M. S. Fedorov

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

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.

Energies of the gas-phase deprotonation of nitro-substituted benzenesulfonic and benzoic acids: The role of the conformation isomerism of sulfonic acids

The Gibbs energies of deprotonation ΔrG298○ of gaseous benzoic acid (BA), benzenesulfonic acid (BSA) and their six mono-, di-, and trinitro-substituted derivatives are calculated by means of B3LYP/cc-pVTZ and MP2/6-311++G**. The dependences of ΔrG298○ on the number and the position of nitro groups in an aromatic ring are revealed, as is the possibility of intramolecular hydrogen bond (IHB) formation in ortho-substituted acids. It is found that the deprotonation of conformers of ortho-nitro-substituted BSA without IHBs requires less energy (by 4–5 kcal/mol) than for conformers with IHBs. It is shown that the ΔrG298○ values for substituted BA are ∼22 kcal/mol higher than the corresponding values for substituted BSA. A trend of diminishing ΔrG298○ for nitro-substituted acids is observed when the number of nitro groups is increased, and di- and trinitro-substituted BSA may therefore be considered superstrong acids.

Relationships between geometrical and electronic structures and optical properties of 1,8-naphthosultam substituents and derivatives: TDDFT study

Geometrical and electronic structure and optical properties of several substituents and derivatives of 1,8-naphthosultam are studied by quantum-chemical DFT and TDDFT. It is found that the substituents –NO2, –CF3, and –N(CH3)2 affect the geometrical and electronic structure the most. It is shown that the Stokes shift is greatest (≈260 nm) for compounds with the strong donor substituent –N(CH3)2, while strong acceptor substituents provide a quite small Stokes shift. The dependence of the Stokes shift on the difference in energies of the frontier orbitals of the ground and excited states of molecules is found. Of the 1,8-naphthosultam substituents considered, compounds with –N(CH3)2 substituent, which emit in the biological window region, can be advised for use in optical bioimaging. The results can be used as a basis for the development and creation of new functional materials and biologically active compounds.

Characteristics of the hydrogen bond and the structure of Н-complexes of p-n-propyloxybenzoic acid and p-n-propyloxy-p′-cyanobiphenyl

The conformational properties of p-n-propyloxybenzoic acid and p-n-propyloxy-p′-cyanobiphenyl molecules, which can exhibit liquid crystalline properties in the formation of Н-complexes, are studied (DFT/B3LYP)/cc-pVTZ method). It is found that a molecule of p-n-propyloxybenzoic acid has 16 conformers that can be divided into four groups with respect to relative energies (0 kcal/mol, 1.6 kcal/mol, 6.5 kcal/mol, and 8.1 kcal/mol), and a molecule of p-n-propyloxy-p′-cyanobiphenyl has six conformers with relative energies of 0 kcal/mol (two conformers, φ(СPh–O–C–C)=180°) and 1.6 kcal/mol (four conformers, φ(СPh–O–C–C)=64.4°). In all conformers of the 3-AOCB molecule, phenyl rings are turned at 35° relative to each other. A conformation with the planar arrangement of two rings has a higher energy by 1.5 kcal/mol. Barriers to the internal rotation of different groups are determined and it is established that the structural nonrigidity of the molecules is mainly due to the possible rotation of the–C2Н5 moiety about the C–C bond. It is shown that with increasing temperature the vibrational amplitudes of the OC3H7 substituent, which enhance the probabilities of transitions between the conformers, become appreciably larger. It is found that p-n-propyloxybenzoic acid and p-n-propyloxy-p′-cyanobiphenyl can form Н-complexes with the medium hydrogen bond. Two types of the structural organization of Н-complexes are considered: linear and angular. The similarity of energies of Н-complexes with different structures (NBO analysis) can be the reason for the occurrence of two liquid crystalline subphases of p-n-propyloxybenzoic acid and p-n-propyloxy-p′-cyanobiphenyl system.

Manifestation of the gauche effect in conformers of benzenesulfonic acid hydrazide

Conformational properties of a benzenesulfonic acid hydrazide molecule and its para-nitro and para-methyl derivatives, which have found wide application as porofors and biologically active compounds, are studied. It is found that the benzenesulfonic acid hydrazide molecule has six conformers with relative energies of 0//0 kcal/mol, 0.34//0.98 kcal/mol, 2.51//2.25 kcal/mol, 2.54//2.56 kcal/mol, 2.90//3.28 kcal/mol, 6.64//6.43 kcal/mol (MP2//DFT(B3LYP) with the cc-pVTZ basis set), each conformer has enantiomer. The conformers differ from each other in the relative orientation of the fragments of the–SO2NHNH2 group, the energies of the frontier orbitals, the direction and value of the dipole moments. It is shown that the introduction of a nitro or methyl group into the para-position practically does not affect the conformational properties of the sulfonyl hydrazide group. Change in the structure of benzenesulfonic acid hydrazide during the crystal–gas transition is considered and it is revealed that in the crystal the conformation similar in structure to one of the high-energy conformers of the free molecule is stabilized. The NBO analysis of the electron density distribution is performed and it is shown that the occurrence of the gauche effect in all conformers of the molecules under study can be interpreted by the manifestation of the total action of strong anomeric effects between the lone pairs of nitrogen atoms and antibonding orbitals of S=O, N–H, C–S, and N–S bonds.