Vibrational circular dichroism spectrum of 2-methyloxetane

The infrared absorption (vapor phase and solution) and Raman (liquid phase) spectra of bicyclo[3.2.1]octane, 8-oxabicyclo[3.2.1]octane, 6-oxabicyclo[3.2.1]octane, 6,8-dioxabicyclo[3.2.1]octane, and the 7,7-dideutero-substituted derivatives of the last two compounds are reported in the region 100–1500 cm−1 for the first time. The vibrational spectra are assigned almost completely with the guidance of ab initio 3-21G geometries and scaled force fields. A total of 14 force-field scale facors are transferred from smaller molecules, predicting the frequencies with an average error of 7.6 cm−1 (1.2%) for 196 assigned transitions. After optimizing the factors in an overlay refinement involving all six molecules, the frequencies are within 5.7 cm−1 (0.75%) of experiment. The ab initio absorption and Raman intensities are calculated with the 3-21G basis set and are demonstrated to be of such accuracy as to be useful for the spectral assignments. These intensities are calculated with uniformly and nonuniformly scaled force fields and compared to the experimental spectra. The intensities derived from the latter force fields are superior, meaning that nonuniform scaling is preferable at this level of theory for both vibrational frequencies and normal mode descriptions.

Section: Introductionmentioning

confidence: 99%

“…5 and 1 1 . 'Calculated with uniformly scaled force field ( I ) , transferred scale factors (2). and fully refined scale factors (3); see text.…”

mentioning

confidence: 99%

The vibrational spectra (100–1500 cm⁻¹) of a series of bicyclo[3.2.1]octanes assigned by means of scaled 3-21G ab initio harmonic force fields

Eggimann¹,

Ibrahim²,

Shaw³

et al. 1993

“…The decision to permit such flexibility in the scaling procedure was motivated partially as a means of resolving certain ambiguities in the assignments. More importantly in the present context, we have suggested previously that such an approach should also optimize the predictive power of the scaling factors (4)(5)(6).…”

Section: Introductionmentioning

confidence: 90%

“…The success of this approach in the accurate prediction of vibrational spectra rests upon the availability of scaling factors suitable for correcting the calculated force constants. Although different types of force constant may require quite different correction factors, it has been shown that for a particular coordinate type the scaling factor is transferable over a range of molecules (3)(4)(5)(6).…”

Section: Introductionmentioning

confidence: 99%

Scaled abinitio force field and vibrational spectra of 2-methylthietane

Shaw¹,

Ibrahim²,

Wieser³

1990

. J. Chem. 68, 90 (1990).The vibrational spectra have been measured for 2-methylthietane over the range 100-1600 cm-' (Raman) and 400-3200 cm-' (solution and vapour phase infrared). Ab initio calculations at the 3-21G and 6-31G* levels indicate that both the axial and equatorial conformers represent minima on the ring inversion potential surface, the latter suggesting the equatorial species to be more stable by 191 cm-I. Vibrational force fields were evaluated for each of the two conformers and scaled using factors previously optimized for thietane and 2-methyloxetane. All of the major experimental features were readily assigned to the equatorial species, and most transitions expected for the axial species were also located. Calculated absorption intensities, although not accurate in an absolute sense, proved useful in predicting the relative intensities for corresponding axial and equatorial transitions.

“…l ]octane molecules exhibit well resolved hot band sequences (6) normally characteristic of large amplitude anharmonic skeletal deformations which should lead to corresponding rotational lines in a number of excited vibrational states. Second, the interpretation of vibrational circular dichroism spectra of certain chiral analogs of the title compound, currently in progress, depends on a reliable force field best obtained by ab initio methods (7), for which in turn the geometry must be ascertained with commensurate accuracy. Only two experimental structures have been reported in the literature for related bicyclo[3.2. lloctane systems (8, 9) which…”

Section: Introductionmentioning

confidence: 99%

The microwave and far-infrared spectra and the geometry of 8-oxabicyclo[3.2.1]octane

Eggimann¹,

Smithson²,

Wieser³

et al. 1990

. Can. J. Chem. 68, 267 (1990). The structure of 8-oxabicyclo[3.2.l]octane was calculated by a b initio means at the STO-3G level, and the structural data used to assign the microwave spectrum of the title compound in the range of 26.5 to 40 GHz fitting 144 lines up to J = 48 by adjusting the rotational and three centrifugal distortion constants in the Watson A reduction. Structural parameters were deduced from the rotational constants by allowing all bond lengths and angles to vary in a least-squares manner. These parameters were in good agreement with those calculated at the STO-3G and 3-21G levels. Of many satellite lines observed, only the most intense could be utilized for locating the two lowest fundamental vibrations, estimated to occur at 190 and 240 cm-'. The far-infrared spectrum is reported in the region of 80 to 300 cm-I, exhibiting only one distinct sequence of Q branches at 264 cm-I. On the basis of unscaled a b initio force fields at both levels of the theory the corresponding fundamentals are assigned, respectively, to a twisting (A") and a bending (A') of the cycloheptane ring.