Cyclopentadecanone 2,4-dinitrophenylhydrazone.

In the title compound, C(21)H(32)N(4)O(4), no disorder is present in the 15-membered hydrocarbon ring, which exists in an unsymmetrical quinquangular [12345] conformation. The 2,4-dinitrophenylhydrazone group is approximately perpendicular to the C(15) ring, with a dihedral angle of 84.66 (1) degrees between their best planes.

Cyclopentadecanone.

In the crystal structure, the title compound (exaltone), C(15)H(28)O, exhibits no disorder, and the 15-membered ring exists in the quinquangular [13353] C(1) symmetry conformation. The crystal exhibits nonmerohedral twinning by twofold rotation about [100], but adjustment of the temperature to 90 K causes the [101] distance to equal the c axial length, allowing the twinning to be treated as pseudo-merohedral. The literature description of the structure as disordered orthorhombic with halved a-axis length is corrected.

Conformational equilibria in formic acid and the adduct of formic acid and hexafluoroacetone, HCO2C(CF3)2OH.

Low-temperature 1H and 13C NMR spectra of formic acid (1) showed separate signals for the E and Z conformations in solvents containing a hydrogen bond acceptor, dimethyl ether. The population of E-1 (6.2% in 3:1:1 CHClF2/CHCl2F/(CH3)2O) was larger than that for 13C-labeled methyl formate in the same solvent (0.2%), which indicated that the relative populations are not determined by steric effects. The free-energy difference between the E and Z conformations of 1 was 0.9 kcal/mol. In a 1:3 CD2Cl2/(CH3)2O solvent mixture, peaks for E and Z conformations were found at low temperatures by 1H and 13C NMR for both formic acid and an adduct with hexafluoroacetone, HCO2C(CF3)2OH (2). The population of E-1 in this solvent mixture was 4.3% by 13C NMR. The carbon spectrum showed two peaks in the carbonyl carbon region of nearly equal intensities at -151.6 degrees C, with E-2 (48%) absorbing downfield of the major Z-2 (52%). The large population of E-2 confirms that electron-withdrawing groups R' in RCO2R' enhance the populations of the E-isomers. The free-energy barriers for 2 of 6.24 (E-to-Z) and 6.26 kcal/mol (Z-to-E) were determined from rate constants obtained by line shape analysis at -143.2 degrees C.

Interconversion of carbon sites in boat-chair-boat cyclodecane; conformations of chlorocyclodecane and cyclodecyl acetate.

Interconversion of carbon sites in boat-chair-boat (BCB) cyclodecane occurs by way of the twist-boat-chair (TBC) conformation, which predicts that C-1 exchanges with C-4, etc. The previously obtained low-temperature 13C spectra could be matched by assuming (1,4) or (1,3) exchange, but not (1,2) exchange. A free-energy barrier of 5.54 kcal/mol was obtained for conversion of BCB to TBC at -137.4 degrees C. The major conformation of chlorocyclodecane and cyclodecyl acetate is assigned to the IIIe BCB.

Cyclodecyl 4-nitrophenylacetate.

Cyclodecyl 4-nitrophenylacetate, C18H25NO4, has its ten-membered ring in the expected diamond-lattice boat-chair-boat [2323] conformation, with the substituent 4-nitrophenylacetoxy group in the BCB IIIe position. The ester unit has the expected Z conformation, with an O=C-O-C torsion angle of -0.3 (3) degrees, and the connection to the benzene ring is nearly perpendicular to the ester, with an O=C-C-C torsion angle of 85.5 (2) degrees. An intermolecular contact exists between the ester C atom and a nitro O atom, having a C...O distance of 2.909 (2) A.

Conformations of cycloundecane.

Cycloundecane (1) was shown to exist at -183.1 degrees C as a mixture of the [12323] (approximately 59%) and [335] (approximately 41%) conformations. Populations were determined from the (13)C NMR spectrum, and assignments were based on the (13)C spectra, calculated free energies and chemical shifts, and information from the literature, including X-ray studies of solid derivatives and calculated barriers.

