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

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.     

Spectroscopic and theoretical investigation of (R)-3-methylcyclopentanone. The effect of solvent and temperature on the distribution of conformers

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.     

Matrix‐isolation IR spectroscopy of R‐(+)‐3‐methylcyclopentanone in para‐hydrogen crystal

High‐resolution infrared (IR) spectra of R‐(+)‐3‐methylcyclopentanone (R3MCP) in para‐hydrogen (pH₂) crystal were recorded and compared with the corresponding IR spectra of R3MCP in Argon (Ar) isolation matrix as well as the IR spectra of the neat crystalline R3MCP at low deposition temperature of 4 ± 0.05 K. Moreover, IR spectra of R3MCP, hosted in pH₂ crystal, were recorded using a high‐resolution Fourier transform IR spectrometer as a function of sample concentration and over the range 10–300 ppm. Furthermore, density functional theory calculations of simulated IR spectra for the optimized geometries of R3MCP equatorial‐methyl and axial‐methyl conformers are compared with experimental spectra for the purpose of investigating molecular conformation. Upon comparison between theoretical and experimental IR spectra, vibrational modes arising from equatorial and axial conformers have been successfully assigned and related to the individual conformer's structure. Copyright © 2015 John Wiley & Sons, Ltd.     

Structural analysis of (R)-3-methylcyclohexanone conformers

Structural Chemistry - The electronic circular dichroism (CD) spectra as a function of temperature for R-(?+)-3-methylcyclohexanone (R3MCH) were reported in 11 solvents of wide polarity range...     

Solvent Effects on IR Modes of (R)-3-Methylcyclopentanone Conformers: A Computational Investigation

Density Functional Theory (DFT) calculations of infrared spectra for the optimized geometries of the R-(+)-3-methylcyclopentanone (R3MCP) equatorial-methyl and axial-methyl conformers were performed in eleven common solvents with a wide polarity range, within the framework of polarizable continuum model (PCM). The DFT correlation function type B3LYP, using a powerful basis set (aug-cc-pVDZ), yielded different linear correlations between solvent polarity and R3MCP equatorial and axial conformers for frequencies of IR modes, intensities, and enthalpies. DFT calculations of the dipole moments of R3MCP equatorial and axial conformer components in 3D were also carried out and found to have a linear correlation with the solvent polarity. An increasing trend for the hypsochromic (blue) shift in the equatorial conformer??s IR frequencies is observed, in comparison to bathochromic (red) shift for the axial-methyl conformer IR modes, as a function of the solvent polarity.     

Isotopic composition of bottled water in Saudi Arabia

The ¹⁸O/¹⁶O and ²H/¹H ratios of 18 water brands representing the most popular bottled water brands in the Saudi market were measured using a system based on the latest advancements in tunable off-axis integrated cavity output diode laser spectroscopy (OA-ICOS) in the near-infrared spectral region. Utilizing δ¹⁸O and the δ²H values of locally produced water samples, a meteoric water line (δ²H?=?7.84 δ¹⁸O?+?2.11) was extracted and found to be consistent with the slope of the global meteoric water line (GMWL) and the geographic location of Saudi Arabia.