Rotational spectra of the bio-mimetic molecule, alanine dipepitide and the double 15N(15N2) isotopomer have been observed using a pulsed-molecular-beam Fourier transform microwave spectrometer. The spectra reveal large 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 asymmetric-rotor Hamiltonians. The infinite-barrier-limit rotational constants of the 15N2 isotopomer are: A=1699.881(5).MHz, B = 986.388(5) MHz, and C = 711.644(5) MHz. The AE-states are analyzed independently using high-barrier torsion-rotation Hamiltonians, yielding observed-minus-calculated standard deviations of < 400 kHz. The fits improve substantially (> 100 fold for the 15N2 isotopomer) when analyzed in non-principal axis frames. The best-fit torsion-rotation parameters provide accurate V3 barriers and C3 rotor axis angles for both methyl groups. The observed angles are shown to uniquely correlate with those calculated for the acetyl and amide methyl groups in the C7eq conformational form. The V3 barriers of the amide and acetyl methyl groups are 86.3(3) and 98.6(3) cm-1 for the 14N2 and 86.3(1) and 98. 9(1) cm-1 for the 15N2 isotopomers, respectively. These results are in good agreement with prior geometry optimizations and with current V3 barrier calculations which predict the C7eq conformation as the lowest energy form in the gas phase. Under certain conditions, the spectrum is dominated by transitions from a decomposition product formed by dehydration of alanine dipeptide. This molecule is tentatively identified as 3,5-dihydro-2,3,5-trimethyl-(9CI)4H imidazole-4-one (CAS registry #32023-93-1).
Citation: Journal of Chemical Physics
Issue: No. 3
Pub Type: Journals
dipeptide, Fourier-Transform Spectroscopy, rotational spectrum, structure, torsional-rotation splitting