Michael 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

Rotational spectra of the biomimetic molecule, alanine dipeptide and the double 15N(15N2) isotopomer have been observed using a pulsed-molecular-beam Fourier transform microwave spectrometer. The spectra reveal tunneling splittings from the torsional mode structure of two of its three methyl rotors. The torsional states assigned include one AA-state and two AE-states (i.e., AE and EA) for each isotopomer. The AA-states are well-fit to A-reduction asymmetricrotor Hamiltonians. The “infinite-barrier-limit” rotational constants of the 14N2 isotopomer are A=1710.97(8) MHz, B=991.89(9) MHz, and C=716.12(6) MHz. The AE-states are analyzed independently using “high-barrier” torsion-rotation Hamiltonians, yielding observedminus-calculated standard deviations of 100-fold for the 15N2 isotopomer) when analyzed in a ρ-axis frame where ρb=ρc=0. The best-fit torsion-rotation parameters provide accurate V3 barriers and C3 rotor axis angles for both methyl groups. The observed ...

A spectroscopic and computational investigation of the conformational structural changes induced by hydrogen bonding networks in the glycidol-water complex.

Rotational spectra were recorded in natural abundance for the (13)C isotopomers of two conformers of glycidol. Moments of inertia from the (13)C isotopomers were used to calculate the substitution coordinates and C-C bond lengths of two glycidol monomer conformations. The structures of seven different conformational minima were found from ab initio (MP2/6-311++G(d,p)) optimizations of glycidol-water. The rotational spectrum of glycidol-water was recorded using microwave spectroscopy, and the rotational constants were determined to be A = 3902.331 (11) MHz, B = 2763.176 (3) MHz, and C = 1966.863 (3) MHz. Rotational spectra were also recorded for glycidol-H(2)(18)O, glycidol-D(b)OH, and glycidol-d(O)-D(2)O. The rotational spectra were assigned to the lowest-energy ab initio structure, and the structure was improved by fitting to the experimental moments of inertia. The best-fit structure shows evidence for structural changes in glycidol to accommodate formation of the intermolecular hydrogen bonding network: the O-C-C-O torsional angle in glycidol was found to increase from 40.8 degrees for the monomer to 49.9 degrees in the water complex.

Effect of solvent on molecular conformation: Microwave spectra and structures of 2-aminoethanol van der Waals complexes

Rotational spectra of the 13C isotopomers of the 2-aminoethanol monomer have been recorded in natural abundance using a Fourier-transform microwave spectrometer. The two sets of 13C isotopomer rotational constants were used to complete the 2-aminoethanol substitution structure. Rotational spectra of the van der Waals complexes 2-aminoethanol–water and 2-aminoethanol–argon were also recorded. Sixteen a-, b-, and c-type transitions were fit to the Watson A-reduction Hamiltonian for 2-aminoethanol–argon yielding A=4986.762(2) MHz, B=1330.693(3) MHz, and C=1143.933(3) MHz. Fifteen a- and b-type transitions for 2-aminoethanol–water were fit to A=4886.451(5) MHz, B=3356.038(2) MHz, and C=2311.715(2) MHz. The spectra are assigned to the lowest-energy ab initio [MP2/6-311++G(d,p)] structures of the two complexes. The conformation of 2-aminoethanol is unchanged in the argon complex, and the argon is 3.775 A from the monomer center of mass. A network of intermolecular hydrogen bonds in the 2-aminoethanol–water comp...

Microwave spectra of the deuterium isotopologues of cis-hexatriene and a semiexperimental equilibrium structure.

Microwave transitions and ground state rotational constants are reported for five newly synthesized deuterium isotopologues of cis-1,3,5-hexatriene (cHTE). These rotational constants along with those of the parent and the three (13)C species are used with vibration-rotation constants calculated from an MP2/cc-pVTZ model to derive an equilibrium structure. That structure is improved by the mixed estimation method. In this method, internal coordinates from good-quality quantum chemical calculations (with appropriate uncertainties) are fit simultaneously with moments of inertia of the full set of isotopologues. The new structure of cHTE is confirmed to be planar and is stabilized by an interaction between the hydrogen atoms H2 and H5, which form a bond and participate in a six-membered ring. cHTE shows larger structural effects of π-electron delocalization than does butadiene with the effects being magnified in the center of the molecule. Thus, strong structural evidence now exists for an increase in π-electron delocalization as the polyene chain lengthens.

Semiexperimental equilibrium structure of the lower energy conformer of glycidol by the mixed estimation method.

Rotational constants were determined for (18)O-substituted isotopologues of the lower energy conformer of glycidol, which has an intramolecular inner hydrogen bond from the hydroxyl group to the oxirane ring oxygen. Rotational constants were previously determined for the (13)C and the OD species. These rotational constants have been corrected with the rovibrational constants calculated from an ab initio cubic force field. The derived semiexperimental equilibrium rotational constants have been supplemented by carefully chosen structural parameters, including those for hydrogen atoms, from medium level ab initio calculations. The combined data have been used in a weighted least-squares fit to determine an equilibrium structure for the glycidol H-bond inner conformer. This work shows that the mixed estimation method allows us to determine a complete and reliable equilibrium structure for large molecules, even when the rotational constants of a number of isotopologues are unavailable.

