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Conformers of β-aminoisobutyric acid probed by jet-cooled microwave and matrix isolation infrared spectroscopic techniques
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10.1063/1.4799456
/content/aip/journal/jcp/138/14/10.1063/1.4799456
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/14/10.1063/1.4799456

Figures

Image of Scheme 1.
Scheme 1.

Schematic representation of BAIBA with indication of its 4 internal torsional degrees of freedom which can give rise to conformational isomers.

Image of FIG. 1.
FIG. 1.

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.

Image of FIG. 2.
FIG. 2.

MP2/6-311++G(d,p) calculated potential energy profile for interconversion between conformers B 2 and B 3. The calculated B 3B 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.

Image of FIG. 3.
FIG. 3.

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.

Image of FIG. 4.
FIG. 4.

Infrared spectrum of BAIBA in a N2 matrix at 13 K.

Image of FIG. 5.
FIG. 5.

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 ).

Image of FIG. 6.
FIG. 6.

Schematic representation of the observed near-IR induced conformational isomerizations of BAIBA isolated in a N2 matrix upon irradiation at 6930 cm−1.

Image of FIG. 7.
FIG. 7.

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).

Tables

Generic image for table
Table I.

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

Generic image for table
Table II.

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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/content/aip/journal/jcp/138/14/10.1063/1.4799456
2013-04-12
2014-04-24
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Conformers of β-aminoisobutyric acid probed by jet-cooled microwave and matrix isolation infrared spectroscopic techniques
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/14/10.1063/1.4799456
10.1063/1.4799456
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