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The Challenging Conformer Assignment of Proline Methyl Ester from Rotational Spectroscopy
The Challenging Conformer Assignment of Proline Methyl Ester from Rotational Spectroscopy
The rotational spectrum of proline methyl ester was published in Physical Chemistry Chemical Physics (PCCP). Molecular moments of inertia, dipole selection rules, and nuclear quadrupole coupling constants were used to assign the spectrum to a conformation with an intramolecular hydrogen bond from the imino hydrogen to the carbonyl oxygen and with the pyrrolidine ring puckered in an envelope configuration with C-gamma endo (structure 2).
D. Marasinghe, M. J. Carrillo, D. Z. Smallridge, K. E. Butts, B. Bagale, and M. J. Tubergen, Phys. Chem. Chem. Phys. 27, 13167 (2025).
Identification of Two Stable Side-Chain Orientations of Valine Methyl Ester by Microwave Spectroscopy.
We reported rotational spectra from the amino acid derivative valine methyl ester and assigned the spectra to 2 conformations with different orientations of the isopropyl side chain. Each rotational spectrum was split into A- and E-components arising from internal-rotation tunneling of the methyl and hyperfine components by 14N nuclear quadrupole coupling. The V3 barriers were found to be 401.64(19) and 409.74(16) cm-1.
D. Marasinghe, R. M. Gurusinghe, and M. J. Tubergen, J. Phys. Chem. A 128, 3266 – 3272 (2024). doi: https://doi.org/10.1021/acs.jpca.4c00388.
A Theoretical and Microwave Spectroscopic Characterization of Cyclobutenone: Planar or Puckered?
We characterized cyclobutenone using high-resolution rotational spectroscopy for the first time. By analyzing the molecular moments of inertia, derived from the fitted rotational constants, we determined that the carbon and oxygen atoms are coplanar. No tunneling splittings were observed, suggesting that cyclobutenone is not undergoing ring-puckering tunneling.
M. J. Carrillo, D. M. Marasinghe, E. B. Feeley, K. M. Sobie, R. J. Zarzycki, K Carter-Fenk, C. J. Fenk, and M. J. Tubergen. J. Phys. Chem. A. 127, 9082 – 9087 (2023). doi: https://doi.org/10.1021/acs.jpca.3c05510.
Structures of Guaiacol and the Guaiacol-Argon van der Waals Complex from Rotational Spectroscopy of Guaiacol Isotopologues.
High-resolution rotational spectra were recorded for guaiacol and eight 13C and D isotopologues as well as the guaiacol-argon van der Waals complex. Atomic coordinates of the carbon and hydroxyl hydrogen atoms were determined from the experimental moments of inertia using the Kraitchman method. The rotational spectrum of guaiacol-argon was used to determine the location of the argon with respect to the guaiacol monomer.
R. M Gurusinghe, A. Fox-Loe, and M. J. Tubergen. J. Mol. Struc. 1246, 131233 (2021).
Microwave Spectra of 2-Phenylethyl Methyl Ether and 2-Phenylethyl Methyl Ether-Argon: Conformation-Dependent Tunneling and Complexation.
High-resolution rotational spectra were recorded for 2-phenylethyl methyl ether and the 2-phenylethyl methyl ether – argon complex using a cavity-based Fourier transform microwave spectrometer. The spectra were assigned to the aao and g‒ao monomer conformations and the g‒ao – argon complex. Small tunneling splittings arising from more than one internal motion were resolved for eight transitions of the aao conformer; no tunneling splittings were observed in the spectra of the g‒ao conformer or the argon complex. The calculated barriers (ωB97XD/6-311++G(d,p)) for methyl and phenyl internal rotations were found to be very similar for both conformers. The observation of tunneling splittings is limited to the aao conformer by relatively large barriers for the internal motions and sensitivity to the rotor axis angles within the two conformers.
