Free-energy barriers of 9.85 and 11.91 ± 0.15 kcal/mol at −70.8 °C were found by dynamic NMR spectroscopy for the E-to-Z and Z-to-E conversions, respectively, of methyl formate (1) enriched in 13C to ...99% for the carbonyl carbon methyl formate 13C (2). These barriers are higher than the literature values reported for −53 °C. The free-energy barrier to 1,3 oxygen-to-oxygen migration of the methyl group in methyl formate was determined by ab initio calculations at several levels. The value of 58.7 kcal/mol obtained at the MP2/6-311+G (df,pd) level was compared to a literature barrier for this process (MINDO/3) and to barriers for related compounds. A free-energy barrier of 63.0 kcal/mol for the oxygen − to − oxygen migration of the CF3 group in trifluoromethyl formate (3) was calculated at the MP2/6-31+G* level.
The 13C spectra of cyclohexene oxide (1) show decoalescence of the peak at lowest frequency, with slow exchange at −187.7 °C and a coalescence temperature slightly above −178.2 °C. The ...low-temperature NMR results are interpreted in terms of two enantiomeric half-chair conformations, 1a and 1b, which could interconvert by way of either the endo-boat (1c) or exo-boat (1d) conformation. Ab initio calculations indicate that the endo-boat is significantly lower in energy than the exo-boat. Both boat conformations are shallow energy minima, as evidenced by the absence of imaginary frequencies. Relative free energies for the three conformations at −187.7 °C obtained from Allinger's MM3 program are in reasonable agreement with the ab initio results for 25 °C. A possible explanation for the greater stability of the endo-boat in terms of less eclipsing for the CH hydrogens of the three-membered ring with the CH2 hydrogens on the adjacent carbons is supported by calculated geometries. The experimental rate constant and free-energy barrier for interconversion of 1a and 1b were 227 s-1 and 4.3 ± 0.2 kcal/mol at −178.2 °C, and the corresponding parameters for the conversion of the half-chair to the endo-boat were 454 s-1 and 4.2 kcal/mol at this temperature. Estimates of the free energy at 25 °C of the transition state leading to the ring inversion were obtained at the HF/6-311G* and MP2/6-311G* levels by using the STQN method and were found to be 1.09 and 0.88 kcal/mol, respectively, above the local endo-boat minima. The corresponding calculated half-chair to endo-boat free-energy barriers at 25 °C were 4.87 and 4.96 kcal/mol, in reasonable agreement with the experimental value at −178.2 °C. Chemical shifts for the carbons of 1a were calculated at the HF/6-311G* and HF/6-311+G(2d,p) levels, using the GIAO method, to assign peaks to specific carbons.
Cyclodecyl 4-nitrophenylacetate Brown, Judge; Pawar, Diwakar M.; Fronczek, Frank R. ...
Acta crystallographica. Section C, Crystal structure communications,
10/2006, Letnik:
62, Številka:
10
Journal Article
Recenzirano
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)°, 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)°. An intermolecular contact exists between the ester C atom and a nitro O atom, having a C⋯O distance of 2.909 (2) Å.
Solutions of 1,2-cycloundecadiene in propane were studied by low-temperature 13C NMR spectroscopy. A total of 17 peaks were observed at −166.7 °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 13C 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 °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.
Low-temperature 13C NMR spectra of cyclodecane (1) showed the presence of a minor conformation, assigned to the twist-boat-chair-chair (TBCC), in addition to the expected boat-chair-boat (BCB). If ...only the TBCC and BCB conformations were assumed to be appreciably populated, then a free-energy difference between the two conformations of 0.73 ± 0.3 kcal/mol could be obtained from the five area measurements over a temperature range of −148.6 to −131.0 °C, with populations of 5.2 and 94.8% for the TBCC and BCB conformations at −146.1 °C. However, an alternative description of the conformations of 1 was suggested by the ab initio calculations, which predicted that the twist-boat-chair (TBC) and TBCC conformations have comparable free energies and populations. Equal amounts of TBCC and TBC would give populations of 5.2, 5.2, and 89.6% and relative free energies of 0.72, 0.72, and 0.00 kcal/mol for the TBCC, TBC, and BCB conformations at −146.1 °C, based on the experimental areas at this temperature. The experimental spectra could neither confirm nor disprove the presence of the TBC. Saunders' calculations of the strain energies of 1 using Allinger's MM3 program were reproduced to obtain a complete set of these parameters and drawings of the conformations, and free energies and populations were obtained at +25 and −171.1 °C. Free energies were also calculated at the HF/6-31G* and HF/6-311G* levels, and chemical shifts were obtained for three conformations at the HF/6-311G* level by the GIAO method. Chlorocyclodecane (2) was shown by 13C and 1H NMR spectroscopy to have three conformations at −165.5 °C. To aid in conformational assignments, the 13C chemical shifts were calculated for all of the BCB and TBCC conformations of 2 using the GIAO method at the HF/6-311G* level. The free energies for each of the possible BCB, TBCC, and TBC conformations were also calculated using Allinger's MM3 program. From the line shape changes in the experimental 13C NMR spectra, the free-energy barriers, a consideration of the X-ray structures of substituted cyclodecanes, and these calculated chemical shifts and free energies, the three conformations of 2 at −165.5 °C were suggested to be 2e BCB (31.2%), 2a BCB (14.9%), and a TBCC conformation (53.9%) (numbering as in Figure ); the 2e and 2a BCB assignments could be reversed. Free-energy barriers for interconversion of BCB conformations of 2 at −159.8 °C were 5.4 ± 0.2 and 5.5 ± 0.2 kcal/mol, and the free-energy barriers at −120.9 °C for equilibration of the TBCC conformation with the rapidly interconverting BCB conformations were 7.07 ± 0.2 and 7.08 ± 0.2 kcal/mol. The 13C NMR spectrum of cyclodecyl acetate (3) at −160.0 °C showed a similar pattern of chemical shifts and intensities for the substituted ring carbon.
