The infrared spectrum of 2-methylaziridine from scaled abinitio force fields

Abstract

The structures of the cis and trans invertomers of 2-methylazirdine were optimized at the RHF/6-31G*(0.3) level of theory and the composition of the equilibrium mixture was determined to be trans/cis = 0.71:0.29, in good agreement with the experimental value, 0.66:0.35. The abinitio force field was scaled and a complete assignment of the vibrational spectra of the two conformers is presented. The IR spectrum of 2-methylazirdine was recorded in the vapor phase and in solution (cyclohexane and CCl4). The predicted IR spectrum of the equilibrium mixture agrees very well with the experimental spectrum. The 6-31G*(0.3) basis set is found to provide better geometries and more uniformly scalable force fields than the conventional 6-31G* basis set. Keywords: abinitio force fields, 2-methylaziridine.

The vibrational circular dichroism spectra of 2-methylaziridine: dominance of the asymmetric centre at nitrogen

“…optical rotatory strengths, R, calculated in the CO and DO gauges using uniformly and optimally scaled force fields, are listed in Tables 1 and 2. The close correspondence between optimum scaling and uniform scaling of 6-31~"'O.~) force fields has been noted (22), and is apparent in the similarities in the dipole and rotary strengths calculated from the two force fields (Tables 1 and 2). Previous experience with V C~l 6 -3 1~* '~. '…”

“…On the other hand, vg, a mixed mode consisting of C-CH3 stretch and ring distortion, is at higher frequency by 12 cm-I for the trans isomer. Lesser perturbations to lower (vl, vg, vlo, ~1 2 , v13, v15, and ~2 5 ) and higher (~1 4 , ~1 7 , ~2 3 , and vz7) frequencies in the trans compared to the cis conformer are resolved in the experimental IR spectrum (22) and therefore may be significant for the in-"ver the period of the last year, the alignment of the beamsplitter and mirrors of the Michaelson interferometer has been worsening, resulting in a loss of energy at the polarization optics. Furthermore, the optimal VCD signal is not located in the optical axis anymore, causing additional energy loss, and a deterioration of the appearance of the recorded VCD spectrum.…”

“…1. Geometric details, relative energies, and complete vibrational assignments of the two inversional diastereomers of 2-methylaziridine have been reported (22). Only details required for the discussion of the VCD spectra are repeated here (Tables 1 and 2).…”

“…basis sets, have been reported previously (22). Bond lengths are calculated to be a little longer by the was purified additionally by distilling over Na (metal).…”

“…2-Methylaziridine exists as an equilibrium mixture of trans and cis diastereoisomeric forms whose composition has been determined by nmr (19) and IR (20) measurements. Following an earlier partial assignment on the basis of an empirical force field (21), a complete assignment of the IR spectrum has been carried out on the basis of scaled ab initio force fields (22).…”