Schematic representation of BAIBA with indication of its 4 internal torsional degrees of freedom which can give rise to conformational isomers.
The 19 lowest energy conformers of BAIBA. Conformers A 1 , A 2 , B 1, and B 2 were observed experimentally by microwave spectroscopy in this study.
MP2/6-311++G(d,p) calculated potential energy profile for interconversion between conformers B 2 and B 3. The calculated B 3–B 2 energy difference ( ) is 1.9 kJ mol−1, while the zero-point corrected MP2/B3LYP/6-311++G(d,p) energy difference ( ) is 1.6 kJ mol−1 (see also Table I ). The barrier to conformational isomerization presented in the figure does not take into account the zero-point corrections.
Broadband CP-FTMW rotational spectrum of BAIBA in the 6–18 GHz region. Top insets: (left) detail of the CP-FTMW spectrum showing the feature ascribed to the 303-212 transition for conformer B 1; (right) high-resolution LA-MB-FTMW spectrum of the same transition, showing the quadrupole components labeled for conformer B 1.
Infrared spectrum of BAIBA in a N2 matrix at 13 K.
Difference IR spectrum (near-IR irradiated BAIBA:N2 matrix at 6930 cm−1 minus as-deposited matrix) (top), compared with the difference simulated spectrum for conformers A 1 (bands pointing up) and B 1 (down) (bottom). In the simulated spectrum, bands were represented by Lorentzian profiles having a full-width at half maximum (FWHM) of 2 cm−1 centered at the calculated frequencies scaled by 0.993 (below 1650 cm−1), 0.965 (above 1650 cm−1) and 0.945 (νO–H and νNH2 stretching modes). Note that even after the applied scaling the calculated νO–H frequency in the O–H⋯N hydrogen bonded conformer A 1 is considerably overestimated (as evidenced by the arrow). The most characteristic new bands emerging upon irradiation are marked with a red asterisk, and are ascribed to conformer D 1 (see text and Figure 6 ).
Schematic representation of the observed near-IR induced conformational isomerizations of BAIBA isolated in a N2 matrix upon irradiation at 6930 cm−1.
Difference spectra of BAIBA monomers isolated in N2 matrix at 13 K, in the B 1 νO–H (left) and νC=O (right) stretching regions. The spectra shown correspond to consecutive irradiations. Profile in black is obtained after subtracting spectrum of the freshly deposited matrix from that obtained after irradiation 1 (λ = 6939 cm−1); profile in blue is the result of subtraction of the spectrum in black from the one obtained after irradiation 2 (λ = 6920 cm−1); and the one in red is obtained after subtracting the blue spectrum from the one obtained as a result of irradiation 3 (λ = 6930 cm−1).
Calculated relative energies (zero-point corrected; ΔEo) and Gibbs free energies at room temperature (RT = 298.15 K) (ΔGo), and predicted populations for the lowest energy conformers of BAIBA. a
Experimental spectroscopic parameters for the four observed conformers of BAIBA compared with those predicted at the MP2/6-311++G(d,p) level of theory. a
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