Linear and nonlinear circular dichroism of R-(+)-3-methylcyclopentanone (R-3MCP) is reported in the gas and liquid phases. Measurements of (2+1) resonance-enhanced multiphoton ionization circular dichroism (REMPICD) for nozzle-jet expanded molecular beams of the equatorial conformer of R-3MCP are presented. Monitoring either mass-selected cations or photoelectrons produced via (2+1) REMPI through the n --> 3s Rydberg transition yielded a REMPICD of +1.5+/-0.5% [REMPICD identical with 2(I(L)-I(R))(I(L)+I(R))], where I(L/R) refers to the ion/electron signal for left/right circularly polarized light. A racemic mixture of 3-methylcyclopentanone showed no significant CD; however, the signal fluctuations were much larger than that observed for the resolved R-(+)-3-methylcyclopentanone as might be expected for the small number of ions produced from slightly unequal numbers of enantiomers in each laser shot. Gas phase, vibrationally resolved, one-photon CD for vapor phase R-(+)-3-methylcyclopentanone (i.e., admixture of five axial and equatorial forms) was measured to be approximately 0 and -0.004 at photon energies corresponding to the one- (nonresonant) and two-(3s resonance) photon energy levels. The one-photon CD (of the room temperature population of conformers) at an energy corresponding to the ionization step was measured previously to be approximately +0.0011 which is of the same sign as the REMPICD. The first step is also near a positive CD region. This suggests that the (2+1) REMPICD is determined primarily by both the initial and continuum steps. The one-photon CDs for the equatorial and axial forms of 3MCP are calculated, using GAUSSIAN03, to be approximately equal but having opposite sign for the transitions of interest. The CD for 3MCP in cyclohexane is found to be strongly temperature dependent as a result of the presence of both the axial and equatorial conformers. The energy difference between the two conformers is determined from a van't Hoff plot of these data to be 3.50+/-0.05 kJ/mole in cyclohexane and is approximately 1 kJ/mole smaller than measurements employing other methods.

Linear and nonlinear circular dichroism of R-(+)-3-methylcyclopentanone.

Conformational Analysis Using ab Initio Vibrational Spectroscopy: 3-Methylcyclohexanone

Temperature-dependent electronic circular dichroism (CD) spectra are reported for (R)-(+)-3-methylcyclopentanone (R3MCP) in 34 solvents. Analysis of these data yielded the enthalpy and entropy differences between axial methyl and equatorial methyl conformers, the dominant conformers for R3MCP. The weakly absorbing n-->pi* transition exhibited a decrease in lambdamax as the solvent polarity increased. Vibrational CD fine structure of the n-->pi* transition was observed in the gas phase in addition to several of the solvents. Vapor-phase CD spectra were compared to both the solution-phase spectra and theoretical calculations of the axial and equatorial methyl conformers. Optical rotation (OR) measurements for R3MCP in cyclohexane solution in the visible region showed excellent agreement with OR DFT calculations of the equatorial and axial methyl conformers. Density functional calculations with B3LYP and the 6-13G* and aug-cc-pVDZ basis sets, which incorporate the solvent dielectric constant, yielded trends in thermodynamic quantities as a function of polarity and solvent dipole moments that are only weakly or not observed in experiments.

Spectroscopic and Theoretical Investigation of (R)-3-Methylcyclopentanone. The Effect of Solvent and Temperature on the Distribution of Conformers

A comparison of theoretical to experimental results of vibrational circular dichroism (VCD) in the CH stretching region and Raman optical activity (ROA) in selected lower frequency modes is reported for (+)‐(3R)‐methylcyclohexanone. Calculations were carried out using the fixed partial charge model of VCD and the atom dipole interaction model of ROA. Regions of detailed comparison have been remeasured in both VCD and ROA and confirm earlier results. The principal features of both VCD and ROA spectra are reproduced by the calculations and are interpreted on the basis of their constituent vibrational motions. Correlations and trends in both the VCD and ROA data, as well as between them, are discussed.

Vibrational optical activity: Comparison of theoretical and experimental results for (+)‐(3R)‐methylcyclohexanone

Circular dichroism of molecules with isotopically engendered chirality

Conformational analysis of 3-methylcyclohexanone assumed a special importance some time ago in connection with the so-named "3-alkylketone effect." Klyne and Robins and Walker noted that in the axial conformation of 3-methylcyclohexanone one of the 1,3-diaxial H:CH3 interactions of methylcyclohexane was missing and concluded that the conformational energy difference between axial and equatorial methyl forms should be lowered from the value of 7.11 kJ/mol (for methylcyclohexane) to one-half the value, or 3.55 kJ/mol. The difference (3.55 kJ/mol) between methyl A-values in methylcyclohexane and 3-methylcyclohexanone is called the 3-alkylketone effect. In order to generate an experimental value, thermochemical equilibration studies of cis-3,5-dimethylcyclohexanone on Pd(C) were carried out by Allinger and Freiberg to yield a conformational energy of ΔH° = 5.69 kJ/mol for the trans to cis epimerization. The authors concluded in favor of a greatly reduced 3-alkylketone effect of ca. 1.7 kJ/mol. The value of ΔH° differs from that of Rickborn (7.5 kJ/mol) derived from isomenthone and menthone equilibrations, but falls near to the data derived by Cotterill and Robinson in their base-catalyzed equilibration studies of 2,5-dimethylcyclohexanone [ΔH(e → a) = +5.4 kJ/mol and ΔS(e → a) = -0.2 eu] and 2-t-butyl-5-methylcyclohexanone [ΔH(e → a) = 6.4 kJ/mol and ΔS(e → a) = -0.5 eu]. Subsequently, Allinger et al. indicated conformational energies associated with the equatorial (E), axial (A) and twist (T) conformers (Fig. 1) of 3-methylcyclohexanone as 0, 5.9, and 13.8 kJ/mol, respectively, with a room temperature conformational mixture of 94, 5, and 1%, respectively. More recently Nakamura and I'Haya calculated rotational strengths using CNDO/2 and INDO methods for various probable conformers of 3-methyl-cyclohexanone and suggested a mixture of 83% E, 9% A, 7% T and 1% twist chair at 300°K, and 100% E at 81°K. Surprisingly, there has been no experimentally derived conformational analysis of 3-methylcyclohexanone itself; so, we report herein our conformational analysis of (+)-(3R)-methylcyclohexanone using variable temperature circular dichroism (CD) spectroscopy.

B. V. Crist

Papers

Conformational Analysis of (+)- (3R)-Methylcyclohexanone from Temperature-dependent Circular Dichroism Measurements