The relative intensity contributions of axial and equatorial CH bonds in the local mode overtone spectra of cyclohexane

Abstract

Vapor phase room temperature overtone spectra of cyclohexane are measured with conventional near infrared spectroscopy for the lower overtones, ΔvCH=2–4, and with intracavity dye laser photoacoustic spectroscopy for the higher overtones, ΔvCH=5–7. The relative intensity of the axial to equatorial peak is explained in terms of the local mode model of harmonically coupled anharmonic oscillators, where all modes but the two CH-stretching modes are neglected. The dipole moment function is expanded in the two CH-stretching coordinates, where the expansion coefficients are determined from ab initio molecular orbital calculations. This simple calculation, which contains no adjustable parameters, can account very well for the observed relative intensity of axial and equatorial pure local mode peaks in the overtone spectra from ΔvCH=3–7. We also investigate different ways of obtaining the dipole moment derivatives, and find that a fourth order series expansion around the equilibrium geometry is a good approximation when calculating the relevant matrix elements.

Calculations of overtone NIR and NIR-VCD spectra in the local mode approximation: Camphor and Camphorquinone

“…We observe that APT zcomponents (longitudinal components) have a negative value and a negative derivative in all cases, and this is in accord with the results of Ref. [26] and with what found from the interpretation of NIR-absorption spectra [28] (see also Ref. [4]).…”

“…(18) and (19), one can observe that the differential operators (as expected) have the form (26) so that the transition dipole moments are given by…”

“…The success of the DFT method in predicting electric anharmonic terms had been already observed in Ref. [26] for the CH bonds of cyclohexane and also for aromatic CHs in paracyclophanes [4]. No clear analysis and meaningful comparison was made for AAT values at z = 0 or their first derivatives, but we report them anyway in Table 1, since they show a trend.…”

“…(27) and (28), we have first studied ab initio the APTs and AATs at different values of CH bond lengths; this approach had previously been applied by Kjaergaard and Henry [26,27] for studying electric dipole moments. From the continuous curve constructed by polynomial interpolation, we finally obtained its derivatives at equilibrium.…”

Quantum chemical shape: new density domain relations for the topology of molecular bodies, functional groups, and chemical bonding

“…The hydrogen density domain DDl(a, H) with a dihedral angle near 180" with the density domain DD3(a, 0 , H) of the OH functional group has the lowest electron density, indicating differences of formal acidities of the terminal H atoms. These results are similar to those derived from overtone spectra on the nonequivalence of CH bonds (22)(23)(24)(25)(26)(27)(28).…”