Fluorescent tags. dansyl amide (CH3)2N–С10Н6–SO2NH2: Reflection of the conformational properties of a free molecule in crystal structures

Abstract

The conformational properties of a dansyl amide molecule (CH3)2N–C10Н6–SO2NH2, which is widely used for fluorescent labeling of compounds, e.g., amino acids or nucleotides and compounds able to form liquid crystals, are studied. It is found that the molecule has six conformers with relative energies 0/0.13, 1.13/1.38, 0.06/0.06, 0.31/0, 2.95/2.70, 0.31/0 (B3LYP/cc-pVTZ and МР2/cc-pVTZ methods respectively). In all conformers the naphthalene core becomes non-planar under the effect of two substituents. The effect of the nature of substituents on the geometric parameters of the conformers is considered. In terms of the NBO analysis a substantial deviation of the S–N bond in the –SO2NH2 substituent from orthogonality and also the asymmetrical position of the –N(СH3)2 group with respect to the naphthalene core are explained. The paths and barriers of conformational transformations of the dansyl amideа molecule are determined. The structure of the (CH3)2N–C10H6–SO2NН–X moiety in the crystals is analyzed and the relationship between the relative energy of the conformers of a free molecule and the probability of the occurrence of their structures in the crystals is determined.