Eric A. Noe

Cyclotridecanone 2,4-dinitrophenylhydrazone.

In cyclotridecanone 2,4-dinitrophenylhydrazone, C(19)H(28)N(4)O(4), the 13-membered carbocycle exists in the triangular [337] conformation. The 2,4-dinitrophenylhydrazone group is almost perpendicular to the 13-membered ring, with a dihedral angle of 82.66 (2) degrees between the mean planes. The dinitrophenylhydrazone rings are packed parallel to each other and separated by 3.28 (1) A. The NH group forms an intramolecular hydrogen bond to a nitro O atom, and there is a weaker C-H...O interaction between a cyclotridecane CH group and a symmetry-related 4-nitro O atom, with a C...O distance of 3.436 (2) A and a 150 degrees angle about the H atom. The structure, in combination with additional evidence, indicates that [337] is the preferred conformation of cyclotridecane and other simple 13-membered rings.

Conformational studies of trans-cycloheptene, trans-cycloheptene oxide, and trans-bicyclo [5.1.0] octane by ab initio calculations

Abstract The conformations of trans-cycloheptene (1), trans-cycloheptene oxide (2), and trans-bicyclo [5.1.0] octane (3), were studied by ab initio calculations. At the MP2/6-31G* level, a global minimum of C1 symmetry was found for each of the three compounds. In addition, a local minimum of high energy and C2 symmetry was found in each case at this level, with free energies at +25 °C relative to the global minimum of 11.1, 9.6, and 8.7xa0kcal/mol for 1, 2, and 3, respectively. The extent of twisting (29.0°) and bending (33.7°) about the double bond in 1 are compared with the literature values for this alkene and for trans-cyclooctene. The geometry for 1 obtained in the present calculations is compared with the MM3 geometry calculated by Saunders and Jimenez–Vazquez, and the MM3 and MP2/6-31G* geometries for 2 obtained in this work are compared. Topomerization of the global minimum conformation of 1 could take place by a pseudorotation process or by conversion to the C2 local minimum, followed by conversion to the topomer. Free-energy barriers of 10.979 and 12.016xa0kcal/mol, respectively, were found for the two processes, indicating that topomerization occurs mainly by pseudorotation.

Conformational studies of trihalomethyl formates by ab initio molecular orbital calculations

Abstract Ab initio calculations for trifluoromethyl formate ( 1 ) and trichloromethyl formate ( 2 ) were performed using Hartree–Fock, DFT, and electron correlation methods. At the MP2/6-311G(df,pd) level, the E -isomer of 1 was calculated to have a free energy of 1.140xa0kcal/mol, relative to the Z . The dipole moments of the Z and E conformations were 2.46 and 2.17xa0D, respectively. Although the difference in dipole moments was calculated to be close to zero, with a slightly more favorable (lower) value for the E -isomer, the Z isomer is predicted to predominate. Free-energy barriers of 7.11 and 5.97xa0kcal/mol for interconversion of conformations were calculated at the same level. A detailed study of rotation about the bond between the CF 3 carbon and the ‘ether’ oxygen indicated that the preferred orientation of the CF 3 group in both the E and Z conformations possessed a COCF dihedral angle of 180°, and electronic energies were calculated for 13 or 7 points at 10° intervals from these conformations. The rotational barriers were found to be 2.50 ( E ) and 4.51 ( Z )xa0kcal/mol. The MP2/6-311G(df,pd) level calculations for 2 predicted that the Z conformation is more stable than the E isomer by 0.857xa0kcal/mol, and the calculated dipole moments were 2.02 ( Z ) and 2.51xa0D ( E ). The rotational barriers for the CCl 3 group were 4.18 and 6.51xa0kcal/mol for the E and Z conformations, respectively. Free-energy barriers of 7.08 and 6.22xa0kcal/mol for interconversion of conformations were calculated.

Dynamic NMR studies of cyclopropyldifluoroborane and vinyldifluoroborane

Abstract The two fluorines of cyclopropyldifluoroborane ( 1 ) and vinyldifluoroborane ( 2 ) show two chemical shifts at low temperatures, due to hindered rotation about the boron–carbon bonds. The rates of rotation were determined at higher temperatures, and the corresponding free-energy barriers at coalescence were found to be 4.85±0.1 and 4.56±0.1 kcal/mol , respectively. The two barriers indicate that the cyclopropyl and vinyl groups are comparable in electron donating ability. Calculated lineshapes suggest that determination of the rotational barrier in phenyldifluoroborane by making use of the difference in coupling constants for an ortho carbon and the two fluorines may not be possible.

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.

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.

Conformational study of oxacyclononan-2-one by dynamic NMR spectroscopy and molecular mechanics calculations

Abstract The nine-membered lactone, oxacyclononan-2-one ( 1 ), was found by low-temperature 1 H and 13 C NMR spectroscopy to have two conformations with populations of 26. 9 and 73. 1 % at −161.0°C. Free-energy barriers of 6.41 and 6.19xa0kcal/mol for interconversion of conformations at −140.1°C were calculated from the 13 C spectrum of the carbonyl carbon. The 1 H NMR spectrum of the CH 2 O protons splits into two bands of equal intensity at low temperatures, due to slowing of the exchange of geminal hydrogens, and the free-energy barrier for this process was 6.68xa0kcal/mol at coalescence (−122.6°C). The NMR results for the compound are discussed in terms of the conformations predicted by Allingers MM3 program.

LOW-TEMPERATURE NMR STUDIES OF PHENYL DITHIOACETATE AND ETHYL DITHIOACETATE

Abstract The 300.52 MHz proton NMR spectrum of phenyl dithioacetate in CD2Cl2 at −91°C shows separate methyl signals for the E and Z conformations, with populations of 0.09 and 0.91, respectively. The free-energy barriers at coalescence (−65°C) are 9.81 and 10.64 kcal/mol for the E → Z and Z → E conversions, respectively. Slow-exchange C13 NMR spectra were also obtained for solutions in several solvents. The populations in 3:1 acetaldehyde acetone -d 6 were 0.12 and 0.88 at −92°C, as obtained from integration of the methyl carbon signals, and free-energy barriers at coalescence (−68°C) were 9.48 and 10.20 kcal/mol. Decoalescence was not observed at low temperatures for ethyl dithioacetate, probably due to a low population of the E isomer. The finding of an observable population of the E isomer for phenyl dithioacetate, but not for ethyl dithioacetate, is suggested to be related to the difference in hybridization and electronegativity of the two groups, with higher electronegativity favoring the E conformation.