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Entropy-driven population distributions in a prototypical molecule with two flexible side chains: -(2-acetamidoethyl)--acetyltyramine
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10.1063/1.2803076
/content/aip/journal/jcp/127/23/10.1063/1.2803076
http://aip.metastore.ingenta.com/content/aip/journal/jcp/127/23/10.1063/1.2803076

Figures

Image of FIG. 1.
FIG. 1.

Disconnectivity diagrams for (a) NMPEA, (b) NPOEA, and (c) OANAT using the general AMBER force field (GAFF). Roman numerals label structures on which higher level calculations were performed. Pairs of structures that optimized to the same geometry at the RIMP2 level of theory are circled. Labels for trans/trans amide structures of OANAT have been omitted for clarity. Absolute energies from GAFF have been shifted by 80.0, 34.0, and for NMPEA, NPOEA, and OANAT, respectively, placing the first node connecting all minima at .

Image of FIG. 2.
FIG. 2.

Schematic of the time-of-flight mass spectrometer used in this work. The lasers are propagated perpendicular to the molecular beam.

Image of FIG. 3.
FIG. 3.

(a) The R2P1 excitation spectrum of NPEA taken over the region near the origin. (b) UV-UV hole-burning spectrum taken with the hole-burn laser fixed as indicated with an asterisk.

Image of FIG. 4.
FIG. 4.

(a) R2P1 excitation spectrum of NMPEA taken over the region near the origins. [(b)–(d)] UV-UV hole-burning spectra taken with the hole-burn laser fixed as indicated by the asterisks.

Image of FIG. 5.
FIG. 5.

(Color online) The lowest-energy structures, with cis orientation of methoxy group pictured, of NMPEA from RIMP2 optimizations. Relative vibrational zero-point corrected energies (kJ/mol) are reported with the relative free energies in parentheses. The trans orientation of the methoxy group is produced by 180° rotation of dihedral.

Image of FIG. 6.
FIG. 6.

RIDIR spectra in the amide NH stretch region of (a) NMPEA and (b) NPOEA.

Image of FIG. 7.
FIG. 7.

(a) R2P1 excitation spectrum of NPOEA taken over the region near the origins. [(b)–(e)] UV-UV hole-burning spectra taken with hole-burn laser fixed on the transitions marked by asterisks.

Image of FIG. 8.
FIG. 8.

(Color online) The five lowest-energy RIMP2 optimized NPOEA structures and one from B3LYP optimization. Relative vibrational zero-point corrected (ZPC) energies (kJ/mol) are reported along with relative free energies in parentheses. ZPC relative and free energies of were obtained from single-point and B3LYP frequency calculations on the B3LYP geometry. Dashed lines indicate potentially stabilizing interactions.

Image of FIG. 9.
FIG. 9.

Evidence used for assigning the observed conformers of NPOEA. Letters denote the observed conformers and Roman numerals indicate calculated data for the structures shown in Fig. 8. The theoretical results are scaled to match NPOEA-I to , with the RICC2 and TDDFT values scaled by 0.9244 and 0.8929, respectively. Vibrational frequencies are scaled by factors given in Table II and Fig. 10.

Image of FIG. 10.
FIG. 10.

RIDIR spectra of NPOEA in the alkyl CH stretch region. RIMP2 alkyl CH stretch frequencies (scaled by 0.9467) and IR intensities are presented above the conformer to which they are assigned, with NPOEA-II included at the bottom. The symmetric methylene stretch fundamentals are indicated with asterisks on the calculated spectra.

Image of FIG. 11.
FIG. 11.

Overview of R2PI and UV-UV hole-burning spectra of OANAT.

Image of FIG. 12.
FIG. 12.

Closeup of the origin regions of the six conformers of OANAT. The UV-UV hole-burning spectra were obtained by fixing the hole-burn laser on the transitions marked with asterisks.

Image of FIG. 13.
FIG. 13.

[(a)–(f)] RIDIR spectra in the NH stretch region of the six conformers of OANAT.

Image of FIG. 14.
FIG. 14.

(Color online) Lowest-energy noninteracting side chain conformers of OANAT. energies (kJ/mol), vibrational zero point corrected using B3LYP frequencies, are given along with free energies in parentheses.

Image of FIG. 15.
FIG. 15.

(Color online) Conformers of OANAT that form interchain hydrogen bonds between amide groups. Optimized energies (kJ/mol), vibrational zero point corrected with B3LYP frequencies, are given with relative free energies in parentheses. Note that OANAT-XVII is a cis/trans amide structure.

Image of FIG. 16.
FIG. 16.

(Color online) Electron density differences for the transitions of (a) NPOEA-I and (b) , calculated at the RICC2 level of theory. Dark (light) indicates areas of increased (decreased) electron density upon electronic excitation.

Image of FIG. 17.
FIG. 17.

Closeup view of the trans/trans amide portion of the OANAT disconnectivity diagram with major branches of structural families identified. Roman numerals label conformations on which higher level electronic structure calculations were performed. Lines colored black indicate conformers containing an interchain H bond.

Image of FIG. 18.
FIG. 18.

Relative energies of the calculated structures of OANAT employing the indicated theoretical methods. Gibbs free energy corrections were calculated using B3LYP frequencies and RIMP2 energies.

Image of FIG. 19.
FIG. 19.

Log of the density of states vs energy for the indicated conformers of OANAT. Black and grey traces are for interacting chain and independent-chain structures, respectively. The energy axis intercept for each trace is the relative zero point corrected RIMP2 energy.

Tables

Generic image for table
Table I.

Observed and calculated, origins, relative energies and free energies of NPEA, NMPEA, NPOEA, and OANAT.

Generic image for table
Table II.

Summary of relevant vibrational frequencies of NPEA, NMPEA, and NPOEA.

Generic image for table
Table III.

Summary of the relevant vibrational frequencies of OANAT.

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/content/aip/journal/jcp/127/23/10.1063/1.2803076
2007-12-20
2014-04-20
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O-(2-acetamidoethyl)-N-acetyltyramine
http://aip.metastore.ingenta.com/content/aip/journal/jcp/127/23/10.1063/1.2803076
10.1063/1.2803076
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