Experimental Vibrational Circular Dichroism Spectra Research Articles

N-Acetyl-L-alanine N′-methylamide: a density functional analysis of the vibrational absorption and vibrational circular dichroism spectra

Abstract Ab initio 6-31G∗ Becke 3LYP DFT optimized geometries, vibrational frequencies, vibrational absorption (VA) intensities and vibrational circular dichroism (VCD) intensities have been calculated for the eight low energy conformers of N-acetyl-L-alanine N′-methylamide (L-AANMA) in the gas phase and one conformer stabilized by the addition of four water molecules. The VA and VCD spectra are calculated with the 6-31G∗ Becke 3LYP force fields (Hessians) and atomic polar tensors (APT); 6-31G∗∗ RHF atomic axial tensors (AAT) for the eight gas phase structures and 6-31G∗/6-31G RHF AAT for the L-AANMA-water complex. The VA and VCD spectra are also calculated using the 6-31G∗ Becke 3LYP Hessians; 6-31G∗∗ RHF APT and AAT for the eight gas phase structures and 6-31G∗/6-31G RHF APT and AAT for the L-AANMA-water complex. The rotational strengths of the amide A, I, II, III, IV, V and VI modes found in proteins as a function of φ and ψ (for various secondary structures) are for the first time reported for an inherently optically active molecule (non-glycine model) using the 6-31G∗∗ and 6-31G∗/6-31G RHF DOG AAT and 6-31G∗ Becke 3LYP Hessians and APT. This is also the first reported VCD calculation of a molecule with the solvent present. The molecule is not completely solvated, but the important hydrogen-bonded interactions are present and the feasibility of the calculation of the Hessian, APT and AAT with solvent molecules present is demonstrated. The VA and VCD spectra are compared to the experimental VA and VCD spectra in the literature and the conformational analysis (CA) and vibrational assignment of L-AANMA are reinvestigated. The rotational strengths of the amide modes for the various conformers are also compared to peptide and protein VCD spectra of molecules with known secondary structures. The agreement between the calculated rotational strengths of the various amide modes for which experimental measurements have been made is very good, in particular, for the right handed α-helical conformation of a poly(L-amino acid) we correctly predict the negative couplet for the amide A band, the positive couplet for the amide I band and the negative monosignate signal for the amide II band as measured by the groups of Nafie and Keiderling. The large change in the amide I region in aqueous solution from this region in carbon tetrachloride or the Ar matrix is also reproduced, documenting that the qualitative features of the vibrational absorption and VCD spectra can reproduced by explicitly adding water molecules to the Hessian and tensor calculations.

Ab initio calculation of vibrational circular dichroism spectra using large basis set MP2 force fields

Ab initio caculations of the vibrational circular dichroism (VCD) spectra of 2,3-trans- d 2-oxirane ( 1), 1,2-trans- d 2-cyclopropane ( 2), 1- 13C-1,2,3-anti- d 3-cyclopropane ( 3), and propylene oxide ( 4) based on [5s4p2d/3s2p] MP2 harmonic force fields are reported. The results are in perfect qualitative and good quantitative agreement with existing experimental VCD spectra except in the CH stretching region of 4 where Fermi resonance occurs. Calculations for 1–3 using a larger [8s6p3d/6s3p] basis set exhibit insensitivity to a substantial increase in basis set size.

Local and framework stereochemical markers in vibrational circular dichroism: 1,2- and 2,3-dimethylaziridines

The vibrational circular dichroism (VCD) spectra of 2,3-dimethylaziridine 1 and 1,2-dimethylaziridine 2 were measured experimentally and interpreted with the help of ab initio Vibronic Coupling Theory (VCT) both in the common origin (CO) and distributed origin (DO) gauge. The absolute magnitudes of the VCD intensities by the VCT-CO method in the mid-IR range are closer to the experimental measurement. Both VCT-DO and VCT-CO methods, however, predict the same signs for almost all VCD bands. In general, both VCT-DO and VCT-CO methods predict VCD and IR spectra that are both in good agreement with the experimental spectra. The VCT method together with the 6-31G*(0.3) basis set can be employed with confidence in determining the absolute configuration of a chiral molecule. The theoretical and experimental VCD spectra of 1 and 2 were compared with the VCD spectra of the isostructural 2,3-dimethyloxirane and -thiirane. One of the Me rocking motions, in the wave number range 1050–1150 cm−1, was found to serve as a possible marker for the stereochemistry of the molecular skeleton. Two other modes were identified whose Cotton effects (CEs) are characteristic of the local asymmetry, and for which chirality rules were proposed. The first is a methyne vibration, ||CH, which is expected to have a (−) CE at a three-ring centre with (R) absolute configuration. The second is an umbrella motion of a methyl group, which is expected to have a (+) CE when attached to a three-ring centre with (R) absolute configuration.