Conformations of Cycloundecane
for 1 were obtained from +17.0 to -183.1 °C with a 5 mm dual
probe over a period of many months. A pulse width of 8.2 µs was
used, corresponding to a tip angle of 83°, and the repetition rate
was 1 s for all recorded temperatures. Spinning was discontinued
below about -140 °C. Because ejecting the sample at lower
temperatures was difficult, due to the ice formation on the inner
wall of the stack, the temperature calibrations were performed
separately, using a copper-constantan thermocouple immersed in
propane solvent contained in a nonspinning dummy sample tube
and under conditions as nearly identical as possible. The emf’s were
measured with a millivolt potentiometer. The uncertainty in the
temperatures was estimated to be (2 °C, although differences in
temperatures are more accurate (perhaps (1 °C).
comparison between these 16 MM2 structures and our 15 MM3
structures (1a-o) showed 14 matches. Our conformation 1o is not
found among Kolossvary and Guida’s 16 conformations, and their
twelfth and thirteenth conformations in order of increasing MM2
6
energies are not among our 15. Barriers from these workers and
5
several from Anet and Rawdah are included in Table S6 (Sup-
6
porting Information). Three conversion numbers (4, 12, and 22)
do not appear because they involve one of the two conformations
of their 16 not found among our 14. The corresponding dihedral
angles for the conversions are shown in Table S7 (Supporting
Information).
Table S8 (Supporting Information) shows that the dihedral angles
obtained for conformations of 1 by different methods are very
similar.
The initial structure for cycloundecane was generated using
16
Spartan 5.0, and the geometry was exported into the MM3
program.17 A default method for searching conformational space
was used with a kick size of 2 Å and 1000 pushes. The structures
obtained were organized in increasing order of strain energy and
The chemical shifts (GIAO, HF/6-311G*) calculated for 1a and
1b and used for Figure 3 are shown in Table S9 (Supporting
Information).
2
All ab initio calculations were done at the Mississippi Center
for Supercomputing Research (MCSR), University of Mississippi,
Oxford, MS, using the Gaussian 94 and 98 series of programs. The
MM3 calculations and geometry visualizations were done at the
molecular modeling laboratory, Jackson State University, funded
by the Army High Performance Computing Research Center. The
MM4 calculations were done using the computational facilities at
the University of Georgia, Athens, GA.
matched the published results of Saunders. The free energies and
lowest frequencies were obtained at 25 °C and -190 °C for the
first 19 conformations (nos. 1-19) which included four transition
states (nos. 7, 9, 14, and 18) and 15 minima. The 15 minimum
energy structures in increasing order of MM3 steric energies are
designated as conformations 1a-o in Table S1 (Supporting
Information). A search of conformational space was done by MM4,
and the strain energies for the conformations within a 5 kcal/mol
window are included in Table S2 (Supporting Information), along
with strain energies from other force fields. The free energies at
two temperatures for the 11 conformations in this window are
shown in Table S3 (Supporting Information).
Acknowledgment. We thank the National Science Founda-
tion (NSF-CREST Grant No. HRD-9805465, to Jackson State
University) for financial support and the Mississippi Center for
Supercomputing Research for a generous amount of computing
time. Some calculations were done using the molecular modeling
laboratory at Jackson State University, funded by the Army High
Performance Computing Research Center. The NMR facility
at Jackson State University used for this study was supported
by the National Institutes of Health (RCMI Grant No.
G12RR13459).
The MM3-optimized geometries were used as inputs for ab initio
calculations. Full-geometry optimizations were performed subse-
quently at the HF/3-21G*, HF/6-31G*, and HF/6-311G* levels.18
These calculations were followed by calculations of vibrational
frequencies (analytical method), and all structures were character-
ized by the absence of imaginary frequencies except for 1f, which
was a minimum energy conformation in MM3 but was characterized
-1
by an imaginary frequency (-30.89 cm ) at the HF/6-311G* level.
Our results for the highest Hartree-Fock level are shown in Table
S4 (Supporting Information), and calculations at the MP2/3-21G*
level19 for 1a-e are included in Table S5 (Supporting Information).
Supporting Information Available: Optimized geometries,
calculated energies, dihedral angles, and magnetic shielding tensors
are provided. This material is available free of charge via the
Internet at http://pubs.acs.org.
6
Kolossvary and Guida reported 17 minima within a 25 kJ/mol
window by MM2, and 37 saddle points were found within a 50
kJ/mol window. However, the associated Supporting Information
provides details for 16 (not 17) local minima. A geometry
JO0608422
(
16) Spartan Version 5.10.; Wavefunction, Inc.: Irvine, CA.
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Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
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I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
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(17) Molecular mechanics (MM3) program version 2000 was used. The
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(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G.
A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
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J. Org. Chem, Vol. 71, No. 17, 2006 6515