R. M. Gurusinghe and M. J. Tubergen. J. Mol. Spectrosc. 346, 13 – 18 (2018).
Probing the Electronic Environment of Methylindoles Using Internal Rotation and 14N Nuclear Quadrupole Coupling.
High-resolution rotational spectra were recorded in the 10.5 – 21.0 GHz frequency range for seven singly methylated indoles. 14N Nuclear quadrupole hyperfine structure and spectral splittings arising from tunneling along the internal rotation of the methyl group were resolved for all indole species. The nuclear quadrupole coupling constants were used to characterize the electronic environment of the nitrogen atom, and the program XIAM was used to fit the barrier to internal rotation to the measured transition frequencies. The best fit barriers were found to be 277.1(2), 374.32(4), 414.(5), 331.6(2), 126.8675(15), 121.413(4), and 426(3) cm-1 for 1-methylindole through 7-methylindole, respectively. The fitted barriers were found to be in good agreement with barriers calculated at the ωB97XD/6-311++G(d,p) level. The complete set of experimental barriers is compared to theoretical investigations of the origins of methyl torsional barriers and confirms that the magnitude of these barriers is an overall effect of individual hyperconjugative and structural interactions of many bonding/antibonding orbitals.
R. M. Gurusinghe and M. J. Tubergen. J. Phys. Chem. A 120, 3491 – 3496 (2016).
Full List of Publications with M. J. Tubergen
D. Marasinghe, M. J. Carrillo, D. Z. Smallridge, K. E. Butts, B. Bagale, and M. J. Tubergen. The Challenging Conformer Assignment of Proline Methyl Ester from Rotational Spectroscopy. Phys. Chem. Chem. Phys. 27, 13167 - 13173 (2025). doi: https://doi.org/10.1039/d5cp00898k.
T. S. Jayasekara, C. Miller, D. Marasinghe, M. J. Carrillo, M. Swann, M. J. Tubergen, I. Kleiner, and R. M. Gurusinghe, Exploring Low Barrier Quantum Tunneling and Structural Planarity in 3-Methylstyrene Conformers: Insights from Microwave Spectroscopy. J. Chem. Phys. 162, 044303 (2025). doi: https://doi.org/10.1063/5.0238251.
D. Marasinghe, R. M. Gurusinghe, and M. J. Tubergen, Identification of Two Stable Side-Chain Orientations of Valine Methyl Ester by Microwave Spectroscopy. J. Phys. Chem. A 128, 3266 – 3272 (2024). doi: https://doi.org/10.1021/acs.jpca.4c00388.
M. J. Carrillo, D. M. Marasinghe, E. B. Feeley, K. M. Sobie, R. J. Zarzycki, K Carter-Fenk, C. J. Fenk, and M. J. Tubergen. A Theoretical and Microwave Spectroscopic Characterization of Cyclobutenone: Planar or Puckered? J. Phys. Chem. A. 127, 9082 – 9087 (2023). doi: https://doi.org/10.1021/acs.jpca.3c05510.
R. M Gurusinghe, A. Fox-Loe, and M. J. Tubergen. Structures of Guaiacol and the Guaiacol-Argon van der Waals Complex from Rotational Spectroscopy of Guaiacol Isotopologues. J. Mol. Struc. 1246, 131233 (2021).
doi: https://doi.org/10.1016/j.molstruc.2021.131233.
R. M. Gurusinghe and M. J. Tubergen. Microwave Spectra of 2-Phenylethyl Methyl Ether and 2-Phenylethyl Methyl Ether-Argon: Conformation-Dependent Tunneling and Complexation. J. Mol. Spectrosc. 346, 13 – 18 (2018).
J. Demaison, N. C. Craig, R. Gurusinghe, M. J. Tubergen, H. D. Rudolph, L. H. Coudert, P. G. Szalay, A. G. Csaszar. Fourier Transform Microwave Spectrum of Propene-3-d(1) (CH2=CHCH2D), Quadrupole Coupling Constants of Deuterium, and a Semiexperimental Equilibrium Structure of Propene. J. Phys. Chem. A 121, 3155 – 3166 (2017).
