The r 0 structural parameters of equatorial and axial chlorocyclobutane, conformational stability from temperature dependent infrared spectra of xenon solutions, and vibrational assignments

Abstract

Variable temperature (-55 to -100°C) studies of the infrared spectra (4,000-400 cm -1 ) of chlorocyclobutane, c-C 4 H 7 Cl, dissolved in liquid xenon have been carried out. The infrared spectrum (4,000-100 cm -1 ) of the gas has also been recorded. For this puckered ring molecule the enthalpy difference between the more stable equatorial conformer and the axial form, has been determined to be 361 ± 17 cm -1 (4.32 ± 0.20 kJ/mol). This stability order is consistent with that predicted by ab initio calculations but the DH is much lower than the average energy value of 646 ± 73 cm -1 obtained from the MP2 ab initio calculations or 611 ± 28 cm -1 from the B3LYP density functional theory calculations. The percentage of the axial conformer present at ambient temperature is estimated to be 15 ± 1%. By utilizing previously reported microwave rotational constants for both conformers combined with ab initio MP2(full)/6-311?G(d,p) predicted structural values, adjusted r 0 parameters have been obtained. The determined heavy atom structural parameters for the equatorial conformer are: the distances C-Cl = 1.783(5), C 1 -C 4 = 1.539(3), C 4 -C 6 = 1.558(3) Å , and angles \C 6 C 4 C 1 = 86.9(5), \C 4 C 1 C 5 = 89.7(5)°, and for the axial conformer are: the distances C-Cl = 1.803(5), C 1 -C 4 = 1.547(3), C 4 -C 6 = 1.557(3) Å , and angles \C 6 C 4 C 1 = 86.3(5), \C 4 C 1 C 5 = 88.9(5) and the puckering angles for the equatorial and axial conformers are 30.7(5)°and 22.3(5)°, respectively. The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations and compared to experimental values where available. The results are discussed and compared to the corresponding properties of some similar molecules.

A 1H NMR and theoretical investigation of the conformations of some monosubstituted cyclobutanes

“…These results are shown in Table 5 and there is a good correlation between the experimental and the theoretical 4 J HH values. This supports the use of the theoretical 4 J HH values calculated for the axial and the equatorial cyclobutane conformers together with the experimental couplings to calculate the conformer energy difference. Any errors in the calculated 4 J HH couplings will be reflected in the values of G, but the errors will be more dependent on the proportions of the conformers.…”

“…4 J eq -eq is ca 5 Hz and 4 J ax -ax ca 0 Hz. Thus, the long range coupling constant ( 4 J HH ) can be used with confidence to determine the conformational preferences in the compounds studied.…”

“…[25] The 4 J HH couplings in compounds 1-12 were also calculated using the CP-DFT/B3LYP methodology [26] as implemented in the Gaussian03 package of programs. All four terms of 4 J HH spin-spin coupling constant [Fermi contact (FC), spin dipolar, paramagnetic spin orbit and diamagnetic spin orbit] were carried out using the EPR-III basis set, [27] which is a triple-zeta type and includes diffuse and polarisation functions. The s part of this basis set is enhanced to better reproduce the electronic density in the nuclear region, since this point is particularly important when calculating the FC term.…”

“…Theoretical calculations for three molecules containing a fixed cyclobutane ring, namely α-pinene, β-pinene and nopinone (Scheme 2) were performed to determine the agreement between theoretical and experimental [30 -32] values for the 4 J HH in the compounds studied. These results are shown in Table 5 and there is a good correlation between the experimental and the theoretical 4 J HH values.…”

“…Theoretical calculations for three molecules containing a fixed cyclobutane ring, namely α-pinene, β-pinene and nopinone (Scheme 2) were performed to determine the agreement between theoretical and experimental [30 -32] values for the 4 J HH in the compounds studied. These results are shown in Table 5 and there is a good correlation between the experimental and the theoretical 4 J HH values. This supports the use of the theoretical 4 J HH values calculated for the axial and the equatorial cyclobutane conformers together with the experimental couplings to calculate the conformer energy difference.…”