The electronic spectra of small peptides in the gas phase

The electronic spectra of the peptides Gly-Trp, Trp-Gly, and Trp-Gly-Gly have been observed in the gas phase. Solid samples of the peptide were introduced into a supersonic expansion by laser desorption, and both resonantly enhanced multiphoton ionization (MPI) spectra and laser-induced fluorescence spectra were observed. The MPI and fluorescence excitation spectra of these peptides had well resolved structure consisting, in part, of long, low-frequency vibrational progressions. In the case of Gly-Trp, emission spectroscopy was used to separate features in the MPI spectrum into contributions from two conformers of the molecule. The emission spectrum of one conformer contained only broad, red-shifted emission, while the emission spectrum of the second conformer contained both sharp and broad features. Although the MPI spectrum of the tripeptide Trp-Gly-Gly appeared to contain features from more than one conformer, all observed emission spectra from Trp-Gly-Gly were similar and contained only broad features. By analogy with the spectroscopy of tryptophan, the peptide spectra were interpreted in terms of mixing between two excited electronic states, one state arising from an intramolecular exciplex-like interaction.

Microwave spectroscopic and ab initio studies of the hydrogen-bonded trimethylamine–hydrogen sulfide complex

Rotational spectra have been recorded for six isotopomers of the trimethylamine–hydrogen sulfide complex using a Fourier-transform microwave spectrometer. The spectra are characteristic of a symmetric top (B+C)/2=1395.463 (1) MHz, and are indicative of free internal rotation of H2S about the trimethylamine symmetry axis. A structure with a single, linear hydrogen bond, with an S–N distance of 3.36 (5) A, best reproduces the moments of inertia of six isotopomers, including three distinct deuterated complexes. The experimental structure is compared to an ab initio structure optimized at the MP2/6-31+G(d,p) level which predicts an S–N distance of 3.328 A. MP2/aug′-cc-pVTZ calculations were used to determine the binding energy of the complex (ΔEe=−5.8 kcal/mol) and the barrier to an internal tunneling motion which exchanges the two H2S hydrogens (3.0 kcal/mol).

Rotational spectra of o-, m-, and p-cyanophenol and internal rotation of p-cyanophenol.

Rotational spectra of p-, m-, and o-cyanophenol have been measured in the range of 10.5-21 GHz and fit using Watsons A-reduction Hamiltonian coupled with nuclear quadrupole coupling interaction terms for the (14)N nuclei. Ab initio calculations at the MP2/6-311++G(d,p) and CCSD(T)/6-311++G(d,p) levels predict the cis conformers of m- and o-cyanophenol to be more stable than the corresponding trans conformers. A natural bond orbital analysis of the hydrogen bonding interaction in o- and m-cyanophenol revealed an intramolecular hydrogen bond that preferentially stabilizes the cis conformer of o-cyanophenol but there was no evidence of hydrogen bonding interactions in cis m-cyanophenol. We recorded 25 a- and b-type rotational transitions for cis o-cyanophenol; the rotational constants are A = 3053.758(2) MHz, B = 1511.2760(3) MHz, and C = 1010.7989(2) MHz. The trans conformer of o-cyanophenol was not observed. We recorded 14 a- and b-type rotational transitions for cis m-cyanophenol and 16 a- and b-type rotational transitions for trans m-cyanophenol. The rotational constants are A = 3408.9200(2) MHz, B = 1205.8269(2) MHz, and C = 890.6672(1) MHz and A = 3403.1196(3) MHz, B = 1208.4903(2) MHz, and C = 891.7241(2) MHz for the cis and trans species, respectively. Rotational transitions of the p-cyanophenol monomer are split due to the internal rotation of the hydroxyl group with respect to the aromatic ring. We recorded 25 a- and b-type rotational transitions for p-cyanophenol; the b-type transitions are split by 40 MHz. The rotational constants are A = 5612.96(2) MHz, B = 990.4283(6) MHz, and C = 841.9363(6) MHz. The ground state spitting DeltaE is 20.1608(6) MHz and the barrier to internal rotation, V(2), is 1413(2) cm(-1) from a fit of the rotational transitions to an internal axis system Hamiltonian. The barrier to internal rotation was modeled at the MP2/6-311++G(d,p) level and the effects of substituents on the phenolic ring and the barriers to internal rotation are discussed.

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. (14)N 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.

High resolution rotational spectroscopy and ring-puckering conformation of 3-hydroxytetrahydrothiophene

Abstract High resolution rotational spectra have been recorded for 3-hydroxytetrahydrothiophene and its 34S and 13C isotopomers using a Fourier-transform microwave spectrometer. The spectroscopic moments of inertia were used to least-squares fit the ring puckering conformation. The structure was found to have C(3) puckered out of plane, and it is stabilized by a 2.634-A hydrogen bond from the hydroxyl group to the thioether. Ab initio calculations, at the MP2/6-31G** level, also found that the C(3) puckered conformation is the lowest energy conformation of the ring. Measurements of the Stark shifts were used to determine the dipole moment (μ=1.556 (4) D) of 3-hydroxytetrahydrothiophene and its projections onto the principal inertial axes (μa=0.024 (5) D, μb=0.944 (5) D, and μc=1.237 (4) D).