Cyclodecyl 4-nitrophenylacetate BROWN, Judge; PAWAR, Diwakar M; FRONCZEK, Frank R ...
Acta crystallographica. Section C, Crystal structure communications,
10/2006, Letnik:
62, Številka:
Pt 10
Journal Article
Recenzirano
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.
The NMR spectrum of cyclopropanecarbaldehyde (1) shows two chemical shifts for the aldehyde proton at −169.2 °C, separated by 1.71 ppm. The populations of the two conformations at this temperature ...are 0.012 and 0.988, corresponding to the aldehyde hydrogen cis (1a) or trans (1b) to the methine hydrogen, respectively. From the averaged chemical shift at room temperature, populations of 1a and 1b were estimated to be 0.27 and 0.73. Free-energy barriers of 5.03 kcal/mol (1a → 1b) and 5.95 kcal/mol (1b → 1a) were determined for the compound at −169.2 °C. Comparison of these barriers with the barrier for benzaldehyde indicates that the π-donating ability of the cyclopropyl group is lower than for phenyl.
Each of the five 13C signals of cis-cyclodecene (1) splits into two peaks of equal intensity at lower temperatures, with a free-energy barrier of 6.64 kcal/mol for the process at −139.7 °C. The 1H ...NMR spectrum of the allylic protons shows decoalescence at −36.4 °C, and a free-energy barrier of 10.9 kcal/mol was determined at this temperature. The MM3 strain-energy calculations of Saunders and Jimenez-Vazquez were reproduced to obtain drawings of these conformations, and relative free energies for 1 at −152.7 °C were calculated using this program. A single conformation of C 1 symmetry resembling boat−chair−boat (BCB) cyclodecane was predicted by these calculations and by ab initio calculations at the HF/6-311G* level. 13C chemical shifts for the preferred conformation of 1 were calculated at the HF/6-311G* level using the GIAO method. cis-Cyclodecene oxide (2) also shows decoalescence in the 13C spectrum, with ΔG ⧧ = 7.4 kcal/mol at −125.4 °C. The epoxide has a time-averaged plane of symmetry at room temperature, which no longer exists at −152.6 °C. Similarly, the lower barrier of 6.64 kcal/mol for 1 is suggested to be associated with a process which gives the compound a time-averaged plane of symmetry when it is rapid on the NMR time scale.
The 13C NMR spectrum of the olefinic carbons of cis-cyclononene (1) at −189.3 °C showed three peaks, still exchange-broadened, which were interpreted in terms of two conformations. Relative free ...energies of conformations were estimated from ab initio calculations at the HF/6-311G* level and molecular mechanics (MM3) calculations, and possible assignments were made on the basis of these calculations and the experimental data. The major conformation has C 1 symmetry and a population of 66% at −189.3 °C. The structure suggested to be the minor conformation also has C 1 symmetry, but the deviation from C s symmetry is small, resulting in a time-averaged plane of symmetry, even at −189.3 °C. Free-energy barriers of 4.18 ± 0.15 and 4.28 ± 0.15 kcal/mol for interconversion of the two conformations at −183.1 °C were calculated from the rate constants derived from line shape matching of the 13C spectrum of the olefinic carbons at this temperature. Major-to-minor free-energy barriers for four temperatures from −178.3 to −189.3 °C were within the range of 4.28 to 4.35 kcal/mol. Interconversion of sites within the major conformation occurs through conversion to the minor conformation. The effects of a second process were observed in the 1H NMR spectra of the allylic protons of 1. Two multiplets of equal intensity were found by −124.8 °C, and a free-energy barrier of 8.05 ± 0.2 kcal/mol was calculated at coalescence (−102.3 °C). The changes were suggested to be due to slowing the exchange of geminal hydrogen positions.