Conformational study of cis-1,4-di-tert-butylcyclohexane by dynamic NMR spectroscopy and computational methods. Observation of chair and twist-boat conformations.

[reaction: see text] Low-temperature 13C NMR spectra of cis-1,4-di-tert-butylcyclohexane (1) showed signals for the twist-boat (1a) and chair (1b) conformations. 13C NMR signals were assigned to specific carbons based on the different populations, different symmetries (time-averaged C(2v) for 1a and time-averaged C(s) for 1b), and calculated chemical shifts (GIAO, HF/6-311+G*). In addition to slow ring inversion and interconversion of the chair and twist-boat conformations, slow rotation of the tert-butyl groups was found. Most of the expected 13C peaks were observed. Free-energy barriers of 6.83 and 6.35 kcal/mol were found for interconversion of 1a (major) and 1b (minor) at -148.1 degrees C. Conformational space was searched with Allinger's MM3 and MM4 programs, and free energies were obtained for several low-energy conformations 1a-c. Calculations were repeated with ab initio methods up to the HF/6-311+G* level. Molecular symmetries, relative free energies, relative enthalpies and entropies, frequencies, and NMR chemical shifts were obtained. A boat conformation (1d; C(2v) symmetry) was generated and optimized as a transition state by ab initio, MM3, and MM4 calculations.

Conformational study of 1,2-cycloundecadiene by dynamic NMR spectroscopy and computational methods.

Solutions of 1,2-cycloundecadiene in propane were studied by low-temperature (13)C NMR spectroscopy. A total of 17 peaks were observed at -166.7 degrees C, corresponding to two conformations of similar populations, one of C(1) symmetry (11 peaks) and the other of C(2) symmetry. The line shapes show that the predominant pathway for exchange of the topomers (C(1) and C(1)') of the C(1) conformation does not include the C(2) conformation. From the (13)C spectra, free-energy barriers of 8.38 +/- 0.15, 9.45 +/- 0.15, and 9.35 +/- 0.15 kcal/mol were determined for the C(1) to C(1)', (C(1) + C(1)') to C(2), and C(2) to (C(1) + C(1)') conversions, respectively, at -72.2 degrees C. The NMR results for this compound are discussed in terms of the conformations predicted by molecular mechanics calculations obtained with Allinger's MM3 program. Ab initio calculations of free energies are also reported at the HF/ 6-311G level for 25 conformations.

Conformational Study of trans-Cyclododecene by Dynamic NMR Spectroscopy and Computational Methods.

The (13)C spectrum of trans-cyclododecene (1) dissolved in propane showed seven peaks for the olefinic carbons at -164.5 degrees C, corresponding to three conformations of C(1) symmetry and a fourth conformation, with a population of 20.1%, of C(2) symmetry. The populations of the C(1) conformations were 57.0, 18.6, and 4.3%. Conformational space was searched for 1 using MM2. Free energies and populations were calculated by MM3, and the results of the low-temperature NMR study are discussed in terms of these calculations and the (13)C chemical shifts calculated for eight conformations by the GIAO method at the HF/6-311G level. The conformations of 1 and cyclododecane (2) are compared.

Conformational Study of Oxacyclooctan-2-one by Dynamic NMR Spectroscopy and Computational Methods.

The slow-exchange (13)C NMR spectrum of the eight-membered lactone, oxacyclooctan-2-one (1), at -154.2 degrees C shows the presence of two conformations with populations of 25.6 and 74.4%. Free-energy barriers of 6.98 and 7.23 kcal/mol were determined at the coalescence temperature for the carbonyl carbon peak (-128.6 degrees C). The (1)H NMR spectrum of the CH(2)O protons splits into two multiplets of equal intensity by -100 degrees C, and the corresponding free-energy barrier of 9.16 kcal/mol at -85.2 degrees C was suggested to be associated with exchange of the geminal protons. Relative free energies and populations at +25 and -160 degrees C were estimated by using Allinger's MM3 program, and free energies and dipole moments were obtained for three conformations using ab initio calculations at the HF/6-311G and MP2/6-311G levels. The spectra were interpreted in terms of two conformations having the E configuration of the lactone group and resembling the boat-chair conformation of cyclooctane.