N. C. Craig, P. Groner, A. R. Conrad, R. M. Gurusinghe, M. J. Tubergen. Microwave spectra for the three 13C(1) isotopologues of propene and new rotational constants for propene and its 13C(1) isotopologues. J. Mol. Spectrosc. 328, 1 – 6 (2016).
R. M. Gurusinghe and M. J. Tubergen. Probing the Electronic Environment of Methylindoles Using Internal Rotation and 14N Nuclear Quadrupole Coupling. J. Phys. Chem. A 120, 3491 – 3496 (2016).
B. S. Deodhar, R. E. Brenner, J. J. Klaassen, M. J. Tubergen, J. R. Durig. Microwave, structural, conformational, vibrational studies and ab initio calculations of fluoroacetyl chloride. Spectrochimica Acta A 148, 289 – 298 (2015).
J. R. Durig, J. J. Klaassen, D. K. Sawant, B. S. Deodhar, S. S. Panikar, R. M. Gurusinghe, I. D. Darkhalil, M. J. Tubergen. Microwave, Structural, Conformational, Vibrational Studies and Ab Initio Calculations of Isocyanocyclopentane. Spectrochimica Acta A 136A, 3 – 15 (2015).
I. D. Darkhalil, N. Nagels, W. A. Herrebout, B. J. van der Veken, R. M. Gurusinghe, M. J. Tubergen, J. R. Durig. Microwave spectra and conformational studies of ethylamine from temperature dependent Raman spectra of xenon solutions and ab initio calculations. J. Mol. Struct. 1068, 101 – 111 (2014).
I. D. Darkhalil, J. J. Klaassen, N. Nagels, W. A. Herrebout, B. J. van der Veken, R. M. Gurusinghe, M. J. Tubergen, J. R. Durig. Raman and Infrared, Microwave Spectra, Conformational Stability, Adjusted r(0) Structural Parameters, and Vibrational Assignments of Cyclopentylamine. J. Raman Spec. 45, 392 – 406 (2014).
N. C. Craig, Y. H. Chen, H. A. Fuson, H. F. Tian, H. van Besien, A. R. Conrad, M. J. Tubergen, H. D. Rudolph, J. Demaison. Microwave Spectra of the Deuterium Isotopologues of cis-Hexatriene and a Semiexperimental Equilibrium Structure. J. Phys. Chem. A 117, 9391 – 9400 (2013).
J. J. Klaassen, I. D. Darkhalil, B. S. Deodhar, T. K. Gounev, R. M. Gurusinghe, M. J. Tubergen, P. Groner, J. R. Durig. Microwave and Infrared Spectra, Adjusted r0 Structural Parameters, Conformational Stabilities, Vibrational Assignments, and Theoretical Calculations of Cyclobutylcarboxylic Acid Chloride. J. Phys. Chem. A 117, 6508 – 6524 (2013).
S. S. Panikar, G. A. Guirgis, M. T. Eddens, H. W. Dukes, A. R. Conrad, M. J. Tubergen, T. K. Gounev, J. R. Durig. Microwave, Infrared and Raman Spectra, Adjusted r0 Structural Parameters, Conformational Stability, and Vibrational Assignment of Cyclopropylfluorosilane. Chem. Phys. 415, 124 – 132 (2013).
S. X. Zhou, R. M. Ward, M. J. Tubergen, R. M. Gurusinghe, J. R. Durig. Microwave, Infrared, and Raman Spectra, r0 Structural Parameters, Conformational Stability and ab initio Calculations of Cyclohexylisocyanate. Chem. Phys. 415, 44 – 55 (2013).
