyl Isocyanide (C₆H5NC). J. Chem. Phys. 2019, 151, 024301.  View Article

We routinely use computational chemistry through Gaussian and CFOUR to support our prediction and interpretation of experimental results.  Using the computed potential energy surface shown to the right, we rationalized the diastereoselectivity observed in the synthesis of E-1-cyano-1,3-butadiene via the Curtin-Hammett principle.

Recent Publication:

Kougias, S. M.; Knezz, S. N.; Owen, A. N.; Sanchez, R. A.; Hyland, G. E.; Lee, D. J.; Patel, A. R.; Esselman, B. J.; Woods, R. C.; McMahon, R. J. Synthesis and Characterization of Cyanobutadiene Isomers – Molecules of Astrochemical Significance. J. Org. Chem. 2020, 85 (9), 5787-5798.  View Article

We routinely predict IR, UV-Vis, and EPR spectra to support our interpretation of our matrix-isolation spectroscopy.  Due to the reactive nature of the species of interest, there are rarely any authentic spectra available to aid in our characterization.  Thus, we rely on computed spectra to help make conclusive spectroscopic assignments.

Recent Publications:

Nunes, C. M.; Knezz, S. N.; Reva, I.; Fausto, R.; McMahon, R. J. Evidence of a Nitrene Tunneling Reaction: Spontaneous Rearrangement of 2‑Formyl Phenylnitrene to an Imino Ketene in Low‑Temperature Matrices. J. Am. Chem. Soc. 2016, 138 (47) 15287-15290.  View Article

Pharr, C. R.; Kopff, L. A.; Bennett, B.; Reid, S. A.; McMahon, R. J. Photochemistry of Furyl and Thienyl Diazomethanes: Spectroscopy and Characterization of Triplet 3-Thienyl Carbene.  J. Am. Chem. Soc. 2012, 134, 6443-6454. View Article

Our analysis of rotational spectroscopy relies upon our ability to computationally predict rotational, centrifugal distrotion, vibration-rotation interaction, and Coriolis-coupling constants.  These are used for a priori predictions of experimental spectra, providing computed values for those that cannot be determined experimentally, and for lending support for the correctness of our spectroscopic analysis.

Recent Publications:

Dorman, P. M.; Esselman, B. J.; Park, J. E.; Woods, R. C.; McMahon, R. J. Millimeter-Wave Spectrum of 4-Cyanopyridine in its Ground State and Lowest-Energy Vibrationally Excited Dyad, ν₂₀ and ν₃₀. J. Mol. Spectrosc. 2020, 369, 111274.  View Article

Higgins, P. M.; Esselman, B. J.; Zdanovskaia, M. A.; Woods, R. C.; McMahon, R. J., Millimeter-Wave Spectroscopy of the Chlorine Isotopologues of Chloropyrazine and Twenty-two of their Vibrationally Excited States. J. Mol. Spectrosc. 2019, 364, 111179.  View Article

The rotational spectrum of a molecule is highly sensitive to its vibrational state and isotopic substitution.  Analyzing the rotational spectrum results in spectroscopic constants that provide the moments of inertia of the molecule.  By measuring the rotational spectra of multiple isotopologues, it is possible to extract highly precise information regarding the molecule structure.  While this was previously done using Kraitchman’s equations to give a substitution structure (r_s), modern computational capabilities enable rapid least-squares fitting of the moments of inertia for numerous isotopologues with additional corrections to determine a semi-experimental equilibrium structure (r_e^SE).  Specifically this method corrects for vibration-rotation interaction and electron mass, the latter becoming an important contributor to molecules with extended π systems and substantial out-of-plane electron density.

Using the synthetic capabilities of our group, we substantially over-determine all of the experimental parameters and determine very precise rotational constants.  In HN3 (shown to the left), we synthesized a total of 14 isotopologues providing 28 independent moments of inertia to determine five structural parameters.

Recent publication:

Amberger, B. K.; Esselman, B. J.; Stanton, J. F.; Woods, R. C.; McMahon, R. J. Precise Equilibrium Structure Determination of Hydrazoic Acid (HN₃) by Millimeter-wave Spectroscopy. J. Chem. Phys. 2015, 143, 104310.  View Article

For pyrimidine, we synthesized a total of 16 isotopologues and determined a very precise r_e^SE structure.  In collaboration with Professor John F. Stanton, we obtained a high-level computational equilibrium structure at the CCSD(T)/cc-pCV5Z level with high level corrections for correlation effects beyond CCSD(T), basis set effects beyond cc-pCV5Z, relativistic effects and the diagonal Born-Oppenheimer correction.  The resulting computed parameters all fall within the statistical uncertainty of the r_e^SE experimental parameters, which itself is determined to a precision on the order of a nuclear diameter (0.0001 Å).

Recent Publication:

Heim, Z. N.; Amberger, B. K.; Esselman, B. J.; Stanton, J. F.; Woods, R. C.; McMahon, R. J. Molecular Structure Determination: Equilibrium Structure of Pyrimidine (m-C₄H₄N₂) from Rotational Spectroscopy (r_e^SE) and High-Level Ab-initio Calculation (r_e) Agree within the Uncertainty of Experimental Measurement. J. Chem. Phys. 2020, 152, 104303.  View Article