S. X. Zhou, G. A. Guirgis, K. K. Gause, A. R. Conrad, M. J. Tubergen, J. R. Durig. Microwave, Raman, and Infrared Spectra, r(0) Structural Parameters, Conformational Stability, and ab initio Calculations of Cyclobutylisocyanate. Structural Chemistry 24, 201 – 214 (2013).
J. Demaison, N. C. Craig, A. R. Conrad, M. J. Tubergen, H. D. Rudolph. Semiexperimental Structure of the Lower Energy Conformer of Glycidol by the Mixed Estimation Method. J. Phys. Chem. A 116, 9116 – 9122 (2012).
J. R. Durig, J. J. Klaassen, B. S. Deodhar, T. K. Gounev, A. R. Conrad, M. J. Tubergen. Microwave, Infrared, and Raman Spectra, r(0) Sturctural Parameters, Conformational Stability, and Vibrational Assignment of Allyl Thiol. Spectrochimica Acta A 87, 214 – 227 (2012).
A. R. Conrad, H. A. Hassanin, M. J. Tubergen, I. Fabian, N. E. Brasch. The Effects of Ligand Decomposition on the Pseudo First-Order Profile of a Ligand Substitution Reaction: A “Silent Killer” in the Background. New J. Chemistry 36, 1408 – 1412 (2012).
A. R. Conrad, H. L. Seedhouse, R. J. Lavrich, M. J. Tubergen. Rotational Spectra and Computational Analysis of Two Conformers of Leucinamide. J. Phys. Chem. A 115, 9676 – 9681 (2011).
D. C. McKean, M. M. Law, P. Groner, A. R. Conrad, M. J. Tubergen, D. Feller, M. C. Moore, N. C. Craig. Infrared Spectra of CF2=CHD and CF2=CD2: Scaled Quantum-Chemical Force Fields and an Equilibrium Structure for 1,1-Difluoroethylene. J. Phys. Chem. A 114, 9309 – 9318 (2010).
J. R. Durig, R. M. Ward, A. R. Conrad, M. J. Tubergen, G. A. Guirgis. Microwave, Raman, and Infrared Spectra; Adjusted r0 Structural Parameters; Conformational Stability; and Vibrational Assignment of Germylcyclohexane. J. Phys. Chem. A 114, 9289 – 9299 (2010).
J. R. Durig, R. M. Ward, A. R. Conrad, M. J. Tubergen, K. G. Nelson, T. K. Gounev. Microwave, Infrared, and Raman Spectra, r0 Structural Parameters, Conformational Stability, and Vibrational Assignment of Ethynylcyclohexane. J. Mol. Struct. 975, 5 – 16 (2010).
A. R. Conrad, N. Z. Barefoot, M. J. Tubergen. Rotational Spectra of o-, m-, and p-Cyanophenol and Internal Rotation of p-Cyanophenol. Phys. Chem. Chem. Phys. 12, 8350 – 8356 (2010).
J. R. Durig, R. M. Ward, A. R. Conrad, M. J. Tubergen, K. G. Nelson, P. Groner, T. K. Gounev. Microwave, Raman, and Infrared Spectra, r0 Structural Parameters, Conformational Stability, and Vibrational Assignment of Cyanocyclohexane. J. Mol. Struct. 967, 99 – 111 (2010).
A. R. Conrad, N. H. Teumelsan, P. E. Wang, M. J. Tubergen. A Spectroscopic and Computational Investigation of the Conformational Changes Induced by Hydrogen Bonding Networks in the Glycidol-Water Complex. J. Phys. Chem. A, 114, 336 – 342 (2010).
J. R. Durig, R. M. Ward, A. R. Conrad, M. J. Tubergen, G. A. Giurgis, T. K. Gounev. Microwave Spectra, r0 Structural Parameters, and Conformational Stability from Xenon Solutions of Silylcyclohexane. J. Mol. Struct. 922, 19 - 29 (2009).
M. J. Tubergen, A. R. Conrad, R. E. Chavez III, I. Hwang, R. D. Suenram, J. J. Pajski, B. H. Pate. Gas-Phase Conformational Distributions for the 2-Alkylalcohols 2-Pentanol and 2-Hexanol from Microwave Spectroscopy. J. Mol. Spectrosc. 251, 330 - 338 (2008).
Tubergen, M. J.; Lavrich, R. J.; Plusquellic, D. F.; Suenram, R. D. Rotational Spectra and Conformational Structures of 1-Phenyl-2-propanol, Methamphetamine, and 1-Phenyl-2-propanone. J. Phys. Chem. A 110, 13188 – 13194 (2006).
Tubergen, M. J.; Lesarri, A.; Suenram, R. D.; Samuels, A. C.; Jensen, J. O.; Ellzy, M. W.; Lochner, J. M. Rotational Spectra, Nuclear Quadrupole Hyperfine Tensors, and Conformational Structures of the Mustard Gas Simulent 2-Chloroethyl Ethyl Sulfide. J. Mol. Spectrosc. 233, 180 - 188 (2005).
N. C. Craig, P. Groner, D. C. McKean, M. J. Tubergen. Equilibrium Structures for the cis and trans Isomers of 1,2-Difluoroethylene and the cis, trans Isomer of 1,4-Difluorobutadiene. Int. J. Quantum Chem. 95, 837 - 852 (2003).
M. J. Tubergen, C. R. Torok, R. J. Lavrich. Effect of Solvent on Molecular Conformation: Microwave Spectra and Structures of 2-Aminoethanol van der Waals Complexes. J. Chem. Phys. 119, 8397 - 8403 (2003).
R. J. Lavrich, D. F. Plusquellic, R. D. Suenram, G. T. Fraser, A. R. Hight Walker, M. J. Tubergen. Experimental Studies of Peptide Bonds: Identification of the C7eq Conformation of the Alanine Dipeptide Analog N-Acetyl-Alanine N’-Methylamide from Torsion-Rotation Interactions. J. Chem. Phys. 118, 1253 - 1265 (2003).
C. R. Torok, R. J. Lavrich, M. J. Tubergen. High Resolution Rotational Spectroscopy and Ring-Puckering Conformation of 3-Hydroxytetrahydrothiophne. Chem. Phys. 283, 125 - 133 (2002).
R. J. Lavrich, C. R. Torok, M. J. Tubergen. Effect of the Bulky Side Chain on the Backbone Structure of the Amino Acid Derivative Valinamide. J. Phys. Chem. A 106, 8013 - 8018 (2002).
N. C. Craig, L. V. McCarty, P. T. Ligenfelter, A. S. Osmani, O. Rathore, M. J. Tubergen, R. L. Kuczkowski. Complete Structure of trans-3,4-Difluorocyclobutene from Microwave Spectroscopy. J. Phys. Chem. A 106, 6637 - 6642 (2002).
C. R. Torok, R. J. Lavrich, M. J. Tubergen. Spectroscopic and ab initio Structure of the Heterocyclic Ring Tetrahydrothiophen-3-one. J. Mol. Struct. 612, 223 - 230 (2002).
N. C. Craig, C. M. Oertel, D. C. Oertel, M. J. Tubergen, R. J. Lavrich, A. M. Chaka. Structure of cis,trans-1,4-Difluorobutadiene from Microwave Spectroscopy. J. Phys. Chem. A 106, 4230 - 4235 (2002).
R. J. Lavrich, C. R. Torok, and M. J. Tubergen. A Microwave Spectroscopic Investigation of the Networked Structure of 3-Hydroxytetrahydrofuran-H2O. J. Phys. Chem. A 105, 8317 - 8322 (2001).
M. J. Tubergen, R. J. Lavrich, and J. W. McCargar. Infrared Spectrum and Group Theoretical Analysis of the Vibrational Modes of Carbonyl Sulfide. J. Chem. Educ. 77, 1637 - 1639 (2000).
R. J. Lavrich and M. J. Tubergen. Conformation and Hydrogen Bonding in the Alaninamide-Water van der Waals Complex. J. Am. Chem. Soc. 122, 2938 - 2943 (2000).
R. J. Lavrich, R. L. Rhea, J. W. McCargar, and M. J. Tubergen. Rotational Spectroscopy and Ring-Puckering Conformation of 3-Hydroxytetrahydrofuran. J. Mol. Spectrosc. 199, 138 - 143 (2000).
R. J. Lavrich, J. O. Farrar, and M. J. Tubergen. Heavy-Atom Structure of Alaninamide from Rotational Spectroscopy. J. Phys. Chem. A 103, 4659 - 4663 (1999).
K. A. Kuhls, C. A. Centrone, and M. J. Tubergen. Microwave Spectroscopy of the Twist Cb-exo/Cg-endo Conformation of Prolinamide. J. Am. Chem. Soc. 120, 10194 -10198 (1998).
M. J. Tubergen, J. E. Flad, and J. E. Del Bene. Microwave Spectroscopic and ab initio Studies of the Hydrogen-Bonded Trimethylamine-Hydrogen Sulfide Complex. J. Chem. Phys. 107, 2227 - 2231 (1997).
M. J. Tubergen and R. L. Kuczkowski. Hydrogen Bonding to Dimethylamine: The Microwave Spectrum and Structure of the Dimethylamine-Water Complex. J. Mol. Struct. 352/353, 335 - 344 (1995).
X.-Q. Tan, L.-W. Xu, M. J. Tubergen, and R. L. Kuczkowski. The Microwave Spectrum, Structure, and Large-Amplitude Motions of the Methylacetylene-SO2 Complex. J. Chem. Phys. 101, 6512 - 6522 (1994).
M. J. Tubergen and R. L. Kuczkowski. Microwave Spectrum and Structure of the Dimethylamine Dimer: Evidence for a Cyclic Structure. J. Chem. Phys. 100, 3377 - 3383 (1994).
M. J. Tubergen and R. L. Kuczkowski. Microwave Spectroscopic Characterization of a Strong Hydrogen Bond: Trimethylamine-Water. J. Am. Chem. Soc. 115, 9263 - 9266 (1993).
M. J. Tubergen, A. M. Andrews, and R. L. Kuczkowski. Microwave Spectrum and Structure of a Hydrogen-Bonded Pyrrole-Water Complex. J. Phys. Chem. 97, 7451 - 7457 (1993).
M. J. Tubergen and D. H. Levy. Spectroscopy of Indole van der Waals Complexes: Evidence for a Conformation-Dependent Excited State. J. Phys. Chem. 95, 2175 - 2181 (1991).
M. J. Tubergen, J. R. Cable, and D. H. Levy. Substituent Effects on the Electronic Spectroscopy of Tryptophan Derivatives in Jet Expansions. J. Chem. Phys. 92, 51 - 60 (1990).
J. R. Cable, M. J. Tubergen, and D. H. Levy. Fluorescence Spectroscopy of Jet-Cooled Tryptophan Peptides. J. Am. Chem. Soc. 111, 9032 - 9039 (1989).
J. R. Cable, M. J. Tubergen and D. H. Levy. The Electronic Spectroscopy of Small Peptides in the Gas Phase. Faraday Discuss. Chem. Soc. 86, 143 - 152 (1988).
J. R. Cable, M. J. Tubergen, and D. H. Levy. Electronic Spectroscopy of Small Tryptophan Peptides in Supersonic Molecular Beams. J. Am. Chem. Soc. 110, 7349 - 7355 (1988).
J. R. Cable, M. J. Tubergen, and D. H. Levy. Laser Desorption Molecular Beam Spectroscopy: The Electronic Spectra of Tryptophan Peptides in the Gas Phase. J. Am. Chem. Soc. 109, 6198 - 6199 (1987).
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